UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

 WASHINGTON D.C. 20460 

 

OFFICE OF THE ADMINISTRATOR 

 SCIENCE ADVISORY BOARD 

May 30, 2006 

EPA-SAB-06-006 

The Honorable Stephen L. Johnson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: SAB Review of EPA’s Draft Risk Assessment of Potential Human 

Health Effects Associated with PFOA and Its Salts 

Dear Administrator Johnson: 

 In response to a request from EPA’s Office of Pollution Prevention and Toxics (OPPT), 

the Science Advisory Board (SAB) convened an expert panel to conduct a peer review of EPA's 

Draft Risk Assessment of Potential Human Health Effects Associated with Perfluorooctonoic 

Acid (PFOA) and Its Salts (dated January 4, 2005). PFOA is a synthetic (man-made) chemical 

used in the manufacture of several commercially important products. PFOA has been detected in 

the blood of the general U.S. population although it is not fully understood how individuals are 

exposed to the chemical. To determine whether environmental exposure to PFOA might pose a 

risk to human health, EPA’s draft assessment provided an evaluation of available information on 

the health effects and human exposure to PFOA. The draft assessment also compared measured 

human blood levels with the estimated PFOA blood levels that are not anticipated to produce (or 

can produce minimal) toxicities based on data in tested laboratory animals. 

 The SAB was asked to comment on: (a) EPA’s analysis of how PFOA causes tumors in 

rats and its relevance for human health and the weight-of- evidence conclusion about the 

potential for PFOA to cause cancer in humans; (b) the selection of health effects endpoints for 

risk assessment; (c) the adequacy of available data to provide information on exposure of the 

general population to PFOA; and (d) EPA’s risk assessment approach including the use of 

kinetic models to estimate PFOA blood levels in available laboratory animals studies. 

 In general, the SAB Panel endorsed EPA’s risk assessment approach, particularly, the 

inclusion of multiple non-cancer health endpoints for risk assessment, and the use of PFOA 

blood levels as a measure of estimated dose in place of the administered dose in toxicologic 

studies. The Panel recommended the inclusion of additional non-cancer health endpoints for risk 

assessment, and the use of the Benchmark Dose method to better estimate the lowest observed 
2

effect levels and no observed effect levels for risk assessment. Three-quarters of the Panel 

judged that the weight-of-evidence conclusion for the potential of PFOA to cause cancer in 

humans was more aligned and consistent with the hazard descriptor of “likely to be 

carcinogenic” as described in the Agency’s cancer guidelines (i.e., 2003 EPA Guidelines for 

Carcinogen Risk Assessment). They also recommended that a risk assessment be conducted for 

carcinogenic effects. About one-quarter of the Panel agreed with EPA’s conclusion regarding the 

potential cancer hazard of PFOA to humans and the designation of the cancer descriptor of 

“suggestive evidence of carcinogenicity, but not sufficient to assess human carcinogenic 

potential”. Three quarters of the Panel considered the available human biomonitoring studies 

adequate to characterize environmental risk to PFOA for the general population. However, 

about one-quarter of the Panel believed that the available studies are inadequate for risk 

assessment of subpopulations possibly more highly exposed to PFOA. The scientific rationales 

for these viewpoints along with specific recommendations on these issues are detailed in the 

Panel’s report. 

 The SAB strongly urges the Agency to strengthen its risk assessment by considering 

verified and peer reviewed new information found to be relevant and critical to the assessment. 

We look forward to receiving your response to this review and appreciate the opportunity to 

provide EPA with advice on this important subject. We stand ready to assist the Agency in any 

future efforts in updating the draft risk assessment. 

 

 Sincerely, 

 /signed/ /signed/ 

 Dr. M. Granger Morgan Dr. Deborah Cory-Slechta 

 Chair Chair 

 EPA Science Advisory Board PFOA Risk Assessment Review Panel 

 EPA Science Advisory Board 
i

NOTICE 

 This report has been written as part of the activities of the EPA Science Advisory Board 

(SAB), a public advisory group providing extramural scientific information and advice to the 

Administrator and other officials of the Environmental Protection Agency. The SAB is 

structured to provide balanced, expert assessment of scientific matters related to problems facing 

the Agency. This report has not been reviewed for approval by the Agency and, hence, the 

contents of this report do not necessarily represent the views and policies of the Environmental 

Protection Agency, nor of other agencies in the Executive Branch of the Federal government, nor 

does mention of trade names of commercial products constitute a recommendation for use. 

Reports of the SAB are posted on the EPA website at http://www.epa.gov/sab. 
ii

U.S. Environmental Protection Agency 

Science Advisory Board 

Perfluorooctanoic Acid Review Panel 

CHAIR 

Dr. Deborah Cory-Slechta, Director, Environmental and Occupational Health Sciences 

Institute, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New 

Jersey and Rutgers State University, Piscataway, NJ 

MEMBERS 

Dr. Ernest Abel, Professor of Obstetrics and Gynecology; Professor of Psychology; Director, 

Reproductive Toxicology, Obstetrics and Gynecology Department, School of Medicine / C.S. 

Mott Center for Human Growth and Development, Wayne State University, Detroit, MI 

Dr. Melvin Andersen, Director, Department of Biomathematics and Physical Science, Centers 

for Health Research, CIIT, Research Triangle Park, NC 

Dr. George Corcoran, Chairman and Professor, Department of Pharmaceutical Sciences, 

School of Pharmacy & Health Sciences, Eugene Applebaum College, Wayne State University, 

Detroit, MI 

Dr. Norman Drinkwater, Professor and Chair of Oncology, McArdle Laboratory for Cancer 

Research, University of Wisconsin Medical School, Madison, WI 

Dr. William L. Hayton, Professor and Associate Dean, Division of Environmental Health 

Science, College of Pharmacy, School of Medicine and Public Health, Ohio State University, 

Columbus, OH 

Dr. Michael A. Kamrin, Professor Emeritus, Institute of Environmental Toxicology, Michigan 

State University, Haslett, MI 

Dr. James Kehrer, Head, Division of Pharmacology and Toxicology, College of Pharmacy, 

University of Texas at Austin, Austin, TX 

Dr. James E. Klaunig, Professor and Director, Department of Pharmacology and Toxicology, 

School of Medicine, Indiana University, Indianapolis, IN 

Dr. Matthew P. Longnecker, MD ScD, Division of Intramural Research National Institute of 

Environmental Health Sciences, Epidemiology Branch, National Institute of Environmental 

Health Sciences, Research Triangle Park, NC 

Dr. Ronald Melnick, Director of Special Programs, Environmental Toxicology Program, 

National Institute of Environmental Health Sciences, Research Triangle Park, NC 
iii

Dr. Franklin L. Mink, President, MAI, Lake Orion, MI 

Dr. David M. Ozonoff, Professor, Department of Environmental Health, School of Public 

Health, Boston University, Boston, MA 

Dr. Stephen Roberts, Professor, Center for Environmental and Human Toxicology, University 

of Florida, Gainesville, FL 

Dr. Anne Sweeney, Associate Professor, Department of Epidemiology and Biostatistics, School 

of Rural Public Health, Texas A&M University, Bryan, TX 

Dr. Thomas T. Zoeller, Professor, Biology Dept., Morrill Science Center, University of 

Massachusetts at Amherst, Amherst, MA 

SCIENCE ADVISORY BOARD STAFF 

Dr. Sue Shallal, Designated Federal Official, Science Advisory Board Staff Office, Washington, 

DC
iv

TABLE OF CONTENTS 

EXECUTIVE SUMMARY .........................................................................................................................................1

ISSUE 1: RODENT PPAR-ALPHA MODE OF ACTION FOR HEPATOCARCINOGENESIS ...........................................1

ISSUE 2: DESCRIPTOR FOR CARCINOGENIC POTENTIAL........................................................................................3

ISSUE 3: SELECTION OF ENDPOINTS........................................................................................................................4

ISSUE 4: RISK ASSESSMENT APPROACH..................................................................................................................5

ISSUE 4A: PHARMACOKINETIC MODELING AND USE OF AUC AS A MEASURE OF INTERNAL DOSE ....................5

Issue 4b: Cross Species Extrapolation ...............................................................................................................6

Issue 4c: Human Biomonitoring Data ...............................................................................................................7

INTRODUCTION .......................................................................................................................................................8

BACKGROUND ..........................................................................................................................................................8

CHARGE QUESTIONS................................................................................................................................................8

RESPONSES TO THE CHARGE QUESTIONS ...................................................................................................13

ISSUE 1: RODENT PPAR-ALPHA MODE OF ACTION FOR HEPATOCARCINOGENESIS AND LIVER TOXICITY.....13

Question 1. Please comment on the weight of evidence and adequacy of the data available to identify the key 

events for the PPAR alpha agonist induced rodent liver toxicity and hepatocarcinogenesis for PFOA. Discuss 

whether the uncertainties and limitations of these data have been adequately characterized...........................13

ISSUE 2: DESCRIPTOR FOR CARCINOGENIC POTENTIAL .....................................................................................15

Question 2. Please comment on the proposed descriptor for the carcinogenic potential of PFOA. ................15

ISSUE 3: SELECTION OF ENDPOINTS .....................................................................................................................20

Question 3. Please comment on the selection of these toxicity endpoints for the risk assessment....................20

Question 4. Given the available data to date, please comment on the most appropriate 

lifestage/gender/species for assessing human risk.............................................................................................22

Question 5. Please comment on the appropriateness of the available animal models. Please comment on 

whether additional animal models should be investigated, and if so, what information would better enable us 

to ascertain potential human risks. ....................................................................................................................23

ISSUE 4A: PHARMACOKINETIC MODELING AND USE OF AUC AS A MEASURE OF INTERNAL DOSE..................24

Question 6. Please comment on use of the one-compartment pharmacokinetic model. ...................................24

Question 7. Please comment on the use of the AUC as a measure of internal dose for rats and humans for 

calculation of the MOE. .....................................................................................................................................25

ISSUE 4B: CROSS SPECIES EXTRAPOLATION........................................................................................................27

Question 8. Please comment on the need to use or modify the default value of 10 for cross species 

extrapolation given the pharmacokinetic analysis.............................................................................................27

ISSUE 4C: HUMAN BIOMONITORING DATA ..........................................................................................................28

Question 9. Please comment on the adequacy of the human exposure data for use in calculating a MOE. ....28

REFERENCES ..........................................................................................................................................................31
1 

EXECUTIVE SUMMARY 

 EPA’s Office of Pollution Prevention and Toxics (OPPT) requested that the Science 

Advisory Board review the “Draft Risk Assessment of the Potential Human Health Effects 

Associated with Exposure to Perfluorooctanoic Acid (PFOA) and its Salts” (hereafter referred to 

as the “draft PFOA risk assessment document”) which was made available publicly in January 

2006. The PFOA Review Panel of the EPA Science Advisory Board met in February 2005 at 

which time nine charge questions raised by OPPT were deliberated. These questions focused on 

four issues including, a peroxisome proliferator-activated receptor " (PPAR-alpha) mode of 

action (MOA) for rodent liver tumors, carcinogenicity descriptors, useful models for evaluation 

of health effects, toxicokinetic considerations and reliance on currently available human 

biomonitoring exposure data for calculation of margins of exposure (MOEs). Further 

discussions of the entire Panel were held during a conference call in July 2005. 

