IPCS conceptual framework for evaluating a mode of action for chemical carcinogenesis.

The International Programme on Chemical Safety (IPCS) is leading an activity to harmonize approaches to cancer risk assessment as a part of its larger project on the Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals. Through a series of workshops and the evaluation of case studies, a number of key components of risk assessments relating to harmonization were identified: transparency, terminology, weight of evidence, flexibility, and accessibility/communication. A major impediment to harmonization identified in the consideration of weight of evidence was the evaluation of mode of action. To address this need, a conceptual framework was developed, based on the general principles involved in considering the chemical induction of a specific tumor in animals. This is based partly on the Bradford Hill criteria for causality as modified by Faustman et al. (1997) for developmental toxicity. The framework is described in this paper followed by a worked example. It is recognized that the framework addresses only one stage in the overall characterization of hazard to humans of chemical carcinogens. Another important but separate step is the assessment of relevance to humans. This is a priority area for future work in this project.

The OECD program to validate the rat uterotrophic bioassay to screen compounds for in vivo estrogenic responses: phase 1.

The Organisation for Economic Co-operation and Development has completed the first phase of an international validation program for the rodent uterotrophic bioassay. This uterotrophic bioassay is intended to identify the in vivo activity of compounds that are suspected agonists or antagonists of estrogen. This information could, for example, be used to help prioritize positive compounds for further testing. Using draft protocols, we tested and compared two model systems, the immature female rat and the adult ovariectomized rat. Data from 19 participating laboratories using a high-potency reference agonist, ethinyl estradiol (EE), and an antagonist, ZM 189,154, indicate no substantive performance differences between models. All laboratories and all protocols successfully detected increases in uterine weights using EE in phase 1. These significant uterine weight increases were achieved under a variety of experimental conditions (e.g., strain, diet, housing protocol, bedding, vehicle). For each protocol, there was generally good agreement among laboratories with regard to the actual EE doses both in producing the first significant increase in uterine weights and achieving the maximum uterine response. Furthermore, the Hill equation appears to model the dose response satisfactorily and indicates general agreement based on calculated effective dose (ED)(10) and ED(50) within and among laboratories. The feasibility of an antagonist assay was also successfully demonstrated. Therefore, both models appear robust, reproducible, and transferable across laboratories for high-potency estrogen agonists such as EE. For the next phase of the OECD validation program, both models will be tested against a battery of weak, partial estrogen agonists.

Endocrine modulators: risk characterization and assessment.

Over the last several years, information has been accumulating that suggests that adverse effects are being induced in certain wildlife species, and perhaps also in humans, as a consequence of exposure to man-made chemicals that have been released into the environment. Many of these effects have been attributed to interactions with various hormone systems in endocrine tissues. Most often the effects observed have been effects on reproduction and development, although there are also (often conflicting) data regarding an association between exposure and certain kinds of cancer, particularly cancers of reproductive tissues such as breast and testis; effects on the immune system have also been noted. The substances to which these attributes have been ascribed have come to be known as "endocrine modulators" or "endocrine disruptors." The full nature and scope of the "problem" of endocrine modulators/disruptors is currently a matter of great debate, both within and outside of the scientific community. Regulatory authorities around the world are being asked what their position is on this issue and what, if any, regulatory strategies they are developing to address the problem. In many cases, because of the nature of the legislation under which governments manage chemicals, regulatory decisions must be informed by risk assessment. This presentation will describe the general approach to the risk assessment of endocrine modulators/disruptors as practiced by the US government, with particular focus on the current practices/policies of the US Environmental Protection Agency.

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response.

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response. Copyright 1998 Academic Press.

Endocrine disruptor risk characterization: an EPA perspective.

The characterization of risk to endocrine-disruptive agents may prove to be one of the greatest challenges that the risk assessment/regulatory community has ever faced. Why is this so? The endocrine system is actually many systems, having complex interactions and interdependencies. Normal endocrine function is often dependent on cyclical events, rather than steady-state. Timing is everything, as evidenced by significant differences in adverse outcome as a function of age and stage of development. Further, the consequences of concomitant exposures to endocrine-active substances in the diet or as therapeutic agents are poorly understood. So, how should risk characterization to these agents be approached? This presentation will include the description of current practices for addressing hormonally mediated cancer and noncancer effects and offer speculation on modifications to these approaches that might be necessary as our knowledge of this area increases.

