IDO1 and metastatic melanoma: Currently, IDO is being investigated as a target in cellular/vaccine therapy approaches, including a PD-L1/IDO peptide vaccine in metastatic melanoma (NCT03047928), and another combination PD-L1 plus IDO targeted vaccine (NCT05155254) which includes an IDO peptide vaccine (IO102) which is currently being studied in several actively recruiting trials (clinicaltrials.gov) in melanoma, bladder, head and neck, and lung cancers.