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The oxidative metabolism of metoprolol in human liver microsomes: inhibition by the selective serotonin reuptake inhibitors. 1998

F M Belpaire, and P Wijnant, and A Temmerman, and B B Rasmussen, and K Brøsen
Heymans Institute of Pharmacology, University of Gent Medical School, Belgium. frans.belpaire@rug.ac.be

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 Vmax and kM for the high affinity site are for the alpha-hydroxylation in human liver HL-1 32 pmol mg(-1) min(-1) and 75 micromol x l(-1) respectively, and in human liver HL-9 39 pmol mg(-1) x min(-1) and 70 micromol x l(-1) respectively; for the O-demethylation in HL-1 131 pmol mg(-1) min(-1) and 95 micromol x l(-1) respectively, and in HL-9 145 pmol mg(-1) min(-1) and 94 micromol x 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 micromol x l(-1). Fluoxetine, norfluoxetine and paroxetine are likewise potent inhibitors, with Ki values ranging from 0.30 to 2.1 micromol x l(-1) fluvoxamine, sertraline, desmethylsertraline, citalopram and desmethylcitalopram are less potent inhibitors, with K(i) values above 10 micromol x l(-1). CONCLUSIONS 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.

UI MeSH Term Description Entries
D007700 Kinetics The rate dynamics in chemical or physical systems.
D008297 Male Males
D008745 Methylation Addition of methyl groups. In histo-chemistry methylation is used to esterify carboxyl groups and remove sulfate groups by treating tissue sections with hot methanol in the presence of hydrochloric acid. (From Stedman, 25th ed) Methylations
D008790 Metoprolol A selective adrenergic beta-1 blocking agent that is commonly used to treat ANGINA PECTORIS; HYPERTENSION; and CARDIAC ARRHYTHMIAS. Beloc-Duriles,Betaloc,Betaloc-Astra,Betalok,CGP-2175,H 93-26,Lopressor,Metoprolol CR-XL,Metoprolol Succinate,Metoprolol Tartrate,Seloken,Spesicor,Spesikor,Toprol,Toprol-XL,Beloc Duriles,Betaloc Astra,CGP 2175,CGP2175,H 93 26,H 9326,Metoprolol CR XL,Toprol XL
D008862 Microsomes, Liver Closed vesicles of fragmented endoplasmic reticulum created when liver cells or tissue are disrupted by homogenization. They may be smooth or rough. Liver Microsomes,Liver Microsome,Microsome, Liver
D010084 Oxidation-Reduction A chemical reaction in which an electron is transferred from one molecule to another. The electron-donating molecule is the reducing agent or reductant; the electron-accepting molecule is the oxidizing agent or oxidant. Reducing and oxidizing agents function as conjugate reductant-oxidant pairs or redox pairs (Lehninger, Principles of Biochemistry, 1982, p471). Redox,Oxidation Reduction
D011802 Quinidine An optical isomer of quinine, extracted from the bark of the CHINCHONA tree and similar plant species. This alkaloid dampens the excitability of cardiac and skeletal muscles by blocking sodium and potassium currents across cellular membranes. It prolongs cellular ACTION POTENTIALS, and decreases automaticity. Quinidine also blocks muscarinic and alpha-adrenergic neurotransmission. Adaquin,Apo-Quinidine,Chinidin,Quincardine,Quinidex,Quinidine Sulfate,Quinora,Apo Quinidine,Sulfate, Quinidine
D002648 Child A person 6 to 12 years of age. An individual 2 to 5 years old is CHILD, PRESCHOOL. Children
D005473 Fluoxetine The first highly specific serotonin uptake inhibitor. It is used as an antidepressant and often has a more acceptable side-effects profile than traditional antidepressants. Fluoxetin,Fluoxetine Hydrochloride,Lilly-110140,N-Methyl-gamma-(4-(trifluoromethyl)phenoxy)benzenepropanamine,Prozac,Sarafem,Lilly 110140,Lilly110140
D006801 Humans Members of the species Homo sapiens. Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man

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