A patient with carcinoid syndrome has severe diarrhea and flushing. Octreotide provides partial relief. The physician adds telotristat ethyl, which significantly reduces diarrhea. What is the mechanism of action of telotristat ethyl?

• A Tryptophan hydroxylase inhibitor reducing peripheral serotonin synthesis ✓
• B 5-HT3 receptor antagonist blocking serotonin action on the gut
• C Somatostatin analog with higher affinity for SSTR2 than octreotide
• D MAO inhibitor preventing serotonin degradation

Explanation

Telotristat ethyl is a tryptophan hydroxylase (TPH) inhibitor that blocks the rate-limiting step in peripheral serotonin biosynthesis. It is specifically approved for carcinoid syndrome diarrhea inadequately controlled by somatostatin analogs. Unlike ondansetron (5-HT3 antagonist) which acts on receptors, telotristat reduces serotonin production itself. TPH exists in two isoforms; telotristat inhibits TPH1 in peripheral tissues (enterochromaffin cells) without affecting central serotonin synthesis.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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