An experimental GlaxoSmithKline drug, designed to fight heart disease in a new way, failed to meet its main goal in a major late-stage clinical study, dealing a blow to one of the company's biggest new treatment hopes.

Darapladib's inability to reduce the overall risk of heart attacks and strokes in the first of two big Phase III studies is disappointing, but not a huge surprise. Many analysts already had low expectations for the project.

Shares in Britain's biggest drugmaker had fallen 1.2 percent on the news by 1050 GMT on Tuesday, and Deutsche Bank analyst Mark Clark said failure of the drug removed some "blue sky fantasy" about potential multibillion-dollar sales.

GSK obtained full rights to darapladib, along with lupus drug Benlysta, when it bought U.S. biotech firm Human Genome Sciences last year for $3 billion.

The once-daily pill did produce statistically significant reductions in serious coronary events in some of the pre-defined secondary endpoints of the trial, which GSK said required more analysis.

Darapladib is designed to prevent heart attacks and strokes in a completely different way from cholesterol-lowering statin drugs by targeting an enzyme called Lp-PLA2 that is linked to artery-clogging plaques.

While some industry analysts have suggested it could become a $10 billion-a-year seller, if it works, few have pencilled in significant sales forecasts so far, given uncertainty about the product.

Current consensus forecasts point to annual sales of only $605 million in 2018, according to Thomson Reuters Pharma.

Panmure Gordon downgraded GSK shares to "hold" from "buy" on the latest news, which it said was the second big disappointment in the year, following similarly negative results with the company's MAGE-A3 cancer vaccine for melanoma in September.

It also predicted GSK would write down 150 million pounds ($240 million) of its investment in darapladib in the fourth quarter, even though a second Phase III trial is continuing.

"Even if that trial was successful, we believe it will be difficult to gain market registration on one trial alone," said Panmure analyst Savvas Neophytou.

MORE DATA IN 2014

Patrick Vallance, GSK's head of pharmaceuticals R&D, said the group would continue to investigate the role of Lp-PLA2 inhibition in coronary heart disease and other conditions, while also poring over the latest trial results.

"We will now work to better understand the data, including evaluation of the patient sub-groups, and await the outcome of a second Phase III study of darapladib in acute coronary syndrome," he said in a statement.

The first trial, known as STABILITY, enrolled nearly 16,000 patients with chronic coronary heart disease in 39 countries and measured the differences in outcomes between giving them darapladib or a placebo pill, in addition to standard drugs such as statins, aspirin and blood pressure medicines.

A second 13,000-patient study, called SOLID-TIMI 52, is due to complete in March 2014 and is looking at patients who have already suffered an acute coronary event. It will assess if darapladib can prevent a secondary attack.

Some medical experts believe darapladib may have a better chance of success in the second study.

GSK has had some notable successes with its new drugs this year, including approvals of medicines for cancer, lung disease and HIV. But its failure to deliver positive results in more ambitious projects like darapladib and MAGE-A3 may cast doubt over its prowess in the most cutting-edge science. – Reuters