The concept of combining two immunotherapy drugs to fight lung cancer — not so long ago one of the hottest ideas in cancer research — has suffered a fresh blow from the failure of an AstraZeneca clinical study.

Adding the company's experimental drug tremelimumab to its existing product Imfinzi failed to slow disease progression or extend life in patients who had already received at least two previous treatments, the drugmaker said on Tuesday.

Shares in AstraZeneca fell 1 percent following the news.

The result of the study, known as ARCTIC, is disappointing but not a huge surprise, since evidence has been building that there may be little value in using a so-called CTLA-4 drug like tremelimumab on top of a PD-L1 such as Imfinzi.

Indeed, since Imfinzi on its own showed some signs of efficacy, whereas adding tremelimumab did not, analysts believe the second drug might actually offset the efficacy that Imfinzi can offer.

An ARCTIC sub-study that was not powered for statistical significance found that Imfinzi monotherapy showed a clinically meaningful reduction in the risk of death compared to chemotherapy.

A year ago, the notion of combining the two medicines was the big hope driving AstraZeneca's stock price - but that hope was dealt a major blow in July when its main first-line lung cancer trial, called MYSTIC, failed to slow disease progression.

It is still possible the two-drug combination might succeed in lengthening first-line patients' lives - a measure called overall survival - but Deutsche Bank analyst Richard Parkes said expectations for this "should now be very low".

AstraZeneca is due to report overall survival data from the MYSTIC trial in the second half of this year.

LUNG CANCER BATTLEGROUND

Lung cancer is the world's most common and deadly form of the disease, spurring a battle among companies including AstraZeneca, Merck & Co, Bristol-Myers Squibb and Roche to develop immune system-boosting treatments.

The latest findings from a number of studies presented at a medical meeting in Chicago this month suggests that adding chemotherapy to immunotherapy, rather than giving two immunotherapies together, is the most promising approach - a development that is benefiting Merck.

Luckily, AstraZeneca is finding success elsewhere and the relative importance of double immunotherapy within its portfolio has reduced as the company prepares for a return to sales growth over the next few years.

Within lung cancer, Imfinzi on its own is also carving out a distinct market in treating patients with inoperable mid-stage disease that has not spread widely around the body.

AstraZeneca has suffered the industry's biggest patent cliff since 2012, wiping out more than half of its sales, but it expects sales to grow this year, helped by a number of new medicines.

Separately on Tuesday, AstraZeneca said it would subscribe for more shares in Circassia by injecting cash into the respiratory drug specialist to take its stake to a maximum of 19.9 percent from 14.2 percent. Shares in Circassia rose 4 percent on the news.