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 Offi ce 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 Hea lth 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 seve ral commercially important produc ts. PFOA has been detected in the blood of the general U.S. popul ation although it is not fully understood how individuals are exposed to the chemical. To determine whethe r environmental exposure to PFOA might pose a risk to human health, EPA’s draft assessment pr ovided 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 da ta 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 ri sk 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 pl ace of the administered dose in toxicologic

studies. The Panel recommended th e inclusion of additional non-ca ncer health endpoints for risk assessment, and the use of the Benchmark Dose method to better estimate the lowest observed2 effect levels and no observed effect levels fo r risk assessment. Thre e-quarters of the Panel judged that the weight-of-evidence conclusion fo r 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 recomm ended that a risk assessment be conducted for carcinogenic effects. About one-qua rter of the Panel agreed with EPA’s conclusion regarding the potential cancer hazard of PFOA to humans and the designati on of the cancer descriptor of “suggestive evidence of carcinogenicity, but not sufficient to assess human carcinogenic potential”. Three quarters of the Panel consider ed 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 recomme ndations on these issues are detailed in the Panel’s report.

The SAB strongly urges the Agency to stre ngthen its risk asse ssment by considering verified and peer reviewed new in formation 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 Boardi NOTICE

This report has been written as part of th e activities of the EPA Science Advisory Board (SAB), a public advisory group providing extramur al scientific information and advice to the

Administrator and other offici als of the Environmental Protection Agency. The SAB is structured to provide balanced, e xpert 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 repr esent the views and policies of the Environmental Protection Agency, nor of other agencies in th e Executive Branch of the Federal government, nor does mention of trade names of commercial pr oducts constitute a r ecommendation 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, Un iversity of Medicine and Dentistry of New Jersey and Rutgers State Un iversity, Piscataway, NJ

MEMBERS

Dr. Ernest Abel , Professor of Obstetrics and Gynecol ogy; Professor of Psychology; Director, Reproductive Toxicology, Obstetrics and Gyneco logy Department, School of Medicine / C.S. Mott Center for Human Growth and Developmen t, Wayne State University, Detroit, MI

Dr. Melvin Andersen , Director, Department of Biomathema tics 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 Oncol ogy, McArdle Laboratory for Cancer Research, University of Wiscons in Medical School, Madison, WI

Dr. William L. Hayton , Professor and Associate Dean, Di vision of Environmental Health Science, College of Pharmacy, School of Medici ne 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 a nd Toxicology, College of Pharmacy, University of Texas at Austin, Austin, TX

Dr. James E. Klaunig , Professor and Director, Departme nt 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, NCiii Dr. Franklin L. Mink , President, MAI, Lake Orion, MI

Dr. David M. Ozonoff , Professor, Department of Envir onmental Health, School of Public Health, Boston University, Boston, MA

Dr. Stephen Roberts , Professor, Center for Environmen tal and Human Toxicology, University of Florida, Gainesville, FL

Dr. Anne Sweeney , Associate Professor, Department of Epidemiology and Biostatistics, School of Rural Public Health, Texa s 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,

DCiv TABLE OF CONTENTS

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

I SSUE 1: R ODENT PPAR- ALPHA M ODE OF A CTION FOR H EPATOCARCINOGENESIS ...........................................1

I SSUE 2: D ESCRIPTOR FOR C ARCINOGENIC P OTENTIAL ........................................................................................3

I SSUE 3: S ELECTION OF E NDPOINTS ........................................................................................................................4

I SSUE 4: R ISK A SSESSMENT A PPROACH ..................................................................................................................5

I SSUE 4 A : P HARMACOKINETIC M ODELING AND U SE OF AUC AS A M EASURE OF I NTERNAL D OSE ....................5 Issue 4b: Cross Species Extrapolation ...............................................................................................................6 Issue 4c: Human Biomonitoring Data ...............................................................................................................7

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

B ACKGROUND

............................................................................................................................... ...........................8

C HARGE Q UESTIONS

............................................................................................................................... .................8

RESPONSES TO THE CH ARGE QUESTIONS .............................................................................................. .....13

I SSUE 1: R ODENT PPAR- ALPHA M ODE OF A CTION FOR H EPATOCARCINOGENESIS AND L IVER T OXICITY .....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 an d hepatocarcinogenesis for PFOA. Discuss whether the uncertainties and limitations of these data have been adequately characterized...........................13

I SSUE 2: D ESCRIPTOR FOR C ARCINOGENIC P OTENTIAL .....................................................................................15 Question 2. Please comment on the propos ed descriptor for the carcinogenic potential of PFOA. ................15

I SSUE 3: S ELECTION OF E NDPOINTS .....................................................................................................................20 Question 3. Please comment on the selection of these to xicity endpoints for the risk assessment....................20 Question 4. Given the available data to date, pl ease comment on the most appropriate lifestage/gender/species for assessing hum an 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 poten tial human risks. ............................................................................................ ........................23

I SSUE 4 A : P HARMACOKINETIC M ODELING AND U SE OF AUC AS A M EASURE OF I NTERNAL D OSE ..................24 Question 6. Please comment on use of the one-co mpartment pharmacok inetic 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

