NTP TECHNICAL REPORT 

ON THE 

TOXICOLOGY AND CARCINOGENESIS
 

STUDIES OF ISOBUTENE
 

(CAS NO. 115-11-7) 

IN F344/N RATS AND B6C3F1 MICE

(INHALATION STUDIES) 

NATIONAL TOXICOLOGY PROGRAM
 

P.O. Box 12233
 

Research Triangle Park, NC 27709
 

December 1998 

NTP TR 487 

NIH Publication No. 99-3977 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
 

Public Health Service
 

National Institutes of Health
 
FOREWORD 

The National Toxicology Program (NTP) is made up of four charter agencies of the U.S. Department of 

Health and Human Services (DHHS): the National Cancer Institute (NCI), National Institutes of Health; the 

National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health; the National 

Center for Toxicological Research (NCTR), Food and Drug Administration; and the National Institute for 

Occupational Safety and Health (NIOSH), Centers for Disease Control. In July 1981, the Carcinogenesis 

Bioassay Testing Program, NCI, was transferred to the NIEHS. The NTP coordinates the relevant 

programs, staff, and resources from these Public Health Service agencies relating to basic and applied 

research and to biological assay development and validation. 

The NTP develops, evaluates, and disseminates scientific information about potentially toxic and hazardous 

chemicals. This knowledge is used for protecting the health of the American people and for the primary 

prevention of disease. 

The studies described in this Technical Report were performed under the direction of the NIEHS and were 

conducted in compliance with NTP laboratory health and safety requirements and must meet or exceed all 

applicable federal, state, and local health and safety regulations. Animal care and use were in accordance 

with the Public Health Service Policy on Humane Care and Use of Animals. The prechronic and chronic 

studies were conducted in compliance with Food and Drug Administration (FDA) Good Laboratory Practice 

Regulations, and all aspects of the chronic studies were subjected to retrospective quality assurance audits 

before being presented for public review. 

These studies are designed and conducted to characterize and evaluate the toxicologic potential, including 

carcinogenic activity, of selected chemicals in laboratory animals (usually two species, rats and mice). 

Chemicals selected for NTP toxicology and carcinogenesis studies are chosen primarily on the bases of 

human exposure, level of production, and chemical structure. The interpretive conclusions presented in this 

Technical Report are based only on the results of these NTP studies. Extrapolation of these results to other 

species and quantitative risk analyses for humans require wider analyses beyond the purview of these 

studies. Selection per se is not an indicator of a chemical’s carcinogenic potential. 

Listings of all published NTP reports and ongoing studies are available from NTP Central Data 

Management, NIEHS, P.O. Box 12233, MD E1-02, Research Triangle Park, NC 27709 (919-541-3419). 

The Abstracts and other study information for 2-year studies are also available at the NTP’s World Wide 

Web site: http://ntp-server.niehs.nih.gov. 
NTP TECHNICAL REPORT 

ON THE 

TOXICOLOGY AND CARCINOGENESIS
 

STUDIES OF ISOBUTENE
 

(CAS NO. 115-11-7) 

IN F344/N RATS AND B6C3F1 MICE

(INHALATION STUDIES) 

NATIONAL TOXICOLOGY PROGRAM
 

P.O. Box 12233
 

Research Triangle Park, NC 27709
 

December 1998 

NTP TR 487 

NIH Publication No. 99-3977 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
 

Public Health Service
 

National Institutes of Health
 
2 Isobutene, NTP TR 487 

CONTRIBUTORS
 

National Toxicology Program 

Evaluated and interpreted results and reported findings 

J.H. Roycroft, Ph.D., Study Scientist 

D.A. Bridge, B.S. 

J.R. Bucher, Ph.D. 

R.E. Chapin, Ph.D. 

J.R. Hailey, D.V.M. 

J.K. Haseman, Ph.D. 

R.A. Herbert, D.V.M., Ph.D. 

R.R. Maronpot, D.V.M. 

G.N. Rao, D.V.M., Ph.D. 

C.S. Smith, Ph.D. 

G.S. Travlos, D.V.M. 

D.B. Walters, Ph.D. 

K.L. Witt, M.S., Integrated Laboratory Systems 

Battelle Pacific Northwest Laboratory 

Conducted studies, evaluated pathology findings for 14-week and 

2-year studies in rats and mice 

B.J. Chou, D.V.M., Ph.D., Principal Investigator 

J.A. Dill, Ph.D. 

S.L. Grumbein, D.V.M., Ph.D. 

R.A. Miller, D.V.M., Ph.D. 

E.W. Morgan, D.V.M. 

H.A. Ragan, D.V.M. 

R.A. Renne, D.V.M. 

S.E. Rowe, D.V.M., M.S. 

R.B. Westerberg, Ph.D. 

Experimental Pathology Laboratories, Inc. 

Provided pathology quality assurance 

J.F. Hardisty, D.V.M., Principal Investigator 

C.C. Shackelford, D.V.M., M.S., Ph.D. 

Dynamac Corporation 

Prepared quality assurance audits 

S. Brecher, Ph.D., Principal Investigator 

NTP Pathology Working Group 

Evaluated slides, prepared pathology report on rats 

(20 March 1997) 

P.K. Hildebrandt, D.V.M., Chairperson 

PATHCO, Inc. 

S. Ching, D.V.M., Ph.D. 

Glaxo-Wellcome 

J. Dillberger, D.V.M., Ph.D., Observer 

Glaxo-Wellcome 

J.R. Hailey, D.V.M. 

National Toxicology Program 

R.A. Herbert, D.V.M., Ph.D. 

National Toxicology Program 

J.R. Leininger, D.V.M., Ph.D. 

National Toxicology Program 

J. Nold, D.V.M., Ph.D., Observer 

Pathology Associates International 

C.C. Shackelford, D.V.M., M.S., Ph.D. 

Experimental Pathology Laboratories, Inc. 

Evaluated slides, prepared pathology report on mice 

(8 April 1997) 

D.G. Goodman, V.M.D., Chairperson 

PATHCO, Inc. 

J. Everitt, D.V.M. 

Chemistry Industry Institute of Toxicology 

V. Geiss, D.V.M., Observer 

National Toxicology Program 

J.R. Hailey, D.V.M. 

National Toxicology Program 

R.A. Herbert, D.V.M., Ph.D. 

National Toxicology Program 

J.R. Leininger, D.V.M., Ph.D. 

National Toxicology Program 

A. Radovsky, D.V.M., Ph.D. 

National Toxicology Program 

C.C. Shackelford, D.V.M., M.S., Ph.D. 

Experimental Pathology Laboratories, Inc. 
Isobutene, NTP TR 487 3 

Analytical Sciences, Inc. Biotechnical Services, Inc. 

Provided statistical analyses Prepared Technical Report 

R.W. Morris, M.S., Principal Investigator S.R. Gunnels, M.A., Principal Investigator 

S.R. Lloyd, M.S. L.M. Harper, B.S. 

N.G. Mintz, B.S. A.M. Macri-Hanson, M.A., M.F.A. 
4 

CONTENTS
 

ABSTRACT ............................................................. 5
 

EXPLANATION OF LEVELS OF EVIDENCE OF CARCINOGENIC ACTIVITY ............. 8
 

TECHNICAL REPORTS REVIEW SUBCOMMITTEE ................................ 9
 

SUMMARY OF TECHNICAL REPORTS REVIEW SUBCOMMITTEE COMMENTS .......... 10
 

INTRODUCTION ......................................................... 11
 

MATERIALS AND METHODS ................................................ 17
 

RESULTS ............................................................... 29
 

DISCUSSION AND CONCLUSIONS ............................................ 51
 

REFERENCES ........................................................... 53
 

APPENDIX A Summary of Lesions in Male Rats in the 2-Year Inhalation Study of Isobutene ..... 59
 

APPENDIX B Summary of Lesions in Female Rats in the 2-Year Inhalation Study of Isobutene ... 95
 

APPENDIX C Summary of Lesions in Male Mice in the 2-Year Inhalation Study of Isobutene ..... 125
 

APPENDIX D Summary of Lesions in Female Mice in the 2-Year Inhalation Study of Isobutene ... 153
 

APPENDIX E Genetic Toxicology ............................................. 187
 

APPENDIX F Hematology and Clinical Chemistry Results ............................. 193
 

APPENDIX G Organ Weights and Organ-Weight-to-Body-Weight Ratios .................. 199
 

APPENDIX H Reproductive Tissue Evaluations and Estrous Cycle Characterization ........... 203
 

APPENDIX I 2-Hydroxyisobutyric Acid — Biomarker of Exposure ...................... 207
 

APPENDIX J Chemical Characterization and Generation of Chamber Concentrations ......... 211
 

APPENDIX K Ingredients, Nutrient Composition, and Contaminant Levels 

in NIH-07 Rat and Mouse Ration .................................... 223
 

APPENDIX L Sentinel Animal Program ......................................... 227
 
 

5 

ABSTRACT 

CH3 

CH3 C CH2 

ISOBUTENE 

CAS No. 115-11-7 

Chemical Formula: C 4 8 H Molecular Weight: 56.10

Synonyms: (-butylene, isobutylene, liquified petroleum gas, 2-methylpropene 

Isobutene is produced during the fractionation of 

refinery gases or through the catalytic cracking of 

methyl-t-butyl ether. Isobutene is primarily used to 

produce diisobutylene, trimers, butyl rubber, and 

other polymers. In addition, it is used in the production of isooctane, high-octane aviation gasoline, 

methyl-t-butyl ether, and copolymer resins with 

butadiene and acrylonitrile. Isobutene was selected 

for evaluation because of the potential for human 

exposure due to its large production volume and the 

lack of adequate data on its carcinogenic potential. 

The toxicity and carcinogenicity of isobutene were 

determined in male and female F344/N rats and 

B6C3F 1 mice exposed to isobutene (greater than 98%

pure) by inhalation for 14 weeks or 2 years. The 

mutagenicity of isobutene was assessed in Salmonella 

typhimurium, and the frequency of micronuclei was 

determined in the peripheral blood of mice exposed by 

inhalation for 14 weeks. 

14-WEEK STUDIES 

Groups of 10 male and 10 female F344/N rats and 

B6C3F1 mice were exposed to isobutene at concentrations of 0, 500, 1,000, 2,000, 4,000, or 8,000 ppm 

6 hours per day, 5 days per week, for 14 weeks. 

Concentrations greater than 8,000 ppm isobutene were 

not used because of the danger of explosion. All rats 

and mice survived to the end of the study. The final 

mean body weights and body weight gains of all 

exposed groups were similar to those of the chamber 

controls. No exposure-related gross lesions were 

observed in male or female rats or mice at necropsy. 

Microscopically, minimal hypertrophy of goblet cells 

lining the nasopharyngeal duct in the most caudal nose 

section was observed in some rats in each exposed 

group of males and females. 

2-YEAR STUDIES 

Based on the lack of significant exposure-related 

toxicologic effects in the 14-week rat and mouse 

studies, 8,000 ppm was selected as the highest exposure concentration in the 2-year studies. Concentrations of 0, 500, 2,000, and 8,000 ppm were 

selected for rats and mice with the 500 and 2,000 ppm 

selection based on published metabolic elimination 

rates for Sprague-Dawley rats and B6C3F1 mice. 

Rats 

Groups of 50 male and 50 female F344/N rats were 

exposed to isobutene at concentrations of 0, 500, 

2,000, or 8,000 ppm 6 hours per day, 5 days per 

week, for 105 weeks. Survival of exposed males and 

females was similar to that of the chamber controls. 

Mean body weights of exposed groups were generally 

similar to those of the chamber controls throughout 

the study. 
__________ 

6 Isobutene, NTP TR 487 

2-Hydroxyisobutyric Acid — 

Biomarker of Exposure 

2-Hydroxyisobutyric acid (HIBA), the major urinary 

metabolite of isobutene, was measured in the urine of 

male and female rats as an indicator of isobutene 

exposure at 6, 12, and 18 months. The amount of 

HIBA excreted increased with increasing exposure 

concentration. However, when HIBA concentration 

was normalized to isobutene exposure concentration, 

the relative amount of HIBA excreted decreased with 

increasing exposure concentration, implying nonlinear 

kinetics. 

Pathology Findings 

The incidence of thyroid gland follicular cell carcinoma in male rats exposed to 8,000 ppm was increased compared to the chamber control group and 

exceeded the historical control range. The incidences 

of hyaline degeneration of the olfactory epithelium 

were marginally increased in exposed rats; however, 

the severities of hyaline degeneration increased with 

increasing exposure concentration in males and 

females. 

Mice 

Groups of 50 male and 50 female B6C3F1 mice were 

exposed to isobutene at concentrations of 0, 500, 

2,000, or 8,000 ppm 6 hours per day, 5 days per 

week, for 105 weeks. Survival of exposed males and 

females was similar to that of the chamber controls. 

Mean body weights of exposed mice were generally 

similar to those of the chamber controls throughout 

the study except for female mice exposed to 2,000 or 

8,000 ppm, which weighed slightly less than chamber 

controls from about week 52 until week 92. 

2-Hydroxyisobutyric Acid — 

Biomarker of Exposure 

HIBA was measured in the urine of male and female 

mice as an indicator of isobutene exposure at 6, 12, 

and 18 months. The amount of HIBA excreted 

increased with increasing exposure concentration. 

However, when HIBA concentration was normalized 

to isobutene exposure concentration, the relative 

amount of HIBA excreted decreased with increasing 

exposure concentration, implying nonlinear kinetics. 

Pathology Findings 

The incidences of hyaline degeneration of the respiratory epithelium in all groups of exposed males and 

females were significantly greater than those in the 

chamber control groups. The incidences of hyaline 

degeneration of the olfactory epithelium in 2,000 and 

8,000 ppm mice were greater than those in the chamber controls. 

GENETIC TOXICOLOGY 

Isobutene was not mutagenic in any of four strains of 

S. typhimurium, with or without S9 metabolic 

activation, and no increase in the frequency of micronucleated erythrocytes was seen in peripheral blood of 

male or female mice treated with isobutene by inhalation for 14 weeks. 

CONCLUSIONS 

Under the conditions of these 2-year inhalation studies, there was some evidence of carcinogenic activity* 

of isobutene in male F344/N rats based on an increased incidence of follicular cell carcinoma of the 

thyroid gland. There was no evidence of carcinogenic 

activity of isobutene in female F344/N rats or male or 

female B6C3F mice exposed to 500, 2,000, or 1 

8,000 ppm. 

Exposure to isobutene by inhalation for 2 years 

resulted in increased incidences and/or severities of 

nasal lesions including hyaline degeneration of the 

olfactory epithelium in male and female rats and mice 

and hyaline degeneration of the respiratory epithelium 

in male and female mice. 

* Explanation of Levels of Evidence of Carcinogenic Activity is on page 8. A summary of the Technical Reports Review Subcommittee 

comments and the public discussion on this Technical Report appears on page 10. 
Isobutene, NTP TR 487 7 

Summary of the 2-Year Carcinogenesis and Genetic Toxicology Studies of Isobutene 

Male Female Male Female
 

F344/N Rats F344/N Rats B6C3F Mice B6C3F Mice
 1 1 

Concentrations	 0, 500, 2,000, or 0, 500, 2,000, or 0, 500, 2,000, or 0, 500, 2,000, or 

in air 8,000 ppm 8,000 ppm 8,000 ppm 8,000 ppm

Body weights Exposed groups similar to Exposed groups similar to Exposed groups similar to 2,000 and 8,000 ppm groups 

chamber control group chamber control group chamber control group slightly less than chamber 

control group 

Survival rates	 7/50, 5/50, 6/50, 8/50 23/50, 19/50, 33/50, 22/50 28/50, 32/50, 27/50, 28/50 32/50, 31/50, 39/50, 33/50 

Nonneoplastic	 Nose: severity of olfactory Nose: severity of olfactory Nose: respiratory epithelial Nose: respiratory epithelial 

epithelial hyaline epithelial hyaline hyaline degeneration (6/50, hyaline degeneration (21/47, effects 

degeneration (1.3, 1.4, 2.2, degeneration (1.5, 2.4, 2.8,	 19/49, 29/50, 39/48); 39/50, 41/49, 48/50); 

2.6) 2.8)	 olfactory epithelial hyaline olfactory epithelial hyaline 

degeneration (6/50, 7/49, degeneration (17/47, 19/50, 

16/50, 17/48) 24/49, 27/50) 

Neoplastic	 Thyroid gland: follicular None None None 

cell carcinoma (1/48, 0/48, effects 

0/48, 5/50) 

Level of Some evidence No evidence No evidence No evidence 

evidence of 

carcinogenic 

activity 

Genetic toxicology 

Salmonella typhimurium gene mutations: Negative in strains TA97, TA98, TA100, and TA1535, with and without S9
 

Micronucleated erythrocytes
 

Mouse peripheral blood in vivo: Negative in male and female mice
 
8 Isobutene, NTP TR 487 

EXPLANATION OF LEVELS OF EVIDENCE OF CARCINOGENIC ACTIVITY 

The National Toxicology Program describes the results of individual experiments on a chemical agent and notes the strength of the evidence for 

conclusions regarding each study. Negative results, in which the study animals do not have a greater incidence of neoplasia than control 

animals, do not necessarily mean that a chemical is not a carcinogen, inasmuch as the experiments are conducted under a limited set of 

conditions. Positive results demonstrate that a chemical is carcinogenic for laboratory animals under the conditions of the study and indicate that 

exposure to the chemical has the potential for hazard to humans. Other organizations, such as the International Agency for Research on Cancer, 

assign a strength of evidence for conclusions based on an examination of all available evidence, including animal studies such as those conducted 

by the NTP, epidemiologic studies, and estimates of exposure. Thus, the actual determination of risk to humans from chemicals found to be 

carcinogenic in laboratory animals requires a wider analysis that extends beyond the purview of these studies. 

Five categories of evidence of carcinogenic activity are used in the Technical Report series to summarize the strength of the evidence observed 

in each experiment: two categories for positive results (clear evidence and some evidence); one category for uncertain findings (equivocal 

evidence); one category for no observable effects (no evidence); and one category for experiments that cannot be evaluated because of major 

flaws (inadequate study). These categories of interpretative conclusions were first adopted in June 1983 and then revised in March 1986 for 

use in the Technical Report series to incorporate more specifically the concept of actual weight of evidence of carcinogenic activity. For each 

separate experiment (male rats, female rats, male mice, female mice), one of the following five categories is selected to describe the findings. 

These categories refer to the strength of the experimental evidence and not to potency or mechanism. 

•	 Clear evidence of carcinogenic activity is demonstrated by studies that are interpreted as showing a dose-related 

(i) increase of malignant neoplasms, (ii) increase of a combination of malignant and benign neoplasms, or (iii) marked increase of 

benign neoplasms if there is an indication from this or other studies of the ability of such tumors to progress to malignancy. 

•	 Some evidence of carcinogenic activity is demonstrated by studies that are interpreted as showing a chemical-related increased 

incidence of neoplasms (malignant, benign, or combined) in which the strength of the response is less than that required for clear 

evidence. 

•	 Equivocal evidence of carcinogenic activity is demonstrated by studies that are interpreted as showing a marginal increase of
 

neoplasms that may be chemical related.
 

•	 No evidence of carcinogenic activity is demonstrated by studies that are interpreted as showing no chemical-related increases in 

malignant or benign neoplasms. 

•	 Inadequate study of carcinogenic activity is demonstrated by studies that, because of major qualitative or quantitative limitations, 

cannot be interpreted as valid for showing either the presence or absence of carcinogenic activity. 

When a conclusion statement for a particular experiment is selected, consideration must be given to key factors that would extend the actual 

boundary of an individual category of evidence. Such consideration should allow for incorporation of scientific experience and current 

understanding of long-term carcinogenesis studies in laboratory animals, especially for those evaluations that may be on the borderline between 

two adjacent levels. These considerations should include: 

• adequacy of the experimental design and conduct;
 

• occurrence of common versus uncommon neoplasia;
 

•	 progression (or lack thereof) from benign to malignant neoplasia as well as from preneoplastic to neoplastic lesions; 

•	 some benign neoplasms have the capacity to regress but others (of the same morphologic type) progress. At present, it is 

impossible to identify the difference. Therefore, where progression is known to be a possibility, the most prudent course is to 

assume that benign neoplasms of those types have the potential to become malignant; 

•	 combining benign and malignant tumor incidence known or thought to represent stages of progression in the same organ or tissue; 

•	 latency in tumor induction; 

•	 multiplicity in site-specific neoplasia; 

•	 metastases; 

•	 supporting information from proliferative lesions (hyperplasia) in the same site of neoplasia or in other experiments (same lesion in 

another sex or species); 

•	 presence or absence of dose relationships; 

•	 statistical significance of the observed tumor increase; 

•	 concurrent control tumor incidence as well as the historical control rate and variability for a specific neoplasm; 

•	 survival-adjusted analyses and false positive or false negative concerns; 

•	 structure-activity correlations; and 

•	 in some cases, genetic toxicology. 
Isobutene, NTP TR 487 9 

NATIONAL TOXICOLOGY PROGRAM BOARD OF SCIENTIFIC COUNSELORS
 

TECHNICAL REPORTS REVIEW SUBCOMMITTEE
 

The members of the Technical Reports Review Subcommittee who evaluated the draft NTP Technical Report on isobutene on
 

10 December 1997 are listed below. Subcommittee members serve as independent scientists, not as representatives of any institution, company,
 

or governmental agency. In this capacity, subcommittee members have five major responsibilities in reviewing the NTP studies:
 

• to ascertain that all relevant literature data have been adequately cited and interpreted, 

• to determine if the design and conditions of the NTP studies were appropriate, 

• to ensure that the Technical Report presents the experimental results and conclusions fully and clearly, 

• to judge the significance of the experimental results by scientific criteria, and 

• to assess the evaluation of the evidence of carcinogenic activity and other observed toxic responses. 

Gary P. Carlson, Ph.D., Chairperson John M. Cullen, Ph.D., V.M.D. 

School of Health Sciences Department of Microbiology, Parasitology, and Pathology 

Purdue University College of Veterinary Medicine 

West Lafayette, IN North Carolina State University 

Raleigh, NC 

A. John Bailer, Ph.D., Principal Reviewer 

Department of Mathematics and Statistics Susan M. Fischer, Ph.D.

Miami University M.D. Anderson Cancer Center 

Oxford, OH University of Texas 

Smithville, TX

Steven A. Belinsky, Ph.D., Principal Reviewer 

Inhalation Toxicology Research Institute Thomas L. Goldsworthy, Ph.D. Kirkland Air Force Base Integrated Laboratory Systems Albuquerque, NM Research Triangle Park, NC 

James S. Bus, Ph.D. Irma Russo, M.D. Health and Environmental Sciences Fox Chase Cancer Center Dow Chemical Company Philadelphia, PA Midland, MI 

Linda A. Chatman, D.V.M. 

Pfizer, Inc. 

Groton, CT 

Special Reviewers 

Stephen S. Hecht, Ph.D. Jose Russo, M.D. 

University of Minnesota Cancer Centers Fox Chase Cancer Center 

Minneapolis, MN Philadelphia, PA 

Michele Medinsky, Ph.D., Principal Reviewer 

Chemical Industry Institute of Toxicology 

Research Triangle Park, NC 
10 Isobutene, NTP TR 487 

SUMMARY OF TECHNICAL REPORTS REVIEW SUBCOMMITTEE COMMENTS
 

On 10 December 1997, the draft Technical Report on information on exposure assessment for chemicals of 

the toxicology and carcinogenesis studies of isobutene interest to the NTP, including those evaluated by this 

received public review by the National Toxicology Subcommittee. 

Program’s Board of Scientific Counselors’ Technical 

Reports Review Subcommittee. The review meeting Dr. Medinsky, the second principal reviewer, agreed 

was held at the National Institute of Environmental with the proposed conclusions. She complimented 

Health Sciences, Research Triangle Park, NC. the NTP for the use of pharmacokinetic data. 

Dr. Medinsky said it would be useful to include 

Dr. J.H. Roycroft, NIEHS, introduced the toxicology a metabolic scheme for isobutene so that the 

and carcinogenesis studies of isobutene by discussing reader could readily see where the biomarker, 

the uses of the chemical and the rationale for study, 3-hydroxyisobutyric acid, fits in the metabolic fate of 

describing the experimental design, reporting on isobutene relative to what in fact might be the toxic 

survival and body weight effects, and commenting metabolite for this chemical. Dr. Roycroft said that a 

on compound-related neoplastic and nonneoplastic metabolic scheme could be included, that the monolesions in rats and mice. The proposed conclusions epoxide is a putative metabolite, and that a metabolic 

for the 2-year studies were some evidence of modeling effort has been started on isobutene to precarcinogenic activity in male F344/N rats and no dict some blood concentrations of the epoxide. 

evidence of carcinogenic activity in female F344/N 

rats or male or female B6C3F mice. Dr. Belinsky, the third principal reviewer, agreed 1 

with the proposed conclusions. He thought it interDr. Bailer, a principal reviewer, agreed in principle esting that there was no apparent precursor lesion, 

with the proposed conclusions. He said that because hyperplasia, for the thyroid gland neoplasms. 

there were neoplasms at only one site in the Dr. R.A. Herbert, NIEHS, commented that hyper8,000 ppm group in just one species, he would be plasia can be considered a preneoplastic lesion for 

comfortable as well with equivocal evidence of thyroid gland neoplasms and that this has been seen in 

carcinogenic activity in male rats. Dr. Bailer said that other studies; however, in this study, the incidences of 

he would like to see information on typical levels of follicular cell hyperplasia were not significantly 

human exposure for the potentially exposed workers. increased. 

Dr. Roycroft responded that there were no human 

exposure data, and only a limited amount of data in Dr. Bailer moved that the Technical Report on isothe literature provides even a hint of how people butene be accepted with the revisions discussed and 

might be exposed to isobutene; for example, 1% of the conclusions as written for male rats, some evigasoline may be isobutene. Dr. G.W. Lucier, dence of carcinogenic activity, and for female rats and 

NIEHS, reported that the NIEHS/NTP has recently male and female mice, no evidence of carcinogenic 

established an interagency agreement with the Centers activity. Dr. Medinsky seconded the motion, which 

for Disease Control and Prevention, to provide was accepted unanimously with eight votes. 
 

11 

INTRODUCTION 

CH3 

CH3 C CH2 

ISOBUTENE 

CAS No. 115-11-7 

Chemical Formula: C 4 8 H Molecular Weight: 56.10

Synonyms: (-butylene, isobutylene, liquified petroleum gas, 2-methylpropene 

CHEMICAL AND PHYSICAL PROPERTIES 

Isobutene is a colorless, volatile liquid or easily liquified gas with a coal-gas odor. Isobutene has a melting 

point of S139E C, a boiling point of S6.9E C, a 

density of 0.5942, a vapor density of 2.01, and a 

vapor pressure of 400 mm Hg at 21.6E C. It is 

insoluble in water but very soluble in alcohol, benzene, ether, and sulfuric acid. Isobutene is highly 

flammable with a flash point of S76.11E C and an 

explosion limit range of 1.8% to 9.6%. Isobutene 

reacts vigorously with oxidizing materials and polymerizes easily (Material Safety Data Sheet, 1988; 

Merck Index, 1989; Sax’s, 1992; Hawley’s, 1993; 

Patty’s, 1994). 

PRODUCTION, USE, 

AND HUMAN EXPOSURE 

Isobutene is produced during the fractionation of 

refinery gases or through the catalytic cracking of 

methyl-t-butyl ether. Isobutene is primarily used to 

produce diisobutylene, trimers, butyl rubber, and 

other polymers. In addition, it is used in the production of isooctane, high-octane aviation gasoline, 

methyl-t-butyl ether, and copolymer resins with 

butadiene and acrylonitrile (Merck Index, 1989; 

Hawley’s, 1993; Patty’s, 1994). It was estimated that 

approximately 1.3 billion pounds of isobutene were 

produced in the United States in 1992 (HSDB, 1997). 

Although isobutene has been detected in urban atmosphere at low concentrations (Patty’s, 1994), human 

exposure occurs primarily in the workplace. According to the National Occupational Exposure Survey, 

approximately 7,000 employees were potentially 

exposed to isobutene from 1981 until 1983 (NIOSH, 

1990). Currently, no occupational exposure limits 

have been established. Although isobutene is a 

component of petroleum and natural gas, there are no 

reports of the detection of this water-insoluble, 

volatile gas in drinking water or wastewater. 

ABSORPTION, DISTRIBUTION, 

METABOLISM, AND EXCRETION 

Frank et al. (1980) detected isobutene in exhaled 

breath of fasted Sprague-Dawley rats given 5 g 

ethanol/kg body weight intraperitoneally. In addition, 

using a closed inhalation exposure system with 

Sprague-Dawley rats, Frank et al. (1980) determined 

that the isobutene elimination half-life (at an initial 

concentration of 5,000 ppm) was 1.3 hours. 

Csanády et al. (1991), also using a closed inhalation 

exposure system, investigated the metabolism and 

elimination of isobutene over a 5-hour period by male 

Sprague-Dawley rats and B6C3F mice exposed to 1 

isobutene concentrations of 100 to 12,000 ppm. The 

metabolism of isobutene in both species followed 

Michaelis-Menten kinetics. At concentrations up to 
12 Isobutene, NTP TR 487 

500 ppm isobutene, the metabolic elimination in rats in the lung and the rapid formation and disappearance 

and mice was directly proportional to exposure in the liver. 

concentration. Maximum metabolic elimination rates 

(V max ) were determined as 340 μmol/kg per hour for Henderson et al. (1993) investigated the disposition of 

rats and 560 μmol/kg per hour for mice. The atmo- isobutene in male Sprague-Dawley rats exposed to 0, 

spheric concentration at which V max /2 was reached 40, 400, or 4,000 ppm isobutene by nose-only inhawas 1,200 ppm for rats and 1,800 ppm for mice. The lation for 2 hours. Blood levels of isobutene were 

higher Vmax for mice can be explained by the higher linearly related to exposure concentration between 40 

microsomal monooxygenase activity in mice. These and 400 ppm but increased in a supralinear fashion at 

investigations also demonstrated that the epoxide the highest concentration, suggesting that in the rat, 

1,1-dimethyloxirane (2-methyl-1,2-epoxypropane) was metabolism of isobutene may become saturated at 

formed as the primary reactive intermediate during the exposure concentrations between 400 and 4,000 ppm. 

metabolism of isobutene in rats and was detected in These investigators also studied the total uptake, 

exhaled air of these rats at a maximum concentration excretion patterns, and metabolism of isobutene in 

of about 0.05 ppm. Under conditions of saturation, rats exposed to 0, 2, 40, 400, or 4,000 ppm 

14 the epoxide was only about 7% of that observed for [ C]-isobutene by inhalation for up to 6 hours. Based 

ethene oxide (epoxide of ethene) and about 1% of that on the recovered dose of radioactivity, isobutene 

observed for 1,2-epoxy-3-butene (epoxide of absorption was linear up to an exposure concentration 

1,3-butadiene). In another study, these authors of 40 ppm with approximately 8% of the inhaled dose 

showed that isobutene was metabolized to the epoxide being absorbed. However, absorption dropped to 

1,1-dimethyloxirane with mouse liver homogenates approximately 5% at 400 ppm and 2% at 4,000 ppm. 

(Cornet et al., 1991), and this epoxidation was The amount of isobutene metabolized was also linear 

cytochrome P dependent. Accumulation of the up to an exposure concentration of 40 ppm but de­ 450 

epoxide was not linear with time and reached a creased at higher concentrations; over 90% of the 

maximum after 20 minutes. This kinetic behavior was absorbed isobutene was metabolized at concentrations 

explained by the immediate actions of epoxide hydro- up to 400 ppm, but at 4,000 ppm, approximately 20% 

lase and glutathione-S-transferase converting the of the absorbed dose was exhaled as unmetabolized 

epoxide to 2-methyl-1,2-propanediol and to a gluta- isobutene. In addition, rat urinary metabolites of 

thione conjugate, respectively. These enzymes are the isobutene were identified as isobutenediol and 

most important epoxide detoxification mechanisms 2-hydroxyisobutyric acid. Two other urinary metapresent in liver tissue (Figure 1). bolites were tentatively identified as sulfate conjugates 

of isobutanediol. 

Peter et al. (1987) investigated the inhalation pharCornet et al. (1995a) compared the biotransformation macokinetics of isoprene, which is structurally related 

of isobutene to the epoxide in lung and liver homo- to isobutene, in male Wistar rats and male B6C3F 1 

genates taken from male Sprague-Dawley rats. Lung mice in closed desiccator jars equipped with an 

homogenates produced less epoxide than liver homo- oxygen supply and soda lime for carbon dioxide 

genates (15% to 20%) in the first 30 minutes of absorption. Metabolism of isoprene was linear in rats 

incubation, and lung epoxide concentrations at 1 hour and mice up to an atmospheric concentration of about 

were about 40% of those in the liver. Over the course 300 ppm. Saturation of isoprene metabolism was 

of incubation, lung production of epoxide was rela- nearly complete at about 1,000 ppm in rats and at 

tively constant while that of the liver rose sharply and about 2,000 ppm in mice (Peter et al., 1990). The 

declined steadily after 30 minutes. The liver cyto- maximal metabolic elimination rate of inhaled isochrome P 2E1 (CYP2E1) content was 25 times that prene in mice (400 μmol/hr per kg body weight) was 450 

of the lung, and glutathione-S-transferase and epoxide about three times greater than that in rats 

hydrolase activities were about 10 times greater in the (130 μmol/hr per kg body weight). The half-life of 

liver than the lung. These factors may explain the isoprene was 6.8 minutes in rats and 4.4 minutes in 

relatively slow formation and persistence of epoxide mice. 
 

Isobutene, NTP TR 487 13 

FIGURE 1 

Biotransformation of Isobutene 

Cornet et al. (1995b) investigated the in vitro metabolism 

of isobutene in human liver as well as livers from rats and 

mice and showed that all three formed the epoxide 

2-methyl-1,2-epoxypropane in liver homogenates. 

However, there were quantitative differences in epoxide 

formation with the highest concentrations being found 

with the mouse tissue,followed by rat (one-half that of the 

mouse) and human (one-tenth that of the mouse). 

Conversion of isobutene to its epoxide was mediated by 

CYP2E1 in each of these species. The concentration of 

CYP2E1 in the liver of humans, rats, and mice showed a 

pattern similar to that of the interspecies epoxide ­

concentrations in that mice had the highest activity of this 

enzyme. Human liver, however, contained higher concentrations of the microsomal epoxide-detoxifying enzyme 

epoxide hydrolase, and rat liver had about one-third the 

concentration observed in humans. The mouse liver had a 

very low activity of only 5% of that in human liver but had 

three times as much glutathione-S-transferase activity as 

rat and human livers. No other studies on the absorption, 
14 Isobutene, NTP TR 487 

distribution, metabolism, or excretion of isobutene in 

humans were found in the available literature 

(National Library of Medicine, 1997). 

Isoprene is metabolized to the epoxides 3,4-epoxy3-methyl-1-butene and 3,4-epoxy-2-methyl-1-butene 

by liver microsomal cytochrome P450-dependent 

monooxygenases (primarily CYP2E1) obtained from 

Swiss mice, Wistar rats, New Zealand rabbits, Syrian 

golden hamsters, and human cell lines (Del Monte 

et al., 1985; Longo et al., 1985; Gervasi and Longo, 

1990; Bogaards et al., 1996). The 2-methyl epoxide 

is produced at about 20% to 25% the rate of the 

3-methyl epoxide. The epoxide intermediates of 

isoprene biotransformation may undergo hydrolysis to 

form vicinal diols (catalyzed by epoxide hydrolase), 

may be conjugated with glutathione (catalyzed 

by glutathione-S-transferase), or may be further 

oxidized to isoprene diepoxide (2-methyl-1,2:3,4­

diepoxybutane) (Wistuba et al., 1994). Both isoprene 

monoepoxides are further oxidized to isoprene 

diepoxide by rat, mouse, and human liver microsomes 

(Wistuba et al., 1994; Bogaards et al., 1996). The 

Vma value for isoprene oxidation to the 3-methyl x 

epoxide in mice is about seven times that in rats, 

whereas the apparent Km values are similar in these 

two species. Inhibition of epoxide hydrolase activity 

results in nearly similar rates of monoepoxide formation by rat, mouse, and human liver microsomes 

(Bogaards et al., 1996). The biotransformation of 

isoprene is similar to that of 1,3-butadiene, which 

involves initial oxidation to 1,2-epoxy-3-butene 

followed by hydrolysis to 3-butene-1,2-diol, conjugation with glutathione, or further oxidation to 

diepoxybutane (Malvoisin et al., 1979; Malvoisin and 

Roberfroid, 1982; Csanády et al., 1992). As with 

isobutene, CYP2E1 is the major enzyme involved in 

the hepatic metabolism of isoprene and 1,3-butadiene. 

TOXICITY 

Experimental Animals 

There is little information in the literature on the 

toxicity of isobutene in animals. The 4-hour LC50 in 

3 rats is 620 g/m (270,000 ppm), and the 2-hour LC50 3 in mice is 415 g/m (180,000 ppm) (Sax’s, 1992). 

Humans 

No epidemiological studies or reports of health effects 

related to exposure to isobutene were found in the 

literature (National Library of Medicine, 1997). 

CARCINOGENICITY 

Experimental Animals 

No information on the carcinogenicity of isobutene in 

experimental animals was found in a search of the 

available literature (National Library of Medicine, 

1997). 

Several structurally similar compounds which, like 

isobutene, form epoxide intermediates have been 

tested in rodent bioassays. There was no evidence of 

carcinogenicity in male or female F344/N rats or 

B6C3F mice exposed by inhalation for 2 years to 1 

5,000 or 10,000 ppm propylene (NTP, 1985a). 

However, male and female F344/N rats exposed to 

400 ppm propylene oxide for 2 years had increased 

incidences of papillary adenoma of the nasal turbinates, and male and female B6C3F1 mice exposed to 

400 ppm had increased incidences of hemangioma or 

hemangiosarcoma of the nasal turbinates (NTP, 

1985b). A marginal increase in the incidence of 

interstitial cell adenoma of the testis was observed in 

male F344/N rats exposed to 700, 2,200, or 

7,000 ppm isoprene vapor 6 hours per day, 5 days per 

week for 6 months and then allowed to recover for 

6 months without exposure to isoprene (Melnick 

et al., 1994; NTP, 1995). In addition, increased 

incidences of neoplasms of the liver, lung, forestomach, and harderian gland were observed in male 

B6C3F1 mice exposed by inhalation to 700 ppm or 

higher concentrations of isoprene for 6 months followed by a 6-month recovery period. Exposurerelated increased incidences of liver, lung, harderian 

gland, and forestomach neoplasms were also observed 

in male B6C3F1 mice exposed to isoprene vapor at 

concentrations ranging from 10 to 2,200 ppm for 4 or 

8 hours per day, 5 days per week, for 20, 40, or 

80 weeks followed by a recovery period until 

week 105 (Placke et al., 1996). Increased incidences 

of hemangiosarcoma of the spleen and heart and of 

histiocytic sarcoma were also detected in exposed 

mice. Male and female F344/N rats exposed to 220, 

700, or 7,000 ppm isoprene for 2 years had increased 
Isobutene, NTP TR 487 15 

incidences of mammary gland fibroadenoma (males 

and females) and carcinoma (males), renal tubule 

adenoma (males), and testicular adenoma (NTP, 

1998). 

Male and female Sprague-Dawley rats exposed to 

1,000 or 8,000 ppm 1,3-butadiene for 2 years had 

dose-related neoplasms at multiple sites (Owen et al., 

1987). Male rats had increased incidences of 

pancreatic exocrine neoplasms and Leydig cell tumors 

of the testis. Female rats exposed to 1,3-butadiene 

had uterine stromal sarcomas, Zymbal’s gland carcinomas, mammary gland fibroadenomas and 

carcinomas, and thyroid gland follicular cell 

neoplasms. Exposure to 6.25, 20, 62.5, 200, 

or 625 ppm 1,3-butadiene for 2 years produced 

dose-related neoplasms at multiple sites in male and 

female B6C3F mice (NTP, 1993). Exposed 1 

mice had increased incidences of neoplasms of the 

hematopoietic system, brain (males), forestomach, 

harderian gland, heart, kidney (males), liver, 

lung, mammary gland (females), ovary, and preputial gland. 

Humans 

No epidemiology studies of isobutene in humans were 

found in the available literature (National Library of 

Medicine, 1997). 

GENETIC TOXICITY 

Little information is available on the mutagenicity of 

isobutene. Because it is a gaseous substance, modifications to standard testing protocols are necessary to 

achieve adequate and accurate exposure. An early 

abstract reported negative results in genetic toxicity 

tests with isobutene in several strains of Salmonella 

typhimurium and in L5178Y mouse lymphoma cells, 

with and without rat liver S9 metabolic activation 

enzymes (Staab and Sarginson, 1984). It was not 

stated whether experimental controls for volatility 

were employed in these experiments. However, two 

recent investigations which employed airtight exposure systems also found no mutagenicity in 

S. typhimurium strain TA100, TA102, or TA1535 

(Cornet et al., 1992) at concentrations up to 

50,000 ppm isobutene and no induction of micronuclei 

in human lymphocytes exposed to isobutene in vitro 

(Jorritsma et al., 1995). Both studies, however, 

found positive results for each endpoint (gene mutation in S. typhimurium and chromosome damage in 

lymphocytes) when the primary metabolite of isobutene, 2-methyl-1,2-epoxypropane, was tested in the 

absence of metabolic activation enzymes. Cornet 

et al. (1992) reported that no bacterial toxicity was 

noted in any of the three tester strains used to determine isobutene mutagenicity, but toxicity was noted in 

each strain at 75,000 ppm, the highest concentration 

of 2-methyl-1,2-epoxypropane tested. Further investigations by this same laboratory of the mutagenic activity of 2-methyl-1,2-epoxypropane in 

S. typhimurium strains TA100 and TA1535 also 

showed significant increases in mutations at concentrations as low as 250 ppm without S9 (Castelain 

et al., 1993). Mutagenic activity with S9 required 

approximately a fourfold increase in the concentration 

of 2-methyl-1,2-epoxypropane. In addition, an earlier 

report of a mutagenicity test with 2-methyl-1,2-epoxypropane showed a doubling of the mutation rate in 

Klebsiella pneumoniae exposed to 5 or 10 mmol of 

2-methyl-1,2-epoxypropane/L (Voogd et al., 1981). 

STUDY RATIONALE 

Isobutene was nominated by the National Cancer 

Institute for study based on the potential for human 

exposure to the chemical due to its large production 

volume, the lack of test data, and structural relationship to the known animal carcinogens isoprene 

and 1,3-butadiene. Inhalation was chosen as the route 

of exposure because human exposure occurs primarily 

via this route. 
16 Isobutene, NTP TR 487
 
17 

MATERIALS AND METHODS
 

PROCUREMENT AND 

CHARACTERIZATION OF ISOBUTENE 

Isobutene was manufactured by Exxon, Inc. 

(Baytown, TX), supplied by Specialty Gas Concepts 

(La Porte, TX), and shipped through Norco 

(Kennewick, WA) in two lots (SGC051091ECA and 

SGC020594ECA). Lot SGC051091ECA was used 

during the 14-week and 2-year studies, and 

lot SGC020594ECA was used during the 2-year 

studies. Identity, purity, and stability analyses were 

conducted by the study laboratory, Battelle Pacific 

Northwest Laboratories (Richland, WA). Reports on 

analyses performed in support of the isobutene studies 

are on file at the National Institute of Environmental 

Health Sciences. 

The chemical, a colorless vapor at room temperature, 

was identified as isobutene by infrared and nuclear 

magnetic resonance spectroscopy. The purity of each 

lot was determined by gas chromatography relative to 

a reference standard with a declared purity of 99.9% 

purchased from Matheson Gas Products (East 

Rutherford, NJ). Major peak comparisons by two 

systems indicated a relative purity of 100.0% by one 

system and 100.6% by the second system for 

lot SGC051091ECA relative to the reference standard. 

For lot SGC020594ECA, gas chromatographic peak 

comparison indicated a relative purity of greater 

than 98.4% by each system. The overall purity 

of lot SGC051091ECA was determined to be 

greater than 99%, and the overall purity of lot 

SGC020594ECA was greater than 98%. 

Additional analyses of each lot were performed with 

gas chromatography/mass spectrometry to identify and 

quantify the impurities indicated by the manufacturer 

or by gas chromatography. The mass spectrum 

pattern of the major peak was consistent with isobutene. The following impurities were detected for 

lot SGC051091ECA: propane/propene, 42 ppm; 

isobutane, 140 ppm; 1,3-butadiene, 6 ppm; trans2-butene, 213 ppm; and cis-2-butene, 286 ppm. 

Butane and n-butene coeluted with the major peak and 

were not quantified. Lot SGC020594ECA contained 

an estimated 11 ppm propane/propene, 339 ppm 

isobutane, 15 ppm trans-2-butene, 11 ppm cis-2­

butene, and no 1,3-butadiene. Samples from each 

cylinder of each lot of isobutene were analyzed for 

1,3-butadiene before the cylinder was used for exposure by gas chromatography/mass spectrometry 

with the same gas chromatography system. All 

cylinders used in the 14-week studies contained less 

than 50 ppm 1,3-butadiene, well within the set limit of 

100 ppm; the maximum concentration detected in 

cylinders used in the 2-year studies was 15 ppm. 

Cylinders used in the 14-week studies were also 

screened individually for other impurities; results 

indicated less than 1% impurities by peak area, with 

no impurity present greater than 0.1%. 

In a 4-day pilot study, the stability of isobutene was 

monitored in grab-bag samples taken from the distribution manifold at the beginning and the end of 

6-hour generation periods until approximately 94% of 

the cylinder was exhausted. The results from samples 

taken over 8 test generation days showed no significant enhancement of any volatile impurities, and no 

additional impurities were detected with relative areas 

of 0.1% or greater relative to isobutene. Based on 

these results, approximately 90% of the contents of 

each cylinder were used during the 14-week and 

2-year studies. 

During the studies, the bulk chemical was stored in its 

original shipping cylinders at approximately 22E C. 

Stability was monitored throughout the studies by the 

study laboratory with gas chromatography. No degradation of the bulk chemical was detected. 

VAPOR GENERATION 

AND EXPOSURE SYSTEM 

Isobutene was distributed under regulated pressure. 

The cylinder in use and a backup cylinder were 

connected in parallel to the exposure system. Warm 

circulating-water blankets surrounding the cylinders 

provided additional heat to replace the heat lost due to 

isobutene vaporization. The gas passed through a 
18 Isobutene, NTP TR 487 

filter via a main on/off pneumatic valve and then was 

distributed by a manifold to five (14-week studies) or 

six (2-year studies) pairs of metering valves with 

corresponding flow meters. Isobutene was delivered 

to each exposure chamber through these flow meters 

via three-way solenoid valves located at the chamber 

end of the vapor delivery line. Isobutene vapor was 

diluted with conditioned air as it was injected into the 

chamber inlet duct. The study laboratory designed the 

inhalation exposure chamber (Harford Systems Division, Aberdeen, MD) so that uniform vapor concentrations could be maintained throughout the chamber with the catch pans in place. 

VAPOR CONCENTRATION MONITORING 

Chamber concentrations of isobutene were monitored 

by an on-line gas chromatograph. Samples were 

drawn from each chamber approximately every 

20 minutes during exposures by a computercontrolled, 12-port, stream select valve. The on-line 

gas chromatograph was calibrated by direct analysis 

of volumetrically prepared gas bag standards during 

the 14-week studies and by using validated, commercially prepared standards during the 2-year studies. An on-line standard of isobutene in nitrogen was 

used to monitor instrument drift. Vapor concentration 

uniformity in the exposure chambers without animals 

present was measured using the on-line gas chromatograph before each of the studies began. Chamber 

concentration uniformity was maintained throughout the studies. Summaries of the chamber concentrations for the 14-week and 2-year studies are presented in Tables J1 and J2. 

CHAMBER ATMOSPHERE 

CHARACTERIZATION 

The times for the exposure concentration to build up 

to 90% of the final exposure concentration (T ) and 90 

to decay to 10% of the exposure concentration (T ) 10 

were measured with and without animals present in 

the chambers. In the 14-week and 2-year studies, 

with and without animals in the chambers, T90 values 

ranged from 8 to 15 minutes, and T10 values ranged 

from 6 to 12 minutes. The T90 value selected for all 

studies was 12 minutes. 

Studies of isobutene degradation and monitoring for 

impurities were conducted throughout the studies by 

gas chromatography. No significant degradation of 

isobutene or enhancement of impurities was observed 

during the studies. 

14-WEEK STUDIES 

The 14-week studies were conducted to evaluate the 

cumulative toxic effects of repeated exposure to 

isobutene and to determine the appropriate exposure 

concentrations to be used in the 2-year studies. 

Male and female F344/N rats and B6C3F1 mice were 

obtained from Taconic Farms (Germantown, NY). 

On receipt, the rats and mice were 5 weeks old. 

Animals were quarantined for 11 to 13 days and were 

6 weeks old on the first day of the studies. Before 

initiation of the studies, five male and five female rats 

and mice were randomly selected for parasite evaluation and gross observation for evidence of disease. 

At the end of the studies, serologic analyses were 

performed on five male and five female sentinel rats 

and control mice using the protocols of the NTP 

Sentinel Animal Program. 

Groups of 10 male and 10 female rats and mice were 

exposed to isobutene by inhalation at concentrations of 

0, 500, 1,000, 2,000, 4,000, or 8,000 ppm for 

6 hours plus T90 (12 minutes) per day, 5 days per 

week, for 14 weeks; 8,000 ppm was chosen as the 

highest exposure concentration that could be generated 

safely. It is less than 50% of the lower explosion 

limit of isobutene (approximately 18,000 ppm; 

Patty’s, 1994). Groups of 10 male and 10 female 

special study rats were exposed to the same concentrations for 23 days. Feed was available ad libitum 

except during exposure periods. Water was available 

ad libitum. Rats and mice were housed individually. 

Clinical findings were recorded weekly for rats and 

mice. The animals were weighed initially, weekly, 

and at the end of the studies. Details of the study 

design and animal maintenance are summarized in 

Table 1. 

Clinical pathology studies were performed on 10 male 

and 10 female special study rats per group on days 3 

and 23 and on all core study rats at study termination. 

At all time points, rats were anesthetized with CO2 

and blood was drawn from the retroorbital sinus. 

Blood for hematology determinations was placed in 

tubes containing potassium EDTA as the anticoagulant. Blood for clinical chemistry analyses was 
Isobutene, NTP TR 487 19 

placed in tubes without anticoagulant, allowed to clot 

at room temperature, and centrifuged, and the serum 

was separated. The hematology and clinical chemistry endpoints evaluated are listed in Table 1. 

Hematology determinations, including erythrocyte, 

leukocyte, and platelet counts, hemoglobin concentration, hematocrit, mean cell volume, mean cell 

hemoglobin, and mean cell hemoglobin concentration, 

were performed with an Ortho ELT-8/ds 9000 hematology analyzer (Ortho Diagnostic Systems, 

Westwood, MA). Manual microhematocrit determinations were made using a Damon/IEC microcapillary centrifuge and reader (International Equipment Company, Needham Heights, MA). Leukocyte 

differential and nucleated erythrocyte counts 

and morphologic evaluation of blood cells were determined by light microscopic examination of blood 

films stained with a modified Wright’s stain using a 

Wescor 7100 Aerospray slide stainer (Wescor, Logan, 

UT). Smears made from preparations of equal 

volumes of new methylene blue and whole blood and 

incubated for at least 20 minutes at room temperature 

were examined microscopically, using the Miller disc 

method, for the quantitative determination of reticulocytes. Clinical chemistry endpoints were determined on a Cobas Fara chemistry analyzer (Roche 

Diagnostic Systems, Inc., Montclair, NJ) and an 

Abbott VP bichromatic chemistry instrument (Abbott 

Laboratories, Irving, TX) using reagents and methods 

obtained from the manufacturers. 

At the end of the 14-week studies, samples were 

collected for sperm motility and vaginal cytology 

evaluations on core study rats and mice exposed to 0, 

2,000, 4,000, or 8,000 ppm. The parameters evaluated are listed in Table 1. Methods used were those 

described in the NTP’s sperm morphology and vaginal 

cytology evaluations protocol (NTP, 1987). For 

12 consecutive days prior to scheduled terminal 

sacrifice, the vaginal vaults of the females were 

moistened with saline, if necessary, and samples of 

vaginal fluid and cells were stained. Relative numbers of leukocytes, nucleated epithelial cells, and large 

squamous epithelial cells were determined and used to 

ascertain estrous cycle stage (i.e., diestrus, proestrus, 

estrus, and metestrus). Male animals were evaluated 

for sperm count and motility. The left testis and left 

epididymis were isolated and weighed. The tail of the 

epididymis (cauda epididymis) was then removed 

from the epididymal body (corpus epididymis) and 

weighed. Test yolk (rats) or modified Tyrode’s buffer 

(mice) was applied to slides and a small incision was 

made at the distal border of the cauda epididymis. 

The sperm effluxing from the incision were dispersed 

in the buffer on the slides, and the numbers of motile 

and nonmotile spermatozoa were counted for five 

fields per slide by two observers. Following completion of sperm motility estimates, each left cauda 

epididymis was placed in buffered saline solution. 

Caudae were finely minced, and the tissue was 

incubated in the saline solution and then heat fixed at 

65E C. Sperm density was then determined microscopically with the aid of a hemacytometer. To 

quantify spermatogenesis, the testicular spermatid 

head count was determined by removing the tunica 

albuginea and homogenizing the left testis in 

phosphate-buffered saline containing 10% dimethyl 

sulfoxide. Homogenization-resistant spermatid nuclei 

were counted with a hemacytometer. 

A necropsy was performed on all animals. The heart, 

liver, lung, right kidney, right testis, and thymus were 

weighed. Tissues for microscopic examination were 

fixed and preserved in 10% neutral buffered formalin, 

processed and trimmed, embedded in paraffin, sectioned to a thickness of 5 to 6 μm, and stained with 

hematoxylin and eosin. A complete histopathologic 

examination was performed on chamber control and 

8,000 ppm rats and mice, and the nose was examined 

to the no-effect level in rats. Table 1 lists the tissues 

and organs routinely examined. 

2-YEAR STUDIES 

Study Design 

Groups of 50 male and 50 female rats and mice were 

exposed to isobutene by inhalation at concentrations of 

0, 500, 2,000, or 8,000 ppm 6 hours plus T90 

(12 minutes) per day, 5 days per week, for 

105 weeks. 

Source and Specification of Animals 

Male and female F344/N rats and B6C3F1 mice were 

obtained from Taconic Farms (Germantown, NY) for 

use in the 2-year studies. Rats and mice were 

quarantined for 14 days before the beginning of the 

studies. Five male and five female rats and mice were 

randomly selected for parasite evaluation and gross 

observation of disease. Rats and mice were approximately 6 weeks old at the beginning of the studies. 
20 Isobutene, NTP TR 487 

The health of the animals was monitored during the 

studies according to the protocols of the NTP Sentinel 

Animal Program (Appendix L). 

Animal Maintenance 

Rats and mice were housed individually. Feed was 

available ad libitum except during exposure and urine 

collection periods. Water was available ad libitum. 

Cage batteries were rotated weekly. Further details of 

animal maintenance are given in Table 1. Information 

on feed composition and contaminants is provided in 

Appendix K. 

Clinical Examinations and Pathology 

All animals were observed twice daily, 7 days per 

week. Clinical findings were recorded initially, at 

weeks 5, 6, and 9 (mice), every 4 weeks from week 

4 through week 91 (rats) or from week 13 through 

week 92 (mice), and every 2 weeks until the end of 

the study. Body weights were recorded initially, 

weekly through week 12 (rats) or week 13 (mice), 

every 4 weeks from week 15 (rats) or week 16 (mice) 

through week 91 (rats) or week 92 (mice), and every 

2 weeks until the end of the study. 

Five male and five female rats and mice from the 

control groups and 10 male and 10 female rats 

and mice from each exposed group were evaluated at 

6, 12, and 18 months for determination of 

2-hydroxyisobutyric acid (HIBA) in urine. Rats and 

mice were housed individually in metabolism cages 

for 16 hours after exposure while urine samples were 

collected over ice, after which samples were weighed 

and stored at S20E C in glass vials until analysis. All 

urine samples were analyzed for HIBA and creatinine 

and were prepared for analysis by combining a 

weighed aliquot of urine with sulfuric acid/sodium 

sulfate solution containing an internal standard 

(chloroacetic acid). This mixture was aspirated 

through a C -bonded solid-phase extraction tube and 18 

then extracted with ethyl acetate. Diazomethane was 

added to the ethyl acetate extract to derivatize HIBA 

and chloroacetic acid to their respective methyl esters. 

Samples were analyzed using an HP-5890 Series II 

gas chromatograph equipped with an HP-7673 autosampler and an HP-5971 mass selective detector 

(Hewlett-Packard Company, Wilmington, DE). 

Separation of analytes was achieved using a deactivated, fused silica capillary column (J&W 

Scientific DB-1701) with on-column injection and a 

helium carrier gas at 3 psi constant pressure. Detection and quantitation limits were calculated using 

analysis results from spiked urine standards. The 

parameters measured are listed in Table 1. 

A complete necropsy and microscopic examination 

were performed on all rats and mice. At necropsy, all 

organs and tissues were examined for grossly visible 

lesions, and all major tissues were fixed and preserved 

in 10% neutral buffered formalin, processed and 

trimmed, embedded in paraffin, sectioned to a thickness of 5 to 6 μm, and stained with hematoxylin and 

eosin for microscopic examination. For all paired 

organs (i.e., adrenal gland, kidney, ovary), samples 

from each organ were examined. Tissues examined 

microscopically are listed in Table 1. 

Microscopic evaluations were completed by the study 

laboratory pathologist, and the pathology data were 

entered into the Toxicology Data Management System. The slides, paraffin blocks, and residual wet 

tissues were sent to the NTP Archives for inventory, 

slide/block match, and wet tissue audit. The slides, 

individual animal data records, and pathology tables 

were evaluated by an independent quality assessment 

laboratory. The individual animal records and tables 

were compared for accuracy, the slide and tissue 

counts were verified, and the histotechnique was 

evaluated. For the 2-year rat studies, a quality assessment pathologist evaluated slides from all tumors and 

all potential target organs which included the nose and 

lung of males and females; the heart, liver, spleen, 

and thyroid gland of males; the ovaries of females; 

and all neoplasms in all organs. For the 2-year mouse 

studies, a quality assessment pathologist evaluated 

slides from all tumors and all potential target organs 

which included the nose of males and females, the 

thyroid gland of females, and all neoplasms in all 

organs. In addition, the epididymis was reviewed 

when the diagnosis of sperm granuloma occurred. 

