This clinical trial is focused on studying a type of cancer called Head and Neck Squamous Cell Carcinoma (HNSCC), which can occur in areas such as the mouth, throat, and voice box. The study is looking at patients whose cancer has come back or spread to other parts of the body and who have a specific protein called PD-L1 present in their tumors. The trial will test the effectiveness of a new treatment combination using two medications: INBRX-106 and Pembrolizumab, also known by its brand name KEYTRUDA. Pembrolizumab is a type of medicine called a monoclonal antibody, which helps the immune system fight cancer. INBRX-106 is another type of antibody that targets a specific part of the immune system to help it work better against cancer.

The purpose of the study is to compare the effectiveness of the combination of INBRX-106 and Pembrolizumab against Pembrolizumab alone. Some patients will receive the combination treatment, while others will receive only Pembrolizumab. A third group will receive a placebo, which looks like the treatment but does not contain any active medicine. The study will help determine if adding INBRX-106 to Pembrolizumab provides better outcomes for patients with this type of cancer.

Participants in the study will receive their assigned treatment through an intravenous infusion, which means the medicine is given directly into a vein. The study will last for a period of up to 105 weeks, during which time patients will be monitored regularly to assess how well the treatment is working and to check for any side effects. The goal is to find out if the new combination treatment can improve the chances of controlling the cancer and extending the lives of patients with recurrent or metastatic HNSCC.