1. Niclosamide is an old anthelmintic drug that shows potential in fighting against cancers. Here, we characterized the metabolism of niclosamide by cytochrome P450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) using human liver microsomes (HLM) and expressed enzymes. 2. NADPH-supplemented HLM (and liver microsomes from various animal species) generated one hydroxylated metabolite (M1) from niclosamide; and UDPGA-supplemented liver microsomes generated one mono-O-glucuronide (M2). The chemical structures of M1 (3-hydroxy niclosamide) and M2 (niclosamide-2-O-glucuronide) were determined through LC-MS/MS and/or NMR analyses. 3. Reaction phenotyping revealed that CYP1A2 was the main enzyme responsible for M1 formation. The important role of CYP1A2 in niclosamide metabolism was further confirmed by activity correlation analyses as well as inhibition experiments using specific inhibitors. 4. Although seven UGT enzymes were able to catalyze glucuronidation of niclosamide, UGT1A1 and 1A3 were the enzymes showed the highest metabolic activities. Activity correlation analyses demonstrated that UGT1A1 played a predominant role in hepatic glucuronidation of niclosamide, whereas the role of UGT1A3 was negligible. 5. In conclusion, niclosamide was subjected to efficient metabolic reactions hydroxylation and glucuronidation, wherein CYP1A2 and UGT1A1 were the main contributing enzymes, respectively.
