Background: Diarrhea associated with carcinoid syndrome has been attributed to tumor production of serotonin. Telotristat etiprate, (LX1032, LX1606), is an oral inhibitor of peripheral serotonin synthesis. This study explored the safety, tolerability, and efficacy of telotristat etiprate in carcinoid patients with octreotide-refractory diarrhea. Methods: Carcinoid patients with ≥4 bowel movements (BMs)/day on octreotide were randomized 3:1 to receive telotristat etiprate or placebo as double-blind treatment. Patients enrolled in sequential, escalating dose cohorts of 150, 250, 350, or 500 mg tid, followed by a 500 mg tid expansion cohort. Patients were followed for toxicity, 24-hr urinary 5-HIAA (u5-HIAA), BM frequency, and self-reported bowel-related symptoms. Subjects were asked “In the past 7 days, have you had adequate relief of your carcinoid syndrome bowel complaints such as diarrhea, urgent need to have a bowel movement, abdominal pain or discomfort?” Responses (yes or no) were analyzed as categorical variables. Results: 16 patients enrolled in the 4 escalating dose cohorts and 7 in the expansion cohort; 18 on telotristat etiprate and 5 on placebo. Median age: 62 yrs; mean 6.3 BMs/day (range, 4-10). AEs included primarily mild-moderate diarrhea, nausea, and abdominal discomfort. In treated subjects, adequate relief was reported as: Week 1 - 6/18 (33.3%), Week 2 - 5/16 (31.3%), Week 3 - 5/15 (33.3%), and Week 4 - 6/13 (46.0%). No placebo subjects reported improvement at any timepoint. Biochemical response (≥50%reduction in u5-HIAA) and BM response (≥30% reduction in daily BMs for 2 weeks) were associated with reporting of adequate relief. For evaluable telotristat etiprate-treated patients, 9/16 (56%) experienced a biochemical response and 5/18 (28%) experienced a clinical (BM) response; no placebo subjects achieved either biochemical or clinical response. Conclusions: Treatment with telotristat etiprate was associated with decreases in u5-HIAA and BM frequency, and with self-reported relief of bowel related symptoms. Treatment in an extension phase with open-label telotristat etiprate is ongoing.