NTP TECHNICAL REPORT

ON THE

TOXICOLOGY AND CARCINOGENESIS

STUDY OF CHLORAL HYDRATE

(AD LIBITUM AND DIETARY CONTROLLED)

(CAS NO. 302-17-0)

IN MALE B6C3F1 MICE

(GAVAGE STUDY)

NATIONAL TOXICOLOGY PROGRAM

P.O. Box 12233

Research Triangle Park, NC 27709

December 2002

NTP TR 503

NIH Publication No. 03-4437

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 and Prevention. 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 NCTR 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.

Details about ongoing and completed NTP studies are available at the NTP’s World Wide Web site: 

http://ntp-server.niehs.nih.gov. Abstracts of all NTP Technical Reports and full versions of the most recent

reports and other publications are available from the NIEHS’ Environmental Health Perspectives (EHP)

http://ehp.niehs.nih.gov (800-315-3010 or 919-541-3841). In addition, printed copies of these reports are

available from EHP as supplies last. A listing of all the NTP Technical Reports printed since 1982 appears on

the inside back cover.
NTP TECHNICAL REPORT

ON THE

TOXICOLOGY AND CARCINOGENESIS

STUDY OF CHLORAL HYDRATE

(AD LIBITUM AND DIETARY CONTROLLED)

(CAS NO. 302-17-0)

IN MALE B6C3F1 MICE

(GAVAGE STUDY)

NATIONAL TOXICOLOGY PROGRAM

P.O. Box 12233

Research Triangle Park, NC 27709

December 2002

NTP TR 503

NIH Publication No. 03-4437

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

National Institutes of Health
2

CONTRIBUTORS

The studies on chloral hydrate were conducted at the FDA’s National Center for Toxicological Research under an interagency agreement

between the FDA and the NIEHS. The studies were designed and monitored by a Toxicology Study Selection and Review Committee, composed

of representatives from the NCTR and other FDA product centers, NIEHS, and other ad hoc members from other government agencies and

academia. The interagency agreement was designed to use the staff and facilities of the NCTR in testing of FDA priority chemicals and to

provide FDA scientists and regulatory policymakers information for hazard identification and risk assessment. 

Toxicology Study Selection

and Review Committee

B.A. Schwetz, D.V.M., Ph.D., Chairperson

National Center for Toxicological Research

W.T. Allaben, Ph.D.

National Center for Toxicological Research

F.A. Beland, Ph.D.

National Center for Toxicological Research

J.R. Bucher, Ph.D.

National Institute of Environmental Health Sciences

J.F. Contrera, Ph.D.

Center for Drug Evaluation, Food and Drug Administration

D.W. Gaylor, Ph.D.

National Center for Toxicological Research

K.J. Greenlees, Ph.D.

Center for Veterinarian Medicine, Food and Drug Administration

R.J. Lorentzen, Ph.D.

Center for Food Safety and Applied Nutrition, 

Food and Drug Administration

F.D. Sistare, Ph.D.

Center for Drug Evaluation, Food and Drug Administration

Bionetics

Prepared animal feed and cared for mice

J. Carson, B.S.

A. Matson, B.S.

M. Moore

M.L. Nichols

National Center for Toxicological Research,

Food and Drug Administration

Conducted studies, evaluated and interpreted results and pathology

findings, and reported findings

J.E.A. Leakey, Ph.D., Study Scientist

W.T. Allaben, Ph.D.

J.R. Appleget, B.S.

F.A. Beland, Ph.D.

D.W. Gaylor, Ph.D.

N.A. Littlefield, Ph.D.

J.M. Reed, M.S.

J.E. Seng, Ph.D.

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

W.M. Witt, D.V.M., Ph.D.

Conducted chemical analysis of feed and purity of the agent

W.M. Cooper, B.S.

F.E. Evans, Ph.D.

J.P. Freeman, Ph.D.

T.M. Heinze, M.S.

T.C. Schmitt, B.S.

P.H. Siitonen, B.S.

Pathology Associates International

Evaluated pathology findings

T.J. Bucci, V.M.D., Ph.D.

