title: Metabolism of alfentanil by cytochrome p4503a (cyp3a) enzymes.
abstract: The synthetic opioid alfentanil is an analgesic and an in vivo probe for hepatic and first-pass CYP3A activity. Alfentanil is a particularly useful CYP3A probe because pupil diameter change is a surrogate for plasma concentrations, thereby affording noninvasive assessment of CYP3A. Alfentanil undergoes extensive CYP3A4 metabolism via two major pathways, forming noralfentanil and N-phenylpropionamide. This investigation evaluated alfentanil metabolism in vitro to noralfentanil and N-phenylpropionamide, by expressed CYP3A5 and CYP3A7 in addition to CYP3A4, with and without coexpressed or exogenous cytochrome b(5). Effects of the CYP3A inhibitors troleandomycin and ketoconazole were also determined. Rates of noralfentanil and N-phenylpropionamide formation by CYP3A4 and 3A5 in the absence of b(5) were generally equivalent, although the metabolite formation ratio differed, whereas those by CYP3A7 were substantially less. CYP3A4 and 3A5 were equipotently inhibited by troleandomycin, whereas ketoconazole was an order of magnitude more potent toward CYP3A4. Cytochrome b(5) qualitatively and quantitatively altered alfentanil metabolism, with b(5) coexpression having a greater effect than exogenous addition. Addition or coexpression of b(5) markedly stimulated the formation of both metabolites and changed the formation of noralfentanil but not N-phenylpropionamide from apparent single-site to multisite Michaelis-Menten kinetics. These results demonstrate that alfentanil is a substrate for CYP3A5 in addition to CYP3A4, and the effects of the CYP3A inhibitors troleandomycin and ketoconazole are CYP3A enzyme-selective. Alfentanil is one of the few CYP3A substrates that is metabolized in vitro as avidly by both CYP3A4 and 3A5. Polymorphic CYP3A5 expression may contribute to inter-individual variability in alfentanil metabolism.
authors.affiliation: Department of Anesthesiology, Box 356540, University of Washington, 1959 NE Pacific, RR-442, Seattle, WA 98195, USA.
authors.forename: Theresa Mariero
authors.initials: TM
authors.lastname: Klees
authors.forename: Pamela
authors.initials: P
authors.lastname: Sheffels
authors.forename: Ola
authors.initials: O
authors.lastname: Dale
authors.forename: Evan D
authors.initials: ED
authors.lastname: Kharasch
mesh_terms: D015760:Alfentanil / Q000378:metabolism*; D000701:Analgesics, Opioid / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051544:Cytochrome P-450 CYP3A; D015786:Cytochromes b5 / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000378:metabolism; D007654:Ketoconazole / Q000494:pharmacology; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014217:Troleandomycin / Q000494:pharmacology
chemical_list: D000701:Analgesics, Opioid; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D015760:Alfentanil; D015786:Cytochromes b5; D014217:Troleandomycin; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating; D007654:Ketoconazole