Proposed OPPTS Science Policy: PPAR  Mediated Hepatocarcinogenesis in Rodents and Relevance to Human Health Risk Assessments November 5, 2003 Office of Prevention, Pesticides & Toxic Substances U. S. Environmental Protection Agency Washington, D. C. 20460 Page 2 of 39 PREFACE The U. S. Environmental Protection Agency ( EPA) uses two assumptions concerning the assessment of laboratory animal tumors ( USEPA, 1999; 2003). The first is that the tumor findings in the experimental animals are relevant to the assessment of potential cancer hazards and risks in humans. The second is that, if the animal evidence is sufficient to support a conclusion that a carcinogenic response has occurred in the test species, and the mode of action ( mechanistic) information on the way( s) a chemical may induce tumors either is absent or fails to support a non linear dose response, a linear dose response extrapolation is used to estimate risks at environmental exposure levels. Each of these assumptions is rebuttable in the face of convincing scientific information.

EPA has issued general guidance on the use of animal and other data to assess the human carcinogenic potential of environmental agents ( USEPA, 1986) and has proposed updates to that guidance more recently ( USEPA, 1996; USEPA, 1999; USEPA, 2003). To date, EPA has also developed science policies on the interpretation for three specific animal tumor responses. These science policies addressed proliferative lesions in the rat liver ( Rinde et al. 1987), male rat limited kidney tumors associated with accumulation of alpha 2u globulin ( USEPA, 1991), and thyroid tumors resulting from disruption of thyroid pituitary homeostasis ( USEPA, 1997). Accumulated information on rodent liver tumors that are induced through the activation of the peroxisome proliferator activated receptor   ( PPAR ) led the International Life Sciences Institute Risk Science Institute ( ILSI RSI) to establish an expert working group to update the state of the science on PPAR  agonist induced carcinogenesis in rodents and the human relevance of such animal tumors ( ILSI, in press). The report of this ILSI RSI workgroup provides the current scientific understanding of the mode( s) of action of three of the PPAR  agonist induced tumors observed in rodent bioassays; liver tumors in rats and mice and Leydig cell and pancreatic acinar cell tumors in rats.

There are a number of pesticides and industrial chemicals that increase peroxisomes ( number and size) and liver tumors in rodents via activation of PPAR . This proposed guidance document is intended to provide direction to the scientists in the Office of Pollution, Prevention and Toxic Substances in evaluating liver tumor data observed following exposure to this subgroup of carcinogens. This document also responds to the EPA policy concerning risks to infants and children ( USEPA, 1995; USEPA, 2003) which requires that each risk assessment present findings explicit for these lifestages. OPPTS will depart from the science policy within this document where the facts or circumstances warrant. In such cases, OPPTS will explain why a different course was taken. Page 3 of 39 TABLE OF CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 2 of 39 I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 4 of 39 II. Overview of Peroxisome Proliferation and PPAR  Agonism . . . Page 5 of 39 III. Establishing PPAR  Agonism as a Mode of Action for Rodent Hepatocarcinogenicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6 of 39 IV. Human and Non human Primate Response to PPAR  Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 8 of 39 V. Development of PPAR  Activity and Responses to PPAR  Agonists in the Fetus and Neonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12 of 39 Ontogeny of PPAR   Expression, Peroxisomal Assemblage, Peroxisomal Numerical or Volume Density, and Peroxisomal Enzyme Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12 of 39 PPAR  Agonism in the Fetus and Neonate . . . . . . . . . . . . . . . . . . Page 13 of 39 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 14 of 39 VI. Proposed Science Guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 14 of 39 Proposed Science Policy Statements . . . . . . . . . . . . . . . . . . . . . . Page 14 of 39 Framework to Establish the PPAR  Agonist MOA . . . . . . . . . . . . . Page 15 of 39 Data Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 16 of 39 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 18 of 39 APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 25 of 39 A 1. PPAR  Activity and Peroxisome Assemblage/ Content in Fetal Liver Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 25 of 39 A 2. Peroxisome Assemblage/ Content and Peroxisomal Enzyme Activity in Neonatal Liver Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 27 of 39 A 3. Response of the fetal liver to the PPAR  agonist clofibrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 28 of 39 A 4. Response of Neonates Following Lactational Exposures to PPAR  Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 32 of 39 A 5. Response of Neonates Following Direct Exposures to PPAR  Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 37 of 39 Page 4 of 39 I. Introduction There has been substantial scientific interest regarding the role of peroxisome proliferation in rodent hepatocarcinogenesis and its relevance for human carcinogenesis at this and other potential sites. Several scientific groups have examined the state of the science on PPAR  agonist  induced rodent liver tumors over the years. A workgroup convened under the auspices of the International Agency for Research on Cancer concluded that the mode of action ( MOA) for liver tumors induced in rodents by PPAR  agonists is unlikely to be operative in humans ( IARC, 1995). The participants of a workshop held under the auspices of the International Life Sciences Institute Health and Environmental Sciences Institute concluded that although it appeared unlikely that PPAR  agonists could induce liver tumorigenesis in humans, the possibility could not be ruled out ( Cattley et al. 1998).

Recent scientific developments have led to a reevaluation of the state of the science to characterize the mode( s) of action ( e. g., PPAR  agonism) and human relevance of rodent tumors induced by certain peroxisome proliferating agents. To that end, the ILSI Risk Science Institute convened a workgroup to upgrade the state of the science for PPAR  agonist induced rodent liver tumors, as well as to evaluate the mode( s) of action for Leydig cell and pancreatic acinar cell tumors, which also are observed frequently in rats with PPAR  agonists. The workgroup provided a detailed analysis of the key events in the mode of action of PPAR  agonist induced liver tumors in rodents, and then proceeded with a concordance analysis of the evidence that these key events can occur in humans. The workgroup concluded that while the PPAR  receptor can be activated in humans, there are substantial species differences in toxicodynamics that make it very unlikely that the downstream key events and therefore hepatocarcinogenesis would occur in humans. Finally, the workgroup concluded that there is insufficient information at this time to firmly establish a mode of action for the Leydig cell and pancreatic acinar cell tumors in rats ( ILSI, in press). It should be noted that the ILSI expert panel's report on " PPAR  Agonist Induced Rodent Tumors: Mode( s) of Action and Human Relevance" was peer reviewed by an independent group of 17 scientists from academia, government and industry.

The purpose of this OPPTS guidance document is to describe the approach the Office will proposes to use to evaluate the scientific information regarding the mode of action of PPAR  agonists in rodent hepatocarcinogenesis and the relevance of this mode of action for human hepatocarcinogenesis. Other tumor types ( e. g. Leydig cell and pancreatic acinar cell tumors) that may be associated with PPAR  agonists are briefly described. The document provides an overview of the evidence for a PPAR  agonist mode of action for liver tumors in rodents, and an overview of all known age and species differences in the key events. Finally, the document will provide guidance in: 1) using a framework to describe and to present the proposed PPAR  mediated mode of action; 2) data needed to demonstrate that rodent liver tumors have arisen as a result of a PPAR  agonist mode of action; and 3) the relevance of this mode of action for hepatocarcinogenesis in humans. OPPTS will depart from this proposed science policy where the facts and circumstances warrant. Page 5 of 39 II. Overview of Peroxisome Proliferation and PPAR  Agonism Events leading to carcinogenic response following exposure to an environmental agent are varied and may range from mutations of the genome to the activation/ deactivation of genes and/ or receptors. An example of this latter category is PPAR  agonism. This event is key in rodent liver carcinogenesis triggered by administration of certain peroxisome proliferating agents. An increase in the number and volume of peroxisomes present in liver cells is a key characteristic of PPAR  agonism. It is generally accepted that peroxisome proliferation is associated with liver tumor formation in rodents although this phenomenon has not been established as a causal effect.

