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.

Environmental endocrine disruption: an effects assessment and analysis.

This report is an overview of the current state of the science relative to environmental endocrine disruption in humans, laboratory testing, and wildlife species. Background information is presented on the field of endocrinology, the nature of hormones, and potential sites for endocrine disruption, with specific examples of chemicals affecting these sites. An attempt is made to present objectively the issue of endocrine disruption, consider working hypotheses, offer opposing viewpoints, analyze the available information, and provide a reasonable assessment of the problem. Emphasis is placed on disruption of central nervous system--pituitary integration of hormonal and sexual behavioral activity, female and male reproductive system development and function, and thyroid function. In addition, the potential role of environmental endocrine disruption in the induction of breast, testicular, and prostate cancers, as well as endometriosis, is evaluated. The interrelationship of the endocrine and immune system is documented. With respect to endocrine-related ecological effects, specific case examples from the peer-reviewed literature of marine invertebrates and representatives of the five classes of vertebrates are presented and discussed. The report identifies some data gaps in our understanding of the environmental endocrine disruption issue and recommends a few research needs. Finally, the report states the U.S. Environmental Protection Agency Science Policy Council's interim position on endocrine disruption and lists some of the ongoing activities to deal with this matter.

Evaluation of reduced protocols for carcinogenicity testing of chemicals: report of a joint EPA/NIEHS workshop.

The current U.S. Environmental Protection Agency (EPA) and other national/international guidelines specify the use of two species and two sexes rodents (usually the rat and the mouse) for carcinogenicity testing of chemicals. In view of the enormous number of chemicals to be tested, the high cost of testing, and the large number of animals used in the present protocol, many academic, industrial, and government authorities are examining the possibility of using a reduced protocol (less than two species and two sexes of rodents) for carcinogenicity testing of chemicals. The use of a reduced protocol offers many advantages as well as some disadvantages. To pursue further the potential implications and impacts of using a reduced protocol for carcinogenicity testing on the processes of hazard identification and risk assessment, a workshop entitled "Evaluation of Reduced Protocols for Carcinogenicity Testing of Chemicals" was held at the Embassy Suites Hotel in Alexandria, Virginia on September 22 and 23, 1992. It was cosponsored by EPA's Office of Prevention, Pesticides and Toxic Substances (OPPTS) and the National Toxicology Program of the National Institutes of Environmental Health Sciences (NTP/NIEHS) and attended by more than 60 participants from government, industry, academia, and the general public. The Expert Consensus Panel and most of the participants supported the use of reduced protocols in carcinogenicity testing. However, it was recognized that reduced protocols may not be appropriate for the testing of all chemicals and that additional analyses/data may be needed for selection of the most appropriate reduced protocol for certain chemicals/chemical classes.

Interpretation of male rat renal tubule tumors.

Based on an analysis of recent scientific studies, a Technical Panel of the U.S. Environmental Protection Agency's (EPA) Risk Assessment Forum recently advised EPA risk assessors against using information on certain male rat renal tubule tumors to assess human risk under conditions specified in a new Forum report. Risk assessment approaches generally assume that chemicals producing tumors in laboratory animals are a potential cancer hazard to humans. For most chemicals, including classical rodent kidney carcinogens such as N-ethyl-N-hydroxyethylnitrosamine, this extrapolation remains appropriate. Some chemicals, however, induce accumulation of alpha 2u-globulin (alpha 2u-g), a low molecular weight protein, in the male rat kidney. The alpha 2u-g accumulation initiates a sequence of events that appears to lead to renal tubule tumor formation. Female rats and other laboratory mammals administered the same chemicals do not accumulate low molecular weight protein in the kidney, and they do not develop renal tubule tumors. Because humans appear to be more like other laboratory animals than like the male rat, in this special situation, the male rat is not a good model for assessing human risk. The Forum report stresses the need for full scrutiny of a substantial set of data to determine when it is reasonable to presume that renal tumors in male rats are linked to a process involving alpha 2u-g accumulation and to select appropriate procedures for estimating human risks under such circumstances.

Hazard evaluation of chemicals that cause accumulation of alpha 2u-globulin, hyaline droplet nephropathy, and tubule neoplasia in the kidneys of male rats.

This review paper examines the relationship between chemicals inducing excessive accumulation of alpha 2u-globulin (alpha 2u-g) (CIGA) in hyaline droplets in male rat kidneys and the subsequent development of nephrotoxicity and renal tubule neoplasia in the male rat. This dose-responsive hyaline droplet accumulation distinguishes CIGA carcinogens from classical renal carcinogens. CIGA carcinogens also do not appear to react with DNA and are generally negative in short-term tests for genotoxicity, CIGA or their metabolites bind specifically, but reversibly, to male rat alpha 2u-g. The resulting complex appears to be more resistant to hydrolytic degradation in the proximal tubule than native, unbound alpha 2u-g. Single cell necrosis of the tubule epithelium, with associated granular cast formation and papillary mineralization, is followed by sustained regenerative tubule cell proliferation, foci of tubule hyperplasia in the convoluted proximal tubules, and renal tubule tumors. Although structurally similar proteins have been detected in other species, including humans, renal lesions characteristic of alpha 2u-g nephropathy have not been observed. Epidemiologic investigation has not specifically examined the CIGA hypothesis for humans. Based on cancer bioassays, hormone manipulation studies, investigations in an alpha 2u-g-deficient strain of rat, and other laboratory data, an increased proliferative response caused by chemically induced cytotoxicity appears to play a role in the development of renal tubule tumors in male rats. Thus, it is reasonable to suggest that the renal effects induced in male rats by chemicals causing alpha 2u-g accumulation are unlikely to occur in humans.

