IDO1 and cancer: The combination therapy of ICIs with small molecules that target various pathways, including epidermal growth factor receptor (EGFR) inhibitors, vascular endothelial growth factor (VEGF) inhibitors, indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors, and Bruton’s tyrosine kinase (BTK) inhibitors are under investigation as novel anti-cancer strategies [158,171].