 This Executive Summary highlights the outcome of the Panel’s deliberations. It includes 

the context for the charge questions and issues raised for consideration by EPA, and the 

conclusions reached by the SAB Review Panel. The Panel reviewed and discussed the draft risk 

assessment and the data referenced therein with the understanding that further risk assessment 

will proceed as more data on PFOA health effects become available. In instances where the 

views of Panel members diverged, the summary and charge question responses focus to a greater 

extent on the view expressed by about three quarters of the Panel members since the view of 

about one-quarter of Panel members coincided with that already expressed in EPA’s Draft Risk 

Assessment. During the review period, new information1

 was presented to the Panel for their 

consideration. The Panel encourages EPA to consider new information that has been verified and 

peer-reviewed prior to use in their revision of the Draft Risk Assessment. 

 

Issue 1: Rodent PPAR-alpha Mode of Action for Hepatocarcinogenesis 

In rats, PFOA induces liver adenomas, Leydig cell tumors (LCT) and pancreatic acinar 

cell tumors (PACT). The draft document concludes that these tumors constitute a triad and are 

the result of a PPAR-alpha agonism MOA. In this MOA, activation of PPAR-alpha leads to cell 

proliferation and decreased apoptosis, clonal expansion of preneoplastic foci and subsequent 

tumors. The draft document premises its conclusions about this MOA on studies showing that 

PFOA is a potent peroxisome proliferator in liver of rats and mice and, like other peroxisome 

proliferators, induces hepatomegaly in rats. In addition, requisite dose-response and temporal 

associations for some key events for this MOA have been reported. 

 

Comment on the Weight of Evidence and Adequacy of the Data Available to Identify the 

Key Events for the PPAR-alpha agonist-induced Rodent Liver Toxicity and 

Hepatocarcinogenesis for PFOA. 

 

1

 This information included, for example: A) a report entitled, “Pathology Peer Review and Pathology Working 

Group Review of Mammary Glands from a Chronic Feeding Study in Rats with PFOA Report” conducted by 

Experimental Pathology Laboratories, Inc. and submitted to the SAB by Dr. Larry Zobel of 3M Medical Department 

and B) data and documents submitted to the SAB by Mr. Robert Bilott of Taft, Stettinius & Hollister LLP. 
2 

 The Panel’s charge was to determine whether it agreed with the weight of evidence 

supporting a PPAR-alpha MOA for rodent liver toxicity and hepatocarcinogenesis. Panel 

members agreed that, considered collectively, evidence to date was consistent with an 

interpretation that liver tumor induction likely results from a PPAR-alpha MOA. This is based 

on the observations that PFOA activates the receptor, results in peroxisome proliferation, 

increases beta-oxidation and produces hepatomegaly, with dose and temporal responses 

consistent with the PPAR-alpha MOA. These events, moreover, depend upon a functional 

PPAR-alpha receptor, and no other known MOA, e.g., DNA reactivity or mutagenicity, has been 

identified. 

However, with respect to uncertainties and limitations related to concluding that PPARalpha is the sole MOA for rodent liver tumor induction and toxicity, Panel views diverged. 

About three quarters of the Panel members believed that at the current time, sufficient 

uncertainties and limitations of the data still exist with respect to reaching such a conclusion, 

given that: 1) In contrast to what would be predicted, administration of PFOA, but not the 

prototype PPAR-alpha agonist WY-14,643, increased liver weights in PPAR-alpha receptor 

knockout mice, i.e., in mice where PPAR-alpha activation was precluded, raising the possibility 

that PFOA-induced liver tumors could occur by PPAR-alpha independent effects. The 

significance of this finding currently remains uncertain in the absence of a corresponding 

assessment of histopathology or replication by another laboratory. 2) There is as yet no 

published evidence that the induction of PPAR-alpha by PFOA results in clonal expansion of 

pre-neoplastic foci which is considered a critical step in the proposed MOA. 3) There are no data 

demonstrating increased cell proliferation and/or decreased apoptosis in the liver of PFOAtreated rats, key causative events in the proposed MOA. 

These Panel members also viewed two additional issues as requiring further 

consideration. One is the relevance of the PPAR-alpha MOA to humans. Given that human 

exposures to PFOA and related chemicals appear ubiquitous, uncertainties and limitations of the 

data for children have not been adequately characterized to be able to conclude that the PPARalpha MOA is not operative in this young age group. A secondary issue thought to require 

additional characterization in the PFOA response was the potential role of Kupffer cells, resident 

macrophages in the liver that do not express PPAR-alpha, but are activated by peroxisome 

proliferators. 

A different view expressed by the remaining one quarter of the Panel members was that 

the observation in PPAR-alpha knockout mice of increased liver weights in response to PFOA, 

but not to the prototype PPAR-alpha agonist WY-14,643, was not sufficiently significant to 

undermine the view that PPAR-alpha agonism is the sole MOA for PFOA-induced rodent liver 

tumors. 

In summary, Panel members agreed that collectively the weight of evidence supports the 

hypothesis that liver tumor induction in rodents by PFOA is mediated by a PPAR-alpha agonism 

MOA. Most Panel members, however, also felt, based on current evidence, that it is possible that 

PPAR-alpha agonism may not be the sole MOA for PFOA, that not all steps in the pathway of 

PPAR-alpha activation- induced liver tumors have been demonstrated, that other 
3 

hepatoproliferative lesions require clarification, and that extrapolation of this MOA across the 

age range in humans is not supported. A few panel members did not share these reservations 

about a PPAR-alpha agonism MOA for PFOA-induced rodent liver tumors. 

Issue 2: Descriptor for Carcinogenic Potential 

The draft document reaches the conclusion of ‘suggestive’ evidence of carcinogenicity 

but not sufficient to assess human carcinogenic potential of PFOA. This conclusion was based 

upon: 1) a PPAR-alpha MOA for liver tumors in rodents that was considered not relevant to 

humans because of their decreased sensitivity to PPAR-alpha agonism when compared to 

rodents, 2) the absence of hepatic cell proliferation in a 6 month study of PFOA administration in 

cynomologous monkeys, the species considered closest in physiology to humans; 3) the absence 

of a strong association between PFOA exposure and tumors in human studies as interpreted in 

the draft document; 4) the belief that the LCT and PACT tumors produced by PFOA in rats were 

probably not relevant to humans based on the lower levels of expression of the mediators 

leutinizing hormone (LCT) and choleocystokinin growth factor receptors (PACT) in humans, as 

well as differences in quantitative toxicodynamics between rats and humans; and 5) the view that 

mammary fibroadenomas reported in female rats are equivocal based on their comparable rates 

of occurrence relative to a historical control group. 

 

Comment on the Proposed Descriptor for the Carcinogenic Potential of PFOA 

About three quarters of the Panel members concluded that the experimental weight of 

evidence with respect to the carcinogenicity of PFOA was stronger than proposed in the draft 

document, and suggested that PFOA cancer data are consistent with the EPA guidelines 

descriptor ‘likely to be carcinogenic to humans’. According to EPA’s Guidelines for Carcinogen 

Risk Assessment2 (also known as EPA’s Cancer Guidelines), this descriptor is typically applied 

to agents that have tested positive in more than one species, sex, strain, site or exposure route, 

with or without evidence of carcinogenicity in humans. Conclusions of these Panel members 

were based on the following: 

• While human data are ambiguous, two separate feeding studies in rats demonstrate that 

PFOA is a multi-site carcinogen. 

• Uncertainties still exist (see Issue 1 comments) as to whether PPAR-alpha agonism 

constitutes the sole MOA for PFOA effects on liver. This was based on the fact that 

PFOA, but not the prototypical PPAR-alpha agonist, WY-14,643, increases liver 

weights in PPAR-alpha knockout mice, a finding of uncertain significance in the 

absence of liver histopathology and replication of this finding. Further, mitochondrial 

proliferation was suggested in the document as a basis of liver toxicity in monkeys 

exposed to PFOA. 

• The exclusion of mammary tumors in the draft document based on comparisons to 

historical control levels from other laboratories was deemed inappropriate, since the 

most appropriate control group is a concurrent control group. Using that comparison, 

 2

 In March 2005, EPA published final Cancer Guidelines and Supplemental Guidance which can be found at the 

following URL: http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=116283. 
4 

increases in both fibroadenomas (22%, 42% and 48% for rats treated with 0, 30 and 

300 ppm APFO (ammonium perfluorooctanoate or C8, the ammonium salt of PFOA), 

respectively) and adenocarcinomas (15, 31% and 11%, respectively) were seen in the 

Sibinski et al. (1987) 2 yr PFOA feeding study. 

• Insufficient data are currently available to determine the MOA for the observed Leydig 

cell tumors, pancreatic acinar cell tumors and mammary gland tumors. In the absence 

of a defined MOA for these tumor types, they must be presumed to be relevant to 

humans, as suggested by EPA’s Cancer Guidelines. 

Given the current data base, these Panel members were not willing to ascribe an 

associated probability value to the potential for PFOA-induced carcinogencity. Nevertheless, 

based on available evidence to date, most Panel members believed that risk assessments for each 

of the PFOA-induced tumors are appropriate at the current time. 

A different view expressed by the remaining one-quarter of the Panel members was that 

currently available evidence does not exceed the descriptor “suggestive” of carcinogenicity, 

based on the belief that PPAR-alpha agonism does serve as the sole MOA for PFOA-induced 

rodent liver tumors (Issue 1) and that mammary tumors were not demonstrated in animals when 

compared to historical controls. Thus, these members did not believe the evidence exceeded the 

draft document descriptor of “suggestive”. 

Issue 3: Selection of Endpoints 

 The draft document proposes the use of multiple endpoints from several life stages, 

species and gender for risk assessment. No specific recommendations on the most appropriate 

parameters are stipulated at the current time. 

 

Comment on the: 

Selection of Toxicity Endpoints for the Risk Assessment 

The Most Appropriate Lifestage/Gender/Species for Assessing Human Risk 

The Appropriateness of the Available Animal Models 

The Panel agreed with the current approach of inclusivity, given the current uncertainties 

noted above with respect to carcinogenicity, as well as the paucity of information on potential 

PFOA effects on non-cancer endpoints. Similarly, no exclusion of species should be considered 

at present, and differences between genders as demonstrated in rat studies again suggest multiple 

MOAs for PFOA. The use of multiple animal models is appropriate particularly in light of the 

reported differences in toxicokinetics in rats, non-human primates and humans. Resolution of 

most appropriate parameters must await additional research, but the process will be facilitated by 

the ability to measure internal dose. 

Panel members did not reach full agreement as to endpoints that should be included for 

risk assessment and the significance of occupational biomonitoring data. About three quarters of 

the Panel members supported the inclusion of multiple cancer endpoints and liver histopathology 

as well as consideration of the data from occupational and epidemiological studies. While the 

draft document notes that the occupational studies suffer from the fact that they involve 
5 

multiplicity of exposures, other studies have shown a high correlation among fluorinated 

compounds in biological samples from the general population and occupational cohorts. 

Therefore, these human studies could be advantageous for assessing potential interactions among 

these compounds that may be associated with adverse human health effects. These Panel 

members also believed that epidemiologic and occupational studies could not be disqualified 

without disqualifying virtually all such studies in the risk assessment process. Moreover, it is 

clear that occupationally-exposed populations have experienced the highest levels of exposure 

and therefore reported health effects in these studies merit consideration. 

 A contrasting view expressed by the remaining one-quarter of the Panel members was 

that the outcomes from studies of human health effects of PFOA were equivocal, and thus these 

endpoints should not be incorporated into the risk assessment process. 

Panel members agreed on the need for additional research, including PPAR-alpha 

mediated and independent effects of PFOA. Non-carcinogenicity endpoints merit additional 

attention for several reasons. It is not yet known whether carcinogenicity will represent the most 

sensitive endpoint for PFOAs. Immunotoxicity has been reported, and derivations of MOEs for 

such effects were encouraged by many Panel members. Given the prevalence in brain of PPAR 

receptors, including PPAR-alpha, effects on nervous system structure and function warrant 

attention. Moreover, no information currently exists with respect to critical periods; therefore, it 

is important to evaluate effects across age groups. The observations of hormonal alterations in 

treated animals also deserve further study to assess their importance. 

