PDCD1 and central nervous system cancer: Some IDO1 inhibitors have been developed and applied in antitumor therapy in preclinical and clinical trials.50 Although IDO1 inhibitors have not demonstrated antitumor activity as single agents in orthotopic glioma animals, a unique synergy when combined with radiotherapy, temozolomide, or anti-programmed cell death protein 1 antibody has been noted.28–30,51–53 TDO inhibitors have not been tested in glioma animals.