A cancer immunotherapy drug under development at Gaithersburg-based MedImmune Inc. received "breakthrough" status from the Food and Drug Administration for its use in patients with metastatic bladder cancer. MedImmune, the research and development arm for AstraZeneca (NYSE: AZN), has multiple clinical trials in its pipeline for the drug candidate durvalumab, or MEDI4736, to treat non-small cell lung cancer, as well as head and neck, bladder, gastric, pancreatic, liver and blood cancers. The drug candidate inhibits what's known as a programmed death-ligand, or PD-L1 protein, which cancer uses to suppress the immune system.