title: Enantiomeric metabolic interactions and stereoselective human methadone metabolism.
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.
authors.affiliation: Department of Medicinal Chemistry, University of Washington. Seattle, Washington, USA.
authors.forename: Rheem A
authors.initials: RA
authors.lastname: Totah
authors.forename: Kyle E
authors.initials: KE
authors.lastname: Allen
authors.forename: Pamela
authors.initials: P
authors.lastname: Sheffels
authors.forename: Dale
authors.initials: D
authors.lastname: Whittington
authors.forename: Evan D
authors.initials: ED
authors.lastname: Kharasch
mesh_terms: D000465:Algorithms; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D052246:Cyclooxygenase 2 Inhibitors / Q000494:pharmacology; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003640:Dealkylation; D004790:Enzyme Induction / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005231:Fatty Acids, Unsaturated; D006801:Humans; D006834:Hydrazines / Q000494:pharmacology; D007700:Kinetics; D008691:Methadone / Q000737:chemistry / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D009294:Narcotics / Q000737:chemistry / Q000378:metabolism*; D009578:Nitrobenzenes / Q000494:pharmacology; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism; D012293:Rifampin / Q000494:pharmacology; D013237:Stereoisomerism; D013449:Sulfonamides / Q000494:pharmacology; D013481:Superoxides / Q000378:metabolism; D017392:Thiobarbituric Acid Reactive Substances
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D052246:Cyclooxygenase 2 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D005231:Fatty Acids, Unsaturated; D006834:Hydrazines; D009294:Narcotics; D009578:Nitrobenzenes; D013449:Sulfonamides; D017392:Thiobarbituric Acid Reactive Substances; D013481:Superoxides; C080955:N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide; C045749:SQ 29548; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C045793:CYP2C19 protein, human; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D008691:Methadone; D012293:Rifampin