IDO1 and neoplasm: More importantly, NO can react with ROS to generate peroxynitrite (ONOO−), thus directly killing malignant tumor cells,[23, 24] but also reversibly inhibiting the activity of IDO1, regulating the tumor immune microenvironment.[24] Therefore, it is reasonable to consider that the combination strategy of CDT with NO treatment may improve the results of ICB antitumor immune therapy related to IDO1 blockade.