I SSUE 4 B : C ROSS S PECIES E XTRAPOLATION ........................................................................................................27 Question 8. Please comment on the need to use or modify the default value of 10 for cross species extrapolation given the ph armacokinetic analysis............................................................................... ..............27

I SSUE 4 C : H UMAN B IOMONITORING D ATA ..........................................................................................................28 Question 9. Please comment on the adequacy of the human exposure data for use in calculating a MOE. ....28

REFERENCES ..................................................................................................................... .....................................311 

### EXECUTIVE SUMMARY

EPA’s Office of Pollution Prevention and T oxics (OPPT) requested that the Science Advisory Board review the “Draft Risk Asse ssment of the Potential Human Health Effects Associated with Exposure to Perf luorooctanoic Acid (PFOA) and it s 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, carcinogenici ty descriptors, useful models for evaluation of health effects, toxicokinetic considerat ions and reliance on currently available human biomonitoring exposure data for calculation of margins of exposure (MOEs). Further

discussions of the entire Panel were he ld during a conferen ce call in July 2005.

This Executive Summary highlights the outcome of the Panel’s deliberations. It includes the context for the charge questions and issu es raised for consider ation by EPA, and the conclusions reached by the SAB Review Panel. Th e 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 effect s become available. In instances where the views of Panel members diverged, the summary a nd charge question responses focus to a greater extent on the view expressed by about three quart ers 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 revi ew period, new information 1 was presented to the Panel for their consideration. The Panel encourages EPA to consid er new information that has been verified and peer-reviewed prior to use in their revi sion 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 expa nsion of preneoplastic foci and subsequent tumors. The draft document premises its conclu sions about this MOA on studies showing that PFOA is a potent peroxisome proliferator in live r of rats and mice and, like other peroxisome proliferators, induces hepatomegaly in rats. In addition, requisite dos e-response and temporal associations for some key events fo r this MOA have been reported.

Comment on the Weight of Evidence and Adeq uacy 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 whethe r 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 rece ptor, results in peroxisome proliferation, increases beta-oxidation and produces hepatome galy, 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 beli eved that at the current time, sufficient

uncertainties and limitations of th e 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 activ ation was precluded, ra ising the possibility

that PFOA-induced liver tumors could occu r by PPAR-alpha independent effects. The significance of this finding curre ntly remains uncertain in th e absence of a corresponding assessment of histopathology or replication by another laboratory. 2) There is as yet no published evidence that the induc tion of PPAR-alpha by PFOA resu lts in clonal expansion of pre-neoplastic foci which is cons idered a critical step in the pr oposed MOA. 3) There are no data demonstrating increased cell proliferation and/ or decreased apoptosis in the liver of PFOAtreated rats, key causative ev ents in the proposed MOA.

These Panel members also viewed two a dditional 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 charac terized to be able to conclude that the PPARalpha MOA is not operative in this young ag e 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 in creased liver weights in response to PFOA, but not to the prototype PPA R-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 coll ectively the weight of evidence supports the hypothesis that liver tumor induc tion in rodents by PFOA is me diated by a PPAR-alpha agonism MOA. Most Panel members, however, also felt, ba sed 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 other3 hepatoproliferative lesions require clarification, and that extra polation of this MOA across the age range in humans is not s upported. 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 consid ered not relevant to

humans because of their decreased sensitivit y to PPAR-alpha agonism when compared to rodents, 2) the absence of hepatic cell prolifer ation in a 6 month study of PFOA administration in cynomologous monkeys, the species considered clos est 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 gr owth factor receptors (PACT) in humans, as well as differences in quantitati ve toxicodynamics between rats a nd 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 fo r the Carcinogenic Potential of PFOA

About three quarters of the Panel members c oncluded that the experimental weight of evidence with respect to the ca rcinogenicity of PFOA was stronge r than proposed in the draft document, and suggested that PFOA cancer data are consistent with the EPA guidelines descriptor ‘likely to be carci nogenic to humans’. According to EPA’s Guidelines for Carcinogen

Risk Assessment 2 (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 eviden ce of carcinogenicity in humans. Conclusions of these Panel members were based on the following:

• While human data are ambiguous, two separate feeding studies in ra ts demonstrate that PFOA is a multi-site carcinogen.

• Uncertainties still exist (s ee 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 fi nding of uncertain significance in the absence of liver histopathology and replicati on 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 fr om other laboratories was deemed inappropriate, since the most appropriate control group is a concu rrent control group. Us ing that comparison,

2 In March 2005, EPA published final Cancer Guidelines and Supplemental Guidance wh ich 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 tr eated 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 potentia l for PFOA-induced carcinogencity. Nevertheless,

based on available evidence to date , most Panel members believed th at risk assessments for each of the PFOA-induced tumors are ap propriate 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 th e 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) a nd 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 sp ecific recommendations on the most appropriate parameters are stipulated at the current time.