The quality assessment report and the reviewed slides 

were submitted to the NTP Pathology Working Group 

(PWG) chairperson, who reviewed the selected tissues 

and addressed any inconsistencies in the diagnoses 

made by the laboratory and quality assessment pathologists. Representative histopathology slides containing examples of lesions related to chemical administration, examples of disagreements in diagnoses 

between the laboratory and quality assessment pathologist, or lesions of general interest were presented by 

the chairperson to the PWG for review. The PWG 
Isobutene, NTP TR 487 21 

consisted of the quality assessment pathologist and 

other pathologists experienced in rodent toxicologic 

pathology. This group examined the tissues without 

any knowledge of dose groups or previously rendered 

diagnoses. When the PWG consensus differed from 

the opinion of the laboratory pathologist, the diagnosis 

was changed. Final diagnoses for reviewed lesions 

represent a consensus between the laboratory pathologist, reviewing pathologist(s), and the PWG. Details 

of these review procedures have been described, in 

part, by Maronpot and Boorman (1982) and Boorman 

et al. (1985). For subsequent analyses of the pathology data, the decision of whether to evaluate the 

diagnosed lesions for each tissue type separately or 

combined was generally based on the guidelines of 

McConnell et al. (1986). 
22 Isobutene, NTP TR 487 

TABLE 1 

Experimental Design and Materials and Methods in the Inhalation Studies of Isobutene 

14-Week Studies 2-Year Studies 

Study Laboratory 

Battelle Pacific Northwest Laboratories 

(Richland, WA) 

Strain and Species 

Rats: F344/N 

Mice: B6C3F1 

Animal Source 

Taconic Farms 

(Germantown, NY) 

Time Held Before Studies 

Rats: 11 days (males) or 12 days (females) 

Mice: 12 days (males) or 13 days (females) 

Average Age When Studies Began 

6 weeks 

Date of First Exposure 

Rats: 20 January 1992 (males) 

21 January 1992 (females) 

Mice: 21 January 1992 (males) 

22 January 1992 (females) 

Duration of Exposure 

6 hours plus T90 (12 minutes) per day, 5 days per week, for 

14 weeks 

Date of Last Exposure 

Rats: 20 April 1992 (males) 

21 April 1992 (females) 

Mice: 22 April 1992 (males) 

23 April 1992 (females) 

Necropsy Dates 

Rats: 21 April 1992 (males) 

22 April 1992 (females) 

Mice: 23 April 1992 (males) 

24 April 1992 (females) 

Average Age at Necropsy 

20 weeks 

Size of Study Groups 

10 males and 10 females 

Method of Distribution 

Animals were distributed randomly into groups of approximately 

equal initial mean body weights. 

Animals per Cage 

Method of Animal Identification 

Tail tattoo 

Battelle Pacific Northwest Laboratories 

(Richland, WA) 

Rats: F344/N 

Mice: B6C3F1 

Taconic Farms 

(Germantown, NY) 

14 days 

6 weeks 

Rats: 11 March 1993 

Mice: 4 March 1993 

6 hours plus T90 (12 minutes) per day, 5 days per week, for 

105 weeks 

Rats: 10 March 1995 

Mice: 3 March 1995 

Rats: 13-15 March 1995 

Mice: 6-9 March 1995 

Rats: 111 weeks 

Mice: 111-112 weeks 

50 males and 50 females 

Same as 14-week studies 

1 

Tail tattoo 

1 
Isobutene, NTP TR 487 23 

TABLE 1 

Experimental Design and Materials and Methods in the Inhalation Studies of Isobutene 

14-Week Studies 2-Year Studies 

Diet 

NIH-07 open formula pelleted diet (Zeigler Brothers, Inc., 

Gardners, PA), available ad libitum except during exposure 

periods, changed weekly 

Water 

Softened tap water (Richland municipal supply) via automatic 

watering system (Edstrom Industries, Waterford, WI), available 

ad libitum 

Cages 

Stainless steel wire bottom (Hazleton Systems, Inc., Aberdeen, 

MD), changed weekly 

Chamber Air Supply Filters 

Charcoal filter
 

Single HEPA (Northland Filter Systems International,
 

Mechanicville, NY), checked twice per year
 

Purafil® (Environmental Systems, Lynnwood, MA)
 

Chambers 

Stainless steel (Lab Products, Inc., Harford Systems Division, 

Aberdeen, MD), changed weekly 

Chamber Environment 

Temperature: 23.7E-24.4E C 

Relative humidity: 53%-58% 

Room fluorescent light: 12 hours/day 

Chamber air changes: 15/hour 

Exposure Concentrations 

0, 500, 1,000, 2,000, 4,000, or 8,000 ppm 

Type and Frequency of Observation 

Observed twice daily; animals were weighed initially, weekly, and 

at the end of the studies; clinical findings were recorded weekly. 

Method of Sacrifice 

70% CO2 

Necropsy 

Necropsy was performed on all animals. Organs weighed were the 

heart, liver, lung, right kidney, right testis, and thymus. 

NIH-07 open formula pelleted diet (Zeigler Brothers, Inc., 

Gardners, PA), available ad libitum except during exposure and 

urine collection periods, changed weekly 

Same as 14-week studies 

Same as 14-week studies 

Charcoal filter 

Single HEPA (Flanders Filters, Inc., San Rafael, CA), checked 

twice per year 

Purafil® (Environmental Systems, Lynnwood, MA) 

Same as 14-week studies 

Temperature: 23.9E-24.2E C 

Relative humidity: 53%-58% 

Room fluorescent light: 12 hours/day 

Chamber air changes: 15/hour 

0, 500, 2,000, or 8,000 ppm 

Observed twice daily; clinical findings were recorded initially, at 

weeks 5, 6, and 9 (mice), every 4 weeks from week 4 through 

week 91 (rats) or from week 13 through week 92 (mice), and every 

2 weeks until the end of the studies; animals were weighed initially, 

weekly through week 12 (rats) or week 13 (mice), every 4 weeks 

from week 15 (rats) or week 16 (mice) through week 91 (rats) or 

week 92 (mice) and every 2 weeks until the end of the studies. 

Same as 14-week studies 

Necropsy was performed on all animals. 
24 Isobutene, NTP TR 487 

TABLE 1 

Experimental Design and Materials and Methods in the Inhalation Studies of Isobutene 

14-Week Studies 2-Year Studies 

Clinical Pathology 

Blood was collected from the retroorbital plexus of 10 male and 

10 female special study rats at day 3 and day 23 and of core study 

rats at terminal sacrifice for hematology and clinical chemistry 

analyses. 

Hematology: hematocrit (automated and manual); hemoglobin 

concentration; erythrocyte, reticulocyte, nucleated erythrocyte, and 

platelet counts; mean cell volume; mean cell hemoglobin; mean cell 

hemoglobin concentration; leukocyte count and differentials 

Clinical chemistry: urea nitrogen, creatinine, serum glucose, total 

protein, albumin, globulin, A/G ratio, alanine aminotransferase, 

alkaline phosphatase, creatine kinase, sorbitol dehydrogenase, bile 

acids 

Histopathology 

Complete histopathology was performed on all core study chamber 

control and 8,000 ppm rats and mice. In addition to gross lesions 

and tissue masses, the following tissues were examined: adrenal 

gland, bone with marrow, brain, clitoral gland, esophagus, 

gallbladder (mice), heart, large intestine (cecum, colon, rectum), 

small intestine (duodenum, jejunum, ileum), kidney, larynx, liver, 

lung, lymph nodes (mandibular, mesenteric, bronchial, mediastinal), 

mammary gland (with adjacent skin), nose, ovary, pancreas, 

parathyroid gland, pituitary gland, preputial gland, prostate gland, 

salivary gland, spleen, stomach (forestomach and glandular), testes 

(with epididymis and seminal vesicle), thymus, thyroid gland, 

trachea, urinary bladder, and uterus. In addition, the nose of all 

male and female rats was examined. 

Sperm Motility and Vaginal Cytology 

At the end of the studies, sperm samples were collected from all 

male animals in the 0, 2,000, 4,000, and 8,000 ppm exposure 

groups for sperm motility evaluations. The following parameters 

were evaluated: spermatid heads per testis, spermatid count, and 

epididymal spermatozoal concentration and motility. The left cauda 

epididymis, epididymis, and testis were weighed. Vaginal samples 

were collected for up to 12 consecutive days prior to the end of the 

studies from all females exposed to 0, 2,000, 4,000, and 8,000 ppm 

for vaginal cytology evaluations. The following parameters were 

evaluated: estrous cycle length and relative frequency of estrous 

stages. 

2-Hydroxyisobutyric Acid — Biomarker of Exposure 

None 

None 

Complete histopathology was performed on all rats and mice. In 

addition to gross lesions and tissue masses, the following tissues 

were examined: adrenal gland, bone with marrow, brain, clitoral 

gland, esophagus, gallbladder (mice), heart, large intestine (cecum, 

colon, rectum), small intestine (duodenum, jejunum, ileum), kidney, 

larynx, liver, lung, lymph nodes (mandibular, mesenteric, bronchial, 

mediastinal), mammary gland (with adjacent skin), nose, ovary, 

pancreatic islets, parathyroid gland, pituitary gland, preputial gland, 

prostate gland, salivary gland, spleen, stomach (forestomach and 

glandular), testes (with epididymis and seminal vesicle), thymus, 

thyroid gland, trachea, urinary bladder, and uterus. 

None 

Five male and five female rats and mice from the chamber control 

groups and 10 male and 10 female rats and mice from each exposed 

group were evaluated at 6, 12, and 18 months for determination of 

HIBA in urine. Rats and mice were housed individually in 

metabolism cages for 16 hours after exposure while urine samples 

were collected over ice. Parameters evaluated included urinary 

excretion, creatinine, and HIBA. 
Isobutene, NTP TR 487 25 

STATISTICAL METHODS 

Survival Analyses 

The probability of survival was estimated by the 

product-limit procedure of Kaplan and Meier (1958) 

and is presented in the form of graphs. Animals 

found dead of other than natural causes were censored 

from the survival analyses; animals dying from 

natural causes were not censored. Statistical analyses 

for possible dose-related effects on survival used 

Cox’s (1972) method for testing two groups for 

equality and Tarone’s (1975) life table test to identify 

dose-related trends. All reported P values for the 

survival analyses are two sided. 

Calculation of Incidence 

The incidences of neoplasms or nonneoplastic lesions 

are presented in Tables A1, A5, B1, B4, C1, C4, D1, 

and D4 as the number of animals bearing such lesions 

at a specific anatomic site and the number of animals 

with that site examined microscopically. For 

calculation of statistical significance, the incidences of 

most neoplasms (Tables A3, B3, C3, and D3) and all 

nonneoplastic lesions are given as the numbers of 

animals affected at each site examined microscopically. However, when macroscopic examination 

was required to detect neoplasms in certain tissues 

(e.g., harderian gland, intestine, mammary gland, and 

skin) before microscopic evaluation, or when 

neoplasms had multiple potential sites of occurrence 

(e.g., leukemia or lymphoma), the denominators 

consist of the number of animals on which a necropsy 

was performed. Tables A3, B3, C3, and D3 also give 

the survival-adjusted neoplasm rate for each group 

and each site-specific neoplasm. This survivaladjusted rate (based on the Poly-3 method described 

below) accounts for differential mortality by assigning 

a reduced risk of neoplasm, proportional to the third 

power of the fraction of time on study, to animals that 

do not reach terminal sacrifice. 

Analysis of Neoplasm 

and Nonneoplastic Lesion Incidences 

The Poly-k test (Bailer and Portier, 1988; Portier and 

Bailer, 1989; Piegorsch and Bailer, 1997) was used to 

assess neoplasm and nonneoplastic lesion prevalence. 

This test is a survival-adjusted quantal-response 

procedure that modifies the Cochran-Armitage linear 

trend test to take survival differences into account. 

More specifically, this method modifies the denominator in the quantal estimate of lesion incidence to 

approximate more closely the total number of animal 

years at risk. For analysis of a given site, each 

animal is assigned a risk weight. This value is one if 

the animal had a lesion at that site or if it survived 

until terminal sacrifice; if the animal died prior to 

terminal sacrifice and did not have a lesion at that site, 

its risk weight is the fraction of the entire study time 

that it survived, raised to the kth power. 

This method yields a lesion prevalence rate that 

depends only upon the choice of a shape parameter for 

a Weibull hazard function describing cumulative 

lesion incidence over time (Bailer and Portier, 1988). 

Unless otherwise specified, a value of k=3 was used 

in the analysis of site-specific lesions. This value was 

recommended by Bailer and Portier (1988) following 

an evaluation of neoplasm onset time distributions for 

a variety of site-specific neoplasms in control F344 

rats and B6C3F1 mice (Portier et al., 1986). Bailer 

and Portier (1988) showed that the Poly-3 test gave 

valid results if the true value of k was anywhere in the 

range from 1 to 5. A further advantage of the Poly-3 

method is that it does not require lesion lethality 

assumptions. Variation introduced by the use of risk 

weights, which reflect differential mortality, was 

accommodated by adjusting the variance of the Poly-3 

statistic as recommended by Bieler and Williams 

(1993). 

Tests of significance included pairwise comparisons of 

each exposed group with controls and a test for an 

overall exposure-related trend. Continuity-corrected 

tests were used in the analysis of lesion incidence, and 

reported P values are one sided. Values of P greater 

than 0.5 are presented as 1-P with the letter N added 

to indicate a lower incidence or negative trend in 

neoplasm occurrence relative to the control group 

(e.g., P=0.99 is presented as P=0.01N). 

Analysis of Continuous Variables 

Two approaches were employed to assess the significance of pairwise comparisons between exposed and 

control groups in the analysis of continuous variables. 

Organ and body weight data, which have approximately normal distributions, were analyzed with the 

parametric multiple comparison procedures of Dunnett 

(1955) and Williams (1971, 1972). Hematology, 

clinical chemistry, urinalysis, spermatid, and 
26 Isobutene, NTP TR 487 

epididymal spermatozoal data, which have typically resolved or were otherwise addressed during the 

skewed distributions, were analyzed using the non- preparation of this Technical Report. 

parametric multiple comparison methods of Shirley 

(1977) and Dunn (1964). Jonckheere’s test 

(Jonckheere, 1954) was used to assess the significance 

of the dose-related trends and to determine whether a 

trend-sensitive test (Williams’ or Shirley’s test) was 

more appropriate for pairwise comparisons than a test 

that does not assume a monotonic dose-related trend 

(Dunnett’s or Dunn’s test). Prior to statistical analysis, extreme values identified by the outlier test of 

Dixon and Massey (1951) were examined by NTP 

personnel, and implausible values were eliminated 

from the analysis. Average severity values were 

analyzed for significance with the Mann-Whitney 

U test (Hollander and Wolfe, 1973). Because vaginal 

cytology data are proportions (the proportion of the 

observation period that an animal was in a given 

estrous stage), an arcsine transformation was used to 

bring the data into closer conformance with a normality assumption. Treatment effects were investigated by applying a multivariate analysis of variance 

(Morrison, 1976) to the transformed data to test for 

simultaneous equality of measurements across exposure concentrations. 

Historical Control Data 

Although the concurrent control group is always the 

first and most appropriate control group used for 

evaluation, historical control data can be helpful in the 

overall assessment of neoplasm incidence in certain 

instances. Consequently, neoplasm incidences from 

the NTP historical control database, which is updated 

yearly, are included in the NTP reports for neoplasms 

appearing to show compound-related effects. 

QUALITY ASSURANCE METHODS 

The 14-week and 2-year studies were conducted in 

compliance with Food and Drug Administration Good 

Laboratory Practice Regulations (21 CFR, Part 58). 

In addition, as records from the 2-year studies were 

submitted to the NTP Archives, these studies were 

audited retrospectively by an independent quality 

assurance contractor. Separate audits covered completeness and accuracy of the pathology data, pathology specimens, final pathology tables, and a draft of 

this NTP Technical Report. Audit procedures and 

findings are presented in the reports and are on file at 

NIEHS. The audit findings were reviewed and 

assessed by NTP staff, so all comments had been 

GENETIC TOXICOLOGY 

The genetic toxicity of isobutene was assessed by 

testing the ability of the chemical to induce mutations 

in various strains of Salmonella typhimurium and 

increases in the frequency of micronucleated erythrocytes in peripheral blood of mice. The protocols for 

these studies and the results are given in Appendix E. 

The genetic toxicity studies of isobutene are part of a 

larger effort by the NTP to develop a database that 

would permit the evaluation of carcinogenicity in 

experimental animals from the molecular structure and 

the effects of the chemical in short-term in vitro and 

in vivo genetic toxicity tests. These genetic toxicity 

tests were originally developed to study mechanisms 

of chemical-induced DNA damage and to predict 

carcinogenicity in animals, based on the electrophilicity theory of chemical mutagenesis and the 

somatic mutation theory of cancer (Miller and Miller, 

1977; Straus, 1981; Crawford, 1985). 

There is a strong correlation between a chemical’s 

potential electrophilicity (structural alert to DNA 

reactivity), mutagenicity in Salmonella, and carcinogenicity in rodents. The combination of 

electrophilicity and Salmonella mutagenicity is highly 

correlated with the induction of carcinogenicity in rats 

and mice and/or at multiple tissue sites (Ashby and 

Tennant, 1991). Other in vitro genetic toxicity tests 

correlate less well with rodent carcinogenicity 

(Tennant et al., 1987; Zeiger et al., 1990), although 

these other tests can provide information on the types 

of DNA and chromosome effects that can be induced 

by the chemical being investigated. Data from NTP 

studies show that a positive response in Salmonella is 

the most predictive in vitro test for rodent carcinogenicity (89% of the Salmonella mutagens are rodent 

carcinogens), and that there is no complementarity 

among the in vitro genetic toxicity tests. That is, no 

battery of tests that included the Salmonella test 

improved the predictivity of the Salmonella test alone. 

The predictivity for carcinogenicity of a positive 

response in bone marrow chromosome aberration or 

micronucleus tests appears to be less than the 

Salmonella test (Shelby et al., 1993; Shelby and 

Witt, 1995). Positive responses in long-term 
Isobutene, NTP TR 487 27 

peripheral blood micronucleus tests have not been damage and adverse effects in somatic and germ cells, 

formally evaluated for their predictivity for rodent the deter-mination of in vivo genetic effects is imporcarcinogenicity. But, because of the theoretical and tant to the overall understanding of the risks assoobserved associations between induced genetic ciated with exposure to a particular chemical. 
28 Isobutene, NTP TR 487
 
29 

RESULTS
 

RATS 

14-WEEK STUDY 

All male and female rats survived to the end of the hematologic or clinical chemistry indices attributed to 

study (Table 2). The final mean body weights and isobutene exposure (Table F1). There were no biobody weight gains of all exposed groups of males and logically significant effects on male or female reprofemales were similar to those of the chamber control ductive endpoints as a result of exposure to isobutene 

groups. There were no clinical findings or effects on (Tables H1 and H2). 

TABLE 2 

Survival and Body Weights of Rats in the 14-Week Inhalation Study of Isobutene 

Concentration 

(ppm) 

Survivala Initial 

Mean Body Weight (g) b

Final Change 

Final Weight 

Relative to Controls 

(%) 

Male

 0 

500 

1,000 

2,000 

4,000 

8,000 

10/10 

10/10 

10/10 

10/10 

10/10 

10/10 

139 ± 5 

138 ± 5 

140 ± 6 

141 ± 4 

142 ± 7 

145 ± 7 

354 ± 7 

361 ± 6 

359 ± 7 

357 ± 6 

359 ± 7 

356 ± 6 

215 ± 8

223 ± 7 

219 ± 8 

216 ± 6 

217 ± 7 

211 ± 9 

102 

101 

101 

101 

101 

Female

 0 

500 

1,000 

2,000 

4,000 

8,000 

10/10 

10/10 

10/10 

10/10 

10/10 

10/10 

117 ± 3 

115 ± 3 

119 ± 2 

116 ± 2 

116 ± 2 

120 ± 3 

205 ± 4 

200 ± 2 

212 ± 4 

211 ± 4 

205 ± 4 

213 ± 5 

88 ± 4

85 ± 3

93 ± 5 

95 ± 4 

88 ± 4 

92 ± 4 

98 

103 

103 

100 

104 

a Number of animals surviving at 14 weeks/number initially in group b Weights and weight changes are given as mean ± standard error. Differences from the chamber control group were not significant 

by Williams’ or Dunnett’s test. 

The absolute right kidney weights of 4,000 and weights of all exposed groups of females were greater 

8,000 ppm males and the relative right kidney weights (up to 20%) than those of the chamber controls; 

of all exposed groups of males were greater than those however, the increases in absolute and relative liver 

of the chamber controls; however, the differences weights did not occur in a concentration-related 

were no greater than 10% and 8%, respectively manner. There were no histopathologic effects sup­

(Table G1). The absolute liver weights of females porting increased kidney or liver weights as a result of 

exposed to 1,000 ppm and above and the relative liver isobutene exposure. 
30 Isobutene, NTP TR 487 

No exposure-related gross lesions were observed in 

male or female rats at necropsy. Microscopically, 

minimal hypertrophy of goblet cells lining the nasopharyngeal duct in the most caudal nose section was 

observed in some rats in each exposed group of males 

and females (males: chamber control, 0/10; 500 ppm, 

4/10; 1,000 ppm, 7/10; 2,000 ppm, 9/10; 4,000 ppm, 

8/10; 8,000 ppm, 9/10; females: 0/10, 4/10, 8/10, 

8/10, 7/10, 10/10). 

Exposure Concentration Selection Rationale: Based 

on the lack of significant exposure-related toxicologic 

effects, 8,000 ppm was selected as the highest exposure concentration in the 2-year study. A higher 

concentration could not be used because of the danger 

of explosion. The 2-year study exposure concentrations of 0, 500, 2,000, and 8,000 ppm were based 

on published metabolic elimination rates for SpragueDawley rats and B6C3F mice (Csanády et al., 1991). 1 

These rates indicated that 500 ppm would be within 

the linear range for metabolic elimination, 2,000 ppm 

would be near the linear range, and 8,000 ppm would 

be out of the linear range. 
31 

c 

Isobutene, NTP TR 487 

2-YEAR STUDY 

Survival	 Body Weights and Clinical Findings 

Estimates of 2-year survival probabilities for male and Mean body weights of exposed male and female rats 

female rats are shown in Table 3 and in the Kaplan- were generally similar to those of the chamber conMeier survival curves (Figure 2). Survival of exposed trols throughout the study (Figure 3 and Tables 4 and 

males and females was similar to that of the chamber 5). There were no clinical findings attributed to 

controls. isobutene exposure. 

TABLE 3 

Survival of Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Male 

Animals initially in study	 50 50 50 50 

Moribund 39 40 37 41 

Natural deaths 4 5 7 1 

Animals surviving to study termination 7 5 6 8 a Percent probability of survival at end of study 14 10 12 16 

Mean survival (days)b 630 610 612 649 

c Survival analysis	 P=0.282N P=0.346 P=0.450 P=0.718N 

Female 

Animals initially in study	 50 50 50 50 

d Accidental death 0 1 0 0 

Moribund 21 25 15 23 

Natural deaths 6 5 2 5 

Animals surviving to study termination 23 19 33 22 

Percent probability of survival at end of study 46 39 66 44 

Mean survival (days) 667 635 696 658 

Survival analysis	 P=0.926 P=0.469 P=0.069N P=0.909 

a Kaplan-Meier determinations b	 Mean of all deaths (uncensored, censored, and terminal sacrifice) 

The result of the life table trend test (Tarone, 1975) is in the chamber control column, and the results of the life table pairwise comparisons 

(Cox, 1972) with the chamber controls are in the exposed group columns. A negative trend or lower mortality in an exposure group is 

indicated by N. d	 Censored from survival analyses 
 

32 Isobutene, NTP TR 487 

FIGURE 2
 

Kaplan-Meier Survival Curves for Male and Female Rats
 

Exposed to Isobutene by Inhalation for 2 Years
 
 

Isobutene, NTP TR 487 33 

FIGURE 3
 

Growth Curves for Male and Female Rats Exposed
 

to Isobutene by Inhalation for 2 Years
 
 

34 Isobutene, NTP TR 487 

TABLE 4 

Mean Body Weights and Survival of Male Rats in the 2-Year Inhalation Study of Isobutene 

Weeks Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

on Av. Wt. No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of 

Study (g) Survivors (g) controls) Survivors (g) controls) Survivors (g) controls) Survivors 

1 147 50 148 101 50 147 100 50 146 99 50 

2 192 50 194 101 50 193 100 50 193 100 50 

3 220 50 223 101 50 223 101 50 221 101 50 

4 244 50 247 101 50 248 101 50 245 100 50 

5 265 50 269 101 50 269 102 50 266 100 50 

6 282 50 285 101 50 287 102 50 283 100 50 

7 300 50 300 100 50 303 101 50 300 100 50 

8 314 50 315 100 50 317 101 50 311 99 50 

9 326 50 327 100 50 328 101 50 322 99 50 

10 339 50 339 100 50 340 100 50 335 99 50 

11 350 50 350 100 50 352 100 50 345 99 50 

12 359 50 359 100 50 361 101 50 351 98 50 

15 383 50 383 100 50 383 100 50 376 98 50 

19 409 50 413 101 50 413 101 50 406 99 50 

23 433 50 433 100 50 438 101 50 428 99 50 

27 452 50 450 100 50 453 100 50 446 99 50 

31 465 50 462 99 50 465 100 50 457 98 50 

35 472 50 470 100 50 475 101 50 469 99 50 

39 483 50 489 101 49 487 101 50 483 100 50 

43 493 50 496 101 49 498 101 50 492 100 50 

47 501 50 506 101 49 504 101 50 501 100 50 

51 511 50 511 100 49 514 101 49 509 100 50 

55 520 49 515 99 49 520 100 49 512 99 50 

59 524 49 523 100 48 525 100 49 516 99 50 

63 524 49 525 100 48 526 100 49 519 99 50 

67 525 48 529 101 47 528 101 48 520 99 50 

71 531 45 526 99 45 523 98 47 525 99 49 

75 528 44 523 99 42 540 102 40 519 98 48 

79 526 40 533 101 37 523 99 39 522 99 44 

83 526 36 526 100 34 517 98 32 523 100 42 

87 522 33 523 100 29 516 99 27 514 98 38 

91 498 28 512 103 23 520 104 20 502 101 33 

93 500 24 500 100 21 500 100 20 498 100 29 

95 500 22 468 94 19 519 104 16 484 97 26 

97 472 20 455 96 14 519 110 15 475 101 20 

99 479 14 452 94 10 503 105 13 452 94 14 

101 456 12 459 101 7 498 109 10 452 99 9 

103 468 8 450 96 5 508 109 6 446 95 9 

Mean for weeks 

1-13 278 280 101 281 101 277 100 

14-52 460 461 100 463 101 457 99 

53-103 506 501 99 518 102 499 99 
Isobutene, NTP TR 487 35 

TABLE 5 

Mean Body Weights and Survival of Female Rats in the 2-Year Inhalation Study of Isobutene 

Weeks Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

on Av. Wt. No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of 

Study (g) Survivors (g) controls) Survivors (g) controls) Survivors (g) controls) Survivors 

1 111 50 110 100 50 111 100 50 109 98 50 

2 134 50 132 98 50 135 100 50 131 98 49 

3 144 50 143 99 49 146 101 50 143 99 49 

4 154 50 152 99 49 155 101 50 152 99 49 

5 162 50 162 100 49 164 102 50 161 100 49 

6 167 50 167 100 49 170 101 50 166 99 49 

7 172 50 173 101 49 175 102 50 170 99 49 

8 178 50 177 100 49 179 100 50 175 99 49 

9 183 50 183 100 49 186 102 50 181 99 49 

10 189 50 188 99 49 190 101 50 185 98 49 

11 193 50 191 99 49 194 101 50 190 98 49 

12 194 50 192 99 49 194 100 50 189 97 49 

15 203 50 202 100 49 202 100 50 198 98 49 

19 210 50 213 101 49 213 102 50 208 99 49 

23 220 50 225 102 49 224 102 50 218 99 49 

27 231 50 234 102 49 232 101 50 226 98 49 

31 238 50 241 101 49 238 100 50 231 97 49 

35 249 50 250 100 48 248 99 50 240 96 49 

39 257 50 259 101 48 259 101 50 251 98 49 

43 265 50 270 102 48 268 101 50 261 98 49 

47 279 50 284 102 48 282 101 50 274 98 49 

51 295 50 298 101 48 296 100 50 288 98 49 

55 306 50 307 100 48 305 100 50 296 97 49 

59 313 49 316 101 48 315 100 50 308 98 48 

63 320 48 325 102 47 325 102 49 316 99 48 

67 324 47 327 101 45 328 101 49 318 98 48 

71 334 45 339 102 43 337 101 49 329 99 47 

75 337 44 342 101 42 343 102 48 337 100 44 

79 340 43 347 102 39 347 102 47 340 100 43 

83 352 40 353 100 37 360 102 45 347 99 42 

87 353 39 355 101 35 361 102 44 350 99 39 

91 356 38 358 101 31 362 102 42 346 97 37 

93 356 35 355 100 31 362 102 41 340 96 36 

95 361 34 357 99 30 367 102 40 343 95 33 

97 355 33 358 101 29 367 103 40 355 100 29 

99 353 31 358 101 27 372 105 37 358 101 28 

101 352 29 361 102 25 370 105 35 360 102 26 

103 350 27 355 101 20 369 105 34 361 103 25 

Mean for weeks 

1-13 165 164 99 167 101 163 99 

14-52 245 248 101 246 100 240 98 

53-103 341 345 101 349 102 338 99 
36 Isobutene, NTP TR 487 

2-Hydroxyisobutyric Acid — 

Biomarker of Exposure 

2-Hydroxyisobutyric acid (HIBA), the major urinary 

metabolite of isobutene, was measured in the urine of 

male and female rats as an indicator of isobutene 

exposure at 6, 12, and 18 months (Table I1). The 

amount of HIBA excreted increased with increasing 

exposure concentration (Figures 4 and 5). However, 

when HIBA concentration was normalized to isobutene exposure concentration, the relative amount of 

HIBA excreted decreased as exposure concentrations 

increased (Figures 6 and 7). This was true at all three 

collection intervals for males and females. Exposure 

to isobutene had no effect on the quantity of urine or 

the amount of creatinine excreted. 
 

 

Isobutene, NTP TR 487 3 7

FIGURE 4 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Acid Concentration Normalized 

to Creatinine for Male Rats in the 2-Year Inhalation Study of Isobutene 

FIGURE 5 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Acid Concentration Normalized 

to Creatinine for Female Rats in the 2-Year Inhalation Study of Isobutene 
 

 

38 Isobutene, NTP TR 487 

FIGURE 6 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Excretion Normalized to Creatinine 

and Exposure Concentration for Male Rats in the 2-Year Inhalation Study of Isobutene 

FIGURE 7 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Excretion Normalized to Creatinine 

and Exposure Concentration for Female Rats in the 2-Year Inhalation Study of Isobutene 
 

Isobutene, NTP TR 487 39 

Pathology and Statistical Analyses 

This section describes the statistically significant or 

biologically noteworthy changes in the incidences of 

neoplasms and/or nonneoplastic lesions of the thyroid 

gland, nose, and heart and of mononuclear cell leukemia. Summaries of the incidences of neoplasms 

and nonneoplastic lesions, individual animal tumor 

diagnoses, statistical analyses of primary neoplasms 

that occurred with an incidence of at least 5% in at 

least one animal group, and historical incidences for 

the neoplasms mentioned in this section are presented 

in Appendix A for male rats and Appendix B for 

female rats. 

Thyroid Gland: The incidence of follicular cell 

carcinoma in male rats exposed to 8,000 ppm was 

greater than that in the chamber control group and 

exceeded the historical control range (Tables 6, A3, 

and A4). The histomorphology of the carcinomas that 

were observed in chamber control and exposed rats 

was similar to the morphologic spectrum typical of 

spontaneously developing follicular cell carcinomas. 

Histologically, all were unilateral, well-delineated, 

medium-sized to large (0.3 to 1.3 cm) nodular masses 

that partially or completely replaced the affected lobe 

(Plates 1 through 6). There was clear focal to extensive invasion of the capsule and, in some cases, the 

immediately adjacent tissue (Plates 3 through 5). The 

carcinomas consisted of follicular, papillary, or solid 

patterns or variable combinations of these patterns and 

were surrounded by a prominent scirrhous reaction. 

Carcinomas with predominantly follicular or papillary 

patterns had relatively uniform cells and often multiple prominent, cystic, colloid-filled follicles and 

spaces (Plate 7). The neoplastic cells in carcinomas 

having solid areas or that were predominantly solid 

were more anaplastic than the cells in follicular or 

papillary patterns and were characterized by moderate 

variation in cellular size and shape (pleomorphism) 

and low to high numbers of mitotic figures (Plates 8 

and 9). One carcinoma had neoplastic cell emboli 

within the vasculature and had metastasized to the 

lungs (Plates 10 and 11). Concurrent increases in the 

incidences of follicular cell adenoma and hyperplasia 

did not occur in male rats (Tables 6, A1, and A5), nor 

were the incidences of proliferative lesions of the 

thyroid gland increased in female rats (follicular cell 

hyperplasia: chamber control, 0/50; 500 ppm, 2/50; 

2,000 ppm, 0/49; 8,000 ppm, 0/49; follicular cell 

carcinoma: 1/50, 0/50, 0/49, 0/49; Tables B1 and 

B4). 

Nose: Although the incidences of hyaline degeneration of the olfactory epithelium in males and 

females were only slightly increased in exposed rats 

(males: 43/49, 45/49, 46/50, 49/49; females: 44/50, 

47/50, 48/50, 47/49; Tables A5 and B4), the severities of hyaline degeneration increased with increasing exposure concentration (males: 1.3, 1.4, 

2.2, 2.6; females: 1.5, 2.4, 2.8, 2.8). Hyaline 

degeneration was characterized by accumulation of 

variably sized, brightly eosinophilic globules in the 

cytoplasm of sustentacular cells in the ethmoid turbinate olfactory epithelium. In inhalation studies, 

hyaline degeneration is a commonly observed change 

in the epithelium of the nasal cavity, the incidence and 

severity of which may increase with increasing 

exposure concentration. The accumulation of these 

protein globules is considered a nonspecific adaptive 

response to prolonged inhalation of irritant material 

and has no adverse effect on affected animals. 

Heart: The incidences of atrial thrombosis in males 

occurred with a positive trend. The incidence of this 

lesion in 8,000 ppm males was significantly greater 

than that in the chamber control group (0/50, 1/50, 

3/50, 6/50; Table A5). Histologically, thrombi 

occurred in the left atrium and were somewhat varied 

in size and character with no common unifying morphologic features. Atrial thrombosis is a common 

spontaneous lesion observed at a low incidence in 

chronic rat studies, and an incidence of zero in the 

controls is unusual. The marginal increase observed 

in this study was not considered to be related to 

isobutene exposure. 

Mononuclear Cell Leukemia: The incidences of 

mononuclear cell leukemia in male rats occurred with 

a positive trend (21/50, 21/50, 20/50, 31/50; 

Table A3). This marginal increase most likely represents a spurious occurrence. The incidence in the 

8,000 ppm group was not significant, was well 

within the historical control range [520/905 

(57.5% ± 9.4%), range, 34%-70%], and is not 

considered to be related to isobutene exposure. In 

addition, the incidences of mononuclear cell leukemia 

in exposed female rats were similar to that in the 

chamber controls (18/50, 16/50, 22/50, 17/50; 

Table B3). 
40 

c 

Isobutene, NTP TR 487 

TABLE 6 

Incidences of Follicular Cell Neoplasms and Nonneoplastic Lesions of the Thyroid Gland in Male Rats 

in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Number Examined Microscopically 48 48 48 50 

Hyperplasiaa 0 2 (3.0)b 0 1 (4.0) 

Carcinomac 

Overall rated 1/48 (2%) 0/48 (0%) 0/48 (0%) 5/50 (10%) e Adjusted rate 3.0% 0.0% 0.0% 13.5% 

Terminal ratef 0/7 (0%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 661 —h — 618 

Poly-3 testg P=0.004 P=0.519N P=0.521N P=0.125 

a	 Number of animals with lesion 

b	 Average severity grade of lesions in affected animals: 1=minimal, 2=mild, 3=moderate, 4=marked 

Historical incidence for 2-year inhalation studies with chamber control groups (mean ± standard deviation): adenoma: 7/892 

(0.8% ± 1.2%), range 0%-4%; carcinoma: 9/892 (1.0% ± 1.2%), range 0%-4%; adenoma or carcinoma: 16/892 (1.8% ± 1.7%), 

range 0%-6% d	 Number of animals with neoplasm per number of animals with thyroid gland examined microscopically e Poly-3 estimated neoplasm incidence after adjustment for intercurrent mortality f Observed incidence at terminal kill 

g	 Beneath the chamber control incidence is the P value associated with the trend test. Beneath the exposed group incidence are the P values 

corresponding to the pairwise comparisons between the chamber controls and that exposed group. The Poly-3 test accounts for differential 

mortality in animals that do not reach terminal sacrifice. A lower incidence in an exposure group is indicated by N. h	 Not applicable; no neoplasms in animal group 
Isobutene, NTP TR 487 41 

MICE 

14-WEEK STUDY 

All male and female mice survived to the end of the There were no clinical findings or biologically sigstudy (Table 7). The final mean body weights and nificant effects on male or female reproductive endbody weight gains of all exposed groups of males and points attributed to isobutene exposure (Tables H3 and 

females were similar to those of the chamber controls. H4). 

TABLE 7 

Survival and Body Weights of Mice in the 14-Week Inhalation Study of Isobutene 

Concentration 

(ppm)

Survivala Initial 

Mean Body Weight (g) b

Final Change 

Final Weight 

Relative to Controls 

(%) 

Male

 0 

500 

1,000 

2,000 

4,000 

8,000 

10/10 

10/10 

10/10 

10/10 

10/10 

10/10 

25.0 ± 0.4 

24.8 ± 0.3 

25.2 ± 0.4 

25.0 ± 0.3 

25.2 ± 0.4 

25.0 ± 0.3 

35.9 ± 1.2 

36.9 ± 0.8 

38.5 ± 0.8 

35.9 ± 0.9 

36.1 ± 0.6 

35.7 ± 0.9 

10.9 ± 1.0

12.1 ± 0.6 

13.3 ± 0.5 

10.9 ± 0.8 

10.9 ± 0.5 

10.8 ± 0.7

103 

107 

100 

101 

99 

Female

 0 

500 

1,000 

2,000 

4,000 

8,000 

10/10 

10/10 

10/10 

10/10 

10/10 

10/10 

19.8 ± 0.2 

20.0 ± 0.3 

20.1 ± 0.3 

20.1 ± 0.2 

20.2 ± 0.3 

20.1 ± 0.1 

30.6 ± 0.6 

30.6 ± 0.7 

32.7 ± 1.1 

31.9 ± 0.9 

31.5 ± 1.0 

30.8 ± 0.6 

10.8 ± 0.5

10.7 ± 0.7 

12.6 ± 0.9 

11.8 ± 0.8 

11.3 ± 0.9 

10.7 ± 0.6 

100 

107 

104 

103 

101 

a Number of animals surviving at 14 weeks/number initially in group b Weights and weight changes are given as mean ± standard error. Differences from the chamber control group were not significant 

by Williams’ or Dunnett’s test. 

The absolute and relative right kidney weights of effects, 8,000 ppm was selected as the highest ex8,000 ppm males were greater (approximately 11%) posure concentration in the 2-year study. A higher 

than those of the chamber controls (Table G2). The concentration of isobutene could not be used because 

absolute and relative right kidney weights of all of the danger of explosion. The 2-year study exgroups of exposed females were greater (up to 18%) posure concentrations of 0, 500, 2,000, and 

than those of the chamber controls, but, in general, 8,000 ppm were based on published metabolic elimwere not exposure concentration related. There were ination rates for Sprague-Dawley rats and B6C3F 

no lesions detected grossly at necropsy or micro- mice (Csanády et al., 1991). These rates indicated 

scopically that supported these increases. that 500 ppm would be within the linear range for 

metabolic elimination, 2,000 ppm would be near the 

Exposure Concentration Selection Rationale: Based linear range, and 8,000 ppm would be out of the 

on the lack of significant exposure-related toxicologic linear range. 

1 
42 

c 

Isobutene, NTP TR 487 

2-YEAR STUDY 

Survival 

Estimates of 2-year survival probabilities for male and 

female mice are shown in Table 8 and in the 

Kaplan-Meier survival curves (Figure 8). Survival of 

exposed males and females was similar to that of the 

chamber controls. 

TABLE 8 

Body Weights and Clinical Findings 

Mean body weights of exposed male mice and 

500 ppm females were generally similar to those of 

the chamber controls throughout the study; however, 

the mean body weights of 2,000 and 8,000 ppm 

females were slightly less than those of the chamber 

controls from about week 52 to week 92 (Tables 9 and 

10 and Figure 9). There were no clinical findings 

attributed to isobutene exposure. 

Survival of Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Male 

Animals initially in study 50 50 50 50 

Moribund 16 9 20 13 

Natural deaths 6 9 3 9 

Animals surviving to study termination 28 32 27 28 

Percent probability of survival at end of studya 

Mean survival (days)b 

56 

676 

64 

692 

54 

663 

56 

656 

Survival analysisc P=0.642 P=0.488N P=0.835 P=0.977 

Female 

Animals initially in study 50 50 50 50 

Moribund 13 16 6 13 

Natural deaths 5 3 5 4 

Animals surviving to study termination 32 31 39 33 

Percent probability of survival at end of study 64 62 78 66 

Mean survival (days) 681 695 708 710 

Survival analysis P=0.802N P=1.000 P=0.182N P=0.820N 

a Kaplan-Meier determinations b	 Mean of all deaths (uncensored, censored, and terminal sacrifice) 

The result of the life table trend test (Tarone, 1975) is in the chamber control column, and the results of the life table pairwise comparisons 

(Cox, 1972) with the chamber controls are in the exposed group columns. A negative trend or lower mortality in an exposure group is 

indicated by N. 
 

Isobutene, NTP TR 487 43 

FIGURE 8
 

Kaplan-Meier Survival Curves for Male and Female
 

Mice Exposed to Isobutene by Inhalation for 2 Years
 
44 Isobutene, NTP TR 487 

TABLE 9 

Mean Body Weights and Survival of Male Mice in the 2-Year Inhalation Study of Isobutene 

Weeks Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

on Av. Wt. No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of 

Study (g) Survivors (g) controls) Survivors (g) controls) Survivors (g) controls) Survivors 

1 25.6 50 25.9 101 50 25.6 100 50 25.5 100 50 

2 27.5 50 27.6 100 50 27.4 100 50 27.4 100 50 

3 28.4 50 28.5 100 50 28.3 100 50 28.2 99 50 

4 29.3 50 29.3 100 50 29.1 99 50 29.2 100 50 

5 30.2 50 30.1 100 50 29.9 99 50 29.9 99 50 

6 30.8 50 30.6 99 50 30.2 98 50 30.4 99 50 

7 31.3 50 31.1 99 50 30.7 98 50 31.0 99 50 

8 31.7 50 31.5 99 50 31.4 99 50 31.5 99 50 

9 32.7 50 32.6 100 50 32.7 100 50 32.6 100 50 

10 33.2 50 33.1 100 50 33.1 100 50 33.2 100 50 

11 34.0 50 34.0 100 50 33.9 100 50 33.8 99 50 

12 34.6 50 34.8 101 50 34.6 100 50 34.6 100 50 

13 35.4 50 35.3 100 50 35.2 99 50 35.2 99 50 

16 36.3 50 37.4 103 50 36.8 101 50 37.6 104 50 

20 38.7 50 38.8 100 50 38.9 101 50 39.9 103 50 

24 41.5 50 41.8 101 50 41.5 100 50 43.2 104 50 

28 43.7 50 44.5 102 50 42.8 98 50 45.3 104 50 

32 44.8 50 45.4 101 50 44.2 99 50 46.3 103 50 

36 46.4 50 47.1 102 50 46.2 100 50 48.2 104 50 

40 47.0 50 47.5 101 50 46.2 98 50 48.0 102 49 

44 47.7 50 48.7 102 50 47.4 99 50 49.6 104 49 

48 49.1 50 49.8 101 50 48.7 99 50 50.6 103 48 

52 48.9 50 50.0 102 50 48.7 100 50 50.6 104 47 

56 49.7 50 50.6 102 50 49.0 99 50 50.9 102 46 

60 50.5 48 51.4 102 49 50.2 99 48 51.3 102 46 

64 51.0 47 52.0 102 49 50.6 99 47 51.1 100 46 

68 50.1 47 51.0 102 49 49.8 99 47 50.6 101 45 

72 50.8 46 51.1 101 49 50.4 99 46 50.8 100 45 

76 50.3 46 51.6 103 47 50.9 101 42 51.1 102 42 

80 50.2 45 51.6 103 46 50.8 101 42 52.0 104 40 

84 50.7 43 51.5 102 45 51.0 101 40 52.8 104 39 

88 50.5 39 51.5 102 42 50.5 100 38 51.9 103 38 

92 49.9 38 50.0 100 42 50.2 101 35 51.5 103 36 

94 49.4 38 50.6 102 39 50.6 102 33 52.0 105 35 

96 49.9 35 49.8 100 39 49.8 100 32 50.9 102 35 

98 49.7 35 49.8 100 37 49.4 99 32 51.1 103 33 

100 49.4 34 49.6 100 37 49.0 99 30 50.0 101 31 

102 49.8 31 48.8 98 36 48.3 97 29 49.8 100 29 

104 49.8 29 49.2 99 32 47.5 95 28 49.5 99 29 

Mean for weeks 

1-13 31.1 31.1 100 30.9 99 31.0 100 

14-52 44.4 45.1 102 44.1 99 45.9 103 

53-104 50.1 50.6 101 49.9 100 51.1 102 
Isobutene, NTP TR 487 45 

TABLE 10 

Mean Body Weights and Survival of Female Mice in the 2-Year Inhalation Study of Isobutene 

Weeks Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

on Av. Wt. No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of Av. Wt. Wt. (% of No. of 

Study (g) Survivors (g) controls) Survivors (g) controls) Survivors (g) controls) Survivors 

1 20.0 50 19.8 99 50 20.0 100 50 19.8 99 50 

2 21.6 50 21.3 99 50 21.6 100 50 21.9 101 50 

3 23.0 50 22.6 98 50 22.8 99 50 22.9 100 50 

4 23.3 50 23.3 100 50 23.3 100 50 23.6 101 50 

5 24.2 50 24.0 99 50 24.4 101 50 24.2 100 50 

6 25.0 50 24.9 100 50 25.1 100 50 24.8 99 50 

7 25.6 50 25.4 99 50 25.3 99 50 25.2 98 50 

8 26.0 50 25.5 98 50 25.5 98 50 25.5 98 50 

9 26.8 50 26.3 98 50 26.5 99 50 26.5 99 50 

10 27.1 50 27.0 100 50 27.7 102 50 27.4 101 50 

11 28.2 50 27.8 99 50 27.6 98 50 28.0 99 50 

12 29.2 50 28.6 98 50 28.3 97 50 29.3 100 50 

13 29.5 50 28.9 98 50 28.7 97 50 29.2 99 50 

16 30.9 50 31.5 102 50 30.8 100 50 31.3 101 50 

20 34.9 50 34.3 98 50 33.8 97 50 34.7 99 50 

24 36.9 50 37.5 102 50 37.1 101 50 37.9 103 50 

28 39.9 50 40.6 102 50 39.8 100 50 39.4 99 50 

32 41.2 50 42.1 102 50 40.9 99 50 40.5 98 50 

36 43.9 49 44.5 101 50 41.9 95 50 43.0 98 50 

40 44.9 49 45.1 100 50 42.3 94 50 43.1 96 50 

44 46.3 49 47.7 103 50 44.7 97 50 45.6 99 50 

48 48.3 48 49.3 102 50 46.2 96 50 45.7 95 50 

52 48.8 48 49.7 102 50 46.0 94 50 45.2 93 50 

56 50.4 47 51.7 103 50 47.3 94 50 47.1 94 50 

60 52.8 47 53.7 102 50 50.1 95 50 49.4 94 50 

64 54.9 47 55.2 101 50 52.2 95 50 51.7 94 50 

68 56.2 46 55.0 98 50 52.8 94 50 52.0 93 50 

72 57.3 46 56.3 98 49 54.5 95 49 53.0 93 50 

76 57.9 46 57.2 99 48 55.1 95 48 53.5 92 50 

80 57.9 45 56.2 97 48 55.5 96 47 54.7 95 49 

84 58.1 44 56.9 98 46 56.6 97 47 55.4 95 48 

88 57.6 42 54.9 95 45 54.6 95 46 53.3 93 46 

92 55.9 41 54.2 97 42 54.3 97 43 52.7 94 45 

94 55.6 40 55.5 100 38 54.3 98 43 53.0 95 44 

96 54.1 39 55.4 102 36 53.8 99 43 52.3 97 43 

98 53.8 37 54.6 102 36 53.5 99 42 52.4 97 42 

100 53.6 36 53.6 100 35 52.5 98 40 51.0 95 41 

102 52.5 36 54.5 104 33 51.3 98 40 50.3 96 39 

104 52.0 34 53.2 102 33 50.8 98 39 51.1 98 34 

Mean for weeks 

1-13 25.3 25.0 99 25.1 99 25.3 100 

14-52 41.6 42.2 101 40.4 97 40.6 98 

53-104 55.0 54.9 100 53.1 97 52.1 95 
 

46 Isobutene, NTP TR 487 

FIGURE 9
 

Growth Curves for Male and Female Mice
 

Exposed to Isobutene by Inhalation for 2 Years
 
Isobutene, NTP TR 487 47 

2-Hydroxyisobutyric Acid — 

Biomarker of Exposure 

HIBA was measured in the urine of male and female 

mice as an indicator of isobutene exposure at 6, 12, 

and 18 months (Table I2). The amount of HIBA 

excreted increased with increasing exposure concentration (Figures 10 and 11). However, when 

HIBA concentration was normalized to isobutene 

exposure concentration, the relative amount of HIBA 

excreted decreased as exposure concentrations 

increased (Figures 12 and 13). This was true at all 

three collection intervals for males and females. 

Exposure to isobutene had no effect on the quantity of 

urine or the amount of creatinine excreted. 
48 Isobutene, NTP TR 487 

FIGURE 10 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Acid Concentration Normalized to 

Creatinine for Male Mice in the 2-Year Inhalation Study of Isobutene 

FIGURE 11 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Acid Concentration Normalized to 

Creatinine for Female Mice in the 2-Year Inhalation Study of Isobutene 
 

Isobutene, NTP TR 487 4 9

FIGURE 12 


Mean (± Standard Deviation) 2-Hydroxyisobutyric Excretion Normalized to Creatinine
 

and Exposure Concentration for Male Mice in the 2-Year Inhalation Study of Isobutene
 

FIGURE 1 3 
 

Mean (± Standard Deviation) 2-Hydroxyisobutyric Excretion Normalized to Creatinine
 

and Exposure Concentration for Female Mice in the 2-Year Inhalation Study of Isobutene
 
50 Isobutene, NTP TR 487 

Pathology and Statistical Analyses 

This section describes the statistically significant or 

biologically noteworthy changes in the incidences of 

nonneoplastic lesions of the nose. Summaries of the 

incidences of neoplasms and nonneoplastic lesions, 

individual animal tumor diagnoses, and statistical 

analyses of primary neoplasms that occurred with an 

incidence of at least 5% in at least one animal group 

are presented in Appendix C for male mice and 

Appendix D for female mice. 

Nose: The incidences of hyaline degeneration of the 

respiratory epithelium in all groups of exposed males 

and females were significantly greater than those in 

the chamber control groups and occurred with positive 

trends (Tables 11, C4, and D4). The incidences of 

hyaline degeneration of the olfactory epithelium in 

males occurred with a positive trend. The incidences 

of this lesion in 2,000 and 8,000 ppm males were 

significantly greater than that in the chamber controls. 

The incidences of hyaline degeneration of the 

olfactory epithelium in females also occurred with a 

positive trend; however, the incidences were not 

statistically different from that in the chamber 

controls. 

TABLE 11 

Incidences of Nonneoplastic Lesions of the Nose in Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Male 

Number Examined Microscopically 

Respiratory Epithelium, Degeneration Hyalinea 

Olfactory Epithelium, Degeneration Hyaline 

50 

6 

6 

(1.0)b 

(1.0) 

49 

19** (1.2) 

7 (1.1) 

50 

29** (1.5) 

16** (1.6) 

48 

39** (1.8) 

17** (1.4) 

Female 

Number Examined Microscopically 

Respiratory Epithelium, Degeneration Hyaline 

Olfactory Epithelium, Degeneration Hyaline 

47 

21 

17 

(1.8) 

(1.5) 

50 

39** (1.5) 

19 (1.2) 

49 

41** (1.6) 

24 (1.1) 

50 

48** (2.3) 

27 (1.2) 

** Significantly different (P#0.01) from the chamber control group by the Poly-3 test a Number of animals with lesion 

b Average severity grade of lesions in affected animals: 1=minimal, 2=mild, 3=moderate, 4=marked 

GENETIC TOXICOLOGY 

Isobutene (0.001 to 0.027 mol/desiccator) was not normochromatic erythrocytes was seen in peripheral 

mutagenic in Salmonella typhimurium strain TA97, blood samples from male and female mice adminTA98, TA100, or TA1535, with or without induced istered isobutene via inhalation for 14 weeks 

rat or hamster liver S9 enzymes (Table E1). In vivo, (Table E2). 

no increase in the frequency of micronucleated 
PLATE 1 

Follicular cell carcinoma (C) of the thyroid gland from a control 

male rat in the 2-year inhalation study of isobutene. The 

carcinoma has a follicular/papillary pattern and has completely 

replaced an entire lobe of the thyroid gland. T indicates the 

trachea. H&E; 10 × 

PLATE 3 

Follicular cell carcinoma (C) of the thyroid gland from a male 

rat exposed to 8,000 ppm isobutene by inhalation for 2 years. 

The carcinoma has a solid pattern and has completely replaced 

an entire lobe of the thyroid gland. The contralateral lobe of the 

thyroid is normal (arrows). T indicates the trachea. H&E; 10 × 

PLATE 2 

Follicular cell carcinoma (C) of the thyroid gland from a male 

rat exposed to 8,000 ppm isobutene by inhalation for 2 years. 

The carcinoma has a follicular/papillary pattern and has 

completely replaced an entire lobe of the the thyroid gland. T 

indicates the trachea. H&E; 10 × 

PLATE 4 

Follicular cell carcinoma of the thyroid gland from a male rat 

exposed to 8,000 ppm isobutene by inhalation for 2 years. Note 

the mixed follicular/papillary pattern, the unilateral replacement 

of an entire lobe by the carcinoma, and locally extensive 

invasion of the adjacent peritracheal tissue (arrowheads). The 

contralateral lobe of the thyroid is normal (arrows). T indicates 

the trachea. H&E; 6 × 
PLATE 5 

Follicular cell carcinoma of the thyroid gland from a male rat 

exposed to 8,000 ppm isobutene by inhalation for 2 years. Note 

the primarily solid pattern, the unilateral replacement of an 

entire lobe by the carcinoma, and invasion of the adjacent 

peritracheal tissue (arrowheads). The contralateral lobe of the 

thyroid is normal (arrows). T indicates the trachea. H&E; 6 × 

PLATE 7 

Higher magnification of Plate 4. The neoplastic cells lining 

follicles (arrows) are uniformly cuboidal to low columnar. H&E; 

175 × 

PLATE 6 

Follicular cell carcinoma of the thyroid gland from a male rat 

exposed to 8,000 ppm isobutene by inhalation for 2 years. Note 

the primarily solid pattern and partial replacement of the lobe by 

the carcinoma. The contralateral lobe of the thyroid is normal. T 

indicates the trachea. H&E; 12 × 

PLATE 8 

Higher magnification of Plate 5 illustrating the solid pattern. 

The neoplastic cells are densely packed and there is moderate 

variation in cellular size and shape (pleomorphism). Note the 

high number of mitotic figures (arrows). H&E; 175 × 
PLATE 9 PLATE 10 

Higher magnification of Plate 3 illustrating the solid pattern. Embolus of neoplastic cells (arrows) within the vasculature 

Note invasion (arrows) of the surrounding scirrhous capsule adjacent ot the follicular cell carcinoma illustrated in Plate 5. 

(arrowheads). H&E; 175 × H&E; 230 × 

PLATE 11 

Lung from the rat with the thyroid follicular cell carcinoma 

illustrated in Plate 5. Note a focus of metastatic neoplastic cells 

(arrows) adjacent to a terminal bronchiole (TB). Alveoli are 

indicated by asterisks. H&E; 230 × 
51 

DISCUSSION AND CONCLUSIONS
 

Isobutene was evaluated for toxicity and carcinogenicity in 14-week and 2-year inhalation studies in 

male and female F344/N rats and B6C3F mice. The 1 

highest concentration (8,000 ppm) that could be 

generated safely was used in all studies. This concentration is somewhat less than 50% of the lower explosion limit of isobutene (Patty’s, 1994). 

In the 14-week studies, there were no exposurerelated deaths, body weight changes, clinical findings, 

effects on hematologic or clinical chemistry indices 

(rats), or biologically significant effects on male or 

female reproductive endpoints. No exposure-related 

increases in micronucleated erythrocytes were seen in 

mice. Kidney weights of male rats and male and 

female mice exposed to 8,000 ppm were greater than 

those of the chamber controls. For male rats and 

mice, these increases were approximately 10% 

or less, while the increases in female rats and 

mice were greater (about 20%). There were no 

histopathologic effects in the kidneys of exposed rats 

or mice. Liver weights of all exposed groups of 

female rats were also greater than those of the chamber controls; however, the increases did not occur in 

a concentration-related fashion, nor were there histopathologic effects in the liver. There were no 

exposure-related gross lesions in rats; however, a 

minimal hypertrophy of goblet cells lining the nasopharyngeal duct in the most caudal section of the nasal 

cavity was observed in all groups of exposed male and 

female rats. In mice, no lesions were detected grossly 

at necropsy or microscopically. 

In the 2-year rat study, neither survival rates nor body 

weight gains were significantly affected by isobutene 

exposure. There were no exposure-related clinical 

findings. Isobutene exposure caused an increased 

incidence of thyroid gland follicular cell carcinoma in 

the 8,000 ppm male group compared to the chamber 

controls. The morphology of the carcinomas in this 

group was similar to the morphologic spectrum typical 

of spontaneously developing follicular cell carcinomas. There were no concurrent increases in the 

incidences of thyroid gland follicular cell hyperplasia 

or adenoma in male rats, nor were there increased 

incidences of proliferative lesions of the thyroid gland 

in exposed female rats compared to the chamber 

controls. The historical control range for follicular 

cell carcinoma in male rats in inhalation studies is 0% 

to 4%, and the highest historical control incidence of 

this neoplasm by any route for male rats is 3/50 (6%) 

(in a dosed feed study). The five carcinomas in the 

8,000 ppm male group were considered treatment 

related because of the significant increase over historical control rates for inhalation studies as well as all 

other routes of administration. 

