A model describing the disposition of phenytoin in isolated rat hepatocytes

The use of isolated rat hepatocytes in studies of drug metabolism has become well documented in the past few years. However, in part because of modelling difficulties due to the simultaneously occurring substrate transferring processes, its predictability of in vivo situations has not been emphasized. Much controversy surrounds the metabolism of phenytoin (5,5-diphenylhydantoin), a widely used anticonvulsant, and an appropriate pharmacokinetic model to describe the disposition of this drug still lacks general acceptance. In the present study, metabolism of phenytoin in the isolated rat hepatocyte system was followed by assaying either the unchanged drug or the pooled metabolites in both the suspending medium and the cells. A model was developed which can describe the time course of the different species sampled. Inhibition of biotransformation by the major metabolic product [5-(p-hydroxyphenyl)-5-phenylhydantoin or p-HPPH] and the uptake and release of the latter were also studied, in order to elucidate the role of product inhibition in determining the dose-dependent pharmacokinetic behaviour of the drug. The results obtained strongly suggest that only concentrations of p-HPPH higher than the ones attained by phenytoin biotransformation alone can significantly inhibit the main enzymatic reaction.