Abstract
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.
Footnotes
-
This work was supported by National Institutes of Health Grants R01DA14211 and K24DA00417 (E.D.K.). R.A.T. is supported in part by a University of Washington School of Pharmacy Drug Metabolism, Transporter and Pharmacogenomics Research Program (DMTPR) funded by gifts from Abbott, Allergan, Amgen, Bristol-Myers Squibb, Eli Lilly, Hoffman La Roche, Johnson & Johnson, Merck, and Pfizer. K.E.A. was supported by the Pharmacological Predoctoral Training Grants GM07750 and PO1 GM32165.
-
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
-
doi:10.1124/jpet.106.117580.
-
ABBREVIATIONS: EDDP, 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolidine; d3-EDDP, [ethyl-2′,2′,2′-2H3]-3,3-diphenyl-2-ethyl-5-methyl-1-pyrroline hydrochloride; R-methadone, (R)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride; S-methadone, (S)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride; RS-methadone, (RS)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride; P450, cytochrome P450.
- Received November 23, 2006.
- Accepted January 24, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|