IDO1 and cancer: IDO1 inhibitors have been tested in a variety of anti-cancer treatments, including epacadostat (Table 1), indoximod, and navoximod, but compared with the results of preclinical trials, the efficacy of IDO1 inhibitors alone have not been objectively reflected in the clinical trials (Soliman et al., 2016; Beatty et al., 2017; Nayak-Kapoor et al., 2018).