Mode of action and the assessment of chemical hazards in the presence of limited data: use of structure-activity relationships (SAR) under TSCA, Section 5.

Section 5 of the Toxic Substances Control Act (TSCA) requires that manufacturers and importers of new chemicals must submit a Premanufacture Notification (PMN) to the U.S. Environmental Protection Agency 90 days before they intend to commence manufacture or import. Certain information such as chemical identity, uses, etc., must be included in the notification. The submission of test data on the new substance, however, is not required, although any available health and environmental information must be provided. Nonetheless, over half of all PMNs submitted to the agency do not contain any test data; because PMN chemicals are new, no test data is generally available in the scientific literature. Given this situation, EPA has had to develop techniques for hazard assessment that can be used in the presence of limited test data. EPA's approach has been termed "structure-activity relationships" (SAR) and involves three major components: the first is critical evaluation and interpretation of available toxicity data on the chemical; the second component involves evaluation of test data available on analogous substances and/or potential metabolites; and the third component involves the use of mathematical expressions for biological activity known as "quantitative structure-activity relationships" (QSARs). At present, the use of QSARs is limited to estimating physical chemical properties, environmental toxicity, and bioconcentration factors. An important overarching element in EPA's approach is the experience and judgment of scientific assessors in interpreting and integrating the available data and information. Examples are provided that illustrate EPA's approach to hazard assessment for PMN chemicals.

Characterization and properties of the DNA adducts formed from N-methyl-4-aminoazobenzene in rats during a carcinogenic treatment regimen.

Chronic oral administration of the carcinogenic aminoazo dye N-methyl-4-aminoazobenzene (MAB) to rats is known to result in the induction of liver tumors. In order to assess the role of carcinogen-DNA adduct formation in MAB hepatocarcinogenesis, male rats were fed 0.06% [3'-3H]MAB in the diet for 1, 3 or 5 weeks. Groups were sacrificed at 0, 24 and 72 h after dosing, and DNA was isolated from the liver and from two non-target tissues, the kidney and spleen. Upon enzymatic hydrolysis of the DNA, [3H]aminoazo dye-nucleoside adduct levels in these tissues were determined by h.p.l.c. Rats concurrently administered unlabeled MAB for 5 weeks and continued on a control diet for 9 months developed hepatocellular carcinomas (16/30 animals). No tumors were observed in 21 rats given only control diets. After chronic administration of [3H]MAB, three major MAB-DNA adducts were found in vivo: N-(deoxyguanosin-8-yl)-MAB (C8-dG-MAB), 3-(deoxyguanosin-N2-yl)-MAB (N2-dG-MAB) and 3-(deoxyadenosin-N6-yl)-MAB (N6-dA-MAB). In addition, several minor products were identified as: an (8,9)-purine ring-opened derivative of C8-dG-MAB that may represent an intermediate in DNA repair; N-guanosin-8-yl-MAB which is present due to trace RNA contamination; cis isomers of C8-dG-MAB and N-guanosin-8-yl-MAB, formed by photo-illumination during analyses; and N-(guanin-8-yl)-MAB, a deribosylated product resulting from thermal depurination of C8-dG-MAB. In addition, N-(deoxyguanosin-8-yl)-4-aminoazobenzene (C8-dG-AB), a major adduct previously detected in mouse liver after a single dose of 4-aminoazobenzene, was found in rat liver but appeared to be present in significant amounts only after chronic treatment with MAB. This product co-chromatographed with N6-dA-MAB but could be removed by selective decomposition in 0.1 N NaOH. For all tissues examined N2-dG-MAB and C8-dG-MAB were the major adducts observed with each accounting for 40-50% of the total carcinogen bound to DNA in rats that were sacrificed immediately after MAB feeding for 1, 3 or 5 weeks. The levels of total MAB-DNA adducts in the liver were 2-10 times greater than in the kidney or spleen and appeared to increase 2- to 3-fold over the dosing period.(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation of liver tumor morphology and weight to incidence of pulmonary metastases in the mouse.

Benzidine dihydrochloride administered in water at 0, 30, 60, 120, 200 or 400 ppm to 1664 mice induced a total of 368 hepatocellular carcinomas, of which 34 or 9.2% metastasized to the lungs. Liver tumors which were larger and heavier were more prone to metastasize and all that metastasized contained areas of prominent trabecular formation.

Causative agents in the induction of bladder cancer.

A survey of many of the known or suspected causative agents in the etiology of bladder cancer is presented. In several instances, a comparison of the epidemiological evidence and the results of animal bioassays is made. Difficulties in the interpretation of animal studies and the extrapolation of animal data to estimate human risk is discussed. Each class of chemicals, such as the aromatic aminatic amines or the non-nutritive sweetners, presents its unique problems when attempts are made to evaluate its potential as a causative agent in the etiology of bladder cancer.