Abstract
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 ofO-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 ofO-desethylreboxetine correlated (r2 = 0.99; p < .001) with CYP3A-selective testosterone 6-β-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 KM for theO-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 (Ki = 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.
Footnotes
-
Send reprint requests to: Larry C. Wienkers, Drug Metabolism and Disposition Research, Pharmacia & Upjohn, 7265-300-313, 301 Henrietta St., Kalamazoo, MI 49007. E-mail:larry.c.wienkers{at}am.pnu.com
-
↵2 The unambiguous identification of the unknown reboxetine in vitro metabolite, UK1, is currently under further investigation.
- Abbreviations:
- NARIs
- noradrenaline reuptake inhibitors
- CYP or P-450
- cytochrome P-450
- T1/2
- terminal half-life of elimination
- QUIN
- quinidine
- KETO
- ketoconazole
- MEPH
- (S)-mephenytoin
- ABT
- 1-aminobenzotriazole
- FURF
- furafylline
- SULF
- sulfaphenazole
- COUM
- coumarin
- ORPH
- orphenadrine
- PAPV
- papaverine
- NITR
- para-nitrophenol
- LC/ESI/MS
- liquid chromatography electrospray mass spectrometry
- Received April 13, 1999.
- Accepted August 3, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|