In addition to inducing hepatocarcinogenesis in rodents, PPAR  agonists have also been observed to induce pancreatic acinar cell and Leydig cell tumors in rats. Of 15 PPAR  agonists tested to date, nine have been shown to induce all three tumors in non F344 rat strains but not in mice. In the case of Leydig cell tumor formation, two potential MOAs based on activation of PPAR  have been proposed. One MOA invokes the induction of hepatic aromatase activity leading to an increase in serum estradiol level. The second MOA purports that PPAR  agonists inhibit testosterone biosynthesis. Although agonism of PPAR  may lead to the induction of aromatase or inhibition of testosterone biosynthesis, the data available to date are insufficient to support which, if either, of these two proposed MOAs is operative. For pancreatic acinar cell tumor ( PACT) formation, a MOA has been proposed in which PPAR  agonists cause a decrease in bile acid synthesis and/ or change the composition of the bile acid resulting in cholestasis. These steps increase the level of the growth factor cholecystokinin ( CCK) which then binds to its receptor, CCK A, leading to acinar cell proliferation. Some evidence exists to support this proposed MOA and there does not appear to be evidence of any other MOA operating in the formation of PACTs after exposure to PPAR  agonists. However, the data are not considered sufficient to establish a MOA with confidence, because it has only been described for two chemicals, PFOA and WY14643, in one laboratory. As a result, the evidence is considered insufficient to infer that this MOA may be generalized to all PACT inducing PPAR  agonists. Page 6 of 39 III. Establishing PPAR  Agonism as a Mode of Action for Rodent Hepatocarcinogenicity Although the precise mechanistic steps leading to hepatic carcinogenesis in rodents after PPAR  agonist exposure are not completely understood, knowledge of the mode of action has been characterized ( ILSI, in press). As shown in Figure 1, it has been proposed that PPAR  agonists activate PPAR  ( which regulates the transcription of genes involved in peroxisome proliferation, cell cycle control, apoptosis, and lipid metabolism). Activation of PPAR  leads to an increase in cell proliferation and a decrease in apoptosis, which in turn leads to preneoplastic cells and further clonal expansion and formation of liver tumors. As depicted in Figure 1, the biological events in PPAR  induced hepatocarcinogenesis may be classified as either causal ( i. e., required for this MOA) or associative ( i. e., markers of PPAR  agonism but not shown to be directly involved in the etiology of liver tumors). PPAR  activation, changes in rates of liver cell proliferation/ apoptosis, and selective clonal expansion are causally related to PPAR  mediated liver tumor formation. Of these, only PPAR  activation is highly specific for this MOA while cell proliferation/ apoptosis and clonal expansion are common to other modes of action. Among the associative events are peroxisome proliferation ( a highly specific indicator that this MOA is operative) and peroxisomal gene expression. Peroxisomal proliferation may also result in hepatocyte oxidative stress which may contribute to the mode of action by causing indirect DNA damage and leading to mutations, or by stimulating cell proliferation. However, increases in oxidative damage to DNA have not been unambiguously demonstrated for PPAR  agonists. Oxidative stress is a general phenomenom, and thus does not represent a highly specific marker for PPAR  agonist induced liver carcinogenesis. For a more detailed description of the PPAR  agonist mode of carcinogenic action see ILSI ( in press). Page 7 of 39 Causative Events PPAR  Agonist Activation of PPAR   Cell Proliferation Decreased Apoptosis Preneoplastic Foci Clonal Expansion Liver Tumors ° Expression of Peroxisomal Genes ° Increase in Peroxisomes ( number & size) Associative Events*

* Although there are other biological events ( e. g., Kupffer cell mediated events, inhibition of gap junctions), the measurements of peroxisome proliferation and peroxisomal enzyme activity ( in particular acyl CoA) are widely used as reliable markers of PPAR  activation. Figure 1. Key Events in the Mode of Action for PPAR  Agonist Induced Rodent Liver Tumors When postulating a PPAR  agonist mediated MOA for rodent liver tumorigenesis, one of the first indications that this MOA may be operative is the increase in the number and size of peroxisomes in the cytoplasm of hepatocytes. It should be noted that although peroxisome proliferation itself has not been shown to be a causal event in liver tumorigenesis , it is a highly specific biomarker of the PPAR  agonist MOA. Evidence of peroxisome proliferation ( and by extension PPAR  agonism) has also been obtained through biochemical assays  conducted as part of long term cancer studies or as independent mechanistic studies  that evaluate the activity of peroxisome specific enzymes involved in the metabolism of fatty acids. These assays include activity level measurements for acyl CoA oxidase, palmitoyl CoA oxidase, carnitine acetyl transferase, and CYP4A. Increases in palmitoyl CoA, cartinine acetyl transferase, and CYP4A activity have been noted in conjunction with liver tumorigenesis after exposure to certain peroxisome proliferators ( PP) such as clofibrate, lactofen, trichloroacetic acid, oxidiazon, and di( 2 ethylhexyl) phthalate ( DEHP)( ILSI, in press). Furthermore, the observation that acyl CoA oxidase induction after PPAR  agonist exposure is not seen in PPAR  null mice, has served to identify changes in this enzyme activity as a key event in the induction of rodent liver tumors by PPAR  agonists and a biomarker for the MOA ( Ward et al. 1998). In addition to biochemical alterations, other liver changes noted after exposure to PPAR  agonists Page 8 of 39 include: hepatocyte hypertrophy, increased liver weights, and increased liver cell mitotic activity ( reviewed in Cattley et al. 1998).

The discovery and cloning of the PPAR  in 1990 by Isseman and Green provided insight into the role of PPAR  in the processes leading to hepatocarcinogenesis in rodents ( Isseman and Green, 1990). Conclusive evidence of the pivotal/ causal role of PPAR  in the induction of liver tumors by certain PPAR  agonists was obtained from studies on PPAR  null knockout mice. In a series of seminal experiments, Lee et al. ( 1995) and Peters et al. ( 1997) exposed wild type (+/+) and PPAR  null (/) mice to either WY14643 or clofibrate  two known PPAR  agonists  at doses known to induce hepatocarcinogenesis in rodents. Short term exposure ( 2 weeks) to either one of these PPAR  agonists failed to elicit in PPAR  null mice the early hepatic effects characteristic of liver tumor induction such as increases in liver weights, hepatic peroxisome proliferation, CYP4A induction as well as induction of other peroxisomal enzymes. In contrast, wild type (+/+) mice displayed all these indicators of hepatocellular alteration. These findings were corroborated by experiments using longer periods of exposure ( 5 weeks to 11 months) in which WY14643 exposure failed to elicit in PPAR  null mice the increases in acyl CoA oxidase, CDK 1, CDK 4, and c myc seen in wild type mice. WY14643 treatment also failed to induce replicative DNA synthesis ( as measured through BrDU labeling indices) in PPAR  null mice under conditions that elicited this effect in the wild type mice. Moreover, WY14643 exposure for 11 months led to a 100% incidence of hepatic neoplasms in wild type mice while the PPAR  null mice showed none. PPAR  null mice exposed to DEHP for 24 weeks did not manifest any of the changes associated with PPAR  mediated liver tumors such as peroxisome proliferation, increased peroxisomal enzyme activity, or increased CYP4A mRNA levels ( Ward et al. 1998). Experiments on null mice after WY14643, clofibrate, or DEHP exposure provide persuasive evidence that a PPAR  mediated MOA is operating in the induction of hepatocellular neoplasms.

IV. Human and Non human Primate Response to PPAR  Agonists Studies conducted in numerous test species indicate that while rodents ( mice and rats) are highly responsive to PPAR  agonist induced hepatocarcinogenicity, other species ( e. g., hamster, dogs, guinea pigs, New and Old World primates, and humans) appear to be refractory ( Cattley et al. 1998; Doull et al. 1999). For example, a wide range of PPAR  agonists which produce peroxisome proliferation in rodent hepatocytes, have little or no effect in guinea pig or monkeys when the test compounds were evaluated by changes in peroxisome proliferation and/ or enzyme activities ( e. g., palmitoyl CoA) ( Doull et al. 1999). Furthermore, liver tumors are not found in Syrian hamsters after 40 60 weeks administration of WY14643 and nafenopin ( Lake et al. 1993). A variety of studies have been conducted to characterize the response of humans and non human primates to PPAR  agonists, and to determine the cause( s) of the apparent refractory nature of the response. These studies are summarized below. Page 9 of 39 Two major epidemiology studies have been conducted to assess the effect of prolonged exposure to the hypolipidemic drugs gemfibrozil or clofibrate. In the Helsinki Heart Study over 4000 men with high cholesterol were treated with gemfibrozil or a placebo for five years. Although substantial decreases in the serum lipid levels were noted in the gemfibrozil treated group compared to the placebo group, the death rate and liver cancer rates in both groups were comparable. It is noteworthy, however, that in this study the liver cancer incidence was reported in conjunction with gall bladder and intestinal cancers ( Frick et al., 1987; Huttunen et al., 1994).

In the second study, over 15,000 men with ischemic heart disease received clofibrate or a placebo for five years, and their health status was followed for eight years after cessation of treatment ( CPI, 1984). During the follow up conducted 4 years after treatment stopped, the group treated with clofibrate had a 24% increase in mortality relative to the high cholesterol patients treated with placebo. These mortalities were attributed to diseases ( including malignancies) of the gall bladder, intestines, liver and, pancreas. Eight years after treatment was stopped, however, the cancer death rate between the two groups was comparable. As was the case in the Helsinki Heart Study, cancer data were not provided specifically for the liver but rather they were presented in combination with the gall bladder, pancreatic, and intestinal cancer incidences. Thus, neither of these epidemiological studies has provided evidence of PPAR  mediated hepatocarcinogenesis; however, these data are inconclusive because of the limited durations of exposure The apparent species differences between rodents and human and nonhuman primates in the response to PPAR  agonists is also supported by a long term study in monkeys. Marmosets exposed for 6.5 years to clofibrate at relatively high doses ( 94 mg/ kg/ day or higher) did not develop liver tumors ( Tucker and Orton, 1995). Although the duration of this study did not represent a lifetime exposure, the results strongly suggest that primates appear to be refractory to PPAR  agonists ( Doull et al., 1999 and ILSI, in press).

