CD8A and neoplasm: For tryptophan catabolites, daily gavage with IPA (60 mg/kg) increases plasma and intratumoral IPA levels in multiple mouse transplant tumor models, boosts CD8+ T-cell infiltration/effector cytokines, and further enhances αPD-1 efficacy; similar anti-tumor effects can also be achieved by direct intratumoral injection of 5 μM IPA [107, 194].