LONDON (Reuters) - An experimental heart drug from GlaxoSmithKline Plc (GSK.L: Quote, Profile, Research) effectively blocks an enzyme linked to heart attack risk and a higher dose has a bigger effect, scientists said on Sunday.

The news is an incremental advance for darapladib -- a potential blockbuster that has been in development for years -- although Glaxo has yet to demonstrate the drug helps patients in a clinically significant way.

The first-in-class product, which Glaxo discovered with Human Genome Sciences Inc (HGSI.O: Quote, Profile, Research), targets an enzyme called Lp-PLA2.

Industry analysts at UBS think it could potentially be huge, with annual sales of $5 billion to $10 billion. However, the brokerage views the project as high risk and is only modelling a 20 percent chance it will successfully get to market.

Low confidence is shared by many other analysts, following the recent failure of other novel heart drugs like Pfizer Inc's (PFE.N: Quote, Profile, Research) torcetrapib and AtheroGenics Inc's (AGIX.O: Quote, Profile, Research) AGI-1067, which the U.S. biotech company was working on with AstraZeneca Plc (AZN.L: Quote, Profile, Research).

Results of a darapladib dose-ranging study, involving 959 patients with heart disease, who were already taking Pfizer's cholesterol fighter Lipitor, were presented by researchers from the University of Pennsylvania at the annual meeting of the American College of Cardiology in Chicago.

They showed that three different doses of darapladib inhibited Lp-PLA2 and there were no major safety concerns. The highest 160 mg dose also reduced levels of two biomarkers associated with inflammation.

More significant imaging data, which will assess the impact of darapladib on arterial plaque by peering inside blood vessels, are expected to be published in a leading medical journal later this year.

Positive results from the imaging trial would boost belief in darapladib. Still, Glaxo may yet have to conduct further lengthy clinical outcomes tests that industry analysts say could take around another four years.

(Reporting by Ben Hirschler; editing by Sue Thomas)