Issue 4: Risk Assessment Approach 

Issue 4a: Pharmacokinetic Modeling and Use of AUC as a Measure of Internal Dose 

 The draft document compares internal dose metrics from animal toxicology studies and 

human biomonitoring studies for purposes of ultimately generating margin of exposure (MOE) 

information. Area under the concentration curve (AUC) was calculated from PFOA serum levels 

in human biomonitoring studies assuming a steady state. In some of the rat studies, serum PFOA 

concentrations were available, or it was considered that sufficient pharmacokinetic information 

was available to estimate serum levels. For this purpose, AUC was estimated from a 

pharmacokinetic model. Specifically, compartmental modeling of serum concentrations using 

single dose rat oral exposure studies were used to estimate internal dosimetry for the longer term 

dosing studies based upon the premise that pharmacokinetic information for rats and humans is 

sufficient for this purpose and that this approach does not exceed the limits of the available data. 

 

Comment on the Use of the One Compartment Pharmacokinetic Model 

 The Panel concluded that the empirical model used in the draft document was adequate 

for predicting blood levels resulting from repeated dosing, but that this fitting procedure is 

specific to this limited data set and this particular application. Concern was expressed, therefore, 

that use of the descriptor “one compartment” to describe PFOA pharmacokinetics in the draft 

document is misleading, given the actual complexities in many of the available datasets, and the 

term should be removed or replaced unless carefully qualified. 
6 

 

Comment on the use of the AUC as a Measure of Internal Dose for Rats and Humans for 

Calculation of the MOE 

 The Panel observed that while calculating blood AUC may be an appropriate method to 

estimate internal dose, it is important to note that at the current time information on PFOA health 

effects is limited. As additional data become available, other measures may also be appropriate, 

such as the Cmax, the integrated dose above a minimum concentration, etc. Regardless of the 

choice for the measure of internal dose, a clearer rationale needs to be presented for the approach 

taken, and, importantly, for any choice adopted, the impact of the internal dose measure on the 

magnitude of the MOE should be described. The Panel also believes that caution should be 

exercised in assuming that the form of PFOA in blood, i.e., free compound or PFOA bound to 

various proteins or lipids is constant in serum across the period of observation, given the current 

information on metabolism. 

Issue 4b: Cross Species Extrapolation 

 In extrapolating data from animal experiments to humans, a default value of 10 is 

typically applied, with a factor of 3 for differences in toxicodynamics and a value of 3 for 

toxicokinetic differences. In the PFOA draft risk assessment document, internal doses from 

animal toxicology studies and human biomonitoring studies were compared. Derivation of data 

from animal toxicology studies included both measured PFOA serum levels from non-human 

primates and derived values from pharmacokinetic modeling from rat studies. The reliance on 

internal dose metrics was considered by OPPT to be sufficient to reduce uncertainties and 

therefore raised the question of the ability to either eliminate or reduce the default values for 

cross species extrapolation. 

 

Comment on the Need to Use or Modify the Default Value of 10 for Cross Species 

Extrapolation Given the Pharmacokinetic Analysis 

 The use of internal dose metrics in this analysis was considered by the Panel to be a 

significant step toward reducing uncertainty related to the toxicokinetic uncertainty associated 

with interspecies extrapolation. Nevertheless, it did not believe that the direct use of blood 

concentration in the assessment sufficiently reduced the overall uncertainly to eliminate or 

modify the current default value. Significant uncertainties still remain, including the measured 

internal dose that best predicts adverse effects in human and other species, the bias inherent in 

measurement/modeling errors, the lack of information about non-cancer endpoints, 

developmental vulnerability and the impact of gender, and the multiple PFOA environmental 

exposures that occur in humans vs. animals, among others. The assumption that PFOA serum 

levels are at steady state in children 2-12 years of age has not been tested and may not be valid. 

The Panel likewise stressed that bench mark dose methodologies would be preferable to the 

reliance in the draft document on LOAEL-driven MOE calculations. 

 
7 

Issue 4c: Human Biomonitoring Data 

 Currently available data on PFOA levels in humans includes occupational biomonitoring 

studies as well as three population studies within the U.S. The measurements from the population 

studies come from: 1) samples from 6 American Red Cross blood banks; 2) a study of 

Streptococcal A infection in children; and 3) elderly volunteers in a cognitive study in Seattle. 

The draft EPA document only utilizes data derived from 1 and 2 above in its calculation of the 

MOE. Occupational biomonitoring data were excluded from the calculation because it was stated 

that sample sizes were small, data on gender were not available, and that blood monitoring data 

obtained from 2000 would overestimate current serum levels, since PFOA exposure of this group 

ceased in 2002. Measured levels from the elderly population were not utilized because values 

were considerably lower, for unknown reasons, than those reported in the other population 

studies for adults and children. From the other two population studies utilized in the draft 

document, geometric means and 90th percentiles were calculated across genders for calculation of 

MOEs. 

 

Comment on the Adequacy of the Human Exposure Data for Use in Calculating a MOE 

 Panel members were not in full agreement as to the adequacy of the human exposure data for 

inclusion in the MOE calculation. Many Panel members shared concerns about the approach adopted in 

the draft document. One concern related to the generality of the populations currently included in the 

MOE calculation. It was noted, for example, that use of the blood donor and pediatric biomonitoring 

data may be acceptable if the purpose is to assess whether there is a potential health effect to the 

“general” population, although there is some question as to the size of other non-occupational 

populations that might be more highly exposed and the assumption that PFOA serum levels are at steady 

state may not be valid for children or fetuses. About three quarters of Panel members agreed that 

existing subpopulations of the general public are likely to be more highly exposed than those previously 

reported and results from occupational studies should be included in the MOE calculation. A differing 

view expressed by the remaining one quarter of the Panel members was that the human biomonitoring 

data are equivocal and thus not useful to MOE calculation. 

Three different summary statistics are presented in the draft document and used in the 

calculation of the MOE. Of these, the Panel questioned the use of mean values, particularly 

geometric means in the calculations. Additionally, no rationale was provided for the choice of 

the 90th percentile as a summary statistic, rather than the use of a higher value. Whatever the 

approach adopted, justification must be provided for the chosen summary measure and an 

explicit objective for the MOE analysis described.
8 

INTRODUCTION 

Background 

 This report was prepared by the Science Advisory Board (SAB) PFOA Risk Assessment 

Review Panel (the “Panel”) in response to a request by EPA’s Office of Pollution Prevention and 

Toxics (OPPT) to review their Draft Risk Assessment of the Potential Human Health Effects 

Associated With Exposure to Perfluorooctanoic Acid (PFOA) and Its Salts. According to the 

document, OPPT has been investigating PFOA and its salts to try to understand the health and 

environmental issues presented by fluorochemicals, in the wake of unexpected toxicological and 

bioaccumulation discoveries with respect to perfluorooctane sulfonates (PFOS). PFOA and its 

salts are fully fluorinated organic compounds that can be produced synthetically or through the 

degradation or metabolism of other fluorochemical products. PFOA is primarily used as a 

reactive intermediate, while its salts are used as processing aids in the production of 

fluoropolymers and fluoroelastomers and in other surfactant uses. PFOA and its salts are 

persistent in the environment. 

 

 OPPT identified 4 issues where they were seeking the SAB’s advice and 

recommendations. These included the proposed mode of action, carcinogenicity descriptors, 

toxicological endpoints selected and the pharmacokinetic modeling methods used in the risk 

assessment. OPPT’s assessment focused on the potential human health effects associated with 

exposure to PFOA and its salts. Several toxicological endpoints and hypothesized modes of 

action were considered. Internal dose metrics were estimated for animal toxicology studies with 

pharmacokinetic modeling, and were obtained from human biomonitoring studies, assuming 

steady state. Margin of Exposure (MOE) values were calculated from the internal dose metrics. 

The SAB PFOA Review Panel was asked to comment on the scientific soundness of this risk 

assessment. 

 

 The Panel deliberated on the charge questions during their February 22-23, 2005 face-toface meeting and during a conference call on July 6, 2005. The responses that follow represent 

the views of the Panel. In all cases, there was agreement by a majority of the panel members as 

to a particular recommendation. In some cases, there were one or more panel members that had 

a differing point of view; these instances have been noted throughout the report. The specific 

charge questions to the Panel are as follows: 

 

Charge Questions 

Issue 1: Rodent PPAR-alpha Mode of Action for Hepatocarcinogenesis 

 

The postulated mode of action (MOA) of PPAR"-agonist induced liver toxicity and liver 

tumors in rodents involves four causal key events. The first key event is activation of PPAR"

(which regulates the transcription of genes involved in peroxisome proliferation, cell cycle 

control, apoptosis, and lipid metabolism). Activation of PPAR" leads to an increase in cell 

proliferation and a decrease in apoptosis, which in turn leads to preneoplastic cells and further 
9 

clonal expansion and formation of liver tumors. Of these key events, only PPAR" activation is 

highly specific for this MOA while cell proliferation/apoptosis and clonal expansion are 

common to other modes of action. There are also several “associative” events that are markers 

of PPAR" agonism but are not directly involved in the etiology of liver tumors. These include 

peroxisome proliferation (a highly specific indicator that this MOA is operative) and 

peroxisomal gene expression. 

 

Information that provides evidence that any specific chemical is inducing liver toxicity 

and tumors via a PPAR" agonist MOA includes in vitro evidence of PPAR" agonism (i.e., 

evidence from an in vitro receptor assay), in vivo evidence of an increase in number and size of 

peroxisomes, increases in the activity of acyl CoA oxidase, and hepatic cell proliferation. The in 

vivo evidence should demonstrate dose-response and temporal concordance between precursor 

events and liver tumor formation. Other information that is desirable and may strengthen the 

weight of evidence for demonstrating that a PPAR" agonist MOA is operative includes data on 

hepatic CYP4A1 induction, palmitoyl CoA activity, hepatocyte hypertrophy, increase in liver 

weights, decrease in the incidence of apoptosis, increase in microsomal fatty acid oxidation, and 

enhanced formation of hydrogen peroxide. 

 

OPPT has proposed that there is sufficient weight of evidence to establish that the mode 

of action for the liver tumors (and precursor effects) observed in rats following exposure to 

PFOA is PPAR" agonism. 

 

Question 1 - Please comment on the weight of evidence and adequacy of the data available to 

identify the key events for the PPAR" agonist-induced rodent liver toxicity and 

hepatocarcinogenesis for PFOA. Discuss whether the uncertainties and limitations of these data 

have been adequately characterized. 

 

 

Issue 2: Descriptor for Carcinogenic Potential 

 

Carcinogenicity studies in Sprague-Dawley rats show that PFOA induces a “tumor triad” 

similar to a number of other PPARα agonists. This “tumor triad” includes liver tumors, Leydig 

cell tumors (LCT), and pancreatic acinar cell tumors (PACT). OPPT has proposed that there is 

sufficient evidence to conclude that the liver tumors are due to PPAR"-agonist MOA, and that 

this MOA is unlikely to occur in humans based on quantitative differences between rats and 

humans. In addition, the LCT and PACT induced in the rat by PFOA probably do not represent 

a significant cancer hazard for humans because of quantitative toxicodynamic differences 

between the rat and the human. Overall, based on no adequate human studies and uncertain 

human relevance of the tumor triad (liver, Leydig cell and pancreatic acinar cell tumors) from 

the rat studies, OPPT has proposed that the PFOA cancer data may be best described as 

providing “suggestive evidence of carcinogenicty, but not sufficient to assess human 

carcinogenic potential” under the interim 1999 EPA Guidelines for Carcinogen Risk 

Assessment, as well as the 2003 draft EPA Guidelines for Carcinogen Risk Assessment. 

