AstraZeneca Plc’s key cholesterol drug Crestor failed to beat Pfizer Inc’s Lipitor in a head-to-head clinical study, limiting any protection the British company will get as its U.S. rival faces competition from cheap generic copies.

Although imaging tests showed patients on Crestor had a greater reduction in the percentage of plaque clogging their heart arteries than those on Lipitor, the difference in this main goal of the study was not statistically significant, AstraZeneca said on Friday.

Crestor did show a significant improvement over Lipitor in a secondary endpoint of the trial, which used ultrasound to measure the condition of coronary arteries in around 1,300 patients.

“Top-line results this morning are not the slam-dunk AstraZeneca might have been hoping for,” said Seymour Pierce analyst Mike Mitchell.

Shares in AstraZeneca fell 3.7 percent by 4:25 a.m. EDT, underperforming a 1.8 percent decline in the European drug sector.

The stock sell-off would have been greater without the encouraging signs described in the ‘top-line’ statement on the study results, which did not disclose full details.

While AstraZeneca failed to get a clear result that it could have used to justify Crestor’s superiority over Lipitor, analysts at Jefferies said the result was not an outright negative.

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Crestor is already AstraZeneca’s biggest-selling drug, with 2010 sales of $5.7 billion, and clear superiority would have helped cement its market position.

The absence of a definitive win may now dent confidence in its prospects, just as the Pfizer medicine is about to go off patent in the United States from the end of November.

Industry analysts currently have widely differing views about prospects for Crestor and how the drug will fare when cut-price versions of Lipitor - the world’s top-selling prescription drug - start to flood the market.

The consensus is currently for Crestor sales of $7.1 billion in 2015, but this incorporates a range from $5.4 billion up to $9.0 billion, according to Thomson Reuters data.

Tim Anderson of Bernstein said he was lowering his 2015 Crestor sales forecast by 5 percent to $6.5 billion, which would translate into a 4 percent reduction in 2015 earnings per share.

With patents on other drugs expiring and the group facing setbacks with a number of new products, AstraZeneca needs Crestor to keep growing strongly if it is to achieve its goal of annual group sales of $28 billion to $34 billion between 2010 and 2014.

The company’s decision to run a head-to-head clinical study was always viewed as a risk, but it was one AstraZeneca had reasonable grounds to think it could win.

Past studies have suggested high doses of Crestor are slightly better than high doses of Lipitor at reducing “bad” LDL cholesterol and far better at raising “good” HDL.

Full results of the so-called Saturn study comparing Crestor and Lipitor will be presented at the annual meeting of the American Heart Association on November 15.

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By Paul Hoskins and Ben Hirschler

LONDON/ZURICH