Design, synthesis, and biological evaluation of novel molecules as potent inhibitors of indoleamine 2,3-dioxygenase 1.

Tumoral immune escape is an obstacle to successful cancer therapy. Tryptophan catabolism mediated by indoleamine 2,3-dioxygenase (IDO1) is an important mechanism of peripheral immune tolerance contributing to tumoral immune resistance, and IDO1 inhibition is an active area of drug development. Several classes of small molecule-based IDO1 inhibitors have already been reported. Still, only a few compounds are currently being evaluated in various stages of clinical trials as adjuvants or in combination with chemo- and radiotherapies. In this study, a novel series of 1,2,5-oxadiazole-3-carboximidamide derivatives were designed, synthesized, and evaluated for inhibitory activities against IDO1, and their structure-activity relationship was investigated. Notably, several compounds (11c, 11j, 11o, and 11u) showed powerful anti-tumor effects in the low micromolar range. Among them, compound 11u exhibited excellent inhibitory potency against hIDO1 (IC₅₀ = 42.2 ± 2.23 nM) and in Hela cells expressing hIDO1 (IC₅₀ = 4.35 ± 0.13 nM). Combined with favorable in vitro potency, pharmacokinetic profile, and in vivo efficacy, the promising antitumor drug candidate 11u has subsequently advanced into preclinical research. These compounds provide valuable ideas and information for developing new cancer immunotherapy.