IDO1-IN-27 (Compound I-1) is an inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1), displaying an IC50 of 0.3951 µM against recombinant hIDO1. It also inhibits hIDO1 expression in HeLa cells with an EC50 of 62 nM. By reducing the mRNA expression of pro-inflammatory factors (TNF-alpha, IL-6, and IL-1beta), IDO1-IN-27 effectively stimulates T cell proliferation and simultaneously inhibits the growth of LLC cells.