Pembrolizumab Compared with Standard-of-Care Observation in Treating Patients with Completely Resected Stage I–III Merkel Cell Cancer (STAMP)

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine neoplasm with a dramatically increasing incidence of up to 5–10% per year since the 1990s.¹ These tumors are considered both highly metastatic and immunogenic, with Merkel-cell polyomavirus (MCPyV) and ultraviolet-mediated cell damage as their major causative factors.^1,2 In the tumor microenvironment, approximately 50% of both tumor-infiltrating lymphocytes (TILs) and tumor cells express programmed death-1 (PD-1) and programmed death-ligand 1 (PD-L1), respectively.³ As such, pembrolizumab, a PD-1 immune checkpoint inhibitor, has been US FDA-approved as first-line therapy for advanced MCCs due to an objective response rate of 56% (KEYNOTE-017).^3,4 There is however a paucity of data on the use of pembrolizumab in the adjuvant setting for patients with non-metastatic MCC, unlike melanoma where a recent study documented improved progression-free survival for patients treated with this checkpoint inhibitor after surgery.⁵ Currently, all patients with resectable MCC undergo regional nodal staging via a sentinel lymph node biopsy, with node-positive patients (stage III) proceeding to a formal lymph node dissection and/or adjuvant radiotherapy.⁶ …