Indoleamine 2,3-dioxygenase 1 (IDO1) is a potently immunosuppressive protein that inhibits antitumor immunity through both tryptophan metabolism and non-enzymatic functions. Pharmacological therapies targeting IDO1 enzyme activity have generally failed to improve the overall survival of patients with cancer. Developing new therapeutic agents that are capable of neutralizing both enzyme-and non-enzyme-derived immunosuppressive IDO1 effects is therefore of high interest. We previously described the development of a novel Proteolysis Targeting Chimera (PROTAC), NU223612, that degrades IDO1 in cultured human glioblastoma (GBM) cells, as well as in well-established brain tumors,