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.

Results: The V max, Km, and CLint values for the formation of EDDP from rac-, (R)- and (S)-methadone were in the ranges of 20-77 nmol mg-1 protein h-1, 125-252 microm, and 91-494 ml h-1 g-1 protein. Km and CLint values for (R)- and (S)-methadone were not statistically significantly different (P >0.05), while V max values for (S)-methadone were 15% (P=0.045) lower than for (R)-methadone. Expressed CYP3A4 and CYP2C19 showed similar reaction rates for both (R)- and (S)-methadone, while CYP2D6 did not catalyse this reaction. Selective chemical inhibitors of CYP3A (troleandomycin, ketoconazole) and monoclonal human CYP3A4 antibodies significantly inhibited (P<0.05) the formation of EDDP in a concentration dependent manner by up to 80%. Sulphaphenazole (CYP2C9) also significantly inhibited (P<0.05) EDDP formation (range 14-25%). 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.

Figure 1

Chemical structure and N-demethylation pathway of rac-methadone (*indicates chiral carbon atom).

Figure 2

Eadie-Hofstee representations of EDDP formation from rac-, (R)-, and (S)-methadone by microsomes prepared from HLS# 31.

Figure 3

Effect of chemical inhibitors on EDDP formation from (▪) rac-, (□) (R)-, and () (S)-methadone by human liver microsomes (n=3). Error bars indicate s.d. *Indicates statistically significant inhibition compared with control (P<0.05). No statistically significant differences (P0.05) were observed between rac-, (R)-, and (S)-methadone.

Figure 4

Comparison of EDDP formation from (□) (R)-methadone and () (S)-methadone by microsomes from a human liver (HLS#5) and human lymphoblastoid cells containing the expressed individual CYP450 isoforms CYP3A4 and CYP2C19 (n=2). Error bars indicate s.d.