Stereoselective glucuronidation of 5-(4'-hydroxyphenyl)-5phenylhydantoin by human UDP-Glucuronosyltransferase (UGT) 1A1, UGT1A9, and UGT2B15: Effects of UGT-UGT interactions

5-(4 '-Hydroxyphenyl)-5-phenylhydantoin (4 '-HPPH), a major metabolite of phenytoin in human, is exclusively metabolized to a glucuronide. 4 '-HPPH has a chiral center. (S)-4 '-HPPH is a predominant form produced from phenytoin in humans, and (R)-4 '-HPPH is an extremely toxic form with respect to gingival hyperplasia. In the present study, we investigated stereoselective 4 '-HPPH O-glucuronide formation in human liver microsomes. Human liver microsomes predominantly formed (S)-4 '-HPPH O-glucuronide rather than ( R)-4 '-HPPH O-glucuronide from racemic 4 '- HPPH. Among human UDP-glucuronosyltransferase (UGT) enzymes, UGT1A1, UGT1A9, and UGT2B15 showed 4 '-HPPH O-glucuronide formation. Interestingly, UGT1A1 stereoselectively formed (R)-4 '-HPPH O-glucuronide, whereas UGT1A9 and UGT2B15 stereoselectively formed (S)-4 '-HPPH O-glucuronide from racemic 4 '-HPPH. By using UGT1A double-expression systems in HEK293 cells that we previously established, the effects of UGT-UGT interactions on 4 '-HPPH O-glucuronide formation were investigated. It was demonstrated that coexpression of UGT1A4 increased the V-max values of (S)- and (R)-4 '-HPPH O-glucuronide formation catalyzed by UGT1A1 but decreased the V-max values of (S)- and (R)-4 '-HPPH O-glucuronide formation catalyzed by UGT1A9. Coexpression of UGT1A6 increased the S-50 values and decreased the Vmax values of (S)- and (R)-4 '-HPPH glucuronide formation catalyzed by UGT1A1 and UGT1A9. However, the interaction did not alter the stereoselectivity. In conclusion, we found that 4 '-HPPH O-glucuronide formation in human liver microsomes is catalyzed by UGT1A1, UGT1A9, and UGT2B15 in a stereoselective manner, being modulated by interaction with other UGT1A isoforms.