Recent advances in the discovery of indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors

Indoleamine 2,3-dioxygenase 1 (IDO1), an important immunoregulatory enzyme ubiquitously expressed in various tissues and cells, plays a key role in tryptophan metabolism via the kynurenine pathway and has emerged as an attractive therapeutic target for the treatment of cancer and other diseases, such as Alzheimer's disease and arthritis. IDO1 has diverse biological roles in immune suppression and tumor progression by tryptophan catabolism. In addition, IDO1-mediated immune tolerance assists tumor cells in escaping the immune surveillance. Recently, extensive and enormous investigations have been made in the discovery of IDO1 inhibitors in both academia and pharmaceutical companies. In this review, IDO1 inhibitors are grouped as tryptophan derivatives, inhibitors with an imidazole, 1,2,3-triazole or tetrazole scaffold, inhibitors with quinone or iminoquinone, N-hydroxyamidines and other derivatives, and their enzymatic inhibitory activity, selectivity and other biological activities are also introduced and summarized.