Isobutene, like the structurally related compounds 

propylene, ethylene, 1,3-butadiene, and isoprene, is 

metabolized to an epoxide, 2-methyl-1,2-epoxypropane (1,1-dimethyloxirane). Although isobutene 

is not mutagenic in Salmonella typhimurium itself, the 

isobutene epoxide is mutagenic, but only at high 

concentrations. It was also clastogenic to human 

lymphocytes in vitro (Jorritsma et al., 1995). The 

epoxides of other olefins are mutagenic, and many are 

considered to be responsible for the carcinogenicity of 

the parent compound. For example, propylene was 

not carcinogenic in rats or mice (NTP, 1985a), whereas propylene oxide caused increased incidences of 

papillary adenoma of the nasal turbinates in male and 

female B6C3F mice (NTP 1985b). 1,3-Butadiene 1 

(Owen et al., 1987; NTP, 1993) and isoprene 

(Melnick et al., 1994; NTP, 1998) were multisite 

carcinogens in rats and mice. Of importance is the 

fact that 1,3-butadiene caused thyroid gland follicular 

cell neoplasms in female rats following a 2-year 

exposure to concentrations up to 8,000 ppm. However, whether the isobutene epoxide 2-methyl1,2-epoxypropane was responsible for the thyroid 

gland follicular cell carcinomas in male rats exposed 

to 8,000 ppm is not known, because the isobutene 

epoxide (at conditions of saturation) is detected in the 

exhaled air of rats at about 1% of that observed for 

1,3-butadiene epoxide (1,2-epoxy-3-butene) or 7% of 

that observed for ethene oxide (Csanády et al., 1991). 
__________ 

52 Isobutene, NTP TR 487 

Exposure of rats to isobutene caused an increase, 

although marginal, in the incidences of hyaline degeneration of the olfactory epithelium of the nose in 

males and females; more importantly, the severities of 

this lesion (mild to moderate) were increased in 

exposed males and females in a concentration-related 

fashion. No nasal neoplasms were observed in 

exposed male or female rats. 

During the 2-year mouse study, neither survival rates 

nor body weight gains of males were significantly 

affected by isobutene exposure. Although survival 

rates for female mice were not affected by exposure, 

female mice exposed to 2,000 or 8,000 ppm weighed 

slightly less than the chamber controls in the second 

year of the study. There were no treatment-related 

clinical findings in mice. The only lesions associated 

with exposure in mice were nonneoplastic nasal 

lesions in all exposed groups of males and females. 

Nasal lesions included hyaline degeneration of the 

respiratory and olfactory epithelium; the lesions were 

minimal to mild in severity and the incidences increased with increasing exposure concentration. 

Although they were not observed in the 14-week 

mouse study, these lesions are fairly common in longterm inhalation studies. No nasal neoplasms were 

observed in male or female mice. 

Exposure to isobutene for 6, 12, or 18 months had no 

effect on urine output or the concentration of urinary 

creatinine in male or female rats or mice. During the 

course of the study, there was no consistent timerelated trend relative to the duration of exposure and 

the amount of 2-hydroxyisobutyric acid (HIBA) 

excreted per exposure concentration. Although the 

amount of HIBA/μg creatinine did increase with 

increasing exposure concentration at each of the three 

collection time points, when normalized to exposure 

concentration (ng HIBA/μg creatinine per ppm isobutene), it decreased with increasing exposure concentration. The fact that the urinary excretion of HIBA 

was not directly proportional to exposure concentration indicates nonlinear toxicokinetics. This was 

true for male and female rats and mice. This finding 

was consistent with the findings of Csanády et al. 

(1991), who determined that isobutene metabolism 

was directly proportional to its concentration at 

concentrations of 500 ppm and less. Their assumptions were based on isobutene elimination from a 

closed inhalation exposure system. These data were 

used to set exposure concentrations for the 2-year rat 

and mouse isobutene studies in lieu of significant 

exposure-related toxicological effects following 

14-week exposure to 8,000 ppm. Based on the 2-year 

results and the studies of Csanády et al. (1991) and 

Henderson et al. (1993), concentrations of isobutene 

greater than 500 ppm should result in kinetic events 

that are saturated. 

CONCLUSIONS 

Under the conditions of these 2-year inhalation studies, there was some evidence of carcinogenic activity* 

of isobutene in male F344/N rats based on an increased incidence of follicular cell carcinoma of the 

thyroid gland. There was no evidence of carcinogenic 

activity of isobutene in female F344/N rats or male or 

female B6C3F mice exposed to 500, 2,000, or 1 

8,000 ppm. 

Exposure to isobutene by inhalation for 2 years 

resulted in increased incidences and/or severities of 

nasal lesions including hyaline degeneration of the 

olfactory epithelium in male and female rats and mice 

and hyaline degeneration of the respiratory epithelium 

in male and female mice. 

* Explanation of Levels of Evidence of Carcinogenic Activity is on page 8. A summary of the Technical Reports Review Subcommittee 

comments and the public discussion on this Technical Report appears on page 10. 
53 

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58 Isobutene, NTP TR 487
 
59 

APPENDIX A 

SUMMARY OF LESIONS IN MALE RATS 

IN THE 2-YEAR INHALATION STUDY 

OF ISOBUTENE 

TABLE A1 

TABLE A2 

TABLE A3 

TABLE A4 

TABLE A5 

Summary of the Incidence of Neoplasms in Male Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 

Individual Animal Tumor Pathology of Male Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 

Statistical Analysis of Primary Neoplasms in Male Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 

Historical Incidence of Thyroid Gland Follicular Cell Neoplasms 

in Chamber Control Male F344/N Rats .................................. 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 

60 

64 

84 

88 

89 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

60 Isobutene, NTP TR 487 

TABLE A1 

Summary of the Incidence of Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 39 40 37 41 

Natural deaths 4 5 7 1 

Survivors 

Terminal sacrifice 7 5 6 8 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Intestine large, colon (49) (48) (48) (49) 

Intestine large, rectum (47) (49) (49) (49) 

Polyp adenomatous 1 (2%) 

Intestine large, cecum (48) (47) (49) (49) 

Intestine small, jejunum (47) (46) (47) (49) 

Carcinoma 1 (2%) 1 (2%) 

Hemangiosarcoma 1 (2%) 

Intestine small, ileum (47) (46) (47) (49) 

Leiomyosarcoma 1 (2%) 

Liver (50) (50) (50) (50) 

Histiocytic sarcoma 1 (2%) 2 (4%) 

Osteosarcoma, metastatic, bone 1 (2%) 

Mesentery (15) (10) (8) (11) 

Liposarcoma 1 (10%) 

Oral mucosa (1) 

Pharyngeal, squamous cell papilloma 1 (100%) 

Pancreas (50) (50) (50) (50) 

Adenoma 2 (4%) 1 (2%) 

Salivary glands (50) (50) (50) (50) 

Stomach, forestomach (50) (50) (50) (50) 

Stomach, glandular (50) (49) (50) (50) 

Tongue (1) (1) 

Hemangiosarcoma 1 (100%) 

Cardiovascular System 

Heart (50) (50) (50) (50) 

Endocrine System 

Adrenal cortex (50) (49) (50) (50) 

Adenoma 1 (2%) 1 (2%) 

Osteosarcoma, metastatic, bone 1 (2%) 

Adrenal medulla (50) (49) (50) (50) 

Pheochromocytoma malignant 1 (2%) 1 (2%) 2 (4%) 

Pheochromocytoma benign 13 (26%) 7 (14%) 8 (16%) 11 (22%) 

Bilateral, pheochromocytoma malignant 1 (2%) 

Bilateral, pheochromocytoma benign 10 (20%) 11 (22%) 8 (16%) 12 (24%) 

Islets, pancreatic (50) (49) (50) (50) 

Adenoma 4 (8%) 1 (2%) 4 (8%) 

Carcinoma 4 (8%) 2 (4%) 1 (2%) 4 (8%) 

Pituitary gland (50) (49) (50) (49) 

Pars distalis, adenoma 43 (86%) 40 (82%) 40 (80%) 41 (84%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 61 

TABLE A1 

Summary of the Incidence of Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Endocrine System (continued) 

Thyroid gland 

C-cell, adenoma

C-cell, carcinoma

Follicular cell, carcinoma

(48) 

 5 (10%) 

 1 (2%) 

(48) 

 4 (8%) 

(48) 

 5 (10%) 

 2 (4%) 

(50) 

 7 (14%) 

 1 (2%) 

 5 (10%) 

General Body System 

None 

Genital System 

Epididymis (50) (50) (50) (50) 

Preputial gland (50) (50) (49) (50) 

Adenoma 4 (8%) 2 (4%) 2 (4%) 1 (2%) 

Carcinoma 1 (2%) 2 (4%) 2 (4%) 3 (6%) 

Bilateral, carcinoma 1 (2%) 

Prostate (50) (49) (50) (50) 

Adenoma 1 (2%) 

Seminal vesicle (50) (47) (50) (50) 

Testes (50) (50) (50) (50) 

Bilateral, interstitial cell, adenoma 13 (26%) 14 (28%) 17 (34%) 14 (28%) 

Interstitial cell, adenoma 10 (20%) 14 (28%) 12 (24%) 11 (22%) 

Hematopoietic System 

Bone marrow (50) (48) (50) (50) 

Histiocytic sarcoma 2 (4%) 

Lymph node (4) (11) (10) (10) 

Histiocytic sarcoma 1 (25%) 1 (10%) 

Iliac, osteosarcoma, metastatic, bone 1 (25%) 

Pancreatic, histiocytic sarcoma 1 (10%) 

Lymph node, bronchial (30) (41) (35) (45) 

Histiocytic sarcoma 1 (3%) 2 (6%) 

Lymph node, mandibular (47) (47) (49) (49) 

Lymph node, mesenteric (50) (50) (49) (50) 

Lymph node, mediastinal (46) (42) (44) (45) 

Carcinoma, metastatic, thyroid gland 1 (2%) 

Histiocytic sarcoma 1 (2%) 

Spleen (50) (49) (50) (50) 

Histiocytic sarcoma 1 (2%) 

Thymus (40) (42) (38) (42) 

Integumentary System 

Mammary gland (41) (41) (40) (44) 

Carcinoma 1 (2%) 

Carcinoma, multiple 1 (2%) 

Fibroadenoma 1 (2%) 1 (3%) 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

62 Isobutene, NTP TR 487 

TABLE A1 

Summary of the Incidence of Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Integumentary System (continued) 

Skin (50) (50) (50) (50) 

Basal cell adenoma 2 (4%) 

Basal cell carcinoma 2 (4%) 

Keratoacanthoma 3 (6%) 4 (8%) 2 (4%) 4 (8%) 

Squamous cell papilloma 1 (2%) 2 (4%) 1 (2%) 

Sebaceous gland, adenoma 2 (4%) 1 (2%) 

Subcutaneous tissue, fibroma 1 (2%) 2 (4%) 

Subcutaneous tissue, histiocytic sarcoma 1 (2%) 

Subcutaneous tissue, lipoma 1 (2%) 

Subcutaneous tissue, rhabdomyosarcoma 1 (2%) 

Subcutaneous tissue, sarcoma 1 (2%) 

Subcutaneous tissue, schwannoma benign 1 (2%) 

Subcutaneous tissue, schwannoma malignant 1 (2%) 

Subcutaneous tissue, pinna, melanoma malignant 1 (2%) 

Musculoskeletal System 

Bone (50) (50) (50) (50) 

Osteosarcoma 1 (2%) 1 (2%) 

Skeletal muscle (1) (1) (1) 

Histiocytic sarcoma 1 (100%) 

Nervous System 

Brain (50) (50) (50) (50) 

Astrocytoma malignant 1 (2%) 

Histiocytic sarcoma 1 (2%) 

Oligodendroglioma malignant 1 (2%) 

Respiratory System 

Lung (50) (50) (50) (50) 

Alveolar/bronchiolar adenoma 2 (4%) 2 (4%) 2 (4%) 

Alveolar/bronchiolar carcinoma 2 (4%) 1 (2%) 1 (2%) 

Carcinoma, metastatic, thyroid gland 1 (2%) 

Histiocytic sarcoma 1 (2%) 2 (4%) 

Osteosarcoma, metastatic, bone 1 (2%) 

Mediastinum, alveolar/bronchiolar carcinoma, 

metastatic, lung 1 (2%) 

Nose (49) (49) (50) (49) 

Special Senses System 

Eye (3) (4) 

Zymbal’s gland (2) (1) 

Carcinoma 2 (100%) 1 (100%) 

Urinary System 

Kidney (50) (50) (50) (50) 

Histiocytic sarcoma 1 (2%) 

Stromal nephroma 1 (2%) 1 (2%) 

Renal tubule, adenoma 2 (4%) 1 (2%) 2 (4%) 

Renal tubule, carcinoma 2 (4%) 

Urinary bladder (50) (49) (50) (50) 
 

 

 

63 

c 

Isobutene, NTP TR 487 

TABLE A1 

Summary of the Incidence of Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Systemic Lesions 

Multiple organsb (50) (50) (50) (50) 

Histiocytic sarcoma 1 (2%) 2 (4%) 

Leukemia mononuclear 21 (42%) 21 (42%) 20 (40%) 31 (62%) 

Mesothelioma malignant 3 (6%) 2 (4%) 

Neoplasm Summary 

Total animals with primary neoplasmsc 50 48 49 50 

Total primary neoplasms 155 143 138 165 

Total animals with benign neoplasms 48 46 49 49 

Total benign neoplasms 117 103 104 113 

Total animals with malignant neoplasms 31 33 28 40 

Total malignant neoplasms 38 40 34 52 

Total animals with metastatic neoplasms 1 1 1 

Total metastatic neoplasms 4 1 2 

a Number of animals examined microscopically at the site and the number of animals with neoplasm b Number of animals with any tissue examined microscopically 

Primary neoplasms: all neoplasms except metastatic neoplasms 
 

 

 

 

 

 

 

 

 

 

 

 

 

64 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 

Number of Days on Study 7 6 7 7 9 1 3 3 4 4 5 6 7 7 8 8 9 1 1 2 2 3 3 3 4 

9 7 6 7 5 6 9 9 0 4 2 0 4 6 0 0 0 4 6 1 8 3 6 9 5 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Carcass ID Number 0 0 2 0 4 3 3 4 4 0 1 0 2 3 2 4 3 5 2 4 2 0 1 0 0 

9 7 4 8 0 1 2 1 7 4 5 2 3 8 2 2 3 0 6 3 0 5 7 6 1 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon + + + + + + + + + A + + + + + + + + + + + + + + + 

Intestine large, rectum + + + + + + + + + A + I + + + + + + + + + + + + + 

Polyp adenomatous X 

Intestine large, cecum + + + + + + + + + A + + A + + + + + + + + + + + + 

Intestine small, duodenum + + + + + + + + + A + + + + + + + + + + + + + + + 

Intestine small, jejunum + A + + + + + + + A + + A + + + + + + + + + + + + 

Carcinoma X 

Intestine small, ileum + A + + + + + + + A + + A + + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Osteosarcoma, metastatic, bone 

Mesentery + + + + + + + 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 

Tongue 

Tooth + + 

Cardiovascular System 

Blood vessel 

Heart 

+ 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Osteosarcoma, metastatic, bone 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign X X X X X X X 

Bilateral, pheochromocytoma benign 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Carcinoma X 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + M + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X 

Thyroid gland + A + + + + + + + + + + A + + + + + + + + + + + + 

C-cell, adenoma X 

Follicular cell, carcinoma 

General Body System 

None 

+: Tissue examined microscopically M: Missing tissue X: Lesion present 

A: Autolysis precludes examination I: Insufficient tissue Blank: Not examined 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 65 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 4 5 6 6 7 7 8 8 8 8 9 9 0 0 0 1 1 2 3 3 3 3 3 3 3 

8 0 1 6 0 8 1 3 3 4 1 2 5 6 6 9 9 0 3 3 3 3 3 3 3 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Total 

Carcass ID Number 4 1 4 1 2 3 3 2 4 3 1 4 4 2 3 1 3 1 0 1 1 1 2 2 3 Tissues/ 

9 3 8 4 9 0 4 5 5 5 6 6 4 1 7 8 9 9 3 0 1 2 7 8 6 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + I + + + + 47 

Polyp adenomatous 1 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Carcinoma 1 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 1 

Osteosarcoma, metastatic, bone X 1 

Mesentery + + + + + + + + 15 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X X 2 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Tongue + 1 

Tooth 2 

Cardiovascular System 

Blood vessel 

Heart 

+

+ + + + + + + + + + + + + + + + + + + + + + + + + 

2 

50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Osteosarcoma, metastatic, bone X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma benign X X X X X X 13 

Bilateral, pheochromocytoma benign X X X X X X X X X X 10 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X X X X 4 

Carcinoma X X X 4 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X X X X 43 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

C-cell, adenoma X X X X 5 

Follicular cell, carcinoma X 1 

General Body System 

None 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

66 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 

Number of Days on Study 7 6 7 7 9 1 3 3 4 4 5 6 7 7 8 8 9 1 1 2 2 3 3 3 4 

9 7 6 7 5 6 9 9 0 4 2 0 4 6 0 0 0 4 6 1 8 3 6 9 5 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Carcass ID Number 0 0 2 0 4 3 3 4 4 0 1 0 2 3 2 4 3 5 2 4 2 0 1 0 0 

9 7 4 8 0 1 2 1 7 4 5 2 3 8 2 2 3 0 6 3 0 5 7 6 1 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma X X 

Carcinoma X 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 

Bilateral, interstitial cell, adenoma X X X X 

Interstitial cell, adenoma X X X 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node + + 

Histiocytic sarcoma 

Iliac, osteosarcoma, metastatic, bone 

Lymph node, bronchial M M + M M + + M M M + M M M + + + + + M M M + + + 

Histiocytic sarcoma 

Lymph node, mandibular + + + + + + + + + + + + + + + + + + + + + + + M + 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node, mediastinal + + + + + + + + + + M + + + + + + + + + + + + + M 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 

Thymus + + + M M + + M + + + + + M + + + + + + + + M M + 

Integumentary System 

Mammary gland M M M M + M + M + M + M + + + + + + + + M + + + + 

Carcinoma, multiple 

Fibroadenoma 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 

Keratoacanthoma X 

Subcutaneous tissue, fibroma 

Subcutaneous tissue, lipoma X 

Subcutaneous tissue, sarcoma X 

Subcutaneous tissue, schwannoma benign 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Osteosarcoma 

Skeletal muscle + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Astrocytoma malignant X 

Oligodendroglioma malignant X 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 

Alveolar/bronchiolar adenoma X 

Histiocytic sarcoma 

Osteosarcoma, metastatic, bone 

Nose + + + + + + + + + + + + A + + + + + + + + + + + + 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 67 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 4 5 6 6 7 7 8 8 8 8 9 9 0 0 0 1 1 2 3 3 3 3 3 3 3 

8 0 1 6 0 8 1 3 3 4 1 2 5 6 6 9 9 0 3 3 3 3 3 3 3 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Total 

Carcass ID Number 4 1 4 1 2 3 3 2 4 3 1 4 4 2 3 1 3 1 0 1 1 1 2 2 3 Tissues/ 

9 3 8 4 9 0 4 5 5 5 6 6 4 1 7 8 9 9 3 0 1 2 7 8 6 Tumors 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X X 4 

Carcinoma 1 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Bilateral, interstitial cell, adenoma X X X X X X X X X 13 

Interstitial cell, adenoma X X X X X X X 10 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node + + 4 

Histiocytic sarcoma X 1 

Iliac, osteosarcoma, metastatic, bone X 1 

Lymph node, bronchial + + M + M + + + + + + + + + M M + + + + + M M + M 30 

Histiocytic sarcoma X 1 

Lymph node, mandibular + + + + + + + + + + + + + + M + + + M + + + + + + 47 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node, mediastinal + + + + + + + + + + + + + + + + + + + + + M + M + 46 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Thymus + + M M M + + + + + + + + + + + + + + + + M + + + 40 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 41 

Carcinoma, multiple X 1 

Fibroadenoma X 1 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Keratoacanthoma X X 3 

Subcutaneous tissue, fibroma X 1 

Subcutaneous tissue, lipoma 1 

Subcutaneous tissue, sarcoma 1 

Subcutaneous tissue, schwannoma benign X 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Osteosarcoma X 1 

Skeletal muscle 1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Astrocytoma malignant 1 

Oligodendroglioma malignant 1 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X 2 

Histiocytic sarcoma X 1 

Osteosarcoma, metastatic, bone X 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

 

 

68 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 

Number of Days on Study 7 6 7 7 9 1 3 3 4 4 5 6 7 7 8 8 9 1 1 2 2 3 3 3 4 

9 7 6 7 5 6 9 9 0 4 2 0 4 6 0 0 0 4 6 1 8 3 6 9 5 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Carcass ID Number 0 0 2 0 4 3 3 4 4 0 1 0 2 3 2 4 3 5 2 4 2 0 1 0 0 

9 7 4 8 0 1 2 1 7 4 5 2 3 8 2 2 3 0 6 3 0 5 7 6 1 

Special Senses System 

Eye + 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 

Stromal nephroma X 

Renal tubule, adenoma 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Leukemia mononuclear X X X X X X 

Mesothelioma malignant X X 
 

 

 

 

 

 

Isobutene, NTP TR 487 69 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 4 5 6 6 7 7 8 8 8 8 9 9 0 0 0 1 1 2 3 3 3 3 3 3 3 

8 0 1 6 0 8 1 3 3 4 1 2 5 6 6 9 9 0 3 3 3 3 3 3 3 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Total 

Carcass ID Number 4 1 4 1 2 3 3 2 4 3 1 4 4 2 3 1 3 1 0 1 1 1 2 2 3 Tissues/ 

9 3 8 4 9 0 4 5 5 5 6 6 4 1 7 8 9 9 3 0 1 2 7 8 6 Tumors 

Special Senses System 

Eye + + 3 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stromal nephroma 1 

Renal tubule, adenoma X X 2 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 1 

Leukemia mononuclear X X X X X X X X X X X X X X X 21 

Mesothelioma malignant X 3 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

70 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

2 3 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 

Number of Days on Study 4 8 6 6 8 0 1 2 2 2 2 3 4 5 6 7 8 8 9 0 0 1 2 2 2 

4 9 0 9 2 7 4 3 6 6 7 7 7 2 0 9 3 3 2 4 7 4 4 5 6 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 

Carcass ID Number 4 2 4 3 3 4 5 2 1 1 4 2 0 2 0 0 1 3 2 2 2 2 0 4 3 

3 6 6 8 3 8 0 1 0 1 2 5 5 3 3 2 8 6 2 7 9 0 9 1 9 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon + A + + A + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, cecum + A + + A A + + + + + + + + + + + + + + + + + + + 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine small, jejunum + A + + A A + + + + + + + + + + + + + + + + + + + 

Intestine small, ileum + A + + A A + + + + + + + + + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Mesentery + + 

Liposarcoma 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + A + + + + + + + + + + + + + + + + + + + + + + + 

Tooth + + 

Cardiovascular System 

Blood vessel 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + A + + + + + + + + + + + + + + + + + + + + + + + 

Adrenal medulla + A + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma malignant 

Pheochromocytoma benign X 

Bilateral, pheochromocytoma benign X 

Islets, pancreatic + A + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pituitary gland + A + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X 

Thyroid gland + + + + A + + + + + + + + + + + + + + + + + + + + 

C-cell, adenoma X 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma X 

Carcinoma X 

Bilateral, carcinoma 

Prostate + A + + + + + + + + + + + + + + + + + + + + + + + 

Seminal vesicle + A + + A + + + + + + + + + + + + + + + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 

Bilateral, interstitial cell, adenoma X X X X 

Interstitial cell, adenoma X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 71 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 

Number of Days on Study 2 3 4 4 6 6 6 6 7 7 7 7 8 8 8 9 9 0 0 1 3 3 3 3 3 

7 2 1 2 0 2 4 4 0 1 1 8 4 4 4 2 2 2 6 6 3 3 3 3 3 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Total 

Carcass ID Number 1 1 3 0 1 0 0 3 1 1 1 1 2 3 3 0 4 3 4 4 0 2 3 4 4 Tissues/ 

7 9 2 4 2 6 1 0 5 3 4 6 4 1 5 7 4 4 0 9 8 8 7 5 7 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine large, rectum + + A + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, duodenum + + A + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, jejunum + + A + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, ileum + + A + + + + + + + + + + + + + + + + + + + + + + 46 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Mesentery + + + + + + + + 10 

Liposarcoma X 1 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Tooth + 3 

Cardiovascular System 

Blood vessel 

Heart 

+ +

+ + + + + + + + + + + + + + + + + + + + + + + + + 

2 

50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma malignant X 1 

Pheochromocytoma benign X X X X X X 7 

Bilateral, pheochromocytoma benign X X X X X X X X X X 11 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Carcinoma X X 2 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X X 40 

Thyroid gland + + A + + + + + + + + + + + + + + + + + + + + + + 48 

C-cell, adenoma X X X 4 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 2 

Carcinoma X 2 

Bilateral, carcinoma X 1 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Seminal vesicle + + A + + + + + + + + + + + + + + + + + + + + + + 47 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Bilateral, interstitial cell, adenoma X X X X X X X X X X 14 

Interstitial cell, adenoma X X X X X X X 14 
 

 

 

 

 

 

 

 

 

 

 

 

 

72 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

2 3 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 

Number of Days on Study 4 

4 

8 

9 

6 

0 

6 

9 

8 

2 

0 

7 

1 

4 

2 

3 

2 

6 

2 

6 

2 

7 

3 

7 

4 

7 

5 

2 

6 

0 

7 

9 

8 

3 

8 

3 

9 

2 

0 

4 

0 

7 

1 

4 

2 

4 

2 

5 

2 

6 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 

Carcass ID Number 4 2 4 3 3 4 5 2 1 1 4 2 0 2 0 0 1 3 2 2 2 2 0 4 3 

3 6 6 8 3 8 0 1 0 1 2 5 5 3 3 2 8 6 2 7 9 0 9 1 9 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Thymus 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + 

M M + M M M + + M M M + M + + + + + + + + + + + + 

+ A + + + + + + M + + M + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ A M + + + + + + + M + M + + + + M M + + + + + + 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

M A + + + + + + + M + + + + + + + + + + + + + + + 

Integumentary System 

Mammary gland 

Carcinoma 

Skin 

Basal cell carcinoma 

Keratoacanthoma 

Squamous cell papilloma 

Sebaceous gland, adenoma 

Subcutaneous tissue, schwannoma malignant 

Subcutaneous tissue, pinna, melanoma malignant 

M M + M M M + + + + + + + + + + + + + + + + + + M 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X 

Musculoskeletal System 

Bone 

Osteosarcoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Mediastinum, alveolar/bronchiolar carcinoma, 

metastatic, lung 

Nose 

Trachea 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Zymbal’s gland 

Carcinoma 

+ 

X 

Urinary System 

Kidney 

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Leukemia mononuclear 

Mesothelioma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X X 

X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 73 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 

Number of Days on Study 2 

7 

3 

2 

4 

1 

4 

2 

6 

0 

6 

2 

6 

4 

6 

4 

7 

0 

7 

1 

7 

1 

7 

8 

8 

4 

8 

4 

8 

4 

9 

2 

9 

2 

0 

2 

0 

6 

1 

6 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Total 

Carcass ID Number 1 

7 

1 

9 

3 

2 

0 

4 

1 

2 

0 

6 

0 

1 

3 

0 

1 

5 

1 

3 

1 

4 

1 

6 

2 

4 

3 

1 

3 

5 

0 

7 

4 

4 

3 

4 

4 

0 

4 

9 

0 

8 

2 

8 

3 

7 

4 

5 

4 

7 

Tissues/ 

Tumors 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Thymus 

+ + A + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + A + + + M + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + A + M + + + M + + + + + + + + + M M + + + + + 

48 

11 

41 

47 

50 

42 

49 

42 

Integumentary System 

Mammary gland 

Carcinoma

Skin 

Basal cell carcinoma 

Keratoacanthoma 

Squamous cell papilloma 

Sebaceous gland, adenoma 

Subcutaneous tissue, schwannoma malignant 

Subcutaneous tissue, pinna, melanoma malignant 

+ M + + + + + + + + + M + + + + + + + M + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

X X X

X

X X

1 

X

41 

1 

50 

2 

4 

1 

2 

1 

Musculoskeletal System 

Bone 

Osteosarcoma

+ + + + + + + + + + + + + + + + + + + + + + + + + 50 

1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Mediastinum, alveolar/bronchiolar carcinoma,

 metastatic, lung 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

X X

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

49 

50 

2 

2 

1 

49 

50 

Special Senses System 

Zymbal’s gland 

Carcinoma 

+

X

 2 

2 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Renal tubule, adenoma X 1 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Leukemia mononuclear X X X X X X X X X X X 21 

Mesothelioma malignant X 2 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

74 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 

Number of Days on Study 5 5 8 9 9 0 1 1 2 2 5 5 5 5 6 6 7 7 8 8 8 9 0 1 1 

2 3 0 6 7 0 2 6 0 0 1 2 5 8 5 8 4 9 0 3 5 0 7 4 4 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

Carcass ID Number 2 4 3 3 0 2 0 1 2 3 1 3 4 0 0 5 0 1 2 4 0 1 3 0 3 

3 6 8 7 1 6 9 1 1 6 6 3 1 7 2 0 8 2 5 8 4 8 0 5 2 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma 

Hemangiosarcoma 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Mesentery + 

Oral mucosa + 

Pharyngeal, squamous cell papilloma X 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 

Tooth + 

Cardiovascular System 

Blood vessel 

Heart 

+ 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma malignant X 

Pheochromocytoma benign X X 

Bilateral, pheochromocytoma benign X X 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Carcinoma 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + A + + + + + + + + 

C-cell, adenoma 

C-cell, carcinoma X 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + M + + + + + + + + + + + 

Adenoma 

Carcinoma X X 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 

Bilateral, interstitial cell, adenoma X X 

Interstitial cell, adenoma X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 75 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 1 2 2 2 2 5 5 5 5 7 7 8 0 0 0 0 0 1 1 3 3 3 3 3 3 

4 0 7 8 8 0 2 4 4 1 8 0 0 1 5 6 6 4 4 3 3 3 3 3 3 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Total 

Carcass ID Number 3 0 4 3 4 0 2 2 4 1 1 2 4 1 2 1 1 1 4 2 2 3 3 4 4 Tissues/ 

4 3 4 5 0 6 8 2 9 7 3 9 5 9 4 0 5 4 2 0 7 1 9 3 7 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon A + + + + + + + + + + + + + A + + + + + + + + + + 48 

Intestine large, rectum + + + + + + + + + + + + + + A + + + + + + + + + + 49 

Intestine large, cecum + + + + + + + + + + + + + + A + + + + + + + + + + 49 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine small, jejunum A + + + + + + + A + + + + + A + + + + + + + + + + 47 

Carcinoma X 1 

Hemangiosarcoma X 1 

Intestine small, ileum A + + + + + + + A + + + + + A + + + + + + + + + + 47 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 2 

Mesentery + + + + + + + 8 

Oral mucosa 1 

Pharyngeal, squamous cell papilloma 1 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Tooth 1 

Cardiovascular System 

Blood vessel + 2 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma malignant 1 

Pheochromocytoma benign X X X X X X 8 

Bilateral, pheochromocytoma benign X X X X X X 8 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Carcinoma X 1 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + M 49 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X 40 

Thyroid gland + + + + + + + + A + + + + + + + + + + + + + + + + 48 

C-cell, adenoma X X X X X 5 

C-cell, carcinoma X 2 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma X X 2 

Carcinoma 2 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Bilateral, interstitial cell, adenoma X X X X X X X X X X X X X X X 17 

Interstitial cell, adenoma X X X X X 12 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

76 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 

Number of Days on Study 5 5 8 9 9 0 1 1 2 2 5 5 5 5 6 6 7 7 8 8 8 9 0 1 1 

2 3 0 6 7 0 2 6 0 0 1 2 5 8 5 8 4 9 0 3 5 0 7 4 4 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

Carcass ID Number 2 4 3 3 0 2 0 1 2 3 1 3 4 0 0 5 0 1 2 4 0 1 3 0 3 

3 6 8 7 1 6 9 1 1 6 6 3 1 7 2 0 8 2 5 8 4 8 0 5 2 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Lymph node + + + 

Histiocytic sarcoma 

Pancreatic, histiocytic sarcoma 

Lymph node, bronchial M + M + M + M M M + + M M M M + + + M + + + M + + 

Histiocytic sarcoma X 

Lymph node, mandibular + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node, mediastinal M M + + + + + M + + + + + + + M M + + + + M + + + 

Histiocytic sarcoma X 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Thymus + M M M M + + + + + + + + + + + + M + + + + + + M 

Integumentary System 

Mammary gland + M + + + M + + + M + + + + + + + + + + M + + + + 

Fibroadenoma 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 

Keratoacanthoma X 

Squamous cell papilloma 

Sebaceous gland, adenoma 

Subcutaneous tissue, fibroma 

Subcutaneous tissue, histiocytic sarcoma 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Skeletal muscle 

Histiocytic sarcoma 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 

Alveolar/bronchiolar adenoma X 

Alveolar/bronchiolar carcinoma 

Histiocytic sarcoma X 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye 

Zymbal’s gland 

Carcinoma 

+ 
 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 77 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 1 

4 

2 

0 

2 

7 

2 

8 

2 

8 

5 

0 

5 

2 

5 

4 

5 

4 

7 

1 

7 

8 

8 

0 

0 

0 

0 

1 

0 

5 

0 

6 

0 

6 

1 

4 

1 

4 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Total 

Carcass ID Number 3 

4 

0 

3 

4 

4 

3 

5 

4 

0 

0 

6 

2 

8 

2 

2 

4 

9 

1 

7 

1 

3 

2 

9 

4 

5 

1 

9 

2 

4 

1 

0 

1 

5 

1 

4 

4 

2 

2 

0 

2 

7 

3 

1 

3 

9 

4 

3 

4 

7 

Tissues/ 

Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 

Lymph node 

Histiocytic sarcoma 

Pancreatic, histiocytic sarcoma 

Lymph node, bronchial 

Histiocytic sarcoma 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Histiocytic sarcoma

Spleen 

Histiocytic sarcoma

Thymus 

X

+ + + + + + + 

X

X

+ + + + + + + + + + + M + + M + + + + + + + + M + 

X

+ + + + + + + + + + + + + + + + M + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + M + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

M + M + + M M + + + + M + + + + + + M + + + + + + 

2 

10 

1 

1 

35 

2 

49 

49 

44 

1 

50 

1 

38 

Integumentary System 

Mammary gland 

Fibroadenoma 

+ M + M M + + + + + + + + + + + + M + + + + M I 

X

+ 40 

1 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Keratoacanthoma X 2 

Squamous cell papilloma 

Sebaceous gland, adenoma 

Subcutaneous tissue, fibroma 

Subcutaneous tissue, histiocytic sarcoma 

X 

X 

X

X

X

X

 2 

1 

2 

1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Skeletal muscle + 1 

Histiocytic sarcoma X 1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Histiocytic sarcoma 

Nose 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

X

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

50 

50 

2 

1 

2 

50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye 

Zymbal’s gland 

Carcinoma 

+ + 

+

X

+ 4 

1 

1 
 

 

 

78 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

3 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 

Number of Days on Study 5 5 8 9 9 0 1 1 2 2 5 5 5 5 6 6 7 7 8 8 8 9 0 1 1 

2 3 0 6 7 0 2 6 0 0 1 2 5 8 5 8 4 9 0 3 5 0 7 4 4 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

Carcass ID Number 2 4 3 3 0 2 0 1 2 3 1 3 4 0 0 5 0 1 2 4 0 1 3 0 3 

3 6 8 7 1 6 9 1 1 6 6 3 1 7 2 0 8 2 5 8 4 8 0 5 2 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Renal tubule, carcinoma 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Leukemia mononuclear X X X X X X X X 
 

 

 

 

Isobutene, NTP TR 487 79 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 1 2 2 2 2 5 5 5 5 7 7 8 0 0 0 0 0 1 1 3 3 3 3 3 3 

4 0 7 8 8 0 2 4 4 1 8 0 0 1 5 6 6 4 4 3 3 3 3 3 3 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Total 

Carcass ID Number 3 0 4 3 4 0 2 2 4 1 1 2 4 1 2 1 1 1 4 2 2 3 3 4 4 Tissues/ 

4 3 4 5 0 6 8 2 9 7 3 9 5 9 4 0 5 4 2 0 7 1 9 3 7 Tumors 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 1 

Renal tubule, carcinoma X X 2 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 2 

Leukemia mononuclear X X X X X X X X X X X X 20 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

80 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

Number of Days on Study 9 0 2 3 3 4 6 7 8 0 0 0 0 1 1 1 3 3 3 4 4 5 5 5 6 

0 5 5 4 4 0 0 6 3 4 4 7 8 0 1 8 1 6 6 7 8 0 0 8 4 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

Carcass ID Number 3 4 2 0 1 2 2 1 1 1 4 4 2 3 0 4 3 3 4 1 0 0 4 3 0 

9 5 1 3 4 7 6 7 8 9 8 6 8 1 9 2 5 8 9 6 1 6 4 2 2 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon + + + + + + + + A + + + + + + + + + + + + + + + + 

Intestine large, rectum + + + + + + + + A + + + + + + + + + + + + + + + + 

Intestine large, cecum + + + + + + + + A + + + + + + + + + + + + + + + + 

Intestine small, duodenum + + + + + + + + A + + + + + + + + + + + + + + + + 

Intestine small, jejunum + + + + + + + + A + + + + + + + + + + + + + + + + 

Intestine small, ileum + + + + + + + + A + + + + + + + + + + + + + + + + 

Leiomyosarcoma 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Mesentery + + + + + 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 

Tongue + 

Hemangiosarcoma X 

Tooth 

Cardiovascular System 

Blood vessel 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma X 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma malignant X 

Pheochromocytoma benign X X X X X 

Bilateral, pheochromocytoma malignant 

Bilateral, pheochromocytoma benign X X 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma X X X 

Carcinoma X 

Parathyroid gland + + + + + + + + M + + + + + + M + + + + + + + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

C-cell, adenoma X X X X 

C-cell, carcinoma X 

Follicular cell, carcinoma X X 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Carcinoma X 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 81 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 6 6 6 6 7 7 8 8 8 8 9 9 0 0 0 0 2 3 3 3 3 3 3 3 3 

4 4 6 7 0 8 8 8 8 9 0 2 0 1 1 2 0 3 3 3 3 3 3 3 3 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Total 

Carcass ID Number 0 3 1 1 1 2 0 1 2 2 3 4 5 1 3 4 4 0 0 2 2 2 3 3 4 Tissues/ 

5 0 5 0 1 9 4 3 0 4 6 7 0 2 7 1 3 7 8 2 3 5 3 4 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Leiomyosarcoma X 1 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Mesentery + + + + + + 11 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Tongue 1 

Hemangiosarcoma 1 

Tooth + 1 

Cardiovascular System 

Blood vessel + + 2 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma malignant X 2 

Pheochromocytoma benign X X X X X X 11 

Bilateral, pheochromocytoma malignant X 1 

Bilateral, pheochromocytoma benign X X X X X X X X X X 12 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 4 

Carcinoma X X X 4 

Parathyroid gland + + + + + + M + + + + + + + + + + + + + + + + + + 47 

Pituitary gland + + + + + + + M + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X 41 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

C-cell, adenoma X X X 7 

C-cell, carcinoma 1 

Follicular cell, carcinoma X X X 5 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Carcinoma X X 3 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

 

 

 

 

 

 

 

 

82 Isobutene, NTP TR 487 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

Number of Days on Study 9 

0 

0 

5 

2 

5 

3 

4 

3 

4 

4 

0 

6 

0 

7 

6 

8 

3 

0 

4 

0 

4 

0 

7 

0 

8 

1 

0 

1 

1 

1 

8 

3 

1 

3 

6 

3 

6 

4 

7 

4 

8 

5 

0 

5 

0 

5 

8 

6 

4 

Carcass ID Number 

6 

3 

9 

6 

4 

5 

6 

2 

1 

6 

0 

3 

6 

1 

4 

6 

2 

7 

6 

2 

6 

6 

1 

7 

6 

1 

8 

6 

1 

9 

6 

4 

8 

6 

4 

6 

6 

2 

8 

6 

3 

1 

6 

0 

9 

6 

4 

2 

6 

3 

5 

6 

3 

8 

6 

4 

9 

6 

1 

6 

6 

0 

1 

6 

0 

6 

6 

4 

4 

6 

3 

2 

6 

0 

2 

Genital System (continued) 

Testes 

Bilateral, interstitial cell, adenoma 

Interstitial cell, adenoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X 

X X X X 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Carcinoma, metastatic, thyroid gland 

Spleen 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + 

M + + + + M M M + + + + + + + + + + + + + + + + + 

+ + + + + + + + + M + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + M + + M + + + + + + + + + + + + + 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ M M + + + M + + M + + + + + + + + + + + + M + + 

Integumentary System 

Mammary gland 

Fibroadenoma 

Skin 

Basal cell adenoma 

Keratoacanthoma 

Squamous cell papilloma 

Subcutaneous tissue, rhabdomyosarcoma 

+ + + + + + + + + M + + + + + + + + + + M + + + + 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Musculoskeletal System 

Bone 

Skeletal muscle 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar carcinoma 

Carcinoma, metastatic, thyroid gland 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + + + + A + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

None 

Urinary System 

Kidney 

Stromal nephroma 

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Leukemia mononuclear 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 83 

TABLE A2 

Individual Animal Tumor Pathology of Male Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 6 

4 

6 

4 

6 

6 

6 

7 

7 

0 

7 

8 

8 

8 

8 

8 

8 

8 

8 

9 

9 

0 

9 

2 

0 

0 

0 

1 

0 

1 

0 

2 

2 

0 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

Carcass ID Number 

6 

0 

5 

6 

3 

0 

6 

1 

5 

6 

1 

0 

6 

1 

1 

6 

2 

9 

6 

0 

4 

6 

1 

3 

6 

2 

0 

6 

2 

4 

6 

3 

6 

6 

4 

7 

6 

5 

0 

6 

1 

2 

6 

3 

7 

6 

4 

1 

6 

4 

3 

6 

0 

7 

6 

0 

8 

6 

2 

2 

6 

2 

3 

6 

2 

5 

6 

3 

3 

6 

3 

4 

6 

4 

0 

Total 

Tissues/ 

Tumors 

Genital System (continued) 

Testes 

Bilateral, interstitial cell, adenoma 

Interstitial cell, adenoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X 

X X X X X X X 

50 

14 

11 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Carcinoma, metastatic, thyroid gland

Spleen 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + 

+ M + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ M + + + + + + + + + + + + + + + + M M + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

M + + + + M + + + + + + + + + + + + + + + M + + + 

50 

10 

45 

49 

50 

45 

1 

50 

42 

Integumentary System 

Mammary gland 

Fibroadenoma

Skin 

Basal cell adenoma 

Keratoacanthoma 

Squamous cell papilloma 

Subcutaneous tissue, rhabdomyosarcoma 

+ + + + + + + + + + + + + M + + M + + M + + + M + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X

X X X X

X

X

44 

1 

50 

2 

4 

1 

1 

Musculoskeletal System 

Bone 

Skeletal muscle 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+

50 

1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar carcinoma 

Carcinoma, metastatic, thyroid gland

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

49 

50 

1 

1 

49 

50 

Special Senses System 

None 

Urinary System 

Kidney 

Stromal nephroma

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

50 

1 

2 

50 

Systemic Lesions 

Multiple organs 

Leukemia mononuclear 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X X X X X X X X X X 

50 

31 
84 Isobutene, NTP TR 487 

TABLE A3 

Statistical Analysis of Primary Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Adrenal Medulla: Benign Pheochromocytoma a Overall rate 23/50 (46%) 18/49 (37%) 16/50 (32%) 23/50 (46%) 

Adjusted rateb 60.0% 52.1% 46.2% 56.9% c Terminal rate 6/7 (86%) 4/5 (80%) 4/6 (67%) 6/8 (75%) 

First incidence (days) 539 526 496 525 

Poly-3 testd P=0.497 P=0.309N P=0.146N P=0.476N 

Adrenal Medulla: Malignant Pheochromocytoma 

Overall rate 0/50 (0%) 1/49 (2%) 1/50 (2%) 3/50 (6%) 

Adjusted rate 0.0% 3.2% 3.2% 8.3% 

Terminal rate 0/7 (0%) 0/5 (0%) 0/6 (0%) 1/8 (13%) e First incidence (days) — 664 453 631 

Poly-3 test P=0.085 P=0.485 P=0.489 P=0.132 

Adrenal Medulla: Benign or Malignant Pheochromocytoma 

Overall rate 23/50 (46%) 18/49 (37%) 17/50 (34%) 24/50 (48%)
 

Adjusted rate 60.0% 52.1% 48.0% 59.1%
 

Terminal rate 6/7 (86%) 4/5 (80%) 4/6 (67%) 6/8 (75%)
 

First incidence (days) 539 526 453 525
 

Poly-3 test P=0.399 P=0.309N P=0.187N P=0.564N
 

Lung: Alveolar/bronchiolar Adenoma or Carcinoma 

Overall rate 2/50 (4%) 4/50 (8%) 3/50 (6%) 1/50 (2%) 

Adjusted rate 5.9% 12.6% 9.5% 2.8% 

Terminal rate 0/7 (0%) 1/5 (20%) 0/6 (0%) 1/8 (13%) 

First incidence (days) 580 579 607 733 (T) 

Poly-3 test P=0.196N P=0.301 P=0.466 P=0.482N 

Pancreatic Islets: Adenoma 

Overall rate 4/50 (8%) 0/49 (0%) 1/50 (2%) 4/50 (8%) 

Adjusted rate 11.8% 0.0% 3.2% 10.8% 

Terminal rate 1/7 (14%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 666 — 714 618 

Poly-3 test P=0.276 P=0.071N P=0.203N P=0.599N 

Pancreatic Islets: Carcinoma 

Overall rate 4/50 (8%) 2/49 (4%) 1/50 (2%) 4/50 (8%) 

Adjusted rate 11.6% 6.4% 3.2% 10.8% 

Terminal rate 1/7 (14%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 477 664 620 534 

Poly-3 test P=0.451 P=0.383N P=0.206N P=0.609N 

Pancreatic Islets: Adenoma or Carcinoma 

Overall rate 8/50 (16%) 2/49 (4%) 2/50 (4%) 8/50 (16%)
 

Adjusted rate 22.8% 6.4% 6.4% 21.0%
 

Terminal rate 2/7 (29%) 0/5 (0%) 0/6 (0%) 0/8 (0%)
 

First incidence (days) 477 664 620 534
 

Poly-3 test P=0.238 P=0.060N P=0.058N P=0.540N
 

Pituitary Gland (Pars Distalis): Adenoma 

Overall rate 43/50 (86%) 40/49 (82%) 40/50 (80%) 41/49 (84%)
 

Adjusted rate 93.0% 89.2% 87.1% 86.9%
 

Terminal rate 6/7 (86%) 3/5 (60%) 5/6 (83%) 6/8 (75%)
 

First incidence (days) 476 460 453 490
 

Poly-3 test P=0.280N P=0.377N P=0.240N P=0.233N
 
Isobutene, NTP TR 487 85 

TABLE A3 

Statistical Analysis of Primary Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Preputial Gland: Adenoma 

Overall rate 4/50 (8%) 2/50 (4%) 2/49 (4%) 1/50 (2%) 

Adjusted rate 11.4% 6.4% 6.4% 2.8% 

Terminal rate 1/7 (14%) 1/5 (20%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 467 583 628 700 

Poly-3 test P=0.170N P=0.386N P=0.390N P=0.167N 

Preputial Gland: Carcinoma 

Overall rate 1/50 (2%) 3/50 (6%) 2/49 (4%) 3/50 (6%) 

Adjusted rate 2.9% 9.4% 6.3% 8.2% 

Terminal rate 0/7 (0%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 560 526 520 540 

Poly-3 test P=0.431 P=0.278 P=0.474 P=0.331 

Preputial Gland: Adenoma or Carcinoma 

Overall rate 5/50 (10%) 5/50 (10%) 4/49 (8%) 4/50 (8%) 

Adjusted rate 14.1% 15.5% 12.4% 10.9% 

Terminal rate 1/7 (14%) 1/5 (20%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 467 526 520 540 

Poly-3 test P=0.383N P=0.572 P=0.562N P=0.477N 

Skin: Keratoacanthoma 

Overall rate 3/50 (6%) 4/50 (8%) 2/50 (4%) 4/50 (8%) 

Adjusted rate 8.8% 12.5% 6.4% 11.1% 

Terminal rate 1/7 (14%) 1/5 (20%) 1/6 (17%) 2/8 (25%) 

First incidence (days) 621 552 574 700 

Poly-3 test P=0.523 P=0.463 P=0.538N P=0.530 

Skin: Squamous Cell Papilloma or Keratoacanthoma 

Overall rate 3/50 (6%) 5/50 (10%) 3/50 (6%) 5/50 (10%)
 

Adjusted rate 8.8% 15.6% 9.5% 13.9%
 

Terminal rate 1/7 (14%) 1/5 (20%) 1/6 (17%) 3/8 (38%)
 

First incidence (days) 621 552 574 700
 

Poly-3 test P=0.442 P=0.318 P=0.631 P=0.384 

Skin: Squamous Cell Papilloma, Keratoacanthoma, Basal Cell Adenoma, or Basal Cell Carcinoma 

Overall rate 3/50 (6%) 7/50 (14%) 3/50 (6%) 7/50 (14%)
 

Adjusted rate 8.8% 21.5% 9.5% 19.2%
 

Terminal rate 1/7 (14%) 2/5 (40%) 1/6 (17%) 3/8 (38%)
 

First incidence (days) 621 552 574 688
 

Poly-3 test P=0.275 P=0.128 P=0.631 P=0.176 

Testes: Adenoma 

Overall rate 23/50 (46%) 28/50 (56%) 29/50 (58%) 25/50 (50%)
 

Adjusted rate 59.1% 70.4% 74.2% 61.1%
 

Terminal rate 5/7 (71%) 3/5 (60%) 6/6 (100%) 6/8 (75%)
 

First incidence (days) 379 460 496 583
 

Poly-3 test P=0.343N P=0.180 P=0.087 P=0.522 

Thyroid Gland (C-cell): Adenoma 

Overall rate 5/48 (10%) 4/48 (8%) 5/48 (10%) 7/50 (14%)
 

Adjusted rate 14.8% 12.8% 16.5% 18.3%
 

Terminal rate 2/7 (29%) 0/5 (0%) 1/6 (17%) 0/8 (0%)
 

First incidence (days) 540 583 678 534
 

Poly-3 test P=0.360 P=0.549N P=0.563 P=0.468 
86 Isobutene, NTP TR 487 

TABLE A3 

Statistical Analysis of Primary Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Thyroid Gland (C-cell): Adenoma or Carcinoma 

Overall rate 5/48 (10%) 4/48 (8%) 7/48 (15%) 8/50 (16%) 

Adjusted rate 14.8% 12.8% 22.7% 20.7% 

Terminal rate 2/7 (29%) 0/5 (0%) 2/6 (33%) 0/8 (0%) 

First incidence (days) 540 583 520 534 

Poly-3 test P=0.290 P=0.549N P=0.310 P=0.365 

Thyroid Gland (Follicular Cell): Carcinoma 

Overall rate 1/48 (2%) 0/48 (0%) 0/48 (0%) 5/50 (10%) 

Adjusted rate 3.0% 0.0% 0.0% 13.5% 

Terminal rate 0/7 (0%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 661 — — 618 

Poly-3 test P=0.004 P=0.519N P=0.521N P=0.125 

All Organs: Malignant Mesothelioma 

Overall rate 3/50 (6%) 2/50 (4%) 0/50 (0%) 0/50 (0%) 

Adjusted rate 8.7% 6.3% 0.0% 0.0% 

Terminal rate 0/7 (0%) 0/5 (0%) 0/6 (0%) 0/8 (0%) 

First incidence (days) 544 547 — — 

Poly-3 test P=0.081N P=0.540N P=0.138N P=0.110N 

All Organs: Mononuclear Cell Leukemia 

Overall rate 21/50 (42%) 21/50 (42%) 20/50 (40%) 31/50 (62%) 

Adjusted rate 54.9% 55.5% 52.4% 71.1% 

Terminal rate 6/7 (86%) 4/5 (80%) 2/6 (33%) 7/8 (88%) 

First incidence (days) 467 514 497 505 

Poly-3 test P=0.028 P=0.575 P=0.504N P=0.075 

All Organs: Benign Neoplasms 

Overall rate 48/50 (96%) 46/50 (92%) 49/50 (98%) 49/50 (98%) 

Adjusted rate 98.3% 98.3% 99.8% 98.4% 

Terminal rate 7/7 (100%) 5/5 (100%) 6/6 (100%) 8/8 (100%) 

First incidence (days) 379 460 453 490 

Poly-3 test P=0.793N P=0.898 P=0.659 P=0.821 

All Organs: Malignant Neoplasms 

Overall rate 31/50 (62%) 33/50 (66%) 28/50 (56%) 40/50 (80%) 

Adjusted rate 72.1% 78.4% 68.5% 87.1% 

Terminal rate 7/7 (100%) 5/5 (100%) 5/6 (83%) 7/8 (88%) 

First incidence (days) 379 469 453 505 

Poly-3 test P=0.030 P=0.314 P=0.445N P=0.040 
87 

c 

Isobutene, NTP TR 487 

TABLE A3 

Statistical Analysis of Primary Neoplasms in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

All Organs: Benign or Malignant Neoplasms 

Overall rate 50/50 (100%) 48/50 (96%) 49/50 (98%) 50/50 (100%) 

Adjusted rate 100.0% 99.6% 99.8% 100.0% 

Terminal rate 7/7 (100%) 5/5 (100%) 6/6 (100%) 8/8 (100%) 

First incidence (days) 

Poly-3 test 

379 

P=1.000 

460 

P=1.000N 

453 

P=1.000N 

490 

—f 

(T)Terminal sacrifice 

a Number of neoplasm-bearing animals/number of animals examined. Denominator is number of animals examined microscopically for adrenal gland, lung, 

pancreatic islets, pituitary gland, preputial gland, testes, and thyroid gland; for other tissues, denominator is number of animals necropsied. b Poly-3 estimated neoplasm incidence after adjustment for intercurrent mortality 

Observed incidence at terminal kill 

d	 Beneath the chamber control incidence are the P values associated with the trend test. Beneath the exposed group incidence are the P values corresponding to 

pairwise comparisons between the chamber controls and that exposed group. The Poly-3 test accounts for differential mortality in animals that do not reach 

terminal sacrifice. A negative trend or a lower incidence in an exposure group is indicated by N. e Not applicable; no neoplasms in animal group f Value of statistic cannot be computed. 
88 Isobutene, NTP TR 487 

TABLE A4 

Historical Incidence of Thyroid Gland Follicular Cell Neoplasms in Chamber Control Male F344/N Ratsa 

Incidence in Controls 

Study Adenoma Carcinoma Adenoma or Carcinoma 

Historical Incidence at Battelle Pacific Northwest Laboratories 

Acetonitrile 0/48 0/48 0/48 

2-Chloroacetophenone 1/45 0/45 1/45 

Cobalt Sulfate Heptahydrate 0/49 1/49 1/49 

l-Epinephrine Hydrochloride 0/50 2/50 2/50 

Hexachlorocyclopentadiene 0/49 0/49 0/49 

Isobutyraldehyde 0/50 0/50 0/50 

Molybdenum Trioxide 1/50 0/50 1/50 

Nitromethane 0/50 1/50 1/50 

o-Chlorobenzalmalononitrile (CS-2) 0/48 0/48 0/48 

Ozone 0/49 1/49 1/49 

Tetrafluoroethylene 1/50 0/50 1/50 

Tetrahydrofuran 2/50 1/50 3/50 

Overall Historical Incidence 

Total 7/892 (0.8%) 9/892 (1.0%) 16/892 (1.8%) 

Standard deviation 1.2% 1.2% 1.7% 

Range 0%-4% 0%-4% 0%-6% 

Data as of 15 October 1996 a 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 89 

TABLE A5 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 39 40 37 41 

Natural deaths 4 5 7 1 

Survivors 

Terminal sacrifice 7 5 6 8 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Esophagus (50) (50) (50) (50) 

Inflammation, suppurative 1 (2%) 1 (2%) 2 (4%) 

Intestine large, colon (49) (48) (48) (49) 

Diverticulum 1 (2%) 

Mineralization 1 (2%) 

Parasite metazoan 4 (8%) 1 (2%) 2 (4%) 2 (4%) 

Intestine large, rectum (47) (49) (49) (49) 

Necrosis 1 (2%) 

Parasite metazoan 2 (4%) 3 (6%) 3 (6%) 

Intestine large, cecum (48) (47) (49) (49) 

Inflammation, acute 1 (2%) 

Necrosis 2 (4%) 1 (2%) 

Parasite metazoan 3 (6%) 2 (4%) 6 (12%) 4 (8%) 

Intestine small, duodenum (49) (49) (50) (49) 

Necrosis 2 (4%) 2 (4%) 

Liver (50) (50) (50) (50) 

Angiectasis 3 (6%) 1 (2%) 1 (2%) 

Basophilic focus 17 (34%) 14 (28%) 14 (28%) 22 (44%) 

Clear cell focus 3 (6%) 1 (2%) 3 (6%) 3 (6%) 

Cyst 2 (4%) 

Degeneration, cystic 13 (26%) 10 (20%) 14 (28%) 16 (32%) 

Degeneration, fatty 14 (28%) 18 (36%) 18 (36%) 14 (28%) 

Eosinophilic focus 6 (12%) 2 (4%) 5 (10%) 5 (10%) 

Hepatodiaphragmatic nodule 4 (8%) 2 (4%) 1 (2%) 2 (4%) 

Inflammation, granulomatous 1 (2%) 

Mineralization 1 (2%) 1 (2%) 

Mixed cell focus 3 (6%) 2 (4%) 4 (8%) 

Necrosis 2 (4%) 3 (6%) 1 (2%) 

Regeneration 2 (4%) 

Thrombosis 2 (4%) 

Vacuolization cytoplasmic, focal 1 (2%) 

Bile duct, hyperplasia 23 (46%) 27 (54%) 23 (46%) 29 (58%) 

Centrilobular, necrosis 10 (20%) 9 (18%) 10 (20%) 15 (30%) 

Mesentery (15) (10) (8) (11) 

Mineralization 1 (7%) 

Pigmentation 1 (7%) 

Artery, inflammation, chronic active 1 (10%) 1 (9%) 

Artery, mineralization 1 (7%) 1 (10%) 1 (9%) 

Fat, hemorrhage 1 (10%) 1 (9%) 

Fat, necrosis 14 (93%) 9 (90%) 8 (100%) 8 (73%) 

a Number of animals examined microscopically at the site and the number of animals with lesion 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

90 Isobutene, NTP TR 487 

TABLE A5 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Alimentary System (continued) 

Pancreas (50) (50) (50) (50) 

Atrophy 22 (44%) 17 (34%) 26 (52%) 18 (36%) 

Basophilic focus 1 (2%) 1 (2%) 1 (2%) 2 (4%) 

Hyperplasia 4 (8%) 3 (6%) 4 (8%) 3 (6%) 

Artery, inflammation 1 (2%) 

Artery, mineralization 1 (2%) 1 (2%) 

Salivary glands (50) (50) (50) (50) 

Artery, mineralization 1 (2%) 

Stomach, forestomach (50) (50) (50) (50) 

Inflammation, chronic 1 (2%) 

Mineralization 1 (2%) 1 (2%) 1 (2%) 1 (2%) 

Necrosis 17 (34%) 16 (32%) 19 (38%) 10 (20%) 

