IDO1 and cancer: The preclinical findings suggest that immune checkpoints are optimal targets for cancer immunotherapy, which stimulated the development of inhibitors targeting these checkpoint receptors and/or their ligands, including antibodies for PD-1 (pembrolizumab, nivolumab, and pidilizumab) [110–112], PD-L1 (MEDI4736, MPDL3280a, and MDX-1105) [113, 114], CTLA-4 (ipilimumab and tremelimumab) [115–117], and IDO (indoximod and INCB024360)[118].