Pembrolizumab in Treating Patients With Advanced Merkel Cell Cancer

PRIMARY OBJECTIVES: I. To determine the clinical efficacy of MK-3475 (pembrolizumab) as the first systemic intervention for patients with advanced Merkel cell carcinoma (MCC). SECONDARY OBJECTIVES: I. To determine the clinical activity of MK-3475 as the first systemic intervention for patients with advanced MCC. TERTIARY OBJECTIVES: I. To determine the immune correlates of the clinical activity of MK-3475. OUTLINE: Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Treatment repeats every 21 days for up to 24 months in the absence of disease progression\* or unacceptable toxicity. \* NOTE: Patients with confirmed disease progression may continue to receive treatment if they are otherwise clinically stable until there is an increase in tumor burden of 25% or more following initial confirmation of progression. Under exceptional circumstances, and with protocol principal investigator (P.I.) and Cancer Immunotherapy Trials Network (CITN) P.I. approval, patients may receive treatment beyond 2 years. After completion of study treatment, patients are followed up at 30 days and then every 3 months for 1 year, every 6 months for 2 years, annually until the patient has completed 3 years of follow up for disease assessment, and then every 12 weeks.