The oxidative metabolism of metoprolol in human liver microsomes: inhibition by the selective serotonin reuptake inhibitors

Objective: Biotransformation of metoprolol to alpha-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) is mediated by CYP2D6. The selective serotonin reuptake inhibitors (SSRIs) are known to inhibit CYP2D6. The aim was to study in vitro the potential inhibitory effect of SSRIs on metoprolol biotransformation. Methods: Using microsomes from two human livers, biotransformation of metoprolol to alpha-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) as a function of the concentrations of the SSRIs and of some of their metabolites was studied. Results: The kinetics of the formation of both metabolites are best described by a biphasic enzyme model. The estimated values of V-max and k(M) for the high affinity site are for the alpha-hydroxylation in human liver HL-1 32 mu mol mg(-1).min(-1) and 75 mu mol.l(-1) respectively, and in human liver HL-9 39 mu mol mg(-1).min(-1) and 70 mu mol.l(-1) respectively; for the O-demethylation in HL-1 131 pmol mg(-1) min(-1) and 95 mu mol.l(-1) respectively, and in HL-9 145 pmol mg(-1) min(-1) and 94 mu mol.l(-1) respectively. Quinidine is for both pathways a potent inhibitor of the high-affinity site, with K-i values ranging from 0.03 to 0.18 mu mol.l(-1). Fluoxetine, norfluoxetine and paroxetine are likewise potent inhibitors, with K-i values ranging from 0.30 to 2.1 mu mol.l(-1) fluvoxamine, sertraline, desmethylsertraline, citalopram and desmethylcitalopram are less potent inhibitors, with K-i values above 10 mu mol.l(-1). Conclusion: The rank order of the SSRIs for inhibition of metoprolol metabolism is comparable to that reported in the literature for other CYP2D6 substrates, with fluoxetine, norfluoxetine and paroxetine being the most potent. These findings need further investigation to determine their clinical relevance.