Received for publication November 5, 2007.
Revised March 19, 2008.
Accepted for publication March 20, 2008.

Role of Flavin-containing Monooxygenase in Oxidative Metabolism of Voriconazole by Human Liver Microsomes

Abstract

Voriconazole is a potent second generation triazole antifungal agent with broad-spectrum activity against clinically important fungi. It is cleared predominantly via metabolism in all species tested including humans. N-oxidation of the fluoropyrimidine ring, its hydroxylation, and hydroxylation of the adjacent methyl group are the known pathways of voriconazole oxidative metabolism, with the N-oxide being the major circulating metabolite in human. In vitro studies have shown that CYP2C19, CYP3A4, and to a lesser extent CYP2C9 contribute to the oxidative metabolism of voriconazole. When CYP-specific inhibitors and antibodies were used to evaluate the oxidative metabolism of voriconazole by human liver microsomes (HLM), the results suggested that CYP-mediated metabolism accounted for ~75% of the total oxidative metabolism. The studies presented here provide evidence that the remaining ~25% of the metabolic transformations are catalyzed by flavin-containing monooxygenase (FMO). This conclusion was based on the evidence that the NADPH-dependent metabolism of voriconazole was sensitive to heat (45 oC for 5 min), a condition known to selectively inactivate FMO without affecting CYP activity. The role of FMO in the metabolic formation of voriconazole N-oxide was confirmed by the use of recombinant FMO enzymes. Kinetic analysis of voriconazole metabolism by FMO1 and FMO3 yielded K_m values of 3.0 mM and 3.4 mM and V_max values of 0.025 pmol/min/pmol and 0.044 pmol/min/pmol, respectively. FMO5 did not metabolize voriconazole effectively. This is the first report of the role of FMO in the oxidative metabolism of voriconazole.

Key words: drug disposition, first-pass metabolism, flavin-containing monooxygenase, liver microsomes