Comment on the: Selection of Toxicity Endpoi nts for the Risk Assessment The Most Appropriate Lifestage/Gende r/Species for Assessing Human Risk The Appropriateness of the Available Animal Models

The Panel agreed with the current approach of inclusivity, given th e current uncertainties noted above with respect to car cinogenicity, as well as the pauc ity 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 toxicoki netics in rats, non-hu man 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 occupatio nal 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 epidem iological studies. While the draft document notes that the occupational studi es suffer from the fact that they involve5 multiplicity of exposures, other studies ha ve shown a high correlation among fluorinated compounds in biological samples from the general population and occupational cohorts. Therefore, these human studies could be advant ageous for assessing pote ntial interactions among these compounds that may be associated with adverse human health effects. These Panel members also believed that epidemiologic and occupational studies coul d not be disqualified without disqualifying virtually all such studies in the risk assess ment process. Moreover, it is clear that occupationally-exposed populations have experienced th e highest levels of exposure and therefore reported health effects in these studies merit consideration.

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

that the outcomes from studies of human health effects of PFOA were e quivocal, and thus these endpoints should not be in corporated 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 me mbers. Given the prevalence in brain of PPAR receptors, including PPAR-alpha, effects on nerv ous system structure and function warrant attention. Moreover, no information currently exis ts with respect to criti cal periods; therefore, it is important to evaluate effects across age gr oups. The observations of hormonal alterations in

treated animals also deserve furthe r study to assess their importance.

Issue 4: Risk Assessment Approach

Issue 4a: Pharmacokinetic Modeling and Us e 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 ulti mately generating margin of exposure (MOE)

information. Area under the concentr ation curve (AUC) was calculat ed from PFOA serum levels in human biomonitoring studies assuming a steady st ate. In some of the rat studies, serum PFOA concentrations were available, or it was cons idered that sufficient pharmacokinetic information was available to estimate serum levels. For this purpose, AUC was estimated from a pharmacokinetic model. Specifically, compartmen tal modeling of serum concentrations using single dose rat oral exposure studies were used to estimate internal dosimet ry for the longer term dosing studies based upon the premise that pharm acokinetic 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 m odel used in the draft document was adequate for predicting blood levels result ing from repeated dosing, but th at 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 pharmac okinetics in the draft document is misleading, given the actual complexiti es in many of the available datasets, and the term should be removed or repl aced unless carefu lly 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 calculati ng blood AUC may be an appropriate method to estimate internal dose, it is important to note th at 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 C

max , 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 c hoice 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 PFO A in blood, i.e., free compound or PFOA bound to various proteins or lipids is c onstant in serum across the period of observation, given the current

information on metabolism.

Issue 4b: Cross Species Extrapolation

In extrapolating data from animal experi ments to humans, a default value of 10 is typically applied, with a factor of 3 for differe nces in toxicodynamics and a value of 3 for

toxicokinetic differences. In the PFOA draft ri sk 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 pharmacokine tic 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 e ither 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 unc ertainty related to the toxicokinetic uncertainty associated with interspecies extr apolation. Nevertheless, it did not be lieve that the direct use of blood concentration in the assessment sufficiently reduced the overall uncertainly to eliminate or modify the current default value. Significant unc ertainties 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 test ed 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 L OAEL-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 populati on studies within the U.S. The measurements from the population studies come from: 1) samples from 6 Am erican Red Cross blood banks; 2) a study of Streptococcal A infection in children; and 3) el derly volunteers in a cogn itive 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 exclude d from the calculation because it was stated that sample sizes were small, data on gender we re not available, and th at blood monitoring data

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

ceased in 2002. Measured levels from the elderl y 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 othe r two population studies utilized in the draft document, geometric means and 90 th percentiles were calculated acro ss genders for calculation of

MOEs.

Comment on the Adequacy of the Human Expos ure 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 rela ted to the generality of the popul ations 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 wh ether there is a potential health effect to the “general” population, although there is some ques tion as to the size of other non-occupational populations that might be more highly exposed and th e assumption that PFOA seru m levels are at steady state may not be valid for children or fetuses. About three quarters of Pa nel members agreed that existing subpopulations of the genera l public are likely to be more highl y exposed than those previously reported and results from occupational studies shou ld be included in the MOE calculation. A differing view expressed by the remaining one quarter of th e Panel members was that the human biomonitoring data are equivocal and thus not useful to MOE calculation.

Three different summary statistics are presen ted in the draft document and used in the

calculation of the MOE. Of thes e, the Panel questioned the use of mean values , particularly geometric means in the calculations. Additiona lly, no rationale was provided for the choice of

the 90 th percentile as a summary statis tic, rather than the use of a higher value. Whatever the approach adopted, justification must be provi ded 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 unexp ected toxicological and bioaccumulation discoveries with respect to perf luorooctane sulfonates (PFOS). PFOA and its salts are fully fluorinated organic compounds that can be produced synthetically or through the degradation or metabolism of other fluorochemi cal products. PFOA is primarily used as a reactive intermediate, while its salts are used as processi ng aids in the production of fluoropolymers and fluoroelastomers and in othe r surfactant uses. PF OA 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 pharmac okinetic modeling methods used in the risk assessment. OPPT’s assessment focused on the pot ential human health effects associated with exposure to PFOA and its salts. Several t oxicological endpoints a nd hypothesized modes of

