This study aimed to characterize the inactivation kinetics of cytochrome P450 3A4 (CYP3A4) by erythromycin, which involves mechanism-based inhibition (MBI), in detail. In addition to an MBI assay based on the conventional method in which erythromycin and recombinant CYP3A4 were pre-incubated for 15 min, the study also evaluated the long-term MBI kinetics of this reaction by pre-incubation for 120 min. Mechanism-based inhibition profiles were obtained using three typical substrates, testosterone, midazolam and nifedipine. In the long-term assay, erythromycin evoked a time-dependent biphasic reduction in enzyme activity, but some residual activity (α) was detected in the terminal phase. The inactivation rate constant obtained in the presence of 30 μm erythromycin using nifedipine as a substrate was 1.44-fold higher than that acquired using testosterone, while there was no difference among the α values obtained with the three substrates. In the short-term assay, time-dependent monophasic inactivation was observed. To extrapolate these data to in vivo, the extent of the increase in the area under the curve (AUC ratio) induced by erythromycin was estimated from the results of the conventional short-term experiment and the long-term experiment examining residual activity. The AUC ratio estimated from the long-term kinetics (2.92) was closer to the clinically reported values (3.3–4.42). In conclusion, the relatively long-term evaluation of the kinetics of CYP3A4 inactivation revealed that the enzyme was not fully inactivated by erythromycin. To improve the estimation of the extent of the drug–drug interactions induced by MBI from in vitro data, longer-term investigations of the target enzyme's inactivation profile might be necessary.
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