Physiologically based pharmacokinetic analyses of simple mixtures.

Exposure to multiple chemicals may cause significant alterations of tissue dose of the toxic moiety of one or more of the individual chemicals. The change in target tissue dose of a chemical present in simple mixtures can be predicted when the determinants of disposition of each chemical, and the mechanism of toxicokinetic interaction between chemicals are understood at a quantitative level. Determinants of disposition include physiological (e.g., breathing rates, cardiac output, tissue volumes, blood flow rates), biochemical (e.g., kinetic constants for metabolism and protein binding), and physicochemical factors (e.g., blood air and tissue blood partition coefficients). Mechanisms of toxicokinetic interactions refer to the manner in which coexposure alters these determinants of disposition as compared to exposure to the individual chemicals. Interactions between chemicals can be described quantitatively with physiologically based pharmacokinetic (PBPK) models, which integrate these mechanic determinants and permit prediction of alterations in tissue dose for various exposure situations by computer simulation. PBPK modeling studies of binary chemical interactions conducted so far indicate that inhibitory rather than potentiating metabolic interactions are more likely to be observed during multiple chemical exposures. As PBPK models of representative binary, tertiary and quaternary mixtures are developed, it will become increasingly possible to draw reliable conclusions about the risk associated with human exposure to chemical mixtures.