Glucuronidation of HMR1098 in human microsomes: evidence for the involvement of UGT1A1 in the formation of S-glucuronides.

HMR1098, a novel KATP-blocking agent, is metabolized to form an S-glucuronide in rat and dog bile. Synthesis of the S-glucuronide metabolite was studied in human liver and kidney microsomes. Recombinant UPD-glucuronosyltransferases (UGTs) were screened for activity, and kinetic analysis was performed to identify the isoform or isoforms responsible for the formation of this novel S-glucuronide in humans. S-Glucuronidation is relatively rare, but from this study it appears that S-glucuronides are not generated exclusively by a single UGT isoform. From the panel of recombinant isoforms used, both UGT1A1 and UGT1A9 catalyzed the glucuronidation of HMR1098. The Vmax values in both instances were similar, but the Km for UGT1A1 was substantially lower than that measured for UGT1A9, 82 microM compared with 233 microM, respectively. Liver and kidney microsomes displayed similar Km values, but the Vmax in kidney was more than 20-fold less than in liver microsomes, which is suggestive of a significant role for the bilirubin UGT in catalysis of HMR1098, although other UGTs may play a secondary role.