Finding new uses for an old drug is one surefire way to boost sales. That’s what Merck & Co. Inc. (MRK) is doing with its immune therapy drug Keytruda.Approved for advanced melanoma in September 2014 and for advanced non-small cell lung cancer last October, Keytruda is now under FDA review for the proposed treatment of patients with recurrent or metastatic head and neck squamous cell carcinoma, with a decision expected on August 9, 2016.Keytruda clinched sales totaling $566 million in 2015, and new indications for the drug can further drive its sales.