2866 Antitumor activity of the Anti-PD-1 antibody pembrolizumab in subgroups of patients with recurrent/metastatic head and neck squamous cell carcinoma (R/M HNSCC): Exploratory analyses from KEYNOTE-012. (September 2015) - Chow, L.; Mehra, R.; Haddad, R.; Gupta, S.; Weiss, J.; Berger, R.; Burtness, B.; Tahara, M.; Keam, B.; Le, D.; Cheng, J.; Heath, K.; et al.