Pembrolizumab is a humanized antibody used in cancer immunotherapy. It is an IgG4 isotype antibody that blocks a protective mechanism of cancer cells, and allows the immune system to destroy those cancer cells. It targets the programmed cell death 1 (PD-1) receptor of lymphocytes. The FDA initially approved it to treat metastatic melanoma. In 2017 the FDA approved it for any unresectable or metastatic solid tumor with certain genetic anomalies. It was the first time the FDA had approved a cancer drug based on tumor genetics rather than tissue type or tumor site.