Pembrolizumab Compared to Standard of Care Observation in Treating Patients With Completely Resected Stage I-III Merkel Cell Cancer

Inclusion Criteria:

• Patients must have an Eastern Cooperative Oncology Group (ECOG) performance status: 0, 1, or 2 (However, those patients with a performance state of 3 because they are wheel chair bound due to congenital or traumatic events more than one year before the diagnosis of Merkel cell carcinoma are eligible).
• Women must not be pregnant or breast-feeding due to the unknown effects of the study drug in this setting. All women of childbearing potential must have a blood test or urine study within 2 weeks prior to registration to rule out pregnancy. A female of childbearing potential is any woman, regardless of sexual orientation or whether they have undergone tubal ligation, who meets the following criteria: 1) has achieved menarche at some point, 2) has not undergone a hysterectomy or bilateral oophorectomy; or 3) has not been naturally postmenopausal (amenorrhea following cancer therapy does not rule out childbearing potential) for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months).
• 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.
• White blood count >= 2000/uL (within 4 weeks prior to randomization).
• Absolute neutrophil count (ANC) >= 1000/Ul (within 4 weeks prior to randomization).
• Platelets >= 75 x 10^3/uL (within 4 weeks prior to randomization).
• Hemoglobin >= 8 g/dL (>= 80 g/L; may be transfused) (within 4 weeks prior to randomization).
• Creatinine =< 2.0 x upper limit of normal (ULN) (within 4 weeks prior to randomization).
• Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) =< 2.5 x ULN (within 4 weeks prior to randomization).
• Total bilirubin =< 2.0 x ULN, (except patients with Gilbert's syndrome, who must have a total bilirubin less than 3.0 mg/dL) (within 4 weeks prior to randomization).
• 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.