QUANTITATIVE CONTRIBUTION OF CYP2D6 AND CYP3A TO OXYCODONE METABOLISM IN HUMAN LIVER AND INTESTINAL MICROSOMES

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 μM 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 μM 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 μl/min/mg versus 1.5 μl/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.