action were considered. Internal dose metrics were estimated for animal toxicology studies with pharmacokinetic modeling, and were obtained fr om 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 comm ent 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 wa s 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 even ts. The first key event is activation of PPAR " (which regulates the transcrip tion of genes involved in pero xisome 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 further9 clonal expansion and formation of liver tu mors. Of these key events, only PPAR " activation is highly specific for this MOA while cell prolif eration/apoptosis and clonal expansion are

common to other modes of action. There are also several “associative” ev ents 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 sp ecific 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 a nd temporal concordance between precursor

events and liver tumor formation. Other inform ation that is desirable and may strengthen the weight of evidence for demonstrating that a PPAR " agonist MOA is operati ve includes data on hepatic CYP4A1 induction, palmitoyl CoA activit y, hepatocyte hypertrop hy, increase in liver weights, decrease in the inciden ce of apoptosis, increase in micros omal fatty acid oxidation, and enhanced formation of hydrogen peroxide.

OPPT has proposed that there is sufficient wei ght of evidence to establish that the mode of action for the liver tumors (and precursor e ffects) 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 th e 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 PPAR α agonists. This “tumor triad” includes liver tumors, Leydig cell tumors (LCT), and pancreatic acinar cell tumors (PACT). O PPT 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 base d on quantitative differenc es 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 desc riptor for the carcinogenic potential of

PFOA.10 Issue 3: Selection of Endpoints

OPPT has proposed the use of several endpoi nts 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 prov ide transparency.

For adults, endpoints were selected from the non-human primate and rat studies; the endpoints included liver toxicity and possibl y 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 toxic ity 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 toxic ity 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 weani ng (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 th ese toxicity endpoints for the risk assessment.

Question 4 - Given the available data to date, please comment on the most appropriate lifestage/gender/species for a ssessing human risk.

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

comment on whether additional animal models should be investigated, and if so, what information would better e nable 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 chem ical. The MOE is calculated as the ratio of the NOAEL or LOAEL for a specific en dpoint to the estimated human exposure level. The MOE does not provide an estimate of population ris k, but simply describes the relative “distance” between the exposure level and the NOAEL or LOA EL. In this risk assessment there is no information on the sources or pathways of huma n exposure. However, serum levels of PFOA,

which are indicative of cumulativ e exposure, were available from human biomonitoring studies. In addition, serum levels of PFOA were availabl e for many of the animal toxicology studies or there was sufficient pharmacokinetic information to estimate serum levels. Thus, in this11 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 Us e 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 fr om measured PFOA serum levels in human biomonitoring studies, assuming steady state. For the rat toxicology studies, the area under the concentration curve (AUC) and C

max 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 exposu res. PBPK modeling is perhaps the ideal approach for addressing PFOA for purpose s of cross-species extr apolation. Extensive pharmacokinetic studies have b een undertaken in rats de monstrating complex phenomena including high tissue concen trations in liver, kidney and serum and enterohepatic recirculation of the parent compound. These could be addressed us ing PBPK modeling for the rats, but the more limited information in monkeys and humans woul d either require substantial assumptions or preclude use of this approach. Classical comp artmental modeling can be used to analyze the existing data on blood concentrations in rats, mo nkeys, and humans. Currently, the available pharmacokinetic information for rats and hum ans is sufficient to support compartmental modeling. Comparisons of seru m 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 acr oss sexes in rats and across species, compartmental mode ling of serum concentrations provides a sound approach for estimating internal dosimetry without exceeding the li mits of the available data, so this approach

was selected for this risk assessment.

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

Question 7 - Please comment on the use of the AU C 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 extrap olation. 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 a nd human biomonitoring studies were compared. For humans, the internal doses were base d 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 rat12 toxicology studies, pharmacokine tic modeling was used to estim ate 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 th e pharmacokinetic analysis.

Issue 4c: Human Biomonitoring Data

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

available for adults and children. Similar anal ytical 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; Minneapo lis/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 Dist rict 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 eviden ce and adequacy of the data available to identify the key events for the PPAR alpha agoni st induced rodent liver toxicity and hepatocarcinogenesis for PFOA. Discuss whether th e 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 Perf luorooctanoic 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 even t is (1) activation of PPAR-alpha, which regulates the expression of genes involved in peroxisome pr oliferation, 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 resul ting (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 enzy mes, and liver and hepatocyte hypertrophy.

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

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

adult rats that have be en associated with PPAR-alpha activat ion. As discussed in the draft PFOA risk assessment, some of the key elements to establish this MOA have been demonstrated by appropriate experiments. In vi tro studies demonstrate that PF OA is a PPAR-alpha agonist, and treatment of rats and/or mice results in peroxi some proliferation, incr eased beta-oxidation, and hepatomegaly, with dose and temporal responses consistent with this MOA for liver tumor induction. Studies comparing PPAR-alpha null a nd wild-type mice showed that PFOA-induced peroxisome proliferation, beta-oxidation, a nd immunotoxicity depend on the presence of a functional receptor. Further, no other established modes of acti on of liver cancer-induction have been reported for PFOA. PFOA is neither DNA r eactive nor mutagenic, a nd thus not involved in a genotoxic mode of action; nor is the liver neoplastic effect due to the induction of repeated hepatocyte death and compensato ry regeneration (a cytotoxic mode of action). No PPAR-alpha independent MOA for the rat liver tu mor induction has been proposed.

