Abstract

Atypical (non-Michaelis-Menten) kinetics are commonly observed with CYP3A4 substrates in vitro. If relevant in vivo, cytochrome P450 heteroactivation could give rise to increased drug clearance. To test the possible in vivo relevance of atypical cytochrome P450 kinetics, we investigated the role of heteroactivation in the therapeutically relevant drug interaction between the anti-epileptics felbamate and carbamazepine. Felbamate heteroactivates CYP3A4-mediated formation of carbamazepine-10,11-epoxide (carbamazepine-ep), the major metabolite of carbamazepine, in human liver microsomes and recombinant CYP3A4 at relevant in vivo concentrations of both drugs (maximum activation 98% at 10 μM carbamazepine, 1 mM felbamate). Felbamate (50–500 μM) did not induce CYP3A4, as based on mRNA measurements in human liver slices. The further metabolism of carbamazepine-ep was inhibited (38% by 500 μM felbamate) in human liver slices. We propose a methodology to predict changes in steady-state plasma concentrations (Css) of parent drug and metabolite from in vitro heteroactivation and inhibition data, including prediction of the increase in fraction metabolized. A meta-analysis of reported in vivo effects of felbamate on Css_{carbamazepine} was performed to allow evaluation of this approach. The predicted effect of in vitro heteroactivation on Css_{carbamazepine} corresponds well to that observed in vivo. Combining the effect of heteroactivation on the fraction metabolized to carbamazepine-ep, and inhibition of its further metabolism, predicts a change in Css_{carbamazepine-ep} that falls within the range observed in vivo. Our results strongly suggest that in vivo heteroactivation of CYP3A4 is a possible mechanism of the clinically observed drug interaction between felbamate and carbamazepine.