Examination of liver biopsies from patients receiving hypolipidemic drugs ( clofibrate or ciprofibrate) to treat hyperlipidemia provide another line of evidence for human response to PPAR  agonists. Clofibrate exposure  at clinically relevant doses  has been shown only to induce a 1.5 fold increase in the number of hepatic peroxisomes and a 23% increase in the volume density of these organelles. Similar findings have been reported for ciprofibrate where liver biopsies from patients treated with the drug for 6 months to two years exhibited a 30% increase in the volume density of hepatic peroxisomes ( Bentley et al., 1993; Hinton et al., 1983). In addition, there was no evidence of liver tumors from liver biopsies that were conducted on patients treated with gemfibrozil, clofibrate or fenofibrate for periods ranging for up to 5.3 years and followed up for an additional 7.9 years ( Blumcke et al., 1983; De La Iglesia et al., 1982; Gariot et al., 1987).

Further evidence that humans appear to be refractory to the hepatocarcinogenic properties of PPAR  agonists is provided by experiments in which exposure to Page 10 of 39 ciprofibrate, clofibrate, gemfibrozil, and fenofibrate ( hypolipidemic drugs known to be PPAR  agonists) failed to elicit a response in human hepatocytes that is observed in rodent hepatocytes. Human hepatocytes cultured in the presence of these compounds did not exhibit increases in replicative DNA synthesis or acyl CoA oxidase activity and/ or expression ( i. e. mRNA levels), suppression of apoptosis, or increased peroxisome proliferation. These findings are consistent with the results obtained after exposure to other PPAR  agonists that are not hypolipidemic drugs such as mono 2 ethylhexyl phthalate ( MEHP) ( Baker et al., 1996; Kamendulis et al., 2002; Hasmall et al. 1999 and 2000).

In vitro and in vivo data on monkeys ( cynomolgus, marmoset, and Rhesus) support the findings from human in vitro and liver biopsy studies. Palmitoyl CoA oxidase activity was evaluated in monkeys after in vivo exposure to a variety of PPAR  agonists ( e. g., bezafibrate, clofibrate, di 2 ethylhexyl phthalate ( DEHP), MEHP, fenofibrate, nafenopin, and LY171883). The changes noted in enzymatic activity after PPAR  agonist exposure were minimal or non existent relative to control ( ILSI, in press). Moreover, cynomolgus monkeys exposed in vivo to DEHP, di isononyl phthalate and clofibrate also failed to exhibit an increase in DNA synthesis ( Pugh et al., 2000). It should be noted, however, that cynomolgus monkeys treated for two weeks with clinically relevant doses of fenofibrate or ciprofibrate exhibited a 3 fold increase in the number of hepatic peroxisomes, a 2 fold increase in liver size, and hepatocellular hypertrophy ( Qualls et al., 2003). Although these changes appear to be similar to the response in rodents to PPAR  agonist exposure, there was no evidence of increases in peroxisomal enzyme activity ( e. g., acyl CoA oxidase, catalase, or carnitine acetyltransferase).

Research on several aspects of PPAR  agonism has been undertaken to attempt to ascertain the basis for the differential response between rodents and humans after exposure to PPAR  agonists. These investigations have focused on PPAR  gene expression, PPAR  gene structure, and peroxisome proliferator response element ( PPRE) structure. After the discovery and cloning of the gene encoding the human PPAR , Tugwood et al. ( 1996) and Palmer et al. ( 1998) demonstrated that PPAR  expression in humans is 10 fold lower than that seen in rodents. While this may account in part for the unresponsiveness of humans to the hepatocarcinogenic properties of PPAR  agonists, it is unknown how much it contributes to the differences between the species.

Transfection experiments have demonstrated that human PPAR  ( hPPAR ) is very similar to the rodent PPAR  and capable of transactivating reporter genes ( containing a rodent PPRE in their promoter region) following clofibrate exposure, thereby establishing that hPPAR" is functional ( Sher et al., 1993; Mukherjee et al., 1994; Pineau et al., 1996). This finding was confirmed by in vivo experiments in which PPAR  null mice were infected by an adenovirus carrying the hPPAR  gene. The hPPAR  gene was able to " rescue" these knockout mice as evidenced by the increases in peroxisome proliferation and peroxisomal enzyme gene expression ( Yu et al., 2001). Page 11 of 39 Other studies have focused on the potential variability in the structure of the PPAR  gene in human populations. The hPPAR  8/ 14 variant identified by Tugwood et al. ( 1996) results in a truncated hPPAR , while hPPAR  6/ 29, identified by Roberts et al. ( 1998), results in a four amino acid substitution. The hPPAR  8/ 14 variant has been detected in every sample examined to date in two laboratories while hPPAR  6/ 29 is extremely rare having been isolated only once ( Roberts et al., 2000). Two point mutations ( PPAR * 3 and PPAR * 2) were identified by Sapone et al. ( 2000). PPAR * 3 is relatively common in Northern India populations while PPAR * 2 is relatively rare. Relative to wild type, PPAR * 3 results in unresponsiveness to low concentrations of ligand as well as a lower non ligand dependent trans activating ( i. e. constitutive) activity. In contrast, PPAR * 2 exhibits a slightly higher constitutive activity than wild type. However, PPAR * 2 activity after WY14643 exposure is comparable to wild type. Precisely how these mutations may render humans less sensitive to PPAR  agonists is not known.

Another factor that may contribute to the apparent lack of response in humans to the hepatocarcinogenic inducing properties of PPAR  agonists may be related to a difference in the hPPREs response to hPPAR  induction. This theory stems from transfection studies in which Sher et al. ( 1993) demonstrated that hPPAR  can transactivate reporter genes containing a murine PPRE. However, the gene and protein expression of various biomarkers indicative of PPAR  mediated liver tumorigenesis and regulated by hPPRE, including acyl CoA oxidase, were not increased in human hepatocytes following exposure to several PPAR  agonists. These findings were corroborated by data collected after patients were exposed at clinically relevant doses to various fibrates ( e. g., bezafibrate, fenofibrate, and gemfibrozil) which indicate that acyl CoA oxidase mRNA levels were unaffected ( Roglans et al., 2002). In contrast, other human genes regulated by PPAR  agonists  but not implicated in hepatic carcinogenesis  ( e. g. , lipoprotein kinase, carnitine palmitoyl transferase, etc.) are effectively regulated by PPAR  agonists ( discussed in ILSI, in press). Moreover, in experiments in which HepG2 cells were transfected with hPPAR  ( at the levels of PPAR  seen in rodents) and treated with fibrate did not lead to increases in acyl CoA oxidase mRNA levels ( Hsu et al., 2001; Lawrence et al., 2001a). The possibility that the hPPRE for acyl CoA oxidase may differ from the rodent in its ability to bind hPPAR  has been further suggested by experiments showing that this peroxisomal enzyme cannot be activated in cells transfected with a PPAR  expression vector and that it may require high concentrations of PPAR  agonists to be induced ( Woodyatt et al., 1999; Varanasi et al., 1998; Rodriguez et al., 2000). Furthermore, Lambe et al. ( 1999) have shown that when rat acyl CoA oxidase PPRE is disrupted by site directed mutagenesis, the hPPRE for this enzyme could not rescue the null phenotype in reporter gene assays. Although it appears that the downstream effects of PPAR  agonism such as induction of acyl CoA oxidase, CYP4A1, DNA replication, or apoptosis suppression do not occur in humans, hPPAR  in COS 1 or NIH 3T3 cells can be activated by exposure to MEHP as indicated by the increases in luciferase reporter gene activity ( Dirven et al., 1993; Maloney & Waxman, 1999; Hasmall et al., 2000). Page 12 of 39 In summary, although humans possess a functional PPAR , and the human receptor can be activated by peroxisome proliferators, humans ( and non human primates) appear to be refractory to the key events associated with the induction of liver tumors by PPAR  agonists. It is the totality of evidence that provides a strong argument that PPAR  induced liver carcinogenesis is not likely to occur in humans. Human and non human primate hepatocytes treated with a range of different PPAR  agonists do not exhibit increased replicative DNA synthesis, suppression of apoptosis, increased expression of marker mRNAs and proteins including acyl CoA oxidase or peroxisome proliferation. In vivo studies with monkeys showed that treatment with the hypolipdemic drugs fenofibrate or ciprofibrate induced an increase in liver weights ( up to 2 fold) and peroxisome numbers ( up to about 3 fold), but only slight or no increases in peroxisomal enzyme activities. Similarly, analyses of biopsies from liver tissue of patients treated with hypolipodemic drugs showed only slight increases or no increases in peroxisome numbers or volume density. Although the epidemiological literature and cancer studies in nonhuman primates are inconclusive, they do not provide evidence for the potential of PPAR  agonists to induce liver tumors in humans.

V. Development of PPAR  Activity and Responses to PPAR  Agonists in the Fetus and Neonate Previous sections of this document provided an overview of the evidence and basis for the establishment of PPAR  agonism as a mode of action for the induction of liver tumors observed in adult rodents. This section focuses on the ontogeny of the response to PPAR  agonists during fetal and postnatal development and the sensitivity of the developing organism relative to the adult. No data on the ontogeny of the response during human development were found in the literature. Thus, conclusions concerning the ontogeny of the response to PPAR  agonists and the relative sensitivity of developing humans must rely on evidence provided by laboratory studies in animals.