With respect to the weight of evidence and th e adequacy of consideration of uncertainties and limitations, however, the Panel did not reac h full agreement. About three quarters of the Panel members believed that data gaps still exis t and not all of the causa l events in the PPARalpha MOA have been demonstrated for PFOA. Th ese include the induction of cell proliferation in the liver at early times following PFOA tr eatment and/or modulat ion of apoptosis in hepatocytes. They also shared the belief that while the PFOA Draft Risk Assessment in general appropriately discusses the uncer tainties and limitations of the da ta that support a PPAR-alpha MOA for PFOA-induced liver tumors in adult rats, it fails to consider thre e 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 immunotoxici ty, PPAR-alpha null mice exhibited >2-fold increases in liver weight but no peroxisomal acyl CoA oxidase induction in re sponse to PFOA. No increase in liver weight was observed in PPAR-alpha nu ll 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 PF OA risk assessment. Secondly, uncertainties exist with respect to the relevance to exposed fetuses, infants and children of the PPAR-alpha agonist MOA for induc tion 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 ar e 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 su ggest that the difference between humans and rats or mice may be a consequence of a lower number of PPAR-alpha re ceptors such that the PPAR-alpha MOA is not considered likely to yi eld a similar hepatic cancer response in adult humans. However, exposures of fetuses, ne onates 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 in formation in fetuses, neonates and children is minimal, this same extrapolation cannot be ma de in humans. Given that human exposures to PFOA and related chemicals appear ubiquitous, un certainties 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 assessmen t states (page 76 lines 15-16) that the “[a]ctivation of PPAR-alpha leads to an increase in cell proliferat ion and a decrease in apoptosis, which in turn leads to preneopl astic cells ...” Questions were ra ised as to whether there is available experimental evidence th at the induction of PPAR-alpha re sults in an increase in the number of preneoplastic foci. The effect of the PPAR-alpha activa tion 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 M OA, has not been shown in rats treated with

PFOA.

Thirdly, some Panel members felt that the ro le of Kupffer cells (s hown 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 essentia l role of Kupffer cells in signaling peroxisome proliferator-induced hepatocyte pr oliferation. Studies have shown that hepatocyte proliferation and peroxisome proliferation occur by different mechanisms. Parzefall et al. (2001) and Hasmall

et al. (2001) demonstrated that pero xisome proliferators had no e ffect on DNA synthesis but still induced peroxisomal acyl CoA oxidase activity in cultured rat and mouse hepatocytes that had been purified to remove contaminating Kupf fer cells. Kupffer cells, which are resident macrophages in the liver, are a major source of gr owth factors (tumor necrosis factor alpha, interleukins) that induce DNA synt hesis or suppress apoptosis in purified hepatocytes. A key15 finding relevant to the proposed MOA is that Ku pffer cells do not express PPAR-alpha (Peters et
al. , 2000), but are activated by peroxisome prolifer ators. 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 Ku pffer cells. Recognizing th e role of Kupffer cell

activation in the inductio n of DNA synthesis and subsequent neoplastic development by PPARalpha agonists, some members of the FIFRA Scie nce Advisory Panel (2003) [SAP Minutes No. 2003-05] noted that the interplay between PPAR-al pha agonism and Kupffe r cells has not been characterized. Thus, the results from the PPAR-al pha 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 ev idence was adequate to support a PPAR-alpha agonism mode of

action for PFOA-induced rodent liver tumors. In this view, the observa tion 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 toxi city, and the observation represents a single replication without corresponding hi stopathology at the current ti me. Nor was the possible role of Kupffer cells considered to be significan t. 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 th at the PFOA cancer data may be best described as providing “ suggestive evidence of carcinogenici ty, 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 ba sed 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 rais es uncertainties about the human relevance of the “tumor triad” response (liver tumors, Leydi g cell tumors, and pancreat ic acinar cell tumors) of PPAR-alpha agonist ac tivation in rats.

The determination of an appr opriate descriptor for the ca rcinogenic 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 th e 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 the16 conclusion that the current evidence fails to exceed the descri ptor “suggestive” of carcinogenicity.

Cancer studies on PFOA

Carcinogenicity studies in Sprague-Dawle y 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 inciden ces of testicular Leydig cell tumors (LCT) (0% vs. 11%), pancreatic acinar ce ll 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 we re 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 doe s 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%, a nd 6%. Hyperplastic nodules may be part of the continuum of pr oliferative lesions in the live r 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 lo w dose group than in controls (15% in controls, 31% at 30 pp m, and 11% at 300 ppm). The draft PFOA risk assessment did not consider the mammary gla nd neoplasms to represent a compound-related effect because of high background rates reported for fibroadenomas in Sprague-Dawley rats in historical control data (37%) repo rted for female rats in the Hask ell Laboratory in 1987 (Sykes). Three-quarters of the Panel members did not believ e that historical cont rol 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 car e procedures that may af fect 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 c ontrols 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 appa rent 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 mammar y gland that should not be dismissed. In addition, while new information 3 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 histopathol ogy 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 Department17 The remaining Panel members believed that the comparison of the Sibinski et al. (1987) mammary tumor data to the historical control da ta (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 PPA R-alpha agonist MOA (Klaunig et
al. , 2003), and that this MOA is unl ikely to occur in humans based on quantitative differences in the numbers of PPAR-alpha receptors between ra ts and humans. In addition, the PFOA draft risk assessment proposes that the Leydig cell tu mors (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 th at the collective evidence is consistent with the hypothesis of a PPAR-alpha agonist MOA for PFOA with associated peroxisomal β

oxidation activity, increases in abso lute and relative liver weight, and liver tumors in SpragueDawley rats. Issues on which the Panel member s 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 s uppression of apoptosis in rats exposed to PFOA. Many Panel

members believed this to be a critical deficienc y, because these are key events in the proposed MOA linking activation of PPAR-alpha to the liver tumor response.

