IDO1 and neoplasm: NLG919, a potent indoleamine‐2,3‐dioxygenase (IDO) inhibitor, was co‐encapsulated into PpIX‐NLG@Lipo along with PS (PpIX) (Table 2, entry 15).[181] The IDO inhibitor blocked the checkpoint by reliving the immunosuppressive microenvironment and activating the host immune system, and enhance PDT‐induced immune responses by increasing the penetration of CD8+ T lymphocytes into the tumor site.