 

Question 2 - Please comment on the proposed descriptor for the carcinogenic potential of 

PFOA. 
10 

 

Issue 3: Selection of Endpoints 

 

OPPT has proposed the use of several endpoints from several life stages, species and 

gender for the risk assessment. For this draft assessment, OPPT has not made specific 

recommendations on the most appropriate endpoint/lifestage/species/gender. Rather, all have 

been presented to provide transparency. 

 

For adults, endpoints were selected from the non-human primate and rat studies; the 

endpoints included liver toxicity and possibly mortality for the non-human primates and 

decreased body weight for rats. 

 

For developmental endpoints, OPPT relied upon the definition of developmental toxicity 

outlined in the Agency’s Developmental Toxicity Risk Assessment Guidelines. These 

guidelines state that the period of exposure for developmental toxicity is prior to conception to 

either parent, through prenatal development and continuing until sexual maturation. (In contrast, 

the period during which a developmental effect may be manifested includes the entire lifespan of 

the organism). Based on this definition of developmental exposure, OPPT considered 

developmental effects in the rat two-generation reproductive toxicity study to include reductions 

in F1 mean pup body weight (sexes combined) on lactation days 1, 5 and 8, an increase in 

mortality during the first few days after weaning (both sexes), a delay in the timing of sexual 

maturation (both sexes), and a reduction in mean body weight postweaning (F1 males only). 

 

Question 3 - Please comment on the selection of these toxicity endpoints for the risk assessment. 

 

Question 4 - Given the available data to date, please comment on the most appropriate 

lifestage/gender/species for assessing human risk. 

 

Question 5 - Please comment on the appropriateness of the available animal models. Please 

comment on whether additional animal models should be investigated, and if so, what 

information would better enable us to ascertain potential human risks. 

 

 

Issue 4: Risk Assessment Approach 

 

A margin of exposure (MOE) approach can be used to describe the potential for human 

health effects associated with exposure to a chemical. The MOE is calculated as the ratio of the 

NOAEL or LOAEL for a specific endpoint to the estimated human exposure level. The MOE 

does not provide an estimate of population risk, but simply describes the relative “distance” 

between the exposure level and the NOAEL or LOAEL. In this risk assessment there is no 

information on the sources or pathways of human exposure. However, serum levels of PFOA, 

which are indicative of cumulative exposure, were available from human biomonitoring studies. 

In addition, serum levels of PFOA were available for many of the animal toxicology studies or 

there was sufficient pharmacokinetic information to estimate serum levels. Thus, in this 
11 

assessment internal doses from animal and human studies were compared; this is analogous to a 

MOE approach which uses external exposure estimates. 

 

Issue 4a: Pharmacokinetic Modeling and Use of AUC as a Measure of Internal Dose 

 

As noted above, internal dose metrics from animal toxicology studies and human 

biomonitoring studies were compared in this draft assessment. For humans, the area under the 

concentration curve (AUC) was calculated from measured PFOA serum levels in human 

biomonitoring studies, assuming steady state. For the rat toxicology studies, the area under the 

concentration curve (AUC) and Cmax were estimated from a pharmacokinetic model. The 

pharmacokinetic analysis could be done using a number of approaches including non-parametric 

analysis, physiologically based pharmacokinetic (PBPK) modeling, and classical compartmental 

modeling. Each has strengths and limitations given the available data. Non-parametric analyses 

provide a description of the data that have been collected, but have fairly limited ability to make 

predictions across species or to account for variations in exposures. PBPK modeling is perhaps 

the ideal approach for addressing PFOA for purposes of cross-species extrapolation. Extensive 

pharmacokinetic studies have been undertaken in rats demonstrating complex phenomena 

including high tissue concentrations in liver, kidney and serum and enterohepatic recirculation of 

the parent compound. These could be addressed using PBPK modeling for the rats, but the more 

limited information in monkeys and humans would either require substantial assumptions or 

preclude use of this approach. Classical compartmental modeling can be used to analyze the 

existing data on blood concentrations in rats, monkeys, and humans. Currently, the available 

pharmacokinetic information for rats and humans is sufficient to support compartmental 

modeling. Comparisons of serum protein binding across species indicated a high degree of 

binding in all species but also interspecies differences in the percentage of unbound PFOA in 

plasma. In light of the documented differences in clearance of PFOA across sexes in rats and 

across species, compartmental modeling of serum concentrations provides a sound approach for 

estimating internal dosimetry without exceeding the limits of the available data, so this approach 

was selected for this risk assessment. 

 

Question 6 - Please comment on the use of the one compartment pharmacokinetic model. 

 

Question 7 - Please comment on the use of the AUC as a measure of internal dose for rats and 

humans for calculation of the MOE. 

 

Issue 4b: Cross Species Extrapolation 

Judgments about the “adequacy” of a MOE are based on many considerations including 

uncertainty associated with cross species extrapolation. Typically, a value of 10 is considered 

which consists of a value of 3 for toxicodynamics and a value of 3 for toxicokinetics. Each of 

these can be decreased or increased if there are data to warrant it. In this draft assessment, 

internal doses from animal toxicology studies and human biomonitoring studies were compared. 

For humans, the internal doses were based on measured PFOA serum levels in human 

biomonitoring studies. For the non-human primate toxicology studies, internal doses associated 

with the NOAEL and/or LOAEL were based on measured PFOA serum levels. For the rat 
12 

toxicology studies, pharmacokinetic modeling was used to estimate an internal dose metric 

associated with a NOAEL or LOAEL. 

 

Question 8 - Please comment on the need to use or modify the default value of 10 for cross 

species extrapolation given the pharmacokinetic analysis. 

 

Issue 4c: Human Biomonitoring Data 

 

For this draft assessment, human biomonitoring data of PFOA serum levels were 

available for adults and children. Similar analytical methods were used to measure the PFOA 

levels in both sets of blood samples. The adult data included 645 U.S. adult blood donors (332 

males, 313 females) from 2000-2001, ages 20-69, obtained from six American Red Cross blood 

banks located in: Los Angeles, CA; Minneapolis/St. Paul, MN; Charlotte, NC; Boston, MA; 

Portland, OR, and Hagerstown, MD. Each blood bank provided approximately 10 samples per 

10-year age interval (20-29, 30-39, etc.) for each sex. 

 

The children’s data included a sample of 598 children, ages 2-12 years old, who had 

participated in a study of group A streptococcal infections. The samples collected in 1994-1995 

from children residing in 23 states and the District of Columbia were analyzed for PFOA in 

2002. 

 

Question 9 - Please comment on the adequacy of the human exposure data for use in calculating 

a MOE. 
13 

RESPONSES TO THE CHARGE QUESTIONS 

Issue 1: Rodent PPAR-alpha Mode of Action for Hepatocarcinogenesis and Liver Toxicity

 

Question 1. Please comment on the weight of evidence and adequacy of the data available to 

identify the key events for the PPAR alpha agonist induced rodent liver toxicity and 

hepatocarcinogenesis for PFOA. Discuss whether the uncertainties and limitations of these data 

have been adequately characterized. 

 

 As discussed in the EPA Draft Risk Assessment of the Potential Human Health Effects 

Associated with Exposure to Perfluorooctanoic Acid and its Salts, a sequence of four key events 

define the mode of action (MOA) by which PPAR-alpha agonists induce rodent liver tumors. 

According to this MOA, the initial causal event is (1) activation of PPAR-alpha, which regulates 

the expression of genes involved in peroxisome proliferation, cell cycle control, apoptosis, and 

lipid metabolism. These transcriptional events lead to (2) increased cell proliferation and/or 

decreased cell death. The chronic increase in cell growth occurs primarily in the preneoplastic 

focal lesions in the liver resulting (3) in the clonal expansion of the preneoplastic lesions, which 

ultimately results (4) in the development of hepatocellular neoplasms. In addition, there are 

“associative” events that may or may not be causally linked to the PPAR-alpha MOA for 

hepatocarcinogenesis which include blockage of cell to cell communication, an increase in 

peroxisomes, an increase in peroxisomal enzymes, and liver and hepatocyte hypertrophy. 

 

 The Panel agreed that, considered collectively, the weight of evidence to date is 

consistent with the assertion that PFOA is a PPAR-alpha agonist and can induce liver changes in 

adult rats that have been associated with PPAR-alpha activation. As discussed in the draft PFOA 

risk assessment, some of the key elements to establish this MOA have been demonstrated by 

appropriate experiments. In vitro studies demonstrate that PFOA is a PPAR-alpha agonist, and 

treatment of rats and/or mice results in peroxisome proliferation, increased beta-oxidation, and 

hepatomegaly, with dose and temporal responses consistent with this MOA for liver tumor 

induction. Studies comparing PPAR-alpha null and wild-type mice showed that PFOA-induced 

peroxisome proliferation, beta-oxidation, and immunotoxicity depend on the presence of a 

functional receptor. Further, no other established modes of action of liver cancer-induction have 

been reported for PFOA. PFOA is neither DNA reactive nor mutagenic, and thus not involved in 

a genotoxic mode of action; nor is the liver neoplastic effect due to the induction of repeated 

hepatocyte death and compensatory regeneration (a cytotoxic mode of action). No PPAR-alpha 

independent MOA for the rat liver tumor induction has been proposed. 

With respect to the weight of evidence and the adequacy of consideration of uncertainties 

and limitations, however, the Panel did not reach full agreement. About three quarters of the 

Panel members believed that data gaps still exist and not all of the causal events in the PPARalpha MOA have been demonstrated for PFOA. These include the induction of cell proliferation 

in the liver at early times following PFOA treatment and/or modulation of apoptosis in 

hepatocytes. They also shared the belief that while the PFOA Draft Risk Assessment in general 

appropriately discusses the uncertainties and limitations of the data that support a PPAR-alpha 

MOA for PFOA-induced liver tumors in adult rats, it fails to consider three issues contrary to 

this MOA in sufficient detail. 
14 

First, in a study by Yang et al. (2002) cited in the report in the context of the receptor 

dependence of PFOA immunotoxicity, PPAR-alpha null mice exhibited >2-fold increases in 

liver weight but no peroxisomal acyl CoA oxidase induction in response to PFOA. No increase 

in liver weight was observed in PPAR-alpha null mice treated with the well-characterized 

prototype PPAR-alpha agonist, WY-14,643. While this finding is of uncertain significance, due 

to the lack of histopathology and the absence of a second study showing such an effect, it 

nevertheless raises the possibility that PFOA may induce some of its effects in mouse liver by a 

PPAR-alpha-independent pathway. This observation and the associated uncertainty were not 

mentioned in the context of liver tumor induction in the draft PFOA risk assessment. Secondly, 

uncertainties exist with respect to the relevance to exposed fetuses, infants and children of the 

PPAR-alpha agonist MOA for induction of liver tumors in adults. Humans are refractory to 

some, but not all, PPAR-alpha activation effects. Data from studies using PPAR-alpha receptor 

knockout mice have shown that these receptors are essential for the rapid induction of liver 

neoplasms after exposure to WY-14,643. However, humans do have functional PPAR-alpha 

receptors, leaving unanswered the question as to why they respond so differently from rats and 

mice to PPAR-alpha agonists. Available data suggest that the difference between humans and 

rats or mice may be a consequence of a lower number of PPAR-alpha receptors such that the 

PPAR-alpha MOA is not considered likely to yield a similar hepatic cancer response in adult 

humans. However, exposures of fetuses, neonates and children to PFOA remain a potential 

concern. Rat studies suggest similar PPAR-alpha receptor levels in neonates and adults, but 

because adult humans have so few receptors, and information in fetuses, neonates and children is 

minimal, this same extrapolation cannot be made in humans. Given that human exposures to 

PFOA and related chemicals appear ubiquitous, uncertainties and limitations of the data for 

children have not been adequately characterized to be able to conclude that the PPAR-alpha 

MOA is not operative in this young age group. 

