Background: Limited systemic treatment options are available for progressive well-differentiated neuroendocrine tumors (NET), also called carcinoid tumors. Given emerging evidence for immunotherapy response in high grade NET including small cell lung cancer, we sought to determine the efficacy of combination immunotherapy with ipilimumab and nivolumab in patients with advanced, progressive, well-differentiated NET in an open label phase II clinical trial. Methods: Eligible patients had well-differentiated, nonfunctional NET of lung, pancreas, or GI origin that had progressed within the past 12 months after at least one line of prior therapy. Patients received nivolumab 240 mg every 2 weeks and ipilimumab 1mg/kg every 6 weeks for up to 2 years. Primary endpoint was objective response rate (ORR) by RECIST v1.1. Using a Simon’s 2-stage design, the study planned to accrue up to 56 patients. Based on published response rates to everolimus of 5%, we hypothesized that this regimen would be considered promising if the true ORR is > 15%. Results: Nine patients were enrolled prior to study closure due to funding, including 6 patients with NET of lung origin, 2 pancreatic, and 1 small bowel (Table). Median age was 71 years. All patients had distant metastatic disease at enrollment, with an average of 2 prior lines of therapy. Four of 9 patients achieved the primary endpoint of confirmed objective response, all of whom have ongoing response with a median duration of 15.4 months. Five of 9 patients, including all 4 responders, experienced immune-related toxicity requiring treatment modification or discontinuation. The trial did not accrue the target of 56 patients, however, objective response in 4 of 9 patients (ORR 44.4%, 90% CI: 16.9-74.9%) excluded the response rate target (15%). Conclusions: The impressive ORR of 44% with a median duration of response exceeding 15 months in this small clinical trial warrants further study of combination CTLA-4 and PD-1 inhibition in previously treated well-differentiated NET. Our ongoing immunologic and genomic correlative analysis in responders and non-responders will help inform future study of immunotherapy in this patient population in need of new systemic therapy approaches. Clinical trial information: NCT03420521

*Response ongoing. Int, intermediate; G2, grade 2; G3, grade 3; PR, partial response; SD, stable disease; PD, progressive disease; NE, not evaluable.