Vol. 27, Issue 11, 1334-1340, November 1999

Cytochrome P-450-Mediated Metabolism of the Individual Enantiomers of the Antidepressant Agent Reboxetine in Human Liver Microsomes

Larry C. Wienkers, Cecilia Allievi, Michael J. Hauer, and Michael A. Wynalda

Department of Drug Metabolism, Pharmacia & Upjohn, Kalamazoo, Michigan

In vitro studies were conducted to identify the hepatic cytochrome P-450 (CYP) enzymes responsible for the oxidative metabolism of the individual enantiomers of reboxetine. In human liver microsomes, each reboxetine enantiomer was metabolized to one primary metabolite, O-desethylreboxetine, and three minor metabolites, two arising via oxidation of the ethoxy aromatic ring and a third yet unidentified metabolite. Over a concentration range of 2 to 200 µM, the rate O-desethylreboxetine formation for either enantiomer conformed to monophasic Michaelis-Menten kinetics. Evidence for a principal role of CYP3A in the formation of O-desethylreboxetine for (S,S)-reboxetine and (R,R)-reboxetine was based on the results from the following studies: 1) inhibition of CYP3A activity by ketoconazole markedly decreased the formation of O-desethylreboxetine, whereas inhibitors selective for other CYP enzymes did not inhibit reboxetine metabolism, 2) formation of O-desethylreboxetine correlated (r² = 0.99; p < .001) with CYP3A-selective testosterone 6--hydroxylase activity across a population of human livers (n = 14). Consistent with inhibition and correlation data, O-desethylreboxetine formation was only detectable in incubations using microsomes prepared from a Baculovirus-insect cell line expressing CYP3A4. Furthermore, the apparent K_M for the O-desethylation of reboxetine in cDNA CYP3A4 microsomes was similar to the affinity constants determined in human liver microsomes. In addition, (S,S)-reboxetine and (R,R)-reboxetine were found to be competitive inhibitors of CYP2D6 and CYP3A4 (K_i = 2.5 and 11 µM, respectively). Based on the results of the study, it is concluded that the metabolism of both reboxetine enantiomers in humans is principally mediated via CYP3A.