By Kim Dixon

CHICAGO, June 25 (Reuters) - Controversy over the popular diabetes pill Avandia will take center stage among the world's experts on the disease on Monday, when researchers debate findings that the drug could boost heart-attack risk.

The GlaxoSmithKline Plc <GSK.L> drug has been under a cloud since an analysis published in the New England Journal of Medicine last month found that taking Avandia led to a 43 percent boost in heart attack risk and a 64 percent rise in risk of heart death.

U.S. regulators have since asked for stiffer warnings on Avandia and a similar drug called Actos. Some 60 million prescriptions have been written for Avandia, which generates about $3 billion in annual sales for Glaxo. Sales have dropped sharply since the report.

The report by Cleveland Clinic's cardiology chief, Steven Nissen, has sparked debate because it is an analysis that looked back at dozens of studies.

Nissen's study of the drug, known generically as rosiglitazone, is based on conclusions from pooled data of 42 trials involving nearly 28,000 patients.

"The remaining question is, if you take a study not designed to look at heart attacks, can you make a conclusion?" said John Buse, chief of endocrinology at the University of Chapel Hill, North Carolina and president-elect of the diabetes group.

Nissen will headline a panel later on Monday at the American Diabetes Association's (ADA) annual meeting, attended by about 13,000 doctors and researchers.

Diabetes is a chronic disease in which the pancreas is unable to produce enough of or properly use the hormone insulin, which regulates blood sugar. The drug treats type II diabetes -- the most common form of the disease that afflicts about 180 million people worldwide.

Philip Home, professor of diabetes medicine at Britain's Newcastle University, will make a presentation along with Nissen at the panel and then open it up to an expected packed audience for questions.

The Nissen study "doesn't answer a question, it asks a question," said Home, a lead investigator on a separate ongoing 4,400-patient clinical trial of the drug.

That study, called Record and funded by GlaxoSmithKline, suggests Avandia "should still have a role in the glucose-lowering armory," he said.

But he added that "a clinically significant increase in MI (heart attack) not causing death cannot be ruled out," Home said.

Avandia and Actos, sold by Japan's Takeda Pharmaceutical Co. <4502.T>, are part of a class of drugs known as thiazolidinediones, or glitazones. The first drug in the class, Rezulin, was pulled from the market in 2000 after more than 60 people died from acute liver failure.

Glaxo has disagreed with the findings and said there was compelling data from other trials showing Avandia was just as safe as other diabetes medicines.

An advisory panel of the U.S. Food and Drug Administration is due to study the issue of Avandia's safety at a meeting on July 30.

Some doctors fear a rush to judgment could put patients in jeopardy and is disrupting patient care.

Dr. Joel Zonszein, head of the Clinical Diabetes Center at Montefiore Medical Center in New York City, is on an advisory board at Takeda, which sells Actos.

He said Nissen's study and the media attention are premature because the findings have not been confirmed by randomized prospective trials.

"It is very difficult to prescribe a drug when you turn on the TV and the lawyers are already soliciting. One thing is what the science shows and the other thing is what is happening in real life," he said.

(Additional reporting by Julie Steenhuysen)

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