Updated from 12:14 p.m. EST

A two-year study shows that AstraZeneca's ( AZN) cholesterol fighter Crestor reversed plaque buildup in the arteries of patients with evidence of coronary artery disease.

AstraZeneca says the results mean that for the first time, a drug from the class known as statins has shown an ability to reverse atherosclerosis in a major clinical study.

Atherosclerosis occurs when deposits of fatty substances, cholesterol or cellular waste products collect in the inner lining of an artery, forming a buildup called plaque. If plaque ruptures, it can block the flow of blood to organs such as the heart or the brain and can lead to a heart attack or stroke.

Data from a trial called Asteroid were presented at the American College of Cardiology meeting in Atlanta. According to the study, the plaque content in patients' arteries was reduced by 7% to 9%. Crestor also contributed to a 53% reduction in LDL, or "bad" cholesterol and a 15% increase in HDL cholesterol, the "good" kind.

Pfizer ( PFE), whose Lipitor is not only the top-selling statin, but the biggest-selling drug in the world, took exception to some of AstraZeneca's statements.

"Only Lipitor has demonstrated significant LDL reductions and cardiovascular benefits for a wide range of patients, with an excellent efficacy and safety profile across the full dose range," the company said in a press release responding to the Crestor data. "This is not the first time a statin has demonstrated plaque regression."

Pfizer said in a trial that compared Lipitor with another lipid-lowering agent over 18 months, a 5.9% total plaque reduction was seen in a subset of patients. Companies selling cholesterol-reducing drugs are battling for position in what is a huge market. Crestor's sales totaled $1.27 billion last year. By comparison, Pfizer sold more than $12 billion of Lipitor in 2005.

Other well-known statins include Pravachol from Bristol-Myers Squibb ( BMY) and Zocor from Merck ( MRK).

At any rate, Howard Hutchinson, vice president of clinical research for AstraZeneca, said in a statement that multiple clinical studies "have clearly documented that Crestor provides better LDL-C lowering and significant increases in HDL-C compared to other statins, establishing it as an important, effective and well tolerated treatment. The data from this study -- demonstrating that Crestor regressed plaque in the arteries of four out of five patients -- is an important new finding."

The AstraZeneca study involved 507 patients who had undergone coronary angiography and who had evidence of coronary artery disease. Crestor is indicated for the treatment of high cholesterol but isn't approved by regulators for atherosclerosis. More than 6 million patients have taken the drug.

AstraZeneca closed Monday up $1.14, or 2.3%, at $50.94. Pfizer slipped 11 cents, or 0.4%, to $25.97.

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