Used in cancer immunotherapy, particularly in clinical trials for solid tumors. It works by inhibiting the IDO1 (indoleamine 2,3-dioxygenase 1) enzyme, which plays a key role in suppressing the immune system’s ability to attack cancer cells. By blocking IDO1, epacadostat helps enhance T-cell activation and improve the body’s immune response against tumors. It has been studied in combination with immune checkpoint inhibitors like pembrolizumab and nivolumab in melanoma, prostate cancer, and other advanced cancers. Despite promising early results, some phase III trials did not meet primary endpoints, leading to ongoing research into patient selection and combination strategies.