Phase II study of pembrolizumab-based therapy in previously treated extrapulmonary poorly differentiated neuroendocrine carcinomas: Results of Part A (pembrolizumab alone).

363Background: Immune checkpoint inhibitor (CPI) efficacy has not been established in extrapulmonary poorly-differentiated neuroendocrine carcinomas (EP-PDNECs). In small cell lung cancer, promising antitumor activity of CPI led to accelerated approval of nivolumab in 8/2018. We investigated the efficacy and safety of pembrolizumab (PEM)-based therapy in biomarker-unselected EP-PDNECs. Methods: Open label, multicenter, phase 2 study of PEM-based therapy in patients (pts) with EP-PDNECs, excluding Merkel cell carcinoma and well differentiated grade 3 NET, with progression on first-line systemic therapy, ECOG 0-1, and adequate hepatic and renal function. Enrollment via an adaptive Simon’s 2-stage design. Plan for 14 pts treated with PEM alone (Part A Stage 1) 200 mg IV every 3 weeks. If > 2 of 14 pts respond by week 18, then 21 additional pts enroll in Part A Stage 2, corresponding to H0 10% vs. H1 26% response rate (RR) at type I error 0.05 with power 80%. Otherwise study proceeds to Part B: PEM plus chemo...