* Second darapladib study to enrol 11,500 patients

* Drug could be a blockbuster, but prospects uncertain

LONDON - GlaxoSmithKline has started a second large-scale clinical trial with experimental heart drug darapladib, increasing its investment in the new type of medicine for clogged arteries.

The latest Phase III study will include 11,500 patients with acute coronary syndrome in 40 countries and follows the launch of a first 15,000-patient trial a year ago, the company said on Wednesday.

With more than 27,000 patients, the combined Phase III clinical programme will be one of the largest ever conducted for a heart drug.

The new study will test whether darapladib affects the chances of a patient with heart problems having a cardiovascular event, such as a heart attack or stroke.

Glaxo had always said it planned to start the second trial in late 2009.

The move comes at a time when some large drugmakers are cutting back on developing new cardiovascular treatments -- but Glaxo believes its drug could be a winner because it works in a different way to others.

Darapladib, which Glaxo discovered with Human Genome Sciences, is the first in a new class of drugs targeting an enzyme called Lp-PLA2 which is linked to artery-clogging plaques.

It is designed to offer something above and beyond the hugely successful statin class of cholesterol-lowering treatments, by reducing the risk of plaques rupturing and blocking blood vessels, thereby triggering heart attacks.

Industry analysts estimate the drug could sell billions of dollars a year, if it succeeds in late-stage tests. But the project remains high-risk and darapladib is still many years from reaching the market. (Reporting by Ben Hirschler)

Medicine Glaxosmithkline