Metabolism of sulfinpyrazone sulfide and sulfinpyrazone by human liver microsomes and cDNA-expressed cytochrome P450s

Human liver microsomes catalyze the oxidation of sulfinpyrazone sulfide (SP ZS) to a variable mixture of sulfinpyrazone (SPZ) enantiomers and two minor phenolic metabolites. In one, the thiophenyl ring is hydroxylated, whereas in the second an N-phenyl ring is hydroxylated. SPZ is further oxidized to sulfinpyrazone sulfone (SPZO) and a minor polar metabolite that also has a n N-phenyl ring hydroxylated. Determination of the metabolism of SPZ and SP ZS under modified incubation conditions of prior heat treatment, higher pH, and the presence of detergent indicated that the formation of SPZ was cyto chrome P450 (P450)- but not flavin monooxygenase-dependent. Specific P450 i nhibitors (sulfaphenazole, quinidine sulfate, coumarin, diethyldithiocarbam ic acid, troleandomycin, and furafylline) and specific cDNA-expressed P450s were used to identify the major isoforms responsible for the oxidation of SPZS to SPZ and SPZ to SPZO. Both P450 2C9 and P450 3A4 were responsible fo r the oxidation of SPZS to SPZ, whereas P450 3A4 alone catalyzed the furthe r oxidation of SPZ to SPZO. SPZS was found to be metabolized by P450 2C9 to SPZ with a high degree of enantiomeric selectivity (9:1) and a K-m compara ble with its previously determined K-i for inhibition of the P450 2C9-depen dent 7-hydroxylation of (S)-warfarin (WARF). In contrast, the P450 3A4-cata lyzed oxidation of SPZS to SPZ proceeded with the same enantioselectivity b ut to a much lesser degree (58:42). These results provide evidence that the metabolism of both (S)- WARF and SPZS is mediated by a common enzyme, P450 2C9, which is central to understanding the WARF-SPZ interaction and SPZS-m ediated drug interactions in general.