Ontogeny of PPAR   Expression, Peroxisomal Assemblage, Peroxisomal Numerical or Volume Density, and Peroxisomal Enzyme Activities A detailed review of studies available on the ontogeny of peroxisomes and peroxisomal enzyme activities during development in the rodent liver is provided in Appendices A 1 and A 2. These studies indicate that expression of the PPAR  gene, assemblage of peroxisomal proteins into plasmids, the content of peroxisomes, and peroxisomal enzyme activity appears to occur late in rodent fetal development ( i. e., gestational day 15 or later) ( Braissant and Wahli, 1998; Wilson et al, 1991; Tsukada, et al., 1968; Stefanini et al., 1989; Stefanini, et al., 1985; Cibelli, et al., 1988). Thus, it appears that the direct effects of a PPAR  agonist on PPAR  and secondary effects on peroxisomal proliferation and enzyme activites would not be operative prior to gestational day 15 or later.

In the neonatal rat, peroxisomal assemblage, peroxisomal numerical density and the volume of peroxisomes have been reported to be similar among neonatal and adult rats, as have peroxisomal enzyme activities ( Stefanini et al., 1999; Stefanini et al., Page 13 of 39 1995; Cimini et al., 1994; Singh and Lazo, 1992; Dostal et al., 1987; Staubli et al., 1977; Weibel et al., 1969).

Based on the available evidence, it appears that PPAR  expression is low or absent in the rodent fetus until just before birth. In neonatal rodents, the expression of PPAR  mRNA is comparable to or less than that seen in the adult rodent. Thus, it is plausible that rodent embryos would not respond to the effects of a PPAR  agonist until late in development whereas the neonate might be expected to respond to a PPAR  agonist like adult rodents.

PPAR  Agonism in the Fetus and Neonate Several studies have been conducted to examine the effects of exposure to PPAR  agonists during fetal and postnatal development in rats and mice. These studies are reviewed in detail in Appendices A 3  A 5. Administration of the PPAR  agonist clofibrate to rats or mice during pregnancy can accentuate peroxisome assemblage or peroxisomal proliferation in the liver of their fetuses late in development ( i. e., during gestational days 17 21)( Wilson et al., 1991; Stefanini et al., 1989; Cibelli, et al., 1988). Markers of peroxisomal assemblage, the proteins PMP 70 and DHAP AT, are increased, relative to controls, to an equal or greater extent in the mouse fetus compared to clofibrate treated dams. However, the total levels of these assemblage proteins ( i. e., on a µ g/ mg or units/ mg protein basis) are lower in the fetus than in treated dams. Peroxisomal numbers also increased in rat or mouse fetuses from dams treated with clofibrate, particularly in gestational day 19 fetuses. The activity of catalase, a peroxisomal enzyme which is also found in the cytoplasm, increases somewhat in gestational day 19 to 21 fetuses and neonates from mothers treated with clofibrate but the specific activity of the enzyme does not exceed the level of activity observed in clofibrate treated rat or mouse dams. Similarly, palmitoyl CoA oxidase activity was enhanced as much as 8 fold in gestational day 21 rat fetuses but, again, the enhanced specific activity of the enzyme was about equal to that seen in the clofibrate treated dams.

Although limited, these data when considered together suggest that: 1) the rodent fetus responds to a PPAR  agonist like the adult rodent dam ( i. e., there are increases in peroxisomal assemblage/ proliferation and peroxisomal enzyme activities; 2) effects of a PPAR  agonist on the fetus occur late in development; and 3) although the enhancement, relative to controls, of peroxisomal numbers and peroxisomal enzyme activities during fetal development may exceed that observed in the PPAR  agonist treated pregnant rat or mouse, the increased levels of peroxisomal protein or peroxisomal enzyme activities do not exceed the levels found in the pregnant dam. Thus, it does not appear that the fetal rodent is more sensitive than the adult rodent dam to the effects of a PPAR  agonist.

Treatment of rat dams during lactation also leads to an enhancement of peroxisomal assemblage, peroxisomal enzyme activities, and the numerical density of peroxisomes in their nursing pups ( Stefanini et al., 1999; Stefanini et al., 1995; Cimini Page 14 of 39 et al., 1994; Singh and Lazo, 1992; Fahl et al., 1983). Generally, nursing neonates appear no more sensitive than their PPAR  agonist treated dams because increases in these parameters are similar to the increases observed in the treated dams. Furthermore, increases in liver weights, an effect characteristic of PPAR  agonism, in nursing neonates exposed to a PPAR  agonist were no greater or less than that reported in treated dams ( Singh and Lazo, 1992; Cimini et al., 1994; Osterburg et al., 1992; Schroeder, 1983). At this time, however, it is unknown how much of the compound is metabolized by the dams and/ or transferred to the neonates via the milk; hence it is possible that a difference in the internal dose may play a role in the differential responses noted between nursing neonates and their dams.

Direct exposure of neonates to PPAR  agonists results in an increase in peroxisomal enzyme activities and an increase in the numerical density or volume of peroxisomes; the increases in these parameters are comparable to those observed in young adults or adult rats ( Yu et al., 2001; Yamoto, 1996; Dostal et al., 1987; Staubli et al., 1977;). At a dose of a PPAR  agonist that affects peroxisome enzyme activity or peroxisome numbers, no effect on liver weights was observed ( Yu et al., 2001) or the increase in liver weights observed in neonates was no greater than that observed in treated adults ( Dostal et al., 1997; Yamoto, 1996)

Conclusions The data available on the effects of PPAR  agonist in the rodent fetus or neonate ( e. g., increases in peroxisome numbers and size, peroxisome enzyme activities, and liver weights) provides support that there is not an increased sensitivity relative to an adult to hepatocarcinogenicity during fetal or neonatal development. Any conclusions regarding this hepatocarcinogenic mode of action in adult rodents would also appear to apply to young rodents, and similarly any conclusions regarding the relevance of this mode of action for human hepatocarcinogenesis would apply to the young, as well as the adults.

VI. Proposed Science Guidance Proposed Science Policy Statements Although the precise mechanism for the formation of liver tumors by a PPAR  agonist has not been established, key events for the mode of action leading to hepatocarcinogenesis have been identified. Key events for the mode of action that have been causally related to liver tumor formation include: activation of PPAR , perturbation of cell proliferation and apoptosis, and selective clonal expansion. Key events that are associated with PPAR  agonism and liver tumor formation and that are reliable markers that a chemical has induced PPAR  include expression of peroxisomal genes ( e. g., palmitoyl CoA oxidase, acyl CoA oxidase) and peroxisome proliferation ( i. e., an increase in the number and size of peroxisomes). Page 15 of 39 It has been well established that chemicals that are PPAR  agonists can induce liver tumors, and perhaps other tumors, in rats and mice but the potential for PPAR  agonists to induce liver tumors in other species, including humans, appears to be unlikely. This is because evidence obtained from in vivo and in vitro studies with hamsters, guinea pigs, non human primates, and humans ( i. e., cells in culture or biopsies) shows that, quantitatively, these other species are apparently refractory to the effects of a PPAR  agonist. Lines of evidence supporting this presumption include minimal or no effects on peroxisome proliferation, peroxisomal enzyme activity, or hepatocellular proliferation in species other than rats or mice. Moreover, epidemiological studies have not provided evidence of increased incidence of liver neoplasms in humans; however, these data are inconclusive because of the limited durations of exposure.

Recognizing the mode of action data that show a linkage between PPAR  agonism and liver tumor formation, OPPTS proposes to adopt the following science policy:

° When liver tumors are observed in long term studies in rats and mice, and 1) the data are sufficient to establish that the liver tumors are a result of a PPAR  agonist MOA and 2) other potential MOAs have been evaluated and found not operative, the evidence of liver tumor formation in rodents should not be used to characterize potential human hazard.

° There is limited evidence that a chemical may induce pancreatic and Leydig cell tumors through a PPAR  agonist mode of action. However, the evidence is inadequate at this time to support a linkage between PPAR  agonism and formation of these tumor types. Thus, it is presumed that chemicals in this subclass that induce pancreatic or Leydig cell tumors may pose a carcinogenic hazard for humans.

Different types of data on a chemical may be provided that indicate that a chemical induces liver tumors via a PPAR  agonist mode of action. The approach to establishing that a PPAR  agonist mode of action is operative and the data needed to support this presumption are discussed below.

Framework to Establish the PPAR  Agonist MOA The Agency uses an analytical framework for judging whether available evidence for an agent supports a mode of action for tumor induction in animals ( USEPA, 1999, Sonich Mullin et al., 2000). This framework was considered in evaluating the postulated mode of action for PPAR  agonists.

