UPDATE 1-Glaxo bets big on drug for clogged arteries

* 15,000-patient Phase III darapladib trial launched

* Second 12,000-patient trial planned for late 2009

* Shares in partner Human Genome Sciences up 10 pct

(Adds Human Genome Science share price, detail on royalty deal)

By Ben Hirschler

LONDON, Dec 18 (Reuters) - GlaxoSmithKline Plc (GSK.L) placed a big bet on a new type of medicine for clogged arteries on Thursday by launching a costly three-year clinical trial involving more than 15,000 patients worldwide.

The world's second biggest drugmaker said it expected the Phase III study of darapladib to last about three years, with a second large trial involving around 12,000 subjects due to start in late 2009.

Glaxo wants to find out if patients live longer and have fewer heart attacks and strokes when given the drug.

The move comes at a time when some other large drugmakers are cutting back on developing new cardiovascular treatments.

Darapladib, which Glaxo discovered with U.S. biotech company Human Genome Sciences Inc (HGSI) (HGSI.O), is the first in a new class of drugs targeting an enzyme called Lp-PLA2 which is linked to artery-clogging plaques.

Shares in HGSI, which will earn a 10 percent royalty if darapladib makes it to market, rose 10 percent in early trade on Nasdaq. Glaxo stock was unchanged in London.

The drug is designed to offer additional benefits 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.

UBS analysts estimate annual sales might eventually top 5 billion pounds ($7.7 billion) a year if the drug works. But the project remains high-risk and darapladib is still many years from reaching the market.

Glaxo's decision to invest heavily in the new approach contrasts with a move this year by its biggest rival Pfizer Inc (PFE.N) to stop hunting for new drugs for heart disease.

"We are prepared to take a risk in an area where risk-taking is necessary if we're going to change the treatment paradigm," Patrick Vallance, Glaxo's head of drug discovery, told Reuters.

He declined to comment on the exact cost of the Phase III trial programme but acknowledged it would be expensive.