Study of Front Line Pembrolizumab and Valemetostat in PD-L1 Positive, HPV-Negative Recurrent/Metastatic Squamous Cell Carcinoma (SCC) of the Head and Neck: The PANTHERAS

Background: Head and neck squamous cell carcinoma (HNSCC) is the 6th most common cancer worldwide. These cancers have different causes, with smoking/tobacco exposure and human papilloma virus infection being the most common. . When HNSCC occurs in people who are not infected with HPV, the cancers are more likely to return after treatment; when this happens, overall survival is only about 10 months, thus better treatments are needed. Objective: To test a combination treatment using 2 drugs (valemetostat and pembrolizumab) in people with HNSCC. Phase 1b of the study will determine a recommended dose of the 2 drugs and evaluate how safe the combination is.; this will include patients with HPV-positive and HPV-negative HNSCC, as well as squamous cell NSCLC that have progressed on anti-PD-1/anti-PD-L1 therapies.Phase II will determine how effective the combination is and will focus on patients with HPV-negative HNSCC. Eligibility: People aged 18 years and older with HPV-negative HNSCC, sinonasal carcinoma of the head and neck, or squamous non-small cell lung cancer (NSCLC). Design: Participants will be screened. They will have a physical exam. They will have blood and urine tests and tests of their heart function. They will have imaging scans. They may have a biopsy: A small sample of tissue will be removed from the tumor. Treatment will be given in 21-day cycles. Pembrolizumab is administered through a tube attached to a needle inserted into a vein in the arm. Participants will receive pembrolizumab on the first day of each cycle. Valemetostat is a tablet taken by mouth. Participants will take the tablet once a day at home. They will record the date and time of each dose in a diary. They will also write down any adverse effects they experience. Participants may remain in the study up to 2 years.