title: Role of cytochrome P450 2D6 (CYP2D6) in the stereospecific metabolism of E- and Z-doxepin.
abstract: The tricyclic antidepressant, doxepin, is formulated as an irrational mixture of E (trans) and Z (cis) stereoisomers (85%: 15%). We examined the stereoselective metabolism of doxepin in vitro, with the use of human liver microsomes, recombinant CYP2D6 and gas chromatography-mass spectrometry. In human liver microsomes over the concentration range 5-1500 microM, the rate of Z-doxepin N-demethylation exceeded that of E-doxepin above 100 microM in two of three livers. Eadie-Hofstee plots were curvilinear indicating the involvement of several enzymes in N-demethylation. Coincubation of doxepin with 7,8-naphthoflavone and ketoconazole reduced the rates of N-demethylation of E- and Z-doxepin by 30-50% and 40-60%, respectively, suggesting the involvement of CYP1A and CYP3A4, whilst quinidine had little effect on N-demethylation. In contrast, doxepin hydroxylation was exclusively stereo-specific; E-doxepin and E-N-desmethyldoxepin were hydroxylated with high affinity in liver microsomes and by recombinant CYP2D6 (Km in the range of 5-8 microM), but there was no evidence of Z-doxepin hydroxylation. In 'metabolic consumption' experiments with liver microsomes (having measurable CYP2D6 activity) and initial substrate concentration of 1 microM, the consumption of E-doxepin was greater (P < 0.05, n = 5) than that of Z-doxepin. Quinidine inhibited the consumption of E-doxepin but did not affect the consumption of Z-doxepin. With N-desmethyldoxepin, quinidine inhibited the consumption of E-N-desmethyl-doxepin whereas Z-N-desmethyldoxepin appeared to be a terminal oxidative metabolite. In summary, CYP2D6 is a major oxidative enzyme in doxepin metabolism; predominantly catalysing hydroxylation with an exclusive preference for the E-isomers. The relatively more rapid metabolism of E-isomeric forms, and the limited metabolic pathways for the Z-isomers may explain the apparent enrichment of Z-N-desmethyldoxepin that is observed in vivo.
authors.affiliation: Key Centre for Applied and Nutritional Toxicology, RMIT-University, Victoria, Australia.
authors.forename: V S
authors.initials: VS
authors.lastname: Haritos
authors.forename: H
authors.initials: H
authors.lastname: Ghabrial
authors.forename: J T
authors.initials: JT
authors.lastname: Ahokas
authors.forename: M S
authors.initials: MS
authors.lastname: Ching
mesh_terms: D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism*; D004316:Doxepin / Q000031:analogs & derivatives / Q000493:pharmacokinetics*; D006801:Humans; D006900:Hydroxylation; D008745:Methylation; D008862:Microsomes, Liver / Q000201:enzymology; D011802:Quinidine / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D011994:Recombinant Proteins; D004316:Doxepin; D019389:Cytochrome P-450 CYP2D6; C028701:desmethyldoxepin; D011802:Quinidine