0090-9556/97/2506-0716-0724$02.00/0
DRUG METABOLISM AND DISPOSITION
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
Vol. 25, No. 6

CYTOCHROME P450 RESPONSIBLE FOR THE STEREOSELECTIVE S-OXIDATION OF FLOSEQUINAN IN HEPATIC MICROSOMES FROM RATS AND HUMANS

Eiji Kashiyama, Tsuyoshi Yokoi, Masaaki Odomi, Yoshihiko Funae, Kazuaki Inoue, and Tetsuya Kamataki

Division of Drug Metabolism (E.K., T.Y., T.K.), Faculty of Pharmaceutical Sciences, Hokkaido University; Department of Drug Metabolism (M.O.), Tokushima Research Institute, Otsuka Pharmaceutical Co., Ltd.; Laboratory of Chemistry (Y.F.), Osaka City University; and Department of Pathology (K.I.), Hokkaido University Hospital

The forms of cytochrome P450 involved in the stereoselective S-oxidation of flosequinan [(±)-7-fluoro-1-methyl-3-methylsulfinyl-4-quinolone] were investigated in vitro using liver microsomes from rats and humans. Rat liver microsomes supplemented with NADPH catalyzed the four different S-oxidations, which were from flosequinan sulfide (FS; 7-fluoro-1-methyl-3-methylthio-4-quinolone) to R(+)- and S()-flosequinan (R-FSO and S-FSO, respectively) and from R-FSO and S-FSO to flosequinan sulfone (FSO₂; 7-fluoro-1-methyl-3-methylsulfonyl-4-quinolone). The activities of all the S-oxidases in liver microsomes from male rats were higher than those from female rats. The activities of the S-oxidases measured at a high substrate concentration (1 mM) were induced by treatment of rats with phenobarbital and dexamethasone. Treatment of rats with 3-methylcholanthrene also induced the activities, but only at a low substrate concentration (50 µM), except for the S-oxidase catalyzing the reaction from FS to R-FSO. Enzymes induced by clofibrate and ethanol were not involved in the oxidations at a low substrate concentration. The activities of S-oxidases were correlated with the contents of cytochrome P450 (CYP)3A enzymes. Anti-CYP3A2 antisera inhibited the activities of the S-oxidases catalyzing the reactions from FS to R-FSO (40%) and to S-FSO (60%) at the high substrate concentration and inhibited the activities of the S-oxidases, thus catalyzing reactions from R-FSO and S-FSO to FSO₂ (70%) at both high and low substrate concentrations. These results suggest that CYP3A is the major enzyme involved in all S-oxidation pathways in flosequinan metabolism in rats. On the other hand, except for the S-oxidation of FS to R-FSO, the rates of the other three S-oxidations by liver microsomes from 30 individual humans correlated highly with each other, suggesting that the same enzyme would be involved in the three S-oxidations. Anti-CYP3A2 antisera inhibited the activities of all the S-oxidases in human liver microsomes ranging from 40 to 80%, suggesting that CYP3A is also involved in all of the S-oxidations of flosequinan in humans.