Chloroethylene mixtures: pharmacokinetic modeling and in vitro metabolism of vinyl chloride, trichloroethylene, and trans-1,2-dichloroethylene in rat.

Environmental and occupational exposures are typically to mixtures of chemicals, although most toxicity information is for individual compounds. Interactions between chemicals may involve pharmacokinetic and/or pharmacodynamic effects resulting in modulation of toxicity. Therefore, physiologically based pharmacokinetic modeling has been used to analyze data describing the metabolism of vinyl chloride (VC) and trichloroethylene (TCE) mixtures in rats. A single saturable pathway was modeled, representing cytochrome P450 2E1. This was partially validated using preexposure to trans-1,2-dichloroethylene (tDCE) which virtually eliminated in vivo metabolism of both VC and TCE at low concentrations. Microsomes from tDCE-exposed animals showed inhibition of metabolism of P450 2E1 substrates (chlorzoxazone, p-nitrophenol, and TCE) and no effect on 7-ethoxycoumarin deethylation. Studies with liver microsomes from VC-exposed animals found that neither suicide inhibition nor induction occurred during 6-hr exposures to high concentrations. Therefore, these effects were not modeled. Modeling of mixtures of VC and TCE was successful only using competitive inhibition, as might be predicted for cytochrome P450 2E1 substrates, and not uncompetitive or noncompetitive inhibition. These results were further confirmed by determining the depletion of glutathione due to VC metabolism. The validation of a detailed model for the inhibition kinetics of metabolism of these two compounds permits better understanding of the implications of coexposures for toxicity. It is notable that competitive inhibition only becomes significant at relatively high concentrations (tens of ppm), while at typical low environmental concentrations (ppb), absorption is perfusion limited and enzyme is in excess so that the chemicals will be metabolized independently.