Application of in Vitro Biotransformation Data and Pharmacokinetic Modeling to Risk Assessment
The adverse biological effects of toxic substances are dependent upon the exposure concentration and the duration of exposure. Pharmacokinetic models can quantitatively relate the external concentration of a toxicant in the environment to the internal dose of the toxicant in the target tissues of an exposed organism. The exposure concentration of a toxic substance is usually not the same as the concentration of the active form of the toxicant that reaches the target tissues following absorption, distribution, and biotransformation of the parent toxicant. Biotransformation modulates the biological activity of chemicals through bioactivation and detoxication pathways. Many toxicants require biotransformation to exert their adverse biological effects. Considerable species differences in biotransformation and other pharmacokinetic processes can make extrapolation of toxicity data from laboratory animals to humans problematic. Additionally, interindividual differences in biotransformation among human populations with diverse genetics and lifestyles can lead to considerable variability in the bioactivation of toxic chemicals. Compartmental pharmacokinetic models of animals and humans are needed to understand the quantitative relationships between chemical exposure and target tissue dose as well as animal to human differences and interindividual differences in human populations. The data-based compartmental pharmacokinetic models widely used in clinical pharmacology have little utility for human health risk assessment because they cannot extrapolate across dose route or species. Physiologically based pharmacokinetic (PBPK) models allow such extrapolations because they are based on anatomy, physiology, and biochemistry. In PBPK models, the compartments represent organs or groups of organs and the flows between compartments are actual blood flows. The concentration of a toxicant in a target tissue is a function of the solubility of the toxicant in blood and tissues (partition coefficients), blood flow into the tissue, metabolism of the toxicant in the tissue, and blood flow out of the tissue. The appropriate degree of biochemical detail can be added to the PBPK models as needed. Comparison of model simulations with experimental data provides a means of hypothesis testing and model refinement. In vitro biotransformation data from studies with isolated liver cells or subcellular fractions from animals or humans can be extrapolated to the intact organism based upon protein content or cell number. In vitro biotransformation studies with human liver preparations can provide quantitative data on human interindividual differences in chemical bioactivation. These in vitro data must be integrated into physiological models to understand the true impact of interindividual differences in chemical biotransformation on the target organ bioactivation of chemical contaminants in air and drinking water.