Research needs for the risk assessment of health and environmental effects of endocrine disruptors: a report of the U.S. EPA-sponsored workshop.

The hypothesis has been put forward that humans and wildlife species adverse suffered adverse health effects after exposure to endocrine-disrupting chemicals. Reported adverse effects include declines in populations, increases in cancers, and reduced reproductive function. The U.S. Environmental Protection Agency sponsored a workshop in April 1995 to bring together interested parties in an effort to identify research gaps related to this hypothesis and to establish priorities for future research activities. Approximately 90 invited participants were organized into work groups developed around the principal reported health effects-carcinogenesis, reproductive toxicity, neurotoxicity, and immunotoxicity-as well as along the risk assessment paradigm-hazard identification, dose-response assessment, exposure assessment, and risk characterization. Attention focused on both ecological and human health effects. In general, group felt that the hypothesis warranted a concerted research effort to evaluate its validity and that research should focus primarily on effects on development of reproductive capability, on improved exposure assessment, and on the effects of mixtures. This report summarizes the discussions of the work groups and details the recommendations for additional research.

Impact of the Delaney Clause in the EPA.

The U.S. Environmental Protection Agency serves as the lead agency of the Federal government for the regulation of pesticide use in the United States. Regulatory responsibilities are mandated in the Federal Insecticide Fungicide and Rodenticide Act (FIFRA) and several sections of the Federal Food Drug and Cosmetic Act (FFDCA). In order for a pesticide to be used in the U.S., it must either be granted a full registration under Section 3, or offered special consideration under Section 18 or 24 of FIFRA. Tolerances (maximum allowable residue limits) for pesticides to be used on agricultural commodities meant for human consumption or animal feed are established under Sections 408 and/or 409 of FFDCA. This presentation will describe the underlying conflict between Section 409 of FFDCA and both Section 408 and FIFRA, the recommendations made by the National Academy of Sciences in its 1987 publication Regulating Pesticides in Food--The Delaney Paradox, EPA's attempts to implement these recommendations, the true impact of "Delaney" on the ability of EPA to grant tolerances on raw or processed foods for chemicals determined to meet the "induce cancer" criterion and the status of legislation designed to "fix" Delaney.

Risk assessment of carcinogens in food with special consideration of non-genotoxic carcinogens. Scientific arguments for use of risk assessment and for changing the Delaney Clause specifically.

The document "Risk Assessment of Carcinogens in Food with Special Consideration of Non-Genotoxic Carcinogens" was produced by the International Federation of Societies of Toxicologic Pathologists on the occasion of its triannual meeting in Tours, France, April 23-26, 1995. Subsequently, it was endorsed by the North American Society of Toxicologic Pathologists at its annual meeting in San Diego, CA, USA, June 11-15, 1995. This document was written to address up-to-date risk assessment of carcinogens and anachronisms in the Delaney Clause of the US Federal Food, Drug and Cosmetic Act which have become evident since its enactment in 1958. In the intervening years, major progress has been made in understanding mechanisms of cancer induction and in recognizing causes of human cancer. The Clause in conjunction with its present legal interpretation and implementation does not provide for rational, scientific evaluation of carcinogens. It ignores the fact that the diverse mechanisms now known to underlie cancer increases in rodents exposed to high doses of chemicals are often inapplicable to man. In this regard, current evaluation of chemicals based on the tenets of the Delaney Clause is irrational in many cases. The document presents several examples of chemicals to which humans may be exposed through food and which illustrate the need for science-based risk assessment. Appropriate risk assessment methods are available to provide assurance of negligible risk, and accordingly, it is recommended that the Delaney Clause be rescinded as it has outlived its usefulness. This will enable US governmental agencies to regulate the use of chemicals in foods by using appropriate current scientific methods on a case by case basis within the context of other relevant legislation.

Evaluation of the carcinogenic potential of pesticides. 4. Chloroalkylthiodicarboximide compounds with fungicidal activity.

