Britain's biggest drugmaker - already a world leader in respiratory medicine - said as a result it planned to move the new drug into final Phase III development before the end of 2012.

The injectable antibody treatment mepolizumab is designed to help a small group of people with asthma in which white blood cells called eosinophils cause inflammation of lung airways. It reduces the number of eosinophils by inhibiting an immune system signaling chemical called interleukin-5.

Ian Pavord from Britain's University Hospitals of Leicester National Health Service Trust, who led a mid-stage study of the medicine, said it was a potentially important advance for patients for whom conventional treatment was inadequate.

"It seems to be a safe and effective treatment option for patients with eosinophilic asthma that is associated with frequent flare-ups, and may reduce the need for conventional treatment with oral corticosteroids that can have serious side effects including osteoporosis, high blood pressure and impaired growth in children," he said.

Pavord's findings from the Phase IIb trial, which was funded by GSK, were published in The Lancet medical journal on Friday and will be presented at the European Respiratory Society annual congress next month.

Severe refractory asthma only affects around 4 percent of patients with the disease, so the drug may not become a major seller for GSK but could consolidate the group's strong grip on the market for lung drugs.

"Mepolizumab is one of a number of medicines in GSK's respiratory pipeline and highlights our commitment to develop a broad respiratory portfolio which meets the needs of different patient populations," a company spokesman said.

The year-long study involving 621 patients found that the rate in patients on mepolizumab of clinically significant exacerbations - defined as episodes requiring oral corticosteroids or a hospital visit - was around half that of those on placebo.

But while monthly intravenous injections of the drug cut attacks they failed to produce consistent improvements in symptoms or lung function, suggesting that this aspect of the illness might require different treatment.

Simone Hashimoto and Elisabeth Bel from the University of Amsterdam, who were not involved in the research, said in an accompanying commentary that the overall results were "very promising".

GSK is also assessing the drug as a treatment for Churg-Strauss syndrome, a rare systemic autoimmune disease. Earlier plans to win approval in hypereosinophilic syndrome were abandoned after European regulators asked for more data in 2009.

Novartis and Roche already market another injectable antibody medicine called Xolair for treating patients with severe asthma, although this works in a different way.

(Editing by Greg Mahlich)

• A Permanent Economic Emergency