The ( R )-omeprazole hydroxylation index reflects CYP2C19 activity in healthy Japanese volunteers

Purpose Omeprazole has ( R )- and ( S )-enantiomers, which exhibit different pharmacokinetics (PK) among patients with cytochrome P450 (CYP) 2C19 genotype groups. The aim of this study was to investigate whether the 1-point, 4-h postdose ( R )-omeprazole hydroxylation index (HI) of racemic omeprazole reflects the three CYP2C19 genotype groups in Japanese individuals. Methods Ninety healthy Japanese individuals were enrolled and classified into the three different CYP2C19 genotype groups: homozygous extensive metabolizers (hmEMs; n = 34), heterozygous EMs (htEMs; n = 44), and poor metabolizers (PMs; n = 12). Blood samples were drawn 4 h after the intake of an oral dose of omeprazole 40 mg, and plasma levels of omeprazole and its metabolites were analyzed by high-performance liquid chromatography (HPLC) using a chiral column. Results Mean plasma concentrations of ( R )- and ( S )-omeprazole in PMs were significantly higher than those in hmEMs and htEMs, and similar results were obtained in the case of omeprazole sulfone. Additionally, there was a significant difference in plasma concentrations of ( R )-5-hydroxyomeprazole among CYP2C19 genotype groups, whereas no significant differences were observed in that of ( S )-5-hydroxyomeprazole. Similarly, ( R )-omeprazole HI in hmEMs, htEMs, and PMs were 5.6, 3.1, and 0.3, respectively, which were significantly different, but no significant difference was present in the ( S )-omeprazole HI. Conclusion Our findings demonstrate that ( R )-omeprazole HI correlated better with CYP2C19 genotype groups than racemic-omeprazole HI, and these results may be useful for classification among patients in CYP2C19 genotype groups prior to omeprazole treatment.