A Phase III Randomized Trial Comparing Adjuvant MK-3475 (Pembrolizumab) to Standard of Care Observation in Completely Resected Merkel Cell Carcinoma

Inclusion Criteria

Women must not be pregnant or breast-feeding due to the unknown effects of the study drug in this setting

Women of childbearing potential, and sexually active males, on Arm A MK-3475 (pembrolizumab must use accepted and effective method(s) of contraception or abstain from sex from time of registration, while on study treatment, and continue for 120 days after the last dose of study treatment. For patients on Arm B only receiving radiation therapy, contraception use should be per institutional standard

Patient must have a histological confirmation of diagnosis of Merkel cell carcinoma (MCC), pathologic stages (American Joint Committee on Cancer [AJCC] version 8) I-IIIb.

Stage I patients with negative sentinel lymph node biopsy are ineligible. Patients who have a positive biopsy or for whom no biopsy was done are eligible.

Patients with distant metastatic disease (stage IV) are not eligible.

The primary tumor must have negative margins.

Patients with all macroscopic Merkel cell carcinoma (either identified by physical exam or imaging) have been completely resected by surgery within 8 weeks before registration

All patients must have disease-free status documented by a complete physical examination and conventional imaging studies within 4 weeks prior to registration

Patient must have no currently present metastases (as confirmed by standard imaging studies)

Patient must have no previous systemic therapy or radiation therapy for Merkel cell carcinoma

 Patients with inoperable disease who have received radiation are not eligible

Patients who are human immunodeficiency virus (HIV)+ with undetectable HIV viral load are eligible provided they meet all other protocol criteria for participation

Patients with hepatitis B virus (HBV) or hepatitis C virus (HCV) infection are eligible provided viral loads are undetectable

Patients on suppressive therapy are eligible

Patients must not be on active immunosuppression, have a history of life threatening virus, have had other (beside non-melanoma skin cancers, or recent indolent cancers e.g.: resected low grade prostate cancer) cancer diagnoses in the last two years, or have had immunotherapy of any kind within the last 2 years

Patients must not have a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis

Operative notes from patient's surgical resection must be accessible.