N-Benzyl/Aryl Substituted Tryptanthrin as Dual Inhibitors of Indoleamine 2,3-Dioxygenase and Tryptophan 2,3-Dioxygenase.

Indoleamine 2,3-dioxygenase 1 (IDO1), which catalyzes the initial and rate-limiting step of the kynurenine pathway of tryptophan catabolism, has emerged as a key target in cancer immunotherapy because of its role in enabling cancers to evade the immune system. Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase 2 (IDO2) catalyze the same reaction and play a potential role in cancer immunotherapy. Starting from our previously discovered tryptanthrin IDO1 inhibitor scaffold, we synthesized novel N-benzyl/aryl substituted tryptanthrin derivatives and evaluated their inhibitory efficacy on IDO1, TDO, and IDO2. Most compounds showed similar high inhibitory activities on both IDO1 and TDO, which were significantly superior over that of IDO2 with magnitude difference. We showed that N-benzyl/aryl substituted tryptanthrin directly interacted with IDO1, TDO, and IDO2, significantly augmented the proliferation of T cells in vitro, blocked the kynurenine pathway, and suppressed tumor growth when administered to LLC and H22 tumor-bearing mice.