(Reuters) - U.S. regulators have approved AstraZeneca’s key immunotherapy drug durvalumab as a treatment for bladder cancer, marking the first commercial green light for a product the company hopes will go on to sell billions of dollars.

FILE PHOTO: The logo of AstraZeneca is seen on a medication package at a pharmacy in London April 28, 2014. REUTERS/Stefan Wermuth/File Photo

The approval, while expected, marks a milestone for the British company, which expects new cancer drugs to help revive its fortunes following patent losses on older blockbuster products like cholesterol pill Crestor and Nexium for heartburn.

Bladder cancer itself is a relatively small initial market, where AstraZeneca is lagging behind rivals Bristol-Myers Squibb and Roche whose immunotherapies are already approved for the condition.

Durvalumab’s big commercial opportunity lies in previously untreated lung cancer, where key clinical trial results, including with combination therapy, are due in June or July.

Leerink analyst Seamus Fernandez sees durvalumab capturing a modest 10 percent of the estimated $2.3 billion global bladder cancer market, while AstraZeneca in 2014 put the drug’s peak sales in all cancers at $6.5 billion, including combination use.

The U.S. Food and Drug Administration (FDA) said late on Monday it granted accelerated approval to AstraZeneca’s drug to treat advanced bladder cancer in patients whose disease had progressed despite chemotherapy.

The drug, which will have the brand name Imfinzi, works by helping the body’s immune cells kill cancer, offering an alternative to toxic chemotherapy. While not without side effects, such immuno-oncology treatment has the potential of longer-lasting efficacy, although it comes at a high price.

AstraZeneca said the average wholesale acquisition cost of durvalumab would be around $15,000 a month.

“This first approval for Imfinzi is an important milestone in our return to growth,” said AstraZeneca Chief Executive Pascal Soriot.

The drug belongs to a new class of medicines called PD-L1 inhibitors that block a mechanism tumors use to evade detection from the immune system.

It was approved by the FDA for use in patients with locally advanced or metastatic urothelial carcinoma, by far the most common form of bladder cancer, regardless of their status for the amount of PD-L1 protein on their cancer cells.

Durvalumab won accelerated approval, which enables the use of therapies for serious conditions to fill an unmet medical need based on data the FDA believes is likely to predict a clinical benefit. AstraZeneca is required to conduct trials to confirm actual benefit to patients.

The FDA also approved a complementary diagnostic from Roche that can be used with the drug to assess PD-L1 levels. Studies have shown patients with high PD-L1 are more likely to do well on durvalumab, although such a test is not required for its use.

Durvalumab is being tested on its own and also in combination with another immune system-boosting therapy called tremelimumab in various cancers.

The medicine is the latest immunotherapy to be approved by the FDA, after nods for treatments developed against various cancers by Bristol-Myers Squibb, Merck & Co, Roche, and a collaboration between Germany’s Merck KGaA and Pfizer.

AstraZeneca shares were up 0.3 percent in early London trading on Tuesday.