The Respective Roles of CYP3A4 and CYP2D6 in the Metabolism of Pimozide to Established and Novel Metabolites.
Pimozide is a dopamine receptor antagonist indicated for the treatment of Tourette syndrome. Prior in vitro studies characterized N-dealkylation of pimozide to 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI) via CYP3A4 and, to a lesser extent, CYP1A2 as the only notable routes of pimozide biotransformation. However, drug-drug interactions between pimozide and CYP2D6 inhibitors and CYP2D6 genotype-dependent effects have since been observed. To reconcile these incongruities between the prior in vitro and in vivo studies, we characterized two novel pimozide metabolites: 5-hydroxypimozide and 6-hydroxypimozide. Notably, 5-hydroxypimozide was the major metabolite produced by recombinant CYP2D6 (Km ∼82 nM, V max ∼0.78 pmol/min per picomoles), and DHPBI was the major metabolite produced by recombinant CYP3A4 (apparent Km ∼1300 nM, V max ∼2.6 pmol/min per picomoles). Kinetics in pooled human liver microsomes (HLMs) for the 5-hydroxylation (Km ∼2200 nM, V max ∼59 pmol/min per milligram) and N-dealkylation (Km ∼3900 nM, V max ∼600 pmol/min per milligram) reactions were also determined. Collectively, formation of DHPBI, 5-hydroxypimozide, and 6-hydroxypimozide accounted for 90% of pimozide depleted in incubations of NADPH-supplemented pooled HLMs. Studies conducted in HLMs isolated from individual donors with specific cytochrome P450 isoform protein abundances determined via mass spectrometry revealed that 5-hydroxypimozide (r 2 = 0.94) and 6-hydroxypimozide (r 2 = 0.86) formation rates were correlated with CYP2D6 abundance, whereas the DHPBI formation rate (r 2 = 0.98) was correlated with CYP3A4 abundance. Furthermore, the HLMs differed with respect to their capacity to form 5-hydroxypimozide relative to DHPBI. Collectively, these data confirm a role for CYP2D6 in pimozide clearance via 5-hydroxylation and provide an explanation for a lack of involvement when only DHPBI formation was monitored in prior in vitro studies. SIGNIFICANCE STATEMENT: Current CYP2D6 genotype-guided dosing information in the pimozide label is discordant with available knowledge regarding the primary biotransformation pathways. Herein, we characterize the CYP2D6-dependent biotransformation of pimozide to previously unidentified metabolites. In human liver microsomes, formation rates for the novel metabolites and a previously identified metabolite were determined to be a function of CYP2D6 and CYP3A4 content, respectively. These findings provide a mechanistic basis for observations of CYP2D6 genotype-dependent pimozide clearance in vivo.
Drug metabolism and disposition: the biological fate of chemicals
Chapron BD, Dinh JC, Toren PC, Gaedigk A, Leeder JS. The Respective Roles of CYP3A4 and CYP2D6 in the Metabolism of Pimozide to Established and Novel Metabolites. Drug Metab Dispos. 2020;48(11):1113-1120. doi:10.1124/dmd.120.000188