The framework for analyzing mode of action begins with a summary description of the postulated mode( s) of action. This is followed by questions to be addressed to the available empirical data and experimental observations anticipated to be pertinent. The areas of inquiry in the framework are: Page 16 of 39 ( i) identification of key event( s). A " key event" is defined as an empirically observable, precursor step that is a necessary element of the mode of action, or is a marker for such an element ( e. g., increased cell growth and organ weight, hyperplasia, cellular proliferation, hormone or other protein perturbations, receptor ligand changes, DNA or chromosome effects, cell cycle effects).,

( ii) strength, consistency, specificity of association ( e. g., causality is supported by a significant statistical and biological association between key events and a tumor response in well conducted studies and with consistent observations in a number of such studies, with differing experimental designs),

( iii) dose response relationships ( i. e., key event( s) and tumor response increase correlatively with dose),

( iv) temporal relationships ( i. e., if an event( s) is an essential element of tumorigenesis, it should precede tumor appearance),

( v) biological plausibility and coherence ( i. e., is the mode of action consistent with what is known about carcinogenesis in general and for the case specifically?),

( vi) other modes of action ( i. e., have alternative modes of action for the tumor response been considered and are they supported by the data?).

It should be emphasized that the topics described above for analysis should not be regarded as a checklist of necessary " proofs". The judgment whether a postulated mode of action is supported by available data takes into account the weight of the evidence and the analysis as a whole.

Data Needs Chemicals can produce tumors at a given site by more than one mode of action. Thus, before a PPAR  agonist MOA can be defined as a cause of the liver tumors, it is also critical to ensure that other MOAs do not contribute significantly to the development of the tumors. For instance, it is important to ensure that direct DNA reactivity is not the source of the carcinogenic findings. The results of in vitro and in vivo short term tests for mutagenicity and the evaluation of the presence of structural alerts and structure activity relationships are helpful. Likewise, chemicals producing rodents liver tumors exclusively through PPAR  activation do not cause cytotoxicity; such findings need to be evaluated at doses that have produced PPAR  agonist precursor effects and liver tumors to ensure that cytotoxicity is not prominent.

Parameters chosen to demonstrate that activation of PPAR  is the mode of action for the induction of rodent liver tumors must be both sensitive and specific. In other words, precursor events, whether causally related to or associated with liver tumor formation, must clearly show that tumors are due to a PPAR  agonist MOA and Page 17 of 39 exclude other potential MOAs. Valuable data to address specificity can also be obtained from PPAR  knockout mouse bioassays. Demonstration of the absence of hepatocarcinogenicity and related liver toxicity in a PPAR  knockout mouse but the presence of liver tumors and toxic responses in the wild type mouse not only provides evidence that a PPAR  agonist MOA is operating in the induction of the liver tumors but also demonstrates that other MOAs ( e. g., direct mutagenic effects, cytotoxicity) are not major contributors to the onset of hepatocarcinogenesis. However, it is recognized that a PPAR  knockout mouse is not generally used to demonstrate that a chemical induces liver tumors via a PPAR   agonist MOA. Thus, other data may be used to demonstrate that a PPAR   agonist MOA for liver tumor formation is operative.

Demonstration that a PPAR  agonist MOA is operative can be shown by a data set that 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 be collected from studies designed to provide the data needed to show dose response and temporal concordance between precursor events and liver tumor formation.

Because some chemicals have been shown to induce little or no effect on PPAR  activity but produce other effects associated with PPAR  agonisms ( ILSI, in press), a receptor assay should be used to demonstrate that a compound is a PPAR  agonist and is not inducing effects via other PPAR receptors. Evidence of peroxisome proliferation is a fundamental aspect of PPAR  agonists and along with evidence from an in vitro reporter assay provides definitive evidence that a PPAR  agonist MOA is operative. Increases in peroxisomal enzyme activities are commonly used markers for peroxisome proliferation and data from measurements of peroxisomal enzyme activity, when combined with direct evidence of peroxisome proliferation, enhance the ability to establish temporal and dose response concordance between key events and liver tumor formation. Finally, hepatic cell proliferation is a key, causal event leading to the formation of liver tumors by PPAR  agonists and evidence of hepatic cell proliferation, when combined with other evidence, may also provide important information on the temporal aspect of tumor development and the dose response concordance of precursor events and tumor formation.

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Yamoto, T., Ohashi, Y., Furukawa, T., Teranishi, M., Manabe, S., and Makita, T. ( 1996). Change of the sex dependent response to clofibrate in F344 rat liver during postnatal development. Toxicol. Lett. 85: 77 83.

Yu, S., Cao, W. Q., Kashireddy, P., Meyer, K., Jia, Y., Hughes, D. E., Tan, Y., Feng, J., Yeldandi, A. V., Rao, M. S., Costa, R. H., Gonzalez, F. J., Reddy, J. K. ( 2001). Human peroxisome proliferator activated receptor { alpha} ( PPAR{ alpha}) supports the induction of peroxisome proliferation in PPAR { alpha} deficient mouse liver. J. Biol. Chem. 276, 42485 42491. Page 25 of 39 APPENDIX A 1. PPAR  Activity and Peroxisome Assemblage/ Content in Fetal Liver Tissue PPAR  activity is expressed late in rodent embryonic development. In a study using in situ hybridization with antisense and sense riboprobes, expression of PPAR   in E8.5 ( gestation day 8.5), E11.5, E13.5, E15.5, and E18.5 Sprague Dawley rat embryos was determined ( Braissant and Wahli 1998). Low levels of transcripts of the PPAR  gene were first detected in the livers of E13.5 embryos and became highly expressed in E18.5 embryos.

Wilson et al., ( 1991) assayed liver tissue of gestational day 13 to 19 Swiss Webster fetal mice for peroxisomal matrix, membrane associated, and integral membrane proteins. Markers for these proteins used in the study were: matrix  catalase ( not specific to peroxisomes but also found in the cytoplasm); membraneassociated protein  dihydroxyacetone phosphate acyltransferase ( DHAP AT); and integral membrane protein  peroxisomal membrane protein ( PMP 70). DHAP AT, which provides evidence of peroxisome assemblage during ontogeny, increased about 2 fold in fetuses from gestational day 13 to 19. There were low levels of PMP 70 in 13  and 15 day fetuses; levels of this peroxisomal membrane protein were increased somewhat in 15 and 19 day fetuses. These data, and the fact that the numerical density of peroxisomes could not be quantified in fetal liver tissue due to weak staining of the peroxisomes indicate that the assemblage of peroxisomal proteins to form plasmids has not been completed in the gestational day 19 mouse fetus.

Catalase and palmitoyl CoA oxidase activities were first detected in the gestational day 15 Wistar rat fetus ( Cibelli, et al., 1988). In the liver, there was about a 2 3 fold increase in catalase activity and palmitoyl CoA oxidase activity between gestational days 19 and birth. Though these enzymes are not the products of PPAR  target genes, they are indicative of peroxisome proliferation and therefore useful in establishing the " competency" of these test animals to respond to PPAR  agonist exposure.

Peroxisomes are present ( few to many) in gestational day 19 rat fetuses, few are present in gestational day 15 fetuses, and peroxisomes are not evident in fetal liver tissue before gestational days 14 15 ( Tsukada, et al., 1968; Stefanini et al., 1985; Stefanini et al.,, 1989; Cibelli et al., 1988). In mice, few peroxisomes are found in 15 day fetal livers ( Wilson et al., 1991). Page 26 of 39 Table 1. Summary of Data on Peroxisome Assemblage/ Content and PPAR  Activity in Fetal Liver Tissue Parameter Indicator Reference Assemblage ( membrane proteins) Swiss Webster mice  DHAP AT  levels increased from gestational days 13 19;

PMP 70  low levels in gestational day 13 and 15 fetuses and increased slightly in gestational day 17 to 19 fetuses. Wilson et al., 1991.

Peroxisomal content Few to many in gestational day 19 rat embryos but not evident before gestational days 14 15.

Mean number in thin section of cell increases from 2.5 to 7.8 between 16 and 21 days of development; 7.8 in newborn and 30.1 in adult.

Few found in gestational day 15 mouse embryos; inability to quantify the numerical density in gestational day 19 mouse embryos suggests assemblage of peroxisomal proteins into plasmids has not been completed. Tsukada, et al., 1968; Stefanini et al.,

1989; Cibelli et al., 1988 Stefanini et. al., 1985 Wilson, et al., 1991 PPAR  gene or peroxisomal activity Low levels of PPAR  gene transcripts first detected in gestational day 13.5 SD rat embyros; highly expressed in gestational day 18.5 embryos Catalase and palmitoyl CoA oxidase activities first detected in gestational day 15 embryos and increase 2 to 3 fold at birth. Braissant and Wahli, 1998 Cibelli, et al., 1988 Page 27 of 39 A 2. Peroxisome Assemblage/ Content and Peroxisomal Enzyme Activity in Neonatal Liver Tissue The activities of DHAP AT and alkyl DHAP synthetase, enzymes involved in the synthesis of plasmalogens and that are found in peroxisomes, were reported to be similar in 19 day Sprague Dawley neonates and adult rats ( Singh and Lazo, 1992).

Stefanini et al., ( 1995) found no statistically significant differences in numerical density or volume density of peroxisomes in livers of 14, 21, or 35 day F344 rat neonates and no differences between neonate groups and adult females. In a more recent report ( Stefanini et. al., 1999) no differences were found between 14 or 21 day neonates or between these groups and adult F344 rats in peroxisomal   oxidation; the numerical density and volume density of liver peroxisomes were also comparable among groups. However, from a qualitative standpoint, hepatic peroxisomes were reported to be more numerous and larger in adult females than in neonates. There were gradual increases in catalase and palmitoyl CoA oxidase activities among the age groups ( 14 day neonate < 21 day neonate < adult). No differences were reported for the specific volume density among 7 8.5 10 13 or 17 day Wistar derived rat neonates or among these neonates or when compared with adult data from a separate report ( Staubli, et al., 1977; Weibel, et al., 1969).

