Human CYP2C-mediated stereoselective phenytoin hydroxylation in Japanese: Difference in chiral preference of CYP2C9 and CYP2C19

Regio- and stereoselective hydroxylation of phenytoin was determined in liv er microsomes of nine extensive (EM) and three poor metabolizers (PM) of me phenytoin. Hydroxyphenytoins (HPPH) were isolated and quantified after sepa ration into four regio- and stereoisomers. The total rates of microsomal ph enytoin 4'- hydroxylation were approximately 3-fold higher than those of 3' -hydroxylation, and not significantly different in EM and PM. Formation of 4'-(R)-HPPH was 4.4-fold higher in EM than in PM, whereas no clear differen ces between EM and PM were detected in the formation of 4'-(S)-, 3'- (R)-, and 3'- (S)-HPPH. Cytochrome P450 (CYP)2C9, expressed in a fission yeast, S chizosaccharomyces pombe, catalyzed the formation of 4'-(R)- and 4'-(S)-HPP H stereoselectively, as observed with EM, in which predominantly 4'- (S)-HP PH was formed. Recombinant CYP2C19 was more stereoselective for 4'-(R)-HPPH formation. These results, in addition to inhibition experiments with anti- human CYP2C antibody, indicate that phenytoin hydroxylation is mainly catal yzed by CYP2C9. Furthermore, CYP2C19 showed limited contribution to phenyto in 4'-hydroxylation with a different chiral preference from CYP2C9. BIOCHEM PHARMACOL 57;11:1297-1303, 1999. (C) 1999 Elsevier Science Inc.