title: Inhibition of the enantioselective oxidative metabolism of metoprolol by verapamil in human liver microsomes.
abstract: In an effort to investigate the metabolic basis of previously reported pharmacokinetic interactions between beta-adrenergic antagonists and calcium channel blockers, the effects of verapamil on the oxidative metabolism of metoprolol were studied in microsomes isolated from four human livers. Deuterium-labeled pseudoracemic metoprolol was used to characterize the substrate stereoselectivity of this metabolic interaction. Biphasic kinetics were observed for both the alpha-hydroxylation and O-demethylation of metoprolol, suggesting that multiple P-450 enzymes with differing KMs are involved in the formation of these oxidative metabolites. alpha-Hydroxylation showed a slight preference for S-(-)-metoprolol, and it was largely mediated by a high-affinity enzyme over a wide range of substrate concentrations. The high-affinity component of the O-demethylation reaction exhibited significant selectivity for the R-(+)-enantiomer. The opposite enantioselectivity was observed for the low-affinity component of O-demethylation. Racemic verapamil inhibited both the alpha-hydroxylation and the O-demethylation of metoprolol. The kinetics of inhibition were consistent with competitive effects of verapamil on the high-affinity components of both oxidative pathways, which previously had been suggested to be mediated by CYP2D6. Potent inhibition of the low-affinity component of O-demethylation was also observed. The inhibitory effect of verapamil on the alpha-hydroxylation of metoprolol was not enantioselective. On the other hand, verapamil preferentially inhibited the O-demethylation of R-(+)-metoprolol compared with S-(-)-metoprolol at low substrate concentration.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle 98195.
authors.forename: M
authors.initials: M
authors.lastname: Kim
authors.forename: D D
authors.initials: DD
authors.lastname: Shen
authors.forename: A C
authors.initials: AC
authors.lastname: Eddy
authors.forename: W L
authors.initials: WL
authors.lastname: Nelson
authors.forename: L K
authors.initials: LK
authors.lastname: Roskos
mesh_terms: D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003640:Dealkylation; D003864:Depression, Chemical; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D008790:Metoprolol / Q000378:metabolism*; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D008954:Models, Biological; D010084:Oxidation-Reduction; D013237:Stereoisomerism; D014700:Verapamil / Q000494:pharmacology*
chemical_list: D003577:Cytochrome P-450 Enzyme System; D014700:Verapamil; D008790:Metoprolol