IDO inhibitors, especially targeting IDO1 isoform, are an important class of pharmaceuticals because KYN and its metabolites directly participate in many physiological and pathological processes, including tumor immune escape, development of infections, autoimmune, or metabolic diseases and neurodegeneration [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,37,38,39,40,41,42,43,44,45,46,47,72,77,81,100,184]. Here, IDO1 is linked to neoplasm.