Ketoconazole inhibits the clearance of the enantiomers of the antidepressant reboxetine in humans.

Background: Ketoconazole is a potent inhibitor of the cytochrome P450 3A4 enzyme. Reboxetine, a selective norepinephrine reuptake inhibitor, is metabolized by cytochrome P450 3A4. The potential interaction of reboxetine with this representative from the azole derivative class was examined.

Methods: Eleven healthy volunteers received (1) 4 mg reboxetine orally on the second day of a 5-day regimen of 200 mg ketoconazole once daily and (2) 4 mg reboxetine orally in a crossover design. Plasma concentrations of reboxetine enantiomers [R,R(-)-reboxetine and the more active S,S(+)-reboxetine] were measured by high-performance liquid chromatography-tandem mass spectrometry. Effects of ketoconazole on enantiomer pharmacokinetics were assessed by ANOVA.

Results: Ketoconazole increased R,R(-)-reboxetine and S,S(+)-reboxetine mean area under the plasma concentration-time curves (AUC) by 58% and 43%, respectively (P < .02). Oral clearance of both enantiomers was consequently decreased 34% and 24%, respectively, by ketoconazole (P < .005). Ketoconazole did not significantly affect maximal plasma concentrations (P > .1). Mean terminal half-life after administration of ketoconazole (21.5 hours and 18.9 hours) was significantly longer than after reboxetine alone (14.8 hours and 14.4 hours; P < or = .005). The AUC ratio for R,R(-)-reboxetine to S,S(+)-reboxetine was reduced by ketoconazole administration (2.76 after ketoconazole versus 2.39; P < .003).

Conclusion: Ketoconazole decreases clearance of both reboxetine enantiomers. Although the adverse effect profile for reboxetine was not altered by ketoconazole, the results of this study suggest that caution should be used and that a reduction in reboxetine dose should be considered when the two are coadministered.