Perforation 1 (2%) 

Stomach, glandular (50) (49) (50) (50) 

Mineralization 3 (6%) 3 (6%) 5 (10%) 4 (8%) 

Necrosis 8 (16%) 5 (10%) 11 (22%) 9 (18%) 

Tongue (1) (1) 

Necrosis 1 (100%) 

Tooth (2) (3) (1) (1) 

Developmental malformation 2 (100%) 1 (33%) 1 (100%) 

Inflammation, chronic active 2 (67%) 1 (100%) 

Necrosis 1 (33%) 

Cardiovascular System 

Blood vessel (2) (2) (2) (2) 

Aorta, mineralization 2 (100%) 2 (100%) 2 (100%) 2 (100%) 

Heart (50) (50) (50) (50) 

Cardiomyopathy 39 (78%) 39 (78%) 41 (82%) 41 (82%) 

Artery, mineralization 3 (6%) 4 (8%) 4 (8%) 5 (10%) 

Atrium, thrombosis 1 (2%) 3 (6%) 6 (12%) 

Endocrine System 

Adrenal cortex (50) (49) (50) (50) 

Atrophy 1 (2%) 

Degeneration, cystic 1 (2%) 

Hyperplasia 21 (42%) 23 (47%) 24 (48%) 23 (46%) 

Hypertrophy 7 (14%) 6 (12%) 7 (14%) 4 (8%) 

Necrosis 1 (2%) 1 (2%) 

Vacuolization cytoplasmic 1 (2%) 3 (6%) 

Adrenal medulla (50) (49) (50) (50) 

Atrophy 1 (2%) 

Hyperplasia 14 (28%) 18 (37%) 28 (56%) 15 (30%) 

Bilateral, hyperplasia 1 (2%) 1 (2%) 2 (4%) 3 (6%) 

Islets, pancreatic (50) (49) (50) (50) 

Hyperplasia 4 (8%) 3 (6%) 3 (6%) 

Parathyroid gland (49) (50) (49) (47) 

Hyperplasia 9 (18%) 11 (22%) 12 (24%) 10 (21%) 

Pituitary gland (50) (49) (50) (49) 

Angiectasis 1 (2%) 

Pars distalis, hyperplasia 5 (10%) 5 (10%) 3 (6%) 6 (12%) 

Pars distalis, thrombosis 1 (2%) 

Thyroid gland (48) (48) (48) (50) 

C-cell, hyperplasia 32 (67%) 36 (75%) 34 (71%) 26 (52%) 

Follicular cell, hyperplasia 2 (4%) 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 91 

TABLE A5 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

General Body System 

None 

Genital System 

Epididymis (50) (50) (50) (50) 

Granuloma sperm 1 (2%) 

Preputial gland (50) (50) (49) (50) 

Hyperplasia 2 (4%) 1 (2%) 2 (4%) 1 (2%) 

Inflammation, acute 1 (2%) 

Inflammation, chronic active 3 (6%) 3 (6%) 2 (4%) 

Prostate (50) (49) (50) (50) 

Hyperplasia 6 (12%) 6 (12%) 6 (12%) 7 (14%) 

Inflammation, chronic active 4 (8%) 4 (8%) 4 (8%) 6 (12%) 

Seminal vesicle (50) (47) (50) (50) 

Inflammation, chronic active 1 (2%) 1 (2%) 

Mineralization 1 (2%) 1 (2%) 1 (2%) 

Testes (50) (50) (50) (50) 

Atrophy 7 (14%) 3 (6%) 5 (10%) 

Necrosis 3 (6%) 1 (2%) 

Artery, inflammation, chronic active 4 (8%) 2 (4%) 1 (2%) 3 (6%) 

Artery, mineralization 1 (2%) 

Interstitial cell, hyperplasia 4 (8%) 4 (8%) 4 (8%) 2 (4%) 

Hematopoietic System 

Bone marrow (50) (48) (50) (50) 

Necrosis 1 (2%) 

Lymph node (4) (11) (10) (10) 

Hemorrhage 1 (25%) 

Iliac, ectasia 1 (10%) 

Renal, hemorrhage 1 (9%) 

Lymph node, mandibular (47) (47) (49) (49) 

Hemorrhage 1 (2%) 

Infiltration cellular, plasma cell 1 (2%) 1 (2%) 1 (2%) 

Infiltration cellular, polymorphonuclear 1 (2%) 

Lymph node, mesenteric (50) (50) (49) (50) 

Ectasia 1 (2%) 1 (2%) 

Lymph node, mediastinal (46) (42) (44) (45) 

Hemorrhage 1 (2%) 

Spleen (50) (49) (50) (50) 

Accessory spleen 1 (2%) 1 (2%) 1 (2%) 

Fibrosis 9 (18%) 6 (12%) 10 (20%) 11 (22%) 

Hematopoietic cell proliferation 2 (4%) 5 (10%) 4 (8%) 3 (6%) 

Hemorrhage 1 (2%) 1 (2%) 1 (2%) 

Necrosis 2 (4%) 1 (2%) 

Integumentary System 

Mammary gland (41) (41) (40) (44) 

Galactocele 2 (5%) 7 (17%) 1 (3%) 4 (9%) 

Hyperplasia, atypical 1 (2%) 

Inflammation, chronic 1 (2%) 1 (3%) 1 (2%) 

Skin (50) (50) (50) (50) 

Cyst epithelial inclusion 1 (2%) 

Hyperkeratosis 1 (2%) 1 (2%) 1 (2%) 

Inflammation, chronic active 1 (2%) 1 (2%) 5 (10%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

92 Isobutene, NTP TR 487 

TABLE A5 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Musculoskeletal System 

Bone (50) (50) (50) (50) 

Fibrous osteodystrophy 9 (18%) 7 (14%) 9 (18%) 6 (12%) 

Hyperostosis 1 (2%) 1 (2%) 

Skeletal muscle (1) (1) (1) 

Mineralization 1 (100%) 

Nervous System 

Brain (50) (50) (50) (50) 

Angiectasis 1 (2%) 

Hemorrhage 1 (2%) 

Respiratory System 

Larynx (50) (49) (50) (49) 

Necrosis 1 (2%) 1 (2%) 

Epiglottis, metaplasia, squamous 1 (2%) 

Lung (50) (50) (50) (50) 

Cyst 1 (2%) 

Hemorrhage 1 (2%) 1 (2%) 1 (2%) 

Inflammation, granulomatous 9 (18%) 3 (6%) 1 (2%) 5 (10%) 

Inflammation, suppurative 1 (2%) 1 (2%) 

Metaplasia, squamous 1 (2%) 

Mineralization 2 (4%) 2 (4%) 1 (2%) 1 (2%) 

Alveolar epithelium, hyperplasia 9 (18%) 5 (10%) 7 (14%) 5 (10%) 

Alveolus, infiltration cellular, histiocyte 1 (2%) 

Artery, mediastinum, mineralization 2 (4%) 2 (4%) 1 (2%) 1 (2%) 

Bronchiole, hyperplasia 1 (2%) 

Mediastinum, inflammation, suppurative 1 (2%) 

Nose (49) (49) (50) (49) 

Inflammation, suppurative 14 (29%) 11 (22%) 16 (32%) 9 (18%) 

Thrombosis 4 (8%) 7 (14%) 10 (20%) 6 (12%) 

Lateral wall, metaplasia, squamous 1 (2%) 

Olfactory epithelium, atrophy 3 (6%) 

Olfactory epithelium, degeneration, hyaline 43 (88%) 45 (92%) 46 (92%) 49 (100%) 

Olfactory epithelium, metaplasia 4 (8%) 

Respiratory epithelium, metaplasia, squamous 1 (2%) 

Trachea (50) (50) (50) (50) 

Inflammation, suppurative 2 (4%) 

Special Senses System 

Eye (3) (4) 

Cataract 2 (67%) 3 (75%) 

Degeneration 1 (33%) 

Inflammation, chronic active 2 (50%) 

Retina, atrophy 1 (33%) 3 (75%) 
 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 93 

TABLE A5 

Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Urinary System 

Kidney (50) (50) (50) (50) 

Cyst 3 (6%) 3 (6%) 1 (2%) 

Hydronephrosis 1 (2%) 

Hyperplasia, oncocytic 1 (2%) 

Infarct 1 (2%) 1 (2%) 

Mineralization 4 (8%) 2 (4%) 3 (6%) 4 (8%) 

Nephropathy 50 (100%) 48 (96%) 50 (100%) 50 (100%) 

Papilla, necrosis 1 (2%) 

Pelvis, inflammation, acute 1 (2%) 1 (2%) 2 (4%) 1 (2%) 

Renal tubule, hyperplasia 2 (4%) 4 (8%) 3 (6%) 4 (8%) 

Transitional epithelium, hyperplasia 1 (2%) 2 (4%) 

Urinary bladder (50) (49) (50) (50) 

Hemorrhage 1 (2%) 1 (2%) 

Inflammation, chronic active 1 (2%) 3 (6%) 2 (4%) 

Inflammation, suppurative 1 (2%) 

Necrosis 1 (2%) 

Transitional epithelium, hyperplasia 1 (2%) 1 (2%) 1 (2%) 3 (6%) 
94 Isobutene, NTP TR 487
 
95 

APPENDIX B 

SUMMARY OF LESIONS IN FEMALE RATS 

IN THE 2-YEAR INHALATION STUDY 

OF ISOBUTENE 

TABLE B1 Summary of the Incidence of Neoplasms in Female Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 97 

TABLE B2 Individual Animal Tumor Pathology of Female Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 100 

TABLE B3 Statistical Analysis of Primary Neoplasms in Female Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 118 

TABLE B4 Summary of the Incidence of Nonneoplastic Lesions in Female Rats 

in the 2-Year Inhalation Study of Isobutene ............................... 121 
96 Isobutene, NTP TR 487
 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 97 

TABLE B1 

Summary of the Incidence of Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Accidental death 1 

Moribund 21 25 15 23 

Natural deaths 6 5 2 5 

Survivors 

Terminal sacrifice 23 19 33 22 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Intestine large, colon (49) (50) (49) (49) 

Intestine large, rectum (47) (47) (47) (48) 

Polyp adenomatous 1 (2%) 

Intestine large, cecum (48) (49) (48) (49) 

Intestine small, duodenum (49) (50) (48) (48) 

Intestine small, jejunum (47) (47) (48) (47) 

Leiomyosarcoma 1 (2%) 

Intestine small, ileum (46) (47) (48) (47) 

Liver (50) (50) (50) (49) 

Carcinoma, metastatic, thyroid gland 1 (2%) 

Hepatocellular carcinoma 1 (2%) 

Mesentery (8) (10) (12) (7) 

Oral mucosa (1) 

Pharyngeal, squamous cell papilloma 1 (100%) 

Pancreas (50) (50) (49) (49) 

Salivary glands (50) (50) (50) (50) 

Stomach, glandular (50) (50) (50) (49) 

Cardiovascular System 

Heart (50) (50) (49) (50) 

Endocrine System 

Adrenal cortex (50) (50) (49) (49) 

Adenoma 1 (2%) 1 (2%) 1 (2%) 1 (2%) 

Adrenal medulla (50) (50) (49) (49) 

Pheochromocytoma complex 1 (2%) 

Pheochromocytoma benign 3 (6%) 3 (6%) 9 (18%) 4 (8%) 

Bilateral, pheochromocytoma benign 1 (2%) 

Islets, pancreatic (50) (50) (49) (49) 

Adenoma 1 (2%) 

Pituitary gland (50) (49) (50) (49) 

Pars distalis, adenoma 39 (78%) 33 (67%) 37 (74%) 36 (73%) 

Thyroid gland (50) (50) (49) (49) 

C-cell, adenoma 7 (14%) 6 (12%) 3 (6%) 5 (10%) 

C-cell, carcinoma 2 (4%) 3 (6%) 6 (12%) 2 (4%) 

Follicular cell, carcinoma 1 (2%) 

General Body System 

None 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

98 Isobutene, NTP TR 487 

TABLE B1 

Summary of the Incidence of Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Genital System 

Clitoral gland (50) (49) (47) (48) 

Adenoma 3 (6%) 3 (6%) 6 (13%) 

Carcinoma 5 (11%) 1 (2%) 

Bilateral, carcinoma 1 (2%) 

Ovary (50) (50) (50) (50) 

Granulosa cell tumor malignant 1 (2%) 

Uterus (50) (50) (50) (50) 

Polyp stromal 7 (14%) 9 (18%) 7 (14%) 4 (8%) 

Polyp stromal, multiple 1 (2%) 1 (2%) 1 (2%) 

Vagina (1) (1) (1) 

Polyp 1 (100%) 

Hematopoietic System 

Bone marrow (50) (50) (49) (49) 

Lymph node (6) (3) (3) (2) 

Deep cervical, carcinoma, metastatic, 

thyroid gland 1 (50%) 

Renal, sarcoma 1 (17%) 

Lymph node, bronchial (36) (31) (36) (27) 

Lymph node, mandibular (44) (46) (47) (47) 

Lymph node, mesenteric (50) (50) (49) (49) 

Lymph node, mediastinal (42) (45) (39) (45) 

Plasma cell tumor malignant 1 (2%) 

Spleen (50) (50) (49) (50) 

Hemangiosarcoma 1 (2%) 

Thymus (48) (47) (45) (41) 

Integumentary System 

Mammary gland (50) (50) (50) (50) 

Adenoma 1 (2%) 1 (2%) 1 (2%) 

Carcinoma 2 (4%) 2 (4%) 4 (8%) 4 (8%) 

Fibroadenoma 16 (32%) 14 (28%) 24 (48%) 22 (44%) 

Fibroadenoma, multiple 6 (12%) 5 (10%) 6 (12%) 5 (10%) 

Skin (50) (50) (50) (50) 

Keratoacanthoma 1 (2%) 1 (2%) 

Squamous cell papilloma 1 (2%) 

Subcutaneous tissue, fibroma 1 (2%) 1 (2%) 

Subcutaneous tissue, fibrosarcoma 1 (2%) 

Subcutaneous tissue, fibrous histiocytoma

 malignant 1 (2%) 

Subcutaneous tissue, hemangioma 1 (2%) 

Subcutaneous tissue, melanoma malignant 1 (2%) 

Subcutaneous tissue, sarcoma 1 (2%) 2 (4%) 

Subcutaneous tissue, schwannoma malignant 1 (2%) 

Subcutaneous tissue, pinna, melanoma malignant 1 (2%) 

Musculoskeletal System 

Bone (50) (50) (50) (50) 

Osteosarcoma 1 (2%) 

Sarcoma 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 99 

TABLE B1 

Summary of the Incidence of Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Nervous System 

Brain (50) (50) (50) (50) 

Respiratory System 

Lung 

Adenoma

Alveolar/bronchiolar adenoma

Carcinoma, metastatic, mammary gland

Carcinoma, metastatic, thyroid gland

Fibrosarcoma, metastatic, skin

Plasma cell tumor malignant, metastatic, 

lymph node, mediastinal

Mediastinum, plasma cell tumor malignant, 

metastatic, lymph node, mediastinal

Nose 

(50) 

 2 (4%) 

 1 (2%) 

 1 (2%) 

 1 (2%) 

(50) 

(50) 

 1 (2%) 

 1 (2%) 

(50) 

(50) 

 2 (4%) 

(50) 

(50) 

 1 (2%) 

(49) 

Special Senses System 

Zymbal’s gland

Carcinoma

 (1) 

 1 (100%) 

Urinary System 

Kidney 

Lipoma

Stromal nephroma

Renal tubule, carcinoma

Urinary bladder 

Transitional epithelium, papilloma

(50) 

 1 (2%) 

(50) 

(50) 

 1 (2%) 

(49) 

(49) 

(49) 

 1 (2%) 

(49) 

 1 (2%) 

(49) 

Systemic Lesions 

Multiple organsb 

Leukemia mononuclear

(50) 

 18 (36%) 

(50) 

 16 (32%) 

(50) 

 22 (44%) 

(50) 

 17 (34%) 

Neoplasm Summary 

Total animals with primary neoplasmsc 

Total primary neoplasms 

Total animals with benign neoplasms 

Total benign neoplasms 

Total animals with malignant neoplasms 

Total malignant neoplasms 

Total animals with metastatic neoplasms 

Total metastatic neoplasms 

50 

125 

42 

92 

30 

33 

2 

3 

48 

102 

45 

76 

24 

26 

2 

2 

49 

140 

48 

100 

30 

40 

2 

2 

48 

107 

46 

80 

25 

27 

1 

2 

a Number of animals examined microscopically at the site and the number of animals with neoplasm b Number of animals with any tissue examined microscopically 

Primary neoplasms: all neoplasms except metastatic neoplasms c 
 

 

 

 

 

 

 

 

 

 

 

100 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

Number of Days on Study 

3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 

9 2 4 8 8 9 2 5 5 7 8 0 4 4 4 6 6 7 9 0 0 1 1 2 2 

0 1 0 8 8 6 9 2 2 3 1 8 2 2 8 3 4 8 1 5 5 4 5 0 0 

Carcass ID Number 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

4 2 1 1 2 1 1 1 4 0 1 4 0 2 5 0 3 3 2 1 4 0 2 0 4 

3 2 2 0 4 1 7 6 5 5 5 7 8 8 0 7 7 8 6 9 6 6 9 4 2 

Alimentary System 

Esophagus 

Intestine large, colon 

Intestine large, rectum 

Polyp adenomatous 

Intestine large, cecum 

Intestine small, duodenum 

Intestine small, jejunum 

Intestine small, ileum 

Liver 

Mesentery 

Pancreas 

Salivary glands 

Stomach, forestomach 

Stomach, glandular 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

+ + + + + + + + + + + + + A + + + + + + + + A + + 

X 

+ + + + + + + + + + + + + A + + + + + + + + A + + 

+ + + + + + + + + + + + + + + + + + + + + + A + + 

+ A + + + + + + + + + + + A + + + + + + + + A + + 

+ A + + + + + + + + + + + A + + + + + + A + A + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma complex 

Pheochromocytoma benign 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

C-cell, adenoma 

C-cell, carcinoma X 

Follicular cell, carcinoma 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Bilateral, carcinoma X 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 

Polyp stromal X X X X X X 

Polyp stromal, multiple 

Vagina + 

Polyp X 

+: Tissue examined microscopically M: Missing tissue X: Lesion present 

A: Autolysis precludes examination I: Insufficient tissue Blank: Not examined 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 101 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

2 6 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 3 3 0 0 0 0 1 1 1 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

1 2 1 2 3 9 3 4 8 0 1 3 5 7 0 3 4 5 6 9 0 1 4 8 9 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + I 47 

Polyp adenomatous 1 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Mesentery + + 8 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma complex X 1 

Pheochromocytoma benign X X X 3 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Parathyroid gland + + + M + + + + + + + + + + + M + + + + + + + + + 48 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X X X 39 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

C-cell, adenoma X X X X X X X 7 

C-cell, carcinoma X 2 

Follicular cell, carcinoma X 1 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X X X 3 

Bilateral, carcinoma 1 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Polyp stromal X 7 

Polyp stromal, multiple X 1 

Vagina 1 

Polyp 1 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

102 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 

Number of Days on Study 9 2 4 8 8 9 2 5 5 7 8 0 4 4 4 6 6 7 9 0 0 1 1 2 2 

0 1 0 8 8 6 9 2 2 3 1 8 2 2 8 3 4 8 1 5 5 4 5 0 0 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

Carcass ID Number 4 2 1 1 2 1 1 1 4 0 1 4 0 2 5 0 3 3 2 1 4 0 2 0 4 

3 2 2 0 4 1 7 6 5 5 5 7 8 8 0 7 7 8 6 9 6 6 9 4 2 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node + + + + + + 

Renal, sarcoma X 

Lymph node, bronchial M M M M M + M M M M + + + + + + + + + + + + + + + 

Lymph node, mandibular + + M + + + + M + + + + + + + + + + + + + + + + + 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 

Lymph node, mediastinal + + + + + M + + + M + + + + + + + + + M + + + + M 

Plasma cell tumor malignant X 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 

Thymus + + + + + M + + + + + + + + + + + + + + + + + + M 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Carcinoma X 

Fibroadenoma X X X X X X 

Fibroadenoma, multiple X X 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 

Keratoacanthoma 

Subcutaneous tissue, fibroma 

Subcutaneous tissue, fibrous histiocytoma malignant X 

Subcutaneous tissue, hemangioma 

Subcutaneous tissue, sarcoma 

Subcutaneous tissue, pinna, melanoma malignant 

Musculoskeletal System 

Bone 

Osteosarcoma 

Sarcoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Carcinoma, metastatic, mammary gland 

Plasma cell tumor malignant, metastatic, 

lymph node, mediastinal 

Mediastinum, plasma cell tumor malignant, 

metastatic, lymph node, mediastinal 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 103 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

2 6 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 3 3 0 0 0 0 1 1 1 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

1 2 1 2 3 9 3 4 8 0 1 3 5 7 0 3 4 5 6 9 0 1 4 8 9 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node 6 

Renal, sarcoma 1 

Lymph node, bronchial + M + M + + + + + M + + + + + + M + M + + + + + + 36 

Lymph node, mandibular + + + + + + + + M + + + + + M M + + + + + + + M + 44 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node, mediastinal + + M + + + + + + + + + + + + + + + + M M + + M + 42 

Plasma cell tumor malignant 1 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Thymus + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Carcinoma X 2 

Fibroadenoma X X X X X X X X X X 16 

Fibroadenoma, multiple X X X X 6 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Keratoacanthoma X 1 

Subcutaneous tissue, fibroma X 1 

Subcutaneous tissue, fibrous histiocytoma malignant 1 

Subcutaneous tissue, hemangioma X 1 

Subcutaneous tissue, sarcoma X 1 

Subcutaneous tissue, pinna, melanoma malignant X 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Osteosarcoma X 1 

Sarcoma X 1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X X 2 

Carcinoma, metastatic, mammary gland 1 

Plasma cell tumor malignant, metastatic, 

lymph node, mediastinal 1 

Mediastinum, plasma cell tumor malignant, 

metastatic, lymph node, mediastinal 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye 4 

50 
 

 

104 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 

Number of Days on Study 9 2 4 8 8 9 2 5 5 7 8 0 4 4 4 6 6 7 9 0 0 1 1 2 2 

0 1 0 8 8 6 9 2 2 3 1 8 2 2 8 3 4 8 1 5 5 4 5 0 0 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

Carcass ID Number 4 2 1 1 2 1 1 1 4 0 1 4 0 2 5 0 3 3 2 1 4 0 2 0 4 

3 2 2 0 4 1 7 6 5 5 5 7 8 8 0 7 7 8 6 9 6 6 9 4 2 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 

Renal tubule, carcinoma X 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Leukemia mononuclear X X X X X X X X X 
 

 

Isobutene, NTP TR 487 105 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

2 6 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 3 3 0 0 0 0 1 1 1 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

1 2 1 2 3 9 3 4 8 0 1 3 5 7 0 3 4 5 6 9 0 1 4 8 9 Tumors 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Renal tubule, carcinoma 1 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Leukemia mononuclear X X X X X X X X X 18 
 

 

 

 

 

 

 

 

 

 

 

 

106 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

Number of Days on Study 

0 2 4 4 4 4 4 4 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 

1 1 2 4 5 7 9 9 3 3 4 6 7 8 0 0 2 3 3 6 6 8 8 9 0 

3 6 0 0 5 9 4 6 4 4 8 5 3 0 3 8 1 0 5 2 4 8 8 2 1 

Carcass ID Number 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

0 4 4 1 4 4 1 0 2 3 0 1 3 2 1 1 4 3 4 0 2 1 2 2 3 

5 2 5 1 8 6 8 1 6 6 2 6 3 9 7 3 0 5 1 9 3 9 2 1 7 

Alimentary System 

Esophagus 

Intestine large, colon 

Intestine large, rectum 

Intestine large, cecum 

Intestine small, duodenum 

Intestine small, jejunum 

Intestine small, ileum 

Liver 

Carcinoma, metastatic, thyroid gland 

Mesentery 

Pancreas 

Salivary glands 

Stomach, forestomach 

Stomach, glandular 

Tongue 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + I + + + I + + + + + + + + + 

+ + + + + + + + + + + + + + + + + A + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + A + + A + + + + + + + 

+ + + + + + + + + + + + + + A + + A + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Parathyroid gland + + M + + + + + + + + + + + + + + + + + + + + M + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

C-cell, adenoma 

C-cell, carcinoma X X X 

General Body System 

None 

Genital System 

Clitoral gland 

Adenoma 

Ovary 

Uterus 

Polyp stromal 

Vagina 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X 

+ 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ 

M M M M M + + + + M M + + + M M M + M + M + M M + 

M M + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + M + + + M + + + + + + + + + + M + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + M + + M + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 107 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 0 1 1 1 1 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

8 4 6 8 9 2 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Total 

Carcass ID Number 0 3 2 3 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 Tissues/ 

4 8 7 2 8 3 6 7 0 2 4 5 0 4 5 8 0 1 4 9 3 4 7 9 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, rectum + + + + + + + + + I + + + + + + + + + + + + + + + 47 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine small, jejunum + + + + A + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, ileum + + + + A + + + + + + + + + + + + + + + + + + + + 47 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Carcinoma, metastatic, thyroid gland 1 

Mesentery + + + + + + + 10 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Tongue + 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma benign X X X 3 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + M + 49 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X 33 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

C-cell, adenoma X X X X X X 6 

C-cell, carcinoma 3 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + + I + + + + + + + + + + + + + + 49 

Adenoma X X X 3 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Polyp stromal X X X X 9 

Vagina 1 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node + + 3 

Lymph node, bronchial + M + + + + + + + M M + M + M + + + + + + + + + + 31 

Lymph node, mandibular + + + + + + + + + M + + + M + + + + + + + + + + + 46 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node, mediastinal + + + + M + + + + + + + + M + + + + + + + + + + + 45 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Thymus + + + + + + + + + + + + + + + + + + + + M + + + + 47 

50 
 

 

 

 

 

 

 

 

 

108 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

Number of Days on Study 

0 

1 

3 

2 

1 

6 

4 

2 

0 

4 

4 

0 

4 

5 

5 

4 

7 

9 

4 

9 

4 

4 

9 

6 

5 

3 

4 

5 

3 

4 

5 

4 

8 

5 

6 

5 

5 

7 

3 

5 

8 

0 

6 

0 

3 

6 

0 

8 

6 

2 

1 

6 

3 

0 

6 

3 

5 

6 

6 

2 

6 

6 

4 

6 

8 

8 

6 

8 

8 

6 

9 

2 

7 

0 

1 

Carcass ID Number 

3 

0 

5 

3 

4 

2 

3 

4 

5 

3 

1 

1 

3 

4 

8 

3 

4 

6 

3 

1 

8 

3 

0 

1 

3 

2 

6 

3 

3 

6 

3 

0 

2 

3 

1 

6 

3 

3 

3 

3 

2 

9 

3 

1 

7 

3 

1 

3 

3 

4 

0 

3 

3 

5 

3 

4 

1 

3 

0 

9 

3 

2 

3 

3 

1 

9 

3 

2 

2 

3 

2 

1 

3 

3 

7 

Integumentary System 

Mammary gland 

Adenoma 

Carcinoma 

Fibroadenoma 

Fibroadenoma, multiple 

Skin 

Subcutaneous tissue, fibrosarcoma 

Subcutaneous tissue, sarcoma 

Subcutaneous tissue, schwannoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Adenoma 

Fibrosarcoma, metastatic, skin 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Ear 

Eye + + + 

+ 

Urinary System 

Kidney 

Stromal nephroma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + + + + + + + + + + + + + + + + + + M + + 

Systemic Lesions 

Multiple organs 

Leukemia mononuclear 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 109 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 0 1 1 1 1 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

8 4 6 8 9 2 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Total 

Carcass ID Number 0 3 2 3 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 Tissues/ 

4 8 7 2 8 3 6 7 0 2 4 5 0 4 5 8 0 1 4 9 3 4 7 9 0 Tumors 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Carcinoma X X 2 

Fibroadenoma X X X X X 14 

Fibroadenoma, multiple X X X X X 5 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Subcutaneous tissue, fibrosarcoma X 1 

Subcutaneous tissue, sarcoma X 2 

Subcutaneous tissue, schwannoma malignant 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Fibrosarcoma, metastatic, skin X 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Ear 1 

Eye + 4 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stromal nephroma 1 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Leukemia mononuclear X X X X X X X X 16 

50 

50 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

110 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

Number of Days on Study 

3 

9 

9 

5 

2 

2 

5 

3 

4 

5 

5 

2 

5 

6 

0 

6 

0 

7 

6 

2 

0 

6 

2 

6 

6 

4 

4 

6 

5 

1 

6 

8 

0 

6 

8 

4 

6 

8 

6 

7 

0 

1 

7 

0 

5 

7 

0 

6 

7 

2 

0 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

4 

7 

3 

4 

7 

3 

4 

Carcass ID Number 

5 

3 

7 

5 

0 

3 

5 

5 

0 

5 

4 

7 

5 

4 

3 

5 

2 

1 

5 

2 

3 

5 

2 

6 

5 

3 

8 

5 

4 

6 

5 

3 

6 

5 

3 

5 

5 

4 

2 

5 

0 

2 

5 

2 

4 

5 

0 

7 

5 

4 

4 

5 

0 

1 

5 

0 

4 

5 

0 

5 

5 

0 

6 

5 

0 

8 

5 

0 

9 

5 

1 

0 

5 

1 

1 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon + + + + + + + + + + + + + + A + + + + + + + + + + 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, cecum + + + + + + A + + + + + + + A + + + + + + + + + + 

Intestine small, duodenum + + + + + + A + + + + + + + A + + + + + + + + + + 

Intestine small, jejunum + + + + + + M + + + + + + + A + + + + + + + + + + 

Leiomyosarcoma 

Intestine small, ileum + + + + + + A + + + + + + + A + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Mesentery + + + + + + + + 

Pancreas + + + + + + + + + + + + + + A + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 

Tooth + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + A + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + A + + + + + + + + + + 

Adenoma 

Adrenal medulla + + + + + + + + + + + + + + A + + + + + + + + + + 

Pheochromocytoma benign X X X X X 

Bilateral, pheochromocytoma benign 

Islets, pancreatic + + + + + + + + + + + + + + A + + + + + + + + + + 

Parathyroid gland + + + + + + + + + + + + + + A + + + + + + + + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X 

Thyroid gland + + + + + + + + + + + + + + A + + + + + + + + + + 

C-cell, adenoma X X X 

C-cell, carcinoma X X 

General Body System 

None 

Genital System 

Clitoral gland + + + M + + + + + + + + + + + + + + + + + + + + + 

Adenoma X X X 

Carcinoma X X 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 

Polyp stromal X X X X X 

Polyp stromal, multiple X 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + A + + + + + + + + + + 

Lymph node + + 

Lymph node, bronchial + + + + M + + + + M + + M + + + + M + M + + + + + 

Lymph node, mandibular + + + + + + + + + + + + + + A + + + + + + + + + + 

Lymph node, mesenteric + + + + + + + + + + + + + + M + + + + + + + + + + 

Lymph node, mediastinal M + + M + + + + + + M + + + M + + + + + + + + M + 

Spleen + + + + + + + + + + + + + + A + + + + + + + + + + 

Thymus + + + + + + + + + + + + M + + + + + + + M + + + M 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 111 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Total 

Carcass ID Number 1 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

2 3 4 5 6 7 8 9 0 2 5 7 8 9 0 1 2 3 4 9 0 1 5 8 9 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + I + + + I + + + + I + + + + + + + + + + + 47 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Leiomyosarcoma X 1 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Mesentery + + + + 12 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Tooth 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign X X X X 9 

Bilateral, pheochromocytoma benign X 1 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X 37 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

C-cell, adenoma 3 

C-cell, carcinoma X X X X 6 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + M + + + + + + + + + + + + M + + 47 

Adenoma X X X 6 

Carcinoma X X X 5 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Polyp stromal X X 7 

Polyp stromal, multiple 1 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node + 3 

Lymph node, bronchial + + + + + + + M M M + + + M + M M M M M + + + + + 36 

Lymph node, mandibular + + + + + + + + + + + + + M + + + M + + + + + + + 47 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node, mediastinal + + M + + + M M + + M + M + + + + + M + + + + + + 39 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Thymus + + + + + + + + M + M + + + + + + + + + + + + + + 45 

49 
 

 

 

 

 

 

 

 

 

 

 

112 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

Number of Days on Study 

3 

9 

9 

5 

2 

2 

5 

3 

4 

5 

5 

2 

5 

6 

0 

6 

0 

7 

6 

2 

0 

6 

2 

6 

6 

4 

4 

6 

5 

1 

6 

8 

0 

6 

8 

4 

6 

8 

6 

7 

0 

1 

7 

0 

5 

7 

0 

6 

7 

2 

0 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

3 

7 

3 

4 

7 

3 

4 

7 

3 

4 

Carcass ID Number 

5 

3 

7 

5 

0 

3 

5 

5 

0 

5 

4 

7 

5 

4 

3 

5 

2 

1 

5 

2 

3 

5 

2 

6 

5 

3 

8 

5 

4 

6 

5 

3 

6 

5 

3 

5 

5 

4 

2 

5 

0 

2 

5 

2 

4 

5 

0 

7 

5 

4 

4 

5 

0 

1 

5 

0 

4 

5 

0 

5 

5 

0 

6 

5 

0 

8 

5 

0 

9 

5 

1 

0 

5 

1 

1 

Integumentary System 

Mammary gland 

Adenoma 

Carcinoma 

Fibroadenoma 

Fibroadenoma, multiple 

Skin 

Keratoacanthoma 

Squamous cell papilloma 

Subcutaneous tissue, fibroma 

Subcutaneous tissue, melanoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X X X X X X X X X 

X X X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Carcinoma, metastatic, thyroid gland 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye + + 

Zymbal’s gland + 

Carcinoma X 

Urinary System 

Kidney 

Urinary bladder 

Transitional epithelium, papilloma 

+ + + + + + + + + + + + + + A 

+ + + + + + + + + + + + + + A 

+ + + + + + + + + + 

+ + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Leukemia mononuclear 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 113 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Total 

Carcass ID Number 1 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

2 3 4 5 6 7 8 9 0 2 5 7 8 9 0 1 2 3 4 9 0 1 5 8 9 Tumors 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma 1 

Carcinoma X X X 4 

Fibroadenoma X X X X X X X X X X X X X X X 24 

Fibroadenoma, multiple X X X 6 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Keratoacanthoma 1 

Squamous cell papilloma 1 

Subcutaneous tissue, fibroma X 1 

Subcutaneous tissue, melanoma malignant 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Carcinoma, metastatic, thyroid gland X X 2 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye + + + + 6 

Zymbal’s gland 1 

Carcinoma 1 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Transitional epithelium, papilloma X 1 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Leukemia mononuclear X X X X X X X X X 22 

50 

50 
 

 

 

 

 

 

 

 

 

 

 

 

114 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

Number of Days on Study 

0 4 4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 7 7 

0 0 6 0 1 1 4 5 8 9 9 2 3 4 5 5 6 6 6 7 7 8 9 0 0 

5 7 9 0 1 2 7 9 0 2 7 8 0 2 0 7 3 4 8 1 4 1 2 2 6 

Carcass ID Number 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

3 2 3 1 4 3 1 3 2 4 0 0 1 2 0 4 1 0 2 3 1 3 3 2 0 

9 4 5 9 4 3 1 0 5 3 8 5 3 8 3 8 0 1 3 8 4 6 7 2 2 

Alimentary System 

Esophagus 

Intestine large, colon 

Intestine large, rectum 

Intestine large, cecum 

Intestine small, duodenum 

Intestine small, jejunum 

Intestine small, ileum 

Liver 

Hepatocellular carcinoma 

Mesentery 

Oral mucosa 

Pharyngeal, squamous cell papilloma 

Pancreas 

Salivary glands 

Stomach, forestomach 

Stomach, glandular 

Tongue 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + A + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + A + + + + + + + + + + + + + + + + + + + + 

+ A + + A + + + + + + + + + + + + + + + + + + + + 

+ A + + A + + + + + + + + + + + + + A + + + + + + 

+ A + + A + + + + + + + + + + + + + A + + + + + + 

+ + + + A + + + + + + + + + + + + + + + + + + + + 

+ + + + + 

+ 

X 

+ + + + A + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + A + + + + + + + + + + + + + + + + + + + + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + A + + + + + + + + + + + + + + + + + + + + 

Adenoma X 

Adrenal medulla + + + + A + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign X X 

Islets, pancreatic + + + + A + + + + + + + + + + + + + + + + + + + + 

Parathyroid gland + + + M A + + + + + + + + + + + + + + + + + + + + 

Pituitary gland + + + + A + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X X X X X X X 

Thyroid gland + + + + A + + + + + + + + + + + + + + + + + + + + 

C-cell, adenoma X X 

C-cell, carcinoma 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 

Granulosa cell tumor malignant 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 

Polyp stromal X X 

Polyp stromal, multiple X 

Vagina + 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 115 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

0 0 9 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 Total 

Carcass ID Number 2 4 0 0 0 0 1 1 1 1 1 2 2 2 2 3 3 3 4 4 4 4 4 4 5 Tissues/ 

6 2 7 4 6 9 2 5 6 7 8 0 1 7 9 1 2 4 0 1 5 6 7 9 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + I + + + + + + + + I + + + + + + 48 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Hepatocellular carcinoma X 1 

Mesentery + + 7 

Oral mucosa 1 

Pharyngeal, squamous cell papilloma 1 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Tongue + 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign X X 4 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Parathyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X X X X X X X X X X X X X X X X X X X 36 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

C-cell, adenoma X X X 5 

C-cell, carcinoma X X 2 

General Body System 

None 

Genital System 

Clitoral gland M M + + + + + + + + + + + + + + + + + + + + + + + 48 

Carcinoma X 1 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Granulosa cell tumor malignant X 1 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Polyp stromal X X 4 

Polyp stromal, multiple 1 

Vagina 1 
 

 

 

 

 

 

 

 

 

 

 

 

116 Isobutene, NTP TR 487 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

0 4 4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 7 7 

Number of Days on Study 0 0 6 0 1 1 4 5 8 9 9 2 3 4 5 5 6 6 6 7 7 8 9 0 0 

5 7 9 0 1 2 7 9 0 2 7 8 0 2 0 7 3 4 8 1 4 1 2 2 6 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Carcass ID Number 3 2 3 1 4 3 1 3 2 4 0 0 1 2 0 4 1 0 2 3 1 3 3 2 0 

9 4 5 9 4 3 1 0 5 3 8 5 3 8 3 8 0 1 3 8 4 6 7 2 2 

Hematopoietic System 

Bone marrow + + + + A + + + + + + + + + + + + + + + + + + + + 

Lymph node + 

Deep cervical, carcinoma, metastatic,

 thyroid gland 

Lymph node, bronchial + M M M M + M M + + + + + + M + + M M + M + + M + 

Lymph node, mandibular + + + + M + + + + + + + + + + + + + + + + + + + + 

Lymph node, mesenteric + + + + A + + + + + + + + + + + + + + + + + + + + 

Lymph node, mediastinal + + + M + + + + + + + + + + + + + + + + + + + + + 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 

Hemangiosarcoma 

Thymus + M + + A M M + + + + + + + + + + + + + + + M + + 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma X X 

Fibroadenoma X X X X X X X X X 

Fibroadenoma, multiple 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Skeletal muscle 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma, metastatic, thyroid gland 

Nose + + + + A + + + + + + + + + + + + + + + + + + + + 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye + + 

Lacrimal gland + 

Urinary System 

Kidney + + + + A + + + + + + + + + + + + + + + + + + + + 

Lipoma 

Urinary bladder + + + + A + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Leukemia mononuclear X X X X X X X X X X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 117 

TABLE B2 

Individual Animal Tumor Pathology of Female Rats in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

0 0 9 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 Total 

Carcass ID Number 2 4 0 0 0 0 1 1 1 1 1 2 2 2 2 3 3 3 4 4 4 4 4 4 5 Tissues/ 

6 2 7 4 6 9 2 5 6 7 8 0 1 7 9 1 2 4 0 1 5 6 7 9 0 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node + 2 

Deep cervical, carcinoma, metastatic,

 thyroid gland X 1 

Lymph node, bronchial M + M M + + M M + + + + M M + + + + M M M + M M + 27 

Lymph node, mandibular + + + M + + + + + + + + M + + + + + + + + + + + + 47 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node, mediastinal + M + M + M + + + + + + + + + + + + M + + + + + + 45 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hemangiosarcoma X 1 

Thymus M + + + M + + + + + + + + + M M + + + + + + + + + 41 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Carcinoma X X 4 

Fibroadenoma X X X X X X X X X X X X X 22 

Fibroadenoma, multiple X X X X X 5 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Skeletal muscle + 1 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Carcinoma, metastatic, thyroid gland X 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye + + 4 

Lacrimal gland 1 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lipoma X 1 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Leukemia mononuclear X X X X 17 

50 
118 Isobutene, NTP TR 487 

TABLE B3 

Statistical Analysis of Primary Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Adrenal Medulla: Benign Pheochromocytoma a Overall rate 3/50 (6%) 3/50 (6%) 10/49 (20%) 4/49 (8%) 

Adjusted rateb 7.5% 8.2% 22.7% 10.2% c Terminal rate 3/23 (13%) 2/19 (11%) 7/33 (21%) 2/22 (9%) 

First incidence (days) 733 (T) 722 626 650 

Poly-3 testd P=0.589 P=0.625 P=0.050 P=0.493 

Adrenal Medulla: Benign or Complex Pheochromocytoma 

Overall rate 4/50 (8%) 3/50 (6%) 10/49 (20%) 4/49 (8%) 

Adjusted rate 10.0% 8.2% 22.7% 10.2% 

Terminal rate 4/23 (17%) 2/19 (11%) 7/33 (21%) 2/22 (9%) 

First incidence (days) 733 (T) 722 626 650 

Poly-3 test P=0.516N P=0.545N P=0.101 P=0.638 

Clitoral Gland: Adenoma 

Overall rate 3/50 (6%) 3/49 (6%) 6/47 (13%) 0/48 (0%) 

Adjusted rate 7.5% 8.4% 14.2% 0.0% 

Terminal rate 3/23 (13%) 3/18 (17%) 4/31 (13%) 0/22 (0%) e First incidence (days) 733 (T) 733 (T) 620 —

Poly-3 test P=0.080N P=0.611 P=0.269 P=0.129N
 

Clitoral Gland: Carcinoma 

Overall rate 1/50 (2%) 0/49 (0%) 5/47 (11%) 1/48 (2%) 

Adjusted rate 2.5% 0.0% 12.0% 2.7% 

Terminal rate 0/23 (0%) 0/18 (0%) 4/31 (13%) 1/22 (5%) 

First incidence (days) 664 — 720 733 (T) 

Poly-3 test P=0.622N P=0.523N P=0.109 P=0.746 

Clitoral Gland: Adenoma or Carcinoma 

Overall rate 4/50 (8%) 3/49 (6%) 11/47 (23%) 1/48 (2%) 

Adjusted rate 10.0% 8.4% 26.1% 2.7% 

Terminal rate 3/23 (13%) 3/18 (17%) 8/31 (26%) 1/22 (5%) 

First incidence (days) 664 733 (T) 620 733 (T) 

Poly-3 test P=0.115N P=0.564N P=0.051 P=0.199N 

Mammary Gland: Fibroadenoma 

Overall rate 22/50 (44%) 19/50 (38%) 30/50 (60%) 27/50 (54%)
 

Adjusted rate 52.9% 46.6% 65.6% 62.9%
 

Terminal rate 14/23 (61%) 8/19 (42%) 23/33 (70%) 16/22 (73%)
 

First incidence (days) 581 440 560 511
 

Poly-3 test P=0.144 P=0.357N P=0.152 P=0.230 

Mammary Gland: Fibroadenoma or Adenoma 

Overall rate 22/50 (44%) 20/50 (40%) 31/50 (62%) 27/50 (54%)
 

Adjusted rate 52.9% 49.1% 67.8% 62.9%
 

Terminal rate 14/23 (61%) 9/19 (47%) 24/33 (73%) 16/22 (73%)
 

First incidence (days) 581 440 560 511
 

Poly-3 test P=0.178 P=0.446N P=0.104 P=0.230 

Mammary Gland: Carcinoma 

Overall rate 2/50 (4%) 2/50 (4%) 4/50 (8%) 4/50 (8%) 

Adjusted rate 4.9% 5.5% 9.1% 10.0% 

Terminal rate 1/23 (4%) 2/19 (11%) 3/33 (9%) 2/22 (9%) 

First incidence (days) 488 733 (T) 701 597 

Poly-3 test P=0.284 P=0.658 P=0.377 P=0.331 
Isobutene, NTP TR 487 119 

TABLE B3 

Statistical Analysis of Primary Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Mammary Gland: Adenoma or Carcinoma 

Overall rate 3/50 (6%) 3/50 (6%) 5/50 (10%) 4/50 (8%)
 

Adjusted rate 7.4% 8.2% 11.3% 10.0%
 

Terminal rate 2/23 (9%) 3/19 (16%) 4/33 (12%) 2/22 (9%)
 

First incidence (days) 488 733 (T) 701 597
 

Pol-3 test P=0.478 P=0.616 P=0.403 P=0.494 

Mammary Gland: Fibroadenoma, Adenoma, or Carcinoma 

Overall rate 23/50 (46%) 21/50 (42%) 32/50 (64%) 30/50 (60%)
 

Adjusted rate 54.4% 51.5% 69.8% 68.5%
 

Terminal rate 14/23 (61%) 10/19 (53%) 24/33 (73%) 17/22 (77%)
 

First incidence (days) 488 440 560 511
 

Poly-3 test P=0.081 P=0.481N P=0.092 P=0.117 

Pituitary Gland (Pars Distalis): Adenoma 

Overall rate 39/50 (78%) 33/49 (67%) 37/50 (74%) 36/49 (73%)
 

Adjusted rate 87.8% 77.1% 76.5% 83.7%
 

Terminal rate 21/23 (91%) 13/18 (72%) 25/33 (76%) 21/22 (95%)
 

First incidence (days) 496 455 399 469
 

Poly-3 test P=0.492 P=0.126N P=0.111N P=0.391N
 

Skin (Subcutaneous Tissue): Fibrous Histiocytoma, Fibrosarcoma, or Sarcoma 

Overall rate 2/50 (4%) 3/50 (6%) 0/50 (0%) 0/50 (0%)
 

Adjusted rate 4.9% 8.0% 0.0% 0.0%
 

Terminal rate 1/23 (4%) 1/19 (5%) 0/33 (0%) 0/22 (0%)
 

First incidence (days) 488 548 — —
 

Poly-3 test P=0.117N P=0.462 P=0.219N P=0.244N
 

Skin (Subcutaneous Tissue): Fibroma, Fibrous Histiocytoma, Fibrosarcoma, or Sarcoma 

Overall rate 3/50 (6%) 3/50 (6%) 1/50 (2%) 0/50 (0%)
 

Adjusted rate 7.4% 8.0% 2.3% 0.0%
 

Terminal rate 2/23 (9%) 1/19 (5%) 1/33 (3%) 0/22 (0%)
 

First incidence (days) 488 548 733 (T) —
 

Poly-3 test P=0.079N P=0.625 P=0.276N P=0.123N
 

Thyroid Gland (C-cell): Adenoma 

Overall rate 7/50 (14%) 6/50 (12%) 3/49 (6%) 5/49 (10%)
 

Adjusted rate 17.6% 16.4% 6.8% 12.7%
 

Terminal rate 7/23 (30%) 6/19 (32%) 1/33 (3%) 3/22 (14%)
 

First incidence (days) 733 (T) 733 (T) 552 663
 

Poly-3 test P=0.416N P=0.565N P=0.117N P=0.384N
 

Thyroid Gland (C-cell): Carcinoma 

Overall rate 2/50 (4%) 3/50 (6%) 6/49 (12%) 2/49 (4%) 

Adjusted rate 5.0% 8.0% 13.7% 5.1% 

Terminal rate 0/23 (0%) 0/19 (0%) 4/33 (12%) 1/22 (5%) 

First incidence (days) 720 603 534 729 

Poly-3 test P=0.444N P=0.473 P=0.164 P=0.686 

Thyroid Gland (C-cell): Adenoma or Carcinoma 

Overall rate 9/50 (18%) 9/50 (18%) 9/49 (18%) 7/49 (14%)
 

Adjusted rate 22.5% 23.9% 20.1% 17.7%
 

Terminal rate 7/23 (30%) 6/19 (32%) 5/33 (15%) 4/22 (18%)
 

First incidence (days) 720 603 534 663
 

Poly-3 test P=0.326N P=0.550 P=0.495N P=0.400N
 
120 

c 

Isobutene, NTP TR 487 

TABLE B3 

Statistical Analysis of Primary Neoplasms in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Uterus: Stromal Polyp 

Overall rate 8/50 (16%) 9/50 (18%) 8/50 (16%) 5/50 (10%) 

Adjusted rate 18.9% 23.0% 18.0% 12.1% 

Terminal rate 1/23 (4%) 4/19 (21%) 7/33 (21%) 2/22 (9%) 

First incidence (days) 529 420 626 407 

Poly-3 test P=0.169N P=0.429 P=0.567N P=0.287N 

All Organs: Mononuclear Cell Leukemia 

Overall rate 18/50 (36%) 16/50 (32%) 22/50 (44%) 17/50 (34%) 

Adjusted rate 41.6% 39.9% 46.8% 37.3% 

Terminal rate 7/23 (30%) 7/19 (37%) 13/33 (39%) 3/22 (14%) 

First incidence (days) 421 420 534 407 

Poly-3 test P=0.349N P=0.525N P=0.386 P=0.421N 

All Organs: Benign Neoplasms 

Overall rate 42/50 (84%) 45/50 (90%) 48/50 (96%) 46/50 (92%) 

Adjusted rate 92.7% 94.8% 96.1% 95.7% 

Terminal rate 22/23 (96%) 17/19 (90%) 32/33 (97%) 21/22 (96%) 

First incidence (days) 496 420 399 407 

Poly-3 test P=0.444 P=0.507 P=0.375 P=0.417 

All Organs: Malignant Neoplasms 

Overall rate 30/50 (60%) 24/50 (48%) 30/50 (60%) 25/50 (50%) 

Adjusted rate 63.6% 56.1% 63.6% 54.3% 

Terminal rate 12/23 (52%) 9/19 (47%) 20/33 (61%) 9/22 (41%) 

First incidence (days) 390 216 534 407 

Poly-3 test P=0.257N P=0.303N P=0.585 P=0.237N 

All Organs: Benign or Malignant Neoplasms 

Overall rate 50/50 (100%) 48/50 (96%) 49/50 (98%) 48/50 (96%) 

Adjusted rate 100.0% 98.0% 98.0% 98.0% 

Terminal rate 23/23 (100%) 18/19 (95%) 32/33 (97%) 21/22 (96%) 

First incidence (days) 390 216 399 407 

Poly-3 test P=0.521N P=0.496N P=0.500N P=0.496N 

(T)Terminal sacrifice 

a Number of neoplasm-bearing animals/number of animals examined. Denominator is number of animals examined microscopically for adrenal gland, clitoral 

gland, pituitary gland, thyroid gland, and uterus; for other tissues, denominator is number of animals necropsied. b	 Poly-3 estimated neoplasm incidence after adjustment for intercurrent mortality 

Observed incidence at terminal kill 

d	 Beneath the chamber control incidence are the P values associated with the trend test. Beneath the exposed group incidence are the P values corresponding to 

pairwise comparisons between the chamber controls and that exposed group. The Poly-3 test accounts for differential mortality in animals that do not reach 

terminal sacrifice. A negative trend or a lower incidence in an exposure group is indicated by N. e Not applicable; no neoplasms in animal group 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 121 

TABLE B4 

Summary of the Incidence of Nonneoplastic Lesions in Female Rats in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Accidental death 1 

Moribund 21 25 15 23 

Natural deaths 6 5 2 5 

Survivors 

Terminal sacrifice 23 19 33 22 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Esophagus (50) (50) (50) (50) 

Inflammation, suppurative 1 (2%) 

Intestine large, colon (49) (50) (49) (49) 

Inflammation, acute 1 (2%) 

Parasite metazoan 5 (10%) 1 (2%) 1 (2%) 

Intestine large, rectum (47) (47) (47) (48) 

Parasite metazoan 8 (17%) 5 (11%) 5 (11%) 3 (6%) 

Intestine large, cecum (48) (49) (48) (49) 

Inflammation, acute 1 (2%) 1 (2%) 

Parasite metazoan 5 (10%) 2 (4%) 4 (8%) 3 (6%) 

Intestine small, jejunum (47) (47) (48) (47) 

Inflammation, acute 1 (2%) 

Necrosis 1 (2%) 

Intestine small, ileum (46) (47) (48) (47) 

Inflammation, acute 1 (2%) 

Parasite metazoan 1 (2%) 

Liver (50) (50) (50) (49) 

Angiectasis 1 (2%) 3 (6%) 4 (8%) 3 (6%) 

Basophilic focus 36 (72%) 36 (72%) 43 (86%) 36 (73%) 

Clear cell focus 6 (12%) 9 (18%) 12 (24%) 5 (10%) 

Cytomegaly 1 (2%) 

Degeneration, cystic 2 (4%) 

Degeneration, fatty 12 (24%) 10 (20%) 12 (24%) 12 (24%) 

Eosinophilic focus 6 (12%) 6 (12%) 5 (10%) 9 (18%) 

Hematopoietic cell proliferation 1 (2%) 

Hepatodiaphragmatic nodule 4 (8%) 7 (14%) 5 (10%) 3 (6%) 

Inflammation, granulomatous 1 (2%) 

Mitotic alteration 1 (2%) 

Mixed cell focus 7 (14%) 11 (22%) 9 (18%) 8 (16%) 

Necrosis 2 (4%) 2 (4%) 

Regeneration 1 (2%) 1 (2%) 2 (4%) 3 (6%) 

Thrombosis 1 (2%) 

Bile duct, hyperplasia 9 (18%) 5 (10%) 7 (14%) 10 (20%) 

Centrilobular, necrosis 4 (8%) 4 (8%) 5 (10%) 9 (18%) 

Mesentery (8) (10) (12) (7) 

Thrombosis 1 (8%) 

Artery, inflammation, chronic active 2 (29%) 

Fat, hemorrhage 1 (13%) 

Fat, necrosis 8 (100%) 10 (100%) 12 (100%) 6 (86%) 

a Number of animals examined microscopically at the site and the number of animals with lesion 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

122 Isobutene, NTP TR 487 

TABLE B4 

Summary of the Incidence of Nonneoplastic Lesions in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Alimentary System (continued) 

Pancreas (50) (50) (49) (49) 

Atrophy 9 (18%) 11 (22%) 17 (35%) 10 (20%) 

Basophilic focus 1 (2%) 4 (8%) 3 (6%) 2 (4%) 

Hyperplasia 1 (2%) 1 (2%) 

Metaplasia, hepatocyte 1 (2%) 

Salivary glands (50) (50) (50) (50) 

Atrophy 1 (2%) 1 (2%) 

Stomach, forestomach (50) (50) (50) (50) 

Hyperplasia, basal cell 1 (2%) 

Inflammation, acute 1 (2%) 

Mineralization 1 (2%) 

Necrosis 7 (14%) 7 (14%) 6 (12%) 5 (10%) 

Stomach, glandular (50) (50) (50) (49) 

Mineralization 1 (2%) 

Necrosis 3 (6%) 2 (4%) 1 (2%) 1 (2%) 

Tongue (1) (1) 

Hyperplasia, squamous 1 (100%) 

Tooth (1) 

Necrosis 1 (100%) 

Cardiovascular System 

Heart (50) (50) (49) (50) 

Cardiomyopathy 30 (60%) 29 (58%) 37 (76%) 31 (62%) 

Atrium, thrombosis 1 (2%) 

Endocrine System 

Adrenal cortex (50) (50) (49) (49) 

Atrophy 1 (2%) 1 (2%) 

Degeneration, cystic 2 (4%) 2 (4%) 4 (8%) 2 (4%) 

Hyperplasia 15 (30%) 16 (32%) 24 (49%) 21 (43%) 

Hypertrophy 11 (22%) 8 (16%) 9 (18%) 9 (18%) 

Necrosis 2 (4%) 3 (6%) 1 (2%) 

Thrombosis 1 (2%) 

Vacuolization cytoplasmic 1 (2%) 

Adrenal medulla (50) (50) (49) (49) 

Hyperplasia 10 (20%) 5 (10%) 6 (12%) 10 (20%) 

Islets, pancreatic (50) (50) (49) (49) 

Hyperplasia 1 (2%) 

Pituitary gland (50) (49) (50) (49) 

Angiectasis 1 (2%) 

Pars distalis, angiectasis 1 (2%) 1 (2%) 

Pars distalis, cyst 1 (2%) 

Pars distalis, hyperplasia 7 (14%) 7 (14%) 11 (22%) 5 (10%) 

Thyroid gland (50) (50) (49) (49) 

C-cell, hyperplasia 37 (74%) 34 (68%) 38 (78%) 35 (71%) 

Follicular cell, hyperplasia 2 (4%) 

General Body System 

None 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 123 

TABLE B4 

Summary of the Incidence of Nonneoplastic Lesions in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Genital System 

Clitoral gland (50) (49) (47) (48) 

Hyperplasia 1 (2%) 5 (11%) 1 (2%) 

Inflammation, chronic active 1 (2%) 

Ovary (50) (50) (50) (50) 

Cyst 9 (18%) 4 (8%) 4 (8%) 2 (4%) 

Inflammation, granulomatous 1 (2%) 

Uterus (50) (50) (50) (50) 

Fibrosis 1 (2%) 

Inflammation, chronic active 1 (2%) 

Necrosis 1 (2%) 

Cervix, hypertrophy 1 (2%) 

Vagina (1) (1) (1) 

Inflammation, suppurative 1 (100%) 

Hematopoietic System 

Bone marrow (50) (50) (49) (49) 

Atrophy 1 (2%) 1 (2%) 2 (4%) 

Hyperplasia, histiocytic 1 (2%) 

Hyperplasia, reticulum cell 1 (2%) 

Lymph node (6) (3) (3) (2) 

Pancreatic, ectasia 1 (33%) 

Lymph node, mandibular (44) (46) (47) (47) 

Infiltration cellular, plasma cell 1 (2%) 

Infiltration cellular, polymorphonuclear 1 (2%) 

Lymph node, mesenteric (50) (50) (49) (49) 

Infiltration cellular, plasma cell 1 (2%) 

Spleen (50) (50) (49) (50) 

Accessory spleen 1 (2%) 

Fibrosis 3 (6%) 6 (12%) 2 (4%) 

Hematopoietic cell proliferation 5 (10%) 2 (4%) 3 (6%) 2 (4%) 

Hemorrhage 1 (2%) 2 (4%) 

Inflammation, granulomatous 1 (2%) 

Necrosis 1 (2%) 

Integumentary System 

Mammary gland (50) (50) (50) (50) 

Galactocele 2 (4%) 2 (4%) 1 (2%) 2 (4%) 

Hyperplasia, atypical 1 (2%) 

Inflammation, acute 1 (2%) 

Inflammation, chronic 1 (2%) 

