A drug used to treat certain types of non-small cell lung cancer that have spread to other parts of the body. It is used in patients whose cancer has a certain protein and whose disease got worse during or after treatment with anticancer drugs that included platinum. Pembrolizumab is also used to treat melanoma that cannot be removed by surgery or has spread to other parts of the body. It is used in patients whose disease got worse after being treated with ipilimumab and who may have also been treated with a BRAF inhibitor (a type of anticancer drug). It is also being studied in the treatment of other types of cancer. Pembrolizumab binds to a receptor for a substance 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 an immune checkpoint inhibitor. Also called Keytruda.