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, Vmax ~0.78 pmol/min/pmol) and DHPBI was the major metabolite produced by recombinant CYP3A4 (apparent Km ~1300 nM, Vmax ~2.6 pmol/min/pmol). Kinetics in pooled human liver microsomes (HLMs) for the 5-hydroxylation (Km ~ 2200 nM, Vmax ~59 pmol/min/mg) and N-dealkylation (Km ~3900 nM, Vmax ~600 pmol/min/mg) 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 CYP isoform protein abundances determined via mass spectrometry revealed that 5-hydroxypimozide (r2=0.94) and 6-hydroxypimozide (r2=0.86) formation rates were correlated with CYP2D6 abundance, whereas the DHPBI formation rate (r2=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.