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.

These results suggest a significant role for CYP2B6, but not CYP3A, in stereoselective human methadone metabolism and disposition.

ALF and MDZ have similar intestinal extraction but low and intermediate hepatic extraction, respectively. Systemic and oral clearances of ALF are excellent in vivo probes for hepatic and first-pass CYP3A activities and drug interactions. Miosis was an acceptable surrogate for plasma ALF. ALF miosis may be a suitable noninvasive in vivo probe for both hepatic and first-pass CYP3A.

Methadone is administered as a racemate, although opioid activity resides in the R-enantiomer. Methadone disposition is stereoselective, with considerable unexplained variability in clearance and plasma R/S ratios. N-Demethylation of methadone in vitro is predominantly mediated by cytochrome P450 CYP3A4 and CYP2B6 and somewhat by CYP2C19. This investigation evaluated stereoselectivity, models, and kinetic parameters for methadone N-demethylation by recombinant CYP2B6, CYP3A4, and CYP2C19, and the potential for interactions between enantiomers during racemate metabolism. CYP2B6 metabolism was stereoselective. CYP2C19 was less active, and stereoselectivity was opposite that for CYP2B6. CYP3A4 was not stereoselective. With all three isoforms, enantiomer N-dealkylation rates in the racemate were lower than those of (R)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (R-methadone) or (S)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (S-methadone) alone, suggesting an enantiomeric interaction and mutual metabolic inhibition. For CYP2B6, the interaction between enantiomers was stereoselective, with S-methadone as a more potent inhibitor of R-methadone N-demethylation than R-of S-methadone. In contrast, enantiomer interactions were not stereoselective with CYP2C19 or CYP3A4. For all three cytochromes P450, methadone N-demethylation was best described by two-site enzyme models with competitive inhibition. There were minor model differences between cytochromes P450 to account for stereoselectivity of metabolism and enantiomeric interactions. Changes in plasma R/S methadone ratios observed after rifampin or troleandomycin pretreatment in humans in vivo were successfully predicted by CYP2B6-but not CYP3A4-catalyzed methadone N-demethylation. CYP2B6 is a predominant catalyst of stereoselective methadone metabolism in vitro. In vivo, CYP2B6 may be a major determinant of methadone metabolism and disposition, and CYP2B6 activity and stereoselective metabolic interactions may confer variability in methadone disposition.The chiral opioid methadone, a mainstay in the treatment of opiate addiction and acute and chronic pain, is used clinically as a racemate, although opioid activity resides in the R-enantiomer. Methadone disposition is stereoselective; there is considerable interindividual variability in clearance (up to 100-fold) and plasma R/S ratios (4-fold), and methadone clearance is subject to numerous drug interactions (Plummer et al., 1988;Eap et al., 2002). Nevertheless, the mechanism of stereoselective clearance and metabolism and interindividual variability remains poorly understood. Methadone elimination is due mainly to metabolic clearance (Anggard et al., 1975). The main pathway of methadone metabolism is N-demethylation to inactive 2-ethylidine-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP). A minor reductive route to normethadol is not cytochrome P450-mediated. The significant correlation between methadone clearance and the plasma EDDP/methadone area under the curve ratios suggest that N-dem...