 

 Second, the current draft PFOA risk assessment states (page 76 lines 15-16) that the 

“[a]ctivation of PPAR-alpha leads to an increase in cell proliferation and a decrease in apoptosis, 

which in turn leads to preneoplastic cells ...” Questions were raised as to whether there is 

available experimental evidence that the induction of PPAR-alpha results in an increase in the 

number of preneoplastic foci. The effect of the PPAR-alpha activation appears to be at the level 

of focal lesion clonal expansion (Klaunig et al., 2003), however clonal expansion of focal 

lesions, which is not unique to a PPAR-alpha MOA, has not been shown in rats treated with 

PFOA. 

 Thirdly, some Panel members felt that the role of Kupffer cells (shown in Figure 1, page 

78 of the draft document) should be discussed in the text of the draft PFOA risk assessment. 

There is an extensive literature on the essential role of Kupffer cells in signaling peroxisome 

proliferator-induced hepatocyte proliferation. Studies have shown that hepatocyte proliferation 

and peroxisome proliferation occur by different mechanisms. Parzefall et al. (2001) and Hasmall 

et al. (2001) demonstrated that peroxisome proliferators had no effect on DNA synthesis but still 

induced peroxisomal acyl CoA oxidase activity in cultured rat and mouse hepatocytes that had 

been purified to remove contaminating Kupffer cells. Kupffer cells, which are resident 

macrophages in the liver, are a major source of growth factors (tumor necrosis factor alpha, 

interleukins) that induce DNA synthesis or suppress apoptosis in purified hepatocytes. A key 
15 

finding relevant to the proposed MOA is that Kupffer cells do not express PPAR-alpha (Peters et 

al., 2000), but are activated by peroxisome proliferators. Prevention of Kupffer cell activation by 

glycine inhibited, although not completely, the development of liver tumors by the potent 

peroxisome proliferator, WY-14,643 (Rose et al., 1999). There are no data available on the 

effects of peroxisome proliferators on human Kupffer cells. Recognizing the role of Kupffer cell 

activation in the induction of DNA synthesis and subsequent neoplastic development by PPARalpha agonists, some members of the FIFRA Science Advisory Panel (2003) [SAP Minutes No. 

2003-05] noted that the interplay between PPAR-alpha agonism and Kupffer cells has not been 

characterized. Thus, the results from the PPAR-alpha null mouse are not directly applicable to 

the human situation in which PPAR-alpha is present and can be activated. 

 

A different conclusion was reached by the remaining one quarter of the Panel members 

who found that the weight of evidence was adequate to support a PPAR-alpha agonism mode of 

action for PFOA-induced rodent liver tumors. In this view, the observation of increased liver 

weights in response to PFOA but not to the prototype PPAR-alpha agonist WY-14,643 in PPARalpha knock-out mice as reported in Yang et al. (2002) did not merit significance because the 

study was not designed to evaluate liver toxicity, and the observation represents a single 

replication without corresponding histopathology at the current time. Nor was the possible role 

of Kupffer cells considered to be significant. Based on these considerations, these Panel 

members believed that PPAR-alpha agonism can be considered the sole MOA for PFOAinduced rodent liver tumors. 

 

Issue 2: Descriptor for Carcinogenic Potential 

Question 2. Please comment on the proposed descriptor for the carcinogenic potential of 

PFOA. 

 

The draft PFOA risk assessment proposes that the PFOA cancer data may be best 

described as providing “suggestive evidence of carcinogenicity, but not sufficient to assess 

human carcinogenic potential” under the interim 1999 EPA Guidelines for Carcinogen Risk 

Assessment (US EPA, 1999), as well as the 2003 draft EPA Guidelines for Carcinogen Risk 

Assessment (US EPA, 2003). This opinion is based on the view that human studies on PFOA do 

not provide adequate support of carcinogenicity, as well as on the quantitative differences 

between rats and humans that OPPT believes raises uncertainties about the human relevance of 

the “tumor triad” response (liver tumors, Leydig cell tumors, and pancreatic acinar cell tumors) 

of PPAR-alpha agonist activation in rats. 

 

The determination of an appropriate descriptor for the carcinogenic potential of PFOA 

was discussed by the Panel in the context of the available carcinogenicity data, an evaluation of 

mechanistic or MOA data, and guidance on how EPA applies various descriptors for 

summarizing weight of evidence data. Panel members did not achieve full agreement on the 

appropriate descriptor. Based on the above considerations, the view of about three quarters of the 

Panel members was that the descriptor “likely to be carcinogenic” was more consistent with 

currently available data, while the remaining one quarter of the Panel members reached the 
16 

conclusion that the current evidence fails to exceed the descriptor “suggestive” of 

carcinogenicity. 

Cancer studies on PFOA 

Carcinogenicity studies in Sprague-Dawley rats have shown that PFOA induces 

neoplasms at multiple sites. In male rats exposed to 0 or 300 ppm ammonium perfluorooctanoate 

(APFO) in the feed for 2 years, increased incidences of testicular Leydig cell tumors (LCT) (0% 

vs. 11%), pancreatic acinar cell tumors (PACT) (0% vs. 11%), and liver adenomas (3% vs. 13%) 

were observed in treated animals compared to controls (Biegel et al., 2001). In a 2-year study in 

which male and female Sprague-Dawley rats were fed diets containing 0, 30 or 300 ppm APFO, 

a dose-related increase in LCT was observed (0% in controls, 4% at 30 ppm, 14% at 300 ppm) 

(Sibinski et al., 1987). The draft PFOA risk assessment does not address the effects in the liver 

observed in the Sibinski et al. (1987) study. In that study, the incidences of hepatocellular 

carcinoma in male rats were 6%, 2%, and 10%, and although no adenomas were diagnosed, the 

incidences of hyperplastic nodules in the liver were 0%, 0%, and 6%. Hyperplastic nodules may 

be part of the continuum of proliferative lesions in the liver carcinogenic process. 

 

In female rats, a dose-related increase in mammary gland fibroadenomas was reported 

(22% in controls, 42% at 30 ppm, and 48% at 300 ppm) by Sibinski et al. (1987). In addition, the 

incidence of mammary gland adenocarcinomas was greater in the low dose group than in 

controls (15% in controls, 31% at 30 ppm, and 11% at 300 ppm). The draft PFOA risk 

assessment did not consider the mammary gland neoplasms to represent a compound-related 

effect because of high background rates reported for fibroadenomas in Sprague-Dawley rats in 

historical control data (37%) reported for female rats in the Haskell Laboratory in 1987 (Sykes). 

Three-quarters of the Panel members did not believe that historical control comparisons are as 

reliable as concurrent controls. A number of parameters may contribute to inter-laboratory 

differences in tumor response including differences in diet, animal age at the start and 

termination of studies, animal supply sources and breeding practices, environmental conditions, 

vehicles and routes of administration, animal care procedures that may affect weight gain and 

survival, and the use of different substrains. Thus, in their view, the concurrent control group is 

the most appropriate group for evaluations of chemical-related effects. Moreover, in the 

historical database of Chandra et al. (1992), the incidence in controls of mammary gland 

fibroadenomas was 19.0% and the incidence of adenocarcinomas was 8.8% in female SpragueDawley rats. Therefore, a neoplastic effect in the mammary gland is apparent in the Sibinski 

study in comparison to Chandra et al. (1992). Those Panel members therefore believe that the 

elevated tumor rates observed in female rats in the Sibinski et al. (1987) study raise concerns for 

neoplastic effects induced by PFOA in the mammary gland that should not be dismissed. In 

addition, while new information3

 was submitted to the panel questioning the findings in the 

Sibinski study, about three quarters of the Panel members urged that an independent, 

appropriately-designed histopathology review of the male rat livers and the female mammary 

glands from the Sibinski study be conducted to re-analyze the resulting tumor incidence data. 

 

3

 a report entitled, “Pathology Peer Review and Pathology Working Group Review of Mammary Glands from a 

Chronic Feeding Study in Rats with PFOA Report” conducted by Experimental Pathology Laboratories, Inc. and 

submitted to the SAB by Dr. Larry Zobel of 3M Medical Department 
17 

The remaining Panel members believed that the comparison of the Sibinski et al. (1987) 

mammary tumor data to the historical control data (Sykes, 1987) in the draft risk assessment 

document was valid. 

Mode-of-action analysis, uncertainties, and human relevance 

The PFOA draft risk assessment proposes that there is sufficient evidence to conclude 

that liver tumors induced by PFOA are due to a proposed PPAR-alpha agonist MOA (Klaunig et 

al., 2003), and that this MOA is unlikely to occur in humans based on quantitative differences in 

the numbers of PPAR-alpha receptors between rats and humans. In addition, the PFOA draft 

risk assessment proposes that the Leydig cell tumors (LCT) and pancreatic acinar cell tumors 

(PACT) induced in the rat by PFOA probably do not represent a significant cancer hazard for 

humans because of quantitative toxicodynamic differences between the rat and the human. 

Thus, the panel examined issues related to our understanding of the MOA for the multiple tumor 

types induced by PFOA in rats and the impact of available information on determining the 

human relevance of the animal tumor responses. 

 

Liver adenomas. 

As noted under Issue 1, the Panel concurred that the collective evidence is consistent 

with the hypothesis of a PPAR-alpha agonist MOA for PFOA with associated peroxisomal βoxidation activity, increases in absolute and relative liver weight, and liver tumors in SpragueDawley rats. Issues on which the Panel members opinions diverged related to whether a PPARalpha agonist MOA for liver tumor induction in rats might occur in humans and/or whether 

additional MOAs might be involved. 

 

Key events in the PPAR-alpha agonist MOA. 

 The PFOA risk assessment did not identify dose-response data showing increases in 

hepatocyte proliferation and suppression of apoptosis in rats exposed to PFOA. Many Panel 

members believed this to be a critical deficiency, because these are key events in the proposed 

MOA linking activation of PPAR-alpha to the liver tumor response. 

 

Another observation that influenced most Panel members with respect to potential human 

relevance of the response in rats is the observation that the same early effects actually occur in 

monkeys exposed to PFOA. These effects include the induction of peroxisomal β-oxidation 

activity (2.6 fold), significant increases and positive dose-response trends for absolute and 

relative liver weights (1.6 fold), and the return of relative liver weight to control levels after a 

13-week recovery period. Cell proliferation was evaluated in monkeys but only after 6 months 

of exposure. Unfortunately, neither the rat nor the monkey studies provided data on hepatocyte 

proliferation during the first 1-2 weeks of exposure, or direct measurements of apoptotic cells 

during or after exposure to PFOA was stopped. The lack of data on cell proliferation and 

apoptosis in animals exposed to PFOA makes it impossible to analyze dose-response 

concordance between these key events and tumor induction for PFOA in relation to other PPARalpha agonists. Because the available data for PFOA in rats and monkeys indicate similar 

responses in the livers of rodents and primates (increased liver weight and induction of hepatic 

peroxisomal enzyme activity), about three quarters of the Panel members shared the view that 

human relevance for liver effects induced by PFOA by a PPAR-alpha agonism MOA cannot be 

discounted. 
18 

The remaining panel members, however, considered the increase in liver weight in rats 

exposed to PFOA and the return to control levels following an 8-week recovery period 

(Palazzolo, 1993) to be consistent with an increase in cell proliferation and suppression of 

apoptosis by PFOA during the exposure period. In addition, the lack of an increase in hepatic 

cell proliferation in rats after 1 month or more exposure to PFOA (Biegel et al., 2001) was 

considered consistent with observations of a transient increase in hepatocyte proliferation with 

other peroxisome proliferators. 

