PDCD1 and head and neck squamous cell carcinoma: Based on the results of the clinical trial KEYNOTE-048, on June 10, 2019, the U.S. Food and Drug Administration (FDA) approved the PD-1 monoclonal antibody pembrolizumab (Keytruda) as the first-line therapeutic drug for patients with metastatic, unresectable, and recurrent HNSCC.