The Health Effects Division of the Office of Pesticide Programs (OPP) assessed the carcinogenic potential of three structurally related chloroalkylthiodicarboximide fungicides using a consensus peer review process and EPA's 1986 guidelines for cancer risk assessment. All of the fungicides were categorized as Group B2 (probable human) carcinogens based upon findings of an increased incidence of malignant tumors, or combined malignant and benign tumors, in multiple experiments involving different strains of mice and rats. The primary sites of tumor formation with the chloroalkylthiodicarboximide fungicides in male and/or female mice (CD-1 and B6C3F1) were the gastrointestinal tract (captan, folpet, and captafol), the lymph system (folpet and captafol), and the vascular system (captafol). The main sites of tumor formation in rats of one or both sexes (CR CD, Wistar, or F344 strains) were the kidney (Captan and captafol), uterus (captan), mammary gland and liver (captafol). In addition, positive trends for thyroid, testicular, mammary gland, and lymph node tumors were observed with folpet in the same strains of rats. All three of the compounds exhibited positive mutagenic activity in a variety of in vitro short-term tests for gene mutation, DNA repair, and chromosomal aberrations in prokaryotic and eukaryotic cells, but were not genotoxic in available studies performed under in vivo conditions. The assessment of human cancer risk for captan, folpet, and captafol was made using low-dose extrapolation models.

Regulatory concerns of the United States Environmental Protection Agency.

The US Environmental Protection Agency, under 2 of its legislative mandates, has the authority to require the testing of industrial and pesticide chemicals. Among the testing requirements, particularly in chronic studies, are those relating to hematology, clinical chemistry, and urinalysis. Some of these requirements will be discussed in detail. Comments on the usefulness of the current requirements and recommendations for changes will be solicited from the meeting participants.

Evaluation of the carcinogenic potential of pesticides. 3. Aliette.

Aliette, a fungicide compound, was evaluated for carcinogenic potential by the Health Effects Division of the Office of Pesticide Programs using a consensus peer review process and EPA's guidelines for risk assessment. Aliette was categorized as a group C (possible human) carcinogen based upon evidence of an increased incidence of combined benign and malignant urinary bladder tumors in a single study involving male Charles River (CR) CD rats. The bladder tumors occurred only at the unusually high top dose level of aliette that was tested (40,000/30,000 ppm). The compound was not carcinogenic in female CR-CD rats in the same study, or in CD-1 mice of either sex in a second study. Monosodium phosphite, the main urinary metabolite of aliette, was also not carcinogenic in male or female CR-CD rats. Aliette was not demonstrated to be genotoxic. No structural analogues of aliette were identified. The mechanism of action for the production of bladder tumors was not identified; however, it did not appear to involve a genotoxic effect, a carcinogenic effect of metabolites, or the formation of renal stones. The data were not found to be sufficient to quantify human cancer risk from aliette.

Federal regulatory assessment approach at the U.S. EPA.

For this presentation the data package on acrylonitrile was used as an example of the processes by which risk assessment evaluations would be performed for chemicals regulated by the Environmental Protection Agency, Office of Drinking Water. The discussion spans both regulatory and non-regulatory aspects of regulation. Developing risk assessments for EPA regulatory purposes is similar in all offices and in that context the considerations as well as the methods of assessing risks apply generally to regulatory considerations by other offices within the EPA.

Evaluation of the carcinogenic potential of pesticides. 2. Methidathion.

The carcinogen potential of methidathion, a dimethoxyorganic phosphorus pesticide and cholinesterase inhibitor, was evaluated by the Health Effects Division of the Office of Pesticide Programs using a consensus peer review process and the EPA's guidelines for risk assessment. Methidathion was categorized as a Group C (possible human) carcinogen based upon evidence of an increased incidence of benign and malignant hepatocellular tumors, alone and in combination, in a single study involving male Chr-CD-1 mice. The compound was not carcinogenic in female Chr-CD-1 mice in the same study or in Sprague-Dawley rats of either sex in a second study. Methidathion was not genotoxic in a variety of in vitro or in vivo tests designed to detect DNA damage, chromosome aberrations, gene mutations, and sister chromatid exchange. Although methidathion was identified as being structurally similar to two other organophosphate insecticides, prothidathion and lythidathion, no toxicological data were available on either of these agents for comparative purposes. The biological information on methidathion was reviewed by the agency's FIFRA Scientific Advisory Panel who agreed with the category C designation for methidathion. The data were not found to be sufficient to quantify human risk to methidathion.