Another observation that influenced most Pane l members with respec t 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 effect s include the induction of peroxisomal β -oxidation activity (2.6 fold), significant increases and positive dose-resp onse 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, neit her the rat nor the m onkey studies provided data on hepatocyte proliferation during the first 1-2 weeks of exposur e, 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 make s it impossible to analyze dose-response concordance between these key events and tumo r induction for PFOA in relation to other PPARalpha agonists. Because the available data fo r PFOA in rats and monke ys 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, consider ed the increase in liver weight in rats exposed to PFOA and the return to control le vels following an 8-week recovery period (Palazzolo, 1993) to be consistent with an in crease in cell prolifera tion and suppression of apoptosis by PFOA during the exposure period. In a ddition, 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 tran sient 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 exis t 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 incr ease 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 independe nt 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 indu ction by peroxisome prolif erators 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 resu lts with Wy-14,643 in PPAR-alpha null mice do not predict all of the potential liver effects of PFOA.

Further, while not diminishing th e conclusion that a PPAR-alpha MOA is operative in the rodent liver carcinogenesis induc ed by PFOA, about three quar ters 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 hepato cyte growth factor production in Kupffer cells appear to be essential to the mechan ism of hepatocyte replicative DNA synthesis, suppression of apoptosis, a nd liver tumor development by peroxisome proliferators. Until the interplay between PPAR- alpha agonism and Kupffer cell activation is characterized, negative results from the PPAR-al pha null mouse may not be relevant to the human situation in which Kupffer cells and hepa tocellular PPAR-alpha ar e 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 consolida tion of liver tumors, LCTs, and PACTs into a triad MOA to be justified. They believed that available evidence is inadequate to support a19 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 me mbers that PPAR-alpha agonism represented the sole MOA for PF OA-induced rodent liver tumors.

Application of cancer descriptors
The meaning of terms such as “suggestive ev idence of carcinogenic potential” or “likely to be carcinogenic to humans ” may differ among some in th e general public and the EPA because of differences in perception and intent . Hence, EPA recommends a weight-of-evidence narrative that explains the comple xity of issues influencing an agent’s carcinogenic potential in humans. Descriptors are applied to provide consistency across agen ts that are evaluated for their carcinogenic potential. In deve loping their cancer risk asse ssment 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 wei ght-of-evidence descriptor , do not correspond to a quantifiable probability.

About three quarters of the Panel members shar ed the view that while human cancer data on PFOA are inadequate to support a definitive conc lusion 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 “sugge stive 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 incr ease 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 sign ificant 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 tu mor incidence and dose -response trends were

observed in male and female rats. Some of the tumor responses were obser ved 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 asso ciated probability value for PFOA-induced carcinogenicity; nor do the EPA guidelines requir e a quantifiable probability. This group also encouraged a cancer risk assessment for each of the PFOA-induced tumors where data permit.

The risk characterization narrativ e should address the state of know ledge and uncertainties in theMOA 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 c onclusions 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 associa tion between PFOA exposure and tumors was not demonstrated in human studies, 4) the belief that the testicular and panc reatic acinar tumors in rats were probably not relevant to humans, and 5) the view that mammary tumors reported in female rats are equivocal. Further thes e 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 to xicity endpoints for th e 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 PF OA. With regard to th e 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 dos e 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 ra ther to insure that sensitive effects (i.e., effects oc curring at relatively low doses) are not overlooked or prematurely

excluded from the assessment.

The Panel agreed with inclusion of all of th e endpoints in the curren t draft of the risk

assessment. None were recommended for dele tion. However, caveats regarding the use of organ and body weights as endpoi nts 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 abse nce of information with which to select more specific endpoints (e.g., biochemi cal or histological changes), body and organ weight changes are likely to be indi cative of toxicity.

Many members of the Panel also believed that additional endpoints should be included in the risk assessment, although rec ognizing that this may not be possible for some endpoints21 because of the absence of sufficient information:

• Based on discussion in response to Ques tion 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 confiden ce 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 re cognized 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 di scussed in the risk assessment.

• Immunotoxicity should be considered as an e ndpoint addressed quantit atively 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 re commended 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 encour aged exploration of met hods to identify critical targets for PFOA beyond a PPAR-alpha MOA:

• Other than ataxia, no data on neurotoxic ity endpoints for PFOA are available. Neurotoxicity endpoints, includ ing behavioral measures, shou ld 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 brai n regions (frontal cortex, basal ganglia, reticular formation). It has been proposed th at, in addition to thei r roles common to other tissues, these receptors in brain may have sp ecific 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 consis tent with U.S. EPA guidance regarding developmental studies. However, considera tion should be given to using developmental endpoints in F1 generation animals prior to in itiation of direct dos ing so that potential effects associated with perinatal exposure can be more clearly identified.

