Chlorpropamide 2-hydroxylation is catalysed by CYP2C9 and CYP2C19 in vitro: Chlorpropamide disposition is influenced by CYP2C9, but not by CYP2C19 genetic polymorphism

Aims: We evaluated the involvement of cytochrome P450 (CYP) isoforms 2C9 and 2C19 in chlorpropamide 2-hydroxylation in vitro and in chlorpropamide disposition in vivo. Methods: To identify CYP isoforms(s) that catalyse 2-hydroxylation of chlorpropamide, the incubation studies were conducted using human liver microsomes and recombinant CYP isoforms. To evaluate whether genetic polymorphisms of CYP2C9 and/or CYP2C19 influence the disposition of chlorpropamide, a single oral dose of 250 mg chlorpropamide was administered to 21 healthy subjects pregenotyped for CYP2C9 and CYP2C19. Results: In human liver microsomal incubation studies, the formation of 2-hydroxychlorpropamide (2-OH-chlorpropamide), a major chlorpropamide metabolite in human, has been best described by a one-enzyme model with estimated K_m and V _max of 121.7 ± 19.9 μM and 16.1 ± 5.0 pmol min ^-1 mg^-1 protein, respectively. In incubation studies using human recombinant CYP isoforms, however, 2-OH-chlorpropamide was formed by both CYP2C9 and CYP2C19 with similar intrinsic clearances (CYP2C9 vs. CYP2C19: 0.26 vs. 0.22 μl min^-1 nmol^-1 protein). Formation of 2-OH-chlorpropamide in human liver microsomes was significantly inhibited by sulfaphenazole, but not by S-mephenytoin, ketoconazole, quinidine, or furafylline. In in vivo clinical trials, eight subjects with the CYP2C9*1/*3 genotype exhibited significantly lower nonrenal clearance [*1/*3 vs. *1/*1: 1.8 ± 0.2 vs. 2.4 ± 0.1 ml h^-1 kg^-1, P < 0.05; 95% confidence interval (CI) on the difference 0.2, 1.0] and higher metabolic ratios (of chlorpropamide/2-OH-chlorpropamide in urine: *1/*3 vs. *1/*1: 1.01 ± 0.19 vs. 0.56 ± 0.08, P < 0.05; 95% CI on the difference -0.9, -0.1) than did 13 subjects with CYP2C9*1/ *1 genotype. In contrast, no differences in chlorpropamide pharmacokinetics were observed for subjects with the CYP2C19 extensive metabolizer vs. poor metabolizer genotypes. Conclusions: These results suggest that chlorpropamide disposition is principally determined by CYP2C9 activity in vivo, although both CYP2C9 and CYP2C19 have a catalysing activity of chlorpropamide 2-hydroxylation pathway.