Skin (50) (50) (50) (50) 

Cyst epithelial inclusion 1 (2%) 

Hyperkeratosis 1 (2%) 

Inflammation, acute 1 (2%) 

Inflammation, chronic active 1 (2%) 1 (2%) 2 (4%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

124 Isobutene, NTP TR 487 

TABLE B4 

Summary of the Incidence of Nonneoplastic Lesions in Female Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Musculoskeletal System 

Bone (50) (50) (50) (50) 

Hyperostosis 9 (18%) 7 (14%) 11 (22%) 15 (30%) 

Skeletal muscle (1) 

Artery, inflammation, chronic 1 (100%) 

Nervous System 

Brain (50) (50) (50) (50) 

Hemorrhage 1 (2%) 

Artery, inflammation, chronic 1 (2%) 

Respiratory System 

Larynx (50) (50) (50) (50) 

Epiglottis, metaplasia, squamous 2 (4%) 

Lung (50) (50) (50) (50) 

Inflammation, granulomatous 7 (14%) 5 (10%) 9 (18%) 11 (22%) 

Inflammation, suppurative 1 (2%) 

Alveolar epithelium, hyperplasia 9 (18%) 9 (18%) 7 (14%) 6 (12%) 

Alveolus, infiltration cellular, histiocyte 1 (2%) 

Bronchiole, hyperplasia 1 (2%) 

Nose (50) (50) (50) (49) 

Inflammation, suppurative 7 (14%) 2 (4%) 8 (16%) 8 (16%) 

Thrombosis 2 (4%) 3 (6%) 2 (4%) 3 (6%) 

Lateral wall, metaplasia, squamous 3 (6%) 

Olfactory epithelium, degeneration, hyaline 44 (88%) 47 (94%) 48 (96%) 47 (96%) 

Respiratory epithelium, metaplasia, squamous 1 (2%) 

Special Senses System 

Eye (4) (4) (6) (4) 

Cataract 4 (100%) 3 (75%) 6 (100%) 4 (100%) 

Retina, atrophy 3 (75%) 3 (75%) 6 (100%) 4 (100%) 

Urinary System 

Kidney (50) (50) (49) (49) 

Infarct 1 (2%) 2 (4%) 

Nephropathy 46 (92%) 46 (92%) 46 (94%) 46 (94%) 

Renal tubule, hyperplasia 1 (2%) 

Urinary bladder (50) (49) (49) (49) 

Hemorrhage 1 (2%) 
125 

APPENDIX C 

SUMMARY OF LESIONS IN MALE MICE 

IN THE 2-YEAR INHALATION STUDY 

OF ISOBUTENE 

TABLE C1 Summary of the Incidence of Neoplasms in Male Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 127 

TABLE C2 Individual Animal Tumor Pathology of Male Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 130 

TABLE C3 Statistical Analysis of Primary Neoplasms in Male Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 146 

TABLE C4 Summary of the Incidence of Nonneoplastic Lesions in Male Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 149 
126 Isobutene, NTP TR 487
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 127 

TABLE C1 

Summary of the Incidence of Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 16 9 20 13 

Natural deaths 6 9 3 9 

Survivors 

Terminal sacrifice 28 32 27 28 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Gallbladder (46) (40) (45) (34) 

Intestine large, cecum (47) (45) (49) (42) 

Leiomyoma 1 (2%) 

Intestine small, duodenum (46) (45) (48) (42) 

Histiocytic sarcoma 1 (2%) 

Intestine small, jejunum (45) (43) (48) (43) 

Carcinoma 1 (2%) 1 (2%) 

Intestine small, ileum (46) (43) (48) (42) 

Liver (50) (50) (50) (49) 

Hemangiosarcoma 1 (2%) 2 (4%) 2 (4%) 

Hepatoblastoma 1 (2%) 

Hepatocellular carcinoma 12 (24%) 12 (24%) 10 (20%) 7 (14%) 

Hepatocellular carcinoma, multiple 1 (2%) 1 (2%) 4 (8%) 7 (14%) 

Hepatocellular adenoma 15 (30%) 20 (40%) 13 (26%) 10 (20%) 

Hepatocellular adenoma, multiple 5 (10%) 4 (8%) 5 (10%) 7 (14%) 

Histiocytic sarcoma 1 (2%) 

Pancreas (50) (48) (50) (47) 

Salivary glands (50) (49) (50) (50) 

Stomach, forestomach (50) (49) (50) (48) 

Squamous cell papilloma 1 (2%) 

Tooth (7) (14) (9) (16) 

Odontoma 1 (14%) 3 (21%) 2 (22%) 2 (13%) 

Cardiovascular System 

Heart (50) (50) (50) (50) 

Hemangiosarcoma 1 (2%) 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 (2%) 

Endocrine System 

Adrenal cortex (49) (49) (50) (49) 

Adenoma 1 (2%) 1 (2%) 1 (2%) 

Capsule, adenoma 1 (2%) 2 (4%) 3 (6%) 

Adrenal medulla (49) (49) (50) (49) 

Pheochromocytoma malignant 1 (2%) 

Pheochromocytoma benign 1 (2%) 1 (2%) 2 (4%) 

Islets, pancreatic (50) (48) (50) (48) 

Adenoma 1 (2%) 

Pituitary gland (50) (48) (49) (48) 

Pars intermedia, adenoma 1 (2%) 

Thyroid gland (50) (50) (50) (49) 

Follicular cell, adenoma 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

128 Isobutene, NTP TR 487 

TABLE C1 

Summary of the Incidence of Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

General Body System 

None 

Genital System 

Epididymis (50) (50) (50) (50) 

Prostate (48) (46) (47) (48) 

Seminal vesicle (49) (47) (50) (47) 

Testes (50) (50) (50) (50) 

Interstitial cell, adenoma 1 (2%) 1 (2%) 

Hematopoietic System 

Bone marrow (50) (49) (50) (49) 

Histiocytic sarcoma 1 (2%) 

Lymph node (4) (3) (7) (1) 

Renal, histiocytic sarcoma 1 (25%) 

Lymph node, bronchial (30) (33) (32) (32) 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 (3%) 

Lymph node, mandibular (34) (29) (33) (35) 

Lymph node, mesenteric (48) (49) (48) (49) 

Histiocytic sarcoma 1 (2%) 1 (2%) 

Lymph node, mediastinal (37) (38) (36) (31) 

Histiocytic sarcoma 1 (3%) 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 (3%) 

Spleen (50) (49) (50) (48) 

Hemangiosarcoma 1 (2%) 1 (2%) 

Histiocytic sarcoma 1 (2%) 1 (2%) 1 (2%) 

Thymus (41) (42) (38) (41) 

Integumentary System 

Skin (49) (50) (50) (50) 

Subcutaneous tissue, hemangioma 1 (2%) 

Subcutaneous tissue, sarcoma 1 (2%) 1 (2%) 

Musculoskeletal System 

None 

Nervous System 

None 

Respiratory System 

Lung (50) (50) (50) (50) 

Alveolar/bronchiolar adenoma 9 (18%) 5 (10%) 5 (10%) 3 (6%) 

Alveolar/bronchiolar adenoma, multiple 3 (6%) 2 (4%) 

Alveolar/bronchiolar carcinoma 6 (12%) 5 (10%) 4 (8%) 3 (6%) 

Alveolar/bronchiolar carcinoma, multiple 1 (2%) 3 (6%) 

Hepatocellular carcinoma, metastatic, liver 4 (8%) 4 (8%) 2 (4%) 3 (6%) 

Histiocytic sarcoma 1 (2%) 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 (2%) 
 

 

 

 

 

 

 

 

 

129 

c 

Isobutene, NTP TR 487 

TABLE C1 

Summary of the Incidence of Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Special Senses System 

Harderian gland

Adenoma

Carcinoma

Bilateral, adenoma

 (4)

 2 (50%) 

 1 (25%) 

 1 (25%) 

(4)

 3 (75%) 

(4)

 4 (100%)

 (4) 

 2 (50%) 

 2 (50%) 

Urinary System 

Kidney 

Hepatocellular carcinoma, metastatic, liver

Histiocytic sarcoma

Pheochromocytoma malignant, metastatic, 

adrenal medulla

Renal tubule, adenoma

(50) 

 1 (2%) 

(49) 

 1 (2%) 

 1 (2%) 

(50) 

 1 (2%) 

 2 (4%) 

(50) 

Systemic Lesions 

Multiple organsb 

Histiocytic sarcoma

Lymphoma malignant

(50) 

 1 (2%) 

 4 (8%) 

(50) 

 2 (4%) 

(50) 

 1 (2%) 

 5 (10%) 

(50) 

 1 (2%) 

 5 (10%) 

Neoplasm Summary 

Total animals with primary neoplasmsc 

Total primary neoplasms 

Total animals with benign neoplasms 

Total benign neoplasms 

Total animals with malignant neoplasms 

Total malignant neoplasms 

Total animals with metastatic neoplasms 

Total metastatic neoplasms 

41 

66 

30 

40 

21 

26 

4 

4 

41 

66 

32 

42 

19 

24 

5 

10 

36 

64 

27 

36 

23 

28 

2 

2 

38 

66 

23 

32 

23 

34 

3 

3 

a Number of animals examined microscopically at the site and the number of animals with neoplasm b Number of animals with any tissue examined microscopically 

Primary neoplasms: all neoplasms except metastatic neoplasms 
 

 

 

 

 

 

 

 

 

 

 

130 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 3 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 

Number of Days on Study 8 9 2 8 2 6 8 8 9 0 1 1 5 5 6 8 9 9 0 0 1 2 3 3 3 

7 9 6 3 7 0 0 9 2 8 0 1 3 3 4 1 5 5 8 9 5 3 3 3 3 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Carcass ID Number 4 5 1 0 2 0 3 4 4 3 0 3 2 3 1 1 1 4 3 2 1 3 0 0 0 

0 0 4 3 6 2 2 7 4 4 9 1 8 3 5 2 7 8 8 3 6 5 1 4 5 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder + + A + + + + M + + + + + + + + + + A M + + + + + 

Intestine large, colon + + + + + A + A + + + + + + + + + + A + + + + + + 

Intestine large, rectum + + + + + A + A + + + + + + + + + + A + + + + + + 

Intestine large, cecum + + + + + A + A + + + + + + + + + + A + + + + + + 

Intestine small, duodenum + + A + + A + A + + + + + + + + + + A + + + + + + 

Histiocytic sarcoma X 

Intestine small, jejunum + + A + + A + A A + + + + + + + + + A + + + + + + 

Intestine small, ileum + + A + + A + A + + + + + + + + + + A + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Hemangiosarcoma X 

Hepatocellular carcinoma X X X X X X X X X X 

Hepatocellular carcinoma, multiple X 

Hepatocellular adenoma X X X X X 

Hepatocellular adenoma, multiple X X 

Mesentery + + + 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Squamous cell papilloma 

Stomach, glandular + + + + + + + A + + + + + + + + + + + + + + + + + 

Tooth + + + + 

Odontoma X 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + A + + + + + + + + + + + + + + + + 

Adenoma 

Capsule, adenoma 

Adrenal medulla + + + + + + + + A + + + + + + + + + + + + + + + + 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Parathyroid gland + + + + + + + M M M M M + M + + + + M + + + + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Prostate + + + + + + + A + + + + + + + + + + + + + + + + + 

Seminal vesicle + + + + + + + A + + + + + + + + + + + + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 

+: Tissue examined microscopically M: Missing tissue X: Lesion present 

A: Autolysis precludes examination I: Insufficient tissue Blank: Not examined 
 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 131 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Total 

Carcass ID Number 0 0 0 1 1 1 1 1 2 2 2 2 2 2 2 3 3 3 3 4 4 4 4 4 4 Tissues/ 

6 7 8 0 1 3 8 9 0 1 2 4 5 7 9 0 6 7 9 1 2 3 5 6 9 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Histiocytic sarcoma 1 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 45 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hemangiosarcoma 1 

Hepatocellular carcinoma X X 12 

Hepatocellular carcinoma, multiple 1 

Hepatocellular adenoma X X X X X X X X X X 15 

Hepatocellular adenoma, multiple X X X 5 

Mesentery + 4 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Squamous cell papilloma X 1 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Tooth + + + 7 

Odontoma 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma X 1 

Capsule, adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Parathyroid gland + + + + + + + + + + + + + I + + + M + + + + + + + 41 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Prostate + + + + + + + + + + + I + + + + + + + + + + + + + 48 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

 

 

 

 

 

 

 

 

132 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

3 3 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 

Number of Days on Study 8 

7 

9 

9 

2 

6 

8 

3 

2 

7 

6 

0 

8 

0 

8 

9 

9 

2 

0 

8 

1 

0 

1 

1 

5 

3 

5 

3 

6 

4 

8 

1 

9 

5 

9 

5 

0 

8 

0 

9 

1 

5 

2 

3 

3 

3 

3 

3 

3 

3 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Carcass ID Number 4 5 1 0 2 0 3 4 4 3 0 3 2 3 1 1 1 4 3 2 1 3 0 0 0 

0 0 4 3 6 2 2 7 4 4 9 1 8 3 5 2 7 8 8 3 6 5 1 4 5 

Hematopoietic System 

Bone marrow 

Lymph node 

Renal, histiocytic sarcoma 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Histiocytic sarcoma 

Lymph node, mediastinal 

Spleen 

Histiocytic sarcoma 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + 

X 

M M M + + + M + + M + M M + M M + M + + + + + + M 

+ + + + M + + M + + + + M M M + M M + + + + M M + 

+ + + + + A + + + + + + + + + + + + + + M + + + + 

X 

+ + + M + M + + + M + + + M + + + + M + + + + + M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + + M + + + + + M + M + M + + M + + + + + 

Integumentary System 

Mammary gland 

Skin 

M M M M M M M M M M M M M M M M M M M M M M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar adenoma, multiple 

Alveolar/bronchiolar carcinoma 

Hepatocellular carcinoma, metastatic, liver 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X 

X X 

X X X 

X X X X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye 

Harderian gland 

Adenoma 

Carcinoma 

Bilateral, adenoma 

+ 

+ 

X 

+ 

X 

Urinary System 

Kidney 

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + A + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Histiocytic sarcoma 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 133 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Total 

Carcass ID Number 0 0 0 1 1 1 1 1 2 2 2 2 2 2 2 3 3 3 3 4 4 4 4 4 4 Tissues/ 

6 7 8 0 1 3 8 9 0 1 2 4 5 7 9 0 6 7 9 1 2 3 5 6 9 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node + + 4 

Renal, histiocytic sarcoma 1 

Lymph node, bronchial M M + M M + + + + M M + + + M + + + + + + M + + M 30 

Lymph node, mandibular + + M + + + M + + + + + + + + + M M + M M M + + + 34 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Histiocytic sarcoma 1 

Lymph node, mediastinal + + M M + M + M + M M + + + + M + + + + + + + + + 37 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Thymus + + M + + + + + + + + M M + + + + + M + + + + + + 41 

Integumentary System 

Mammary gland M M M M M M M M M M M M M M M M M M M M M M M M M 

Skin + + + + + I + + + + + + + + + + + + + + + + + + + 49 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X X X X X X 9 

Alveolar/bronchiolar adenoma, multiple X 3 

Alveolar/bronchiolar carcinoma X X X 6 

Hepatocellular carcinoma, metastatic, liver 4 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye + 2 

Harderian gland + + 4 

Adenoma 2 

Carcinoma X 1 

Bilateral, adenoma X 1 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Renal tubule, adenoma X 1 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Lymphoma malignant X X 4 

50 

50 
 

 

 

 

 

 

 

 

 

 

134 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

4 5 5 5 5 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 0 1 2 4 6 8 9 0 3 4 5 6 6 9 0 1 1 1 3 3 3 3 3 3 3 

5 2 2 6 2 3 6 1 9 9 2 7 7 8 9 1 5 6 3 3 3 3 3 3 3 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 

Carcass ID Number 0 3 4 0 3 4 1 4 2 4 2 0 3 3 4 2 1 3 0 0 0 0 0 0 1 

8 7 1 9 9 8 3 2 9 4 3 4 3 1 7 7 7 8 1 2 3 5 6 7 0 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder A + A I + + A A + + + A + A + + + + + + M M + + + 

Intestine large, colon A + A + + + A + + + + + + A + + + + + + + + + + + 

Intestine large, rectum A + A + + + + + + + + + + A + + + + + + I + + + + 

Intestine large, cecum A + A + + + A A + + + + + A + + + + + + + + + + + 

Intestine small, duodenum A + + A + + A + + + + A + A + + + + + + + + + + + 

Intestine small, jejunum A + A A + + A A + + + A + A + + + + + + + + + + + 

Carcinoma X 

Intestine small, ileum A + A A + + A A + + + A + A + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Hepatocellular carcinoma X X X X X X X X X 

Hepatocellular carcinoma, multiple X 

Hepatocellular adenoma X X X X X X X X X X 

Hepatocellular adenoma, multiple X 

Mesentery 

Pancreas A + + + + + + + + + + + + A + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + A + + + + + + + + + + + 

Stomach, forestomach A + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular A + + + + + A + + + + + + A + + + + + + + + + + + 

Tooth + + + 

Odontoma X X 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + A + + + + + + + + + + + 

Adenoma 

Capsule, adenoma 

Adrenal medulla + + + + + + + + + + + + + A + + + + + + + + + + + 

Pheochromocytoma malignant 

Pheochromocytoma benign 

Islets, pancreatic A + + + + + + + + + + + + A + + + + + + + + + + + 

Adenoma 

Parathyroid gland + + M M + + + M + M + M + + + + + + + + + + + + + 

Pituitary gland A + + + + + + + + + + + + A + + + + + + + + + + + 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + A + + + + + + + + + + + 

Prostate A + A + + + + + + + + + + A + + + I + + + + + + + 

Seminal vesicle A + A + + + + + + + + + + A + + + + + + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 135 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Total 

Carcass ID Number 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 5 Tissues/ 

1 2 4 5 6 8 9 0 1 2 4 5 6 8 0 2 4 5 6 0 3 5 6 9 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder + + + + + + + + + + + + + + + + + + + + M + + + + 40 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 45 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 45 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 43 

Carcinoma 1 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 43 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hepatocellular carcinoma X X X 12 

Hepatocellular carcinoma, multiple 1 

Hepatocellular adenoma X X X X X X X X X X 20 

Hepatocellular adenoma, multiple X X X 4 

Mesentery + + 2 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Tooth + + + + + + + + + + + 14 

Odontoma X 3 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma malignant,metastatic, 

adrenal medulla X 1 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma X 1 

Capsule, adenoma X X 2 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma malignant X 1 

Pheochromocytoma benign X 1 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Adenoma X 1 

Parathyroid gland + M + + + + M + + + + M + + + + + + + + + + + + + 42 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + M + + + + + + + + + + + + + 48 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

 

 

 

 

 

 

 

 

136 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

4 5 5 5 5 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 0 

5 

1 

2 

2 

2 

4 

6 

6 

2 

8 

3 

9 

6 

0 

1 

3 

9 

4 

9 

5 

2 

6 

7 

6 

7 

9 

8 

0 

9 

1 

1 

1 

5 

1 

6 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 

Carcass ID Number 0 3 4 0 3 4 1 4 2 4 2 0 3 3 4 2 1 3 0 0 0 0 0 0 1 

8 7 1 9 9 8 3 2 9 4 3 4 3 1 7 7 7 8 1 2 3 5 6 7 0 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Spleen 

Hemangiosarcoma 

Thymus 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

+ + 

M + + M M + M + + M + + + M M + M + M + + + + + + 

M + M M + + M M M + M + M M + + + + + + + + + M + 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

+ + + + M + M + + + M M + M + + M + + + + + M M + 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

X 

+ + M + + + M + + + + + + A + + + + + + + M + + + 

Integumentary System 

Mammary gland 

Skin 

M M M M M M M M M M M M M M M M M M M M M M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar adenoma, multiple 

Alveolar/bronchiolar carcinoma 

Alveolar/bronchiolar carcinoma, multiple 

Hepatocellular carcinoma, metastatic, liver 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Nose 

Trachea 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X X 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

+ + + + + + + + + + + + + A + + + + + + + + + + + 

Special Senses System 

Harderian gland 

Adenoma 

+ 

X 

Urinary System 

Kidney 

Hepatocellular carcinoma, metastatic, liver 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Urinary bladder 

+ + + + + + + + + + + + + A 

X 

A + + + + + A + + + + + + A 

+ + + + + + + + + + + 

+ + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 137 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Total 

Carcass ID Number 1 

1 

1 

2 

1 

4 

1 

5 

1 

6 

1 

8 

1 

9 

2 

0 

2 

1 

2 

2 

2 

4 

2 

5 

2 

6 

2 

8 

3 

0 

3 

2 

3 

4 

3 

5 

3 

6 

4 

0 

4 

3 

4 

5 

4 

6 

4 

9 

5 

0 

Tissues/ 

Tumors 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Spleen 

Hemangiosarcoma

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+

+ + + M + + + M + + M + + M + M M + + + + + + M M 

X 1 

+ M + M M + M + + + + + + + + M M + M + + M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ M + M + + M + + + + + + + + + + + + M + + + + + 

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + M + + + + + + + + + + + + + + + M M + M + + 

49 

3 

33 

29 

49 

38 

1 

49 

1 

42 

Integumentary System 

Mammary gland 

Skin 

M M M M M M M M M M M M M M M M M M M M M M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 50 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar adenoma, multiple 

Alveolar/bronchiolar carcinoma 

Alveolar/bronchiolar carcinoma, multiple 

Hepatocellular carcinoma, metastatic, liver 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X

X X

X X X X

X

X X

X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

49 

50 

5 

2 

5 

1 

4 

1 

49 

49 

Special Senses System 

Harderian gland 

Adenoma 

+ 

X 

+ +

X

 4 

3 

Urinary System 

Kidney 

Hepatocellular carcinoma, metastatic, liver

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 1 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

49 

1 

47 

Systemic Lesions 

Multiple organs 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

50 

2 
 

 

 

 

 

 

 

 

 

 

 

 

 

138 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

3 3 4 4 4 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 

Number of Days on Study 8 8 2 8 9 1 1 2 5 5 8 8 2 3 3 3 4 5 8 8 0 1 2 3 3 

7 9 8 0 9 2 2 7 5 9 3 4 8 1 6 9 5 3 1 6 8 3 9 3 3 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

Carcass ID Number 2 4 3 1 2 1 2 0 0 0 1 0 1 2 3 3 4 4 2 4 2 5 4 0 0 

1 1 1 0 7 8 9 1 4 7 1 5 9 6 7 8 0 7 3 9 5 0 5 2 3 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder + + + A + + M + + + M A + + + + + + + + + + + + + 

Intestine large, colon + + + A + + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, cecum + + + + + + + + + + + A + + + + + + + + + + + + + 

Intestine small, duodenum + + + A + + + + + + + A + + + + + + + + + + + + + 

Intestine small, jejunum + + + A + + + + + + + A + + + + + + + + + + + + + 

Intestine small, ileum + + + A + + + + + + + A + + + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Hemangiosarcoma X X 

Hepatoblastoma 

Hepatocellular carcinoma X X X X X X 

Hepatocellular carcinoma, multiple X X X 

Hepatocellular adenoma X X X X X X 

Hepatocellular adenoma, multiple X 

Histiocytic sarcoma X 

Mesentery + + + 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular + + + A + + + + + + + + + + + + + + + + + + + + + 

Tooth + + 

Odontoma 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Adenoma 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 

Parathyroid gland + + + M M + M + + M + + + + + + + M + + + M + + + 

Pituitary gland + + + + + + + + + + + + + + + + + + + M + + + + + 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Follicular cell, adenoma X 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + +
 

Penis + +
 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + +
 

Prostate + + + + M + I + + + + + + + + + + + + + + + + + +
 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + +
 

Testes + + + + + + + + + + + + + + + + + + + + + + + + +
 

Interstitial cell, adenoma 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 139 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Total 

Carcass ID Number 0 0 0 1 1 1 1 1 1 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 Tissues/ 

6 8 9 2 3 4 5 6 7 0 2 4 8 0 2 3 4 5 6 9 2 3 4 6 8 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder I + + + + + + + + + + + + + + + + + + + + + + + + 45 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hemangiosarcoma 2 

Hepatoblastoma X 1 

Hepatocellular carcinoma X X X X 10 

Hepatocellular carcinoma, multiple X 4 

Hepatocellular adenoma X X X X X X X 13 

Hepatocellular adenoma, multiple X X X X 5 

Histiocytic sarcoma 1 

Mesentery + 4 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Tooth + + + + + + + 9 

Odontoma X X 2 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pheochromocytoma benign X 1 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Parathyroid gland + + + + + + + + + + + + + + + + + M M + + + + + + 42 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Follicular cell, adenoma 1 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Penis 2 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Prostate + + + + + + + M + + + + + + + + + + + + + + + + + 47 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Interstitial cell, adenoma X 1 
 

 

 

 

 

 

 

 

 

 

 

 

 

140 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

3 3 4 4 4 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 

Number of Days on Study 8 

7 

8 

9 

2 

8 

8 

0 

9 

9 

1 

2 

1 

2 

2 

7 

5 

5 

5 

9 

8 

3 

8 

4 

2 

8 

3 

1 

3 

6 

3 

9 

4 

5 

5 

3 

8 

1 

8 

6 

0 

8 

1 

3 

2 

9 

3 

3 

3 

3 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

Carcass ID Number 2 4 3 1 2 1 2 0 0 0 1 0 1 2 3 3 4 4 2 4 2 5 4 0 0 

1 1 1 0 7 8 9 1 4 7 1 5 9 6 7 8 0 7 3 9 5 0 5 2 3 

Hematopoietic System 

Bone marrow 

Histiocytic sarcoma 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Histiocytic sarcoma 

Lymph node, mediastinal 

Histiocytic sarcoma 

Spleen 

Histiocytic sarcoma 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + + + 

+ M M + + + M + + M M + + + + + + + + + M M M M + 

+ + + M + M M M M + + M + + + + + + + M + M + + + 

+ + + + + M + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + M + + + M + + M + M M + + M + + + + M + 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

M + M + + + + + M M + + + M + + + M + M + M + + M 

Integumentary System 

Mammary gland 

Skin 

Subcutaneous tissue, hemangioma 

Subcutaneous tissue, sarcoma 

M M M M M M M M M M M M M M M M M M M M M M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Hepatocellular carcinoma, metastatic, liver 

Histiocytic sarcoma 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Harderian gland 

Adenoma 

+ 

X 

+ 

X 

+ 

X 

Urinary System 

Kidney 

Histiocytic sarcoma 

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + A + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Histiocytic sarcoma 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X X X 
 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 141 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

3 

4 

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Total 

Carcass ID Number 0 

6 

0 

8 

0 

9 

1 

2 

1 

3 

1 

4 

1 

5 

1 

6 

1 

7 

2 

0 

2 

2 

2 

4 

2 

8 

3 

0 

3 

2 

3 

3 

3 

4 

3 

5 

3 

6 

3 

9 

4 

2 

4 

3 

4 

4 

4 

6 

4 

8 

Tissues/ 

Tumors 

Hematopoietic System 

Bone marrow 

Histiocytic sarcoma

Lymph node

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Histiocytic sarcoma

Lymph node, mediastinal 

Histiocytic sarcoma

Spleen 

Histiocytic sarcoma

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ M M M + + M M + M + + M + + + M + + + + + M + + 

+ M + + M + + + M M + M + + + + M + + + + M + M M 

+ + + + + + + + + + + + + + + + + + + + + + + M + 

M + + + M + + + + + M M M + M + + + + + + M + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + M + + + M M + + + + + + 

50 

1 

7 

32 

33 

48 

1 

36 

1 

50 

1 

38 

Integumentary System 

Mammary gland 

Skin 

Subcutaneous tissue, hemangioma 

Subcutaneous tissue, sarcoma

M M M M M M M M M M M M M M M M M M M M M M M M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

50 

1 

1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Hepatocellular carcinoma, metastatic, liver

Histiocytic sarcoma

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X

X X X X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

50 

50 

5 

4 

2 

1 

50 

50 

Special Senses System 

Harderian gland 

Adenoma 

+

X

 4 

4 

Urinary System 

Kidney 

Histiocytic sarcoma

Renal tubule, adenoma 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X

+ + + + + + + + + + + + + + + + + + + + + + + + + 

50 

1 

2 

49 

Systemic Lesions 

Multiple organs 

Histiocytic sarcoma

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X

50 

1 

5 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

142 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

2 3 3 3 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 6 7 7 7 7 7 

Number of Days on Study 5 3 4 8 5 9 0 1 4 4 6 0 3 3 3 6 7 8 8 9 0 2 3 3 3 

1 0 1 6 7 9 8 2 7 7 2 8 1 3 9 7 0 2 9 5 6 7 3 3 3 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

Carcass ID Number 4 3 0 3 1 0 5 2 1 3 1 1 4 1 4 0 3 1 1 4 3 0 0 0 0 

4 8 3 9 0 4 0 3 2 6 6 5 5 3 0 5 7 9 4 3 2 2 1 6 7 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder + A A A + + M + M M + I + + + M A A A + A + + + + 

Intestine large, colon + A A + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + A A + + + + + + + + + + + + + + + A + + + + + + 

Intestine large, cecum A A A + + + A + + + + + + + + + A A A + A + + + + 

Leiomyoma 

Intestine small, duodenum A A A A + + A + + + + + + + + + A A A + + + + + + 

Intestine small, jejunum A A A + + + A + + + + + + + + + A A A + + + + + + 

Carcinoma 

Intestine small, ileum A A A + + + A + + + + + + + + + A A A + A + + + + 

Liver + + + + + + + + + + + + + + + + + A + + + + + + + 

Hemangiosarcoma X 

Hepatocellular carcinoma X X X X X 

Hepatocellular carcinoma, multiple X X X X X 

Hepatocellular adenoma X X X 

Hepatocellular adenoma, multiple X X 

Mesentery + 

Pancreas + + A + + + + + + + + + + + + + + + A + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + A + + + + + + + + + + + + + + + A + + + + + + 

Stomach, glandular + + A + + + + + + + + + + + + + A A A + + + + + + 

Tooth + + + + 

Odontoma 

Cardiovascular System 

Heart 

Hemangiosarcoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + A + + + + + + 

Capsule, adenoma 

Adrenal medulla + + + + + + + + + + + + + + + + + + A + + + + + + 

Pheochromocytoma benign X X 

Islets, pancreatic + + A + + + + + + + + + + + + + + + A + + + + + + 

Parathyroid gland M + + + M + + M + + + + + + + M M M + + + + + + + 

Pituitary gland + + I + + + I + + + + + + + + + + + + + + + + + + 

Pars intermedia, adenoma 

Thyroid gland + + + + + + + + + + + + + + + + + A + + + + + + + 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Prostate A + + + + + + + + + + + + + + M + + + + + + + + + 

Seminal vesicle + + A + + + + + + + + + + + + + + A A + + + + + + 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 

Interstitial cell, adenoma 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 143 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Total 

Carcass ID Number 0 0 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 4 Tissues/ 

8 9 1 7 8 0 1 2 4 5 6 7 8 9 0 1 3 4 5 1 2 6 7 8 9 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder + M M + + M + + + + + + + + + + + + + + + M + + + 34 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + I + 46 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 42 

Leiomyoma X 1 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 42 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 43 

Carcinoma X 1 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 42 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Hemangiosarcoma X 2 

Hepatocellular carcinoma X X 7 

Hepatocellular carcinoma, multiple X X 7 

Hepatocellular adenoma X X X X X X X 10 

Hepatocellular adenoma, multiple X X X X X 7 

Mesentery + 2 

Pancreas + + + + + + + + + + + + I + + + + + + + + + + + + 47 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Tooth + + + + + + + + + + + + 16 

Odontoma X X 2 

Cardiovascular System 

Heart 

Hemangiosarcoma

+ + + + + + + + + + + + + + + + + + + + + + + + + 50 

1 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Capsule, adenoma X X X 3 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign 2 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Parathyroid gland + + + + + + + + + M M + M + + M + + M + M + + + M 37 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Pars intermedia, adenoma X 1 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

General Body System 

None 

Genital System 

Epididymis + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Preputial gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Prostate + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Seminal vesicle + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Testes + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Interstitial cell, adenoma X 1 
 

 

 

 

 

 

 

 

 

 

 

 

144 Isobutene, NTP TR 487 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

2 3 3 3 4 4 5 5 5 5 5 6 6 6 6 6 6 6 6 6 7 7 7 7 7 

Number of Days on Study 5 

1 

3 

0 

4 

1 

8 

6 

5 

7 

9 

9 

0 

8 

1 

2 

4 

7 

4 

7 

6 

2 

0 

8 

3 

1 

3 

3 

3 

9 

6 

7 

7 

0 

8 

2 

8 

9 

9 

5 

0 

6 

2 

7 

3 

3 

3 

3 

3 

3 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

Carcass ID Number 4 3 0 3 1 0 5 2 1 3 1 1 4 1 4 0 3 1 1 4 3 0 0 0 0 

4 8 3 9 0 4 0 3 2 6 6 5 5 3 0 5 7 9 4 3 2 2 1 6 7 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Hemangiosarcoma 

Histiocytic sarcoma 

Thymus 

+ + + + + + + + + + + + + + + + + + A + + + + + + 

+ 

M + M M M M M M + M + + + + + + M + M M M + + + + 

+ M M + + + M + M M + + + + + + M M + + + + M + + 

+ + + + + + + + + + + + + + + + + + A + + + + + + 

+ M + + M + + M + M + + + + + M + + M M M M + M + 

+ + A + + + + + + + + + + + + + + + A + + + + + + 

X 

X 

+ + + M + + + M + M + M + + + + M + A + + M + + + 

Integumentary System 

Mammary gland 

Skin 

Subcutaneous tissue, sarcoma 

M M M M M M M M M M M M M M M M M M M M M M M M + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain 

Peripheral nerve 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Alveolar/bronchiolar carcinoma, multiple 

Hepatocellular carcinoma, metastatic, liver 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + A A + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X X 

+ + A + + + + + + + + + + + + + + A + + + + + + + 

+ + + + + + + + + + + + + + + + + A + + + + + + + 

Special Senses System 

Eye 

Harderian gland + 

Adenoma X 

Carcinoma 

Urinary System 

Kidney 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

A + A + + + + + + + + + + + + + + A A + + + + + + 

Systemic Lesions 

Multiple organs 

Histiocytic sarcoma 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X X 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 145 

TABLE C2 

Individual Animal Tumor Pathology of Male Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Total 

Carcass ID Number 0 0 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 4 Tissues/ 

8 9 1 7 8 0 1 2 4 5 6 7 8 9 0 1 3 4 5 1 2 6 7 8 9 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node 1 

Lymph node, bronchial + + + + M + + + + + + + + + + + M M + + + + M M M 32 

Lymph node, mandibular + + + M M + M + + + + M + + + M + M + M + + + + + 35 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node, mediastinal M + M M + M + M + + + + + M + + M + + + + M + + M 31 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Hemangiosarcoma 1 

Histiocytic sarcoma 1 

Thymus + + + + + M + + + + + + M + + + + + + + + + + + + 41 

Integumentary System 

Mammary gland M M M M M M M M M M M M M M M M M M M M M M M M M 1 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Subcutaneous tissue, sarcoma 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Peripheral nerve 1 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X X X 3 

Alveolar/bronchiolar carcinoma X X 3 

Alveolar/bronchiolar carcinoma, multiple X X X 3 

Hepatocellular carcinoma, metastatic, liver 3 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Special Senses System 

Eye + + 2 

Harderian gland + + + 4 

Adenoma X 2 

Carcinoma X X 2 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Lymphoma malignant X X 5 

50 
146 Isobutene, NTP TR 487 

TABLE C3 

Statistical Analysis of Primary Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Adrenal Cortex: Adenoma a Overall rate 2/49 (4%) 3/49 (6%) 1/50 (2%) 3/49 (6%) 

Adjusted rateb 4.9% 7.0% 2.5% 7.8% c Terminal rate 2/28 (7%) 3/32 (9%) 1/27 (4%) 3/28 (11%) 

First incidence (days) 733 (T) 733 (T) 733 (T) 733 (T) 

Poly-3 testd P=0.432 P=0.520 P=0.511N P=0.474 

Harderian Gland: Adenoma 

Overall rate 3/50 (6%) 3/50 (6%) 4/50 (8%) 2/50 (4%) 

Adjusted rate 7.2% 6.9% 10.0% 5.1% 

Terminal rate 2/28 (7%) 3/32 (9%) 1/27 (4%) 2/28 (7%) 

First incidence (days) 610 733 (T) 631 733 (T) 

Poly-3 test P=0.434N P=0.643N P=0.480 P=0.527N 

Harderian Gland: Adenoma or Carcinoma 

Overall rate 4/50 (8%) 3/50 (6%) 4/50 (8%) 4/50 (8%) 

Adjusted rate 9.6% 6.9% 10.0% 10.2% 

Terminal rate 3/28 (11%) 3/32 (9%) 1/27 (4%) 4/28 (14%) 

First incidence (days) 610 733 (T) 631 733 (T) 

Poly-3 test P=0.477 P=0.477N P=0.624 P=0.612 

Liver: Hepatocellular Adenoma 

Overall rate 20/50 (40%) 24/50 (48%) 18/50 (36%) 17/49 (35%)
 

Adjusted rate 46.9% 51.8% 43.6% 42.6%
 

Terminal rate 14/28 (50%) 17/32 (53%) 12/27 (44%) 13/28 (46%)
 

First incidence (days) 592 522 584 330
 

Poly-3 test P=0.299N P=0.400 P=0.468N P=0.432N
 

Liver: Hepatocellular Carcinoma 

Overall rate 13/50 (26%) 13/50 (26%) 14/50 (28%) 14/49 (29%)
 

Adjusted rate 28.6% 27.4% 31.9% 31.7%
 

Terminal rate 2/28 (7%) 4/32 (13%) 5/27 (19%) 4/28 (14%)
 

First incidence (days) 399 405 389 330
 

Poly-3 test P=0.401 P=0.541N P=0.453 P=0.464 

Liver: Hepatocellular Adenoma or Carcinoma 

Overall rate 30/50 (60%) 32/50 (64%) 28/50 (56%) 29/49 (59%)
 

Adjusted rate 64.2% 65.7% 62.4% 64.9%
 

Terminal rate 15/28 (54%) 19/32 (59%) 15/27 (56%) 16/28 (57%)
 

First incidence (days) 399 405 389 330
 

Poly-3 test P=0.549 P=0.528 P=0.516N P=0.563 

Liver: Hepatocellular Carcinoma or Hepatoblastoma 

Overall rate 13/50 (26%) 13/50 (26%) 14/50 (28%) 14/49 (29%)
 

Adjusted rate 28.6% 27.4% 31.9% 31.7%
 

Terminal rate 2/28 (7%) 4/32 (13%) 5/27 (19%) 4/28 (14%)
 

First incidence (days) 399 405 389 330
 

Poly-3 test P=0.401 P=0.541N P=0.453 P=0.464 

Liver: Hepatocellular Adenoma, Hepatocellular Carcinoma, or Hepatoblastoma 

Overall rate 30/50 (60%) 32/50 (64%) 28/50 (56%) 29/49 (59%)
 

Adjusted rate 64.2% 65.7% 62.4% 64.9%
 

Terminal rate 15/28 (54%) 19/32 (59%) 15/27 (56%) 16/28 (57%)
 

First incidence (days) 399 405 389 330
 

Poly-3 test P=0.549 P=0.528 P=0.516N P=0.563 
Isobutene, NTP TR 487 147 

TABLE C3 

Statistical Analysis of Primary Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Lung: Alveolar/bronchiolar Adenoma 

Overall rate 12/50 (24%) 7/50 (14%) 5/50 (10%) 3/50 (6%) 

Adjusted rate 28.7% 16.1% 12.5% 7.6% 

Terminal rate 9/28 (32%) 6/32 (19%) 4/27 (15%) 3/28 (11%) 

First incidence (days) 592 698 583 733 (T) 

Poly-3 test P=0.031N P=0.125N P=0.060N P=0.013N 

Lung: Alveolar/bronchiolar Carcinoma 

Overall rate 6/50 (12%) 6/50 (12%) 4/50 (8%) 6/50 (12%) 

Adjusted rate 14.2% 13.8% 10.1% 15.3% 

Terminal rate 4/28 (14%) 6/32 (19%) 4/27 (15%) 6/28 (21%) 

First incidence (days) 580 733 (T) 733 (T) 733 (T) 

Poly-3 test P=0.490 P=0.601N P=0.411N P=0.571 

Lung: Alveolar/bronchiolar Adenoma or Carcinoma 

Overall rate 17/50 (34%) 13/50 (26%) 9/50 (18%) 9/50 (18%) 

Adjusted rate 39.7% 29.8% 22.5% 22.9% 

Terminal rate 12/28 (43%) 12/32 (38%) 8/27 (30%) 9/28 (32%) 

First incidence (days) 580 698 583 733 (T) 

Poly-3 test P=0.119N P=0.227N P=0.070N P=0.077N 

Tooth: Odontoma 

Overall rate 1/50 (2%) 3/50 (6%) 2/50 (4%) 2/50 (4%) 

Adjusted rate 2.4% 6.8% 5.1% 5.1% 

Terminal rate 0/28 (0%) 1/32 (3%) 2/27 (7%) 2/28 (7%) 

First incidence (days) 527 649 733 (T) 733 (T) 

Poly-3 test P=0.575 P=0.323 P=0.479 P=0.478 

All Organs: Hemangioma or Hemangiosarcoma 

Overall rate 1/50 (2%) 1/50 (2%) 3/50 (6%) 2/50 (4%) 

Adjusted rate 2.4% 2.3% 7.5% 5.1% 

Terminal rate 0/28 (0%) 0/32 (0%) 2/27 (7%) 2/28 (7%) 

First incidence (days) 664 715 559 733 (T) 

Poly-3 test P=0.405 P=0.750N P=0.291 P=0.481 

All Organs: Malignant Lymphoma 

Overall rate 4/50 (8%) 2/50 (4%) 5/50 (10%) 5/50 (10%) 

Adjusted rate 9.6% 4.6% 12.3% 12.6% 

Terminal rate 2/28 (7%) 2/32 (6%) 1/27 (4%) 3/28 (11%) 

First incidence (days) 611 733 (T) 631 667 

Poly-3 test P=0.266 P=0.318N P=0.481 P=0.469 

All Organs: Benign Neoplasms 

Overall rate 30/50 (60%) 32/50 (64%) 27/50 (54%) 23/50 (46%) 

Adjusted rate 68.5% 68.6% 63.6% 56.5% 

Terminal rate 21/28 (75%) 24/32 (75%) 17/27 (63%) 19/28 (68%) 

First incidence (days) 527 522 583 330 

Poly-3 test P=0.115N P=0.591 P=0.394N P=0.167N 

All Organs: Malignant Neoplasms 

Overall rate 21/50 (42%) 19/50 (38%) 23/50 (46%) 23/50 (46%) 

Adjusted rate 45.0% 39.7% 50.8% 50.9% 

Terminal rate 7/28 (25%) 9/32 (28%) 9/27 (33%) 12/28 (43%) 

First incidence (days) 399 405 389 330 

Poly-3 test P=0.237 P=0.376N P=0.363 P=0.362 
148 

c 

Isobutene, NTP TR 487 

TABLE C3 

Statistical Analysis of Primary Neoplasms in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

All Organs: Benign or Malignant Neoplasms 

Overall rate 41/50 (82%) 41/50 (82%) 36/50 (72%) 38/50 (76%) 

Adjusted rate 85.0% 83.6% 78.4% 83.1% 

Terminal rate 22/28 (79%) 27/32 (84%) 20/27 (74%) 24/28 (86%) 

First incidence (days) 399 405 389 330 

Poly-3 test P=0.530N P=0.537N P=0.281N P=0.516N 

(T)Terminal sacrifice 

a Number of neoplasm-bearing animals/number of animals examined. Denominator is number of animals examined microscopically for adrenal gland, liver, and 

lung; for other tissues, denominator is number of animals necropsied. b	 Poly-3 estimated neoplasm incidence after adjustment for intercurrent mortality 

Observed incidence at terminal kill 

d	 Beneath the chamber control incidence are the P values associated with the trend test. Beneath the exposed group incidence are the P values corresponding to 

pairwise comparisons between the chamber controls and that exposed group. The Poly-3 test accounts for differential mortality in animals that do not reach 

terminal sacrifice. A negative trend or a lower incidence in an exposure group is indicated by N. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 149 

TABLE C4 

Summary of the Incidence of Nonneoplastic Lesions in Male Mice in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 16 9 20 13 

Natural deaths 6 9 3 9 

Survivors 

Terminal sacrifice 28 32 27 28 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Liver (50) (50) (50) (49) 

Basophilic focus 7 (14%) 8 (16%) 6 (12%) 5 (10%) 

Clear cell focus 9 (18%) 4 (8%) 6 (12%) 10 (20%) 

Cyst 1 (2%) 1 (2%) 

Degeneration, fatty 2 (4%) 1 (2%) 2 (4%) 5 (10%) 

Eosinophilic focus 5 (10%) 4 (8%) 11 (22%) 7 (14%) 

Mixed cell focus 2 (4%) 1 (2%) 

Necrosis 1 (2%) 5 (10%) 2 (4%) 1 (2%) 

Vacuolization cytoplasmic, focal 1 (2%) 1 (2%) 1 (2%) 

Centrilobular, necrosis 2 (4%) 

Mesentery (4) (2) (4) (2) 

Fat, inflammation, chronic active 1 (25%) 

Fat, necrosis 3 (75%) 2 (100%) 4 (100%) 2 (100%) 

Pancreas (50) (48) (50) (47) 

Atrophy 1 (2%) 

Basophilic focus 1 (2%) 

Stomach, forestomach (50) (49) (50) (48) 

Hyperplasia 2 (4%) 1 (2%) 1 (2%) 

Inflammation, acute 1 (2%) 1 (2%) 

Stomach, glandular (49) (47) (49) (46) 

Inflammation, acute 1 (2%) 

Necrosis 2 (4%) 1 (2%) 

Tooth (7) (14) (9) (16) 

Malformation 6 (86%) 12 (86%) 7 (78%) 14 (88%) 

Cardiovascular System 

Heart (50) (50) (50) (50) 

Cardiomyopathy 21 (42%) 18 (36%) 16 (32%) 19 (38%) 

Inflammation, chronic 1 (2%) 

Thrombosis 1 (2%) 1 (2%) 

a Number of animals examined microscopically at the site and the number of animals with lesion 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

150 Isobutene, NTP TR 487 

TABLE C4 

Summary of the Incidence of Nonneoplastic Lesions in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Endocrine System 

Adrenal cortex (49) (49) (50) (49) 

Hyperplasia 14 (29%) 10 (20%) 15 (30%) 16 (33%) 

Hypertrophy 32 (65%) 24 (49%) 24 (48%) 29 (59%) 

Adrenal medulla (49) (49) (50) (49) 

Hyperplasia 1 (2%) 1 (2%) 

Islets, pancreatic (50) (48) (50) (48) 

Hyperplasia 1 (2%) 1 (2%) 1 (2%) 

Pituitary gland (50) (48) (49) (48) 

Pars distalis, hyperplasia 1 (2%) 4 (8%) 3 (6%) 4 (8%) 

Thyroid gland (50) (50) (50) (49) 

Follicular cell, hyperplasia 5 (10%) 6 (12%) 3 (6%) 3 (6%) 

General Body System 

None 

Genital System 

Epididymis (50) (50) (50) (50) 

Granuloma sperm 1 (2%) 1 (2%) 5 (10%) 

Penis (2) 

Inflammation, acute 1 (50%) 

Preputial gland (50) (48) (50) (50) 

Cyst 2 (4%) 1 (2%) 

Inflammation, chronic active 2 (4%) 3 (6%) 

Prostate (48) (46) (47) (48) 

Inflammation, chronic active 2 (4%) 4 (9%) 4 (8%) 

Seminal vesicle (49) (47) (50) (47) 

Necrosis 1 (2%) 

Testes (50) (50) (50) (50) 

Atrophy 1 (2%) 1 (2%) 

Mineralization 1 (2%) 

Hematopoietic System 

Bone marrow (50) (49) (50) (49) 

Congestion 1 (2%) 

Hemorrhage 1 (2%) 

Lymph node (4) (3) (7) (1) 

Iliac, infiltration cellular, plasma cell 1 (33%) 2 (29%) 1 (100%) 

Iliac, infiltration cellular,

 polymorphonuclear 1 (14%) 

Lumbar, infiltration cellular, plasma cell 1 (33%) 

Pancreatic, infiltration cellular, plasma

 cell 1 (25%) 

Lymph node, mesenteric (48) (49) (48) (49) 

Angiectasis 1 (2%) 1 (2%) 

Infiltration cellular, plasma cell 1 (2%) 

Spleen (50) (49) (50) (48) 

Hematopoietic cell proliferation 4 (8%) 3 (6%) 5 (10%) 4 (8%) 

Infiltration cellular, histiocyte 1 (2%) 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 151 

TABLE C4 

Summary of the Incidence of Nonneoplastic Lesions in Male Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Integumentary System 

Skin (49) (50) (50) (50) 

Cyst 1 (2%) 

Prepuce, inflammation, chronic active 7 (14%) 6 (12%) 13 (26%) 9 (18%) 

Subcutaneous tissue, edema 1 (2%) 

Musculoskeletal System 

None 

Nervous System 

Peripheral nerve (1) 

Sciatic, degeneration 1 (100%) 

Respiratory System 

Lung (50) (50) (50) (50) 

Hemorrhage 3 (6%) 

Thrombosis 1 (2%) 1 (2%) 

Alveolar epithelium, hyperplasia 5 (10%) 6 (12%) 4 (8%) 3 (6%) 

Nose (50) (49) (50) (48) 

Inflammation, suppurative 1 (2%) 2 (4%) 2 (4%) 

Polyp, inflammatory 1 (2%) 1 (2%) 

Nasolacrimal duct, polyp, inflammatory 1 (2%) 

Olfactory epithelium, atrophy 1 (2%) 1 (2%) 1 (2%) 2 (4%) 

Olfactory epithelium, degeneration, hyaline 6 (12%) 7 (14%) 16 (32%) 17 (35%) 

Respiratory epithelium, degeneration, hyaline 6 (12%) 19 (39%) 29 (58%) 39 (81%) 

Special Senses System 

Eye (2) (2) 

Cataract 1 (50%) 

Cornea, inflammation, chronic active 1 (50%) 

Harderian gland (4) (4) (4) (4) 

Hyperplasia 1 (25%) 

Urinary System 

Kidney (50) (49) (50) (50) 

Cyst 1 (2%) 1 (2%) 

Infarct 1 (2%) 2 (4%) 4 (8%) 

Inflammation, chronic active 1 (2%) 

Metaplasia, osseous 1 (2%) 2 (4%) 

Mineralization 1 (2%) 1 (2%) 

Nephropathy 36 (72%) 39 (80%) 38 (76%) 40 (80%) 

Capsule, inflammation, chronic 1 (2%) 

Papilla, inflammation, suppurative 3 (6%) 3 (6%) 6 (12%) 5 (10%) 

Pelvis, dilatation 3 (6%) 2 (4%) 4 (8%) 4 (8%) 

Renal tubule, hyperplasia 1 (2%) 1 (2%) 

Urinary bladder (49) (47) (49) (46) 

Calculus, gross observation 1 (2%) 

Inflammation, chronic active 4 (8%) 3 (6%) 7 (14%) 6 (13%) 
152 Isobutene, NTP TR 487
 
153 

APPENDIX D 

SUMMARY OF LESIONS IN FEMALE MICE 

IN THE 2-YEAR INHALATION STUDY 

OF ISOBUTENE 

TABLE D1 Summary of the Incidence of Neoplasms in Female Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 154 

TABLE D2 Individual Animal Tumor Pathology of Female Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 158 

TABLE D3 Statistical Analysis of Primary Neoplasms in Female Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 180 

TABLE D4 Summary of the Incidence of Nonneoplastic Lesions in Female Mice 

in the 2-Year Inhalation Study of Isobutene ............................... 184 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

154 Isobutene, NTP TR 487 

TABLE D1 

Summary of the Incidence of Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 13 16 6 13 

Natural deaths 5 3 5 4 

Survivors 

Terminal sacrifice 32 31 39 33 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Intestine large, colon (46) (48) (49) (50) 

Intestine large, cecum (46) (47) (47) (48) 

Intestine small, jejunum (46) (47) (46) (47) 

Carcinoma 1 (2%) 

Intestine small, ileum (46) (48) (46) (47) 

Liver (47) (50) (50) (50) 

Hepatoblastoma 1 (2%) 

Hepatoblastoma, multiple 1 (2%) 

Hepatocellular carcinoma 4 (9%) 8 (16%) 6 (12%) 9 (18%) 

Hepatocellular carcinoma, multiple 1 (2%) 1 (2%) 2 (4%) 

Hepatocellular adenoma 15 (32%) 18 (36%) 11 (22%) 13 (26%) 

Hepatocellular adenoma, multiple 5 (11%) 4 (8%) 8 (16%) 7 (14%) 

Hepatocholangiocarcinoma 1 (2%) 

Histiocytic sarcoma 2 (4%) 4 (8%) 

Mesentery (7) (8) (8) (10) 

Histiocytic sarcoma 1 (13%) 

Lipoma 1 (13%) 

Pancreas (47) (50) (48) (49) 

Salivary glands (48) (50) (50) (50) 

Stomach, forestomach (48) (49) (49) (49) 

Squamous cell carcinoma 1 (2%) 

Stomach, glandular (46) (48) (48) (49) 

Squamous cell carcinoma, metastatic, stomach, 

forestomach 1 (2%) 

Tongue (1) 

Squamous cell papilloma 1 (100%) 

Cardiovascular System 

Heart (50) (50) (50) (50) 

Histiocytic sarcoma 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 155 

TABLE D1 

Summary of the Incidence of Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Endocrine System 

Adrenal cortex (49) (50) (49) (50) 

Hepatocellular carcinoma, metastatic, liver 1 (2%) 

Histiocytic sarcoma 1 (2%) 

Capsule, adenoma 2 (4%) 1 (2%) 

Adrenal medulla (48) (49) (49) (49) 

Pheochromocytoma benign 1 (2%) 2 (4%) 2 (4%) 

Bilateral, pheochromocytoma malignant 1 (2%) 

Islets, pancreatic (47) (49) (48) (49) 

Adenoma 1 (2%) 1 (2%) 1 (2%) 

Pituitary gland (49) (49) (49) (49) 

Pars distalis, adenoma 8 (16%) 13 (27%) 12 (24%) 12 (24%) 

Pars distalis, carcinoma 1 (2%) 

Pars intermedia, adenoma 1 (2%) 

Thyroid gland (48) (50) (49) (50) 

Follicular cell, adenoma 1 (2%) 4 (8%) 3 (6%) 1 (2%) 

Follicular cell, adenoma, multiple 1 (2%) 

Follicular cell, carcinoma 1 (2%) 1 (2%) 

General Body System 

None 

Genital System 

Ovary (48) (50) (48) (50) 

Cystadenoma 3 (6%) 1 (2%) 2 (4%) 

Granulosa cell tumor benign 1 (2%) 2 (4%) 

Granulosa-theca tumor benign 1 (2%) 

Hemangioma 1 (2%) 1 (2%) 

Hemangiosarcoma 1 (2%) 

Histiocytic sarcoma 2 (4%) 3 (6%) 1 (2%) 

Luteoma 1 (2%) 

Teratoma benign 1 (2%) 

Yolk sac carcinoma 1 (2%) 

Uterus (49) (50) (50) (50) 

Histiocytic sarcoma 2 (4%) 1 (2%) 

Polyp stromal 2 (4%) 1 (2%) 1 (2%) 

Polyp stromal, multiple 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

156 Isobutene, NTP TR 487 

TABLE D1 

Summary of the Incidence of Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Hematopoietic System 

Bone marrow (48) (49) (49) (50) 

Hemangiosarcoma 1 (2%) 

Histiocytic sarcoma 2 (4%) 

Lymph node (7) (2) (5) (4) 

Iliac, histiocytic sarcoma 1 (14%) 1 (50%) 

Inguinal, hemangiosarcoma 1 (20%) 

Renal, histiocytic sarcoma 1 (14%) 2 (100%) 

Lymph node, bronchial (38) (33) (44) (44) 

Histiocytic sarcoma 1 (3%) 

Lymph node, mandibular (41) (42) (44) (38) 

Histiocytic sarcoma 2 (5%) 

Lymph node, mesenteric (46) (48) (47) (49) 

Histiocytic sarcoma 2 (4%) 3 (6%) 

Lymph node, mediastinal (38) (42) (36) (42) 

Histiocytic sarcoma 1 (3%) 2 (5%) 

Spleen (49) (50) (50) (50) 

Hemangiosarcoma 3 (6%) 1 (2%) 

Hepatocholangiocarcinoma, metastatic, liver 1 (2%) 

Histiocytic sarcoma 1 (2%) 2 (4%) 

Thymus (47) (44) (48) (46) 

Histiocytic sarcoma 1 (2%) 

Integumentary System 

Mammary gland (48) (49) (49) (50) 

Carcinoma 2 (4%) 2 (4%) 

Skin (49) (50) (49) (50) 

Subcutaneous tissue, fibrosarcoma 2 (4%) 1 (2%) 1 (2%) 

Subcutaneous tissue, fibrous histiocytoma

 malignant 1 (2%) 

Subcutaneous tissue, hemangiosarcoma 1 (2%) 1 (2%) 

Subcutaneous tissue, sarcoma 2 (4%) 2 (4%) 2 (4%) 

Musculoskeletal System 

None 

Nervous System 

Brain (49) (50) (50) (50) 

Astrocytoma benign 1 (2%) 

Carcinoma, metastatic, pituitary gland 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 157 

TABLE D1 

Summary of the Incidence of Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Respiratory System 

Lung (49) (50) (50) (50) 

Alveolar/bronchiolar adenoma 2 (4%) 2 (4%) 2 (4%) 2 (4%) 

Alveolar/bronchiolar carcinoma 4 (8%) 2 (4%) 2 (4%) 1 (2%) 

Alveolar/bronchiolar carcinoma, metastatic, 

lung 1 (2%) 

Carcinoma, metastatic, mammary gland 1 (2%) 

Hepatoblastoma, metastatic, liver 1 (2%) 

Hepatocellular carcinoma, metastatic, liver 1 (2%) 4 (8%) 2 (4%) 1 (2%) 

Histiocytic sarcoma 2 (4%) 4 (8%) 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 (2%) 

Sarcoma, metastatic, skin 1 (2%) 

Nose (47) (50) (49) (50) 

Pleura (1) 

Hepatoblastoma, metastatic, liver 1 (100%) 

Special Senses System 

Harderian gland (2) (2) (1) (2) 

Adenoma 1 (50%) 1 (50%) 1 (100%) 1 (50%) 

Carcinoma 1 (50%) 1 (50%) 

Bilateral, carcinoma 1 (50%) 

Urinary System 

Kidney (49) (50) (49) (50) 

Histiocytic sarcoma 3 (6%) 

Urinary bladder (45) (49) (49) (50) 

Histiocytic sarcoma 1 (2%) 

Systemic Lesions 

Multiple organsb (50) (50) (50) (50) 

