This clinical trial is focused on studying a type of skin cancer called Stage III melanoma. Melanoma is a serious form of skin cancer that can spread to other parts of the body. The study is specifically looking at patients who have had a complete surgical removal of high-risk Stage III melanoma, which means the cancer has spread to the lymph nodes but has been fully removed by surgery. The trial is testing a treatment called pembrolizumab, also known by its code name MK-3475, which is an anti-PD-1 monoclonal antibody. This is a type of medication that helps the immune system recognize and attack cancer cells. The study will compare pembrolizumab to a placebo to see if it can help prevent the cancer from coming back.

The purpose of the study is to find out if pembrolizumab can improve the time patients remain free from cancer after surgery, known as recurrence-free survival. The study will also look at a subgroup of patients whose tumors have a specific protein called PD-L1 to see if pembrolizumab is more effective for them. Participants in the study will receive either pembrolizumab or a placebo through an intravenous infusion, which means the medication is given directly into a vein. The treatment will be administered over a period of up to 52 weeks, and patients will be monitored regularly to check for any signs of the cancer returning.

Throughout the study, patients will undergo regular check-ups and imaging tests like CT or MRI scans to monitor their health and the status of their cancer. The study aims to provide valuable information on whether pembrolizumab can help prevent the recurrence of melanoma after surgery, potentially offering a new treatment option for patients with high-risk Stage III melanoma. The trial is expected to continue until 2026, and the results will help determine the effectiveness of pembrolizumab in improving outcomes for melanoma patients.