Vol. 303, Issue 1, 172-178, October 2002

Stereoselective Metabolism of Bufuralol Racemate and Enantiomers in Human Liver Microsomes

Shizuo Narimatsu, Chie Takemi, Daisuke Tsuzuki, Hiroyuki Kataoka, Shigeo Yamamoto, Noriaki Shimada, Satoshi Suzuki, Tetsuo Satoh, Urs A. Meyer and Frank J. Gonzalez

Laboratories of Health Chemistry (S.N., C.T., D.T., H.K.) and Biomolecular Sciences (S.Y.), Faculty of Pharmaceutical Sciences, Okayama University, Okayama, Japan; Daiichi Pure Chemicals, Tokyo, Japan (N.S.); Biomedical Research Institutes, Chiba, Japan (S.S., T.S.); Biozentrum, University of Basel, Basel Switzerland (U.A.M.); and National Cancer Institute, National Institutes of Health, Bethesda, Maryland (F.J.G.)

A new HPLC method was developed using a chiral column to efficiently separate four 1''-hydroxybufuralol (1''-OH-BF) diastereomers that are major metabolites of bufuralol (BF). Employing this method, we examined diastereomer selectivity in the formation of 1''-OH-BF from BF racemate or enantiomers in four individual samples of human liver microsomes. Three different human liver microsomes showed a selectivity of 1''R-OH < 1''S-OH for BF enantiomers, which was similar to that of recombinant CYP2D6 expressed in insect cell microsomes, whereas one human liver microsomal fraction yielded a selectivity of 1''R-OH > 1''S-OH for BF enantiomers, which was similar to those of recombinant CYP2C19 expressed in insect cell microsomes. Recombinant CYP1A2 and CYP3A4 showed a selectivity similar to that of CYP2D6, but their BF 1''-hydroxylase activities were much lower than those of CYP2D6. In inhibition studies, quinidine, a known CYP2D6 inhibitor, markedly inhibited BF 1''-hydroxylation in the fractions of human liver microsomes that showed the CYP2D6-type selectivity. Furthermore, omeprazole, a known CYP2C19 inhibitor, efficiently suppressed the formation of 1''-OH-BF diastereomers from BF in the microsomal fraction that showed the CYP2C19-type selectivity. From these results, we concluded that the diastereomer selectivity in the formation of 1''-OH-BF from BF differs between CYP2D6 and CYP2C19, both of which can be determinant enzymes in the diastereoselective 1''-hydroxylation of BF in human liver microsomes.