Histiocytic sarcoma 2 (4%) 4 (8%) 2 (4%) 

Lymphoma malignant 16 (32%) 9 (18%) 17 (34%) 19 (38%) 

Neoplasm Summary 

Total animals with primary neoplasmsc 43 45 44 44 

Total primary neoplasms 77 86 83 82 

Total animals with benign neoplasms 29 35 33 32 

Total benign neoplasms 39 53 46 44 

Total animals with malignant neoplasms 29 29 30 31 

Total malignant neoplasms 38 33 37 38 

Total animals with metastatic neoplasms 3 7 4 2 

Total metastatic neoplasms 3 8 5 2 

a Number of animals examined microscopically at the site and the number of animals with neoplasm b Number of animals with any tissue examined microscopically 

Primary neoplasms: all neoplasms except metastatic neoplasms c 
 

 

 

 

 

 

 

 

 

 

 

 

158 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

2 3 3 4 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 2 7 4 4 8 0 1 2 3 5 6 6 8 0 0 2 2 3 3 3 3 3 3 3 

0 2 8 9 1 0 8 0 1 9 3 7 7 1 9 9 3 3 5 5 5 5 5 5 5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

Carcass ID Number 4 1 2 2 3 4 0 3 0 0 2 1 4 3 0 1 2 4 0 0 0 0 0 1 1 

0 0 4 0 7 7 8 9 6 3 7 5 4 1 4 7 6 9 1 2 5 7 9 1 2 

Alimentary System 

Esophagus A + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder A A A + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, colon A A A + A + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum A A A + A + + + + + + + + + + + + + I + + + + + + 

Intestine large, cecum A A A + A + + + + + + + + + + + + + + + + + + + + 

Intestine small, duodenum A A A + A + + + + + + + + + + + + + + + + + + + + 

Intestine small, jejunum A A A + A + + + + + + + + + + + + + + + + + + + + 

Intestine small, ileum A A A + A + + + + + + + + + + + + + + + + + + + + 

Liver A A A + + + + + + + + + + + + + + + + + + + + + + 

Hepatocellular carcinoma X X X 

Hepatocellular carcinoma, multiple X 

Hepatocellular adenoma X X X X X X X 

Hepatocellular adenoma, multiple X 

Hepatocholangiocarcinoma X 

Histiocytic sarcoma 

Mesentery + + + 

Pancreas A A A + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands A + M + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach A + A + + + + + + + + + + + + + + + + + + + + + + 

Stomach, glandular A A A + A + + + + + + + + + + + + + + + + + + + + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex A + + + + + + + + + + + + + + + + + + + + + + + + 

Adrenal medulla A + + + + + + + + + + + + + + I + + + + + + + + + 

Islets, pancreatic A A A + + + + + + + + + + + + + + + + + + + + + + 

Adenoma X 

Parathyroid gland A + + M M + + M + + + + M + + + M M + M + M + + + 

Pituitary gland M + + + + + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X 

Thyroid gland A + A + + + + + + + + + + + + + + + + + + + + + + 

Follicular cell, adenoma X 

General Body System 

None 

Genital System 

Clitoral gland A M + + + + + + + + + I + + + + + + + + + + M + + 

Ovary A + A + + + + + + + + + + + + + + + + + + + + + + 

Granulosa cell tumor benign 

Histiocytic sarcoma 

Teratoma benign X 

Yolk sac carcinoma X 

Uterus A + + + + + + + + + + + + + + + + + + + + + + + + 

Polyp stromal X 

Polyp stromal, multiple 

+: Tissue examined microscopically M: Missing tissue X: Lesion present 

A: Autolysis precludes examination I: Insufficient tissue Blank: Not examined 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 159 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 5 Tissues/ 

3 4 6 8 9 1 2 3 5 8 9 0 2 3 4 5 6 8 1 2 3 5 6 8 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Gallbladder + + + + + + + + + + + + + + + + + + + + + + + + I 46 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 45 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Hepatocellular carcinoma X 4 

Hepatocellular carcinoma, multiple 1 

Hepatocellular adenoma X X X X X X X X 15 

Hepatocellular adenoma, multiple X X X X 5 

Hepatocholangiocarcinoma 1 

Histiocytic sarcoma X X 2 

Mesentery + + + + 7 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Adenoma 1 

Parathyroid gland M + + + + + + + + + M M + M + + M + + + + M + + + 35 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X 8 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Follicular cell, adenoma 1 

General Body System 

None 

Genital System 

Clitoral gland + + I + I + + + + + + + + + + + + + + + + + + + + 44 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Granulosa cell tumor benign X 1 

Histiocytic sarcoma X X 2 

Teratoma benign 1 

Yolk sac carcinoma 1 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Polyp stromal X 2 

Polyp stromal, multiple X 1 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

160 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

2 3 3 4 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 

0 

2 

2 

7 

8 

4 

9 

4 

1 

8 

0 

0 

8 

1 

0 

2 

1 

3 

9 

5 

3 

6 

7 

6 

7 

8 

1 

0 

9 

0 

9 

2 

3 

2 

3 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

Carcass ID Number 4 1 2 2 3 4 0 3 0 0 2 1 4 3 0 1 2 4 0 0 0 0 0 1 1 

0 0 4 0 7 7 8 9 6 3 7 5 4 1 4 7 6 9 1 2 5 7 9 1 2 

Hematopoietic System 

Bone marrow A + A + + + + + + + + + + + + + + + + + + + + + + 

Hemangiosarcoma 

Lymph node 

Iliac, histiocytic sarcoma 

Renal, histiocytic sarcoma 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Histiocytic sarcoma 

Lymph node, mediastinal 

Histiocytic sarcoma 

Spleen 

Hemangiosarcoma 

Hepatocholangiocarcinoma, metastatic, liver 

Histiocytic sarcoma 

Thymus 

Histiocytic sarcoma 

A 

A 

A 

A 

A 

A 

X 

+ + + + 

+ M M M + + + + + + + + + + + M M + + + + M + M 

+ M + + + + + + + + M + + + + M + + + + M + + + 

+ A + + + + + + + + M + + + I + + + + + + + + + 

M + M + + + + + + + M + + M + + + + M + M + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

+ A M + + + + + + + + + + + + + + + + + + + + + 

Integumentary System 

Mammary gland 

Carcinoma 

A + + + + + + + + + + + + + + + + + + + + + + + + 

X X 

Skin A + + + + + + + + + + + + + + + + + + + + + + + + 

Subcutaneous tissue, sarcoma X X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain A + + + + + + + + + + + + + + + + + + + + + + + + 

Astrocytoma benign X 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Carcinoma, metastatic, mammary gland 

Hepatocellular carcinoma, metastatic, liver 

Histiocytic sarcoma 

Nose 

A 

A 

A 

+ + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + 

X X 

X X X 

X 

X 

A A + + + + + + + + + + + + + + + + + + + + + + 

Trachea A + A + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye 

Harderian gland 

Adenoma 

+ 

+ + 

X 

Bilateral, carcinoma X 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 161 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 5 Tissues/ 

3 4 6 8 9 1 2 3 5 8 9 0 2 3 4 5 6 8 1 2 3 5 6 8 0 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Hemangiosarcoma 1 

Lymph node + + + 7 

Iliac, histiocytic sarcoma X 1 

Renal, histiocytic sarcoma X 1 

Lymph node, bronchial + + + M + + + M + + + + M + + + M + + + + + + + + 38 

Lymph node, mandibular + + + + + M + + + + + + M + + + M + + + M + + + + 41 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Histiocytic sarcoma X X 2 

Lymph node, mediastinal + + + + M M + + + + + + + M M + + + + + + + M + + 38 

Histiocytic sarcoma X 1 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Hemangiosarcoma X X 3 

Hepatocholangiocarcinoma, metastatic, liver 1 

Histiocytic sarcoma X 1 

Thymus + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Histiocytic sarcoma X 1 

Integumentary System 

Mammary gland + + + + I + + + + + + + + + + + + + + + + + + + + 48 

Carcinoma 2 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Subcutaneous tissue, sarcoma 2 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Astrocytoma benign 1 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Alveolar/bronchiolar adenoma 2 

Alveolar/bronchiolar carcinoma X 4 

Carcinoma, metastatic, mammary gland 1 

Hepatocellular carcinoma, metastatic, liver 1 

Histiocytic sarcoma X X 2 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Special Senses System 

Eye

Harderian gland

Adenoma

Bilateral, carcinoma

 1 

2 

1 

1 
 

 

162 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

2 3 3 4 5 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 2 7 4 4 8 0 1 2 3 5 6 6 8 0 0 2 2 3 3 3 3 3 3 3 

0 2 8 9 1 0 8 0 1 9 3 7 7 1 9 9 3 3 5 5 5 5 5 5 5 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

Carcass ID Number 4 1 2 2 3 4 0 3 0 0 2 1 4 3 0 1 2 4 0 0 0 0 0 1 1 

0 0 4 0 7 7 8 9 6 3 7 5 4 1 4 7 6 9 1 2 5 7 9 1 2 

Urinary System 

Kidney + + A + + + + + + + + + + + + + + + + + + + + + + 

Urinary bladder A A + + A + + + + + I + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Lymphoma malignant X X X X X X X X X 
 

 

Isobutene, NTP TR 487 163 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: Chamber Control 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Total 

Carcass ID Number 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 5 Tissues/ 

3 4 6 8 9 1 2 3 5 8 9 0 2 3 4 5 6 8 1 2 3 5 6 8 0 Tumors 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Urinary bladder + + + + + + + + + + M + + + + + + + + + + + + + + 45 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X X 2 

Lymphoma malignant X X X X X X X 16 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

164 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

Number of Days on Study 

4 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 

8 0 6 7 9 1 1 1 3 3 3 4 5 5 9 9 9 2 3 3 3 3 3 3 3 

3 6 7 2 4 1 1 5 9 9 9 9 3 3 1 5 5 7 1 5 5 5 5 5 5 

Carcass ID Number 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

2 1 4 3 3 1 2 4 0 3 3 0 1 2 0 0 0 1 1 0 0 0 0 1 1 

9 5 4 5 6 8 8 5 2 0 7 4 0 6 9 3 8 1 2 1 5 6 7 3 4 

Alimentary System 

Esophagus 

Gallbladder 

Intestine large, colon 

Intestine large, rectum 

Intestine large, cecum 

Intestine small, duodenum 

Intestine small, jejunum 

Carcinoma 

Intestine small, ileum 

Liver 

Hepatoblastoma 

Hepatoblastoma, multiple 

Hepatocellular carcinoma 

Hepatocellular adenoma 

Hepatocellular adenoma, multiple 

Histiocytic sarcoma 

Mesentery 

Histiocytic sarcoma 

Pancreas 

Salivary glands 

Stomach, forestomach 

Stomach, glandular 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ A + + + + + + + + + A + + M + M + A + + + + + + 

+ A + + + + + + + + + A + + + + + + + + + + + + + 

+ A + + + + + I + + + A + + I + + + + + + + + + + 

+ A + + + + + + + + + A + + + + + + A + + + + + + 

+ A + + + + + + + + + A + + + + + + + + + + + + + 

+ A + + + + + + + + + A + + + + + + A + + + + + + 

X 

+ A + + + + + + + + + A + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X X X X 

X X X X X X X X 

X 

X X 

+ + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ A + + + + + + + + + + + + + + + + + + + + + + + 

+ A + + + + + + + + + A + + + + + + + + + + + + + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Capsule, adenoma 

Adrenal medulla + A + + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign X 

Islets, pancreatic + + + + + + + + + + + A + + + + + + + + + + + + + 

Adenoma X 

Parathyroid gland M M M + M + + + + + + M + + + + M M + + + + M M + 

Pituitary gland + + + + + + + + + I + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Follicular cell, adenoma X 

General Body System 

None 

Genital System 

Clitoral gland M + M + + + M + + + + + + + + + + + + + + M + + + 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 

Cystadenoma X X 

Granulosa cell tumor benign 

Hemangioma X 

Histiocytic sarcoma X 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Polyp stromal 

Vagina 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 165 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 Tissues/ 

6 7 9 0 1 2 3 4 5 7 1 2 3 4 8 9 0 1 2 3 6 7 8 9 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder + M + + + + + + + + + + + + + + + + + + + + + + M 43 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + I + + 45 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Carcinoma 1 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hepatoblastoma 1 

Hepatoblastoma, multiple X 1 

Hepatocellular carcinoma X X X 8 

Hepatocellular adenoma X X X X X X X X X X 18 

Hepatocellular adenoma, multiple X X X 4 

Histiocytic sarcoma X X 4 

Mesentery + + + + + + 8 

Histiocytic sarcoma X 1 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 1 

Capsule, adenoma X X 2 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign 1 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Adenoma 1 

Parathyroid gland + + + + + + + + + M + + + + + + M + + + + + + + + 39 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X X X 13 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Follicular cell, adenoma X X X 4 

General Body System 

None 

Genital System 

Clitoral gland + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Cystadenoma X 3 

Granulosa cell tumor benign X X 2 

Hemangioma 1 

Histiocytic sarcoma X X 3 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 2 

Polyp stromal X 1 

Vagina + 1 
 

 

 

 

 

 

 

 

 

 

 

 

 

166 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

4 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 

Number of Days on Study 8 0 6 7 9 1 1 1 3 3 3 4 5 5 9 9 9 2 3 3 3 3 3 3 3 

3 6 7 2 4 1 1 5 9 9 9 9 3 3 1 5 5 7 1 5 5 5 5 5 5 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

Carcass ID Number 2 1 4 3 3 1 2 4 0 3 3 0 1 2 0 0 0 1 1 0 0 0 0 1 1 

9 5 4 5 6 8 8 5 2 0 7 4 0 6 9 3 8 1 2 1 5 6 7 3 4 

Hematopoietic System 

Bone marrow + A + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Lymph node + 

Iliac, histiocytic sarcoma X 

Renal, histiocytic sarcoma X 

Lymph node, bronchial M + + + + + + M + M + + M M + + + M + + + + + + M 

Histiocytic sarcoma 

Lymph node, mandibular M + M + + M + M + M + + + + + + + + + + + + + + + 

Histiocytic sarcoma 

Lymph node, mesenteric + + + M + + + + + + + + + + + M + + + + + + + + + 

Histiocytic sarcoma X 

Lymph node, mediastinal + + + + + + M + + M + M + M + + + + + + + + + + + 

Histiocytic sarcoma X 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Thymus M + + + + + + + + + + + + M M + + M + + + + + + + 

Integumentary System 

Mammary gland + + M + + + + + + + + + + + + + + + + + + + + + + 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 

Subcutaneous tissue, fibrosarcoma X 

Subcutaneous tissue, fibrous, histiocytoma 

malignant 

Subcutaneous tissue, hemangiosarcoma 

Subcutaneous tissue, sarcoma X X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 

Alveolar/bronchiolar adenoma x 

Alveolar/bronchiolar carcinoma X 

Alveolar/bronchiolar carcinoma, metastatic, lung X 

Hepatoblastoma, metastatic, liver X 

Hepatocellular carcinoma, metastatic, liver X X X 

Histiocytic sarcoma X X 

Sarcoma, metastatic, skin X 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 

Pleura + 

Hepatoblastoma, metastatic, liver X 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 167 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 Tissues/ 

6 7 9 0 1 2 3 4 5 7 1 2 3 4 8 9 0 1 2 3 6 7 8 9 0 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Histiocytic sarcoma X X 2 

Lymph node + 2 

Iliac, histiocytic sarcoma 1 

Renal, histiocytic sarcoma X 2 

Lymph node, bronchial M + + + + + + M + + + M M + + + + M M + M + M M M 33 

Histiocytic sarcoma X 1 

Lymph node, mandibular + + + + + + + + M + + + + + M + + + M + + + + + + 42 

Histiocytic sarcoma X X 2 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Histiocytic sarcoma 3 

Lymph node, mediastinal M + + M + + + + + M + + + + + + + + M + + + + + + 42 

Histiocytic sarcoma X 2 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X 2 

Thymus + + + M + + + + + + + + + + + + + + + + + + M + + 44 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Subcutaneous tissue, fibrosarcoma X 2 

Subcutaneous tissue, fibrous, histiocytoma 

malignant X 1 

Subcutaneous tissue, hemangiosarcoma X 1 

Subcutaneous tissue, sarcoma 2 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X 2 

Alveolar/bronchiolar carcinoma X 2 

Alveolar/bronchiolar carcinoma, metastatic, lung 1 

Hepatoblastoma, metastatic, liver 1 

Hepatocellular carcinoma, metastatic, liver X 4 

Histiocytic sarcoma X X 4 

Sarcoma, metastatic, skin 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Pleura 1 

Hepatoblastoma, metastatic, liver 1 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

168 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

Number of Days on Study 

4 

8 

3 

5 

0 

6 

5 

6 

7 

5 

7 

2 

5 

9 

4 

6 

1 

1 

6 

1 

1 

6 

1 

5 

6 

3 

9 

6 

3 

9 

6 

3 

9 

6 

4 

9 

6 

5 

3 

6 

5 

3 

6 

9 

1 

6 

9 

5 

6 

9 

5 

7 

2 

7 

7 

3 

1 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

Carcass ID Number 

3 

2 

9 

3 

1 

5 

3 

4 

4 

3 

3 

5 

3 

3 

6 

3 

1 

8 

3 

2 

8 

3 

4 

5 

3 

0 

2 

3 

3 

0 

3 

3 

7 

3 

0 

4 

3 

1 

0 

3 

2 

6 

3 

0 

9 

3 

0 

3 

3 

0 

8 

3 

1 

1 

3 

1 

2 

3 

0 

1 

3 

0 

5 

3 

0 

6 

3 

0 

7 

3 

1 

3 

3 

1 

4 

Special Senses System 

Eye 

Harderian gland 

Adenoma 

Carcinoma 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Urinary bladder + + + + + + + + + + + A + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X X 

Lymphoma malignant X X X X 
 

 

 

 

 

 

Isobutene, NTP TR 487 169 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 500 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 Tissues/ 

6 7 9 0 1 2 3 4 5 7 1 2 3 4 8 9 0 1 2 3 6 7 8 9 0 Tumors 

Special Senses System 

Eye + 1 

Harderian gland + + 2 

Adenoma X 1 

Carcinoma X 1 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X X 3 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Histiocytic sarcoma 1 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma X X 4 

Lymphoma malignant X X X X X 9 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

170 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

4 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 7 0 4 0 1 2 2 7 8 9 0 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

7 9 3 0 8 0 8 5 6 2 9 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

Carcass ID Number 1 2 0 4 2 2 3 4 4 3 3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 

3 3 8 9 6 1 5 1 5 4 6 1 2 3 4 5 6 7 9 0 1 2 4 5 6 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder + + A A + + + + A A + + + + + + + + + + + + M + + 

Intestine large, colon + + A + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + + A + I + + + + + + + + + + + + + + + + + + + + 

Intestine large, cecum + + A A + + + + + A + + + + + + + + + + + + + + + 

Intestine small, duodenum + + A A + + + + A A + + + + + + + + + + + + + + + 

Intestine small, jejunum + + A A + + + + A A + + + + + + + + + + + + + + + 

Intestine small, ileum + + A A + + + + A A + + + + + + + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Hepatocellular carcinoma X X X X 

Hepatocellular carcinoma, multiple X 

Hepatocellular adenoma X X X X X 

Hepatocellular adenoma, multiple X X X 

Mesentery + + 

Lipoma 

Pancreas + + A + + + + + A + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + A + + + + + + + + + + + + + + + + + + + + + + 

Squamous cell carcinoma X 

Stomach, glandular + + A A + + + + + + + + + + + + + + + + + + + + + 

Squamous cell carcinoma, metastatic, stomach, 

forestomach X 

Tooth 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + A + + + + + + + + + + + + + + + + + + + + + + 

Hepatocellular carcinoma, metastatic, liver X 

Adrenal medulla + + A + + + + + + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign 

Islets, pancreatic + + A + + + + + A + + + + + + + + + + + + + + + + 

Adenoma 

Parathyroid gland + + A M + + + M M + M + + + M + M + + + + + + M + 

Pituitary gland + + + + + + + + A + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X 

Pars distalis, carcinoma X 

Pars intermedia, adenoma X 

Thyroid gland + + A + + + + + + + + + + + + + + + + + + + + + + 

Follicular cell, adenoma X X X 

Follicular cell, carcinoma X 

General Body System 

None 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 171 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 Tissues/ 

7 8 9 0 2 4 5 7 8 9 0 1 2 3 7 8 9 0 2 3 4 6 7 8 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder + + + + M M + + + + + + + + + + + + + + + + + + + 43 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Intestine large, rectum + + + + + + + + + + + I + + + + + + + + + + + + + 47 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 46 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hepatocellular carcinoma X X 6 

Hepatocellular carcinoma, multiple 1 

Hepatocellular adenoma X X X X X X 11 

Hepatocellular adenoma, multiple X X X X X 8 

Mesentery + + + + + + 8 

Lipoma X 1 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Squamous cell carcinoma 1 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Squamous cell carcinoma, metastatic, stomach, 

forestomach 1 

Tooth + 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Hepatocellular carcinoma, metastatic, liver 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign X X 2 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Adenoma X 1 

Parathyroid gland + + + + M M + + M + + M + + + + + + M + + + + + + 37 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X 12 

Pars distalis, carcinoma 1 

Pars intermedia, adenoma 1 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Follicular cell, adenoma 3 

Follicular cell, carcinoma 1 

General Body System 

None 
 

 

 

 

 

 

 

 

 

 

172 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

Number of Days on Study 

4 

7 

7 

5 

0 

9 

5 

4 

3 

6 

0 

0 

6 

1 

8 

6 

2 

0 

6 

2 

8 

6 

7 

5 

6 

8 

6 

6 

9 

2 

7 

0 

9 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

Carcass ID Number 

5 

1 

3 

5 

2 

3 

5 

0 

8 

5 

4 

9 

5 

2 

6 

5 

2 

1 

5 

3 

5 

5 

4 

1 

5 

4 

5 

5 

3 

4 

5 

3 

6 

5 

0 

1 

5 

0 

2 

5 

0 

3 

5 

0 

4 

5 

0 

5 

5 

0 

6 

5 

0 

7 

5 

0 

9 

5 

1 

0 

5 

1 

1 

5 

1 

2 

5 

1 

4 

5 

1 

5 

5 

1 

6 

Genital System 

Clitoral gland M + A + + + M + A M + + + + + + + + + + + + + + + 

Ovary + + A + + + + + + + + + + + + + + + + + + + + + + 

Cystadenoma 

Hemangioma X 

Hemangiosarcoma X 

Histiocytic sarcoma X 

Luteoma X 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X 

Polyp stromal X 

Hematopoietic System 

Bone marrow + + A + + + + + + + + + + + + + + + + + + + + + + 

Lymph node 

Inguinal, hemangiosarcoma 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Thymus 

+ + + 

+ M + + + + + + + + + M + + M + M + + + + + + + + 

+ + A + + + + M + + + + + + + + + M M + + + + + + 

M + A + + I + + + + + + + + + + + + + + + + + + + 

+ M M + M + M + + + + M + M + M + + + + + + + M + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

M + A + + + + + + + + + + + + + + + + + + + + + + 

Integumentary System 

Mammary gland 

Carcinoma 

+ + + + + + + + A + + + + + + + + + + + + + + + + 

Skin + + + + + + + + A + + + + + + + + + + + + + + + + 

Subcutaneous tissue, fibrosarcoma 

Subcutaneous tissue, hemangiosarcoma 

X 

X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Carcinoma, metastatic, pituitary gland X 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Hepatocellular carcinoma, metastatic, liver 

Nose 

+ + A + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X 

X 

+ + A + + + + + + + + + + + + + + + + + + + + + + 

Trachea + + A + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Harderian gland 

Adenoma 

+ 

X 
 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 173 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 Tissues/ 

7 8 9 0 2 4 5 7 8 9 0 1 2 3 7 8 9 0 2 3 4 6 7 8 0 Tumors 

Genital System 

Clitoral gland + + + + + + + + + + + + + + + + + + + + + + + + + 45 

Ovary + + + + + + + + + + + I + + + + + + + + + + + + + 48 

Cystadenoma X 1 

Hemangioma 1 

Hemangiosarcoma 1 

Histiocytic sarcoma 1 

Luteoma 1 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 1 

Polyp stromal 1 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node + + 5 

Inguinal, hemangiosarcoma X 1 

Lymph node, bronchial M + + + M + + + + + + + + + + + + + + + + + + + + 44 

Lymph node, mandibular + + + + + + + + + M + + + + + M + + + + + + + + + 44 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Lymph node, mediastinal + + + + + + + + + + M M M + + + + + + M + M + + M 36 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Thymus + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Carcinoma X X 2 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Subcutaneous tissue, fibrosarcoma 1 

Subcutaneous tissue, hemangiosarcoma 1 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Carcinoma, metastatic, pituitary gland 1 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X 2 

Alveolar/bronchiolar carcinoma 2 

Hepatocellular carcinoma, metastatic, liver X 2 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Special Senses System 

Harderian gland 1 

Adenoma 1 

50 
 

 

174 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

4 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 7 0 4 0 1 2 2 7 8 9 0 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

7 9 3 0 8 0 8 5 6 2 9 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 

Carcass ID Number 1 2 0 4 2 2 3 4 4 3 3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 

3 3 8 9 6 1 5 1 5 4 6 1 2 3 4 5 6 7 9 0 1 2 4 5 6 

Urinary System 

Kidney + + A + + + + + + + + + + + + + + + + + + + + + + 

Urinary bladder + + A + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 

Histiocytic sarcoma X X 

Lymphoma malignant X X X X X X X X X 
 

 

Isobutene, NTP TR 487 175 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 2,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Total 

Carcass ID Number 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 Tissues/ 

7 8 9 0 2 4 5 7 8 9 0 1 2 3 7 8 9 0 2 3 4 6 7 8 0 Tumors 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Histiocytic sarcoma 2 

Lymphoma malignant X X X X X X X X 17 
 

 

 

 

 

 

 

 

 

 

 

 

176 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

Number of Days on Study 

5 

2 

7 

5 

6 

5 

5 

8 

3 

5 

9 

6 

6 

1 

7 

6 

4 

5 

6 

5 

3 

6 

7 

2 

6 

8 

7 

7 

0 

1 

7 

0 

3 

7 

0 

9 

7 

0 

9 

7 

1 

5 

7 

2 

1 

7 

2 

1 

7 

2 

9 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

7 

3 

5 

Carcass ID Number 

7 

2 

9 

7 

0 

1 

7 

1 

2 

7 

2 

8 

7 

4 

4 

7 

3 

2 

7 

0 

3 

7 

0 

7 

7 

3 

8 

7 

1 

0 

7 

4 

2 

7 

3 

6 

7 

4 

7 

7 

1 

6 

7 

1 

1 

7 

1 

9 

7 

0 

8 

7 

0 

2 

7 

0 

4 

7 

0 

5 

7 

0 

6 

7 

0 

9 

7 

1 

3 

7 

1 

4 

7 

1 

5 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 

Gallbladder + A + + + + M + + + A + + + + A + + + + + + + + + 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 

Intestine large, rectum + + + I + + + + + + + + + + + A + + + + + + + + + 

Intestine large, cecum + A + + + + + + + + + + + + + A + + + + + + + + + 

Intestine small, duodenum + A + + + + + + + + + + + + + A + + + + + + + + + 

Intestine small, jejunum + A + + + + + + + A + + + + + A + + + + + + + + + 

Intestine small, ileum + A + + + + + + + + A + + + + A + + + + + + + + + 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 

Hepatocellular carcinoma X X X X X X X 

Hepatocellular carcinoma, multiple 

Hepatocellular adenoma X X X X 

Hepatocellular adenoma, multiple X X X 

Mesentery + + + + + + 

Pancreas + A + + + + + + + + + + + + + + + + + + + + + + + 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 

Stomach, forestomach + + + + + + + + + + + + + + + A + + + + + + + + + 

Stomach, glandular + + + + + + + + + + + + + + + A + + + + + + + + + 

Tongue + 

Squamous cell papilloma X 

Tooth + 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 

Capsule, adenoma 

Adrenal medulla + + + + + + + I + + + + + + + + + + + + + + + + + 

Pheochromocytoma benign X 

Bilateral, pheochromocytoma malignant X 

Islets, pancreatic + A + + + + + + + + + + + + + + + + + + + + + + + 

Parathyroid gland + + + + M + + + + + M M + + M + + + + + + + + + + 

Pituitary gland + + + + M + + + + + + + + + + + + + + + + + + + + 

Pars distalis, adenoma X X X X X X 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 

Follicular cell, adenoma X 

Follicular cell, adenoma, multiple 

Follicular cell, carcinoma 

General Body System 

None 

Genital System 

Clitoral gland + + M + + + M + + + + + + + M + + + M + + + + + + 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 

Cystadenoma 

Granulosa-theca tumor benign 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 
 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 177 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 Total 

Carcass ID Number 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 Tissues/ 

7 8 0 1 2 3 4 5 6 7 0 1 3 4 5 7 9 0 1 3 5 6 8 9 0 Tumors 

Alimentary System 

Esophagus + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Gallbladder M + + + + M + + + + + M + + + + + + + + + + + + + 43 

Intestine large, colon + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Intestine large, rectum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine large, cecum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, duodenum + + + + + + + + + + + + + + + + + + + + + + + + + 48 

Intestine small, jejunum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Intestine small, ileum + + + + + + + + + + + + + + + + + + + + + + + + + 47 

Liver + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hepatocellular carcinoma X X 9 

Hepatocellular carcinoma, multiple X X 2 

Hepatocellular adenoma X X X X X X X X X 13 

Hepatocellular adenoma, multiple X X X X 7 

Mesentery + + + + 10 

Pancreas + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Salivary glands + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Stomach, forestomach + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Stomach, glandular + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Tongue 1 

Squamous cell papilloma 1 

Tooth 1 

Cardiovascular System 

Heart + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Endocrine System 

Adrenal cortex + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Capsule, adenoma X 1 

Adrenal medulla + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pheochromocytoma benign X 2 

Bilateral, pheochromocytoma malignant 1 

Islets, pancreatic + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Parathyroid gland + + + + + + M + + M + + M + + + M + + + + M + + + 41 

Pituitary gland + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Pars distalis, adenoma X X X X X X 12 

Thyroid gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Follicular cell, adenoma 1 

Follicular cell, adenoma, multiple X 1 

Follicular cell, carcinoma X 1 

General Body System 

None 

Genital System 

Clitoral gland + + + + M + + + + + + + + + + + + + + + + + + + + 45 

Ovary + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Cystadenoma X X 2 

Granulosa-theca tumor benign X 1 

Uterus + + + + + + + + + + + + + + + + + + + + + + + + + 50 
 

 

 

 

 

 

 

 

 

 

178 Isobutene, NTP TR 487 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

5 5 5 5 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 2 

7 

6 

5 

8 

3 

9 

6 

1 

7 

4 

5 

5 

3 

7 

2 

8 

7 

0 

1 

0 

3 

0 

9 

0 

9 

1 

5 

2 

1 

2 

1 

2 

9 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

3 

5 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Carcass ID Number 2 

9 

0 

1 

1 

2 

2 

8 

4 

4 

3 

2 

0 

3 

0 

7 

3 

8 

1 

0 

4 

2 

3 

6 

4 

7 

1 

6 

1 

1 

1 

9 

0 

8 

0 

2 

0 

4 

0 

5 

0 

6 

0 

9 

1 

3 

1 

4 

1 

5 

Hematopoietic System 

Bone marrow 

Lymph node 

Lymph node, bronchial 

Lymph node, mandibular 

Lymph node, mesenteric 

Lymph node, mediastinal 

Spleen 

Hemangiosarcoma 

Thymus 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + 

+ + + + + M + + + + M + + + + + + + M + + + + + + 

M + + + + M M + + M + + M + + + + + + + + M M + + 

+ + + + + + + + + + + + + + + A + + + + + + + + + 

+ + + + + + M + + + + + + + + + + + + + M + + M M 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

+ + + + + + + + + + + M + + + + + + + + + + + + + 

Integumentary System 

Mammary gland 

Skin 

Subcutaneous tissue, fibrosarcoma 

Subcutaneous tissue, sarcoma 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X X 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx 

Lung 

Alveolar/bronchiolar adenoma 

Alveolar/bronchiolar carcinoma 

Hepatocellular carcinoma, metastatic, liver 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 

Nose 

Trachea 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X 

X 

X 

X 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Special Senses System 

Eye 

Harderian gland 

Adenoma 

Carcinoma 

+ 

+ 

X 

Urinary System 

Kidney 

Urinary bladder 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

Systemic Lesions 

Multiple organs 

Lymphoma malignant 

+ + + + + + + + + + + + + + + + + + + + + + + + + 

X X X X X X X X X 
 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 179 

TABLE D2 

Individual Animal Tumor Pathology of Female Mice in the 2-Year Inhalation Study of Isobutene: 8,000 ppm 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 

Number of Days on Study 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 

5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 Total 

Carcass ID Number 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 Tissues/ 

7 8 0 1 2 3 4 5 6 7 0 1 3 4 5 7 9 0 1 3 5 6 8 9 0 Tumors 

Hematopoietic System 

Bone marrow + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymph node 4 

Lymph node, bronchial + + M + + + + + + + + + + + M + M + + + + + + + + 44 

Lymph node, mandibular + + + M + + + + + M + + + + + + + M + + + + M + M 38 

Lymph node, mesenteric + + + + + + + + + + + + + + + + + + + + + + + + + 49 

Lymph node, mediastinal + + + + + + + M + + + + + M + + + + + M + + + M + 42 

Spleen + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Hemangiosarcoma 1 

Thymus + M + + + + M + + + + + + + + + + + + + + M + + + 46 

Integumentary System 

Mammary gland + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Skin + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Subcutaneous tissue, fibrosarcoma 1 

Subcutaneous tissue, sarcoma 2 

Musculoskeletal System 

Bone + + + + + + + + + + + + + + + + + + + + + + + + + 

Nervous System 

Brain + + + + + + + + + + + + + + + + + + + + + + + + + 

Respiratory System 

Larynx + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lung + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Alveolar/bronchiolar adenoma X 2 

Alveolar/bronchiolar carcinoma 1 

Hepatocellular carcinoma, metastatic, liver 1 

Pheochromocytoma malignant, metastatic, 

adrenal medulla 1 

Nose + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Trachea + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Special Senses System 

Eye 1 

Harderian gland + 2 

Adenoma X 1 

Carcinoma 1 

Urinary System 

Kidney + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Urinary bladder + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Systemic Lesions 

Multiple organs + + + + + + + + + + + + + + + + + + + + + + + + + 50 

Lymphoma malignant X X X X X X X X X X 19 

50 

50 
180 Isobutene, NTP TR 487 

TABLE D3 

Statistical Analysis of Primary Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Adrenal Medulla: Benign or Malignant Pheochromocytoma a Overall rate 0/48 (0%) 1/49 (2%) 2/49 (4%) 3/49 (6%) 

Adjusted rateb 0.0% 2.3% 4.4% 6.7% c Terminal rate 0/32 (0%) 0/31 (0%) 2/39 (5%) 3/33 (9%) e First incidence (days) — 639 735 (T) 735 (T) 

Poly-3 testd P=0.116 P=0.509 P=0.257 P=0.133 

Liver: Hepatocellular Adenoma 

Overall rate 20/47 (43%) 22/50 (44%) 19/50 (38%) 20/50 (40%)
 

Adjusted rate 46.3% 49.2% 40.8% 43.1%
 

Terminal rate 16/32 (50%) 17/31 (55%) 17/39 (44%) 16/33 (48%)
 

First incidence (days) 621 611 543 596
 

Poly-3 test P=0.389N P=0.474 P=0.379N P=0.462N
 

Liver: Hepatocellular Carcinoma 

Overall rate 5/47 (11%) 8/50 (16%) 7/50 (14%) 11/50 (22%)
 

Adjusted rate 11.7% 17.7% 15.0% 22.9%
 

Terminal rate 4/32 (13%) 3/31 (10%) 5/39 (13%) 4/33 (12%)
 

First incidence (days) 610 567 600 527
 

Poly-3 test P=0.133 P=0.315 P=0.442 P=0.132 

Liver: Hepatocellular Adenoma or Carcinoma 

Overall rate 23/47 (49%) 28/50 (56%) 22/50 (44%) 28/50 (56%)
 

Adjusted rate 52.7% 60.0% 46.4% 58.1%
 

Terminal rate 18/32 (56%) 18/31 (58%) 18/39 (46%) 19/33 (58%)
 

First incidence (days) 610 567 543 527
 

Poly-3 test P=0.399 P=0.309 P=0.348N P=0.377 

Liver: Hepatocellular Carcinoma or Hepatoblastoma 

Overall rate 5/47 (11%) 10/50 (20%) 7/50 (14%) 11/50 (22%)
 

Adjusted rate 11.7% 22.0% 15.0% 22.9%
 

Terminal rate 4/32 (13%) 4/31 (13%) 5/39 (13%) 4/33 (12%)
 

First incidence (days) 610 567 600 527
 

Poly-3 test P=0.206 P=0.158 P=0.442 P=0.132 

Liver: Hepatocellular Adenoma, Hepatocellular Carcinoma, or Hepatoblastoma 

Overall rate 23/47 (49%) 28/50 (56%) 22/50 (44%) 28/50 (56%)
 

Adjusted rate 52.7% 60.0% 46.4% 58.1%
 

Terminal rate 18/32 (56%) 18/31 (58%) 18/39 (46%) 19/33 (58%)
 

First incidence (days) 610 567 543 527
 

Poly-3 test P=0.399 P=0.309 P=0.348N P=0.377 

Lung: Alveolar/bronchiolar Carcinoma 

Overall rate 4/49 (8%) 2/50 (4%) 2/50 (4%) 1/50 (2%) 

Adjusted rate 9.3% 4.6% 4.4% 2.2% 

Terminal rate 1/32 (3%) 1/31 (3%) 2/39 (5%) 1/33 (3%) 

First incidence (days) 639 639 735 (T) 735 (T) 

Poly-3 test P=0.193N P=0.331N P=0.311N P=0.161N 

Lung: Alveolar/bronchiolar Adenoma or Carcinoma 

Overall rate 6/49 (12%) 4/50 (8%) 4/50 (8%) 3/50 (6%) 

Adjusted rate 13.8% 9.1% 8.8% 6.5% 

Terminal rate 2/32 (6%) 2/31 (6%) 4/39 (10%) 2/33 (6%) 

First incidence (days) 639 639 735 (T) 701 

Poly-3 test P=0.248N P=0.360N P=0.338N P=0.215N 
Isobutene, NTP TR 487 181 

TABLE D3 

Statistical Analysis of Primary Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Ovary: Cystadenoma 

Overall rate 0/48 (0%) 3/50 (6%) 1/48 (2%) 2/50 (4%) 

Adjusted rate 0.0% 6.8% 2.3% 4.4% 

Terminal rate 0/32 (0%) 1/31 (3%) 1/38 (3%) 2/33 (6%) 

First incidence (days) — 572 735 (T) 735 (T) 

Poly-3 test P=0.486 P=0.125 P=0.509 P=0.255 

Pituitary Gland (Pars Distalis): Adenoma 

Overall rate 8/49 (16%) 13/49 (27%) 12/49 (24%) 12/49 (24%)
 

Adjusted rate 18.8% 29.8% 26.7% 26.2%
 

Terminal rate 6/32 (19%) 10/31 (32%) 11/39 (28%) 8/33 (24%)
 

First incidence (days) 709 615 709 672
 

Poly-3 test P=0.462 P=0.171 P=0.265 P=0.279 

Pituitary Gland (Pars Distalis): Adenoma or Carcinoma 

Overall rate 8/49 (16%) 13/49 (27%) 13/49 (27%) 12/49 (24%)
 

Adjusted rate 18.8% 29.8% 28.4% 26.2%
 

Terminal rate 6/32 (19%) 10/31 (32%) 11/39 (28%) 8/33 (24%)
 

First incidence (days) 709 615 477 672
 

Poly-3 test P=0.476 P=0.171 P=0.206 P=0.279 

Skin (Subcutaneous Tissue): Fibrous Histiocytoma, Fibrosarcoma, or Sarcoma 

Overall rate 2/50 (4%) 5/50 (10%) 1/50 (2%) 3/50 (6%) 

Adjusted rate 4.7% 11.3% 2.2% 6.5% 

Terminal rate 0/32 (0%) 3/31 (10%) 0/39 (0%) 1/33 (3%) 

First incidence (days) 667 611 692 709 

Poly-3 test P=0.578N P=0.228 P=0.476N P=0.532 

Spleen: Hemangiosarcoma 

Overall rate 3/49 (6%) 0/50 (0%) 0/50 (0%) 1/50 (2%) 

Adjusted rate 7.1% 0.0% 0.0% 2.2% 

Terminal rate 2/32 (6%) 0/31 (0%) 0/39 (0%) 1/33 (3%) 

First incidence (days) 709 — — 735 (T) 

Poly-3 test P=0.524N P=0.115N P=0.106N P=0.280N 

Thyroid Gland (Follicular Cell): Adenoma 

Overall rate 1/48 (2%) 4/50 (8%) 3/49 (6%) 2/50 (4%) 

Adjusted rate 2.4% 9.2% 6.6% 4.4% 

Terminal rate 1/32 (3%) 3/31 (10%) 3/39 (8%) 2/33 (6%) 

First incidence (days) 735 (T) 695 735 (T) 735 (T) 

Poly-3 test P=0.499N P=0.187 P=0.330 P=0.527 

Thyroid Gland (Follicular Cell): Adenoma or Carcinoma 

Overall rate 1/48 (2%) 4/50 (8%) 4/49 (8%) 3/50 (6%) 

Adjusted rate 2.4% 9.2% 8.8% 6.6% 

Terminal rate 1/32 (3%) 3/31 (10%) 4/39 (10%) 3/33 (9%) 

First incidence (days) 735 (T) 695 735 (T) 735 (T) 

Poly-3 test P=0.558 P=0.187 P=0.200 P=0.334 

Uterus: Stromal Polyp 

Overall rate 3/50 (6%) 1/50 (2%) 1/50 (2%) 0/50 (0%)
 

Adjusted rate 7.0% 2.3% 2.2% 0.0%
 

Terminal rate 2/32 (6%) 1/31 (3%) 1/39 (3%) 0/33 (0%)
 

First incidence (days) 709 735 (T) 735 (T) —
 

Poly-3 test P=0.132N P=0.298N P=0.280N P=0.106N
 
182 Isobutene, NTP TR 487 

TABLE D3 

Statistical Analysis of Primary Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

All Organs: Hemangiosarcoma 

Overall rate 3/50 (6%) 1/50 (2%) 3/50 (6%) 1/50 (2%) 

Adjusted rate 7.0% 2.3% 6.5% 2.2% 

Terminal rate 2/32 (6%) 1/31 (3%) 2/39 (5%) 1/33 (3%) 

First incidence (days) 709 735 (T) 618 735 (T) 

Poly-3 test P=0.323N P=0.298N P=0.625N P=0.280N 

All Organs: Hemangioma or Hemangiosarcoma 

Overall rate 3/50 (6%) 2/50 (4%) 4/50 (8%) 1/50 (2%) 

Adjusted rate 7.0% 4.6% 8.7% 2.2% 

Terminal rate 2/32 (6%) 2/31 (6%) 3/39 (8%) 1/33 (3%) 

First incidence (days) 709 735 (T) 618 735 (T) 

Poly-3 test P=0.236N P=0.492N P=0.543 P=0.280N 

All Organs: Histiocytic Sarcoma 

Overall rate 2/50 (4%) 4/50 (8%) 2/50 (4%) 0/50 (0%) 

Adjusted rate 4.7% 9.1% 4.3% 0.0% 

Terminal rate 2/32 (6%) 2/31 (6%) 1/39 (3%) 0/33 (0%) 

First incidence (days) 735 (T) 615 628 — 

Poly-3 test P=0.077N P=0.354 P=0.664N P=0.221N 

All Organs: Malignant Lymphoma 

Overall rate 16/50 (32%) 9/50 (18%) 17/50 (34%) 19/50 (38%) 

Adjusted rate 35.9% 20.4% 36.0% 40.7% 

Terminal rate 12/32 (38%) 7/31 (23%) 12/39 (31%) 14/33 (42%) 

First incidence (days) 378 506 543 583 

Poly-3 test P=0.119 P=0.079N P=0.582 P=0.398 

All Organs: Benign Neoplasms 

Overall rate 29/50 (58%) 35/50 (70%) 33/50 (66%) 32/50 (64%) 

Adjusted rate 65.0% 74.8% 70.8% 68.4% 

Terminal rate 21/32 (66%) 24/31 (77%) 30/39 (77%) 26/33 (79%) 

First incidence (days) 449 572 543 596 

Poly-3 test P=0.490N P=0.205 P=0.351 P=0.450 

All Organs: Malignant Neoplasms 

Overall rate 29/50 (58%) 29/50 (58%) 30/50 (60%) 31/50 (62%) 

Adjusted rate 60.9% 60.7% 60.8% 63.2% 

Terminal rate 15/32 (47%) 17/31 (55%) 20/39 (51%) 18/33 (55%) 

First incidence (days) 322 506 477 527 

Poly-3 test P=0.440 P=0.577N P=0.581N P=0.489 
183 

c 

Isobutene, NTP TR 487 

TABLE D3 

Statistical Analysis of Primary Neoplasms in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

All Organs: Benign or Malignant Neoplasms 

Overall rate 43/50 (86%) 45/50 (90%) 44/50 (88%) 44/50 (88%) 

Adjusted rate 87.7% 91.3% 89.2% 89.1% 

Terminal rate 26/32 (81%) 28/31 (90%) 34/39 (87%) 29/33 (88%) 

First incidence (days) 322 506 477 527 

Poly-3 test P=0.568N P=0.397 P=0.534 P=0.541 

(T)Terminal sacrifice 

a Number of neoplasm-bearing animals/number of animals examined. Denominator is number of animals examined microscopically for adrenal gland, liver, 

lung, ovary, pituitary gland, spleen, and thyroid gland; for other tissues, denominator is number of animals necropsied. b	 Poly-3 estimated neoplasm incidence after adjustment for intercurrent mortality 

Observed incidence at terminal kill 

d	 Beneath the chamber control incidence are the P values associated with the trend test. Beneath the exposed group incidence are the P values corresponding to 

pairwise comparisons between the chamber controls and that exposed group. The Poly-3 test accounts for differential mortality in animals that do not reach 

terminal sacrifice. A negative trend or a lower incidence in an exposure group is indicated by N. e Not applicable; no neoplasms in animal group 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

184 Isobutene, NTP TR 487 

TABLE D4 

Summary of the Incidence of Nonneoplastic Lesions in Female Mice in the 2-Year Inhalation Study of Isobutenea 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Disposition Summary 

Animals initially in study 50 50 50 50 

Early deaths 

Moribund 13 16 6 13 

Natural deaths 5 3 5 4 

Survivors 

Terminal sacrifice 32 31 39 33 

Animals examined microscopically 50 50 50 50 

Alimentary System 

Intestine large, rectum (45) (45) (47) (48) 

Artery, inflammation, chronic active 1 (2%) 

Intestine small, ileum (46) (48) (46) (47) 

Inflammation, chronic active 1 (2%) 

Liver (47) (50) (50) (50) 

Angiectasis 1 (2%) 1 (2%) 1 (2%) 

Basophilic focus 4 (9%) 4 (8%) 2 (4%) 5 (10%) 

Clear cell focus 4 (8%) 3 (6%) 4 (8%) 

Cyst 1 (2%) 1 (2%) 

Degeneration, fatty 2 (4%) 1 (2%) 5 (10%) 1 (2%) 

Eosinophilic focus 17 (36%) 11 (22%) 12 (24%) 7 (14%) 

Hematopoietic cell proliferation 1 (2%) 1 (2%) 

Mixed cell focus 1 (2%) 

Necrosis 2 (4%) 3 (6%) 1 (2%) 

Tension lipidosis 1 (2%) 3 (6%) 1 (2%) 

Bile duct, hyperplasia 1 (2%) 

Centrilobular, necrosis 1 (2%) 3 (6%) 

Mesentery (7) (8) (8) (10) 

Artery, inflammation 1 (13%) 1 (10%) 

Fat, inflammation, chronic active 1 (10%) 

Fat, necrosis 7 (100%) 6 (75%) 6 (75%) 10 (100%) 

Pancreas (47) (50) (48) (49) 

Atrophy 1 (2%) 

Basophilic focus 1 (2%) 2 (4%) 

Lipomatosis 1 (2%) 1 (2%) 

Duct, cyst 1 (2%) 1 (2%) 

Stomach, forestomach (48) (49) (49) (49) 

Hyperplasia 2 (4%) 4 (8%) 1 (2%) 1 (2%) 

Inflammation, acute 1 (2%) 2 (4%) 

Stomach, glandular (46) (48) (48) (49) 

Necrosis 1 (2%) 

Artery, inflammation, chronic active 1 (2%) 

Tooth (1) (1) 

Malformation 1 (100%) 1 (100%) 

Cardiovascular System 

Heart (50) (50) (50) (50) 

Cardiomyopathy 10 (20%) 10 (20%) 12 (24%) 8 (16%) 

Mineralization 1 (2%) 

a Number of animals examined microscopically at the site and the number of animals with lesion 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isobutene, NTP TR 487 185 

TABLE D4 

Summary of the Incidence of Nonneoplastic Lesions in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Endocrine System 

Adrenal cortex (49) (50) (49) (50) 

Hematopoietic cell proliferation 1 (2%) 1 (2%) 1 (2%) 

Hyperplasia 3 (6%) 4 (8%) 3 (6%) 5 (10%) 

Hypertrophy 3 (6%) 1 (2%) 4 (8%) 2 (4%) 

Vacuolization cytoplasmic 1 (2%) 2 (4%) 1 (2%) 2 (4%) 

Adrenal medulla (48) (49) (49) (49) 

Hyperplasia 3 (6%) 

Islets, pancreatic (47) (49) (48) (49) 

Hyperplasia 1 (2%) 1 (2%) 3 (6%) 

Pituitary gland (49) (49) (49) (49) 

Pars distalis, hyperplasia 13 (27%) 15 (31%) 19 (39%) 14 (29%) 

Thyroid gland (48) (50) (49) (50) 

Follicular cell, hyperplasia 25 (52%) 16 (32%) 29 (59%) 30 (60%) 

General Body System 

None 

Genital System 

Ovary (48) (50) (48) (50) 

Angiectasis 1 (2%) 3 (6%) 

Cyst 13 (27%) 14 (28%) 11 (23%) 8 (16%) 

Inflammation, suppurative 1 (2%) 

Uterus (49) (50) (50) (50) 

Angiectasis 1 (2%) 

Hydrometra 6 (12%) 7 (14%) 3 (6%) 4 (8%) 

Inflammation, suppurative 1 (2%) 1 (2%) 

Necrosis 1 (2%) 

Thrombosis 1 (2%) 

Endometrium, hyperplasia, cystic 1 (2%) 

Hematopoietic System 

Bone marrow (48) (49) (49) (50) 

Necrosis 1 (2%) 

Lymph node (7) (2) (5) (4) 

Lumbar, hemorrhage 1 (14%) 

Lymph node, bronchial (38) (33) (44) (44) 

Hyperplasia, plasma cell 1 (3%) 

Lymph node, mandibular (41) (42) (44) (38) 

Hyperplasia, lymphoid 1 (2%) 

Infiltration cellular, mixed cell 1 (2%) 

Lymph node, mesenteric (46) (48) (47) (49) 

Angiectasis 1 (2%) 2 (4%) 

Infiltration cellular, plasma cell 1 (2%) 

Infiltration cellular, mixed cell 1 (2%) 

Lymph node, mediastinal (38) (42) (36) (42) 

Infiltration cellular, mixed cell 1 (3%) 

Spleen (49) (50) (50) (50) 

Hematopoietic cell proliferation 4 (8%) 12 (24%) 6 (12%) 5 (10%) 

Infarct 1 (2%) 

Infiltration cellular, histiocyte 1 (2%) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

186 Isobutene, NTP TR 487 

TABLE D4 

Summary of the Incidence of Nonneoplastic Lesions in Female Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Control 500 ppm 2,000 ppm 8,000 ppm 

Integumentary System 

Skin 

Inflammation, acute

Inflammation, chronic active

(49) 

 1 (2%) 

(50) 

 1 (2%) 

 1 (2%) 

(49) (50) 

 1 (2%) 

Musculoskeletal System 

None 

Nervous System 

Brain (49) (50) (50) (50) 

Inflammation, chronic active 1 (2%) 

Respiratory System 

Lung (49) (50) (50) (50) 

Hemorrhage 1 (2%) 

Infiltration cellular, histiocyte 1 (2%) 1 (2%) 

Inflammation, granulomatous 1 (2%) 

Metaplasia, osseous 1 (2%) 

Alveolar epithelium, hyperplasia 4 (8%) 3 (6%) 3 (6%) 1 (2%) 

Nose (47) (50) (49) (50) 

Inflammation, chronic 1 (2%) 

Inflammation, suppurative 1 (2%) 1 (2%) 

Olfactory epithelium, atrophy 1 (2%) 

Olfactory epithelium, degeneration, hyaline 17 (36%) 19 (38%) 24 (49%) 27 (54%) 

Respiratory epithelium, degeneration, hyaline 21 (45%) 39 (78%) 41 (84%) 48 (96%) 

Special Senses System 

Eye (1) (1) (1) 

Degeneration 1 (100%) 

Cornea, metaplasia, squamous 1 (100%) 

Urinary System 

Kidney (49) (50) (49) (50) 

Amyloid deposition 1 (2%) 

Cyst 1 (2%) 

Infarct 1 (2%) 2 (4%) 1 (2%) 

Metaplasia, osseous 1 (2%) 1 (2%) 2 (4%) 

Mineralization 1 (2%) 

Nephropathy 13 (27%) 14 (28%) 18 (37%) 12 (24%) 

Renal tubule, necrosis 1 (2%) 

Urinary bladder (45) (49) (49) (50) 

Artery, inflammation, chronic active 1 (2%) 
187 

APPENDIX E
 

GENETIC TOXICOLOGY
 

SALMONELLA TYPHIMURIUM MUTAGENICITY TEST PROTOCOL ............................. 188
 

MOUSE PERIPHERAL BLOOD MICRONUCLEUS TEST PROTOCOL ............................ 188
 

RESULTS ................................................................ 189
 

TABLE E1 Mutagenicity of Isobutene in Salmonella typhimurium ......................... 190
 

TABLE E2 Frequency of Micronuclei in Peripheral Blood Erythrocytes of Mice
 

Following Treatment with Isobutene by Inhalation for 14 Weeks .................. 191
 
188 Isobutene, NTP TR 487 

GENETIC TOXICOLOGY 

SALMONELLA TYPHIMURIUM MUTAGENICITY TEST PROTOCOL 

Because isobutene is a gas, it was tested in a desiccator (Zeiger et al., 1992). The Salmonella typhimurium 

strains (TA97, TA98, TA100, and TA1535) and buffer or S9 mix (metabolic activation enzymes and 

cofactors from Aroclor 1254-induced male F344 rat or Syrian hamster liver) were incorporated into the top 

agar and poured onto minimal medium plates. The lids of the plates were slightly raised, and the plates 

were stacked in glass desiccator jars. The air was evacuated. A measured amount of isobutene was 

introduced, and air was allowed to enter until atmospheric pressure was reached. The desiccator was then 

sealed and placed in a 37E C incubator for 24 hours, after which time the desiccator was moved to a safety 

hood, the lid was opened, and the residual test gas was allowed to dissipate. The plates were removed from 

the desiccator and incubated at 37E C for an additional 24 hours. The concentration was expressed as moles 

of isobutene per desiccator. 

Each trial consisted of triplicate plates of concurrent positive and negative controls and five doses of 

isobutene. The high dose of 0.027 mol/dessicator was limited by toxicity. 

In this assay, a positive response is defined as a reproducible, dose-related increase in histidine-independent 

(revertant) colonies in any one strain/activation combination. An equivocal response is defined as an 

increase in revertants that is not dose related, is not reproducible, or is not of sufficient magnitude to 

support a determination of mutagenicity. A negative response is obtained when no increase in revertant 

colonies is observed following chemical treatment. There is no minimum percentage or fold increase 

required for a chemical to be judged positive or weakly positive. 

MOUSE PERIPHERAL BLOOD MICRONUCLEUS TEST PROTOCOL 

A detailed discussion of this assay is presented in MacGregor et al. (1990). At the end of the 14-week 

toxicity study, peripheral blood samples were obtained from male and female mice. Smears were 

immediately prepared and fixed in absolute methanol. The methanol-fixed slides were stained with acridine 

orange and coded. Slides were scanned to determine the frequency of micronuclei in 2,000 normochromatic 

erythrocytes (NCEs) in each of five male and five female mice per dose group. 

The results were tabulated as the mean of the pooled results from all animals within a treatment group plus 

or minus the standard error of the mean. The frequency of micronucleated cells among NCEs was analyzed 

by a statistical software package that tested for increasing trend over exposure groups with a one-tailed 

Cochran-Armitage trend test, followed by pairwise comparisons between each exposed group and the 

control group (Margolin et al., 1990). In the presence of excess binomial variation, as detected by a 

binomial dispersion test, the binomial variance of the Cochran-Armitage test was adjusted upward in 

proportion to the excess variation. In the micronucleus test, an individual trial is considered positive if the 

trend test P value is less than or equal to 0.025 or if the P value for any single exposed group is less than or 

equal to 0.025 divided by the number of exposure groups. A final call of positive for micronucleus 

induction is preferably based on reproducibly positive trials (as noted above). Ultimately, the final call is 

determined by the scientific staff after considering the results of statistical analyses, reproducibility of any 

effects observed, and the magnitudes of those effects. 
Isobutene, NTP TR 487 189 

RESULTS 

Isobutene (0.001 to 0.027 mol/desiccator) was not mutagenic in S. typhimurium strain TA97, TA98, 

TA100, or TA1535, with or without induced rat or hamster liver S9 enzymes (Table E1). In vivo, no 

increase in the frequency of micronucleated NCEs was seen in peripheral blood samples from male or 

female mice administered isobutene via inhalation for 14 weeks (Table E2). 
 

