It was previously demonstrated that mibefradil, which shows mechanism-based inhibition in humans, also caused drug-drug interactions (DDIs) with midazolam (MDZ) in rats. In this study, we aimed to quantitatively predict the DDIs observed in rats using a physiologically based pharmacokinetic (PBPK) model from in vitro inactivation parameters. For more precise predictions, contribution ratios of cytochrome P450 (P450) isozymes involved in MDZ metabolism and inactivation parameters of mibefradil against each isozyme were incorporated in the predictive model. The evaluation of metabolic rate using recombinant P450s suggested that CYP3A2 and CYP2C11 contributed to 89 and 11% of MDZ metabolism, respectively. Inactivation studies of mibefradil against the two isozymes showed that the maximal inactivation rate constants (k(inact)) were considerable in both isozymes (0.231-0.565 min(-1)), whereas the inhibitor concentration producing half the k(inact) (K(I, app)) of CYP3A2 (0.263-0.410 μM) was a good deal lower than that for CYP2C11 (6.82-11.4 μM). As a result of predicting the DDIs using the PBPK model, predicted increases in areas under the concentration-time curve of MDZ with coadministration of mibefradil (284 and 510% at 6 and 12 mg/kg mibefradil, respectively) closely corresponded to the observed values (226 and 545%, respectively). From those results, it was thought that the construction of a predictive model for DDIs using the PBPK model in detail would enable us to quantitatively predict in vivo DDIs from in vitro data. This approach to predict DDIs on the basis of the contributing isozymes would be important for predicting clinical DDIs of drugs metabolized by multiple enzymes.