Indoleamine 2,3-dioxygenase 1 (IDO1), a known immunosuppressive enzyme that 
catalyzes the rate-limiting step in the oxidation of tryptophan (Trp) to 
kynurenine (Kyn), has received increasing attention as an attractive 
immunotherapeutic target for cancer therapy. Up to now, eleven small-molecule 
IDO1 inhibitors have entered clinical trials for the treatment of cancers. In 
addition, proteolysis targeting chimera (PROTAC) based degraders also provide 
prospects for cancer therapy. Herein we present a comprehensive overview of the 
medicinal chemistry strategies and potential therapeutic applications of IDO1 
inhibitors in nonclinical trials and IDO1-PROTAC degraders.