190 Isobutene, NTP TR 487 

TABLE E1 

Mutagenicity of Isobutene in Salmonella typhimuriuma 

Strain	 Revertants/plateb

 Concentration !S9 + hamster S9 + rat S9 

(mol/dessicator) Trial 1 Trial 2 10% 30% 10% 30% 

TA100 Air 110 ± 7.5 143 ± 14.3 

0.001 98 ± 1.9 152 ± 5.3

0.002 100 ± 4.0 151 ± 11.0

0.007 101 ± 4.9 135 ± 2.6 

0.013 81 ± 1.9 124 ± 11.9

0.027 68 ± 11.7 84 ± 1.2

131 ± 2.2 127 ± 4.7 

132 ± 13.5 121 ± 8.0 

128 ± 11.6 114 ± 8.6 

130 ± 3.2 117 ± 7.2 

112 ± 11.8 87 ± 4.7 

127 ± 17.1 79 ± 5.9 

142 ± 11.0 141 ± 10.4 

131 ± 12.9 147 ± 12.5 

142 ± 1.9 124 ± 9.1 

146 ± 5.5 134 ± 4.3 

117 ± 10.6 95 ± 0.9 

89 ± 10.4 83 ± 7.2 

Trial summary Negative Negative 

Positive control 432 ± 11.8 383 ± 12.4 c Negative Negative 

332 ± 15.3 859 ± 31.9 

Negative Negative 

914 ± 55.4 583 ± 11.5 

TA1535 Air 10 ± 2.6 8 ± 1.5 

0.001 11 ± 1.2 10 ± 1.2 

0.002 15 ± 2.0 14 ± 2.3 

0.007 13 ± 1.0 11 ± 0.6 

0.013 10 ± 1.8 14 ± 0.9 

0.027 9 ± 0.9 13 ± 2.0 

12 ± 0.9 22 ± 4.2 

13 ± 1.3 15 ± 2.1 

10 ± 2.0 16 ± 0.9 

11 ± 1.3 16 ± 2.0 

13 ± 1.7 16 ± 1.3 

13 ± 3.1 16 ± 2.2 

15 ± 1.2 14 ± 3.2 

11 ± 1.5 20 ± 2.7 

13 ± 1.9 20 ± 3.9 

12 ± 3.2 14 ± 3.7 

13 ± 3.0 16 ± 1.9 

13 ± 2.1 13 ± 2.9 

Trial summary Negative Negative 

Positive control 359 ± 14.5 340 ± 8.8 

Negative Negative 

63 ± 4.8 129 ± 5.7 

Negative Negative 

164 ± 9.6 121 ± 11.6 

TA97 Air 153 ± 7.3 153 ± 3.3 

0.001 142 ± 3.2 140 ± 7.1 

0.002 139 ± 2.7 139 ± 6.8 

0.007 135 ± 9.5 140 ± 7.5 

0.013 134 ± 4.3 155 ± 7.6 

0.027 121 ± 1.2 161 ± 6.6 

134 ± 5.5 185 ± 13.0 

136 ± 9.0 195 ± 13.3 

157 ± 16.1 194 ± 11.6 

135 ± 4.3 194 ± 3.0 

137 ± 8.0 175 ± 5.5 

125 ± 4.4 149 ± 2.0 

155 ± 5.2 212 ± 5.0 

168 ± 8.8 196 ± 1.5 

170 ± 7.6 205 ± 6.1 

162 ± 2.6 225 ± 15.9 

147 ± 8.0 209 ± 16.8 

139 ± 8.7 189 ± 13.5 

Trial summary Negative Negative 

Positive control 583 ± 35.8 432 ± 38.4 

Negative Negative 

2,035 ± 28.9 1,505 ± 74.3 

Negative Negative 

942 ± 19.7 759 ± 44.8 

TA98 Air 17 ± 2.2 15 ± 2.2 

0.001 19 ± 1.5 13 ± 2.3 

0.002 17 ± 1.5 14 ± 0.7 

0.007 15 ± 0.7 14 ± 0.9 

0.013 18 ± 3.9 13 ± 0.6 

0.027 11 ± 1.5 10 ± 1.0 

24 ± 2.9 21 ± 1.5 

19 ± 1.0 23 ± 2.9 

22 ± 1.9 26 ± 3.8 

26 ± 3.8 20 ± 1.2 

19 ± 2.4 19 ± 2.7 

18 ± 2.6 14 ± 1.5 

26 ± 4.6 26 ± 5.5 

17 ± 0.6 25 ± 1.2 

17 ± 2.7 31 ± 2.9 

20 ± 2.0 29 ± 4.6 

27 ± 3.6 19 ± 4.1 

23 ± 0.3 15 ± 1.5 

Trial summary Negative Negative 

Positive control 377 ± 14.1 316 ± 11.1 

Negative Negative 

265 ± 8.0 979 ± 2.0 

Negative Negative 

314 ± 22.1 321 ± 16.7 

a Study was performed at Microbiological Associates, Inc. The detailed protocol is presented in Zeiger et al. (1992). Air was the control. b	 Revertants are presented as mean ± standard error from three plates. 

The positive controls in the absence of metabolic activation were sodium azide (TA100 and TA1535), 9-aminoacridine (TA97), and 

4-nitro-o-phenylenediamine (TA98). The positive control for metabolic activation with all strains was 2-aminoanthracene. 

c 
191 

c 

Isobutene, NTP TR 487 

TABLE E2 

Frequency of Micronuclei in Peripheral Blood Erythrocytes of Mice Following Treatment with Isobutene 

by Inhalation for 14 Weeksa 

Compound Concentration 

(ppm) 

with Erythrocytes 

Scored 

Number of Mice 

Micronucleated NCEs/ 

1,000 NCEsb 

NCEsb (%) 

Male 

Chamber Control 5 1.10 ± 0.37 97.20 ± 0.38 

Isobutene 500 

1,000 

2,000 

4,000 

8,000 

5 

5 

5 

5 

5 

1.40 ± 0.33 

1.70 ± 0.12 

1.70 ± 0.20 

1.60 ± 0.33 

2.00 ± 0.47 

97.42 ± 0.38 

97.20 ± 0.33 

97.68 ± 0.31 

97.58 ± 0.51 

97.50 ± 0.19 

P=0.086c 

Female 

Chamber Control 5 0.80 ± 0.20 97.38 ± 0.15 

Isobutene 500 

1,000 

2,000 

4,000 

8,000 

5 

5 

5 

5 

5 

1.00 ± 0.32 

1.10 ± 0.29 

0.60 ± 0.10 

1.20 ± 0.25 

0.80 ± 0.25 

97.42 ± 0.24 

97.28 ± 0.36 

96.38 ± 0.18 

97.06 ± 0.33 

96.72 ± 0.34 

P=0.541 

a Analysis was performed at Environmental Health Research and Testing, Inc. The detailed protocol is presented in MacGregor et al. (1990). 

NCE=normochromatic erythrocyte b Mean ± standard error
 

Significance of micronucleated NCEs/1,000 NCEs tested by the one-tailed trend test, significant at P 0.025 (Margolin et al., 1990)
 
192 Isobutene, NTP TR 487
 
193 

APPENDIX F
 

HEMATOLOGY AND CLINICAL CHEMISTRY
 

RESULTS
 

TABLE F1	 Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study 

of Isobutene .................................................... 194 
194 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male 

Hematology 

n 10 10 10 10 10 10 

Automated hematocrit (%) 

Day 3 41.6 ± 0.3 41.8 ± 0.4 41.5 ± 0.5 41.5 ± 0.7 42.0 ± 0.5 42.1 ± 0.5 

Day 23 45.7 ± 0.7 45.0 ± 0.4 44.4 ± 0.5 45.4 ± 0.2 45.6 ± 0.3 45.5 ± 0.4 

Week 14 45.8 ± 0.4 45.4 ± 0.3 44.9 ± 0.3 45.4 ± 0.4 45.8 ± 0.4 45.1 ± 0.4 

Manual hematocrit (%) 

Day 3 42.5 ± 0.3 42.6 ± 0.4 42.8 ± 0.4 42.2 ± 0.6 43.3 ± 0.4 43.4 ± 0.4 

Day 23 46.6 ± 0.7 45.9 ± 0.5 45.4 ± 0.6 46.2 ± 0.3 46.4 ± 0.3 46.5 ± 0.3 

Week 14 46.3 ± 0.4 46.8 ± 0.9 45.5 ± 0.3 45.9 ± 0.4 46.6 ± 0.3 46.0 ± 0.3 

Hemoglobin (g/dL) 

Day 3 13.9 ± 0.1 14.1 ± 0.1 13.8 ± 0.2 13.7 ± 0.2 14.0 ± 0.1 13.8 ± 0.2 

Day 23 16.0 ± 0.2 15.7 ± 0.2 15.6 ± 0.2 15.8 ± 0.1 15.7 ± 0.1 15.8 ± 0.1 

Week 14 15.4 ± 0.1 15.3 ± 0.1 15.2 ± 0.1 15.2 ± 0.1 15.3 ± 0.1 15.2 ± 0.1 6 Erythrocytes (10 /μL) 

Day 3 7.47 ± 0.11 7.55 ± 0.11 7.41 ± 0.12 7.33 ± 0.17 7.50 ± 0.12 7.37 ± 0.12 

Day 23 8.64 ± 0.11 8.55 ± 0.12 8.34 ± 0.11 8.50 ± 0.09 8.51 ± 0.06 8.56 ± 0.09 

Week 14 9.18 ± 0.05 9.08 ± 0.06 9.03 ± 0.06 9.05 ± 0.06 9.12 ± 0.06 9.03 ± 0.08 6 Reticulocytes (10 /μL) 

Day 3 0.24 ± 0.05 0.17 ± 0.03 0.18 ± 0.03 0.12 ± 0.02 0.16 ± 0.05 0.27 ± 0.03 

b b b b Day 23 0.10 ± 0.02 0.09 ± 0.02 0.07 ± 0.02 0.09 ± 0.02 0.13 ± 0.03 0.12 ± 0.02
 

Week 14 0.07 ± 0.01 0.05 ± 0.01 0.06 ± 0.01 0.07 ± 0.01 0.06 ± 0.01 0.06 ± 0.01 3 Nucleated erythrocytes (10 /μL) 

Day 3 0.11 ± 0.03 0.07 ± 0.02 0.08 ± 0.03 0.09 ± 0.03 0.11 ± 0.04 0.14 ± 0.05 

Day 23 0.02 ± 0.01 0.01 ± 0.01 0.04 ± 0.02 0.02 ± 0.01 0.01 ± 0.01 0.01 ± 0.01 

Week 14 0.03 ± 0.01 0.03 ± 0.02 0.04 ± 0.02 0.02 ± 0.01 0.03 ± 0.02 0.02 ± 0.02 

Mean cell volume (fL) 

Day 3 55.5 ± 0.7 55.4 ± 0.6 55.9 ± 0.5 56.8 ± 0.6 56.1 ± 0.3 57.2 ± 0.4 

Day 23 52.8 ± 0.4 52.6 ± 0.4 53.2 ± 0.4 53.3 ± 0.4 53.5 ± 0.4 53.1 ± 0.5 

Week 14 49.9 ± 0.3 50.0 ± 0.3 49.8 ± 0.3 50.2 ± 0.2 50.1 ± 0.3 50.1 ± 0.4 

Mean cell hemoglobin (pg) 

Day 3 18.6 ± 0.1 18.7 ± 0.1 18.7 ± 0.1 18.7 ± 0.2 18.7 ± 0.1 18.7 ± 0.1 

Day 23 18.5 ± 0.1 18.4 ± 0.1 18.7 ± 0.1 18.6 ± 0.2 18.5 ± 0.1 18.5 ± 0.1 

Week 14 16.8 ± 0.0 16.9 ± 0.1 16.8 ± 0.0 16.8 ± 0.0 16.8 ± 0.0 16.8 ± 0.1 

Mean cell hemoglobin concentration (g/dL) 

Day 3 33.4 ± 0.2 33.6 ± 0.1 33.4 ± 0.2 32.9 ± 0.2 33.3 ± 0.2 32.7 ± 0.1** 

Day 23 35.1 ± 0.2 34.9 ± 0.2 35.1 ± 0.1 34.8 ± 0.1 34.5 ± 0.2 34.8 ± 0.2 

Week 14 33.7 ± 0.2 33.7 ± 0.2 33.7 ± 0.3 33.5 ± 0.1 33.5 ± 0.1 33.6 ± 0.3 3 Platelets (10 /μL) 

Day 3 922.4 ± 25.1 878.9 ± 26.9 874.8 ± 43.1 815.7 ± 14.4* 836.6 ± 33.9 851.0 ± 14.6 

Day 23 680.2 ± 19.2 672.0 ± 13.6 669.8 ± 14.2 646.3 ± 24.1 681.5 ± 17.9 642.8 ± 30.8 

Week 14 515.5 ± 10.4 530.9 ± 12.0 535.1 ± 7.3 501.6 ± 7.4 525.9 ± 5.3 541.2 ± 7.9 3 Leukocytes (10 /μL) 

Day 3 7.85 ± 0.39 8.76 ± 0.36 8.71 ± 0.31 8.55 ± 0.32 7.84 ± 0.48 7.86 ± 0.34 

Day 23 5.56 ± 0.55 6.12 ± 0.18 5.85 ± 0.35 6.08 ± 0.45 5.87 ± 0.45 6.61 ± 0.35 

Week 14 5.73 ± 0.39 6.14 ± 0.27 6.13 ± 0.36 6.73 ± 0.30 6.04 ± 0.36 5.69 ± 0.37 3 Segmented neutrophils (10 /μL) 

Day 3 0.80 ± 0.09 0.74 ± 0.11 0.91 ± 0.12 0.77 ± 0.10 0.74 ± 0.10 0.92 ± 0.08 

Day 23 0.82 ± 0.14 0.90 ± 0.09 0.78 ± 0.07 0.83 ± 0.07 0.73 ± 0.07 0.84 ± 0.10 

Week 14 0.93 ± 0.08 1.17 ± 0.10 1.33 ± 0.12 0.95 ± 0.12 1.07 ± 0.10 1.02 ± 0.11 3 Lymphocytes (10 /μL) 

Day 3 6.81 ± 0.34 7.88 ± 0.35 7.74 ± 0.26 7.72 ± 0.24 6.87 ± 0.44 6.81 ± 0.31 

Day 23 4.68 ± 0.48 5.20 ± 0.13 5.03 ± 0.37 5.21 ± 0.42 5.11 ± 0.42 5.73 ± 0.38 

Week 14 4.70 ± 0.35 4.86 ± 0.24 4.70 ± 0.30 5.71 ± 0.25 4.93 ± 0.34 4.59 ± 0.32 
Isobutene, NTP TR 487 195 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male (continued) 

Hematology (continued) 

n 10 10 10 10 10 10 

Monocytes (10 /μL) 3 

Day 3 0.20 ± 0.05 0.12 ± 0.04 0.06 ± 0.03 0.02 ± 0.01** 0.15 ± 0.04 0.12 ± 0.03 

Day 23 0.04 ± 0.02 0.01 ± 0.01 0.02 ± 0.02 0.03 ± 0.03 0.01 ± 0.01 0.01 ± 0.01 

Week 14 0.06 ± 0.02 0.05 ± 0.02 0.06 ± 0.02 0.04 ± 0.02 0.03 ± 0.01 0.05 ± 0.02 

Eosinophils (10 /μL) 3 

Day 3 0.03 ± 0.02 0.02 ± 0.02 0.01 ± 0.01 0.03 ± 0.02 0.07 ± 0.02 0.02 ± 0.01 

Day 23 0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01 0.00 ± 0.00 0.02 ± 0.01 0.03 ± 0.01 

Week 14 0.05 ± 0.02 0.06 ± 0.02 0.04 ± 0.02 0.03 ± 0.02 0.01 ± 0.01 0.03 ± 0.02 

Clinical Chemistry 

n 

Day 3 10 10 10 10 9 10 

Day 23 10 10 10 10 10 10 

Week 14 10 10 10 10 10 10 

Urea nitrogen (mg/dL) 

Day 3 12.7 ± 0.7 14.2 ± 0.6 12.3 ± 0.4 11.1 ± 0.5 11.9 ± 0.6 11.5 ± 0.5 

Day 23 15.6 ± 0.4 14.7 ± 0.3 14.6 ± 0.3 15.2 ± 0.5 14.9 ± 0.2 14.6 ± 0.2 

Week 14 20.6 ± 0.7 19.7 ± 0.6 18.8 ± 0.7 20.4 ± 0.7 19.6 ± 0.8 19.2 ± 0.6 

Creatinine (mg/dL) 

Day 3 0.61 ± 0.05 0.62 ± 0.02 0.56 ± 0.03 0.47 ± 0.02** 0.53 ± 0.04* 0.51 ± 0.02* 

Day 23 0.73 ± 0.02 0.72 ± 0.01 0.71 ± 0.02 0.70 ± 0.02 0.72 ± 0.01 0.71 ± 0.02 

Week 14 0.86 ± 0.02 0.88 ± 0.02 0.89 ± 0.01 0.86 ± 0.02 0.86 ± 0.02 0.84 ± 0.02 

Serum glucose (mg/dL) 

Day 3 146 ± 4 158 ± 2 162 ± 4 162 ± 4* 147 ± 3 138 ± 2 

Day 23 157 ± 4 156 ± 10 157 ± 4 150 ± 3 153 ± 2 155 ± 5 

Week 14 153 ± 7 185 ± 12 158 ± 5 171 ± 11 178 ± 13 166 ± 10 

Total protein (g/dL) 

Day 3 6.2 ± 0.1 6.5 ± 0.1 6.2 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.1 ± 0.1 

Day 23 6.8 ± 0.1 6.6 ± 0.1 6.7 ± 0.1 6.8 ± 0.1 6.7 ± 0.1 6.6 ± 0.1 

Week 14 7.6 ± 0.1 7.6 ± 0.1 7.7 ± 0.1 7.5 ± 0.1 7.7 ± 0.1 7.5 ± 0.1 

Albumin (g/dL) 

Day 3 5.0 ± 0.1 5.3 ± 0.1 5.1 ± 0.1 4.8 ± 0.1 4.9 ± 0.1 5.0 ± 0.1 

Day 23 4.9 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 5.0 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 

Week 14 4.8 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 

Globulin (g/dL) 

Day 3 1.1 ± 0.1 1.2 ± 0.1 1.1 ± 0.0 1.1 ± 0.1 1.2 ± 0.1 1.1 ± 0.0 

Day 23 1.9 ± 0.1 1.7 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 

Week 14 2.8 ± 0.1 2.7 ± 0.1 2.9 ± 0.1 2.7 ± 0.1 2.9 ± 0.0 2.7 ± 0.1 

A/G ratio 

Day 3 4.5 ± 0.3 4.6 ± 0.3 4.9 ± 0.3 4.4 ± 0.2 4.4 ± 0.3 4.4 ± 0.2 

Day 23 2.7 ± 0.2 2.9 ± 0.2 2.9 ± 0.2 2.8 ± 0.1 2.8 ± 0.2 3.0 ± 0.2 

Week 14 1.7 ± 0.1 1.8 ± 0.0 1.7 ± 0.1 1.8 ± 0.1 1.7 ± 0.0 1.8 ± 0.1 
196 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male (continued) 

Clinical Chemistry (continued) 

n 

Day 3 10 10 10 10 9 10 

Day 23 10 10 10 10 10 10 

Week 14 10 10 10 10 10 10 

Alanine aminotransferase (IU/L) 

Day 3 35 ± 1 31 ± 1 33 ± 1 38 ± 1 36 ± 1c 37 ± 1 

Day 23 34 ± 1 34 ± 1 33 ± 1 37 ± 3 32 ± 1 33 ± 1 

Week 14 68 ± 5 58 ± 3 54 ± 2 62 ± 5 61 ± 3 56 ± 3 

Alkaline phosphatase (IU/L) 

Day 3 674 ± 59 764 ± 34 755 ± 35 791 ± 29 748 ± 32 726 ± 40 

Day 23 454 ± 16 445 ± 15 460 ± 10 465 ± 13 439 ± 14 443 ± 18 

Week 14 371 ± 18 377 ± 9 363 ± 8 361 ± 11 330 ± 13 364 ± 11 

Creatine kinase (IU/L) 

Day 3 265 ± 46 201 ± 15 235 ± 16 232 ± 11 273 ± 36c 264 ± 23 

Day 23 199 ± 22 247 ± 38 196 ± 24 242 ± 31 215 ± 19 241 ± 26 

Week 14 110 ± 18 111 ± 18 103 ± 11 103 ± 12 96 ± 9 105 ± 9 

Sorbitol dehydrogenase (IU/L) 

Day 3 13 ± 1 12 ± 1 13 ± 0 13 ± 1 14 ± 1 14 ± 1 

Day 23 12 ± 0 12 ± 1 12 ± 1 14 ± 1 12 ± 0 11 ± 0 

Week 14 18 ± 1 16 ± 1 14 ± 0* 16 ± 1 17 ± 1 16 ± 1 

Bile acids (μmol/L) 

Day 3 25.8 ± 2.0 22.6 ± 2.5 23.2 ± 1.8 22.3 ± 1.0 22.6 ± 1.0c 22.6 ± 0.9 

Day 23 17.1 ± 0.7 19.5 ± 1.8 16.7 ± 0.8 19.5 ± 1.1 17.1 ± 0.5 17.4 ± 0.8 

Week 14 23.4 ± 2.1 27.7 ± 3.2 21.3 ± 1.5 27.1 ± 4.8 30.2 ± 3.9 25.0 ± 1.6 

Female 

n 10 10 10 10 10 10 

Hematology 

Automated hematocrit (%) 

Day 3 45.1 ± 0.6 44.9 ± 0.7 43.9 ± 0.7 46.2 ± 0.4 46.3 ± 0.8 45.7 ± 0.6 

Day 23 47.8 ± 0.5 47.5 ± 0.4 47.5 ± 0.5 47.2 ± 0.4 47.3 ± 0.2 48.0 ± 0.4 

Week 14 44.9 ± 0.5 45.4 ± 0.3 45.6 ± 0.3 45.3 ± 0.4 45.2 ± 0.1 45.6 ± 0.4 

Manual hematocrit (%) 

Day 3 45.2 ± 0.4 44.8 ± 0.5 43.6 ± 0.6 45.8 ± 0.2 45.4 ± 0.6 45.2 ± 0.4 

Day 23 47.1 ± 0.4 47.3 ± 0.4 47.0 ± 0.4 47.0 ± 0.4 46.9 ± 0.2 47.4 ± 0.4 

Week 14 45.3 ± 0.4 45.7 ± 0.3 46.2 ± 0.3 45.4 ± 0.4 45.6 ± 0.2 45.8 ± 0.3 

Hemoglobin (g/dL) 

Day 3 15.4 ± 0.2 15.1 ± 0.2 15.0 ± 0.3 15.2 ± 0.2 15.4 ± 0.3 15.3 ± 0.1 

Day 23 16.2 ± 0.0 16.2 ± 0.1 16.1 ± 0.1 16.0 ± 0.1 16.1 ± 0.1 16.0 ± 0.1 

Week 14 15.3 ± 0.2 15.5 ± 0.1 15.5 ± 0.1 15.4 ± 0.1 15.4 ± 0.1 15.5 ± 0.1 

Erythrocytes (10 /μL) 6 

Day 3 8.15 ± 0.11 7.96 ± 0.12 7.94 ± 0.15 8.09 ± 0.08 8.23 ± 0.17 8.12 ± 0.11 

Day 23 8.57 ± 0.04 8.54 ± 0.10 8.53 ± 0.08 8.46 ± 0.07 8.56 ± 0.05 8.46 ± 0.04 

Week 14 8.41 ± 0.09 8.50 ± 0.05 8.58 ± 0.05 8.49 ± 0.06 8.47 ± 0.03 8.50 ± 0.05 

Reticulocytes (10 /μL) 6 

Day 3 0.16 ± 0.03 0.22 ± 0.03 0.20 ± 0.02 0.20 ± 0.03 0.18 ± 0.02 0.15 ± 0.03 

Day 23 

Week 14 

0.07 ± 0.01 

0.05 ± 0.01 

0.07 ± 0.01 

0.05 ± 0.01 

0.06 ± 0.02 

0.05 ± 0.01 

0.06 ± 0.01 

0.05 ± 0.01 

0.08 ± 0.01 

0.05 ± 0.01 

0.05 ± 0.01 

0.05 ± 0.01b 
Isobutene, NTP TR 487 197 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Female (continued) 

n 10 10 10 10 10 10 

Hematology (continued) 

Nucleated erythrocytes (10 /μL) 3 

Day 3 0.05 ± 0.02 0.05 ± 0.02 0.06 ± 0.02 0.06 ± 0.03 0.04 ± 0.02 0.07 ± 0.04 

Day 23 0.03 ± 0.01 0.02 ± 0.02 0.01 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.00 ± 0.00 

Week 14 0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01 0.04 ± 0.01 0.01 ± 0.01 0.01 ± 0.01 

Mean cell volume (fL) 

Day 3 55.3 ± 0.4 56.5 ± 0.4 55.3 ± 0.7 57.1 ± 0.3* 56.4 ± 0.3 56.3 ± 0.6 

Day 23 56.0 ± 0.4 55.6 ± 0.5 55.7 ± 0.4 55.9 ± 0.4 55.3 ± 0.3 56.4 ± 0.3 

Week 14 53.4 ± 0.3 53.6 ± 0.2 53.2 ± 0.3 53.2 ± 0.2 53.3 ± 0.2 53.6 ± 0.2 

Mean cell hemoglobin (pg) 

Day 3 18.9 ± 0.1 19.0 ± 0.1 18.8 ± 0.1 18.8 ± 0.1 18.7 ± 0.1 18.8 ± 0.1 

Day 23 18.9 ± 0.1 19.0 ± 0.1 18.9 ± 0.1 18.9 ± 0.1 18.8 ± 0.1 19.0 ± 0.1 

Week 14 18.2 ± 0.1 18.2 ± 0.1 18.1 ± 0.1 18.1 ± 0.0 18.2 ± 0.1 18.3 ± 0.1 

Mean cell hemoglobin concentration (g/dL) 

Day 3 34.1 ± 0.2 33.7 ± 0.3 34.1 ± 0.4 32.9 ± 0.2* 33.3 ± 0.1 33.5 ± 0.3 

Day 23 34.0 ± 0.3 34.1 ± 0.2 33.9 ± 0.3 33.8 ± 0.2 34.0 ± 0.1 33.5 ± 0.2 

Week 14 34.1 ± 0.2 34.1 ± 0.1 34.1 ± 0.1 34.1 ± 0.2 34.2 ± 0.1 34.0 ± 0.2 

Platelets (10 /μL) 3 

Day 3 797.5 ± 48.0 861.6 ± 63.7 831.0 ± 40.1 871.7 ± 39.4 840.8 ± 54.6 791.4 ± 32.1 

Day 23 705.0 ± 21.4 675.0 ± 30.5 754.5 ± 35.2 721.4 ± 20.5 707.4 ± 11.4 745.5 ± 19.9 

Week 14 599.4 ± 16.2 615.8 ± 26.6 570.2 ± 24.2 530.5 ± 13.4* 575.8 ± 15.1 603.8 ± 19.2 

Leukocytes (10 /μL) 3 

Day 3 8.24 ± 0.39 8.61 ± 0.25 8.45 ± 0.38 8.90 ± 0.28 9.94 ± 0.54* 9.34 ± 0.43* 

Day 23 5.44 ± 0.36 6.26 ± 0.57 5.68 ± 0.34 5.76 ± 0.31 5.97 ± 0.60 5.74 ± 0.43 

Week 14 6.71 ± 0.14 6.95 ± 0.27 6.92 ± 0.45 6.95 ± 0.20 7.26 ± 0.39 6.43 ± 0.40 

Segmented neutrophils (10 /μL) 3 

Day 3 0.78 ± 0.10 0.77 ± 0.10 0.79 ± 0.10 0.78 ± 0.08 1.16 ± 0.13 0.97 ± 0.19 

Day 23 0.72 ± 0.08 0.76 ± 0.12 0.76 ± 0.08 0.76 ± 0.09 0.99 ± 0.12 0.81 ± 0.09 

Week 14 1.03 ± 0.10 0.86 ± 0.08 1.22 ± 0.15 0.95 ± 0.08 1.40 ± 0.12 1.05 ± 0.14 

Lymphocytes (10 /μL) 3 

Day 3 7.33 ± 0.39 7.70 ± 0.20 7.54 ± 0.37 7.99 ± 0.28 8.71 ± 0.46 8.28 ± 0.36 

Day 23 4.65 ± 0.33 5.42 ± 0.49 4.81 ± 0.31 4.95 ± 0.33 4.90 ± 0.52 4.87 ± 0.49 

Week 14 5.59 ± 0.17 6.02 ± 0.27 5.62 ± 0.32 5.90 ± 0.20 5.76 ± 0.35 5.29 ± 0.31 

Monocytes (10 /μL) 3 

Day 3 0.09 ± 0.04 0.11 ± 0.04 0.09 ± 0.03 0.07 ± 0.04 0.03 ± 0.02 0.04 ± 0.03 

Day 23 0.02 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.01 ± 0.01 0.00 ± 0.00 0.01 ± 0.01 

Week 14 0.04 ± 0.02 0.03 ± 0.02 0.04 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 0.03 ± 0.01 

Eosinophils (10 /μL) 3 

Day 3 0.04 ± 0.02 0.03 ± 0.01 0.04 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 

Day 23 0.05 ± 0.02 0.06 ± 0.02 0.07 ± 0.02 0.03 ± 0.01 0.07 ± 0.02 0.03 ± 0.01 

Week 14 0.05 ± 0.02 0.04 ± 0.01 0.04 ± 0.02 0.06 ± 0.02 0.05 ± 0.02 0.08 ± 0.02 

Clinical Chemistry 

Urea nitrogen (mg/dL) 

Day 3 18.3 ± 1.1 13.4 ± 0.5** 14.6 ± 0.7** 14.1 ± 0.3** 13.5 ± 0.5** 12.9 ± 0.5** 

Day 23 14.3 ± 0.4 14.6 ± 0.6 14.0 ± 0.5 13.5 ± 0.6 13.7 ± 0.4 13.1 ± 0.3 

Week 14 19.4 ± 1.0 20.0 ± 0.7 20.0 ± 0.8 19.2 ± 1.1 19.7 ± 0.5 19.0 ± 0.8 

Creatinine (mg/dL) 

Day 3 0.63 ± 0.02 0.63 ± 0.02 0.59 ± 0.02 0.55 ± 0.02** 0.59 ± 0.02* 0.58 ± 0.03* 

Day 23 0.76 ± 0.02 0.84 ± 0.02 0.77 ± 0.03 0.76 ± 0.02 0.77 ± 0.02 0.75 ± 0.02 

Week 14 0.83 ± 0.03 0.81 ± 0.03 0.85 ± 0.02 0.84 ± 0.02 0.83 ± 0.03 0.80 ± 0.02 
198 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Female (continued) 

n 10 10 10 10 10 10 

Clinical Chemistry (continued) 

Serum glucose (mg/dL) 

Day 3 124 ± 4 136 ± 6 141 ± 9 128 ± 3 127 ± 4 122 ± 4 

Day 23 155 ± 4 171 ± 13 166 ± 8 148 ± 6 158 ± 3 164 ± 5 

Week 14 164 ± 5 169 ± 11 176 ± 10 170 ± 7 167 ± 4 169 ± 5 

Total protein (g/dL) 

Day 3 6.1 ± 0.1 6.1 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.1 ± 0.1 

Day 23 6.5 ± 0.1 6.6 ± 0.1 6.5 ± 0.1 6.4 ± 0.1 6.5 ± 0.1 6.6 ± 0.1 

Week 14 7.3 ± 0.1 7.5 ± 0.1 7.6 ± 0.1 7.3 ± 0.1 7.3 ± 0.1 7.3 ± 0.1 

Albumin (g/dL) 

Day 3 5.4 ± 0.1 5.7 ± 0.1 5.5 ± 0.1 5.5 ± 0.0 5.4 ± 0.1 5.3 ± 0.1 

Day 23 4.7 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 4.7 ± 0.0 4.8 ± 0.1 4.7 ± 0.1 

Week 14 5.1 ± 0.1 5.2 ± 0.1 5.1 ± 0.1 5.1 ± 0.1 5.0 ± 0.1 5.0 ± 0.1 

Globulin (g/dL) 

Day 3 0.7 ± 0.1 0.5 ± 0.1 0.5 ± 0.1 0.5 ± 0.0* 0.6 ± 0.1 0.8 ± 0.1 

Day 23 1.8 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 1.7 ± 0.0 1.8 ± 0.1 

Week 14 2.2 ± 0.1 2.3 ± 0.1 2.5 ± 0.1 2.3 ± 0.1 2.4 ± 0.1 2.3 ± 0.1 

A/G ratio 

Day 3 8.1 ± 0.8 14.5 ± 2.7b 12.8 ± 2.0 13.1 ± 1.1 9.9 ± 1.3 7.5 ± 1.0 

Day 23 2.7 ± 0.1 2.6 ± 0.1 2.7 ± 0.1 2.9 ± 0.1 2.8 ± 0.1 2.6 ± 0.1 

Week 14 2.4 ± 0.1 2.3 ± 0.1 2.1 ± 0.1 2.3 ± 0.1 2.1 ± 0.1 2.3 ± 0.1 

Alanine aminotransferase (IU/L) 

Day 3 34 ± 1 34 ± 1 35 ± 2 34 ± 1 33 ± 1 33 ± 1 

Day 23 30 ± 1 34 ± 3 30 ± 1 29 ± 1 29 ± 1 30 ± 1 

Week 14 47 ± 2 65 ± 6* 55 ± 3 66 ± 7* 54 ± 5 60 ± 4 

Alkaline phosphatase (IU/L) 

Day 3 583 ± 22 577 ± 21 569 ± 32 539 ± 20 486 ± 18* 554 ± 26 

Day 23 347 ± 10 391 ± 10* 366 ± 12 366 ± 9 330 ± 10 347 ± 8 

Week 14 307 ± 15 328 ± 11 369 ± 14* 317 ± 15 321 ± 12 306 ± 13 

Creatine kinase (IU/L) 

Day 3 204 ± 16 222 ± 12 210 ± 24 198 ± 18 205 ± 26 277 ± 36 

Day 23 197 ± 35 196 ± 24 162 ± 20 202 ± 27 183 ± 28 221 ± 59 

Week 14 109 ± 9 122 ± 20 112 ± 12 139 ± 17 134 ± 14 146 ± 13 

Sorbitol dehydrogenase (IU/L) 

Day 3 16 ± 0 16 ± 1 14 ± 1* 14 ± 0** 15 ± 1* 14 ± 1* 

Day 23 15 ± 1 19 ± 2 14 ± 1 16 ± 1 15 ± 1 16 ± 0 

Week 14 14 ± 1 17 ± 1 17 ± 1 17 ± 1 15 ± 2 17 ± 1 

Bile acids (μmol/L) 

Day 3 17.9 ± 1.9 13.6 ± 1.3 20.0 ± 3.7 17.8 ± 1.3 16.1 ± 1.2 16.1 ± 1.2 

Day 23 14.6 ± 0.7 21.7 ± 5.7 18.0 ± 2.0 14.7 ± 1.1 14.2 ± 0.6 16.6 ± 1.4 

Week 14 24.8 ± 3.5 24.5 ± 3.4 30.5 ± 6.1 40.1 ± 5.6 32.6 ± 5.4 30.4 ± 4.2 

* Significantly different (P#0.05) from the chamber control group by Dunn’s or Shirley’s test 

** P#0.01 a Mean ± standard error. Statistical tests were performed on unrounded data. b n=9
 

n=8
 c 
194 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male 

Hematology 

n 10 10 10 10 10 10 

Automated hematocrit (%) 

Day 3 41.6 ± 0.3 41.8 ± 0.4 41.5 ± 0.5 41.5 ± 0.7 42.0 ± 0.5 42.1 ± 0.5 

Day 23 45.7 ± 0.7 45.0 ± 0.4 44.4 ± 0.5 45.4 ± 0.2 45.6 ± 0.3 45.5 ± 0.4 

Week 14 45.8 ± 0.4 45.4 ± 0.3 44.9 ± 0.3 45.4 ± 0.4 45.8 ± 0.4 45.1 ± 0.4 

Manual hematocrit (%) 

Day 3 42.5 ± 0.3 42.6 ± 0.4 42.8 ± 0.4 42.2 ± 0.6 43.3 ± 0.4 43.4 ± 0.4 

Day 23 46.6 ± 0.7 45.9 ± 0.5 45.4 ± 0.6 46.2 ± 0.3 46.4 ± 0.3 46.5 ± 0.3 

Week 14 46.3 ± 0.4 46.8 ± 0.9 45.5 ± 0.3 45.9 ± 0.4 46.6 ± 0.3 46.0 ± 0.3 

Hemoglobin (g/dL) 

Day 3 13.9 ± 0.1 14.1 ± 0.1 13.8 ± 0.2 13.7 ± 0.2 14.0 ± 0.1 13.8 ± 0.2 

Day 23 16.0 ± 0.2 15.7 ± 0.2 15.6 ± 0.2 15.8 ± 0.1 15.7 ± 0.1 15.8 ± 0.1 

Week 14 15.4 ± 0.1 15.3 ± 0.1 15.2 ± 0.1 15.2 ± 0.1 15.3 ± 0.1 15.2 ± 0.1 6 Erythrocytes (10 /μL) 

Day 3 7.47 ± 0.11 7.55 ± 0.11 7.41 ± 0.12 7.33 ± 0.17 7.50 ± 0.12 7.37 ± 0.12 

Day 23 8.64 ± 0.11 8.55 ± 0.12 8.34 ± 0.11 8.50 ± 0.09 8.51 ± 0.06 8.56 ± 0.09 

Week 14 9.18 ± 0.05 9.08 ± 0.06 9.03 ± 0.06 9.05 ± 0.06 9.12 ± 0.06 9.03 ± 0.08 6 Reticulocytes (10 /μL) 

Day 3 0.24 ± 0.05 0.17 ± 0.03 0.18 ± 0.03 0.12 ± 0.02 0.16 ± 0.05 0.27 ± 0.03 

b b b b Day 23 0.10 ± 0.02 0.09 ± 0.02 0.07 ± 0.02 0.09 ± 0.02 0.13 ± 0.03 0.12 ± 0.02
 

Week 14 0.07 ± 0.01 0.05 ± 0.01 0.06 ± 0.01 0.07 ± 0.01 0.06 ± 0.01 0.06 ± 0.01 3 Nucleated erythrocytes (10 /μL) 

Day 3 0.11 ± 0.03 0.07 ± 0.02 0.08 ± 0.03 0.09 ± 0.03 0.11 ± 0.04 0.14 ± 0.05 

Day 23 0.02 ± 0.01 0.01 ± 0.01 0.04 ± 0.02 0.02 ± 0.01 0.01 ± 0.01 0.01 ± 0.01 

Week 14 0.03 ± 0.01 0.03 ± 0.02 0.04 ± 0.02 0.02 ± 0.01 0.03 ± 0.02 0.02 ± 0.02 

Mean cell volume (fL) 

Day 3 55.5 ± 0.7 55.4 ± 0.6 55.9 ± 0.5 56.8 ± 0.6 56.1 ± 0.3 57.2 ± 0.4 

Day 23 52.8 ± 0.4 52.6 ± 0.4 53.2 ± 0.4 53.3 ± 0.4 53.5 ± 0.4 53.1 ± 0.5 

Week 14 49.9 ± 0.3 50.0 ± 0.3 49.8 ± 0.3 50.2 ± 0.2 50.1 ± 0.3 50.1 ± 0.4 

Mean cell hemoglobin (pg) 

Day 3 18.6 ± 0.1 18.7 ± 0.1 18.7 ± 0.1 18.7 ± 0.2 18.7 ± 0.1 18.7 ± 0.1 

Day 23 18.5 ± 0.1 18.4 ± 0.1 18.7 ± 0.1 18.6 ± 0.2 18.5 ± 0.1 18.5 ± 0.1 

Week 14 16.8 ± 0.0 16.9 ± 0.1 16.8 ± 0.0 16.8 ± 0.0 16.8 ± 0.0 16.8 ± 0.1 

Mean cell hemoglobin concentration (g/dL) 

Day 3 33.4 ± 0.2 33.6 ± 0.1 33.4 ± 0.2 32.9 ± 0.2 33.3 ± 0.2 32.7 ± 0.1** 

Day 23 35.1 ± 0.2 34.9 ± 0.2 35.1 ± 0.1 34.8 ± 0.1 34.5 ± 0.2 34.8 ± 0.2 

Week 14 33.7 ± 0.2 33.7 ± 0.2 33.7 ± 0.3 33.5 ± 0.1 33.5 ± 0.1 33.6 ± 0.3 3 Platelets (10 /μL) 

Day 3 922.4 ± 25.1 878.9 ± 26.9 874.8 ± 43.1 815.7 ± 14.4* 836.6 ± 33.9 851.0 ± 14.6 

Day 23 680.2 ± 19.2 672.0 ± 13.6 669.8 ± 14.2 646.3 ± 24.1 681.5 ± 17.9 642.8 ± 30.8 

Week 14 515.5 ± 10.4 530.9 ± 12.0 535.1 ± 7.3 501.6 ± 7.4 525.9 ± 5.3 541.2 ± 7.9 3 Leukocytes (10 /μL) 

Day 3 7.85 ± 0.39 8.76 ± 0.36 8.71 ± 0.31 8.55 ± 0.32 7.84 ± 0.48 7.86 ± 0.34 

Day 23 5.56 ± 0.55 6.12 ± 0.18 5.85 ± 0.35 6.08 ± 0.45 5.87 ± 0.45 6.61 ± 0.35 

Week 14 5.73 ± 0.39 6.14 ± 0.27 6.13 ± 0.36 6.73 ± 0.30 6.04 ± 0.36 5.69 ± 0.37 3 Segmented neutrophils (10 /μL) 

Day 3 0.80 ± 0.09 0.74 ± 0.11 0.91 ± 0.12 0.77 ± 0.10 0.74 ± 0.10 0.92 ± 0.08 

Day 23 0.82 ± 0.14 0.90 ± 0.09 0.78 ± 0.07 0.83 ± 0.07 0.73 ± 0.07 0.84 ± 0.10 

Week 14 0.93 ± 0.08 1.17 ± 0.10 1.33 ± 0.12 0.95 ± 0.12 1.07 ± 0.10 1.02 ± 0.11 3 Lymphocytes (10 /μL) 

Day 3 6.81 ± 0.34 7.88 ± 0.35 7.74 ± 0.26 7.72 ± 0.24 6.87 ± 0.44 6.81 ± 0.31 

Day 23 4.68 ± 0.48 5.20 ± 0.13 5.03 ± 0.37 5.21 ± 0.42 5.11 ± 0.42 5.73 ± 0.38 

Week 14 4.70 ± 0.35 4.86 ± 0.24 4.70 ± 0.30 5.71 ± 0.25 4.93 ± 0.34 4.59 ± 0.32 
Isobutene, NTP TR 487 195 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male (continued) 

Hematology (continued) 

n 10 10 10 10 10 10 

Monocytes (10 /μL) 3 

Day 3 0.20 ± 0.05 0.12 ± 0.04 0.06 ± 0.03 0.02 ± 0.01** 0.15 ± 0.04 0.12 ± 0.03 

Day 23 0.04 ± 0.02 0.01 ± 0.01 0.02 ± 0.02 0.03 ± 0.03 0.01 ± 0.01 0.01 ± 0.01 

Week 14 0.06 ± 0.02 0.05 ± 0.02 0.06 ± 0.02 0.04 ± 0.02 0.03 ± 0.01 0.05 ± 0.02 

Eosinophils (10 /μL) 3 

Day 3 0.03 ± 0.02 0.02 ± 0.02 0.01 ± 0.01 0.03 ± 0.02 0.07 ± 0.02 0.02 ± 0.01 

Day 23 0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01 0.00 ± 0.00 0.02 ± 0.01 0.03 ± 0.01 

Week 14 0.05 ± 0.02 0.06 ± 0.02 0.04 ± 0.02 0.03 ± 0.02 0.01 ± 0.01 0.03 ± 0.02 

Clinical Chemistry 

n 

Day 3 10 10 10 10 9 10 

Day 23 10 10 10 10 10 10 

Week 14 10 10 10 10 10 10 

Urea nitrogen (mg/dL) 

Day 3 12.7 ± 0.7 14.2 ± 0.6 12.3 ± 0.4 11.1 ± 0.5 11.9 ± 0.6 11.5 ± 0.5 

Day 23 15.6 ± 0.4 14.7 ± 0.3 14.6 ± 0.3 15.2 ± 0.5 14.9 ± 0.2 14.6 ± 0.2 

Week 14 20.6 ± 0.7 19.7 ± 0.6 18.8 ± 0.7 20.4 ± 0.7 19.6 ± 0.8 19.2 ± 0.6 

Creatinine (mg/dL) 

Day 3 0.61 ± 0.05 0.62 ± 0.02 0.56 ± 0.03 0.47 ± 0.02** 0.53 ± 0.04* 0.51 ± 0.02* 

Day 23 0.73 ± 0.02 0.72 ± 0.01 0.71 ± 0.02 0.70 ± 0.02 0.72 ± 0.01 0.71 ± 0.02 

Week 14 0.86 ± 0.02 0.88 ± 0.02 0.89 ± 0.01 0.86 ± 0.02 0.86 ± 0.02 0.84 ± 0.02 

Serum glucose (mg/dL) 

Day 3 146 ± 4 158 ± 2 162 ± 4 162 ± 4* 147 ± 3 138 ± 2 

Day 23 157 ± 4 156 ± 10 157 ± 4 150 ± 3 153 ± 2 155 ± 5 

Week 14 153 ± 7 185 ± 12 158 ± 5 171 ± 11 178 ± 13 166 ± 10 

Total protein (g/dL) 

Day 3 6.2 ± 0.1 6.5 ± 0.1 6.2 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.1 ± 0.1 

Day 23 6.8 ± 0.1 6.6 ± 0.1 6.7 ± 0.1 6.8 ± 0.1 6.7 ± 0.1 6.6 ± 0.1 

Week 14 7.6 ± 0.1 7.6 ± 0.1 7.7 ± 0.1 7.5 ± 0.1 7.7 ± 0.1 7.5 ± 0.1 

Albumin (g/dL) 

Day 3 5.0 ± 0.1 5.3 ± 0.1 5.1 ± 0.1 4.8 ± 0.1 4.9 ± 0.1 5.0 ± 0.1 

Day 23 4.9 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 5.0 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 

Week 14 4.8 ± 0.1 4.9 ± 0.1 4.9 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 

Globulin (g/dL) 

Day 3 1.1 ± 0.1 1.2 ± 0.1 1.1 ± 0.0 1.1 ± 0.1 1.2 ± 0.1 1.1 ± 0.0 

Day 23 1.9 ± 0.1 1.7 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 

Week 14 2.8 ± 0.1 2.7 ± 0.1 2.9 ± 0.1 2.7 ± 0.1 2.9 ± 0.0 2.7 ± 0.1 

A/G ratio 

Day 3 4.5 ± 0.3 4.6 ± 0.3 4.9 ± 0.3 4.4 ± 0.2 4.4 ± 0.3 4.4 ± 0.2 

Day 23 2.7 ± 0.2 2.9 ± 0.2 2.9 ± 0.2 2.8 ± 0.1 2.8 ± 0.2 3.0 ± 0.2 

Week 14 1.7 ± 0.1 1.8 ± 0.0 1.7 ± 0.1 1.8 ± 0.1 1.7 ± 0.0 1.8 ± 0.1 
196 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Male (continued) 

Clinical Chemistry (continued) 

n 

Day 3 10 10 10 10 9 10 

Day 23 10 10 10 10 10 10 

Week 14 10 10 10 10 10 10 

Alanine aminotransferase (IU/L) 

Day 3 35 ± 1 31 ± 1 33 ± 1 38 ± 1 36 ± 1c 37 ± 1 

Day 23 34 ± 1 34 ± 1 33 ± 1 37 ± 3 32 ± 1 33 ± 1 

Week 14 68 ± 5 58 ± 3 54 ± 2 62 ± 5 61 ± 3 56 ± 3 

Alkaline phosphatase (IU/L) 

Day 3 674 ± 59 764 ± 34 755 ± 35 791 ± 29 748 ± 32 726 ± 40 

Day 23 454 ± 16 445 ± 15 460 ± 10 465 ± 13 439 ± 14 443 ± 18 

Week 14 371 ± 18 377 ± 9 363 ± 8 361 ± 11 330 ± 13 364 ± 11 

Creatine kinase (IU/L) 

Day 3 265 ± 46 201 ± 15 235 ± 16 232 ± 11 273 ± 36c 264 ± 23 

Day 23 199 ± 22 247 ± 38 196 ± 24 242 ± 31 215 ± 19 241 ± 26 

Week 14 110 ± 18 111 ± 18 103 ± 11 103 ± 12 96 ± 9 105 ± 9 

Sorbitol dehydrogenase (IU/L) 

Day 3 13 ± 1 12 ± 1 13 ± 0 13 ± 1 14 ± 1 14 ± 1 

Day 23 12 ± 0 12 ± 1 12 ± 1 14 ± 1 12 ± 0 11 ± 0 

Week 14 18 ± 1 16 ± 1 14 ± 0* 16 ± 1 17 ± 1 16 ± 1 

Bile acids (μmol/L) 

Day 3 25.8 ± 2.0 22.6 ± 2.5 23.2 ± 1.8 22.3 ± 1.0 22.6 ± 1.0c 22.6 ± 0.9 

Day 23 17.1 ± 0.7 19.5 ± 1.8 16.7 ± 0.8 19.5 ± 1.1 17.1 ± 0.5 17.4 ± 0.8 

Week 14 23.4 ± 2.1 27.7 ± 3.2 21.3 ± 1.5 27.1 ± 4.8 30.2 ± 3.9 25.0 ± 1.6 

Female 

n 10 10 10 10 10 10 

Hematology 

Automated hematocrit (%) 

Day 3 45.1 ± 0.6 44.9 ± 0.7 43.9 ± 0.7 46.2 ± 0.4 46.3 ± 0.8 45.7 ± 0.6 

Day 23 47.8 ± 0.5 47.5 ± 0.4 47.5 ± 0.5 47.2 ± 0.4 47.3 ± 0.2 48.0 ± 0.4 

Week 14 44.9 ± 0.5 45.4 ± 0.3 45.6 ± 0.3 45.3 ± 0.4 45.2 ± 0.1 45.6 ± 0.4 

Manual hematocrit (%) 

Day 3 45.2 ± 0.4 44.8 ± 0.5 43.6 ± 0.6 45.8 ± 0.2 45.4 ± 0.6 45.2 ± 0.4 

Day 23 47.1 ± 0.4 47.3 ± 0.4 47.0 ± 0.4 47.0 ± 0.4 46.9 ± 0.2 47.4 ± 0.4 

Week 14 45.3 ± 0.4 45.7 ± 0.3 46.2 ± 0.3 45.4 ± 0.4 45.6 ± 0.2 45.8 ± 0.3 

Hemoglobin (g/dL) 

Day 3 15.4 ± 0.2 15.1 ± 0.2 15.0 ± 0.3 15.2 ± 0.2 15.4 ± 0.3 15.3 ± 0.1 

Day 23 16.2 ± 0.0 16.2 ± 0.1 16.1 ± 0.1 16.0 ± 0.1 16.1 ± 0.1 16.0 ± 0.1 

Week 14 15.3 ± 0.2 15.5 ± 0.1 15.5 ± 0.1 15.4 ± 0.1 15.4 ± 0.1 15.5 ± 0.1 

Erythrocytes (10 /μL) 6 

Day 3 8.15 ± 0.11 7.96 ± 0.12 7.94 ± 0.15 8.09 ± 0.08 8.23 ± 0.17 8.12 ± 0.11 

Day 23 8.57 ± 0.04 8.54 ± 0.10 8.53 ± 0.08 8.46 ± 0.07 8.56 ± 0.05 8.46 ± 0.04 

Week 14 8.41 ± 0.09 8.50 ± 0.05 8.58 ± 0.05 8.49 ± 0.06 8.47 ± 0.03 8.50 ± 0.05 

Reticulocytes (10 /μL) 6 

Day 3 0.16 ± 0.03 0.22 ± 0.03 0.20 ± 0.02 0.20 ± 0.03 0.18 ± 0.02 0.15 ± 0.03 

Day 23 

Week 14 

0.07 ± 0.01 

0.05 ± 0.01 

0.07 ± 0.01 

0.05 ± 0.01 

0.06 ± 0.02 

0.05 ± 0.01 

0.06 ± 0.01 

0.05 ± 0.01 

0.08 ± 0.01 

0.05 ± 0.01 

0.05 ± 0.01 

0.05 ± 0.01b 
Isobutene, NTP TR 487 197 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Female (continued) 

n 10 10 10 10 10 10 

Hematology (continued) 

Nucleated erythrocytes (10 /μL) 3 

Day 3 0.05 ± 0.02 0.05 ± 0.02 0.06 ± 0.02 0.06 ± 0.03 0.04 ± 0.02 0.07 ± 0.04 

Day 23 0.03 ± 0.01 0.02 ± 0.02 0.01 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.00 ± 0.00 

Week 14 0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01 0.04 ± 0.01 0.01 ± 0.01 0.01 ± 0.01 

Mean cell volume (fL) 

Day 3 55.3 ± 0.4 56.5 ± 0.4 55.3 ± 0.7 57.1 ± 0.3* 56.4 ± 0.3 56.3 ± 0.6 

Day 23 56.0 ± 0.4 55.6 ± 0.5 55.7 ± 0.4 55.9 ± 0.4 55.3 ± 0.3 56.4 ± 0.3 

Week 14 53.4 ± 0.3 53.6 ± 0.2 53.2 ± 0.3 53.2 ± 0.2 53.3 ± 0.2 53.6 ± 0.2 

Mean cell hemoglobin (pg) 

Day 3 18.9 ± 0.1 19.0 ± 0.1 18.8 ± 0.1 18.8 ± 0.1 18.7 ± 0.1 18.8 ± 0.1 

Day 23 18.9 ± 0.1 19.0 ± 0.1 18.9 ± 0.1 18.9 ± 0.1 18.8 ± 0.1 19.0 ± 0.1 

Week 14 18.2 ± 0.1 18.2 ± 0.1 18.1 ± 0.1 18.1 ± 0.0 18.2 ± 0.1 18.3 ± 0.1 

Mean cell hemoglobin concentration (g/dL) 

Day 3 34.1 ± 0.2 33.7 ± 0.3 34.1 ± 0.4 32.9 ± 0.2* 33.3 ± 0.1 33.5 ± 0.3 

Day 23 34.0 ± 0.3 34.1 ± 0.2 33.9 ± 0.3 33.8 ± 0.2 34.0 ± 0.1 33.5 ± 0.2 

Week 14 34.1 ± 0.2 34.1 ± 0.1 34.1 ± 0.1 34.1 ± 0.2 34.2 ± 0.1 34.0 ± 0.2 

Platelets (10 /μL) 3 

Day 3 797.5 ± 48.0 861.6 ± 63.7 831.0 ± 40.1 871.7 ± 39.4 840.8 ± 54.6 791.4 ± 32.1 

Day 23 705.0 ± 21.4 675.0 ± 30.5 754.5 ± 35.2 721.4 ± 20.5 707.4 ± 11.4 745.5 ± 19.9 

Week 14 599.4 ± 16.2 615.8 ± 26.6 570.2 ± 24.2 530.5 ± 13.4* 575.8 ± 15.1 603.8 ± 19.2 

Leukocytes (10 /μL) 3 

Day 3 8.24 ± 0.39 8.61 ± 0.25 8.45 ± 0.38 8.90 ± 0.28 9.94 ± 0.54* 9.34 ± 0.43* 

Day 23 5.44 ± 0.36 6.26 ± 0.57 5.68 ± 0.34 5.76 ± 0.31 5.97 ± 0.60 5.74 ± 0.43 

Week 14 6.71 ± 0.14 6.95 ± 0.27 6.92 ± 0.45 6.95 ± 0.20 7.26 ± 0.39 6.43 ± 0.40 

Segmented neutrophils (10 /μL) 3 

Day 3 0.78 ± 0.10 0.77 ± 0.10 0.79 ± 0.10 0.78 ± 0.08 1.16 ± 0.13 0.97 ± 0.19 

Day 23 0.72 ± 0.08 0.76 ± 0.12 0.76 ± 0.08 0.76 ± 0.09 0.99 ± 0.12 0.81 ± 0.09 

Week 14 1.03 ± 0.10 0.86 ± 0.08 1.22 ± 0.15 0.95 ± 0.08 1.40 ± 0.12 1.05 ± 0.14 

Lymphocytes (10 /μL) 3 

Day 3 7.33 ± 0.39 7.70 ± 0.20 7.54 ± 0.37 7.99 ± 0.28 8.71 ± 0.46 8.28 ± 0.36 

Day 23 4.65 ± 0.33 5.42 ± 0.49 4.81 ± 0.31 4.95 ± 0.33 4.90 ± 0.52 4.87 ± 0.49 

Week 14 5.59 ± 0.17 6.02 ± 0.27 5.62 ± 0.32 5.90 ± 0.20 5.76 ± 0.35 5.29 ± 0.31 

Monocytes (10 /μL) 3 

Day 3 0.09 ± 0.04 0.11 ± 0.04 0.09 ± 0.03 0.07 ± 0.04 0.03 ± 0.02 0.04 ± 0.03 

Day 23 0.02 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.01 ± 0.01 0.00 ± 0.00 0.01 ± 0.01 

Week 14 0.04 ± 0.02 0.03 ± 0.02 0.04 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 0.03 ± 0.01 

Eosinophils (10 /μL) 3 

Day 3 0.04 ± 0.02 0.03 ± 0.01 0.04 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 0.05 ± 0.02 

Day 23 0.05 ± 0.02 0.06 ± 0.02 0.07 ± 0.02 0.03 ± 0.01 0.07 ± 0.02 0.03 ± 0.01 

Week 14 0.05 ± 0.02 0.04 ± 0.01 0.04 ± 0.02 0.06 ± 0.02 0.05 ± 0.02 0.08 ± 0.02 

Clinical Chemistry 

Urea nitrogen (mg/dL) 

Day 3 18.3 ± 1.1 13.4 ± 0.5** 14.6 ± 0.7** 14.1 ± 0.3** 13.5 ± 0.5** 12.9 ± 0.5** 

Day 23 14.3 ± 0.4 14.6 ± 0.6 14.0 ± 0.5 13.5 ± 0.6 13.7 ± 0.4 13.1 ± 0.3 

Week 14 19.4 ± 1.0 20.0 ± 0.7 20.0 ± 0.8 19.2 ± 1.1 19.7 ± 0.5 19.0 ± 0.8 

Creatinine (mg/dL) 

Day 3 0.63 ± 0.02 0.63 ± 0.02 0.59 ± 0.02 0.55 ± 0.02** 0.59 ± 0.02* 0.58 ± 0.03* 

Day 23 0.76 ± 0.02 0.84 ± 0.02 0.77 ± 0.03 0.76 ± 0.02 0.77 ± 0.02 0.75 ± 0.02 

Week 14 0.83 ± 0.03 0.81 ± 0.03 0.85 ± 0.02 0.84 ± 0.02 0.83 ± 0.03 0.80 ± 0.02 
198 Isobutene, NTP TR 487 

TABLE F1 

Hematology and Clinical Chemistry Data for Rats in the 14-Week Inhalation Study of Isobutene 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

Female (continued) 

n 10 10 10 10 10 10 

Clinical Chemistry (continued) 

Serum glucose (mg/dL) 

Day 3 124 ± 4 136 ± 6 141 ± 9 128 ± 3 127 ± 4 122 ± 4 

Day 23 155 ± 4 171 ± 13 166 ± 8 148 ± 6 158 ± 3 164 ± 5 

Week 14 164 ± 5 169 ± 11 176 ± 10 170 ± 7 167 ± 4 169 ± 5 

Total protein (g/dL) 

Day 3 6.1 ± 0.1 6.1 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.0 ± 0.1 6.1 ± 0.1 

Day 23 6.5 ± 0.1 6.6 ± 0.1 6.5 ± 0.1 6.4 ± 0.1 6.5 ± 0.1 6.6 ± 0.1 

Week 14 7.3 ± 0.1 7.5 ± 0.1 7.6 ± 0.1 7.3 ± 0.1 7.3 ± 0.1 7.3 ± 0.1 

Albumin (g/dL) 

Day 3 5.4 ± 0.1 5.7 ± 0.1 5.5 ± 0.1 5.5 ± 0.0 5.4 ± 0.1 5.3 ± 0.1 

Day 23 4.7 ± 0.1 4.8 ± 0.1 4.8 ± 0.1 4.7 ± 0.0 4.8 ± 0.1 4.7 ± 0.1 

Week 14 5.1 ± 0.1 5.2 ± 0.1 5.1 ± 0.1 5.1 ± 0.1 5.0 ± 0.1 5.0 ± 0.1 

Globulin (g/dL) 

Day 3 0.7 ± 0.1 0.5 ± 0.1 0.5 ± 0.1 0.5 ± 0.0* 0.6 ± 0.1 0.8 ± 0.1 

Day 23 1.8 ± 0.1 1.8 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 1.7 ± 0.0 1.8 ± 0.1 

Week 14 2.2 ± 0.1 2.3 ± 0.1 2.5 ± 0.1 2.3 ± 0.1 2.4 ± 0.1 2.3 ± 0.1 

A/G ratio 

Day 3 8.1 ± 0.8 14.5 ± 2.7b 12.8 ± 2.0 13.1 ± 1.1 9.9 ± 1.3 7.5 ± 1.0 

Day 23 2.7 ± 0.1 2.6 ± 0.1 2.7 ± 0.1 2.9 ± 0.1 2.8 ± 0.1 2.6 ± 0.1 

Week 14 2.4 ± 0.1 2.3 ± 0.1 2.1 ± 0.1 2.3 ± 0.1 2.1 ± 0.1 2.3 ± 0.1 

Alanine aminotransferase (IU/L) 

Day 3 34 ± 1 34 ± 1 35 ± 2 34 ± 1 33 ± 1 33 ± 1 

Day 23 30 ± 1 34 ± 3 30 ± 1 29 ± 1 29 ± 1 30 ± 1 

Week 14 47 ± 2 65 ± 6* 55 ± 3 66 ± 7* 54 ± 5 60 ± 4 

Alkaline phosphatase (IU/L) 

Day 3 583 ± 22 577 ± 21 569 ± 32 539 ± 20 486 ± 18* 554 ± 26 

Day 23 347 ± 10 391 ± 10* 366 ± 12 366 ± 9 330 ± 10 347 ± 8 

Week 14 307 ± 15 328 ± 11 369 ± 14* 317 ± 15 321 ± 12 306 ± 13 

Creatine kinase (IU/L) 

Day 3 204 ± 16 222 ± 12 210 ± 24 198 ± 18 205 ± 26 277 ± 36 

Day 23 197 ± 35 196 ± 24 162 ± 20 202 ± 27 183 ± 28 221 ± 59 

Week 14 109 ± 9 122 ± 20 112 ± 12 139 ± 17 134 ± 14 146 ± 13 

Sorbitol dehydrogenase (IU/L) 

Day 3 16 ± 0 16 ± 1 14 ± 1* 14 ± 0** 15 ± 1* 14 ± 1* 

Day 23 15 ± 1 19 ± 2 14 ± 1 16 ± 1 15 ± 1 16 ± 0 

Week 14 14 ± 1 17 ± 1 17 ± 1 17 ± 1 15 ± 2 17 ± 1 

Bile acids (μmol/L) 

Day 3 17.9 ± 1.9 13.6 ± 1.3 20.0 ± 3.7 17.8 ± 1.3 16.1 ± 1.2 16.1 ± 1.2 

Day 23 14.6 ± 0.7 21.7 ± 5.7 18.0 ± 2.0 14.7 ± 1.1 14.2 ± 0.6 16.6 ± 1.4 

Week 14 24.8 ± 3.5 24.5 ± 3.4 30.5 ± 6.1 40.1 ± 5.6 32.6 ± 5.4 30.4 ± 4.2 

* Significantly different (P#0.05) from the chamber control group by Dunn’s or Shirley’s test 

** P#0.01 a Mean ± standard error. Statistical tests were performed on unrounded data. b n=9
 

n=8
 c 
199 

APPENDIX G 

ORGAN WEIGHTS AND 

ORGAN-WEIGHT-TO-BODY-WEIGHT RATIOS 

TABLE G1 

TABLE G2 

Organ Weights and Organ-Weight-to-Body-Weight Ratios for Rats 

in the 14-Week Inhalation Study of Isobutene .............................. 

