PT  - JOURNAL ARTICLE
AU  - Robert S Foti
AU  - Dan A Rock
AU  - Josh T Pearson
AU  - Jan L Wahlstrom
AU  - Larry C Wienkers
TI  - Mechanism-Based Inactivation of Cytochrome P450 3A4 by Mibefradil Through Heme Destruction
AID  - 10.1124/dmd.111.038505
DP  - 2011 Jan 01
TA  - Drug Metabolism and Disposition
PG  - dmd.111.038505
4099  - http://dmd.aspetjournals.org/content/early/2011/03/29/dmd.111.038505.short
4100  - http://dmd.aspetjournals.org/content/early/2011/03/29/dmd.111.038505.full
AB  - Mibefradil (Posicor™) was developed as a calcium channel blocker for the treatment of chronic hypertension. The compound was withdrawn from the market in 1998 due to the potential of rhabdomyolysis, renal failure or bradycardia when co-administered with other drugs. Mibefradil has previously been shown to be a potent reversible (IC50 = 0.3 – 2 µM) and mechanism-based (KI = 2.3 µM; kinact = 0.4 min-1) inhibitor of CYP3A4 catalyzed statin metabolism. Currently the mechanism of CYP3A4 inactivation by mibefradil is unknown. Mechanism-based inactivation experiments and spectral studies were used to examine the mechanism of CYP3A4 inactivation by mibefradil and its major metabolite, Ro 40-5966, in vitro. Both mibefradil and Ro 40-5966 were shown to exhibit Type I binding characteristics (Ks = 0.69 ± 0.06 µM and 1.39 ± 0.04 µM, respectively) towards CYP3A4. Complete KI/kinact experiments were performed, revealing a rapid and irreversible decrease in CYP3A4-catalyzed 1’-hydroxymidazolam formation. Approximately 70% of CYP3A4 activity was lost in the first minute of incubation with mibefradil and inactivation was non-linear after 2 minutes. Ro 40-5966 also resulted in time-dependent inhibition of CYP3A4, albeit to a lesser extent than mibefradil. The decrease in CYP3A4 activity in the presence of mibefradil and NADPH was subsequently shown to have a good correlation with the time-dependent loss of CO-binding, which coupled with the lack of stable heme and/or apoprotein adducts suggests heme destruction as the mechanism of inactivation of CYP3A4 by mibefradil.