Indoleamine 2,3-Dioxygenase (IDO) Inhibition as a Strategy to Augment Cancer Immunotherapy

Indoleamine 2,3-dioxygenase (IDO) is an enzyme of interest in immuno-oncology because of the immunosuppressive effects that result from its role in tryptophan catabolism. IDO is upregulated in malignancy and is associated with poor prognosis in multiple cancer types. IDO inhibitors have been developed to target IDO, both directly and indirectly. Pre-clinical data have shown combined IDO and checkpoint inhibition to be an efficacious strategy for tumor control. Clinical trials of IDO inhibitors with chemotherapy or immunotherapy are currently underway. This review describes the function of IDO and its inhibitors and summarizes the efficacy and toxicity data from recent clinical trials with these drugs.