A Study of Pembrolizumab in Japanese Pediatric Participants With Solid Tumors or Lymphomas and Japanese Adult Participants With Merkel Cell Carcinoma (MK-3475-G21/KEYNOTE-G21)

This study is examining how an immunotherapy drug called pembrolizumab (Keytruda) works in Japanese children and teenagers with certain blood cancers or solid tumors, as well as Japanese adults with a rare skin cancer called Merkel cell carcinoma. It aims to confirm the drug's effectiveness and safety in these specific populations. **You may be eligible if...** - You are a Japanese child or young person with relapsed/refractory classical Hodgkin lymphoma, primary mediastinal large B-cell lymphoma, certain solid tumors, or completely resected high-risk melanoma - OR you are a Japanese adult with Merkel cell carcinoma (a rare but aggressive skin cancer) - Your cancer has been confirmed by standard diagnostic tests - You have measurable disease or meet other specified criteria depending on your cancer type **You may NOT be eligible if...** - You have already received pembrolizumab or a similar checkpoint inhibitor drug - You have active autoimmune disease requiring systemic treatment - You have untreated brain metastases - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.