LONDON (Reuters) - AstraZeneca's (>> AstraZeneca plc) 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 (>> GlaxoSmithKline plc) Nucala and Teva's (>> Teva Pharmaceutical Industries Limited) 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 (>> Pfizer Inc.) 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.

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 this area as an important therapeutic area for the future.

Benralizumab was originally licensed in from a unit of Kyowa Hakko Kirin (>> Kyowa Hakko Kirin Co Ltd) and the Japanese company retains rights to the medicine in Japan and certain other Asian countries. AstraZeneca has the rights elsewhere.

(Reporting by Ben Hirschler; editing by Jason Neely and Adrian Croft)

By Ben Hirschler