The U.S. Food and Drug Administration on Friday approved Merck & Co Inc's immunotherapy, Keytruda, for patients with the most common form of lung cancer whose tumors produce a specific biological marker. The FDA approval for Keytruda in advanced non-small cell lung cancer is for patients whose tumors express PD-L1, a protein targeted by the drug, and includes a companion diagnostic, made by a unit of Agilent Technologies Inc, to measure those protein levels.