GlaxoSmithKline’s experimental drug darapladib didn’t reduce the risk of heart attack or stroke in a late-stage study, the second time the medicine disappointed in clinical trials.

The test results mean the drug failed the main goal of the study known as Stability, which tracked more than 15,000 patients suffering from chronic coronary heart disease, London-based Glaxo (NYSE: GSK) said in a statement today.

Human Genome Sciences Inc., which Glaxo bought last year for $3 billion, originally developed darapladib. It was intended to help rid patients with heart disease of the plaque that builds up in their arteries. After the drug didn’t show enough plaque-busting potential in a mid-stage study five years ago, Glaxo decided to test its ability to prevent pieces of plaque from rupturing and causing potentially fatal blood clots, but expectations of success were low.

“It is not surprising that darapladib did not show a benefit,” Andrew Whitney, an analyst at UBS AG, said in a note to investors today. Whitney estimated the drug only had a 20 percent chance of clinical success in the new indication. “Today’s data point is more removal of a potential positive, rather than a real negative.”

Glaxo fell as much as 1.7 percent, the steepest intraday decline since Oct. 23, and was trading down 1.3 percent. That pared the stock’s gain this year to 22 percent, valuing the drugmaker at $126 billion.

Bad Cholesterol

The drugmaker said it will review the test data, which indicated that some subgroups of patients benefited from treatment, and await the results of a second late-stage study in acute coronary syndrome before deciding what to do.

The main goal used in the study described today was a measure of time to first occurence of any major cardiovascular event, which included heart attack, stroke and death, Glaxo said. Patients in the test, which tracked participants since December 2008, took darapladib or a placebo in addition to a cholesterol-lowering statin.

The drug was developed for patients who aren’t helped enough by statins, a class of medicines that includes Pfizer Inc.’s Lipitor and AstraZeneca Plc’s Crestor. About 12 million people in the U.S. and 21 million worldwide don’t achieve the recommended reduction in low-density lipoprotein cholesterol with statins, according to a presentation last year by Sanofi, which is also developing a new drug to reduce cholesterol.

Heart disease and stroke are the two biggest causes of death worldwide, accounting for almost 22 percent of deaths in 2011, according to the Geneva-based World Health Organization. The LDL form of cholesterol only comes from eating animal products, and it raises the risk of the two illnesses by clogging arteries, according to the American Heart Association.

 GSK operates its North American headquarters in RTP.

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