Merck & Co. Inc. (MRK) has sought FDA approval for the expanded use of its blockbuster cancer drug Keytruda to include treatment of relapsed or refractory primary mediastinal large B-cell lymphoma.The regulatory agency’s decision on the proposed new indication is expected on April 3, 2018.Keytruda, co-developed and co-commercialized with AstraZeneca, is already approved for the treatment of melanoma, non-small cell lung cancer, head and neck squamous cell carcinoma, classical Hodgkin lymphoma, urothelial carcinoma, microsatellite instability-high cancer, and gastric cancer.The drug fetched annual revenue of $3.81 billion for Merck in 2017, compared to $1.40 billion in 2016.MRK closed Tuesday’s (Mar.27) trading at $53.75, down 0.54%.