The British drugmaker is running a lung cancer trial known as ATLANTIC that is due to produce results by the end of this year and could, in theory, support a submission seeking accelerated U.S. approval in the first half of 2016.

However, this option is now looking less likely since Bristol-Myers Squibb's Opdivo and Merck's Keytruda have already won U.S. Food and Drug Administration (FDA) clearance in lung cancer, reducing the chances of durvalamb gaining rapid approval.

"The landscape has changed because competing products received approval far faster than could have been expected a bit more than a year ago," AstraZeneca Chief Executive Pascal Soriot told reporters in a post-results conference call.

Roche also has a competing product, atezolizumab, that it plans to file for approval in lung cancer in the first quarter of 2016.

As a result the chances of an accelerated FDA approval based on ATLANTIC have decreased, since there is a reduced unmet medical need. All four drugs work in a similar way to boost the immune system's ability to detect and fight tumour cells.

Durvalumab is AstraZeneca's most important pipeline product and the company said last year, during its bid defence against a takeover attempt by Pfizer, that the medicine could eventually generate annual sales of $6.5 billion, making it a key component of an overall $45 billion sales forecast for 2023.

While ATLANTIC is looking at durvalumab's use on its own in a subset of late-stage lung cancer patients, Soriot said the drug's real promise lay in combining it with other products to help a much wider range of patients.

"Our long-range forecast for 2023 remains the same. In fact, we now have a little bit more for oncology sales and a little bit less for diabetes in our forecasts," he said.

"Getting accelerated approval with our monotherapy was never part of our base plan or our long-range forecasts. We still want to pursue the accelerated approval, but as an upside."

Soriot also said AstraZeneca was considering a marketing partnership deal for its gout drug lesinurad, which was recommended for approval by an FDA advisory panel last month.

"Partnering the gout portfolio is clearly a very strong option that we are considering," he said. "We have to wait for lesinurad's approval and then, based on that, we will pursue discussions with potential partners."

(Editing by Greg Mahlich)

By Ben Hirschler