Totah Rheem A, Allen Kyle E, Sheffels Pamela, Whittington Dale, Kharasch Evan D
Department of Medicinal Chemistry, University of Washington. Seattle, Washington, USA.
J Pharmacol Exp Ther. 2007 Apr;321(1):389-99. doi: 10.1124/jpet.106.117580. Epub 2007 Jan 26.
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.
美沙酮是以消旋体形式给药的，尽管阿片样物质活性存在于R-对映体中。美沙酮的处置具有立体选择性，其清除率和血浆R/S比值存在相当大的无法解释的变异性。美沙酮在体外的N-去甲基化主要由细胞色素P450 CYP3A4和CYP2B6介导，也有一部分由CYP2C19介导。本研究评估了重组CYP2B6、CYP3A4和CYP2C19对美沙酮N-去甲基化的立体选择性、模型和动力学参数，以及消旋体代谢过程中对映体之间相互作用的可能性。CYP2B6的代谢具有立体选择性。CYP2C19活性较低，且立体选择性与CYP2B6相反。CYP3A4没有立体选择性。对于所有三种同工酶，消旋体中美沙酮对映体的N-脱烷基化速率均低于单独的(R)-(6-二甲基氨基-4,4-二苯基-庚烷-3-酮)盐酸盐（R-美沙酮）或(S)-(6-二甲基氨基-4,4-二苯基-庚烷-3-酮)盐酸盐（S-美沙酮），这表明存在对映体相互作用和相互代谢抑制。对于CYP2B6，对映体之间的相互作用具有立体选择性，S-美沙酮对R-美沙酮N-去甲基化的抑制作用比R-对S-美沙酮更强。相比之下，CYP2C19或CYP3A4的对映体相互作用没有立体选择性。对于所有三种细胞色素P450，美沙酮N-去甲基化最好用具有竞争性抑制的双位点酶模型来描述。细胞色素P450之间存在微小模型差异，以解释代谢的立体选择性和对映体相互作用。利福平或醋竹桃霉素预处理后人体体内观察到的血浆美沙酮R/S比值变化可通过CYP2B6催化的美沙酮N-去甲基化成功预测，但不能通过CYP3A4催化的美沙酮N-去甲基化预测。CYP2B6是体外美沙酮立体选择性代谢的主要催化剂。在体内，CYP2B6可能是美沙酮代谢和处置的主要决定因素，且CYP2B6活性和立体选择性代谢相互作用可能导致美沙酮处置的变异性。
