Researchers have discovered a drug that disables a stubborn mutation that made pancreatic cancer a death sentence. Rasonque helped subjects in a late-stage study live more than 13 months, nearly twice as long as those on chemotherapy alone. It has side effects, including a harsh rash across the body and gastrointestinal issues, and costs more than $477,000 a year. The drug could generate more than $20 billion in annual sales.