IDO1 and neoplasm: After intravenous injection, OX/IND-MSNP was first passively targeted to the primary tumor via EPR effect, suppressing the formation of the immunosuppressive microenvironment of the tumor by inhibiting IDO metabolic pathway and downregulating the level of Foxp3+ T cells (Treg cells) mediated by IND, which plays a positive role in the treatment of pancreatic ductal adenocarcinoma.