Pharmacokinetic factors and their implication in the induction of mouse liver tumors by halogenated hydrocarbons

Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene

The guidelines for carcinogen risk assessment recently proposed by the U.S. Environmental Protection Agency (U.S. EPA) provide an increased opportunity for the consideration of pharmacokinetic and mechanistic data in the risk assessment process. However, the greater flexibility of the new guidelines can also make their actual implementation for a particular chemical highly problematic. To illuminate the process of performing a cancer risk assessment under the new guidelines, the rationale for a state-of-the-science risk assessment for trichloroethylene (TCE) is presented. For TCE, there is evidence of increased cell proliferation due to receptor interaction or cytotoxicity in every instance in which tumors are observed, and most tumors represent an increase in the incidence of a commonly observed, species-specific lesion. A physiologically based pharmacokinetic (PBPK) model was applied to estimate target tissue doses for the three principal animal tumors associated with TCE exposure: liver, lung, and kidney. The lowest points of departure (lower bound estimates of the exposure associated with 10% tumor incidence) for lifetime human exposure to TCE were obtained for mouse liver tumors, assuming a mode of action primarily involving the mitogenicity of the metabolite trichloroacetic acid (TCA). The associated linear unit risk estimates for mouse liver tumors are 1.5 x 10(-6) for lifetime exposure to 1 microg TCE per cubic meter in air and 0.4 x 10(-6) for lifetime exposure to 1 microg TCE per liter in drinking water. However, these risk estimates ignore the evidence that the human is likely to be much less responsive than the mouse to the carcinogenic effects of TCA in the liver and that the carcinogenic effects of TCE are unlikely to occur at low environmental exposures. Based on consideration of the most plausible carcinogenic modes of action of TCE, a margin-of-exposure (MOE) approach would appear to be more appropriate. Applying an MOE of 1000, environmental exposures below 66 microg TCE per cubic meter in air and 265 microg TCE per liter in drinking water are considered unlikely to present a carcinogenic hazard to human health.

Physical activity and cancer prevention: animal-tumor models

PURPOSE

The aims of this paper are to briefly review the types of animal and tumor models used in carcinogenesis studies and to consider their utility in studies of physical activity and cancer.

METHODS

Published data from animal studies using tumor models for which corresponding human epidemiologic evidence shows a clear association between physical activity and that cancer (i.e., colon and breast) are reviewed.

RESULTS

A variety of animal-tumor models have been used in cancer studies including spontaneous tumors, chemically induced tumors, orthotopic and syngeneic tumor transplantation, injected tumors, and genetically engineered (transgenic, knockout, and mutation-induced) mice with a predisposition to neoplasia. The most commonly used animal-tumor model in the study of physical activity has been the chemical carcinogenesis model. Methodological limitations of the various animal-tumor models are described including variations in dosing, route of administration, and type of carcinogen used, and forced versus voluntary exercise effects.

CONCLUSIONS

Animal-tumor models are useful for understanding specific aspects of the carcinogenesis process and the interaction of this process with exercise. There is no one animal-tumor model that is ideally suited for studying physical activity interventions. However, animal-tumor models can be viewed as complementary to epidemiologic studies and human clinical trials in the area of physical activity and cancer prevention.

Combination of cancer data in quantitative risk assessments: case study using bromodichloromethane

There are often several data sets that may be used in developing a quantitative risk estimate for a carcinogen. These estimates are usually based, however, on the dose-response data for tumor incidences from a single sex/strain/species of animal. When appropriate, the use of more data should result in a higher level of confidence in the risk estimate. The decision to use more than one data set (e.g., representing different animal sexes, strains, species, or tumor sites) can be made following biological and statistical analyses of the compatibility of the these data sets. Biological analysis involves consideration of factors such as the relevance of the animal models, study design and execution, dose selection and route of administration, the mechanism of action of the agent, its pharmacokinetics, any species- and/or sex-specific effects, and tumor site specificity. If the biological analysis does not prohibit combining data sets, statistical compatibility of the data sets is then investigated. A generalized likelihood ratio test is proposed for determining the compatibility of different data sets with respect to a common dose-response model, such as the linearized multistage model. The biological and statistical factors influencing the decision to combine data sets are described, followed by a case study of bromodichloromethane.