Activation of CYP3A4‐mediated letrozole metabolism by ketoconazole (LB593)

Abstract

The objective was to investigate the effect of ketoconazole, an imidazole‐containing inhibitor of CYP enzymes, on the metabolism of letrozole, a triazole‐containing inhibitor of CYP19. The inhibition of cytochrome P450 enzymes by drugs that contain nitrogen heterocycles, for example, imidazole‐ and triazole‐containing agents, is well documented and involves the ligation of the nitrogen heterocycle to the heme iron. Letrozole metabolism was evaluated in microsomal and recombinant systems by measuring the formation of 4,4'‐hydroxymethylene‐dibenzonitrile, the major metabolite. A 12.3% increase in the formation of 4,4'‐hydroxymethylene‐dibenzonitrile was observed in the presence of ketoconazole (1 µM), a well‐known and potent inhibitor of CYP3A4 and CYP51, at letrozole concentrations within therapeutic plasma concentrations (e.g., 0.5 µM). In microsomal incubations the effect increased with ketoconazole concentration (0.005 µM to 10 µM) up to 26% and was observed in three different sources of human liver microsomes (50 and 150 donor‐pools from BD Gentest and a 50‐donor pool from Celsis). The activation is mediated through CYP3A. Concentration‐dependent activation was also observed in experiments with recombinant CYP3A4 (14.6 ± 3.6 and 25.6 ± 4.0% activation with 0.1 and 1 µM ketoconazole, respectively). Azamulin (1 µM), the CYP3A‐specific inhibitor, eliminated the activation effect in microsomal and recombinant systems. Ketoconazole metabolism in these systems did not generate peaks that coeluted with 4,4'‐hydroxymethylene‐dibenzonitrile, as detected by HPLC‐fluorescence. The surprising results reported here indicate that the nitrogen heterocycle ketoconazole can act as a heterotropic activator of CYP3A4‐mediated letrozole metabolism, suggesting that drugs that contain nitrogen heterocycles can have effects that extend beyond the normally expected CYP inhibition.