Retifanlimab (INCMGA00012) and Telotristat Ethyl for the Treatment of Advanced Neuroendocrine Tumors and Carcinoid Syndrome

Inclusion Criteria: * Grade 1, 2, or 3 (or described as low grade, intermediate grade, well differentiated, or moderately differentiated) neuroendocrine tumor, according to reviewing pathologist or documented interpretation of report by the investigator * Progressive disease over the preceding 12 months * Prior therapy with any number of anticancer therapies, but a somatostatin analogue (such as octreotide, lanreotide, or pasireotide) must be one of the prior therapies * Carcinoid syndrome, as documented by the investigator * Patients using a somatostatin analogue for symptom control must be on stable doses for 56 days prior to enrollment * Signed informed consent form * Age \>= 18 years * Ability to comply with the study protocol, in the investigator's judgment * Measurable disease according to Response Evaluation Criteria in Solid Tumors (RECIST) version (v)1.1 * Previously irradiated lesions can be considered as measurable disease only if progressive disease has been unequivocally documented at that site since radiation * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Absolute neutrophil count \>= 1,500/mm\^3 without granulocyte colony-stimulating factor support (obtained within 28 days prior to initiation of study treatment) * Lymphocyte count \>= 500/mm\^3 (obtained within 28 days prior to initiation of study treatment) * Platelet count \>= 100,000/mm\^3 without transfusion (obtained within 28 days prior to initiation of study treatment) * W…