Background: PRRT using Lutetium Lu 177 Dotatate, has been approved by the FDA for metastatic, progressive gastroenteropancreatic neuroendocrine tumors (GEPNETs). Despite an impressive progression-free survival noted in NETTER-1 study, most patients will eventually progress after PRRT treatment. Increased serotonin secretion is found in a number of slow-growing NETs of the diffuse neuroendocrine system. Serotonin stimulates proliferation in cell lines of a typical (NCI-H727), an atypical (NCI-H720) bronchopulmonary NET, a small intestinal NET (KRJ-I), and a functioning human pancreatic carcinoid cell line (BON). This stimulation of growth by serotonin can be performed in an autocrine manner. Telotristat ethyl is a novel oral inhibitor of tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin biosynthesis. In 2017, results of placebo-controlled phase III study (Telestar) evaluating Telotristat ethyl confirmed that among patients with carcinoid syndrome not adequately controlled by somatostatin analogs, treatment with Telotristat ethyl was generally safe and well tolerated and resulted in significant reductions in bowel movement frequency and urinary 5- hydroxy indoleacetic acid (u5-HIAA). Based on results of the Telestar trial, FDA approved 250 mg PO TID dosage of Telotristat for control of carcinoid syndrome diarrhea. Of note, 500 mg TID arm showed increased decline in urinary 5-HIAA as compared to 250 mg TID arm. Study Hypotheses: We hypothesize that inhibition of serotonin production will lead to cytostatic effect on neuroendocrine tumors and will complement anti-tumor activity of lutetium 177 dotatate. We anticipate our proposed combination will result in improved treatment efficacy as reflected by improved 20-month PFS as compared to historical control (NETTER-1). Methods: This is a single center, open label, randomized, parallel arm, phase II study evaluating two dose levels of Telotristat (Xermelo; 250 mg vs 500 mg) in combination with Lutetium Lu 177 Dotatate (Lutathera) in 70 well-differentiated neuroendocrine tumor patients. Study is currently enrolling patients and is expected to complete accrual in 3 years. Clinical trial information: NCT04543955.