UDP-glucuronosyltransferases (UGT) represent the major phase II drug metabolism pathway in human. UGT enzymes catalyze the transfer of glucuronic acid (derived from UDPGA co-factor) to xenobiotics and endogenous substrates having nucleophilic acceptor groups, making them more polar and readily excreted in urine or bile. To date, at least 18 human UGT enzymes have been cloned and sequenced. Like the P450 enzymes, UGTs possess broad and overlapping substrate specificities. Clinically relevant drug interactions have been identified for UGT isoforms, leading regulatory agencies (FDA) to require drug interaction testing for compounds showing significant UGT metabolism.
UGT1A4 is important for amine glucurondation of numerous drug substrates, forming N-glucuronide metabolites. It shows a preference for tertiary amine substrates compared with primary or secondary amines. The O-glucuronidation of phenolic sapogenins has been reported for UGT1A4; e.g hexogenin is a specific substrate for UGT1A4, and has also been used a potent, selective inhibitor of UGT1A4 activity. The N-glucuronidation of Trifluorperazine is a selective substrate/reaction for UGT1A4 , as well as the N-glucuronidation of 1’-hydroxymidazolam.