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.

人羧酸酯酶（hCEs）是来自丝氨酸水解酶超家族的关键酶。在所有已鉴定的hCE中，人羧酸酯酶2（hCE2）在酯类药物（包括伊立替康和氟他胺）的代谢活化中起关键作用。选择性和有效的hCE2抑制剂可用于减轻hCE2底物药物诱导的毒性。在这项研究中，使用基于荧光的方法，收集了五十多种类黄酮以测定其对hCE2的抑制作用。结果表明，C3和C6羟基对于hCE2抑制是必不可少的，而O-糖基化或C-糖基化将导致hCE2抑制作用的丧失。在所有测试过的类黄酮中，IC ₅₀对hCE2的抑制作用最强，是5,6-二羟基黄酮值为3.50μM。通过实验和对接模拟进一步研究了5,6-二羟基黄酮的抑制机理。所有这些发现对药物化学家设计和开发更有效和高度选择性的类黄酮型hCE2抑制剂非常有帮助。