Characterization and structure-activity relationship studies of flavonoids as inhibitors against human carboxylesterase 2.

Human carboxylesterases (hCEs) are key enzymes from the serine hydrolase superfamily. Among all identified hCEs, human carboxylesterase 2 (hCE2) plays crucial roles in the metabolic activation of ester drugs including irinotecan and flutamide. Selective and potent hCE2 inhibitors could be used to alleviate the toxicity induced by hCE2-substrate drugs. In this study, more than fifty flavonoids were collected to assay their inhibitory effects against hCE2 using a fluorescence-based method. The results demonstrated that C3 and C6 hydroxy groups were essential for hCE2 inhibition, while O-glycosylation or C-glycosylation would lead to the loss of hCE2 inhibition. Among all tested flavonoids, 5,6-dihydroxyflavone displayed the most potent inhibitory effect against hCE2 with the IC50 value of 3.50 μM. The inhibition mechanism of 5,6-dihydroxyflavone was further investigated by both experimental and docking simulations. All these findings are very helpful for the medicinal chemists to design and develop more potent and highly selective flavonoid-type hCE2 inhibitors.