Evidence for oxazepam as an in vivo probe of UGT2B15: oxazepam clearance is reduced by UGT2B15 D85Y polymorphism but unaffected by UGT2B17 deletion

Abstract

Aims: Although in vitro studies indicate that oxazepam is an isoform-selective substrate probe for UDP-glucuronosyltransferase 2B15, the utility of this drug as an in vivo probe is uncertain. The main aim of this study was to determine whether common missense polymorphisms in the UGT2B15 gene (D85Y and K523T) are associated with altered oxazepam pharmacokinetics and pharmacodynamics. We also determined the possible influence of a common deletion polymorphism in the gene encoding UGT2B17, which shows substantial substrate specificity overlap with UGT2B15.

Methods: Thirty healthy male subjects were administered 15 mg of oxazepam by mouth followed by plasma oxazepam concentration monitoring for 36 h, and pharmacodynamic testing for 8 h. Genotypes were determined by genomic polymerase chain reaction and commercial 5'-nuclease assays.

Results: Allele frequencies for D85Y, K523T, UGT2B17del were 47%, 23% and 19%, respectively. Median oxazepam apparent oral clearance was significantly lower in 85YY subjects (1.62 ml min(-1) kg(-1)) compared with 85DD subjects (3.35 ml min(-1) kg(-1); P= 0.003, Student-Newman-Keuls test), whereas 85DY subjects were intermediate (2.34 ml min(-1) kg(-1); P= 0.018 vs. 85DD, P= 0.034 vs. 85YY). Regression analysis indicated that UGT2B15 D85Y genotype accounted for 34% of interindividual variability. However, neither UGT2B15 K523T nor UGT2B17del was associated with altered oxazepam disposition. Furthermore, no differences in pharmacodynamic measures, including quantitative electroencephalography, digit-symbol substitution test, self- or observer-rated visual analogue scales, could be demonstrated for any of the polymorphisms evaluated.

Conclusions: These results identify UGT2B15 D85Y as a major determinant of oxazepam clearance, and indicate that oxazepam may be useful as an in vivo probe for glucuronidation by UGT2B15.

Figure 1

Plasma oxazepam concentrations and associated effect on electroencephalogram (EEG) activity in 30 subjects administered 15 mg oxazepam by mouth. (A) The mean (± SE) concentrations for all 30 subjects at each time point after drug administration. (B) The mean (± SE) changes in EEG β wave amplitudes from predose values at each time point after drug administration for the same 30 subjects

Figure 3

Effect of UGT2B15 D85Y genotype on oxazepam oral clearance in 30 subjects administered a 15-mg oral dose of oxazepam. Shown are the body weight normalized apparent oral clearance values determined for each subject and grouped by UGT2B15 85DD (n= 6), DY (n= 20) and 85YY (n= 4) genotypes. Also shown are the genotype group median values (horizontal line) and P-values for group comparisons by Student–Newman–Keuls test

Figure 2

Effect of UGT2B15 D85Y genotype on plasma oxazepam concentrations in 30 subjects administered a 15-mg oral dose of oxazepam. Shown at each time point after administration are the mean (± SE) concentrations for subjects with either 85DD (n= 6), DY (n= 20) or 85YY (n= 4) genotypes. Derived pharmacokinetic parameters are given in Table 1. D/D (–•–); D/Y (–○–); Y/Y (–▾–)

Figure 4

Effect of UGT2B15 D85Y genotype on electroencephalogram (EEG) activity in 30 subjects administered a 15-mg oral dose of oxazepam. Plotted values represent the mean change from predose values in EEG β wave amplitude for subjects grouped by UGT2B15 85DD (n= 6), DY (n= 20) and 85YY (n= 4) genotypes. D/D (); D/Y (); Y/Y ()