LONDON (Reuters) - AstraZeneca's experimental lupus drug anifrolumab significantly cut disease activity in a mid-stage clinical trial, boosting hopes for a medicine the company believes could become a $1 billion-a-year seller.

The drugmaker said on Tuesday that the new treatment was more effective than another drug, sifalimumab, that it had also been testing in lupus. It has therefore started a 300 mg dose of anifrolumab in a final-stage Phase III program.

Only one new drug - GlaxoSmithKline's Benlysta - has been introduced for lupus in the past 60 years, underlining the difficulties of tackling the condition, in which the immune system attacks joints and organs.

AstraZeneca's anifrolumab, which is given intravenously, is designed for patients with moderate to severe lupus and works in a different way to Benlysta by targeting interferon, a protein involved in inflammation.

Bing Yao, head of respiratory, inflammatory and autoimmune research at AstraZeneca's biotech unit MedImmune, said anifrolumab had produced the most robust effects yet seen with any lupus drug in a mid-stage clinical study.

The Phase IIb test results, being presented at the American College of Rheumatology annual meeting, showed anifrolumab 300 mg produced a response in 34.4 percent of patients after 169 days of treatment and this rose to more than 50 percent after a year.

"We have been eagerly awaiting clinical data of this magnitude for many years," principal investigator Richard Furie, head of rheumatology at the North Shore-LIJ Health System, said.

Anifrolumab also reduced the need for patients to take oral corticosteroids.

On the downside, there was an increase in patients reporting both Herpes zoster, or shingles, and influenza. However, Yao said these conditions were readily treated with antivirals.

In May 2014, during its defense against a takeover attempt by Pfizer, AstraZeneca predicted that its interferon-based approach to lupus could eventually generate annual sales of around $1 billion.

Yao declined to comment any further on likely sales.

Other companies with lupus drugs in development include Anthera Pharmaceuticals and ImmuPharma. Another experimental product from UCB recently failed in Phase III testing.