Contribution of UGT enzymes to human drug metabolism stereoselectivity: a case study of medetomidine, RO5263397, propranolol and testosterone

The enantiomeric forms of chiral compounds have identical physical properties but may vary greatly in their metabolism by individual enzymes. Enantioselectivity in UDP-glucuronosyl transferase (UGT) metabolism has been reported for a number of compounds and with different UGT isoforms involved. However, the impact of such individual enzyme results on overall clearance stereoselectivity is often not clear. The enantiomers of medetomidine, RO5263397, and propranolol and the epimers testosterone and epitestosterone exhibit more than 10-fold difference in glucuronidation rates by individual UGT enzymes. In this study we examined the translation of human UGT stereoselectivity to hepatic drug clearance considering the combination of multiple UGTs to overall glucuronidation, the contribution of other metabolic enzymes such as cytochromes P450, and the potential for differences in protein binding and blood/plasma partitioning. For medetomidine and RO5263397, the high individual enzyme (UGT2B10) enantioselectivity translated into ~3- to >10-fold differences in predicted human hepatic in vivo clearance. For propranolol the UGT enantioselectivity was irrelevant in the context of high cytochrome P450 (CYP) metabolism. For testosterone a complex picture emerged, due to differential epimeric selectivity of various contributing enzymes and potential for extrahepatic metabolism. Quite different patterns of CYP and UGT-mediated metabolism were observed across species, as well as differences in stereoselectivity, indicating that extrapolation from human enzyme and tissue data is essential when predicting human clearance enantioselectivity.