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

C.V. Okerberg, D.V.M., Ph.D.
Chloral Hydrate, NTP TR 503 3

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.

NTP Pathology Working Group

Evaluated slides, prepared pathology report

(August 25, 1999)

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

Experimental Pathology Laboratories

T.J. Bucci, V.M.D., Ph.D.

Pathology Associates International

M.R. Elwell, D.V.M., Ph.D.

Covance Labs

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

National Institute of Environmental Health Sciences

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

Experimental Pathology Laboratories

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

Pathology Associates International

C.V. Okerberg, D.V.M., Ph.D.

Pathology Associates International

R.W. Trotter, D.V.M.

Pathology Associates International

R.O.W. Sciences, Inc.

Provided statistical analyses

M. Austen, M.S.

D.L. Barton, M.S.

B.D. Bryant

K. Carroll

S. Goldman

J.M. Gossett, M.S.

B. Hampton

C.C. McCarty, B.S.

W.A. McCracken, M.S.

B.T. Thorn, M.S.

C.C. Ulmer, M.S.

Biotechnical Services, Inc.

Prepared Technical Report

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

L.M. Harper, B.S. 

E.S. Paal, M.S.J.

D.C. Serbus, Ph.D.

W.D. Sharp, B.A., B.S.

R.A. Willis, B.A., B.S.

P.A. Yount, B.S.
4

CONTENTS

ABSTRACT ......................................................................... 7

EXPLANATION OF LEVELS OF EVIDENCE OF CARCINOGENIC ACTIVITY . . . . . . . . . . . . . . . 10

TECHNICAL REPORTS REVIEW SUBCOMMITTEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

SUMMARY OF TECHNICAL REPORTS REVIEW SUBCOMMITTEE COMMENTS . . . . . . . . . . . 12

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

DISCUSSION AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

APPENDIX A Summary of Lesions in Ad Libitum-Fed Male Mice in the 2-Year Gavage Study

of Chloral Hydrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

APPENDIX B Summary of Lesions in Dietary-Controlled Male Mice in the 2-Year Gavage Study

of Chloral Hydrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

APPENDIX C Organ Weights and Organ-Weight-to-Body-Weight Ratios . . . . . . . . . . . . . . . . . . . . . . 105

APPENDIX D Body Weight Considerations and Dietary Control Techniques . . . . . . . . . . . . . . . . . . . 109

APPENDIX E Hepatic Enzyme Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

APPENDIX F Chemical Characterization and Dose Formulation Studies . . . . . . . . . . . . . . . . . . . . . . 173

APPENDIX G Feed Consumption in the 2-Year Gavage Study of Chloral Hydrate . . . . . . . . . . . . . . . 183

APPENDIX H Ingredients, Nutrient Composition, and Contaminant Levels

in NIH-31 Rat and Mouse Ration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

APPENDIX I Sentinel Animal Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

APPENDIX J Supplemental Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Chloral Hydrate, NTP TR 503 5

SUMMARY

Background: Chloral hydrate is used as a sedative and a sleep aid for children and as an anesthetic for large

animals. We studied the effects of chloral hydrate in male mice to identify potential toxic or carcinogenic

hazards to humans. Because it is known that higher body weight causes a greater risk for some kinds of

cancers, we also examined if the effects of chloral hydrate changed when we reduced the amount of food eaten

by the mice.

Methods: We gave the mice doses of 25, 50, or 100 mg of chloral hydrate per kg of body weight by

depositing the chemical dissolved in water directly into the animals’ stomachs by a tube. Control mice were

dosed with water. In one study, the mice had free access to feed; in a second study, the animals’ feed was

restricted to maintain lower body weights. Groups of 50 animals were dosed five times per week for two

years, and tissues from more than 40 sites were examined for every animal.

Results: In the study where mice ate freely, the animals receiving 25 mg chloral hydrate per kg body weight

had a higher rate of liver neoplasms than did the control animals. In the study where feed was restricted, the

incidence of liver carcinomas was increased in the groups receiving chloral hydrate compared to the control

group.

