TRENTON, N.J. (AP) — Drugmaker GlaxoSmithKline PLC is making a big investment in a new type of drug for the gigantic heart disease market, announcing Thursday it is starting a 15,000-patient, late-stage study of its experimental compound darapladib.

The drug is meant to keep plaque deposits in heart arteries stable, so they don't rupture — the top cause of heart attacks and strokes.

If it is eventually approved, this would be the first drug in a new class that inhibits an enzyme called Lp-PLA2, Patrick Vallance, head of drug discovery at the British pharmaceutical company, told The Associated Press.

That could make them potential complements to existing cholesterol drugs such as the blockbuster Lipitor and other drugs in the class called statins.

Dr. Daniel Rader, director of preventive cardiology at the University of Pennsylvania School of Medicine, said the experimental drug potentially is "tremendously important" for the field, but there's no guarantee it will work out.

He said experts cannot tell from the results of midstage studies whether darapladib will turn out to prevent heart attacks but applauded Glaxo's management for "taking the plunge" with the study.

Cholesterol drugs, now a standard treatment even for many people who don't have known heart disease, were the second-best-selling category worldwide last year, with roughly $34 billion in revenues, according to data firm IMS Health. Some statins have been proven to reduce risk of heart attack and stroke, and most of them will be available as cheap generics before this drug could hit the market in roughly 5 years.

GlaxoSmithKline, the world's No. 2 drugmaker by sales, said the study is expected to run for about three years. Known by the acronym STABILITY, it will include men and women with heart disease in 39 different countries and will track how well the drug prevents heart attacks, strokes and death from cardiovascular disease.

One group will get the experimental compound, a comparison group will get a dummy pill and both groups will also get standard cardiac care, such as blood pressure medicine, aspirin and a statin.

"We believe we've got a medicine which has real potential, based on preclinical studies, epidemiology and clinical studies," said Vallance.

The Lp-PLA2 enzyme produces substances that stimulate the buildup of inflammatory cells on blood vessel walls, according to Vallance. The enzyme is found both in blood and inside plaques.

Epidemiology studies, those looking for connections among large groups of patients, have produced strong data on the association between the enzyme and cardiovascular disease, with higher blood levels of the enzyme predicting higher risk of cardiovascular complications, he said.

A midstage study of the drug produced mixed results, though. There were no significant differences between patients getting a placebo and the experimental drug on the study's primary goals, including controlling how easily the shape of a plaque inside an artery could be changed — possibly causing a rupture.

But darapladib was better than placebo on one of the secondary goals, preventing the core of plaques inside arteries from growing. Vallance said researchers now believe that's more important.

Rader said no other drugs, and none he knows of in development, work by stabilizing plaque.

"Almost the Holy Grail in some ways of preventing heart attacks is a drug that will act directly on the plaque to stabilize it and make it less likely to rupture," he said.

Rader said basic biology, animal research and the small studies in people so far show it's "reasonable" darapladib could be a success.

Vallance said he expects competitors to do research in this area.

"We know that we're the only ones this far along," he said.

GlaxoSmithKline plans to start another late-stage study of darapladib late next year in patients who have had a mild or serious heart attack or unstable chest pain.