U.S. FDA approved ONIVYDE developed by Cambridge-based Merrimack Pharmaceuticals to treat metastatic pancreatic cancer, one of the few new treatments to be approved in decades for the aggressive and devastating disease. The drug will be used as part of a combination regimen with a two-drug chemotherapy. It was approved for U.S. sale to treat patients with pancreatic cancer that progressed after treatment with a different chemotherapy. While the new regimen promises to extend patients’ lives for only months, on average, it is still considered a significant advance in a field where there have been few therapeutic options and little hope. Studies demonstrated that patients treated with ONIVYDE plus a chemotherapy combining the drugs fluorouracil and leucovorin lived an average of 6.1 months, compared to 4.2 months for those treated only with the chemotherapy, though individual patients taking the three-drug combo lived much longer. Pancreatic cancer is among the most difficult diseases to treat. It accounts for about 3 percent of all tumors but is the fourth-leading cause of cancer death, with a five-year survival rate of only 7 percent.