Telotristat ethyl has been approved in the United States and in Europe, under the brand name XERMELO®, for the treatment of carcinoid syndrome (CS) diarrhea in combination with somatostatin analog (SSA) therapy in adults inadequately controlled by SSA therapy. Telotristat ethyl has been shown to lower serotonin (5-HT) levels in the peripheral system by inhibiting tryptophan hydroxylase (TPH). Through inhibition of TPH, telotristat ethyl reduces the production of peripheral 5-HT.

Preclinical data suggests that inhibiting TPH-1 and serotonin synthesis may reduce cancer cell growth in cholangiocarcinoma and gallbladder cancers (collectively known as biliary tract cancers, or BTC). These data suggest an opportunity for telotristat ethyl. A Phase 2 study has been initiated (https://clinicaltrials.gov/ct2/show/NCT03790111).