Abstract

Regioselective and stereoselective oxidations of pseudoracemic metoprolol, (R)-bufuralol, and (S)-bufuralol by microsomes of h2D6v2 cells--a human lymphoblastoma cell line transfected with a cytochrome P4502D6 expression system--were examined. The formation kinetics of O-demethylmetoprolol and alpha-hydroxymetoprolol were characterized in five different lots of the cDNA-expressed P4502D6. Comparison of the Vmax/KM values indicated that formation of the products from (R)-metoprolol was preferred. Although the favored regiomer overall was O-demethylmetoprolol, the regioselectivity for O-demethylation of metoprolol by the cDNA-expressed enzyme was several-fold less than that observed for the P4502D6 enzyme in human liver microsomes at 20 microM pseudoracematic metoprolol concentration. Oxidation of (R)-metoprolol produced more O-demethylmetoprolol than alpha-hydroxymetoprolol; however, for (S)-metoprolol-d2, a slight preference for alpha-hydroxylation was observed. The O-demethylation and alpha-hydroxylation of metoprolol were inhibited at low microM concentrations of (+/-)-verapamil, a known inhibitor of metoprolol oxidation. (R)- and (S)-Bufuralol were oxidized to their respective diastereomeric 1"-hydroxybufuralols by all 4 lots of h2D6v2 microsomal preparations. Diastereomeric (1'R)-hydroxybufuralols were formed in twice the amount as the hydroxylated diastereomers of (1'S)-products. Product stereoselectivity was observed for the (1'R,1"S)- and (1'S,1"R)-isomers. Although the observed enantioselectivity and diastereoselectivity of the bufuralol oxidation seem to be consistent with those previously reported for human liver microsomes, the regioselectivity of the metoprolol oxidations is unexpectedly low.