Identification and Characterization of Human UDP-Glucuronosyltransferases Responsible for the In Vitro

Daphnetin has been developed as an oral medicine for treatment of coagulation disorders and rheumatoid arthritis in China, but its in vitro metabolism remains unknown. In the present study, the UDP-glucuronosyltransferase (UGT) conjugation pathways of daphnetin were characterized. Two metabolites, 7-O-monoglucuronide daphnetin (M-1) and 8-O-monoglucuronide daphnetin (M-2), were identified by liquid chromatography/mass spectrometry and NMR when daphnetin was incubated, respec-tively, with liver microsomes from human (HLM), rat (RLM), and minipig (PLM) and human intestinal microsomes (HIM) in the pres-ence of UDP-glucuronic acid. Screening assays with 12 human recombinant UGTs demonstrated that the formations of M-1 and M-2 were almost exclusively catalyzed by UGT1A9 and UGT1A6, whereas M-1 was formed to a minor extent by UGT1A3, 1A4, 1A7, 1A8, and 1A10 at a high substrate concentration. Kinetics studies