A patient with metastatic neuroendocrine tumour has carcinoid syndrome with disabling diarrhoea despite octreotide and a low-tryptophan diet. A newer oral agent added to control the diarrhoea works by:
- A Blocking 5-HT3 receptors on enteric vagal afferents
- B Inhibiting tryptophan hydroxylase, reducing peripheral serotonin synthesis ✓
- C Antagonising 5-HT2B receptors on mesenteric vessels
- D Activating somatostatin receptors subtype 3 on enterochromaffin cells
Explanation
Telotristat ethyl is a peripheral tryptophan hydroxylase inhibitor that blocks the rate-limiting step of serotonin synthesis, lowering serotonin production in enterochromaffin cells and neuroendocrine tumour tissue. It is approved for carcinoid diarrhoea inadequately controlled by somatostatin analogues. Because it does not cross the blood-brain barrier appreciably, psychiatric effects are limited. Ondansetron targets 5-HT3 receptors and helps nausea more than secretory diarrhoea, and octreotide already occupies somatostatin receptors, mainly subtype 2.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.