Abstract

Ibogaine is a psychoactive alkaloid that possesses potential as an agent to treat opiate and cocaine addiction. The primary metabolite arises via O-demethylation at the 12-position to yield 12-hydroxyibogamine. In this report, evidence is presented that theO-demethylation of ibogaine observed in human hepatic microsomes is catalyzed primarily by the polymorphically expressed cytochrome P-4502D6 (CYP2D6). An enzyme kinetic examination of ibogaineO-demethylase activity in pooled human liver microsomes suggested that two (or more) enzymes are involved in this reaction: one with a low K_Mapp (1.1 μM) and the other with a high K_Mapp (>200 μM). The lowK_Mapp activity comprised >95% of total intrinsic clearance. Human liver microsomes from three individual donors demonstrated similar enzyme kinetic parameters (meanK_Mapp = 0.55 ± 0.09 μM and 310 ± 10 μM for low and high K_Mactivities, respectively). However, a fourth human microsome sample that appeared to be a phenotypic CYP2D6 poor metabolizer possessed only the high K_Mapp activity. In hepatic microsomes from a panel of human donors, the lowK_Mapp ibogaine O-demethylase activity correlated with CYP2D6-catalyzed bufuralol 1′-hydroxylase activity but not with other P450 isoform-specific activities. Quinidine, a CYP2D6-specific inhibitor, inhibited ibogaineO-demethylase (IC₅₀ = 0.2 μM), whereas other P450 isoform-specific inhibitors did not inhibit this activity. Also, of a battery of recombinant heterologously expressed human P450 isoforms, only rCYP2D6 possessed significant ibogaineO-demethylase activity. Thus, it is concluded that ibogaineO-demethylase is catalyzed by CYP2D6 and that this isoform is the predominant enzyme of ibogaine O-demethylation in humans. The potential pharmacological implications of these findings are discussed.