GlaxoSmithKline's experimental drug mepolizumab may help a small group of asthma sufferers whose illness can be difficult to control, researchers said on Wednesday.

Two studies reported in the New England Journal of Medicine said the drug, also known as Bosatria, reduced the number of asthma attacks in patients for whom oral corticosteroid treatment was not very effective, apparently because they had high levels of immune cells known as eosinophils.

Mepolizumab reduces the number of eosinophils by inhibiting an immune system signaling chemical called interleukin-5.

The company, which is developing the drug to combat other conditions with high eosinophil counts, financed the two new studies after tests on a broader group of asthma patients failed to produce major improvement.

"Although up to 40 percent of cases of severe asthma start later in life, the eosinophilic form of asthma probably represents less than 5 percent of the total number of cases of adult-onset asthma," Dr. Sally Wenzel of the University of Pittsburgh wrote in a commentary.

"This is a very difficult-to-manage group of asthmatics," said Dr. Paul O'Byrne of Canada's McMaster University, who worked on one of the studies.

"These are people who are already on a lot of asthma medications and are also taking oral corticosteroids on a maintenance basis to try to control the symptoms they're having."

He and his colleagues found that the drug reduced the need for oral steroids.

"It had a very dramatic benefit in these patients. Most of the patients came off the oral steroids, which they had been on for about 10 years, and they did not have a flare-up or an exacerbation of their asthma control," O'Byrne said in a telephone interview.

"For the folks who took the placebo, we were able to reduce their oral steroid load, but in all but one or two people, this was associated with a worsening of their asthma control."

The other study, led by Dr. Ian Pavord of the University Hospitals of Leicester National Health Service Trust in Britain, produced similar results.

Wenzel noted that the drug had no effect on lung capacity or general symptoms.

"The reason for that is probably because these patients have had very difficult asthma for a long time and their lung capacity gets reduced because of that," O'Byrne said.

"We are starting the drug 10, 15 years after the problem started, so it's very difficult to recover lung capacity once you've damaged the lungs all this time. Treating earlier in the disease might have had a much bigger effect."