AstraZeneca's drive to rebuild its portfolio of new medicines received a boost with positive results for an experimental biotech drug for severe asthma that the company previously flagged as a potential $2 billion-a-year seller.

Benralizumab, which is likely to reach the market next year, was well tolerated and succeeded in reducing asthma attacks in two pivotal late-stage clinical trials, the drugmaker said on Tuesday.

While it will be entering a competitive market, behind recently approved treatments such as GlaxoSmithKline's Nucala and Teva's Cinqair, AstraZeneca thinks benralizumab has the potential to be the best in class.

Analysts are not yet convinced and consensus annual sales forecasts for benralizumab currently stand at just $450 million by 2021, according to Thomson Reuters Cortellis, well below the $2 billion predicted by AstraZeneca in 2014.

The company gave its bullish forecast for benralizumab, along with many other pipeline products, at the time of Pfizer's

unsuccessful attempt to acquire it.

Tom Keith-Roach, head of AstraZeneca's respiratory, inflammation and autoimmune business, declined to give an update on sales expectations but told Reuters that the drug's unique mechanism of action should position it well against rivals.

Benralizumab works directly against cells in the body driving inflammation called eosinophils, leading to their rapid depletion, while rival medicines work less directly.

Deutsche Bank analyst Richard Parkes said the market for severe asthma drugs could grow to be worth more than $7 billion a year and detailed results on benralizumab, potentially at a medical meeting in September, would be important in determining its commercial potential.

AstraZeneca said it planned to submit benralizumab for regulatory approval in the United States and Europe in the second half of 2016.

Like the other injectable asthma drugs, AstraZeneca's product is designed for patients who have a history of severe attacks despite taking existing medications.

AstraZeneca badly needs new drugs to drive future sales growth as it struggles with a wave of patent expiries on older medicines, such as its cholesterol fighter Crestor and stomach acid pill Nexium.

Most attention is focused on its oncology drug portfolio but the company also has a long history in respiratory medicine and sees it as an important therapeutic area for the future.

Benralizumab was originally licensed from a unit of Kyowa Hakko Kirin <4151.T>. The Japanese company retains rights to the medicine in Japan and certain other Asian countries, but AstraZeneca has an option to acquire the Japanese rights once the drug is approved. AstraZeneca already has the rights elsewhere.

(Editing by Keith Weir)