title: Quantitative contribution of CYP2D6 and CYP3A to oxycodone metabolism in human liver and intestinal microsomes.
abstract: Oxycodone undergoes N-demethylation to noroxycodone and O-demethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K(m) of 600 +/- 119 microM and a V(max) that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K(m) of 130 +/- 33 microM and a V(max) that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 microl/min/mg versus 1.5 microl/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.
authors.affiliation: Department of Pharmacy, University of Washington, Box 357630, Seattle, WA 98105, USA.
authors.forename: Bojan
authors.initials: B
authors.lastname: Lalovic
authors.forename: Brian
authors.initials: B
authors.lastname: Phillips
authors.forename: Linda L
authors.initials: LL
authors.lastname: Risler
authors.forename: William
authors.initials: W
authors.lastname: Howald
authors.forename: Danny D
authors.initials: DD
authors.lastname: Shen
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D001711:Biotransformation; D019389:Cytochrome P-450 CYP2D6 / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism; D018076:DNA, Complementary / Q000235:genetics; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000187:drug effects / Q000648:ultrastructure*; D007654:Ketoconazole / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008657:Metabolic Clearance Rate; D008745:Methylation / Q000187:drug effects; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D008862:Microsomes, Liver / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D015394:Molecular Structure; D009019:Morphinans / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D010098:Oxycodone / Q000378:metabolism* / Q000494:pharmacology; D010111:Oxymorphone / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D011802:Quinidine / Q000009:adverse effects / Q000037:antagonists & inhibitors / Q000494:pharmacology
chemical_list: D018076:DNA, Complementary; D009019:Morphinans; D003577:Cytochrome P-450 Enzyme System; C077256:noroxycodone; D010111:Oxymorphone; D010098:Oxycodone; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; C510164:CYP3A5 protein, human; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D011802:Quinidine; D007654:Ketoconazole