(Reuters) - AstraZeneca AZN.L said on Friday its immunotherapy drug Imfinzi did not meet the main goal of improving survival rates for patients with the most advanced form of lung cancer, putting pressure on its shares.

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The study, known as "Mystic", was among the industry's most anticipated clinical experiments and was viewed as central to proving the value of the group's new drug pipeline and its future as an independent company, after it spurned a $118 billion takeover attempt by Pfizer PFE.N in 2014.

The trial looked at stage IV patients -- those with the most advanced form of cancer.

Results from the final Phase III trial found that Imfinzi (durvalumab) on its own and in combination with tremelimumab did not improve overall survival versus chemotherapy in patients with a protein called PD-L1 on 25 percent or more of their cancer cells.

“We are disappointed that these results missed statistical significance,” said Sean Bohen, AstraZenenca’s executive vice president and chief medical officer.

He said the company remained confident in Imfinzi as the cornerstone of its Immuno-Oncology program and continued to evaluate its potential in ongoing lung cancer trials.

AstraZeneca has been seen as having a head start here in the race for cancer treatments, with Imfinzi the new standard of care in treating early inoperable stage III lung cancer. Imfinzi sales have also been a major focus for investors.

FIGHTING LUNG CANCER

The results come after AstraZeneca last week announced a return to sales growth in the third quarter helped by strong demand for new drugs -- including Imfinzi and Tagrisso for lung cancer.

As the leading cause of cancer deaths, lung cancer represents the biggest opportunity for companies seeking to exploit the power of modern cancer immunotherapies.

However, initial results from Mystic had disappointed investors in July 2017 when AstraZeneca’s two drugs proved no more effective at stopping disease progression than chemotherapy.

Immunotherapy drugs are designed to help the body’s immune cells kill cancer and PD-L1 levels are used as a benchmark to determine if they are likely to work for individual patients.

AstraZeneca's shares were down 1.2 percent at 6,243 pence at 0930 GMT, making the stock one of biggest percentage losers on London's bluechip index .FTSE.

AstraZeneca said in September that Imfinzi, the first immunotherapy to be approved in the stage III lung cancer setting, cut the risk of death in patients with mid-stage lung cancer by nearly a third in a closely watched clinical study.

The medicine has been approved for Stage III lung cancer in more than 40 countries.