Organ Weights and Organ-Weight-to-Body-Weight Ratios for Mice 

in the 14-Week Inhalation Study of Isobutene .............................. 

200 

201 
200 Isobutene, NTP TR 487 

TABLE G1 

Organ Weights and Organ-Weight-to-Body-Weight Ratios for Rats in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

n 10 10 10 10 10 10 

Male 

Necropsy body wt 359 ± 8 367 ± 6 365 ± 8 364 ± 6 362 ± 7 366 ± 6 

Heart 

Absolute 0.993 ± 0.022 1.040 ± 0.021 1.090 ± 0.026* 1.031 ± 0.017 1.033 ± 0.015 1.050 ± 0.025 

Relative 2.77 ± 0.05 2.83 ± 0.02 2.99 ± 0.06* 2.84 ± 0.05 2.86 ± 0.06 2.87 ± 0.06 

R. Kidney 

Absolute 1.116 ± 0.019 1.191 ± 0.028 1.187 ± 0.030 1.182 ± 0.021 1.219 ± 0.030** 1.231 ± 0.024** 

Relative 3.12 ± 0.04 3.24 ± 0.04* 3.25 ± 0.03* 3.25 ± 0.06* 3.36 ± 0.05** 3.36 ± 0.04** 

Liver 

Absolute 12.073 ± 0.484 12.419 ± 0.279 12.377 ± 0.301 12.348 ± 0.291 12.438 ± 0.323 12.461 ± 0.364 

Relative 33.58 ± 0.66 33.81 ± 0.53 33.93 ± 0.29 33.91 ± 0.45 34.31 ± 0.43 33.98 ± 0.59 

Lung 

Absolute 1.649 ± 0.074 1.625 ± 0.040 1.728 ± 0.039 1.677 ± 0.055 1.666 ± 0.036 1.785 ± 0.057 

Relative 4.60 ± 0.17 4.43 ± 0.10 4.75 ± 0.14 4.62 ± 0.17 4.61 ± 0.10 4.89 ± 0.20 

R. Testis 

Absolute 1.416 ± 0.031 1.412 ± 0.026 1.417 ± 0.025 1.412 ± 0.011 1.419 ± 0.016 1.445 ± 0.011 

Relative 3.96 ± 0.07 3.84 ± 0.03 3.89 ± 0.06 3.89 ± 0.07 3.93 ± 0.06 3.95 ± 0.06 

Thymus 

Absolute 0.324 ± 0.015 0.314 ± 0.017 0.319 ± 0.010 0.332 ± 0.014 0.296 ± 0.021 0.309 ± 0.014 

Relative 0.91 ± 0.05 0.86 ± 0.05 0.88 ± 0.03 0.91 ± 0.04 0.82 ± 0.06 0.84 ± 0.04 

Female 

Necropsy body wt 207 ± 4 202 ± 2 215 ± 5 213 ± 5 208 ± 4 217 ± 6 

Heart 

Absolute 0.669 ± 0.014 0.655 ± 0.010 0.710 ± 0.017 0.689 ± 0.016 0.680 ± 0.010 0.719 ± 0.015 

Relative 3.23 ± 0.05 3.24 ± 0.04 3.30 ± 0.06 3.23 ± 0.04 3.27 ± 0.06 3.32 ± 0.05 

R. Kidney 

Absolute 0.682 ± 0.023 0.669 ± 0.012 0.746 ± 0.017 0.696 ± 0.018 0.645 ± 0.045 0.725 ± 0.017 

Relative 3.29 ± 0.10 3.31 ± 0.07 3.47 ± 0.05 3.27 ± 0.06 3.09 ± 0.21 3.35 ± 0.04 

Liver 

Absolute 6.101 ± 0.129 6.450 ± 0.133 7.267 ± 0.193** 6.875 ± 0.257** 6.673 ± 0.211** 7.006 ± 0.248** 

Relative 29.45 ± 0.51 31.92 ± 0.65* 33.75 ± 0.64** 32.20 ± 0.73* 32.01 ± 0.65* 32.27 ± 0.53* 

Lung 

Absolute 1.116 ± 0.034 1.014 ± 0.030 1.162 ± 0.069 1.182 ± 0.058 1.173 ± 0.057 1.222 ± 0.045 

Relative 5.38 ± 0.13 5.01 ± 0.11 5.38 ± 0.25 5.55 ± 0.25 5.64 ± 0.27 5.67 ± 0.28 

Thymus 

Absolute 0.235 ± 0.012 0.245 ± 0.007 0.250 ± 0.014 0.236 ± 0.011 0.243 ± 0.015 0.246 ± 0.009 

Relative 1.14 ± 0.06 1.21 ± 0.03 1.16 ± 0.05 1.10 ± 0.03 1.16 ± 0.06 1.14 ± 0.03 

* Significantly different (P#0.05) from the chamber control group by Williams’ or Dunnett’s test 

** P#0.01 a Organ weights (absolute weights) and body weights are given in grams; organ-weight-to-body-weight ratios (relative weights) are given as 

mg organ weight/g body weight (mean ± standard error). 
Isobutene, NTP TR 487 201 

TABLE G2 

Organ Weights and Organ-Weight-to-Body-Weight Ratios for Mice in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 500 ppm 1,000 ppm 2,000 ppm 4,000 ppm 8,000 ppm 

n 10 10 10 10 10 10 

Male 

Necropsy body wt 37.8 ± 1.4 38.2 ± 0.8 39.8 ± 0.8 38.0 ± 0.9 37.5 ± 0.6 37.8 ± 1.1 

Heart 

Absolute 0.164 ± 0.005 0.169 ± 0.004 0.164 ± 0.005 0.165 ± 0.005 0.166 ± 0.005 0.160 ± 0.004 

Relative 4.38 ± 0.16 4.43 ± 0.09 4.15 ± 0.17 4.35 ± 0.08 4.41 ± 0.09 4.25 ± 0.10 

R. Kidney 

Absolute 0.309 ± 0.012 0.340 ± 0.008 0.345 ± 0.010* 0.339 ± 0.007 0.336 ± 0.007 0.345 ± 0.008* 

Relative 8.26 ± 0.38 8.91 ± 0.16 8.70 ± 0.27 8.97 ± 0.24 8.95 ± 0.12 9.15 ± 0.17* 

Liver 

Absolute 1.747 ± 0.076 1.800 ± 0.034 1.819 ± 0.043 1.794 ± 0.046 1.764 ± 0.034 1.815 ± 0.054 

Relative 46.21 ± 0.72 47.23 ± 0.96 45.80 ± 0.81 47.29 ± 0.61 47.00 ± 0.58 48.04 ± 0.67 

Lung 

Absolute 0.228 ± 0.007 0.228 ± 0.016 0.232 ± 0.007 0.239 ± 0.004 0.244 ± 0.006 0.238 ± 0.004 

Relative 6.09 ± 0.21 5.97 ± 0.38 5.85 ± 0.19 6.33 ± 0.17 6.51 ± 0.15 6.34 ± 0.19 

R. Testis 

Absolute 0.121 ± 0.002 0.125 ± 0.002 0.123 ± 0.003 0.124 ± 0.002 0.127 ± 0.003 0.109 ± 0.010 

Relative 3.25 ± 0.12 3.27 ± 0.04 3.11 ± 0.11 3.28 ± 0.08 3.38 ± 0.06 2.91 ± 0.27 

Thymus 

Absolute 0.037 ± 0.003 0.037 ± 0.002 0.037 ± 0.003 0.040 ± 0.002 0.041 ± 0.002 0.036 ± 0.003 

Relative 0.99 ± 0.09 0.97 ± 0.05 0.94 ± 0.08 1.04 ± 0.04 1.10 ± 0.04 0.95 ± 0.08 

Female 

Necropsy body wt 33.8 ± 0.7 32.6 ± 0.9 35.6 ± 1.2 34.8 ± 0.9 33.8 ± 1.0 33.9 ± 0.8 

Heart 

Absolute 0.134 ± 0.003 0.138 ± 0.002 0.142 ± 0.003 0.140 ± 0.003 0.135 ± 0.002 0.137 ± 0.003 

Relative 3.98 ± 0.10 4.25 ± 0.10 4.02 ± 0.13 4.04 ± 0.09 4.02 ± 0.10 4.05 ± 0.10 

R. Kidney 

Absolute 0.198 ± 0.004 0.226 ± 0.005** 0.226 ± 0.006** 0.222 ± 0.005** 0.226 ± 0.005** 0.234 ± 0.003** 

Relative 5.89 ± 0.19 6.95 ± 0.19** 6.38 ± 0.12** 6.40 ± 0.13** 6.72 ± 0.16** 6.93 ± 0.13** 

Liver 

Absolute 1.603 ± 0.036 1.622 ± 0.038 1.697 ± 0.050 1.694 ± 0.047 1.659 ± 0.050 1.665 ± 0.040 

Relative 47.48 ± 0.59 49.80 ± 0.91 47.85 ± 0.76 48.72 ± 0.59 49.14 ± 0.89 49.11 ± 0.45 

Lung 

Absolute 0.232 ± 0.007 0.233 ± 0.005 0.247 ± 0.007 0.248 ± 0.005 0.233 ± 0.007 0.236 ± 0.004 

Relative 6.88 ± 0.20 7.17 ± 0.20 6.97 ± 0.14 7.15 ± 0.13 6.90 ± 0.11 6.99 ± 0.18 

Thymus 

Absolute 0.054 ± 0.002 0.055 ± 0.003 0.062 ± 0.006 0.057 ± 0.002 0.054 ± 0.004 0.055 ± 0.003 

Relative 1.60 ± 0.07 1.68 ± 0.08 1.71 ± 0.10 1.65 ± 0.07 1.60 ± 0.09 1.62 ± 0.06 

* Significantly different (P#0.05) from the chamber control group by Williams’ or Dunnett’s test 

** P#0.01 a Organ weights (absolute weights) and body weights are given in grams; organ-weight-to-body-weight ratios (relative weights) are given as 

mg organ weight/g body weight (mean ± standard error). 
202 Isobutene, NTP TR 487
 
203 

APPENDIX H
 

REPRODUCTIVE TISSUE EVALUATIONS
 

AND ESTROUS CYCLE CHARACTERIZATION
 

TABLE H1 Summary of Reproductive Tissue Evaluations for Male Rats 

in the 14-Week Inhalation Study of Isobutene .............................. 204 

TABLE H2 Summary of Estrous Cycle Characterization for Female Rats 

in the 14-Week Inhalation Study of Isobutene .............................. 204 

TABLE H3 Summary of Reproductive Tissue Evaluations for Male Mice 

in the 14-Week Inhalation Study of Isobutene .............................. 205 

TABLE H4 Summary of Estrous Cycle Characterization for Female Mice 

in the 14-Week Inhalation Study of Isobutene .............................. 205 
204 Isobutene, NTP TR 487 

TABLE H1 

Summary of Reproductive Tissue Evaluations for Male Rats in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 2,000 ppm 4,000 ppm 8,000 ppm 

n 10 10 10 10 

Weights (g) 

Necropsy body wt 

L. cauda epididymis 

L. epididymis 

L. testis 

358 ± 8 

0.1742 ± 0.0063 

0.4650 ± 0.0067 

1.4802 ± 0.0260 

364 ± 6 

0.1906 ± 0.0020 

0.4758 ± 0.0040 

1.4905 ± 0.0079 

362 ± 7 

0.1838 ± 0.0065 

0.4601 ± 0.0107 

1.4913 ± 0.0304 

366 ± 6 

0.1936 ± 0.0046* 

0.4780 ± 0.0067 

1.5185 ± 0.0211 

Spermatid measurements 

Spermatid heads (10 /g testis) 7 

Spermatid heads (10 /testis) 7 

Spermatid count 

(mean/10 mL suspension) -4 

9.89 ± 0.34 

14.63 ± 0.54 

73.13 ± 2.68 

10.11 ± 0.40 

15.07 ± 0.62 

75.35 ± 3.09 

9.95 ± 0.28 

14.82 ± 0.44 

74.08 ± 2.18 

9.97 ± 0.22 

15.15 ± 0.43 

75.75 ± 2.13 

Epididymal spermatozoal measurements 

Motility (%) 

Concentration 

 (10 /g cauda epididymal tissue) 6 

91.15 ± 3.18 

886 ± 55 

89.32 ± 2.01 

836 ± 39 

88.04 ± 1.97 

819 ± 66 

86.53 ± 1.90* 

840 ± 39 

* Significantly different (P#0.05) from the chamber control group by Shirley’s test (motility) or by Dunnett’s test (left caudal weights) a Data are presented as mean ± standard error. Differences from the chamber control group are not significant by Dunnett’s test (body 

weights) or Dunn’s test (spermatid measurements and epididymal spermatozoal measurements). 

TABLE H2 

Summary of Estrous Cycle Characterization for Female Rats in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 2,000 ppm 4,000 ppm 8,000 ppm 

n 10 10 10 10 

Necropsy body wt (g) 207 ± 4 213 ± 5 208 ± 4 217 ± 6 

Estrous cycle length (days) 4.70 ± 0.15 4.80 ± 0.13 4.80 ± 0.11 4.80 ± 0.08 

Estrous stages (% of cycle) 

Diestrus 40.0 38.3 38.3 39.2 

Proestrus 17.5 18.3 18.3 13.3 

Estrus 20.8 20.8 25.0 24.2 

Metestrus 20.8 20.8 18.3 21.7 

Uncertain diagnoses 0.8 1.7 0.0 1.7 

a Necropsy body weight and estrous cycle length data are presented as mean ± standard error. Differences from the chamber control group 

are not significant by Dunnett’s test (body weights) or Dunn’s test (estrous cycle length). By multivariate analysis of variance, exposed 

females do not differ significantly from the chamber control females in the relative length of time spent in the estrous stages. 
 

 

Isobutene, NTP TR 487 205 

TABLE H3 

Summary of Reproductive Tissue Evaluations for Male Mice in the 14-Week Inhalation Study of Isobutenea 

Chamber Control 2,000 ppm 4,000 ppm 8,000 ppm 

n 10 10 10 10 

Weights (g) 

Necropsy body wt 

L. cauda epididymis 

L. epididymis 

L. testis 

37.8 ± 1.4 

0.0167 ± 0.0011 

0.0436 ± 0.0016 

0.1165 ± 0.0020 

38.0 ± 0.9 

0.0187 ± 0.0014 

0.0485 ± 0.0015 

0.1191 ± 0.0010 

37.5 ± 0.6 

0.0196 ± 0.0007 

0.0462 ± 0.0017 

0.1201 ± 0.0018 

37.8 ± 1.1 

0.0173 ± 0.0009 

0.0426 ± 0.0022 

0.1038 ± 0.0093 

Spermatid measurements 

Spermatid heads (10 /g testis) 7 

Spermatid heads (10 /testis) 7 

Spermatid count 

(mean/10 mL suspension) -4 

18.17 ± 0.54 

2.12 ± 0.08 

66.20 ± 2.39 

17.49 ± 0.53 

2.09 ± 0.07 

65.15 ± 2.30 

18.21 ± 0.90 

2.18 ± 0.10 

68.23 ± 3.22 

15.32 ± 1.80 

1.73 ± 0.20 

53.95 ± 6.29 

Epididymal spermatozoal measurements 

Motility (%) 

Concentration 

 (10 /g cauda epididymal tissue) 6 

79.31 ± 2.74 

1,261 ± 142b 

77.65 ± 1.94 

1,534 ± 151 

76.94 ± 1.53 

1,225 ± 88b 

76.07 ± 1.38b 

1,444 ± 228 

a Data are presented as mean ± standard error. Differences from the chamber control group are not significant by Dunnett’s test (body and 

tissue weights) or Dunn’s test (spermatid and epididymal spermatozoal measurements). b n=9 

TABLE H4 

Summary of Estrous Cycle Characterization for Female Mice in the 14-Week Inhalation Study of 

Isobutenea 

Chamber Control 2,000 ppm 4,000 ppm 8,000 ppm 

n	 10 10 10 10 

Necropsy body wt (g) 33.8 ± 0.7 34.8 ± 0.9 33.8 ± 1.0 33.9 ± 0.8 

Estrous cycle length (days) 4.50 ± 0.15 4.11 ± 0.07b 4.56 ± 0.13b 4.20 ± 0.11 c Estrous stages (% of cycle)

Diestrus 27.5 34.2 39.2 33.3 

Proestrus 16.7 21.7 19.2 20.0 

Estrus 37.5 25.8 24.2 29.2 

Metestrus 17.5 18.3 17.5 15.0 

Uncertain diagnoses 0.8 0.0 0.0 2.5 

a Necropsy body weight and estrous cycle length data are presented as mean ± standard error. Differences from the chamber control group 

are not significant by Dunnett’s test (body weights) or Dunn’s test (estrous cycle length). b	 Estrous cycle was longer than 12 days or unclear in 1 of 10 animals. 

Evidence shows that females exposed to 2,000 or 4,000 ppm differ significantly (Wilk’s Criterion, P#0.05) from the chamber control 

females in the relative length of time spent in the estrous stages. Exposed females spent more time in diestrus and less time in estrus than 

chamber control females. 

c 
 

206 Isobutene, NTP TR 487
 
207 

APPENDIX I 

2-HYDROXYISOBUTYRIC ACID — 

BIOMARKER OF EXPOSURE 

TABLE I1 

TABLE I2 

Urinary Biomarker Data for Rats in the 2-Year Inhalation Study of Isobutene 

Urinary Biomarker Data for Mice in the 2-Year Inhalation Study of Isobutene 

....... 

....... 

208 

209 
208 Isobutene, NTP TR 487 

TABLE I1 

Urinary Biomarker Data for Rats in the 2-Year Inhalation Study of Isobutene 

Chamber Trend 

Control 500 ppm 2,000 ppm 8,000 ppm P Valuea 

n 5 10 10 10 

Male 

Urine excretion (g) 

6 Months 11.04 ± 1.67b 9.08 ± 0.46 9.46 ± 0.94 8.80 ± 0.70 NSc 

12 Months 6.83 ± 0.91 7.39 ± 0.69 7.61 ± 0.95 6.93 ± 0.44 NS 

18 Months 8.96 ± 0.69 9.85 ± 1.11 12.11 ± 1.17 9.56 ± 0.73 NS 

Urine creatinine (μg/g urine) 

6 Months 934.0 ± 90.3 1,073.0 ± 54.5 1,083.0 ± 79.5 1,131.0 ± 79.7 NS 

12 Months 1,114.0 ± 95.7 1,240.0 ± 100.1 1,164.0 ± 83.2 1,279.0 ± 61.4 NS 

18 Months 1,044.0 ± 84.5 989.0 ± 83.5 869.0 ± 60.9 1,011.0 ± 44.2 NS 

Urine HIBA/urine (mg/total sample) 

6 Months —d 5.040 ± 0.128 12.859 ± 0.273** 19.920 ± 0.379** P#0.01 

12 Months — 5.353 ± 0.203 15.857 ± 0.643** 24.746 ± 1.352** P#0.01 

18 Months — 4.059 ± 0.243 12.551 ± 1.650** 23.693 ± 1.132** P#0.01 

Urine HIBA/creatinine (μg/μg) 

6 Months — 0.530 ± 0.010 1.348 ± 0.035** 2.103 ± 0.057** P#0.01 

12 Months — 0.630 ± 0.020 1.959 ± 0.114** 2.843 ± 0.070** P#0.01 

18 Months — 0.457 ± 0.018 1.211 ± 0.079** 2.526 ± 0.065** P#0.01 

Urine HIBA/creatinine/isobutene (ng/μg/ppm) 

6 Months — 1.060 ± 0.020 0.674 ± 0.017** 0.263 ± 0.007** P#0.01 

12 Months — 1.260 ± 0.041 0.979 ± 0.057** 0.355 ± 0.009** P#0.01 

18 Months — 0.914 ± 0.035 0.606 ± 0.040** 0.316 ± 0.008** P#0.01 

Female 

Urine excretion (g) 

6 Months 6.70 ± 1.06 8.13 ± 1.37 9.65 ± 1.47 6.26 ± 0.71 NS 

12 Months 6.30 ± 0.61 9.99 ± 3.67 9.71 ± 3.76 5.95 ± 0.86 NS 

18 Months 6.44 ± 0.24 7.06 ± 0.73 9.82 ± 3.33 7.72 ± 0.64 NS 

Urine creatinine (μg/g urine) 

6 Months 652.0 ± 62.2 733.0 ± 121.5 593.0 ± 88.6 693.0 ± 64.6 NS 

12 Months 754.0 ± 65.2 787.0 ± 112.1 752.0 ± 91.1 846.0 ± 74.7 NS 

18 Months 708.0 ± 30.1 652.0 ± 51.7 707.0 ± 83.2 731.0 ± 60.7 NS 

Urine HIBA/urine (mg/total sample) 

6 Months — 3.317 ± 0.146 6.203 ± 0.248** 8.657 ± 0.788** P#0.01 

12 Months — 3.328 ± 0.265 8.541 ± 0.495** 13.356 ± 0.920** P#0.01 

18 Months — 2.684 ± 0.340 8.431 ± 0.780** 15.450 ± 0.747** P#0.01 

Urine HIBA/creatinine (μg/μg) 

6 Months — 0.731 ± 0.025 1.347 ± 0.075** 2.064 ± 0.060** P#0.01 

12 Months — 0.672 ± 0.041 1.831 ± 0.123** 2.916 ± 0.122** P#0.01 

18 Months — 0.564 ± 0.066 1.825 ± 0.122** 2.932 ± 0.222** P#0.01 

Urine HIBA/creatinine/isobutene (ng/μg/ppm) 

6 Months — 1.462 ± 0.050 0.674 ± 0.037** 0.258 ± 0.008** P#0.01 

12 Months — 1.344 ± 0.082 0.916 ± 0.061** 0.365 ± 0.015** P#0.01 

18 Months — 1.128 ± 0.131 0.913 ± 0.061** 0.367 ± 0.028** P#0.01 

** Significantly different (P#0.01) from the 500 ppm group by Dunnett’s test a The trend was assessed by linear regression analysis. b	 Mean ± standard error. HIBA=2-hydroxyisobutyric acid 

Not significant d	 Sample measurement less than limit of quantitation 

c 
Isobutene, NTP TR 487 209 

TABLE I2 

Urinary Biomarker Data for Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Trend 

Control 500 ppm 2,000 ppm 8,000 ppm P Valuea 

Male 

n 

6 Months 5 10 10 9 

12 Months 5 9 7 8 

18 Months 5 10 9 10 

Urine excretion (g) 

6 Months 1.34 ± 0.18b 1.31 ± 0.23 1.15 ± 0.18 1.71 ± 0.33 NSc 

12 Months 1.64 ± 0.26 2.05 ± 0.10 3.62 ± 1.22 1.80 ± 0.22 NS 

18 Months 1.49 ± 0.37 1.84 ± 0.22 1.97 ± 0.32 1.84 ± 0.25 NS 

Urine creatinine (μg/g urine) 

6 Months 314.0 ± 34.9 302.0 ± 17.0 337.5 ± 19.6d 282.2 ± 18.4 NS 

12 Months 244.0 ± 21.8 270.0 ± 25.4 220.0 ± 35.8 220.0 ± 29.5 NS 

18 Months 230.0 ± 40.4 252.0 ± 11.1 242.2 ± 26.2 214.0 ± 21.9 NS 

Urine HIBA/urine (mg/total sample) 

6 Months —e 1.075 ± 0.174 3.224 ± 0.654* 6.169 ± 1.265** P#0.01 

12 Months — 2.046 ± 0.079 7.336 ± 0.334** 10.971 ± 1.757** P#0.01 

18 Months — 1.629 ± 0.134 6.079 ± 0.903** 11.277 ± 1.497** P#0.01 

Urine HIBA/creatinine (μg/μg) 

6 Months — 2.832 ± 0.222 8.665 ± 1.565**d 13.298 ± 1.640** P#0.01 

12 Months — 3.971 ± 0.343 14.226 ± 1.689** 26.269 ± 2.839** P#0.01 

18 Months — 4.159 ± 0.749 11.882 ± 0.833** 25.888 ± 2.127** P#0.01 

Urine HIBA/creatinine/isobutene (ng/μg/ppm) 

6 Months — 5.664 ± 0.444 4.333 ± 0.782d 1.662 ± 0.205** P#0.01 

12 Months — 7.942 ± 0.687 7.113 ± 0.844 3.284 ± 0.355** P#0.01 

18 Months — 8.318 ± 1.498 5.941 ± 0.416 3.236 ± 0.266** P#0.01 

Female 

n 

6 Months 5 10 9 10 

12 Months 5 10 10 10 

18 Months 5 10 10 10 

Urine excretion (g) 

6 Months 1.47 ± 0.14 1.17 ± 0.12 1.39 ± 0.13 1.14 ± 0.17 NS 

12 Months 1.66 ± 0.17 1.62 ± 0.11 2.96 ± 1.21 1.44 ± 0.08 NS 

18 Months 1.60 ± 0.13 1.70 ± 0.15 1.81 ± 0.23 1.55 ± 0.10 NS 

Urine creatinine (μg/g urine) 

6 Months 294.0 ± 18.6 331.0 ± 33.4 331.1 ± 30.2 352.2 ± 28.7f NS 

12 Months 278.0 ± 22.0 217.0 ± 9.3 235.0 ± 23.2 274.0 ± 11.1 NS 

18 Months 264.0 ± 16.3 208.0 ± 15.5 246.0 ± 20.9 241.0 ± 10.6 NS 

Urine HIBA/urine (mg/total sample) 

6 Months — 1.370 ± 0.130 5.441 ± 0.446** 5.843 ± 0.841** P#0.05 

12 Months — 1.640 ± 0.078 5.894 ± 0.475** 8.294 ± 0.779** P#0.01 

18 Months — 1.668 ± 0.137 6.349 ± 0.292** 8.524 ± 0.743** P#0.01 

Urine HIBA/creatinine (μg/μg) 

6 Months — 4.017 ± 0.529 12.341 ± 0.653** 15.977 ± 1.361**f P#0.01 

12 Months — 4.830 ± 0.266 12.989 ± 1.389** 21.375 ± 2.187** P#0.01 

18 Months — 4.983 ± 0.345 15.898 ± 1.205** 22.758 ± 1.859** P#0.01 
210 

c 

Isobutene, NTP TR 487 

TABLE I2 

Urinary Biomarker Data for Mice in the 2-Year Inhalation Study of Isobutene 

Chamber Trend 

Control 500 ppm 2,000 ppm 8,000 ppm P Value 

Female (continued) 

n 

6 Months

12 Months

18 Months

 5 

5 

5 

10

10

10

 9

 10

 10

 10 

10 

10 

Urine HIBA/creatinine/isobutene (ng/μg/ppm) 

6 Months — 

12 Months — 

18 Months — 

8.034 ± 1.057 

9.660 ± 0.532 

9.966 ± 0.691 

6.171 ± 0.326d 

6.495 ± 0.695** 

7.949 ± 0.603 

1.997 ± 0.170**f 

2.672 ± 0.273** 

2.845 ± 0.232** 

P#0.01 

P#0.01 

P#0.01 

* Significantly different (P#0.05) from the 500 ppm group by Dunnett’s test 

** P#0.01 a The trend was assessed by linear regression analysis. b	 Mean ± standard error. HIBA=2-hydroxyisobutyric acid 

Not significant d n=8 e Sample measurement less than limit of quantitation f n=9 
211 

APPENDIX J
 

CHEMICAL CHARACTERIZATION AND
 

GENERATION OF CHAMBER CONCENTRATIONS
 

PROCUREMENT AND CHARACTERIZATION OF ISOBUTENE ................................. 212
 

VAPOR GENERATION AND EXPOSURE SYSTEM ......................................... 213
 

VAPOR CONCENTRATION MONITORING ............................................. 214
 

CHAMBER ATMOSPHERE CHARACTERIZATION ........................................ 214
 

FIGURE J1 Infrared Absorption Spectrum of Isobutene ................................ 216
 

FIGURE J2 Nuclear Magnetic Resonance Spectrum of Isobutene .......................... 217
 

FIGURE J3 Schematic of Generation and Delivery System .............................. 218
 

FIGURE J4 14-Week Inhalation Suite ............................................ 219
 

FIGURE J5 2-Year Inhalation Suite .............................................. 220
 

TABLE J1 Summary of Chamber Concentrations in the 14-Week Inhalation Studies of Isobutene ... 221
 

TABLE J2 Summary of Chamber Concentrations in the 2-Year Inhalation Studies of Isobutene .... 221
 
212 Isobutene, NTP TR 487 

CHEMICAL CHARACTERIZATION AND 

GENERATION OF CHAMBER CONCENTRATIONS 

PROCUREMENT AND CHARACTERIZATION OF ISOBUTENE 

Isobutene was manufactured by Exxon, Inc. (Baytown, TX), supplied by Specialty Gas Concepts (La Porte, 

TX) and shipped through Norco (Kennewick, WA) in two lots (SGC051091ECA and SGC020594ECA). 

Lot SGC051091ECA was used during the 14-week and 2-year studies, and lot SGC020594ECA was used 

during the 2-year studies. Identity, purity, and stability analyses were conducted by the study laboratory, 

Battelle Pacific Northwest Laboratories (Richland, WA). Reports on analyses performed in support of the 

isobutene studies are on file at the National Institute of Environmental Health Sciences. 

The chemical, a colorless vapor at room temperature, was identified as isobutene by infrared and nuclear 

magnetic resonance spectroscopy. The infrared spectrum was consistent with a literature spectrum (Aldrich, 

1985) of isobutene. Both spectra were consistent with those expected for the structure of isobutene and with 

those of a reference standard supplied by Matheson Gas Products (East Rutherford, NJ) at a stated purity of 

99.9%. The infrared and nuclear magnetic spectra are presented in Figures J1 and J2. 

The initial purity of each lot was determined with gas chromatography/flame ionization detection (GC/FID). 

Two systems were used for initial purity determinations on each lot: 

A) Al 2 3 O /KCl PLOT column (10 M × 0.53 mm), with an oven temperature program of 45E C

for 5 minutes, then 45E to 180E C at 20E C per minute and held for 1 minute, using a 

nitrogen carrier gas at a flow rate of 30 mL/minute, 

B) DB-624 fused silica column (30 M × 0.53 mm) with a 3 μm film thickness, with an oven 

temperature program of S15E C for 10 minutes, then S15E to 100E C at 20E C per minute 

and held for 1 minute, and a nitrogen carrier gas at a flow rate of 5 to 15 mL/minute, and 

C) Al O /Na SO PLOT column (50 M × 0.53 mm), with an oven temperature program of 2 3 2 4 

130E C for 5 minutes to 190E C (no hold time) at 10E C/minute using nitrogen as a carrier 

gas at a flow rate of 30 mL/minute. 

Major peak comparisons with GC/FID indicated a relative purity of 100.0% by system A and 100.6% by 

system B for lot SGC051091ECA relative to the reference standard. For lot SGC020594ECA, gas 

chromatographic peak comparison indicated a relative purity of 98.7% by system C and and 98.5% by 

system A. The overall purity of lot SGC051091ECA was determined to be greater than 99%, and the 

overall purity of lot SGC020594ECA was greater than 98%. 

Additional analyses of each lot were performed with gas chromatography/mass spectrometry (GC/MS) to 

identify and quantify the impurities indicated by the manufacturer or by GC/FID. The gas chromatograph 

system included a flame ionization detector, an RTX volatile fused silica column (60 M × 0.32 mm with 

1.5 μm film thickness) with a nitrogen carrier gas at a flow rate of 1.7 mL per minute and an oven 

temperature program of S15E C for 12 minutes, then S15E to 150E C at 15E C per minute and held for 

3 minutes. The mass spectrum pattern of the major peak was consistent with isobutene. The following 

impurities were detected for lot SGC051091ECA (concentrations were estimated): propane/propene, 

42 ppm; isobutane, 140 ppm; 1,3-butadiene, 6 ppm; trans-2-butene, 213 ppm; and cis-2-butene, 286 ppm. 

Butane and n-butene coeluted with the major peak and were not quantified. Lot SGC020594ECA contained 

an estimated 11 ppm propane/propene, 339 ppm isobutane, 15 ppm trans-2-butene, 11 ppm cis-2-butene, 

and no 1,3-butadiene. Samples from each cylinder of each lot of isobutene were analyzed for 1,3-butadiene 
Isobutene, NTP TR 487 213 

before the cylinder was used for exposure by GC/MS with the same system but with an isothermal oven 

temperature of S15E C. All cylinders used in the 14-week studies contained less than 50 ppm 

1,3-butadiene, well within the maximum limit of 100 ppm; the maximum concentration detected in cylinders 

used in the 2-year studies was 15 ppm. Cylinders used in the 14-week studies were also screened 

individually for other impurities; results indicated less than 1% impurities by peak area. 

In a 4-day pilot study, the stability of isobutene was monitored in grab-bag samples taken from the 

distribution manifold at the beginning and the end of 6-hour generation periods until approximately 94% of 

the cylinder was exhausted. Isobutene in the distribution manifold was assumed to be of equivalent purity to 

that in the cylinder headspace because no dilution flow was present. Samples were analyzed using GC/FID 

with a column as in system B (14-week studies) or a column as in system C (2-year studies) and using 

modified temperature programs. The results from samples taken over 8 test generation days showed no 

significant enhancement of any volatile impurities, and no additional impurities were detected with relative 

areas of 0.1% or greater relative to isobutene. Based on these results, approximately 90% of the contents of 

each cylinder were used during the 14-week and 2-year studies. 

During the studies, the bulk chemical was stored in its original shipping cylinders at approximately 22E C. 

Stability was monitored throughout the studies by the study laboratory with GC/FID by system A. No 

degradation of the bulk chemical was detected. 

VAPOR GENERATION AND EXPOSURE SYSTEM 

Diagrams of the isobutene generation and delivery system used in the 14-week and 2-year studies are shown 

in Figure J3. Because isobutene is a vapor at room temperature, it was distributed under regulated 

pressure, and the chemical flow rate to each chamber was monitored and adjusted individually. Isobutene 

was delivered directly from the cylinder. Two cylinders of isobutene were connected in parallel to the 

exposure system; one supplied isobutene for exposures while the other cylinder was available if the first did 

not maintain sufficient gas pressure in the distribution manifold. Warm circulating-water blankets 

surrounding the cylinders provided additional heat to replace the heat lost due to isobutene vaporization. 

The manifold pressure was regulated to approximately 7 psi with a two-stage regulator on the cylinder. The 

gas passed through a filter via a main on/off pneumatic valve, operated either manually or by computer, and 

then was distributed by a manifold to five (14-week studies) or six (2-year studies) pairs of metering valves 

with corresponding flow meters. Isobutene was delivered to each exposure chamber through these flow 

meters via three-way solenoid valves located at the chamber end of the vapor delivery line. Each three-way 

valve was controlled either manually or by computer; when the valve to a chamber was closed, the vapor 

was routed to the exposure system exhaust. 

Isobutene vapor was diluted with conditioned air as it was injected into the chamber inlet duct. The 

concentration in each chamber was controlled by manually adjusting the individual chamber metering 

valves. The generation system was purged with nitrogen at the end of the exposure day. 

Stainless-steel chambers (Hazleton H-2000®) manufactured by Lab Products, Inc. (Harford Systems 

Division, Aberdeen, MD) were used throughout the studies. The total volume of each chamber was 2.3 m ;

3 the active mixing volume of each chamber was 1.7 m . The chamber was designed by the study laboratory 

so that uniform vapor concentrations could be maintained through the chamber when catch pans were in 

place. Diagrams of the inhalation suites are shown in Figures J4 and J5. 

VAPOR CONCENTRATION MONITORING 

Chamber concentrations of isobutene were monitored during both studies by an on-line GC/FID with a 

column as in system A and a modified temperature program. Samples were drawn from each chamber, the 

3 
 

 

 

 

214 Isobutene, NTP TR 487 

exposure room, and the on-line standard approximately every 20 minutes during exposures by a computercontrolled, 12-port stream select valve. 

The on-line GC/FID was calibrated by direct analysis of volumetrically prepared gas-bag standards during 

the 14-week studies and against validated, commercially prepared, certified standards for the 2-year studies. 

Calibrations were performed approximately once a month or if drift occurred in the value of the on-line 

standard. An on-line standard of isobutene in nitrogen was used to monitor instrument drift before the start 

of each exposure day and once during each monitoring cycle. Standard exposure and chamber samples 

were taken in triplicate. A Teflon® line was coupled directly to the calibration port of the on-line monitor 

stream select valve. 

Summaries of the chamber concentrations for the 14-week and 2-year studies are presented in Tables J1 and 

J2. 

CHAMBER ATMOSPHERE CHARACTERIZATION 

The times for the exposure concentration to build up to 90% of the final exposure concentration (T ) and to 90 

decay to 10% of the exposure concentration (T ) were measured. In all studies, T and T were 10 90 10 

measured in all exposure chambers with and without animals present. At a chamber air flow rate of 15 air 

changes per hour, the theoretical value for both T and T was calculated to be 12.5 minutes. The actual 90 10 

chamber air flow rates were maintained at 12 to 18 changes per hour during all studies. 

In the 14-week studies, without animals present, T values and T values ranged from 8 to 9 minutes for 90 10 

rats and mice. With animals present, T values ranged from 9 to 10 minutes for rats and mice; T values 90 10 

ranged from 9 to 11 minutes for rats and mice. In the 2-year studies, without animals present, T90 values 

ranged from 11 to 15 minutes for rats and from 9 to 13 minutes for mice; T10 values ranged from 8 to 

10 minutes for rats and from 6 to 9 minutes for mice. With animals present, T90 values ranged from 9 to 

12 minutes for rats and from 8 to 11 minutes for mice; T10 values ranged from 9 to 12 minutes for rats and 

from 10 to 11 minutes for mice. The T90 value selected for all studies was 12 minutes. 

Vapor concentration uniformity in the exposure chambers without animals present was measured before 

each of the studies began. Concentration uniformity with animals present was measured during the first 

week of the 14-week rat study and at approximately 90-day intervals during the 2-year studies. Vapor 

concentration was measured using the on-line GC/FID with the automatic 12-port sample valve disabled to 

allow continuous monitoring from a single input line. Samples were taken from several positions in each 

chamber without animals and from two positions with animals present. Chamber concentration uniformity 

was maintained throughout the studies. 

During the 14-week and 2-year studies, the persistence of isobutene after exposure ceased was monitored by 

GC/FID in the 8,000 ppm chambers with animals present. Persistence in the chambers with no animals 

present was tested before the 14-week and 2-year studies began. Without animals present, the concentration 

fell to less than 1% of the target concentration within 19 minutes for the 14-week studies and 2-year rat 

study; for the 2-year mouse study, the concentration fell to less than 1% of the target concentration within 

16 minutes. In the 14-week studies with animals present, the concentration decreased to less than 1% 

within 22 minutes. In the 2-year studies with animals present, the concentration decreased to less than 1% 

of the target concentration within 24 minutes (rat chamber) or 25 minutes (mouse chamber). 

Before and during the 14-week and 2-year studies, the stability of isobutene in the 500 and 8,000 ppm 

chambers was analyzed with the on-line GC/FID within the first and last hours of the exposure day. 

Additionally, samples from the 500 and 8,000 ppm chambers were collected with a syringe and transferred 

to a gas sample bag; these samples and samples collected directly from the distribution manifold were 
Isobutene, NTP TR 487 215 

analyzed by an off-line GC/FID with a DB-624 column (14-week studies) or GC/FID with an Al 2 3 2 4 O /Na SO

PLOT column (2-year studies) within the first and last hours of the exposure day. Mixed standard gas bags 

containing equivalent concentrations of each of the expected volatile contaminants (propene, propane, 

isobutane, 1-butene, 1,3-butadiene, n-butane, trans-2-butene, and cis-2-butene) were volumetrically 

prepared in addition to an isobutene reference standard gas bag. During the 14-week study analyses, butene 

was the only contaminant completely unresolved from isobutene on both GC/FIDs. One unidentified 

impurity with a concentration of 0.14% relative to the concentration of isobutene was detected by the offline GC/FID at the beginning of the exposure period in the 8,000 ppm chamber before the 14-week studies 

began; this impurity was not detected at the end of the exposure period. An unidentified impurity was also 

detected in the 500 ppm chamber with a relative concentration of 0.12% at the beginning of exposure and 

0.04% at the end of the exposure period. No other impurities were detected with relative areas of 0.1% or 

greater during the 14-week studies, although several volatile impurities were present in the 500 and 

8,000 ppm chambers and the distribution manifold at concentrations of less than 0.1% relative to the 

isobutene concentrations. No impurities with concentrations of 0.1% or greater relative to the isobutene 

concentration were detected by the on-line or off-line GC/FID before the 2-year studies began. During the 

2-year studies, the on-line GC/FID detected an unidentified impurity at the beginning of the exposure period 

in the 500 ppm rat and mouse chambers at relative concentrations of 0.6% and 0.7%, respectively, and at 

the end of the exposure period in the 500 ppm mouse chamber with a relative concentration of less than 1%. 

No other impurities with concentrations of 0.1% or greater relative to the isobutene concentrations were 

identified during the 2-year studies by either GC/FID. 

Relative concentrations of 1,3-butadiene remained well below 100 ppm. The results from these stability 

studies indicated that isobutene was stable under the conditions used to generate and transport it to the 

exposure chambers and that no significant enhancement of impurities was detected in the distribution 

manifold or the exposure chambers over the course of a typical exposure day. 
216 Isobutene, NTP TR 487
 

FIGURE J1 

Infrared Absorption Spectrum of Isobutene 
Isobutene, NTP TR 487
 217
 

FIGURE J2 

Nuclear Magnetic Resonance Spectrum of Isobutene 
218 Isobutene, NTP TR 487
 

FIGURE J3 

Schematic of Generation and Delivery System 
Isobutene, NTP TR 487
 219 

FIGURE J4 

14-Week Inhalation Suite 
220 Isobutene, NTP TR 487
 

FIGURE J5 

2 Year Inhalation Suite 
 

Isobutene, NTP TR 487 221 

TABLE J1 

Summary of Chamber Concentrations in the 14-Week Inhalation Studies of Isobutene 

Target Concentration Total Number of Readings Average Concentrationa 

(ppm) (ppm) 

Rat Chambers 

500 1,176 495 ± 16 

1,000 1,183 1,010 ± 35 

2,000 1,185 1,990 ± 61 

4,000 1,185 4,010 ± 130 

8,000 1,184 7,970 ± 585 

Mouse Chambers 

500 1,193 495 ± 16 

1,000 1,201 1,010 ± 35 

2,000 1,203 1,990 ± 62 

4,000 1,203 4,010 ± 130 

8,000 1,202 7,980 ± 582 

a Mean ± standard deviation 

TABLE J2 

Summary of Chamber Concentrations in the 2-Year Inhalation Studies of Isobutene 

Target Concentration Total Number of Readings Average Concentrationa 

(ppm) (ppm) 

Rat Chambers 

500 7,467 497 ± 21 

2,000 7,521 1,990 ± 72 

8,000 7,550 7,940 ± 313 

Mouse Chambers 

500 7,476 498 ± 20 

2,000 7,430 1,990 ± 74 

8,000 7,536 7,960 ± 283 

a Mean ± standard deviation 
222 Isobutene, NTP TR 487
 
223 

APPENDIX K 

INGREDIENTS, NUTRIENT COMPOSITION, 

AND CONTAMINANT LEVELS 

IN NIH-07 RAT AND MOUSE RATION 

TABLE K1 

TABLE K2 

TABLE K3 

TABLE K4 

Ingredients of NIH-07 Rat and Mouse Ration .............................. 

Vitamins and Minerals in NIH-07 Rat and Mouse Ration ...................... 

Nutrient Composition of NIH-07 Rat and Mouse Ration ...................... 

Contaminant Levels in NIH-07 Rat and Mouse Ration ........................ 

224 

224 

225 

226 
 

 

224 Isobutene, NTP TR 487 

TABLE K1 

Ingredients of NIH-07 Rat and Mouse Rationa 

Ingredientsb Percent by Weight 

Ground #2 yellow shelled corn 24.50 

Ground hard winter wheat 23.00 

Soybean meal (49% protein) 12.00 

Fish meal (60% protein) 10.00 

Wheat middlings 10.00 

Dried skim milk 5.00 

Alfalfa meal (dehydrated, 17% protein) 4.00 

Corn gluten meal (60% protein) 3.00 

Soy oil 2.50 

Dried brewer’s yeast 2.00 

Dry molasses 1.50 

Dicalcium phosphate 1.25 

Ground limestone 0.50 

Salt 0.50 

Premixes (vitamin and mineral) 0.25 

a NCI, 1976; NIH, 1978 b Ingredients were ground to pass through a U.S. Standard Screen No. 16 before being mixed. 

TABLE K2 

Vitamins and Minerals in NIH-07 Rat and Mouse Rationa 

Amount Source 

Vitamins

 A 5,500,000 IU Stabilized vitamin A palmitate or acetate

 D3 4,600,000 IU D-activated animal sterol 

K3 2.8 g Menadione 

d-"-Tocopheryl acetate 20,000 IU

 Choline 560.0 g Choline chloride

 Folic acid 2.2 g

 Niacin 30.0 g 

d-Pantothenic acid 18.0 g d-Calcium pantothenate

 Riboflavin 3.4 g

 Thiamine 10.0 g Thiamine mononitrate

 B12 4,000 μg 

Pyridoxine 1.7 g Pyridoxine hydrochloride

 Biotin 140.0 mg d-Biotin 

Minerals

 Iron 120.0 g Iron sulfate

 Manganese 60.0 g Manganous oxide

 Zinc 16.0 g Zinc oxide

 Copper 4.0 g Copper sulfate

 Iodine 1.4 g Calcium iodate

 Cobalt 0.4 g Cobalt carbonate 

a Per ton (2,000 lb) of finished product 
 

Isobutene, NTP TR 487 225 

TABLE K3 

Nutrient Composition of NIH-07 Rat and Mouse Ration 

Mean ± Standard

Nutrient Deviation Range Number of Samples 

Protein (% by weight) 22.91±0.48 22.1)23.6 24

 Crude fat (% by weight) 5.34±0.18 5.00)5.80 24

 Crude fiber (% by weight) 3.12±0.29 2.60)4.00 24

 Ash (% by weight) 6.23±0.16 5.72)6.54 24 

Amino Acids (% of total diet)

 Arginine 1.273±0.083 1.100)1.390 12

 Cystine 0.307±0.068 0.181)0.400 12

 Glycine 1.152±0.051 1.060)1.220 12

 Histidine 0.581±0.029 0.531)0.630 12

 Isoleucine 0.913±0.034 0.867)0.965 12

 Leucine 1.969±0.053 1.850)2.040 12

 Lysine 1.269±0.050 1.200)1.370 12

 Methionine 0.436±0.104 0.306)0.699 12

 Phenylalanine 0.999±0.114 0.665)1.110 12

 Threonine 0.899±0.059 0.824)0.985 12

 Tryptophan 0.216±0.146 0.107)0.671 12

 Tyrosine 0.690±0.091 0.564)0.794 12

 Valine 1.079±0.057 0.962)1.170 12 

Essential Fatty Acids (% of total diet)

 Linoleic 2.389±0.223 1.830)2.570 11

 Linolenic 0.273±0.034 0.210)0.320 11
 

Vitamins

 Vitamin A (IU/kg) 6,802±546 5,550)8,800 24

 Vitamin D (IU/kg) 4,450±1,382 3,000)6,300 4 

"-Tocopherol (ppm) 35.24±8.58 22.5)48.9 12

 Thiamine (ppm) 16.87±3.66 13.0)26.0 23

 Riboflavin (ppm) 7.78±0.899 6.10)9.00 12

 Niacin (ppm) 98.73±23.21 65.0)150.0 12

 Pantothenic acid (ppm) 32.94±8.92 23.0)59.2 12

 Pyridoxine (ppm) 9.28±2.49 5.60)14.0 12

 Folic acid (ppm) 2.56±0.70 1.80)3.70 12

 Biotin (ppm) 0.265±0.046 0.190)0.354 12

 Vitamin B (ppb) 41.6±18.6 10.6)65.0 12 12

 Choline (ppm) 2,955±382 2,300)3,430 11 

Minerals

 Calcium (%) 1.15±0.06 1.03)1.27 24

 Phosphorus (%) 0.89±0.02 0.84)0.95 24

 Potassium (%) 0.886±0.059 0.772)0.971 10

 Chloride (%) 0.531±0.082 0.380)0.635 10

 Sodium (%) 0.316±0.031 0.258)0.370 12

 Magnesium (%) 0.165±0.010 0.148)0.180 12

 Sulfur (%) 0.266±0.060 0.208)0.420 11

 Iron (ppm) 348.0±83.7 255.0)523.0 12

 Manganese (ppm) 93.27±5.62 81.7)102.0 12

 Zinc (ppm) 59.42±9.73 46.1)81.6 12

 Copper (ppm) 11.63±2.46 8.09)15.4 12

 Iodine (ppm) 3.49±1.14 1.52)5.83 11

 Chromium (ppm) 1.57±0.53 0.60)2.09 12

 Cobalt (ppm) 0.81±0.27 0.49)1.23 8 
 

 

 

 

 

 

 

 

226 Isobutene, NTP TR 487 

TABLE K4 

Contaminant Levels in NIH-07 Rat and Mouse Rationa

 Mean ± Standard

 Deviationb Range Number of Samples 

Contaminants

 Arsenic (ppm) 0.52±0.17 0.10 )0.80 24

 Cadmium (ppm) 0.04±0.01 0.04 )0.06 24

 Lead (ppm) 0.24±0.06 0.20 )0.40 24

 Mercury (ppm) <0.02 24

 Selenium (ppm) 0.34±0.10 0.10 )0.50 24

 Aflatoxins (ppb) <5.0 24 c Nitrate nitrogen (ppm) 7.76±2.61 2.90 )14.0 24 c Nitrite nitrogen (ppm) 1.37±0.89 0.30 )3.50 24 

BHA (ppm)d 1.32±1.88 0.05 )10.0 24 

BHT (ppm)d 1.68±1.14 0.18 )5.0 24 

Aerobic plate count (CFU/g) 121,708±126,261 20,000 )460,000 24

 Coliform (MPN/g) 136±569 3 )2,800 24 

Escherichia coli (MPN/g) 6±3.5 3 )10 24 

Salmonella (MPN/g) Negative 24 e Total nitrosoamines (ppb) 12.15±4.09 4.0 )23.0 24 e N-Nitrosodimethylamine (ppb) 10.41±3.86 3.0 )21.0 24 e N-Nitrosopyrrolidine (ppb) 1.74±0.79 1.0 )4.0 24 

Pesticides (ppm) 

"-BHC <0.01 24 

$-BHC <0.02 24 

(-BHC <0.01 24 

*-BHC <0.01 24

 Heptachlor <0.01 24

 Aldrin <0.01 24

 Heptachlor epoxide <0.01 24

 DDE <0.01 24

 DDD <0.01 24

 DDT <0.01 24

 HCB <0.01 24

 Mirex <0.01 24

 Methoxychlor <0.05 24

 Dieldrin <0.01 24

 Endrin <0.01 24

 Telodrin <0.01 24

 Chlordane <0.05 24

 Toxaphene <0.10 24

 Estimated PCBs <0.20 24

 Ronnel <0.01 24

 Ethion <0.02 24

 Trithion <0.05 24

 Diazinon <0.10 24

 Methyl parathion <0.02 24

 Ethyl parathion <0.02 24

 Malathion 0.13±0.16 0.02 )0.83 24

 Endosulfan I <0.01 24

 Endosulfan II <0.01 24

 Endosulfan sulfate <0.03 24 

a CFU=colony-forming units; MPN=most probable number; BHC=hexachlorocyclohexane or benzene hexachloride b For values less than the limit of detection, the detection limit is given as the mean. 

Sources of contamination: alfalfa, grains, and fish meal d Sources of contamination: soy oil and fish meal e All values were corrected for percent recovery. 

c 
227 

APPENDIX L 

SENTINEL ANIMAL PROGRAM 

METHODS 

RESULTS 

.............................................................. 

.............................................................. 

228 

229 
228 Isobutene, NTP TR 487 

SENTINEL ANIMAL PROGRAM
 

METHODS 

Rodents used in the Carcinogenesis Program of the National Toxicology Program are produced in optimally 

clean facilities to eliminate potential pathogens that may affect study results. The Sentinel Animal Program 

is part of the periodic monitoring of animal health that occurs during the toxicologic evaluation of chemical 

compounds. Under this program, the disease state of the rodents is monitored via serology on sera from 

extra (sentinel) animals in the study rooms. These animals and the study animals are subject to identical 

environmental conditions. The sentinel animals come from the same production source and weanling 

groups as the animals used for the studies of chemical compounds. 

Serum samples were collected from randomly selected rats and mice during the 2-year studies. Blood from 

each animal was collected and allowed to clot, and the serum was separated. The samples were processed 

appropriately and sent to Microbiological Associates, Inc. (Bethesda, MD), for determination of antibody 

titers. The laboratory serology methods and viral agents for which testing was performed are tabulated 

below; the times at which blood was collected during the studies are also listed. 

Method and Test Time of Analysis 

RATS 

ELISA 

Mycoplasma arthritidis 

Mycoplasma pulmonis 

PVM (pneumonia virus of mice) 

RCV/SDA (rat coronavirus/ 

sialodacryoadenitis virus) 

Sendai 

Study termination 

Study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination

6, 12, and 18 months, study termination 

Immunofluorescence Assay 

M. arthritidis Study termination 

Hemagglutination Inhibition 

H-1 (Toolan’s H-1 virus) 

KRV (Kilham rat virus) 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 
Isobutene, NTP TR 487 229 

Method and Test 

MICE 

ELISA 

Ectromelia virus 

EDIM (epizootic diarrhea of infant mice) 

GDVII (mouse encephalomyelitis virus) 

LCM (lymphocytic choriomeningitis virus) 

Mouse adenoma virus-FL 

MHV (mouse hepatitis virus) 

M. arthriditis 

M. pulmonis 

PVM
 

Reovirus 3
 

Sendai
 

Immunofluorescence Assay
 

EDIM
 

M. arthriditis 

Reovirus 3 

Hemagglutination Inhibition
 

K (papovavirus)
 

MVM (minute virus of mice)
 

Polyoma virus
 

RESULTS 

Time of Analysis 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

Study termination 

Study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

12 months 

Study termination 

18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

6, 12, and 18 months, study termination 

Four rats and six mice had positive titers for M. arthritidis. Further evaluation of samples positive for 

immunoblot and Western blot procedures indicated that the positive titers may have been due to cross 

reaction with antibodies of nonpathogenic Mycoplasma or other agents. There were no clinical findings or 

histopathologic changes of M. arthritidis infection in animals with positive titers. Accordingly, 

M. arthritidis-positive titers were considered false positives. 
230 Isobutene, NTP TR 487