PPAR-alpha -independent liver effects. 

As noted in response to Issue 1, about three quarters of the members of the Panel shared 

the view that significant uncertainties still exist with respect to the predictability of a PPARalpha agonist MOA for human cancer risk associated with exposure to PFOA. In a comparative 

study of PFOA and the prototype PPAR-alpha agonist Wy-14,643, at doses of each chemical that 

produced increases in liver weight and peroxisomal fatty acid acyl-CoA oxidase activity in wildtype mice, only PFOA caused a similar 2-fold increase in liver weight (but no increase in acylCoA oxidase activity) in PPAR-alpha null mice (Yang et al., 2002). While this study was not 

designed to assess liver toxicity, it confirms that PFOA is a PPAR-alpha agonist for peroxisomal 

enzyme induction, and also indicates that liver changes induced by PFOA in rodents can occur 

by a mechanism that is independent of PPAR-alpha activation. The lack of liver enlargement or 

tumor response in PPAR-alpha null mice exposed to Wy-14,643 for 11 months has been cited 

frequently as evidence that liver cancer induction by peroxisome proliferators is mediated by 

PPAR-alpha activation (Peters et al., 1997). The study of Yang et al. (2002) needs to be 

replicated, but appears to suggest that results with Wy-14,643 in PPAR-alpha null mice do not 

predict all of the potential liver effects of PFOA. 

 Further, while not diminishing the conclusion that a PPAR-alpha MOA is operative in the 

rodent liver carcinogenesis induced by PFOA, about three quarters of the Panel members 

expressed concern over the as yet incomplete understanding of the role of Kupffer cells in the 

carcinogenic process. PPAR-alpha independent stimulation of hepatocyte growth factor 

production in Kupffer cells appear to be essential to the mechanism of hepatocyte replicative 

DNA synthesis, suppression of apoptosis, and liver tumor development by peroxisome 

proliferators. Until the interplay between PPAR-alpha agonism and Kupffer cell activation is 

characterized, negative results from the PPAR-alpha null mouse may not be relevant to the 

human situation in which Kupffer cells and hepatocellular PPAR-alpha are present and can be 

activated. 

 The remaining members of the Panel believed that the finding of increased liver weights 

produced by PFOA in PPAR-alpha knockout mice, as noted for Issue 1, were not significant 

enough to undermine the PPAR-alpha agonism MOA, nor did they consider the absence of 

information about Kupffer cell activation to be relevant to a PPAR-alpha agonism MOA for 

PFOA-induced rodent liver tumors. 

LCTs, PACTs, and mammary neoplasms. 

Panel members did not consider the consolidation of liver tumors, LCTs, and PACTs into 

a triad MOA to be justified. They believed that available evidence is inadequate to support a 
19 

PPAR-alpha agonist MOA for the induction of LCTs and PACTs (Klaunig et al., 2003), and, at 

present, available data are insufficient to characterize the MOA for PFOA-induced LCTs and 

PACTs. As such, a specific MOA needs to be worked out for each tumor type. In addition, 

about three quarters of the Panel members felt that the appropriate comparison for mammary 

neoplasms was to concurrent not to historical controls, and in that view subscribe to the 

interpretation that PFOA does increase mammary gland neoplasms, and no MOA data are 

available for the mammary tumor response. As discussed in EPA’s Cancer Guidelines, in the 

absence of sufficient data to establish a MOA, the animal tumor responses are presumed to be 

relevant to humans. 

 The remaining Panel members believed, in contrast to the above view, that the 

comparison of PFOA-induced mammary tumor levels to historical controls was valid, and thus 

deemed the evidence for mammary neoplasms to be insufficient to demonstrate such tumors in 

response to PFOA. This served to support the view of these members that PPAR-alpha agonism 

represented the sole MOA for PFOA-induced rodent liver tumors. 

Application of cancer descriptors 

The meaning of terms such as “suggestive evidence of carcinogenic potential” or “likely 

to be carcinogenic to humans” may differ among some in the general public and the EPA 

because of differences in perception and intent. Hence, EPA recommends a weight-of-evidence 

narrative that explains the complexity of issues influencing an agent’s carcinogenic potential in 

humans. Descriptors are applied to provide consistency across agents that are evaluated for their 

carcinogenic potential. In developing their cancer risk assessment guidelines (US EPA 1999, 

2003), EPA has not provided definitive criteria for choosing a descriptor; however, examples of 

the types of evidence that would be covered by a descriptor are listed. EPA also cautions that 

terms such as “likely,” when used as a weight-of-evidence descriptor, do not correspond to a 

quantifiable probability. 

 

About three quarters of the Panel members shared the view that while human cancer data 

on PFOA are inadequate to support a definitive conclusion of the presence or absence of a causal 

association, the animal data are much stronger than the examples summarized in the EPA’s 

Cancer Guidelines under the descriptor “suggestive evidence of carcinogenic potential.” The 

descriptor “suggestive” is typically applied to agents that show a marginal increase in tumors 

only in a single animal study or a slight increase in a tumor response at a site with a high 

background rate. The animal data for PFOA are consistent with the examples listed by EPA 

under the descriptor “likely to be carcinogenic to humans” applied to agents that tested positive 

in more than one species, sex, strain, site, or exposure route, with or without evidence of 

carcinogenicity in humans; or a positive study that indicates a highly significant result and where 

the response is assumed to be relevant to humans. These members concluded, as described 

above, that data from two separate feeding studies demonstrate that PFOA is a multi-site 

carcinogen in rats. Significant increases in tumor incidence and dose-response trends were 

observed in male and female rats. Some of the tumor responses were observed at sites with low 

background rates; the incidence of PACTs and LCTs in control rats was 0% at both sites. 

Because available data are insufficient to characterize the MOA for PFOA-induced LCTS, 

PACTs, or mammary tumors, the responses at these sites are presumed to be relevant to humans. 

Uncertainties also still exist for the MOA(s) for liver tumors induced by PFOA. 
20 

While opting for the descriptor “likely to be carcinogenic to humans” these Panel 

members were not willing to state an associated probability value for PFOA-induced 

carcinogenicity; nor do the EPA guidelines require a quantifiable probability. This group also 

encouraged a cancer risk assessment for each of the PFOA-induced tumors where data permit. 

The risk characterization narrative should address the state of knowledge and uncertainties in the 

MOA for each tumor site and a range of approaches should be considered in the cancer doseresponse assessment. 

 

 The remaining one quarter of the Panel members did not find the weight of evidence 

strong enough to exceed the descriptor “suggestive”. These Panel members agreed with the 

EPA’s risk assessment which proposed that the PFOA cancer data may be best described as 

providing “suggestive evidence of carcinogenicty, but not sufficient to assess human 

carcinogenic potential” This view was based upon their conclusions that: 1) a mechanism for 

formation of liver tumors in rats considered not relevant to humans, 2) liver cell proliferation 

was absent in monkeys, 3) a strong association between PFOA exposure and tumors was not 

demonstrated in human studies, 4) the belief that the testicular and pancreatic acinar tumors in 

rats were probably not relevant to humans, and 5) the view that mammary tumors reported in 

female rats are equivocal. Further these panel members also believed that the sole MOA for the 

rodent-induced liver tumors was through PPAR-alpha agonism which they believe was not 

relevant to humans. 

Issue 3: Selection of Endpoints 

Question 3. Please comment on the selection of these toxicity endpoints for the risk assessment. 

The Panel generally agreed with the Agency approach of considering multiple endpoints 

and developing multiple margin of exposure (MOE) values at this stage in the assessment of 

potential human health effects associated with PFOA. With regard to the selection of endpoints, 

the initial overall philosophy should be one of inclusivity. That is, endpoints should be 

considered unless evidence for an effect by PFOA is equivocal or the dose associated with the 

effect is sufficiently high that other effects will clearly be of greater concern. The reason for 

being inclusive is not to generate an exhaustive catalog of PFOA effects, but rather to insure that 

sensitive effects (i.e., effects occurring at relatively low doses) are not overlooked or prematurely 

excluded from the assessment. 

The Panel agreed with inclusion of all of the endpoints in the current draft of the risk 

assessment. None were recommended for deletion. However, caveats regarding the use of 

organ and body weights as endpoints were offered. Organ and body weights are often among the 

least sensitive endpoints for chemicals that exert specific effects on physiological or 

developmental systems. Nevertheless, in the absence of information with which to select more 

specific endpoints (e.g., biochemical or histological changes), body and organ weight changes 

are likely to be indicative of toxicity. 

Many members of the Panel also believed that additional endpoints should be included in 

the risk assessment, although recognizing that this may not be possible for some endpoints 
21 

because of the absence of sufficient information: 

• Based on discussion in response to Question 2, PFOA has the potential to produce 

carcinogenic effects in humans and therefore additional cancer endpoints (liver, 

testicular, pancreatic acinar, and mammary) should be included in the risk assessment. 

• Liver histopathology, other than cancer, should be included as an endpoint since it could 

not be concluded with confidence that all liver effects are mediated through PPAR-alpha 

agonism (see response to Question 1), and therefore liver histopathology from PFOA 

may be relevant to humans. The Panel recognized that interpretation of some liver 

changes as adverse effects may not always be apparent (e.g., liver enlargement with no 

other pathology), and this should be discussed in the risk assessment. 

• Immunotoxicity should be considered as an endpoint addressed quantitatively in the risk 

assessment. The Panel recognizes that in order to be incorporated into the risk 

assessment, immunotoxicity data will need to be derived in rats, or approaches developed 

for the estimation of serum PFOA concentrations in mice. 

• Consideration should be given to addition of endpoints related to lipid metabolism (see 

comments under response to Issue 3, question #5). 

 In the view of a few Panel members who believed that data for PFOA was consistent 

with the descriptor “suggestive” rather than “likely” to be carcinogenic in humans, the cancer 

endpoints noted above were not recommended for inclusion in the risk assessment as they were 

deemed not to be independent PFOA effects. 

The Panel agreed that additional research on PFOA was needed, and encouraged studies 

in the areas noted below; the Panel also encouraged exploration of methods to identify critical 

targets for PFOA beyond a PPAR-alpha MOA: 

 

• Other than ataxia, no data on neurotoxicity endpoints for PFOA are available. 

Neurotoxicity endpoints, including behavioral measures, should be added to the risk 

assessment. PPAR-alpha receptors, as well as other PPAR receptors, are found in both 

neurons and glia, and are found in multiple brain regions (frontal cortex, basal ganglia, 

reticular formation). It has been proposed that, in addition to their roles common to other 

tissues, these receptors in brain may have specific functions in the regulation of genes 

involved in neurotransmission (Moreno et al., 2004). This would further suggest their 

importance in behavioral function. 

• The two-generation rat study (Butenhoff et al., 2004) involved both perinatal PFOA 

exposure and direct PFOA dosing of the F1 offspring beginning at weaning. The Panel 

recognized that this approach is consistent with U.S. EPA guidance regarding 

developmental studies. However, consideration should be given to using developmental 

endpoints in F1 generation animals prior to initiation of direct dosing so that potential 

effects associated with perinatal exposure can be more clearly identified. 

• Current data suggest that PFOA might produce hormonal effects that would be important 

to consider, but in most cases the significance of the observations are unclear. For 

example, in a 26-week study of PFOA administration to cynomolgus monkeys, serum 

TSH was slightly but significantly elevated in all treatment groups on the final day of the 
22 

experiment, and serum thyroxin was slightly but significantly reduced (Butenhoff, 2002). 