Conclusions: We conclude that chloral hydrate caused some liver neoplasms in male mice, whether or not the

animals ate restricted diets.
6 Chloral Hydrate, NTP TR 503
Cl OH

Cl C C OH

Cl H

7

ABSTRACT

CHLORAL HYDRATE

CAS No. 302-17-0

Chemical Formula: C2H3Cl3O2 Molecular Weight: 165.42

Synonyms: Trichloracetaldehyde monohydrate; 1,1,1-trichloro-2,2-ethanediol; 2,2,2-trichloro-1,1-ethanediol

Trade names: Aquachloral Supprettes; Noctec; Somnos

Chloral hydrate is used medically as a sedative or

hypnotic and as a rubefacient in topical preparations,

and it is often given to children as a sedative during

dental and other medical procedures. Chloral hydrate

is used as a central nervous system depressant and

sedative in veterinary medicine and as a general

anesthetic in cattle and horses. It is a byproduct of the

chlorination of water and has been detected in plant

effluent after the bleaching of softwood pulp. Chloral,

the anhydrous form of chloral hydrate, is used as a

synthetic intermediate in the production of insecticides

and herbicides. Chloral hydrate was nominated for

study by the Food and Drug Administration based upon

widespread human exposure and its potential

hepatotoxicity and the toxicity of related chemicals. A

dietary control component was incorporated in

response to concerns within the regulatory community

relating to increased background neoplasm incidences

in rodent strains used for toxicity testing and to the

proposed use of dietary restriction to control background neoplasm incidence in rodent cancer studies.

Male B6C3F1 mice (ad libitum-fed or dietarycontrolled) received chloral hydrate (99% pure) by

gavage for 2 years.

2-YEAR STUDY IN MALE MICE

Groups of 120 male mice received chloral hydrate in

distilled water by gavage at doses of 0, 25, 50, or

100 mg/kg 5 days per week for 104 to 105 weeks.

Each dose group was divided into two dietary groups of

60 mice. The ad libitum-fed mice had free access to

feed, and the dietary-controlled mice received feed in

measured daily amounts calculated to maintain body

weight on a previously computed idealized body weight

curve. Twelve mice from each diet and dose group

were evaluated at 15 months.

Survival, Feed Consumption,

and Body Weights

Survival of dosed groups of ad libitum-fed and dietarycontrolled mice was similar to that of the corresponding

vehicle controls. When compared to the ad libitum-fed

groups, dietary control significantly increased survival

in the vehicle controls and 25 and 50 mg/kg groups.

Mean body weights of all dosed groups were similar

to those of the vehicle control groups throughout the

study. The dietary-controlled mice were successfully

maintained at or near their target idealized body
8 Chloral Hydrate, NTP TR 503

weights. There was less individual variation in body

weights in the dietary-controlled groups than in the

corresponding ad libitum-fed groups. Feed consumption by 25 and 50 mg/kg ad libitum-fed mice was

generally similar to that by the vehicle controls

throughout the study. Feed consumption by 100 mg/kg

ad libitum-fed mice was slightly less than that by the

vehicle controls throughout the study.

Hepatic Enzyme Analysis

Chloral hydrate did not significantly induce either

lauric acid T-hydroxylase activity or CYP4A

immunoreactive protein in any of the dosed groups of

ad libitum-fed mice. However, 100 mg/kg did

significantly induce both lauric acid T-hydroxylase

activity and CYP4A immunoreactive protein in the

dietary-controlled mice. Moreover, the induction

response profile of CYP4A was similar to the increase

in the incidence of liver neoplasms at 2 years in the

dietary-controlled mice with the major effect occurring

in the 100 mg/kg group.

The serum enzymes alanine aminotransferase, amylase,

aspartate aminotransferase, and lactate dehydrogenase

were also assayed at 2 years. In the ad libitum-fed

groups there was a significant increase in aspartate

aminotransferase activity in the 50 mg/kg group. There

were no other significant effects in any dosed group,

but in general the dietary-controlled groups exhibited

lower values than the corresponding ad libitum-fed

groups.

