Although many experimental and theoretical studies indicate that IDO-1-based treatment strategies have progressed in recent years, especially in combination with other ICIs, such as pembrolizumab and ipilimumab, more clinical trials should explore the efficacy and safety of D-1-MT and epacadostat in the treatment of several other metastatic tumors, regardless of their use as monotherapies or combination therapies. This evidence concerns the gene IDO1 and metastatic neoplasm.