CYP2D6-Mediated Metabolism of a Novel Acyl Coenzyme A:Cholesterol Acyltransferase Inhibitor, Pactimibe, and Its Unique Plasma Metabolite, R-125528

Abstract

ABSTRACT:Pactimibe sulfate is a novel acyl coenzyme A:cholesterol acyltransferase inhibitor. We conducted metabolic studies of pactimibe and its plasma metabolite, R-125528. Pactimibe had multiple metabolic pathways including indolin oxidation to form R-125528, -1 oxidation, N-dealkylation, and glucuronidation. Among them, the indolin oxidation and the -1 oxidation were dominant and were mainly catalyzed by CYP3A4 and CYP2D6, respectively. The intrinsic clearance (CL int ) values for these pathways in human hepatic microsomes were 0.63 and 0.76 l/min/mg-protein, respectively. On the other hand, the metabolic reaction for R-125528 was restricted. It was demonstrated that -1 oxidation was the only pathway that could eliminate R-125528 from the systemic circulation. Pactimibe sulfate (formerly named CS-505) ( Fig. 1) is a novel acyl coenzyme A:cholesterol acyltransferase inhibitor used to treat hypercholesterolemia and atherosclerotic diseases (Nissen et al., 2006; Kitayama et al., 2006a,b,c). A number of acyl coenzyme A:cholesterol acyltransferase inhibitors have been evaluated by several investigators. However, because of poor pharmacokinetics (Peck et al., 1995), adverse effects such as adrenal toxicity (Vernetti et al., 1993;Reindel et al., 1994;Matsuo et al., 1996), diarrhea (Kashiwa et al., 1997), and hepatotoxicity (Ishi et al., 1994;Nakaya et al., 1994), and elusive efficacies in humans (Harris et al., 1990;Hainer et al., 1994;Tardif et al., 2004), none of these compounds have so far succeeded in clinical development.Pactimibe sulfate was selected as a clinical development candidate showing good oral absorbability and potent pharmacological effects in apolipoprotein E-deficient mice (Terasaka et al., 2007) and Watanabe heritable hyperlipidemic rabbits (Kitayama et al., 2006b) and without showing significant adrenal toxicity even in dogs, the most sensitive animal species. Pactimibe is a weak acidic compound and has several metabolic pathways including oxidation at the indolin ring, -1 oxidation at the octyl chain, N-dealkylation, and glucuronidation on the carboxylic acid. Kinetic studies using human liver microsomes revealed that indolin ring oxidation (formation of R-125528) and -1 oxidation (formation of M-1) were equally dominant and that glucuronidation and N-dealkylation were minor. In addition, none of the metabolites were estimated to be pharmacologically active in vitro.In vivo biotransformation studies in animals demonstrated that only pactimibe and R-125528, the oxidized form of the indolin ring in pactimibe, appeared in the plasma and that none of the other metabolites were observed. After oral administration of pactimibe sulfate at a dose of 1 mg/kg to rats, dogs, and monkeys, AUC R-125528 /AUC pactimibe ratios were calculated to be 3, 14, and 55%, respectively. On the other hand, pactimibe and R-125528 were not detected in the urine or bile but were excreted into the bile as further metabolized forms. R-125528 is quite a unique metabolite that has higher lipophilicity than the parent compound...

Effects of Ketoconazole and Quinidine on Pharmacokinetics of Pactimibe and Its Plasma Metabolite, R-125528, in Humans

“…To our surprise, although R-125528 is an atypical substrate for CYP2D6 because of its acidity, a P450 isozyme identi-fication study using P450 expression microsomes revealed that CYP2D6 was the only isoform that could catalyze the reaction. In addition, the reaction phenotyping study indicated CYP2D6 activity was strongly (r 2 ϭ 0.90) correlated with the formation of M-2 (Kotsuma et al, 2008). Furthermore, R-125528 oxidation is inhibited as much as 90% in the presence of quinidine in vitro (data not shown).…”

“…None of the metabolites was estimated to be pharmacologically active in vitro. Kinetic studies using human liver microsomes (Kotsuma et al, 2008) revealed that intrinsic clearance (CL int ) values for indolin ring oxidation (formation of R-125528), -1 oxidation (formation of M-1), and glucuronidation were 0.63, 0.76, and 0.16 l/min/mg of protein, respectively. Moreover, according to a P450-isozyme identification study, the indolin oxidation and -1 oxidation were found to be catalyzed mainly by CYP3A4 and CYP2D6, respectively.…”

“…We selected a lower dose (25 mg) in the quinidine study than was used (100 mg) for the ketoconazole study, because we had a safety concern about metabolite accumulation in the quinidine treatment group considering the high f m CYP2D6 for R-125528 metabolism. Even though it was known that the pharmacokinetics of pactimibe showed a nonlinear trend slightly between these two doses, we believe that the contribution of the CYP2D6-and CYP3A4-mediated pathway should not be different between two doses, because the K m values for both M-1 and R-125528 formation are as high as 100 M (Kotsuma et al, 2008).…”

Novel Binding Mode of the Acidic CYP2D6 Substrates Pactimibe and Its Metabolite R-125528

“…Thereafter, Kemp et al (2004) docked spirosulfonamide positioned between the two phenylalanine residues, Phe 120 and Phe 483 , in a CYP2D6 homology model. Recently, we found novel substrates for CYP2D6, pactimibe sulfate and its indole metabolite, R-125528, that are acidic compounds with a carboxyl group and no basic nitrogen in their structures (Kotsuma et al, 2008b). In human clinical drug-drug interaction study, the AUC 0 -72 h of pactimibe and R-125528 were increased 1.7-and 5.0-fold, respectively, by cotreatment with quinidine (Kotsuma et al, 2008a).…”

“…Instead, an N-octyl chain was inserted into the active site, exhibiting interactions between hydrophobic residues, such as Phe 120 , Leu 213 , and Leu 484 . Hence, the docking correctly positioned the site of the metabolism of R-125528 at the -1 position of the N-octyl chain (Kotsuma et al, 2008b), above the heme. The distance between the -1 position of the N-octyl chain and the heme was 4.4 Å.…”