Citation:

KENNEKE, J. F., C. S. MAZUR, W. M. HENDERSON, A. W. GARRISON, S. E. RITGER, T. SACK, C. BROWN, AND J. K. AVANTS. XENOBIOTIC METABOLISM RESEARCH AND ITS APPLICATION TO HUMAN AND ECOLOGICAL EXPOSURE AND RISK ASSESSMENT. Presented at Society of Toxicology Annual Meeting, Seattle, WA, March 16 - 20, 2008.

Impact/Purpose:

This task is divided into four major research areas: (1) Development of computational tools and databases for screening-level modeling of the environmental fate of organic chemicals; (2) Metabolism of xenobiotics: Enhancing the development of a metabolic simulator; (3) Metabonomics: The use of advanced analytical tools to identify toxicity pathways; and (4) Software infrastructure to support development and application of transformation/metabolic simulators.

For many chemicals, multiple transformation/metabolic pathways can exist. Consequently, transformation/metabolic simulators must utilize transformation rate data for prioritization of competing pathways. The prioritization process thus requires the integration of reliable rate data. When this data is absent, it is necessary to generate a database with metabolic and transformation rate constants based on: (1) experimentally measured values, including those requiring the use of advanced analytical techniques for measuring metabolic rate constants in vivo and in vitro; (2) rate constants derived from SPARC and mechanistic-based QSAR models; and (3) data mined from the literature and Program Office CBI. A long-term goal of this project is to build this database. This information will be used to enhance the predictive capabilities of the transformation/metabolic simulators. As indicated previously, exposure genomics, which provide early signs of chemical exposure based on changes in gene expression, will be used to guide chemical fate and metabolism studies. The incorporation of exposure genomics into fate studies will provide information concerning (1) the minimal concentrations at which biological events occur; and (2) the identification of biologically relevant chemicals(s) in mixtures.

The capability of categorizing chemicals and their metabolites based on toxicity pathway is imperative to the success of the CompTox Research Program. Metabonomics, which is the multi-parametric measurement of metabolites in living systems due to physiological stimuli and/or genetic modification, provides such a capability. The application of metabonomics to toxicity testing involves the elucidation of changes in metabolic patterns associated with chemical toxicity based on the measurement of component profiles in biofluids, and enables the generation of spectral profiles for a wide range of endogenous metabolites. Metabolic profiles can provide a measure of the real outcome of potential changes as the result of xenobiotic exposure.

Description:

A major uncertainty in risk assessment is determining the exposure of a target organism to a chemical stressor, and a confounding factor is the transformation of the chemical to a toxic metabolite inside the target organism. Physiologically-based pharmacokinetic (PBPK) models are the preferred tool to describe the fate of xenobiotics in physiological systems, since these models can be used to evaluate target tissue dose relative to route of uptake. One major obstacle to fully implementing PBPK models for risk assessment is the lack of xenobiotic metabolism data for a majority of organic chemicals in use. This data gap becomes an even greater obstacle when risk is extrapolated across species such as from rat to human or to an ecologically important organism. In an effort to close this data gap, we develop and apply innovative techniques for elucidating the kinetics and mechanisms of xenobiotic metabolism, and apply these techniques to the understanding and modeling of chemical exposure for informing human health and ecological risk assessment. Unique to our program, concomitant substrate depletion and metabolite formation kinetic parameters (i.e., Vmax and KM) are determined using non-linear models. Specific enzyme inhibitors, purified recombinant human cytochrome P450s (CYPs), stable isotopes, stereoselectivity and molecular docking are used to elucidate the mechanisms of xenobiotic metabolism. Kinetic parameters and mechanistic information are integrated to develop PBPK models, which are then validated in vivo. The entire paradigm, from metabolite map elucidation to PBPK model development, is applied across multiple species (e.g., human, rodent, fish, crustacean and insect) to develop and test cross-species extrapolations for human health and ecological risk assessment. We will illustrate this paradigm and its application to risk assessment using results from 20 conazole fungicides and the piscicide antimycin A.

Record Details: