UDP-Glucuronosyltransferase (UGT) 2B15 Pharmacogenetics: UGT2B15 D85Y Genotype and Gender Are Major Determinants of Oxazepam Glucuronidation by Human Liver

Abstract

Oxazepam is a commonly used 1,4-benzodiazepine anxiolytic drug that is polymorphically metabolized in humans. However, the molecular basis for this phenomenon is currently unknown. We have previously shown that S-oxazepam glucuronide, the major oxazepam metabolite, is selectively formed by UDPglucuronosyltransferase (UGT) 2B15, whereas the minor Roxazepam glucuronide is produced by multiple UGTs other than UGT2B15. Phenotype-genotype studies were conducted using microsomes and DNA prepared from the same set of 54 human livers. Sequencing of the UGT2B15 gene revealed three nonsynonymous polymorphisms, D85Y, T352I, and K523T, with variant allele frequencies of 0.56, 0.02, and 0.40, respectively. D85Y genotype showed a significant effect (p ϭ 0.012) on S-oxazepam glucuronidation with lower median activities in 85Y/Y livers (49 pmol/min/mg protein) compared with 85D/D livers (131 pmol/min/mg), whereas 85D/Y livers were intermediate in activity (65 pmol/min/mg). There was also a significant trend (p ϭ 0.049) for higher S-oxazepam activities in the two 352T/I livers (135 and 210 pmol/min/mg) compared with the remaining 352T/T livers (median, 64 pmol/min/mg). Conversely, K523T genotype had no apparent effect on oxazepam glucuronidation (p Ͼ 0.05). Donor gender also significantly influenced S-oxazepam glucuronidation with higher median activities in male (65 pmol/min/mg) compared with female (39 pmol/min/ mg) livers (p ϭ 0.042). R-Oxazepam glucuronidation was not affected by either genotype or gender (p Ͼ 0.05). In conclusion, gender and D85Y genotype are identified as major determinants of S-oxazepam glucuronidation by human liver and may explain in part polymorphic oxazepam glucuronidation by human subjects.Glucuronidation, catalyzed by the UDP-glucuronosyltransferases (UGTs), is one of the major metabolic pathways responsible for elimination of potentially harmful xenobiotic and endobiotic compounds in mammals. UGT2B15 is one of more than 16 human UGT isoforms that have been discovered to date. Although originally identified as an enzyme with high intrinsic activity for glucuronidation of androgenic steroids (Chen et al., 1993), subsequent studies with recombinant UGT2B15 indicate that this enzyme is also likely to have a significant role in the metabolism of drugs and other xenobiotics (Green et al., 1994).

Mechanism of Gender-Divergent UDP-Glucuronosyltransferase mRNA Expression in Mouse Liver and Kidney

“…Clearance rates of drugs are determined by factors in addition to P450 metabolism, such as conjugation reactions and transport. Many drugs that are primarily glucuronidated show gender-divergent clearance, such as S-oxazepam (M Ͼ F) and fenofibrate (F Ͼ M) (Liu et al, 1991;Court et al, 2004). In rodents, there are examples of gender-divergent glucuronidation of drugs and exogenous chemicals, including MPA and BPA (Takeuchi et al, 2004;Stern et al, 2007).…”

“…For example, the benzodiazepine S-oxazepam, a UGT2B15 substrate, is glucuronidated at a higher rate in human liver microsomes from males than females (Court et al, 2004). In addition, the plasma clearance of S-oxazepam is 40% higher in males versus females (Greenblatt et al, 1980).…”

Sex Differences in the Expression of Hepatic Drug Metabolizing Enzymes

“…Men also show more rapid clearance of CYP2E1 substrates (e.g., chlorzoxazone) and certain CYP2D6 substrates, including propanolol, metoprolol, dextromethorphan, desipramine, and mirtazapine (Franconi et al, 2007;Schwartz, 2007). Increased rates of glucuronidation (UGT2B15 activity with oxazepam as substrate) also characterize male compared with female livers (Court et al, 2004). Overall, women are more likely to experience adverse drug reactions compared with men, due in part to sex differences in metabolic activity (Rademaker, 2001), with notable effects for cancer chemotherapeutic drugs (Mader, 2006;Wang and Huang, 2007) and antiretroviral agents (Ofotokun, 2005).…”

Phase II Drug Metabolism

“…The UGT enzymes of each family share at least 40% homology in DNA sequence, whereas members of UGT subfamilies exert at least 60% identity in DNA sequence (Burchell et al, 1995). As of the time of writing, 22 human UGT proteins can be distinguished: UGT1A1, UGT1A3, UGT1A4, UGT1A5, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2A1, UGT2A2, UGT2A3, UGT2B4, UGT2B7, UGT2B10, UGT2B11, UGT2B15, UGT2B17, UGT2B28, UGT3A1, UGT3A2, and UGT8A1 (Mackenzie et al, 2008;Miners et al, 2006;Court et al, 2004;Patten, 2006;Sneitz et al, 2009). In general, human UGT enzymes apparently exhibit a broad tissue distribution, although the liver is the major site of expression for many UGTs.…”