Keytruda (pembrolizumab) is a PD-1 immune checkpoint inhibitor made by Merck. The US FDA approved it in 2014 as the first PD-1 drug of its kind. The real shift came in 2016, when a trial limited to patients whose tumors expressed PD-L1 at 50 percent or higher showed the drug more than doubled progression-free survival over chemotherapy. By screening for the patients most likely to respond, it became the leading immunotherapy for lung cancer. Most cancer drugs have a quiet backstory behind the headlines. Keytruda is now one of the most prescribed cancer medicines on the planet, but plenty of people only know it as the K drug, or as…