Glucuronidation of fimasartan, a new angiotensin receptor antagonist, is mainly mediated by UGT1A3.

1. Fimasartan is an angiotensin receptor II antagonist used to treat patients with hypertension. This drug is mainly excreted into bile as either the parent compound or a glucuronide conjugate. In this study, we examined the glucuronidation of fimasartan and characterized the UDP-glucuronosyltransferases (UGTs) responsible for the glucuronidation. 2. Only one type of fimasartan glucuronide was observed after incubation with pooled human liver microsomes (HLMs) and was identified as an N2-glucuronide based on comparison with an authentic standard. 3. Among the 12 UGT isoforms tested, UGT1A1, UGT1A3 and UGT2B7 showed catalytic activity toward fimasartan glucuronidation. The intrinsic clearance (CLint) of UGT1A3 was 68.5- and 21.4-fold higher than that of UGT1A1 and UGT2B7, respectively, and the estimated relative contribution of UGT1A3 in human liver was 94.1%. Both chemical inhibition and correlation studies demonstrated that fimasartan glucuronidation activity in HLMs was significantly related with UGT1A3 activity. Fimasartan glucuronide was identified as a substrate for P-glycoprotein (Pgp) and breast cancer response protein (BCRP). 4. These findings collectively indicate that UGT1A3 is the major UGT isoform responsible for the glucuronidation of fimasartan, and this glucuronide is excreted from hepatocytes via MDR1 and BCRP.