There is evidence from other reports that peroxisomal enzyme activities that are markers of PPAR  expression ( e. g., palmitoyl CoA oxidase and carnitine acetyl transferase) are similar among control neonatal rats of different ages and neonatal and adult rats. As shown by Dostal et al.( 1987), untreated Sprague Dawley neonatal or young adult rats ( 10, 18, 25 or 46 days of age) were found to have levels of palmitoyl CoA oxidase activity ( range  3.61  5.31 nmol/ min/ mg protein in neonates and young adults) comparable to adult rats 90 days of age ( 4.8 nmol/ min/ mg protein). Carnitine acetyl transferase activity was also shown to be comparable among neonates of different ages and similar to adults; carnitine acetyl transferase activity was: 6.97 + 0.06, 5.85 + 0.41, 11.4 + 1.0, 8.10 + 0.55, 6.18 + 0.41, and 4.98 + 0.63 nmol/ min/ mg protein for 10, 18, 20, 25, 46, and 90 day rats, respectively. Stefanini et. al.,( 1999) found no differences between 14 or 21 day neonatal F344 rats or between these age groups and adult rats in peroxisomal   oxidation but during development there were increases in catalase ( 14 day neonate about 5 fold less than the adult) and palmitoyl CoA oxidase ( 14 day neonate about 2 fold less than the adult) activities. In another study using F344 rats ( Cimini et al., 1994), palmitoyl CoA oxidase activity was found to be minimal in 14 day neonates ( 0.2 + 0.02 mU/ mg protein) when compared with enzyme activity in adults ( 3.7 + 0.30 mU/ mg protein. DHAP AT was reported to be similar in 14 day neonates and adults and catalase enzyme activity increased about 3 fold between 14 days and adulthood. Page 28 of 39 Table 2. Ontogeny of Peroxisome Assemblage/ Content and PPAR  Activity in Neonatal Liver Tissue Parameter Indicator Reference Assemblage DHAP AT and alkyl DHAP synthetase activities similar in day 19 neonatal and adult rats.

No difference in DHAP AT activity between 14 day neonates and adults Singh and Lazo, 1992 Cimini et al., 1994 Peroxisomal content No differences in numerical density or volume of peroxisomes among 14, 21, or 35 day or among these age groups and adult F344 rats. Qualitatively, peroxisomes more numerous and larger in adults.

No differences in specific volume density of peroxisomes among Wistar derived 7, 8.5, 10, 13, or 17 day rat neonates or between these age groups and adults Stefanini, et al., 1995; Stefanini, et al., 1999 Staubli, et al., 1977; Weibel, et al., 1969 Peroxisomal activity No differences in peroxisomal   oxidation among 14 day, 21 day, and adult F344 rats; palmitoyl CoA oxidase 2 fold less in 14 day neonates than in adults.

Palmitoyl CoA oxidase activity comparable among 10 day, 18 day, 25 day, 46 day and adult SD rats; carnitine acetyl transferase activity similar among different age groups.

Palmitoyl CoA oxidase activity minimal ( about 19 30 fold less than adult activity) in 14 day F344 neonatal rats. Catalase activity ( not specific to peroxisomes) 3 fold less in 14 day neonates than in adults. Stefanini, et al., 1999 Dostal, et al., 1987 Cimini, et al., 1994 A 3. Response of the fetal liver to the PPAR  agonist clofibrate Several studies have shown that treatment of pregnant rats and mice with the PPAR  agonist, clofibrate, can induce peroxisome proliferation in fetal liver tissue.

Cibelli et al ( 1988) administered clofibrate ( 0.8% in the diet) to Wistar rats at various stages of pregnancy for 7 days. On the eighth day, dams were sacrificed, and the maternal and fetal livers were removed; some dams were allowed to deliver for examination of newborn livers. Liver weights were comparable among treated and control dams and among treated and control fetuses. There was a qualitative increase, relative to unexposed fetuses, in the size of peroxisomes in 15 day fetuses from treated dams and many peroxisomes were observed in 19 day fetuses and the newborn. There was no effect on catalase activity in 15 or 17 day embryos but there was a 3 fold increase in catalase activity in the newborn. Palmitoyl CoA oxidase activity was increased 4 fold in 15 day embryos and 6 8 fold in 19 to 21 day embryos. By birth, palmitoyl CoA oxidase activity was similar in the livers of the treated pups and dams. In the dams, palmitoyl CoA oxidase activity increased 3 to 4 fold and catalase activity increased 1.6 to 1.8 fold during days 15 17 of gestation, and during gestation days 17 to 21, palmitoyl CoA oxidase activity increased 4 to 5 fold and catalase activity increased 1.4 to 1.6 fold. Page 29 of 39 Stefanini et. al. ( 1989) treated pregnant Wistar rats with a diet containing 0.8% clofibrate for 7 days, and dams at 13, 15, 17, 19 and 21 days of pregnancy were sacrificed. Delivery was induced in dams close to term. Livers were removed from the fetuses and newborn rats and morphometric analyses were conducted on hepatocyte peroxisomes. The volume density of peroxisomes and the peroxisomal numerical density were significantly increased in all test animals but particularly in fetuses over 19 days and the newborn. Increases in the numerical densities of 15, 17, 19, 21 day fetuses and the newborn, relative to controls, were 1.5, 1.3, 1.8, 2.5, and 2.4 fold, respectively. Increases in the volume density of peroxisomes for these same age groups were 3.4, 3.3, 3.9, 4.3, and 4.6 fold, respectively.

Wilson et al. ( 1991) treated pregnant Swiss Webster mice with 400 mg/ kg/ day clofibrate by gavage from gestation days 6 19. Peroxisomal density was increased 2 fold in maternal mice after 7 days of treatment. There was also an increase in the number of peroxisomes in 15 day fetuses from treated dams versus untreated dams but because of poor staining the peroxisomal density could not be quantified in the fetuses from treated dams. Maximal increases in peroxisomal membrane protein 70 were also observed 7 days after initiation of treatment in maternal liver tissue and general increases in peroxisomal proteins were observed in fetal liver tissues from clofibrate treated dams between 13 and 19 days of gestation. At gestation day 19, DHAP AT and PMP 70 were each increased about 2 fold in clofibrate treated dams; DHAP AT was also increased about 2 fold in gestation day 19 fetuses but PMP 70 was increased about 5 fold. Catalase activity in liver tissue of gestational day 19 fetuses was increased somewhat ( 1.2 fold to 1.8 fold) relative to catalase activity in fetuses from untreated dams but there was no effect on catalase activity in treated dams. Because catalase is found in the cytoplasm as well in peroxisomes, changes in catalase activity can not be ascribed solely to the effects on peroxisomes. Page 30 of 39 Table 3. Fetal Responses to the PPAR  Agonist, Clofibrate.

Treatment Fetal Response ( compared to control) Dams Response ( compared to control) Reference Pregnant Wistar rats treated with 0.8% dietary supplement for 1 week prior to sacrifice gestational day 15

palmitoyl CoA oxidase activity increased 4

fold; catalase activity not affected;

gestational days 19 21

palmitoyl CoA oxidase activity increased 6

to 8 fold; at birth, specific activity about equal to treated dams catalase activity increased 3 fold in the newborn; specific activity less than treated dams peroxisomes  few in gestational day 15 fetuses, many in gestational day 19 fetuses and newborn, relative to controls liver weights  no increases during any stage of development gestational day 15

palmitoyl CoA oxidase activity increased 3.1 fold;

catalase activity increased 1.8 fold gestational days 19 21

palmitoyl CoA oxidase activity increased 5 fold;

catalase activity increased 1.4 to 1.6 fold peroxisomes  not examined in dams liver weights  no statistically significant increases during gestation Cibelli, et al., 1988 Pregnant Wistar rats treated with 0.8% dietary supplement for 1 week prior to sacrifice. Numerical density of peroxisomes

increased 1.3 to 2.5 fold in gestational day 15,

17, 19, 21 fetuses or newborn;

Volume density of peroxisomes  increased 3.4 to 4.6 fold in gestational day 15, 17, 19, 21 fetuses or newborn Not examined Stefanini et al., 1989 Treatment Fetal Response ( compared to control) Dams Response ( compared to control) Reference Page 31 of 39 400 mg/ kg administered by gavage to pregnant Swiss

Webster mice from gestation day 6 to gestation day 19. Dams were sacrificed at 13, 15, 17 and 19 days of gestation, and maternal and fetal livers were removed. Peroxisomal density could not be quantified in fetuses; general increases in peroxisomal proteins in gestational days 14 19 fetuses;

PMP 70 increased > 5 fold in gestation days 14

19 fetuses; no effect in gestational day 13 or 15 fetuses; specific activity in 17 or 19 day fetuses about 4 fold less than that in treated dams DHAP AT increased about 2 fold in gestational day 14 19 fetuses; minimal effect on gestational day 15 fetuses; no effect on gestational day 13 fetuses; specific activity about 50 fold less in gestational day 17 or 19 fetuses than in treated dams Catalase  1.2 to 1.8 fold increase in gestational day 19 fetuses; no effect in gestational day 13, 15, or 17 gestational day fetsuses Peroxisomal density increased 2

fold after 7 days treatment;

PMP 70 and DHAP AT each increased about 2 fold Catalase  unaffected Wilson et al., 1991 Page 32 of 39 A 4. Response of Neonates Following Lactational Exposures to PPAR  Agonists Peroxisomal enzyme activities were increased in nursing pups from Sprague Dawley rat dams that were treated with 0.025% ciprofibrate in the diet from postnatal days 3 19 ( Singh and Lazo, 1992). DHAP AT and alkyl DHAP synthetase activities were increased in the livers of 19 day neonates by 3.9 and 2.6 fold, respectively; corresponding increases in activities of these two enzymes were also seen in the ciprofibrate treated dams ( 4.2 and 3.2 fold, respectively). Liver weights of treated dams were increased 1.8 fold and in 19 day pups liver weights were increased 1.5 fold.

