Discovery of (E)-2-cyano-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)-3-(1H-indol-3-yl)acrylamide as a novel TDO inhibitor for cancer treatment.

Tryptophan 2,3- oxygenase (TDO), an immune-suppressing enzyme, catalyzes the catabolism of tryptophan to N-formylkynurenine. Overexpression of TDO in multiple malignancies associates with immune evasion mechanisms. Therefore, we aimed to discover novel inhibitors targeting TDO as a potential strategy for cancer treatment. In this study, we designed and synthesized a series of (E)-2-cyano-3-(1H-indol-3-yl) acrylamide derivatives. A lead compound, 5c, was identified and exhibited inhibitory activity against TDO, with an IC50 value of 1.25 ± 0.04 μM. 5c demonstrated significant tumor growth suppression with a favorable safety profile in Hepa1-6 hepatocellular carcinoma allograft model. Notably, 5c synergized with the PD-1/PD-L1 inhibitor BMS-202, both in vitro and in vivo. These findings suggested compound 5c could serve as a promising candidate for targeting TDO to address tumor immune tolerance in cancer therapy.