UDP-glucuronosyltransferase (UGT) 1A1 mainly contributes to the glucuronidation of trovafloxacin

Identification of drug-metabolizing enzyme(s) responsible for the metabolism of drugs is an important step to understand not only interindividual variability in pharmacokinetics but also molecular mechanisms of metabolite-related toxicity. While it was reported that the major metabolic pathway of trovafloxacin, which is an antibiotic, was glucuronidation, the UDP-glucuronosyltransferase (UGT) isoform(s) responsible for the trovafloxacin glucuronidation has not been identified yet. In the present study, among the functional human UGT members, UGT1A1, UGT1A3, and UGT1A9 exhibited higher trovafloxacin acyl-glucuronidation activities. While other UGT members such as UGT1A8, UGT2B7, and UGT2B15 showed glucuronidation activity toward trovafloxacin, the metabolic velocity was extremely low. In human liver microsomes, trovafloxacin acyl-glucuronidation followed the Hill equation with S50 value of 95 μM, Vmax value of 243 pmol/min per mg, and a Hill coefficient of 2.0, while the UGT1A1-expressing system displayed Michaelis–Menten kinetics with a substrate inhibition, with Km value of 759 μM and Vmax value of 1160 pmol/min per mg. In human liver microsomes prepared from poor metabolizers (UGT1A1*28/*28), significantly reduced trovafloxacin acyl-glucuronide formation activity was observed, indicating that UGT1A1 mainly, while other UGT members such as UGT1A3 and UGT1A9 partially, contributes to the glucuronidation of trovafloxacin.