Cimini et al. ( 1994) treated F344 damswith 1g/ kg/ day DEHP by gavage for up to 21 days from day of delivery through lactation. Pups were sacrificed on day 14, day 21 or day 35 following 14 days of recovery. Relative liver weights increased 1.65 fold in the dams at weaning, and 1.47 fold in 14 and 21 day pups. At day 21, palmitoyl CoA oxidase increased 9.3 fold in dams, while it increased 6 fold in the nursing pups at 14 days and 4.85 fold at 21 days. However, palmitoyl CoA oxidase activity was substantially less in the pups than in the dams treated with DEHP ( pups, 1.2 mU/ mg protein at 14 days; dams, 34.4 mU/ mg protein at 21 days). DHAP AT was increased about 2 fold in 14 and 21 day neonates, but DHAP AT levels were unaffected in DEHP treated dams. Catalase activity was increased about 2 fold in 14 day and 21 day neonates and adults. Following 14 days of recovery, most enzyme levels returned to normal in the dams and pups, although catalase activity remained slightly higher in both the dams and pups.

In a separate study, pregnant lactating F344 dams were administered by gavage 1 g/ kg/ day DEHP for 21 days beginning at the day of delivery and the nursed pups were sacrificed after 2 or 3 weeks, or following a 14 day recovery period ( Stefanini, et al., 1995). The numerical density or volume density of peroxisomes was increased marginally (< 2 fold), relative to controls, in both pup groups. Dams treated for 21 days with DEHP showed a more pronounced increase in the volume density of peroxisomes ( about 2 fold), but the numerical density of peroxisomes was increased in the dams to the same degree as the 2 or 3 week pups. The increases in volume density or numerical density of peroxisomes did not decline to control levels in the 3 week pups after a 14 day recovery period. Volume density of peroxisomes apparently declined to close to control levels after a recovery period of 8 days in dams treated for threeweeks but there was no apparent decline in the numerical density of peroxisomes. Relative liver weights were increased about equally in 2 and 3 week old neonates and adults ( 1.5 to 1.6 fold).

Neonatal F344 rats exposed for 14 days following birth to nafenopin ( NF) or Wy 14,643 ( WY) through milk showed increases in peroxisomes and peroxisomal enzyme activities ( Fahl, 1983). The dams were treated twice daily by gavage with 100 mg/ kg NF or WY. Increases in peroxisome numbers were comparable in lactating dams treated with nafenopin and their suckling offspring. In pups exposed lactationally to NF for14 days, there was a 3 fold, 35 fold, 29 fold and 14 fold increase in the activities of Page 33 of 39 catalase. carnitine acetyl transferase, peroxisomal enoyl CoA hydratase, and palmitoyl CoA oxidase, respectively; exposure to WY for 14 days resulted in a 3 fold, 15 fold, 46 fold, and 12 fold increase in the activities of catalase. carnitine acetyl transferase, peroxisomal enoyl CoA hydratase, and palmitoyl CoA oxidase, respectively. The increases in peroxisomal enzyme activities catalase and peroxisomal enoyl CoA hydratase were similar to those seen in the dams treated with the PPAR  agonists for 14 days.

Effects of treatment with ciprofibrate on PPAR  expression were also investigated in lactating F344 rats and their pups ( Stefanini, et al., 1999). Dams were treated with ciprofibrate ( 0.025% in the diet) beginning on the day of delivery for 21 days or a week after delivery for 14 days ( i. e., days 7 21 after delivery). Pups from dams treated with ciprofibrate for 21 days were sacrificed at 14 or 21 days; pups from dams treated with ciprofibrate from day 7 to day 21 were sacrificed at 21 days. Palmitoy CoA oxidase activity was increased 12 14 fold, relative to controls, in pups from dams treated with ciprofibrate from either day 7 to day 21 ( 14 days) or for 21 days; the levels of activity ( approximately 4 U/ g protein) were comparable to adults treated with ciprofibrate for 21 days . However, in the 7 21 day pups, greater increases in enzyme activity, relative to controls, were seen than in adults treated with ciprofibrate for 14 days and the level of enzyme activity in the pups exposed lactationally to ciprofibrate was also greater ( about 2X) than the treated dams ( 7 21 day pups  about 4 U/ g protein or a 12 fold increase relative to controls; adults  about 2 U/ mg protein or a 5 fold increase relative to controls). Increases in cyanide insensitive peroxisomal   oxidation activity, up to 10 fold, relative to controls, were seen in 14 day pups from treated dams and in day 7 to day 21 pups. However, the induced levels of   oxidation ( U/ g of liver tissue) in day 1 14 pups and in day 7 21 pups were less than the induced levels of   oxidation seen in dams treated with ciprofibrate for 14 days. The levels of induced   oxidation in 21 day pups from dams treated with ciprofibrate for 21 days were comparable. There was about a 3 to 4 fold increase, relative to controls, in numerical density of liver peroxisomes in day 1 14 pups and in day 7 21 pups. The increase in this parameter in dams treated with ciprofibrate for either 14 days or for 21 days was about 2 fold. Finally, increases in liver weights were somewhat more pronounced in the pups lactationally exposed to DEHP than in treated dams ( relative liver weight increases  day 1 14 pups, 2.25X and day 7 21 adults, 1.47X; day 7 21 pups, 2.63X and day 7 21 adults, 1.47X; day 1 21 pups, 2.63X and day 1 21 adults, 1.94X. In a two generation rat reproduction study with diclofop methyl submitted to the Agency ( Osterburg, 1992), increased liver weights were reported at 100 ppm ( the highest dose tested) in F0 and F1 adults ( males 12% and 31%; females 26% and 13%), no increases in liver weights were reported for F1 male or female day 4 offspring but increased liver weights were reported for F2 male day 4 offspring ( 24%) and F2 female day 4 offspring( 23%). At the highest dose tested, histologic examination showed that there was nuclear swelling of hepatocytes and hepatocyte hypertrophy in both F0 and F1 adults. No hepatic effects were observed in F1 offspring but cellular hypertrophy and nuclear swelling was observed in F2 offspring. Thus, the results of the study with diclofop methyl show that, qualitatively, exposure of dams to a PPAR  Page 34 of 39 agonist can lead to effects on liver weights and liver histology in neonates that are consistent with PPAR  agonism.

In contrast, a two generation reproduction study with lactofen submitted to the Agency showed no effects on liver weights or liver histology in neonates( Schroeder, 1983). In this study, liver weights were increased somewhat ( 113%) in F1 adult females treated with 2000ppm but not in F1 or F2 weanlings. No liver histopathology was observed in F0 adult males ( histopathology was not conducted on female F0 adults) but intrahepatic bile duct proliferation, centrolobular degeneration/ necrosis was observed in male and female rats dying on test. Histopathology examination of liver tissue from F1 or F2 weanlings revealed no histopathologic effects.

