Human hepatic cytochrome P450 2C9 catalyzes the rate-limiting pathway of torsemide metabolism

Tolyl methylhydroxylation is the rate-limiting step in the elimination of torsemide, a newly developed diuretic, in humans in vivo. Kinetic and inhibitor studies with human liver microsomes and complementary DNA-expressed enzyme were performed to identify the cytochrome P450 (CYP) isoform responsible for torsemide tolyl methylhydroxylation to predict factors that might alter clearance in patients receiving torsemide. As in vivo, tolyl methylhydroxylation was the major biotransformation pathway in human liver microsomes. Microsomal tolyl methylhydroxylation kinetics followed Michaelis- Menten kinetics, with the mean apparent K(m) for the reaction being 11.2 ± 1.3 μM. The microsomal reaction was almost completely abolished by the specific CYP2C9 inhibitor sulfaphenazole and was inhibited competitively by the alternative CYP2C9 substrate tolbutamide. Torsemide tolyl methylhydroxylase activity in microsomes from 16 human livers correlated significantly (r(s) = .81-.88) with tolbutamide and phenytoin hydroxylation, both CYP2C9-mediated reactions. Complementary DNA-expressed CYP2C9 catalyzed torsemide tolyl methylhydroxylation with an apparent K(m) (23 μM) similar to that observed for human liver microsomes and the IC₅₀ values for sulfaphenazole inhibition of the reaction were essentially identical for the two enzyme sources. Taken together, these data demonstrate that human hepatic torsemide tolyl methylhydroxylation is catalyzed predominantly, if not solely, by CYP2C9. The implications of this finding for the regulation of torsemide metabolism in vivo are discussed.