REGIOSELECTIVE AND STEREOSELECTIVE OXIDATION OF METOPROLOL AND BUFURALOL CATALYZED BY MICROSOMES CONTAINING CDNA-EXPRESSED HUMAN P4502D6

Regioselective and stereoselective oxidations of pseudoracemic metopro lol, (R)-bufuralol, and (S)-bufuralol by microsomes of h2D6v2 cells-a human lymphoblastoma cell line transfected with a cytochrome P4502D6 e xpression system-were examined, The formation kinetics of O-demethylme toprotol and alpha-hydroxymetoprolol were characterized in five differ ent lots of the cDNA-expressed P4502D6. Comparison of the V-max/K-M va lues indicated that formation of the products from (R)-metoprolol was preferred. Although the favored regiomer overall was O-demethylmetopro lol, the regioselectivity for O-demethylation of metoprolol by the cDN A-expressed enzyme was several-fold less than that observed for the P4 502D6 enzyme in human liver microsomes at 20 mu M pseudoracematic meto prolol concentration. Oxidation of (R)-metoprolol produced more O-deme thylmetoprolol than alpha-hydroxymetoprolol; however, for (S)-metoprol ol-d(2), a slight preference for alpha-hydroxylation was observed. The O-demethylation and alpha-hydroxylation of metoprolol were inhibited at low mu M concentrations of (+/-)-verapamil, a known inhibitor of me toprolol oxidation. (R)- and (S)-Bufuralol were oxidized to their resp ective diastereomeric 1 ''-hydroxybufuralols by ail 4 lots of h2D6v2 m icrosomal preparations. Diastereomeric (1'R)-hydroxybufuralols were fo rmed in twice the amount as the hydroxylated diastereomers of (1'S)-pr oducts. Product stereoselectivity was observed for the (1'R,1 '' S)-an d (1'S,1 '' R)-isomers. Although the observed enantioselectivity and d iastereoselectivity of the bufuralol oxidation seem to be consistent w ith those previously reported for human liver microsomes, the regiosel ectivity of the metoprolol oxidations is unexpectedly low.