Prediction of isoprene diepoxide levels in vivo in mouse, rat and man using enzyme kinetic data in vitro and physiologically-based pharmacokinetic modelling

The present study was designed to explain the differences in isoprene toxic ity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the huma n situation was attempted. For this purpose, enzyme kinetic parameters K-m and V-max were determined in vitro in mouse, rat and human liver microsomes /cytosol for the cytochrome P450-mediated formation of isoprene mono- and d iepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diep oxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences rega rding isoprene diepoxide levels were forecasted. Almost similar isoprene di epoxide liver and lung concentrations were predicted in mouse and rat, whil e predicted levels in humans were about 20-fold lower. However, when interi ndividual variation in enzyme activity was introduced in the human model, t he levels of isoprene diepoxide changed considerably. It was forecasted tha t in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentration s in the liver increased to similar concentrations as predicted for the mou se. However, the interpretation of the latter finding for human risk assess ment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxi de concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining th e extent of isoprene toxicity. In order to confirm this, in vivo experiment s are required in which isoprene epoxide concentrations will be established in rats and mice. (C) 2001 Elsevier Science Ireland Ltd. All rights reserv ed.