PRIMARY OBJECTIVE:~I. To evaluate the efficacy of retifanlimab (A12) + telotristat ethyl (TE).~SECONDARY OBJECTIVE:~I. To evaluate the efficacy of A12 + TE.~SAFETY OBJECTIVES:~I. To evaluate the safety of A12 + TE. II. To evaluate the safety of withholding concurrent somatostatin analogue in patients receiving A12 + TE.~EXPLORATORY BIOMARKER OBJECTIVE:~I. To identify biomarkers that are predictive of response to A12 + TE (i.e., predictive biomarkers), are associated with progression to a more severe disease state (i.e., prognostic biomarkers), are associated with resistance to A12 + TE, are associated with susceptibility to developing adverse events, can provide evidence of study treatment activity, or can increase the knowledge and understanding of disease biology.~OUTLINE:~Patients receive retifanlimab intravenously (IV) over 30-60 minutes on day 1 and telotristat ethyl orally (PO) 3 times daily (TID) on days 1-28. Cycles repeat every 28 days for up to 2 years in the absence of disease progression or unacceptable toxicity.~After completion of study treatment, patients are followed up every 3 months for 2 years.
This phase II trial studies the effect of retifanlimab and telotristat ethyl in treating patients with neuroendocrine tumors that have spread to other places in the body (advanced) and carcinoid syndrome. Retifanlimab is a monoclonal antibody that may interfere with the ability of tumor cells to grow and spread. Telotristat ethyl is a drug used to reduce side effects of carcinoid syndrome. Giving retifanlimab and telotristat ethyl may help to control neuroendocrine tumors in patients who also have carcinoid syndrome.