A drug used to treat certain types of stomach cancer, gastroesophageal junction cancer, urothelial cancer, Hodgkin lymphoma, non-small cell lung cancer, cancer of the head and neck, and melanoma. It is also used to treat some types of solid tumors, including colorectal cancer, that have certain mutations in genes involved in DNA repair. 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. 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.