• Current data suggest that PF OA might produce hormonal effects that would be important to consider, but in most cases the signif icance of the observations are unclear. For example, in a 26-week study of PFOA admi nistration to cynomolgus monkeys, serum TSH was slightly but significan tly elevated in all treatment groups on the final day of the22 experiment, and serum thyroxin was slightly but significantly reduced (Butenhoff, 2002).

It is not clear whether these observations ar e physiologically meaningf ul 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 m easures 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 Pa nel thought that Margins of Exposure (MOEs) should not be calculated fo r hormonal endpoints at this time, but that

additional research to clar ify the hormonal effects of PFOA should be encouraged.

• Adult male rats exhibited a much slower elim ination 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 castra tion increased APFO elimination and testosterone replacement returned the elimina tion 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 cl earance 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 targ ets of APFO because of the pattern of expression of the OATPs that transport it acro ss 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 endpoin ts 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 th is stage in the risk assessment process, no lifestage/gender/species should be excluded from consideration in predicting human risk. Moreover, absence of informati on 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 a like 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 elemen t of the dataset for PFOA which should enlighten

concerns about the use of fema le 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., 90 th

, 95 th , or 99 th percentile) based on a LOAEL in animal m odels. The second consideration related23 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 dr amatic difference in clearance rate. Also, considering empirical measures of exposures in children and adults, this view emphasizes a concern that both developmental and adult endpo ints be captured, and th ese endpoints have not been evaluated in non-human primates. Therefor e, the findings from adult male rats including the 13 week study by Goldenthal (1978) in which liver weight was signifi cantly increased, and the F1 males in the two-generation re productive toxicity study (Butenhoff et al. , 2004) in which body weight was reduced should be considered in fu rther analysis of human health risk.

The second view emphasizes the biological similarities between nonhuman primates and humans for risk assessment. This is particular ly important in the case of PFOA because there are

a number of issues with a rodent model fo r 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 poten tial 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 woul d 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 th at results obtained in models using female rats were informative because they currently provide the only indication of poten tial effects on endpoints specific to females (e.g., reproductio n and developmental effects, mammary tumors). However, some concerns were noted regarding the differenc e in toxicokinetics of PFOA in female versus male rats and monkeys, with female s exhibiting more rapid excretion.

As part of a discussion of additional sources of information and animal m odels to help ascertain potential human risks, the Panel considered observat ions from studies in hu mans. The following specific observations in regard to inclusi on of the epidemiologic data as in formative regarding endpoints were shared by most Panel members:24 • The PFOA Draft Risk Assessment did not us e the occupational biomonitoring data because “data are not available for specifi c occupational exposures.” The Panel points out that neither are data available for “sp ecific 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 human s, 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 (cerebrovascul ar disease, triglycerides and cholesterol) are deemed less convincing, based on small numbers or inconsistencies in the results. It is undeniable that the epidemiology st udies, like the toxicological ones, have some limitations, not

the least of which are uncertainti es 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 ex trapolation is needed; hu mans 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 studi es 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 co mpelling, they are not, conversely, reassuring. The evidence showing increases in cholesterol and tr iglyceride values in wo rker cohorts suggest a possibility of increased risk of cerebrovascular disease mortality.

In responding to charge ques tion #3, therefore, many Panel me mbers shared the view that human cancer and alterations in lipid metabolism da ta 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 Pane l members was that the peer-reviewed human

data on health effects of PFOA were equivocal, thus there was not c onsensus that endpoints suggested by some epidemiologic studies should be us ed as endpoints in the risk assessment.

Issue 4: Risk Assessment Approach

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

Question 6. Please comment on use of the one-c ompartment pharmacokinetic model.25 The purpose of developing a mathematical mode l to fit the serum PFOA time course data from the single dose rat oral dos ing studies in the PFOA Draft Ri sk Assessment document was to

estimate the AUC and C

max values during the longer term t oxicology 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 read ers 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-compart ment model, the chemical distributes evenly throughout a volume of distribution that is itself in rapid equili brium with blood. Elimination

kinetics are first-order and do not change with dose level or w ith time. However, the data indicate that it is clearly ina ppropriate to describe the observe d kinetics of PFOA in rats or monkeys as following a simple one-compart ment 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 m odel. 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 appr opriate, the empirical model used in the document and referred to as a ‘one compartment m odel’ is adequate for predicting blood levels resulting from repeated dosing. However, the do cument needs to make it clear that the fitting procedure is specific to this limited data set and useful for this one appl ication. It is strongly recommended that the terminology ‘one-compart ment’ 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 intern al 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 C

max , the integrated dose above a minimum con centration, 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. Th ese alternatives include receptor occupancy or the concentration above some minimum concentration (C min ) where C min is the concentration required to initiate activation of the receptor-mediate d signaling pathway. In this latter case, the

MOE would be based on the integral of (C t -C min ) rather than just the integral of concentration (C t ).

