Indoleamine 2,3-dioxygenase 1 (IDO1) is emerging as a promising therapeutic target for the treatment of malignant tumors characterized by dysregulated tryptophan metabolism.However,the antitumor efficacy of existing small-molecule IDO1 inhibitors is still unsatisfactory,and the underlying mechanism remains largely undefined.To identify novel IDO1 inhibitors,an in-house natural product library of 2000 natural products was screened for inhibitory activity against recombinant human IDO1.High throughput fluorescence-based screening identified 79 compounds with inhibitory activity ＞ 30％ at 20 μM.Nine natural products were further confirmed to inhibit IDO1 activity by ＞ 30％ using Ehrlich’s reagent reaction.Compounds 2,7,and 8 were demonstrated to inhibit IDO1 activity in a cellular context.Compounds 2 and 7 were more potent against IDO1 than TDO2 in the enzymatic assay.The kinetic studies showed that compound 2 exhibited noncompetitive inhibition,whereas compounds 7 and 8 were graphically well matched with uncompetitive inhibition.Compounds 7 and 8 were found to bind to the ferric-IDO1 enzyme.Docking stimulations showed that the naphthalene ring of compound 8 formed T-shaped π-π interactions with Phe-163 and that the 6-methyl-naphthalene group formed additional hydrophobic interactions with IDO1.Compound 8 was identified as a derivative of tanshinone,and preliminary SAR analysis indicated that tanshinone derivatives may be promising hits for the development of IDO1 inhibitors.This study provides new clues for the discovery of IDO1/TDO2 inhibitors with novel scaffolds.