It is not clear whether these observations are physiologically meaningful or whether they 

were strictly dependent upon treatment per se, since hormone levels appeared to change 

in the control animals during the course of the experiment as well. The analysis of 

Butenhoff data did not include a repeated measures ANOVA, so interactions were never 

pursued. Even that, however, would not have revealed why hormone levels changed over 

the course of the experiment in control animals. One study reported PFOA-induced 

decreases in pituitary weight in the F1 generation female rats, but the functional 

significance of this observation is unclear. Overall, the Panel thought that Margins of 

Exposure (MOEs) should not be calculated for hormonal endpoints at this time, but that 

additional research to clarify the hormonal effects of PFOA should be encouraged. 

• Adult male rats exhibited a much slower elimination of the ammonium salt of PFOA, i.e., 

ammonium perfluorooctanoate (APFO or C8), than did females. This appears to be due 

to gonadal hormones inasmuch as castration increased APFO elimination and 

testosterone replacement returned the elimination rate toward normal levels. Importantly, 

renal elimination was blocked by probenecid, a selective antagonist of organic anion 

transporters (OATPs) (Shitara, 2004). Thus, gender differences in renal OATPs may 

account for the gender differences in renal clearance of APFO. Likewise, the slower 

clearance of APFO in males may account for the observation that lower doses of APFO 

produced adverse effects in males compared to females. For example, the NOAEL for 

APFO in a 13-week study of male CD rats was 0.56 mg/kg-day whereas females 

exhibited a NOAEL of 22.4 mg/kg-day. These results suggest that specific organs (e.g., 

liver, kidney, and perhaps adrenals) are targets of APFO because of the pattern of 

expression of the OATPs that transport it across the cells (OATP1-4 in rat). Research to 

identify the relationship between OATP and PFOA toxicity may offer insight into the 

most important targets for PFOA effects and the best endpoints for evaluation. 

Question 4. Given the available data to date, please comment on the most appropriate 

lifestage/gender/species for assessing human risk. 

In general, there was consensus that at this stage in the risk assessment process, no 

lifestage/gender/species should be excluded from consideration in predicting human risk. 

Moreover, absence of information identifying a “critical period” in development during which 

PFOA may exert adverse effects on development requires inclusion of all life stages, including 

fetal development. Biomonitoring data indicate children and adults alike exhibit measurable 

levels of PFOA in serum, and the half-life of PFOA appears to be around 4 years. Therefore, 

there is no reason to exclude any developmental period from examination. Finally, the inclusion 

of data on internal dose is an important element of the dataset for PFOA which should enlighten 

concerns about the use of female rats, discussed below. 

Two considerations arose in evaluating the current dataset for use in assessing human 

risk. In general, the EPA provides a margin of safety by using exposure values that produce the 

lowest margin of exposure based on observed effect levels, usually NOAELS or LOAELs, 

including those in animal models. There was general agreement that the most appropriate 

criterion for assessing human risk is one that produces the lowest margin of exposure (e.g., 90th

, 

95th, or 99th percentile) based on a LOAEL in animal models. The second consideration related 
23 

to the appropriateness of the non-human primate as a model, generally considered to be most 

comparable to humans. 

With respect to the first, the emphasis is on having data based on the internal dose 

relationships (i.e., serum PFOA levels) in some of the animal studies so that interspecies 

differences in metabolism and clearance are taken into account. In addition, these data also 

allow using both males and females despite a dramatic difference in clearance rate. Also, 

considering empirical measures of exposures in children and adults, this view emphasizes a 

concern that both developmental and adult endpoints be captured, and these endpoints have not 

been evaluated in non-human primates. Therefore, the findings from adult male rats including 

the 13 week study by Goldenthal (1978) in which liver weight was significantly increased, and 

the F1 males in the two-generation reproductive toxicity study (Butenhoff et al., 2004) in which 

body weight was reduced should be considered in further analysis of human health risk. 

The second view emphasizes the biological similarities between nonhuman primates and 

humans for risk assessment. This is particularly important in the case of PFOA because there are 

a number of issues with a rodent model for PFOA exposure; e.g., sexual dimorphism with 

respect to elimination of PFOA, and differences in sensitivity to PPAR-alpha signaling between 

rat and human. However, monkeys also exhibit a different half-life of PFOA than do humans, 

and information about the potential toxicity of PFOA on non-human primates are derived 

primarily from adults. 

 

Question 5. Please comment on the appropriateness of the available animal models. Please 

comment on whether additional animal models should be investigated, and if so, what 

information would better enable us to ascertain potential human risks. 

 

The available animal models are useful, but all are considered uncertain matches for 

humans with respect to PFOA toxicity. Thus, most Panel members supported continued use of 

multiple animal models and the need for additional models. As previously noted, some 

responses to PFOA may occur via modes of action not related to PPAR-alpha agonism. Without 

knowing how these PPAR-alpha independent effects are mediated, the ability to identify the 

specific animal models that would be most useful is limited. Some Panel members suggested the 

development and use of additional animal models without PPAR-alpha, such as transgenic or 

siRNA rats. Use of these animal models would be of assistance for more clearly identifying 

PPAR-alpha independent effects of PFOA. 

 

Overall, the Panel thought that results obtained in models using female rats were 

informative because they currently provide the only indication of potential effects on endpoints 

specific to females (e.g., reproduction and developmental effects, mammary tumors). However, 

some concerns were noted regarding the difference in toxicokinetics of PFOA in female versus 

male rats and monkeys, with females exhibiting more rapid excretion. 

 

As part of a discussion of additional sources of information and animal models to help ascertain 

potential human risks, the Panel considered observations from studies in humans. The following specific 

observations in regard to inclusion of the epidemiologic data as informative regarding endpoints were 

shared by most Panel members: 
24 

• The PFOA Draft Risk Assessment did not use the occupational biomonitoring data 

because “data are not available for specific occupational exposures.” The Panel points 

out that neither are data available for “specific environmental exposures.” The further 

claim that information on “critical factors” like gender, sampling methods and occupation 

are not available for the worker populations does not seem relevant. Gender differences 

are not considered in the PFOA Draft Risk Assessment document’s MOE calculations 

(combined male and female values are used) because, unlike in rats, there are no apparent 

gender differences in PFOA elimination in humans, at least in the sparse published data 

available at the time of this review. 

• In the PFOA Draft Risk Assessment document, limitations of epidemiological studies are 

emphasized, while some associations (cerebrovascular disease, triglycerides and cholesterol) are 

deemed less convincing, based on small numbers or inconsistencies in the results. It is 

undeniable that the epidemiology studies, like the toxicological ones, have some limitations, not 

the least of which are uncertainties regarding exposure. However, there is little doubt that these 

workers are more highly exposed than the general population. A special strength of 

epidemiological studies is that no cross-species extrapolation is needed; humans are the model. It 

is also true that there may be multiple exposures in the occupational studies, but this fact alone 

cannot disqualify them without simultaneously disqualifying virtually all epidemiological 

studies, which doesn’t seem appropriate. If the question addressed by an MOE analysis is “how 

far” are actual human exposures from exposures that are associated with a health effect, any 

health effect in the epidemiological studies imply the answer is “zero distance,” regardless of the 

actual serum values. 

 While conceding the small numbers and short follow-up in the available epidemiological 

studies make the positive results less than compelling, they are not, conversely, reassuring. The 

evidence showing increases in cholesterol and triglyceride values in worker cohorts suggest a 

possibility of increased risk of cerebrovascular disease mortality. 

 In responding to charge question #3, therefore, many Panel members shared the view that 

human cancer and alterations in lipid metabolism data be included in the relevant endpoints for 

consideration. This implies that the rich data base of occupational exposures be added to the 

occupational biomonitoring data to be considered. They are not now included in the PFOA Draft 

Risk Assessment document because the worker epidemiological studies were not considered 

suitable for quantitative risk assessment. 

A contrasting conclusion reached by some Panel members was that the peer-reviewed human 

data on health effects of PFOA were equivocal, thus there was not consensus that endpoints suggested 

by some epidemiologic studies should be used as endpoints in the risk assessment. 

Issue 4: Risk Assessment Approach 

Issue 4a: Pharmacokinetic Modeling and Use of AUC as a Measure of Internal Dose 

Question 6. Please comment on use of the one-compartment pharmacokinetic model. 

 
25 

The purpose of developing a mathematical model to fit the serum PFOA time course data 

from the single dose rat oral dosing studies in the PFOA Draft Risk Assessment document was to 

estimate the AUC and Cmax values during the longer term toxicology studies with daily dosing. 

The internal dose metrics calculated with this model were then compared with human serum 

concentrations to establish an MOE. The equations used to describe these data sets are the same 

as those usually employed in one-compartment models for uptake and elimination and were 

referred to throughout the draft document as a one-compartment model. 

 

However, the Panel was concerned that using the “one-compartment” nomenclature 

without caveats and qualifications will give readers of the Draft Risk Assessment Document the 

impression that PFOA pharmacokinetics follow a one-compartment description when in fact 

they are much more complex. In a one-compartment model, the chemical distributes evenly 

throughout a volume of distribution that is itself in rapid equilibrium with blood. Elimination 

kinetics are first-order and do not change with dose level or with time. However, the data 

indicate that it is clearly inappropriate to describe the observed kinetics of PFOA in rats or 

monkeys as following a simple one-compartment model. The relatively complex 

pharmacokinetic behavior of PFOA is reflected in several of the pharmacokinetic data sets. For 

example, elimination from blood after iv dosing and tissue distribution kinetics after oral dosing 

are poorly characterized by the one-compartment model. In both rats and monkeys, blood levels 

are related in a complex manner to dosage and the duration of treatment. 

 

Although the one-compartment model is not appropriate, the empirical model used in the 

document and referred to as a ‘one compartment model’ is adequate for predicting blood levels 

resulting from repeated dosing. However, the document needs to make it clear that the fitting 

procedure is specific to this limited data set and useful for this one application. It is strongly 

recommended that the terminology ‘one-compartment’ model should be stricken from the 

document unless carefully qualified. 

 

Question 7. Please comment on the use of the AUC as a measure of internal dose for rats and 

humans for calculation of the MOE. 

 

Calculating the ‘blood’ AUC (as a measure of average daily concentration of PFOA) is 

an appropriate method of estimating the internal dose, although it is not the only possible 

measure. In the absence of clear understanding of modes of action (MOA), it is also possible that 

the Cmax, the integrated dose above a minimum concentration, or some other quantity may be a 

more plausible measure of internal dose. For example, if the MOA was receptor-based, as might 

be expected for interactions of PFOA with PPAR or other receptor proteins, one of these other 

measures of dose might also be appropriate. These alternatives include receptor occupancy or 

the concentration above some minimum concentration (Cmin) where Cmin is the concentration 

required to initiate activation of the receptor-mediated signaling pathway. In this latter case, the 

MOE would be based on the integral of (Ct-Cmin) rather than just the integral of concentration 

(Ct). 

 

In light of these other possible internal dose measures, the EPA document would be 

strengthened if a clear rationale for the choice of the AUC were included. Since the inclusion of 

this explanation may involve a detailed discussion of toxicokinetic and toxicodynamic issues, 
26 

such a discussion would best be included as an appendix. While the report does provide an 

example of how the MOE differs when based on the Cmax as compared to the AUC, it would be 

helpful if the impact on the magnitude of the MOE of using each of these other internal dose 

measures was explored in more detail. Calculations of MOEs based on these other measures 

would provide a better idea of the extent of possible variability introduced by different internal 

dose measures that may reflect a variety of possible MOAs. 

 

When estimating an AUC, it is important to note the sample that is being analyzed in the 

various studies. AUCs can be calculated for serum, plasma or whole blood. These are very 

different biological matrices. The document should clearly specify the biological media 

measured in each study in which AUCs are reported. 

