Pembrolizumab (MK-3475) as First-line Therapy for Advanced Merkel Cell Carcinoma (MK-3475-913)

Inclusion Criteria

• Be male or female and at least 12 years of age, at the time of signing the informed consent/assent.
• Have histologically confirmed diagnosis of locoregional MCC that has recurred following standard locoregional therapy with surgery and/or radiation therapy and is not amenable to local therapy or metastatic MCC (Stage IV) as per American Joint Committee on Cancer (AJCC) 8th edition guidelines.
• Have been untreated for advanced or metastatic disease except as follows:
• Prior intratumoral therapy will be permitted.
• Prior adjuvant or neoadjuvant therapy containing systemic chemotherapy will be permitted if treatment concluded at least 3 months prior to Cycle 1 Day 1 (C1D1).
• Prior adjuvant or neoadjuvant therapy containing anti-PD-1/L1 or anti-CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) therapy will not be permitted.
• Have at least 1 measurable lesion by computed tomography (CT) or magnetic resonance imaging (MRI) per RECIST 1.1 criteria as determined by the local site investigator/radiology assessment.
• Toxic effect(s) of the most recent prior therapy have resolved to Grade 1 or less (except alopecia).
• Contraceptive use by men should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
• Contraceptive use by women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
• A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies:
• Is not a woman of childbearing potential (WOCBP)
• OR
• Is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of <1% per year), with low user dependency, or be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis).
• A WOCBP must have a negative highly sensitive pregnancy test (urine or serum as required by local regulations) within 72 hours before the first dose of study intervention.
• Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 or Lansky Play-Performance Scale (LPS) ≥50 for pediatric participants up to and including 16 years of age.
• Have adequate organ function

Exclusion Criteria

• Has a known additional malignancy that is progressing or has required active treatment within the past 2 years with certain exceptions.
• Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis with certain exceptions.
• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to C1D1.
• Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.
• Has an active autoimmune disease that has required systemic treatment in past 2 years (i.e., with use of disease-modifying agents, corticosteroids or immunosuppressive drugs).
• Has a history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease.
• Has an active infection requiring systemic therapy.
• Has a known history of human immunodeficiency virus (HIV) infection.
• Has a known history of Hepatitis B (defined as Hepatitis B surface antigen [HBsAg] reactive) or known active Hepatitis C virus (defined as HCV RNA [qualitative] is detected) infection.
• Has a known history of active tuberculosis (TB; Bacillus tuberculosis).
• Has clinically significant cardiac disease within 6 months of C1D1, including New York Heart Association Class III or IV congestive heart failure, unstable angina, myocardial infarction, cerebral vascular accident, or cardiac arrhythmia associated with hemodynamic instability.
• Has a history or current evidence of any condition, therapy, or laboratory abnor