This study investigates the effectiveness of an investigational medication, with or without stereotactic body radiation therapy, in treating patients with Merkel cell cancer that has spread to other parts of the body. Merkel cell cancer is a rare type of skin cancer that grows quickly and can spread to other areas. The purpose of this study is to compare how well patients do without their disease getting worse when they receive both the investigational medication and radiation therapy versus the medication alone.

Participants in this study will receive either the investigational medication alone or in combination with stereotactic body radiation therapy. Stereotactic body radiation therapy is a method of delivering high doses of radiation to a specific area of the body. The investigational medication is a substance being tested to see if it can treat the cancer. The study will monitor the participants' health and any changes in their cancer.

• Who can participate: Adults with advanced or metastatic Merkel cell carcinoma with evidence of distant metastasis are eligible. Patients should not have received prior immunotherapy for their advanced cancer. Those with controlled brain metastases are allowed but not if the brain is the only site affected.
• Study details: Participants will either receive the investigational medication alone or in combination with stereotactic body radiation therapy. The investigational medication is being tested to see if it can treat cancer. Radiation therapy involves targeting cancer cells with high-energy rays.