Organ Weights and Pathology Findings

The heart weight of ad libitum-fed male mice administered 100 mg/kg and the kidney weights of 50 and

100 mg/kg ad libitum-fed mice were significantly less

than those of the vehicle controls at 2 years. The liver

weights of all dosed groups of ad libitum-fed and

dietary-controlled mice were greater than those of the

vehicle control groups at 2 years, but the increases were

not statistically significant.

The incidence of hepatocellular adenoma or carcinoma

(combined) in ad libitum-fed mice administered

25 mg/kg was significantly greater than that in the

vehicle controls at 2 years. The incidences of hepatocellular carcinoma and of hepatocellular adenoma or

carcinoma (combined) occurred with positive trends in

dietary-controlled male mice at 2 years, and the incidence of hepatocellular carcinoma in 100 mg/kg

dietary-controlled mice was significantly increased.

CONCLUSIONS

Under the conditions used in this 2-year gavage study,

there was some evidence of carcinogenic activity* of

chloral hydrate in male B6C3F1 mice based on

increased incidences of hepatocellular adenoma or

carcinoma (combined) in ad libitum-fed mice and on

increased incidences of hepatocellular carcinoma in

dietary-controlled mice. In the dietary-controlled mice,

induction of enzymes associated with peroxisome

proliferation was observed at higher doses.

__________

* Explanation of Levels of Evidence of Carcinogenic Activity is on page 10. A summary of the Technical Reports Review Subcommittee

comments and the public discussion on this Technical Report appears on page 12.
Chloral Hydrate, NTP TR 503 9

Summary of the 2-Year Carcinogenesis Study of Chloral Hydrate in B6C3F1 Mice

Ad libitum-Fed Groups Dietary-Controlled Groups

Doses in water by gavage Vehicle control, 25, 50, or 100 mg/kg Vehicle control, 25, 50, or 100 mg/kg

Body weights Dosed groups similar to vehicle control group Dosed groups similar to vehicle control group

Survival rates 41/48, 37/48, 36/48, 44/48 45/48, 44/48, 47/48, 41/48

Nonneoplastic effects None None

Neoplastic effects Liver: hepatocellular adenoma or carcinoma Liver: hepatocellular carcinoma

(16/48, 25/48, 23/47, 22/48) (2/48, 5/48, 4/48, 8/48)

Level of evidence of carcinogenic activity

for ad libitum-fed and dietary-controlled groups Some evidence
10 Chloral Hydrate, NTP TR 503

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.

For studies showing multiple chemical-related neoplastic effects that if considered individually would be assigned to different levels of evidence

categories, the following convention has been adopted to convey completely the study results. In a study with clear evidence of carcinogenic

activity at some tissue sites, other responses that alone might be deemed some evidence are indicated as “were also related” to chemical

exposure. In studies with clear or some evidence of carcinogenic activity, other responses that alone might be termed equivocal evidence are

indicated as “may have been” related to chemical exposure.

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.
Chloral Hydrate, NTP TR 503 11

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 chloral hydrate on

May 18, 2000, 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.

A. John Bailer, Ph.D., Chairperson

Department of Mathematics and Statistics

Miami University

Oxford, OH

James S. Bus, Ph.D.

Health and Environmental Sciences

Dow Chemical Company

Midland, MI

Linda A. Chatman, D.V.M., Principal Reviewer

Pfizer, Inc.

Groton, CT

John M. Cullen, Ph.D., V.M.D.

Department of Microbiology, Parasitology, and Pathology

College of Veterinary Medicine

North Carolina State University

Raleigh, NC

Harold Davis, Ph.D.*

Director of Toxicology

Amgen, Inc.

Thousand Oaks, CA

Special Reviewer

D. Gail McCarver, M.D., Principal Reviewer

Department of Pediatrics

Medical College of Wisconsin

Milwaukee, WI

Norman R. Drinkwater, Ph.D., Principal Reviewer

McArdle Laboratory for Cancer Research

University of Wisconsin-Madison

Madison, WI

Susan M. Fischer, Ph.D.*

M.D. Anderson Cancer Center

The University of Texas

Smithville, TX

Stephen S. Hecht, Ph.D.

University of Minnesota Cancer Centers

Minneapolis, MN

Michele Medinsky, Ph.D.

