Role of hepatic and intestinal cytochrome P450 3A and 2B6 in the metabolism, disposition, and miotic effects of methadone

Background The disposition of the long-acting opioid methadone, used to prevent opiate withdrawal and treat short- and long-lasting pain, is highly variable. Methadone undergoes N-demethylation to the primary metabolite 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolinium (EDDP), catalyzed in vitro by intestinal, hepatic, and expressed cytochrome P450 (CYP) 3A4. However, the role of CYP3A4 in human methadone disposition in vivo is unclear. This investigation tested the hypothesis that CYP3A induction (or inhibition) would increase (or decrease) methadone metabolism and clearance in humans. Methods Healthy volunteers were studied in a randomized, balanced, 4-way crossover study. They received intravenous (IV) midazolam (to assess CYP3A4 activity) and then simultaneous oral deuterium-labeled and IV unlabeled methadone after pretreatment with rifampin (INN, rifampicin) (hepatic/intestinal CYP3A induction), troleandomycin (hepatic/intestinal CYP3A inhibition), grapefruit juice (selective intestinal CYP3A inhibition), or nothing. Methadone effects were measured by dark-adapted pupil diameter. CYP isoforms catalyzing methadone metabolism by human liver microsomes and expressed CYPs in vitro were also evaluated. Results Methadone had high oral bioavailability (70%) and low intestinal (22%) and hepatic (9%) extraction, and there was a significant correlation (r = 0.94, P < .001) between oral bioavailability and intestinal (but not hepatic) availability. Rifampin decreased bioavailability and oral and IV methadone plasma concentrations and increased IV clearance (4.42 ± 1.00 mL · kg−1 · min−1 versus 1.61 ± 0.67 mL · kg−1 · min−1, P < .05) and oral clearance (8.50 ± 3.68 mL · kg−1 · min−1 versus 2.05 ± 0.92 mL · kg−1 · min−1, P < .05), EDDP/methadone area under the curve (AUC) ratios, EDDP formation clearances, and hepatic extraction (0.27 ± 0.06 versus 0.09 ± 0.04, P < .05). Troleandomycin and grapefruit juice decreased the EDDP/methadone AUC ratio after oral methadone (0.17 ± 0.10 and 0.14 ± 0.06 versus 0.27 ± 0.20, P < .05) but not IV methadone and had no effect on methadone plasma concentrations, IV clearance (1.29 ± 0.41 mL · kg−1 · min−1 and 1.48 ± 0.55 mL · kg−1 · min−1) or oral clearance (2.05 ± 1.52 mL · kg−1 · min−1 and 1.89 ± 1.07 mL · kg−1 · min−1), or other kinetic parameters. There was no correlation between methadone clearance and hepatic CYP3A4 activity. Pupil diameter changes reflected plasma methadone concentrations. In vitro experiments showed a predominant role for both CYP3A4 and CYP2B6 in liver microsomal methadone N-demethylation. Conclusion First-pass intestinal metabolism is a determinant of methadone bioavailability. Intestinal and hepatic CYP3A activity only slightly affects human methadone N-demethylation but has no significant effect on methadone concentrations, clearance, or clinical effects. Greater rifampin effects, compared with troleandomycin and grapefruit juice, on methadone disposition suggest a major role for intestinal transporters and for other CYPs, such as CYP2B6. Interindividual variability and drug interactions affecting intestinal transporter and hepatic CYP3A and CYP2B6 activity may alter methadone disposition. Clinical Pharmacology & Therapeutics (2004) 76, 250–269; doi: 10.1016/j.clpt.2004.05.003