Identification and Characterization of Human Cytochrome P 450 Isoforms Interacting with Pimozide 1

Using human liver microsomes (HLMs) and recombinant human cytochrome P450 (CYP450) isoforms, we identified the major route of pimozide metabolism, the CYP450 isoforms involved, and documented the inhibitory effect of pimozide on CYP450 isoforms. Pimozide was predominantly N-dealkylated to 1,3dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI). The formation rate of DHPBI showed biphasic kinetics in HLMs, which suggests the participation of at least two activities. These were characterized as high-affinity (Km1 and Vmax1) and lowaffinity (Km2 and Vmax2) components. The ratio of Vmax1 (14 pmol/min/mg protein)/Km1 (0.73 mM) was 5.2 times higher than the ratio of Vmax2 (244 pmol/min/mg protein)/Km2 (34 mM). Km2 was 91 times higher than Km1. The formation rate of DHPBI from 25 mM pimozide in nine human livers correlated significantly with the catalytic activity of CYP3A (Spearman r 5 0.79, P 5 .028), but not with other isoforms. Potent inhibition of DHPBI formation from 10 mM pimozide was observed with ketoconazole (88%), troleandomycin (79%), furafylline (48%) and a combination of furafylline and ketoconazole (96%). Recombinant human CYP3A4 catalyzed DHPBI formation from 10 mM pimozide at the highest rate (V 5 2.2 6 0.89 pmol/min/pmol P450) followed by CYP1A2 (V 5 0.23 6 0.08 pmol/min/pmol P450), but other isoforms tested did not. The Km values derived with recombinant CYP3A4 and CYP1A2 were 5.7 mM and 36.1 mM, respectively. Pimozide itself was a potent inhibitor of CYP2D6 in HLMs when preincubated for 15 min (Ki 5 0.75 6 0.98 mM) and a moderate inhibitor of CYP3A (Ki 5 76.7 6 34.5 mM), with no significant effect on other isoforms tested. Our results suggest that pimozide metabolism is catalyzed mainly by CYP3A, but CYP1A2 also contributes. Pimozide metabolism is likely to be subject to interindividual variability in CYP3A and CYP1A2 expression and to drug interactions involving these isoforms. Pimozide itself may inhibit the metabolism of drugs that are substrates of CYP2D6. Pimozide is a potent neuroleptic that has been used extensively in Europe for the treatment of schizophrenia and other psychiatric diseases (Pinder et al., 1976; Tueth and Cheong, 1993). The use of pimozide in the United States is restricted to the management of motor and phonic tics associated with Tourette’s syndrome (Colvin and Tankanow, 1985). This is because of pimozide-induced prolongation of the cardiac QT interval (Fulop et al., 1987; Shapiro et al., 1989) and the risk of developing potentially fatal arrhythmia of the torsade de pointes type (Krähenbühl et al., 1995). For a number of drugs, including pimozide, there appears to be a link between cardiac adverse events and increased plasma concentrations (Woosley et al., 1993; Antzelevitch et al., 1996; Flockhart et al., 1996). The pharmacokinetics of pimozide show wide intersubject variability (McCreadie et al., 1984; Sallee et al., 1987), which suggests that altered metabolic capacity may have important clinical consequences in terms of efficacy and safety. Data from animals suggest that pimozide undergoes extensive hepatic metabolism (Soudijn and Wijngaarden, 1969), but its human metabolism and the enzymes that are responsible are not known. An understanding of the enzymatic machinery involved is important to predict which patients might be most vulnerable to the adverse effects that result from drug interactions or from genetic polymorphism in metabolic pathways. Indirect evidence from the literature implicates the CYP450 system in the metabolism of pimozide. Six putative metabolites of pimozide have been recovered in urine and feces in rats (Soudijn and Wijngaarden, 1969) and appear to be products of oxidative reactions. In humans, pimozide apparently undergoes oxidative N-dealkylation that cleaves the molecule in two halves (fig. 1). In a study involving three schizophrenic patients (Baro et al., 1972), ;30% of the administered pimozide dose was recovered in urine and feces as Received for publication October 22, 1997. 1 This study was supported by Grant T32–9 M 08386 from the National Institute of General Medical Sciences, Bethesda, MD, and a fellowship award to Dr. Ko from the World Health Organization (WPRO 0630/95). ABBREVIATIONS: CYP450, cytochrome P450; FPBA, 4,4-bis(4-fluorophenyl)butanoic acid; DHPBI, 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one; HLMs, human liver microsomes; HPLC, high-performance liquid chromatography; G-6-P, glucose 6-phosphate; G-6-PDH, glucose 6-phosphate dehydrogenase; NADP, b-nicotinamide adenine dinucleotide phosphate; EDTA, disodium salt of ethylenediaminetetraacetic acid. 0022-3565/98/2852-0428$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 285, No. 2 Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 285:428–437, 1998 428 at A PE T Jornals on M ay 1, 2017 jpet.asjournals. org D ow nladed from