Methadone N‐demethylation in human liver microsomes: lack of stereoselectivity and involvement of CYP3A4

Abstract

Aims To investigate the kinetics of CYP-mediated N-demethylation of methadone in human liver microsomes, and examine the role of stereoselectivity and CYP isoforms involved. Methods The kinetics of 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) formation via N-demethylation of rac-, (R)-and (S)-methadone in human liver microsomes prepared from six liver samples were determined by h.p.l.c., and inhibition of metabolic function was studied using isoform-specific chemical inhibitors and monoclonal antibodies. Microsomes containing expressed CYP3A4, CYP2D6 and CYP2C19 were also used to examine the formation of EDDP. 25%). There were no statistically significant differences in the inhibition observed between the three substrates. Selective inhibitors of CYP1A2 (furafylline), CYP2A6 (coumarin), CYP2C19 ((S)-mephenytoin), CYP2D6 (quinidine) and CYP2E1 (diethyldithiocarbamic acid sodium salt and monoclonal human CYP2E1 antibodies) had no significant ( P >0.05) effect. Conclusions The N-demethylation of methadone in human liver microsomes is not markedly stereoselective, and is mediated mainly by CYP3A4 with the possible involvement of CYP2C9 and CYP2C19. Thus, the large interindividual variation reported for methadone pharmacokinetics may be due to variability in the expression of these CYP isoforms, and the reported stereoselectivity in the systemic clearance of methadone in vivo is not due to stereoselectivity in N-demethylation. ResultsKeywords: CYP3A4, EDDP formation, human liver microsomes, methadone Ndemethylation, stereoselectivity molecule ( Figure 1) of (R)-and (S)-enantiomeric forms Introduction in which (R)-methadone has a 10-fold higher affinity at m and d opioid receptors [1] and possesses up to 50 times Rac-methadone is commonly used in the management of chronic pain and as maintenance therapy for the pharmacothe analgesic activity of (S)-methadone in human and animal models of pain [2]. (R)-methadone has also been logical treatment of opioid dependence. Rac-methadone (6-dimethylamino-4,4-diphenyl-3-heptanone) is a chiral shown to prevent the occurrence of opioid withdrawal symptoms, while (S)-methadone is ineffective [3]. Despite this high eudismic ratio, methadone is used as the significantly in their disposition in humans, with

Cytochrome P4503A Does Not Mediate the Interaction between Methadone and Ritonavir-Lopinavir

“…CYP3A appears not to mediate ritonavirlopinavir effects on methadone disposition, and, more generally, these results do not support a predominant role for CYP3A in clinical N-demethylation and clearance of single-dose methadone. The preponderance of other clinical evidence also vitiates the longheld notions that CYP3A4 activity determines clinical methadone N-metabolism and clearance, CYP3A4 interindividual variability is a major factor in variable methadone bioavailability, and CYP3A4 mediates methadone interactions with antiretrovirals and other drugs (Ferrari et al, 2004;McCance-Katz et al, 2010), which were extrapolated from in vitro methadone N-demethylation by CYP3A4 (Iribarne et al, 1996;Foster et al, 1999;Gerber et al, 2004;Chang et al, 2011). Strong hepatic and first-pass CYP3A inhibition by troleandomycin, and by ritonavir-indinavir, had no significant effects on methadone plasma concentrations, systemic or apparent oral clearance, hepatic clearance, or bioavailability (Kharasch et al, 2004a(Kharasch et al, , 2009a.…”

Differential in vitro inhibition of M3G and M6G formation from morphine by (R)‐ and (S)‐methadone and structurally related opioids

“…Methadone is primarily metabolized by the cytochrome P450 enzyme system, specifically CYP3A4, to 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) [25][26][27]. Although not a substrate for glucuronidation itself, methadone inhibits the glucuronidation of 3 ′ -azido-3 ′ -deoxythmidine (AZT) in vitro [28].…”

“…In this investigation, it was also observed that methadone-maintained subjects had higher plasma morphine concentrations than non-opioid-dependent controls when given the same i.v. dose, suggesting decreased morphine clearance in this population.Methadone is primarily metabolized by the cytochrome P450 enzyme system, specifically CYP3A4, to 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) [25][26][27]. Although not a substrate for glucuronidation itself, methadone inhibits the glucuronidation of 3 ′ -azido-3 ′ -deoxythmidine (AZT) in vitro [28].…”

Review article: Perioperative pain management of patients on methadone therapy

“…The main enzyme responsible for N-demethylation of methadone is CYP3A4, with lesser involvement from CYP1A2 and CYP2D6. 2,18 Current evidence suggests that CYP2B6 may play a very significant role in metabolism as well.…”