Cytochrome P-450-mediated metabolism of the individual enantiomers of the antidepressant agent reboxetine in human liver microsomes

In vitro studies were conducted to identify the hepatic cytochrome P-450 (C YP) enzymes responsible for the oxidative metabolism of the individual enan tiomers of reboxetine. In human liver microsomes, each reboxetine enantiome r was metabolized to one primary metabolite, O-desethylreboxetine, and thre e 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 mu M, the rate O-desethylreboxetine formation for either enantiomer co nformed to monophasic Michaelis-Menten kinetics. Evidence for a principal r ole 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 forma tion of O-desethylreboxetine, whereas inhibitors selective for other CYP en zymes did not inhibit reboxetine metabolism, 2) formation of O-desethylrebo xetine correlated (r(2) = 0.99; p < .001) with CYP3A-selective testosterone 6-beta-hydroxylase activity across a population of human livers (n = 14). Consistent with inhibition and correlation data, O-desethylreboxetine forma tion was only detectable in incubations using microsomes prepared from a Ba culovirus-insect cell line expressing CYP3A4. Furthermore, the apparent K-M for the O-desethylation of reboxetine in cDNA CYP3A4 microsomes was simila r to the affinity constants determined in human liver microsomes. In additi on, (S,S)-reboxetine and (R,R)-reboxetine were found to be competitive inhi bitors of CYP2D6 and CYP3A4 (K-i = 2.5 and 11 mu M, respectively). Based on the results of the study, it is concluded that the metabolism of both rebo xetine enantiomers in humans is principally mediated via CYP3A.