 

Another issue to be considered is that the analyses of serum time course in the document 

are based on the assumption that the analyte in serum is in the same form and the proportion of 

free compound in blood is constant throughout the period of observation. This assumption does 

not always hold true. For example, with some siloxanes, the blood concentrations during and 

after inhalation exposure are primarily free siloxanes that are available for exhalation and 

metabolism. After a period of time in the body, the siloxanes in blood appear to reside in the 

lipid pool within the blood and although they are easily analyzed are no longer available for 

these other clearance processes (see Andersen et al., 2001; Reddy et al., 2003). A situation 

where the PFOA in blood at much longer times after exposure is in a distinctly different 

biological pool would lead to difficulties in comparing rat AUC and human AUC values to 

obtain a MOE. Interspecies differences in PFOA free fraction in plasma may also complicate 

the comparison of AUC values to obtain a MOE. 

The direct use of internal measures of dose by US EPA in this document represents a 

promising and relatively innovative approach for risk assessments of environmental compounds 

compared to the more usual practice based on comparing daily dose rates by various routes of 

administration. This new approach reduces the need to include uncertainties introduced by the 

use of administered or ambient doses as measures of exposure. This type of risk assessment 

methodology is likely to become much more widespread due to advances in analytical chemistry 

and the rapid expansion of human biomonitoring activities throughout the world. Because this 

risk assessment is likely to serve as a prototype for future tissue-dose based risk assessments, 

some important issues raised by this tissue-dose based approach need to be more fully 

considered and adequately contrasted with the more common assessments based on comparisons 

of administered doses. 

 

To address these issues, the EPA should be encouraged to develop documentation 

explaining their rationale guiding the use of these tissue-dose based risk assessment approaches. 

Such documentation should compare current methods based on daily intakes with these 

alternative, ‘tissue-based’ approaches to more explicitly address the risk characterization issues 

that arise in moving to this new approach. Such a document might include discussion of (1) the 

choices of tissue dose measures based on serum concentrations and the risk implications of each 

choice; (2) the impacts of utilizing direct measures of tissue dose on the magnitudes of 

interspecies and interindividual uncertainty factors; (3) the implications of different metrics for 

characterizing distributions of human tissue dose measures on estimates of MOEs; and (4) the 
27 

importance of routine analysis of appropriate blood concentrations; e.g., serum, plasma, etc. in 

providing the information for most appropriately applying the tissue dose approach. 

 

Issue 4b: Cross Species Extrapolation 

Question 8. Please comment on the need to use or modify the default value of 10 for cross 

species extrapolation given the pharmacokinetic analysis. 

 

The internal dose analysis used in this document is considered by the Panel to be a 

significant step toward reducing uncertainty related to cross species extrapolation. Although 

reduced, however, cross species toxicokinetic uncertainty is not eliminated. Sources of 

uncertainty remain, including the lack of information about the measured internal dose that best 

predicts adverse effect in human and other species, and the bias inherent in 

measurement/modeling error. While it is difficult to assign a quantitative value to the magnitude 

of this uncertainty reduction, it can be stated that the toxicokinetic uncertainty value for PFOA 

would fall within the range of one to three, based on the customary scale of a value of 3 for each 

aspect of cross species extrapolation, pharmacokinetics and pharmacodynamics. 

Pharmacodynamics aspects of PFOA cross species scaling are not addressed in a sufficient 

manner to alleviate the application of some type of uncertainty factor/s (addressing 

toxicodynamic equivalence across species). The assumption that PFOA serum levels are at 

steady state in children 2-12 years of age has not been tested and may not be valid. The 

additional complexity of multiple C-8 environmental exposures in humans versus animal 

experiments involving exposures to PFOA specifically, further clouds the overall uncertainty 

analysis. 

While the pharmacokinetic modeling that is presented in the PFOA risk assessment is 

useful, a more comprehensive way to account for biological processes that determine internal 

dose is with the development of a physiologically based toxicokinetic model. The Panel 

encourages EPA to continue to develop toxicokinetic models as they can improve dose-response 

assessment by revealing and describing nonlinear relationships between applied and internal 

dose. 

 

 A discussion of confidence should always accompany the presentation of model results 

and include consideration of model validation and sensitivity analysis, stressing the predictive 

performance of the model. Toxicokinetic modeling results may be presented as the preferred 

method of estimating equivalent human doses or in parallel with default procedures (see Section 

3.1.3), depending on the confidence in the modeling. 

 

 Standard cross-species scaling procedures are available when the data are not sufficient 

to support a toxicokinetic model or when the purpose of the assessment does not warrant 

developing one. The aim is to define dose levels for humans and animals that are expected to 

produce the same degree of effect (U.S. EPA, 1992b), taking into account differences in scale 

between test animals and humans in size and in lifespan. It is useful to recognize two 

components of this equivalence: toxicokinetic equivalence, which determines administered doses 

in animals, and humans that yield equal tissue doses, and toxicodynamic equivalence, which 

determines tissue doses in animals and humans that yield equal lifetime risks (U.S. EPA, 1992b). 
28 

 

It is equally important to note that pharmacodynamics aspects of PFOA cross species 

scaling are not addressed in a sufficient manner to alleviate the application of some type of 

uncertainty factor/s (addressing toxicodynamic equivalence across species). These factors may 

be different for each species extrapolated. By the language used in the U.S. EPA Cancer 

Guidelines, it seems evident that standard default values were never intended to act as complex 

scaling factors when internal doses in human serum are compared to animal internal doses across 

multiple pathways, genders, steady-state serum levels with long human half-lives and/or 

different life stages. 

 

In the case of PFOA the strong reliance on LOAEL-driven MOE calculations instead of 

more appropriate Bench Mark Dose methodologies, and the absence of probabilistic approaches 

to assessing human exposure and risk, was considered by most Panel members as another source 

of dynamic uncertainty. 

 

The use of an uncertainty factor/s based on data variability may be an alternative to the 

traditional scaling factors given the kinetics analysis strength and in light of the larger concerns 

of overall uncertainties related to dynamic analysis (as reflected in the MOE approach). This 

may prove more productive when comparing relatively robust toxicokinetic dose response 

models involving serum concentrations and/or their surrogates. 

 

In conclusion, whereas toxicokinetic uncertainty is possibly reduced in this analysis, care 

must be exercised in the estimation of the overall cross species uncertainty, which further 

dynamic analyses may show falls below or above 10. 

 

Issue 4c: Human Biomonitoring Data 

Question 9. Please comment on the adequacy of the human exposure data for use in calculating a 

MOE. 

Full agreement was not reached by Panel members with respect to the utility of the human 

biomonitoring data for the calculation of the MOE. Most Panel members expressed the view that the 

human exposure information should be utilized in these calculations while a few Panel members 

believed that these data were equivocal and thus not appropriate for the MOE calculations. 

 

Populations used for MOE calculations 

In addition to the occupational biomonitoring data, the PFOA Draft Risk Assessment document 

described three separate study populations from the United States with available individual serum PFOA 

levels. One consists of samples from six American Red Cross blood banks, another from a study of 

Streptococcal A infection in children, and a third from elderly volunteers from Seattle who participated 

in a study of cognitive function. Only the first two study populations were used in calculating the MOE 

for the risk assessment. 

 

A question was raised about reliance on the female blood bank donor population for calculating 

prenatal MOEs, because the influence of pregnancy on serum PFOA levels is not known. Likewise, use 
29 

of the samples obtained from the children for the age span of 2-12 years for the postweaning period 

MOE may not be appropriate because the assumption of steady state used in the MOE analyses may not 

be valid for children. Half-life issues in humans, especially when considering the impact of age at 

exposure (or the critical windows of exposure model), contribute to the questions about adequacy of 

using these samples (Pryor et al., 2000; Selevan et al., 2000; Sweeney et al., 2001). Thus, there are a 

variety of possible problems with using these data to represent the general population, but the Panel 

agreed that they were likely to be reasonably representative and are better than data often available for 

exercises of this nature. 

 

It was suggested that biomonitoring data in highly exposed groups (occupational and 

environmental) be included in the MOE analyses. It was noted that the existence, size and levels of 

exposures of populations which may differ from those studied has yet to be fully determined. Until this 

has been determined, it is not clear what percent of the general population is covered by the MOEs that 

have been calculated. Thus, the appropriateness of relying solely on the blood bank and pediatric 

samples for MOE calculations depends strongly on the purpose of the MOE exercise, i.e., whether it is 

to assess the likelihood that any people could be suffering health effects from PFOA or only the 

“general population.” If the latter case, the biomonitoring data that were used may be appropriate, but 

the sizes of more highly exposed populations remains unknown and this should be acknowledged. 

A few members of the Panel held the view that the human data were equivocal, based on the 

likely multiplicity of exposures of occupational groups, and thus should not be included in the draft 

PFOA risk assessment for MOE calculations for the general population. 

 

Depiction of the biomonitoring data 

The tables and summary statistics that were used in the draft PFOA risk assessment are 

somewhat uninformative and unsatisfactory. It is difficult to determine the distribution of population 

exposures from these given the method of data presentation. A preferable approach would be to use a 

non-parametric data-driven method to display the data (including the occupational data), using, for 

example, some density estimation procedure or smoother. Inclusion of the worker data in these displays 

would allow a clearer understanding of the relationships. Even side-by-side box plots would have been 

preferable to what was provided. This requires having access to the raw data, however. Because such a 

request is easy to satisfy, the Panel recommends that EPA provide more informative displays of the 

biomonitoring data. 

Appropriate summary measures for MOE calculations 

 At least three summary statistics are mentioned in the Draft, the geometric mean, the arithmetic 

mean, and the 90th percentile. 

 

The rationale for the use of “means” should be explained, especially the use of the geometric 

means which seems the least satisfactory, since it is about 25% lower than the arithmetic mean in these 

data. Use of a geometric mean for population inference (to transform a lognormal to a normal 

distribution, for example) might be justified, but not for the purpose of calculating an MOE. Moreover, 

the distribution does not even seem to be lognormal, as judged by the Shapiro-Wilk test. The idea that a 

few censored data points are responsible for failing this test seems highly unlikely, and could have been 

accounted for in the test itself. 
30 

 

Means of any kind don’t seem appropriate for a ubiquitous exposure. Of the three choices, the 

90th percentile seems the most appropriate in that case. At least one Panel member wondered why some 

even higher percentile, say 95th or even a maximum value wouldn’t be better. The maximum value in 

any of the samples is still an underestimate of the maximum value in the population. Even the upper 

99.99th percentile represents 30,000 people in the US. 

 

In summary, the Panel finds that: 

 ● Use of the blood donor and pediatric biomonitoring data may be acceptable if the purpose is to 

assess whether there is a potential health effect to the “general” population, although there is 

some question as to the size of other non-occupational populations that might be more highly 

exposed and the assumption that PFOA serum levels are at steady state may not be valid for 

fetuses, neonates or children; 

 ● Most Panel members believed that occupational biomonitoring data should be included in the 

MOE calculations, especially regarding additional endpoints such as alterations in lipid 

metabolism; 

 ● A few members did not favor this inclusion based on the equivocal findings; 

 ● The biomonitoring data should be presented in a more informative manner, for example, through 

side-by-side box plots or some other method that would better depict the range of values and 

distributions; and 

 ● Thought should be given to what appropriate summary statistic for the biomonitoring 

datasets used in MOE calculations should be. Some panelists believe that 90th percentiles 

or higher, perhaps even maximum values might be most appropriate. In any event, 

justification for use of the chosen summary measure should be made and related to the 

explicit objective of the MOE analysis.
31 

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