Prospective Assessment of Patients With Neuroendocrine Tumors and Current or Prior History of Carcinoid Syndrome or Diarrhea Undergoing Peptide Receptor Radionuclide Therapy With or Without Telotristat Ethyl

Inclusion Criteria:

* Written informed consent and HIPAA authorization for release of personal health information. NOTE: HIPAA authorization may be included in the informed consent or obtained separately.
* Males and females, aged 18 and older
* Histologically-confirmed neuroendocrine tumor (GI or other primary)
* Presence of somatostatin receptors as by either Ga-68 dotatate imaging or Octreoscan or comparable method, which is a requirement for PRRT (Lutathera ®). Disease does not need to be measurable per RECIST since it is not uncommon to have non-target lesions but not meet criteria for RECIST as long as presence of somatostatin receptors can be demonstrated. NOTE: Patients undergoing other types of PRRT would not be eligible for this clinical trial.
* Eligible for treatment with PRRT (Lutathera®) according to institutional practice and product label.
* Patient with current or prior history of symptomatic carcinoid syndrome or carcinoid diarrhea as per investigator assessment. Note: prior history and current controlled carcinoid patients are eligible.
* Demonstrate adequate organ function as defined in the protocol; all screening labs to be obtained within 28 days prior to registration.
* Life expectancy greater than 12 weeks as per investigator opinion
* ECOG performance status 0-2
* Women of childbearing potential (WOCBP) must have a negative pregnancy test (urine or serum βhCG) within 7 days prior to study registration. If a urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. See the protocol for WOCBP definition.

Exclusion Criteria:

* Surgery, radiotherapy, within 4 weeks; chemotherapy, or other investigational therapy within 2 weeks prior to study registration, or 5 half-lives of a drug, whichever is shorter.
* Uncontrolled congestive heart failure prior to study registration. Patients can be considered eligible if the disease is controlled at the date of randomization.
* Subject with another significant medical, psychiatric, or surgical conditions, currently uncontrolled by treatment, which may interfere with completion of the study as per investigator opinion.
* Women of childbearing potential (WOCBP), must agree to use appropriate method(s) of contraception. Women are considered to be of childbearing potential unless are surgically sterile (i.e., bilateral tubal ligation, bilateral oophorectomy, or complete hysterectomy) or postmenopausal (defined as 12 months with no menses without an alternative medical cause).
* WOCBP must agree to use appropriate method(s) of contraception from the time of informed consent until 7 months post-treatment completion. Complete abstinence is also an acceptable form of contraception.
* Men who are sexually active with WOCBP must agree to use appropriate method(s) of contraception from the first dose of study drug until 4 months post-treatment completion. Complete abstinence is also an acceptable form of contraception.
* Telotristat ethyl tablets contain lactose as an excipient. Patients with rare hereditary problems of galactose intolerance, the Lapp lactase deficiency, or glucose-galactose malabsorption should not take telotristat ethyl.
* Concomitant medications: Any other herbal or alternative medicines being used as an anti-cancer treatment are not allowed. NOTE: if patient is receiving a drug that is a CYP3A4 substrate, strongly consider switching to an alternate drug if possible. The latter is more of a recommendation, not an exclusion criterion.