Epacadostat is primarily used in the field of oncology as an investigational drug targeting cancer immunotherapy. It functions as an inhibitor of the enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which plays a role in suppressing the immune system's response to tumors. By blocking IDO1, it aims to enhance the body's immune defense against cancer cells, particularly when used in combination with other immunotherapies like checkpoint inhibitors. Clinical trials have explored its potential in treating various cancers, including melanoma, renal cell carcinoma, and non-small cell lung cancer, with the goal of improving patient outcomes by boosting the effectiveness of the immune system in recognizing and attacking tumors.