Durham, NC

Jose Russo, M.D.*

Fox Chase Cancer Center

Philadelphia, PA

______________

* Did not attend
12 Chloral Hydrate, NTP TR 503

SUMMARY OF TECHNICAL REPORTS REVIEW SUBCOMMITTEE COMMENTS

On May 18, 2000, the draft Technical Report on the

toxicology and carcinogenesis study of chloral hydrate

(ad libitum and feed restricted) received public review

by the National Toxicology Program’s Board of

Scientific Counselors’ Technical Reports Review

Subcommittee. The review meeting was held at the

National Institute of Environmental Health Sciences,

Research Triangle Park, NC.

Dr. J.E.A. Leakey, NCTR, introduced the toxicology

and carcinogenesis study of chloral hydrate by discussing the uses of the chemical and the rationale for

study, describing the experimental design, reporting on

survival and body weight effects, and commenting on

compound-related neoplasms in male mice. The proposed conclusion for the 2-year study was some

evidence of carcinogenic activity in male B6C3F1 mice.

Dr. Leakey noted that the FDA has been interested in

the effects of dietary restriction on neoplasm incidences

in test animals because several corporations are using

dietary restriction to improve survival in long-term

bioassays. Dr. Leakey used data on liver neoplasm risk

in animals whose body weight was reduced by a

restricted diet or treatment with a noncarcinogenic

chemical to illustrate the concern that dietary restriction

will desensitize bioassays by decreasing the rates of

chemical-induced neoplasms. He described the model

that controls body weight to give a predicted neoplasm

rate as well as controlling survival. Dr. Leakey also

discussed the mechanistic studies of liver enzyme

induction and the pharmacokinetic studies of the

metabolite trichloroacetic acid.

Dr. Drinkwater, a principal reviewer, agreed with the

proposed conclusions. However, he thought that the

last sentence, which implies that induction of liver neoplasms in dietary-controlled mice was a result of

peroxisome proliferation, was not proven. Dr. Leakey

responded that a higher top dose would have been

desirable and further study was needed to confirm the

mechanism.

Dr. Chatman, the second principal reviewer, agreed

with the proposed conclusions. She also agreed that the

last sentence was not fully supported and should be

removed. Dr. W.T. Allaben, NCTR, suggested limiting

such speculation to the Discussion, and Dr. Leakey

agreed. Dr. Chatman asked if incidences of preneoplastic lesions such as foci or hypertrophy in the liver

were increased. Dr. T.J. Bucci, Pathology Associates

International, said that preneoplastic lesion incidences

were not increased.

Dr. McCarver, the third principal reviewer, agreed in

principle with the proposed conclusions, and suggested

including the doses at which the increased neoplasms

occurred. She also suggested softening the last

sentence by inserting “somewhat” in front of “similar.”

Dr. Drinkwater wondered if the intermittent bolus

dosing in the current study, compared to the continuous

drinking water exposure in the Daniel et al. (1992a)

study, was responsible for the lower neoplasm

response. Dr. Leakey replied that this could be the case

because trichloroacetic acid plasma levels would fall

between doses, and peroxisome proliferation is quite

reversible. Dr. Bailer suggested that the dose-response

model that looks at concentration gradient could

include body weight as a covariable, and the modeling

mechanism might provide insight into the impact of

body weight.

Dr. Drinkwater moved that the sentence of the

proposed conclusions on peroxisome proliferation be

removed and the remainder of the proposed conclusions

be accepted as written; Dr. Chatman seconded the

motion. Dr. Bus moved to amend the motion by

changing some evidence of carcinogenic activity to

equivocal evidence of carcinogenic activity; Dr. Hecht

seconded the amended motion. After discussion of the

definitions of levels of evidence, the amended motion

was defeated by four no votes to two yes votes.

Dr. Bus then argued for retaining some version of the

last sentence because of current scientific debate about

the relationship between peroxisome proliferation and

human cancers. Dr. Drinkwater agreed to modify his

motion with a revised sentence reading “In the dietarycontrolled mice, induction of enzymes associated with

peroxisome proliferation was observed at higher

doses.” The motion with the revised sentence was

accepted by five yes votes to one no vote.