TDO2 and neoplasm: Based on the tumor-promoting effects of IDO and TDO, studies seeking small-molecule inhibitors for cancer treatment have been ongoing for recent years, including KHK2455, Epacadostat (INCB 024360), Indoximod, Linrodostat (BMS-986205) and Celecoxib (http://www.clinicaltrials.gov/).