IDO1 and cancer: Recent efforts have been made to establish dual inhibitors that antagonize NAMPT and other enzymes involved in NAD biosynthesis, such as indoleamine 2,3-dioxygenase 1 (IDO1), as well as other proteins important in cancer, including histone deacetylases (HDACs), p21 (RAC1)-activated kinase 4 (PAK4) and even IDH1 itself [51,52,53].