A drug used to treat certain types of Hodgkin lymphoma, non-small cell lung cancer, squamous cell cancer of the head and neck, and melanoma. It is also being studied in the treatment of other types of cancer. Pembrolizumab binds to a protein called PD-1, which is found on T cells (a type of white blood cell). Pembrolizumab may block PD-1 and help the immune system kill cancer cells. It is a type of monoclonal antibody and a type of immune checkpoint inhibitor. Also called Keytruda.