Abstract
Recently, genotyping in clinical studies has revealed that UGT2B15 genetic polymorphism has an influence on the clinical pharmacokinetics of sipoglitazar. In this study, the UGT responsible for sipoglitazar was retrospectively identified by in vitro analysis. A study using UGT-expressing supersomes revealed that sipoglitazar glucuronidation was more extensively catalyzed by UGT1A1, 1A3, 1A6, 2B4, and 2B15 than by other UGTs. Enzyme kinetic studies for sipoglitazar glucuronidation and recent findings related to mRNA expression analysis of UGTs narrowed the involved isoforms down to UGT1A1 and UGT2B15 among these five human UGTs. In a correlation study between sipoglitazar glucuronidation and UGT isoform-specific activities, the glucuronidation of S-oxazepam, a specific substrate for UGT2B15, strongly correlated with that of sipoglitazar, as compared with that of β-estradiol, a representative UGT1A1 substrate. The analysis of the species difference strengthens the possibility of UGT2B15 rather than that of UGT1A1. These in vitro findings indicate that UGT2B15 is principally responsible for sipoglitazar glucuronidation. Moreover, the UGT2B15*2 mutation significantly increased the Km value of sipoglitazar in the kinetic analysis using recombinant His-tag UGT2B15*1- or *2-membrane fractions. These results show that sipoglitazar is a good example to elucidate the relationship between phenotype and genotype for UGT2B15 from in vitro analysis.
