IDO1 and cancer: For instance, inhibitors of IDO-1 (indoleamine 2,3-dioxygenase 1, the rate-limiting enzyme involved in tryptophan catabolism), like Epacadostat and Linrodostat, have been tested in cancer trials (often in tandem with immune checkpoint inhibitors) to block tryptophan depletion, and might offer promising intervention routes [27].