AMT-PET in Monitoring Telotristat Etiprate Treatment in Participants With MetastaticNeuroendocrine Neoplasm

PRIMARY OBJECTIVES: I. To evaluate the effect of telotristat etiprate (telotristat ethyl) treatment in patients with advanced neuroendocrine tumors (NETs) using carbon C 11 alpha-methyltryptophan (alpha-\[11C\]methyl-?L-?tryptophan) (AMT)-?positron emission tomography (PET) as measured by changes in tumor maximum standardized uptake value (SUVmax). SECONDARY OBJECTIVES: I. Show that NETs will have increased AMT uptake on PET, as compared to surrounding non-tumor tissue at baseline. II. Use compartmental modeling (in tumors with the left ventricle of the heart in the field-of-view) to measure change in AMT retention. III. Measure change in AMT retention as mean standardized uptake value (SUVmean). OUTLINE: Participants undergo AMT-PET within 7 days prior to, and 9-14 days after start of telotristat etiprate treatment. Participants receive telotristat etiprate orally (PO) three times a day (TID) for 9-14 days. After completion of study treatment, participants are followed up for 3 months.