The results of these two studies show that when dams are treated with a PPAR  agonist ( i. e., diclofop methyl) during gestation and lactation, liver hypertrophy may be induced in the nursing neonate. However, the liver effects of the PPAR  agonist in neonates are no more pronounced than those observed in adults ( i. e., diclofop methyl) or they are absent in the neonate but present in the adult ( i. e., lactofen). Page 35 of 39 Table 4. PPAR  Agonism in Dams and in Neonates Exposed by Lactation to Peroxisome Proliferators Chemical Treatment Neonatal Response Adult Response Reference Ciprofibrate 0.025% in the diet of SD rat dams from postnatal days 3 19 DHAP AT increased 3.9 fold and alkyl DHAP synthetase increased 2.6 fold in 19

day nursing neonates;

Liver weights increased 1.8 fold DHAP AT increased 4.2 fold and alkyl

DHAP synthetase 3.2 fold at postnatal day 19 Liver weights increased 1.5 fold Singh and Lazo, 1992 Ciprofibrate F344 rat dams treated with 0.025% ciprofibrate in the diet from day of delivery to day 21 or a week after delivery for 14 days palmitoyl CoA oxidase activity increased 12 14

fold in 21 day pups from dams treated for 21 days or 21 day pups from dams treated from postnatal days 7 21.

peroxisomal   oxidation increased up to 10 fold in day 14 pups and in day 21 pups from dams treated from day 7 21.

numerical density of peroxisomes increased in 3

to 4 fold in 14 day pups and in day 21 pups from dams treated from day 7  21; palmitoyl CoA oxidase activity increased 5 fold in dams treated for 14 days ( day 7

21); peroxisomal   oxidation increased 6 8

fold ( estimated from fig. 1) in adults treated for 14 days or 21 days;

numerical density of peroxisomes increased 2 fold in dams treated for 14 or 21 days. Stefanini et al., 1999 DEHP F344 rat dams gavaged with 1g/ kg/ day from parturition to day 21. at 14 or 21 days,

palmitoyl CoA oxidase increased 4.85 to 6 fold;

DHAP AT increased about 2 fold; at 21 days and 14 days recovery, DHAP AT remained increased ( about 2 fold).

catalase increased 2 fold.

relative liver weights increased 1.47 fold in 14

day neonates and about the same amount in 21

day neonates at 21 days:

palmitoyl CoA oxidase increased 9 fold;

DHAP AT unchanged; at 21 days and 14 days recovery, DHAP AT returned to control levels.

catalase increased 2 fold.

relative liver weights increased 1.65 fold Cimini et al.,

1994 DEHP F344 rat dams gavage with 1g/ kg/ day from parturition to day 21 numerical density and volume density of peroxisomes increased marginally (< 2 fold) in 2

or 3 week nursing pups; volume or numerical density of peroxisomes in 3 week pups did not decline following a 14 day recovery period. a more pronounced increase in volume density of peroxisomes ( about 2 fold) than 2 or 3 week nursing pups; volume density, but not numerical density, of peroxisomes declined to control levels following a 8 day recovery in dams treated for 21 days. Stefanini et al., 1995 Chemical Treatment Neonatal Response Adult Response Reference Page 36 of 39 Nafenopin ( NF) or Wy

14,643 ( Wy) F344 rat dams gavaged with 100 mg/ kg NF or Wy twice daily from delivery to postnatal day 14 catalase, NF and Wy  3 fold increase at 14 days;

carnitine acetyl transferase, NF 35 fold increase and Wy 15 fold increase;

peroxisomal enoyl CoA hydratase, NF 29 fold increase and Wy 46 fold increase;

palmitoyl CoA oxidase, NF 14 fold increase and Wy 12 fold increase. catalase and peroxisomal enoyl CoA hydratase were similar to those seen in the day 14 neonates;

peroxisome numbers were comparable,

qualitatively, in lactating dams treated with nafenopin and their suckling offspring Fahl et al.,

1983 Diclofop methyl Sprague Dawley male and female rats administered diclofopmethyl via the diet for two consecutive generations;

dose levels  females 0,

0.9, 2.5, or 8.5 mg/ kg/ day liver weights

no increases in F1 male or female day 4 offspring F2 males  increased 24%; F2 females

increased 23%

no hepatic histopathology liver weights

F0 males increased 12%; F1 males increased 31%;

F0 females increased 26%; F1 females increased 13%

nuclear swelling and hepatocyte hypertrophy in both F0 and F1 adults Osterburg,

1992 Lactofen Sprague Dawley male and female rats administered lactofen via the diet for two generations; dose levels

F0 females 0, 3.1, 31.8,

or 121.3 mg/ kg/ day; F1 females 3.3, 32.9, or 121.3 mg/ kg/ day liver weights  no increases in F0 or F1 pups; no hepatic histopathology observed in F0 or F1 offspring liver weights  increased 13% in F1 adult females; centrolobular degeneration/ necrosis was observed in male and female rats dying on test Schroeder,

1983 Page 37 of 39 A 5. Response of Neonates Following Direct Exposures to PPAR  Agonists A study designed to investigate the effects of a PPAR  agonist on neonatal rats of different ages was conducted by Dostal et al. ( 1987). Male Sprague Dawley rats 6, 14, 16, 21, 42, or 86 days of age were administered ( by gavage) daily doses of DEHP for 5 days, and 24 hours after sacrifice activities of hepatic peroxisomal enzymes, palmitoyl CoA oxidase and carnitine acetyltranferase were determined. The doses administered were 0, 10, 100, 1000, or 2000 mg/ kg/ day. Administration of 1000 mg/ kg/ day caused significant decreases in body weight and mortality ( 66 70%) in pups 14 18 days of age, and administration of 2000 mg/ kg/ day caused mortality in virtually all pups of these ages ( the authors concluded that body weight decrements and mortality were not associated with effects on peroxisome proliferation activity). At a non lethal dose ( 100 mg/ kg/ day), absolute liver weight increases relative to those in the controls were 0, 17, 3, 10, and 14% ( 6 10, 14 18, 21 25, 42 46, and 86 89 day old pups and adults, respectively). At 100 mg/ kg/ day, measurements of palmitoyl CoA activity showed that there was a greater increase only in the 14 18 day pups when compared with 86 90 day adults ( 6.9 fold increase versus a 3.98 fold increase). A greater increase in carnitine acetyl transferase also was shown at this dose level only for 14 18 day pups when compared with 86 90 day adults ( 7.8 fold increase versus a 4.4 fold increase). The data on increased liver weights and peroxisomal enzyme activities from this study indicate that there is little difference in the response of neonatal or young adult rats compared with adult rats to treatment with DEHP.

Administration of clofibrate to 4, 8, or 12 week old male or female F344 rats for 7 days ( 200 mg/ kg/ day) induced increases in liver weights, peroxisomal   oxidation, and the percentage of peroxisomal area relative to hepatocellular cytoplasm ( Yamoto, 1996). Increases in these parameters among the 4, 8, and 12 week old rats, respectively, were: relative liver weights  males  108, 161, and 168%; females  108, 119, and 117%; palmitoyl CoA oxidation  males  206, 589, and 1072% and females  145, 152, and 312%; percentage of peroxisomal area to hepatocellular cytoplasm  males  134, 479, and 657% and females  168, 236, 169%. All increases were statistically significant ( p < 0.05). The results of this study indicate that the effects of the PPAR  agonist clofibrate are weak in the immature rat and that susceptibility to the effects of the chemical increase as rats approach adulthood.

Peroxisome volume density was increased 6 fold and peroxisome number was increased 2 fold following treatment by intubation of neonatal Wistar derived rats with 100 mg/ kg/ day nafenopin from lactation days 5 through 9 ( Staubli, et al., 1977). Following a recovery period of 7 days, peroxisome volume density and number closely approached, but did not attain, control values. Page 38 of 39 Table 5. Data on liver effects in Neonates or Weanlings Exposed Directly to PPAR  agonists Chemical Treatment Neonatal/ Weanling Response Adult Response Reference DEHP male Sprague Dawley rats 6, 14, 16, 21, 42, or 86 days of age gavaged with 0, 100,

1000, or 2000 mg/ kg/ day for 5 days ( NOTE: 1000 and 2000 mg/ kg/ day were lethal doses) at dose = 100 mg/ kg/ day:

Palmitoyl CoA oxidase increase 6 10 day 3X 14 18 day 7X 21 25day 2X carnitine acetyl transferase increase 6 10 day 2.7X 14 18 day 7.8X 21 25 day 2.4X increases in peroxisomes  similar at all ages ( qualitatively)

liver weight increases 14 18 day  1.2X at dose = 100 mg/ kg/ day:

Palmitoyl CoA oxidase increase 42 46 day 2.5X 86 90 day 4X carnitine acetyl transferase increase 42 46 day young adult  3.6X 86 90 day adult  4.4X increases in peroxisomes  similar at all ages ( qualitatively)

liver weight increases 42 46 day young adult  1.1X 86 90 day adult  1.1X Dostal et al.,

1987 Nafenopin 5 day Wistar derived rat pups intubated with 100 mg/ kg/ day from day 5 through day 9 peroxisome volume density increase

6 fold in day 9 neonates peroxisome number increase  2 fold in 9 day neonates no data Staubli, et al.,

1977 Chemical Treatment Neonatal/ Weanling Response Adult Response Reference Page 39 of 39 Clofibrate 4, 8, or 12 week male and female F344 rats gavaged with 200 mg/ kg/ day for 7 days. palmitoyl CoA oxidase increase males 4 week weanling 206 %

8 week young adult  589% females 4 week weanling  145%

8 week young adult  152%

percentage of peroxisomal area to cytoplasm  increase males 4 week weanling 134%

8 week young adult 479% females 4 week weanling 168%

8 week young adult 236%

increased liver weights males 4 week weanling  108%

8 week young adult  161% females 4 week weanling  108%

8 week young adult  119% palmitoyl CoA oxidase increase males 12 week adult 1072% females 12 week adult  312%

percentage of peroxisomal area to cytoplasm  increase males 12 week adult 657% females 12 week adult  169%

increased liver weights males 12 week adult  168% females 12 week adult  117% Yamoto, 1996