In light of these other po ssible 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 discussi on 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 C

max 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. Calcul ations of MOEs based on these other measures would provide a better idea of the extent of pos sible 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 se rum, 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 se rum is in the same form and the proportion of free compound in blood is constant throughout th e period of observation. This assumption does not always hold true. For example, with some siloxanes, the blood c oncentrations during and after inhalation exposure are pr imarily free siloxanes that ar e available for exhalation and metabolism. After a period of time in the body, th e siloxanes in blood appear to reside in the lipid pool within the blood and although they are easily analy zed 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 af ter 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 dos e by US EPA in this document represents a promising and relatively innovative approach for risk assessme nts of environmental compounds compared to the more usual practice based on co mparing daily dose rates by various routes of administration. This new approach reduces the need to include uncerta inties 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 widespr ead due to advances in analytical chemistry and the rapid expansion of human biomonitoring activities throughout the world. Because this risk assessment is likely to serv e as a prototype for future tiss ue-dose based risk assessments, some important issues raised by this tissue-do se based approach need to be more fully considered and adequately contrasted with th e 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 base d 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 ne w approach. Such a document might include discussion of (1) the

choices of tissue dose measures based on serum con centrations 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 factor s; (3) the implications of different metrics for characterizing distributions of human tissue dose measures on estimates of MOEs; and (4) the27 importance of routine analysis of appropriate bl ood concentrations; e.g., serum, plasma, etc. in providing the information for most appropr iately applyi ng the tissue dose approach.

Issue 4b: Cross Species Extrapolation

Question 8. Please comment on the need to use or m odify the default value of 10 for cross species extrapolation given the pharmacokinetic analysis.

The internal dose analysis used in this doc ument 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 informa tion about the measured in ternal dose that best predicts adverse effect in human and ot her species, and the bias inherent in measurement/modeling error. While it is difficult to assign a quantitative value to the magnitude of this uncertainty reduc tion, it can be stated that the toxi cokinetic uncertainty value for PFOA would fall within the range of one to three, base d on the customary scale of a value of 3 for each aspect of cross species extrapolation, pharmacokinetics and pharmacodynamics. Pharmacodynamics aspects of PFOA cross specie s scaling are not addressed in a sufficient manner to alleviate the application of some type of uncertainty factor/s (addressing toxicodynamic equivalence across species). Th e assumption that PFOA serum levels are at steady state in children 2-12 years of age ha s 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 cl ouds 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 physiologi cally 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 consideratio n of model validation an d sensitivity analysis, stressing the predictive performance of the model. Toxicokinetic mode ling results may be pres ented 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 ar e available when the data are not sufficient to support a toxicokinetic model or when th e 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 us eful to recognize two components of this equivalence: toxicokinetic eq uivalence, which determines administered doses in animals, and humans that yield equal tissu e 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 e quivalence 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 defau lt values were never intended to act as complex scaling factors when internal doses in human seru m 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 Do se methodologies, and the absence of probabilistic approaches to assessing human exposure and risk, was consid ered by most Panel members as another source of dynamic uncertainty.

The use of an uncertainty factor/s based on da ta variability may be an alternative to the traditional scaling factors given the kinetics analys is 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 re latively robust toxicokinetic dose response models involving serum concentratio ns and/or their surrogates.

In conclusion, whereas toxicokinetic uncertainty is possibly reduced in this analysis, care

must be exercised in the estimation of the ove rall 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 calculat ions 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 popula tions from the United States with available individual serum PFOA levels. One consists of samples from six Amer ican Red Cross blood banks, another from a study of Streptococcal A infection in children, and a third fr om 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 fe male blood bank donor population for calculating prenatal MOEs, because the influence of pregnanc y on serum PFOA levels is not known. Likewise, use29 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, es pecially when considering the impact of age at exposure (or the critical windows of exposure model), contribute to th e 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 be tter 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 th e 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 th e 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 we re 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 di fficult to determine the distribution of population exposures from these given the method of data presen tation. A preferable approach would be to use a non-parametric data-driven method to display the da ta (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-b y-side box plots would have been preferable to what was provided. This requires ha ving 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 90 th percentile.

The rationale for the use of “means” should be explained, especially th e 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 in ference (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 th is 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 ubi quitous exposure. Of th e three choices, the

90 th percentile seems the most appropriate in that case. At least one Panel member wondered why some even higher percentile, say 95 th 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

99.99 th percentile represents 30,000 people in the US.

In summary, the Panel finds that:

● Use of the blood donor and pediatric biomonito ring data may be acceptable if the purpose is to assess whether there is a potenti al health effect to the “gen eral” population, although there is some question as to the size of other non-occupa tional populations that mi ght be more highly exposed and the assumption that PFOA serum leve ls are at steady state may not be valid for fetuses, neonates or children;

● Most Panel members believed th at occupational biomonitoring data should be included in the MOE calculations, especially regarding additi onal endpoints such as alterations in lipid metabolism;

● A few members did not favor this in clusion 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 appropria te summary statistic for the biomonitoring datasets used in MOE calculations should be. Some panelists believe that 90 th percentiles or higher, perhaps even maximum values mi ght 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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