The designation, which is meant to expedite the development of therapies that provide benefits over existing options, was based on an early clinical data from a phase I trial in patients with advanced metastatic urothelial bladder cancer whose tumour has progressed during or after one standard platinum-based regimen.
Durvalumab is an investigational human monoclonal antibody directed against PD-L1. Signals from PD-L1 help tumours prevent detection by the immune system.
The antibody blocks those signals to counter the immune-evading tactics of the tumour.
AstraZeneca senior vice president, head of immuno-oncology, global medicines development Robert Iannone said: "Metastatic bladder cancer is an area of enormous unmet medical need. We are encouraged by this Breakthrough Therapy designation.
"We look forward to working closely with the FDA to bring durvalumab to bladder cancer patients as soon as possible."
The company is also testing durvalumab in first-line bladder cancer as a monotherapy as well as in combination with tremelimumab.
Urothelial bladder cancer is the ninth most common cancer globally. In 2013, there were 400,000 cases of bladder cancer and 173,000 deaths worldwide.
Metastatic bladder cancer has five-year survival rates of less than 15%.
Last December, Celgene and AstraZeneca started the Fusion clinical development program of durvalumab to treat hematologic disorders.