Description: Pembrolizumab (formerly known as MK-3475 and lambrolizumab, trade name Keytruda) is a humanized antibody and PD-1/PD-L1 inhibitou used in cancer immunotherapy. It is an IgG4 isotype antibody that blocks a protective mechanism of cancer cells and thereby, allows the immune system to destroy them. It targets the programmed cell death protein 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 (mismatch repair deficiency or microsatellite instability). This was the first time the FDA approved a cancer drug based on tumor genetics rather than tissue type or tumor site; therefore, pembrolizumab is a so-called tissue-agnostic drug.