Response to Anti-PD-1 Therapy in Metastatic Merkel Cell Carcinoma Metastatic to the Heart and Pancreas.

Metastatic Merkel cell carcinoma (MCC) is a lethal, Merkel cell polyomavirus (MCPyV) cancer with no currently available effective therapy. Harnessing the immune system through an immune checkpoint blockade is an attractive option because the immune system appears to be dysfunctional in the Merkel cell tumor microenvironment. Although MCPyV is expressed in 80% of MCCs and serves as a powerful antigen for stimulating host immune response, intratumoral CD8+ T-cell infiltration is seen only in 18% of MCCs. In contrast, about 50% of MCPyV-positive MCCs express the programmed death-ligand 1 (PD-L1) on multiple cell types in the tumor microenvironment. We present a case of a young patient with MCC involving the heart and pancreas that showed an impressive response after treatment with four cycles of the anti-PD-1 monoclonal antibody, nivolumab.