AstraZeneca Plc's experimental blood thinner goes before U.S. advisers on Wednesday, facing questions over why a trial of the potential blockbuster drug failed to cut heart attacks and deaths in North American patients.

Brilinta, with expected annual sales of more than a billion dollars, is seen as key to AstraZeneca's growth as the company looks to new medicines to make up for lost sales from coming patent expirations on some of its best-selling medicines.

Members of a Food and Drug Administration advisory panel will hear from company officials and agency reviewers before voting Wednesday afternoon on whether to recommend approval of Brilinta.

FDA staff offered mixed views of Brilinta in documents released Monday ahead of the meeting, but investors pushed AstraZeneca's U.S.-listed shares up nearly 4 percent on optimism the advisers would back the pill.

The FDA is due to make a final decision on Brilinta by September 16. The agency usually follows panel recommendations. In an 18,000-patient trial of the drug, Brilinta showed a sharp reduction overall in deaths and heart attacks compared with current standard anti-clotting pill Plavix, sold by Sanofi-Aventis and Bristol-Myers Squibb.

But no benefit was seen from Brilinta in the relatively small group of North American patients, who made up about 9 percent of the trial.

Some FDA reviewers questioned the British drugmaker's theory that higher aspirin doses used routinely in the United States accounted for the regional differences.

Other FDA staff seemed willing to accept the varying results given that overall findings were so strong.

Reviewers in the agency's pharmacology office recommended FDA clearance for Brilinta with a post-approval study to examine effectiveness in U.S. patients.

AstraZeneca is counting on Brilinta to help offset the looming loss of patent protection on key drugs such as heartburn treatment Nexium and antipsychotic Seroquel.

Consensus 2014 sales forecasts for Brilinta stand at $1.1 billion, according to Thomson Reuters data. The company's total sales topped $32 billion in 2009.

Brilinta would compete with Plavix, the world's second-best selling drug with annual sales of about $9 billion.

In AstraZeneca's study, Brilinta proved better than Plavix in preventing heart-related problems linked to blood clots. After one year, 9.8 percent of Brilinta patients had suffered a heart attack, strokes or cardiovascular-related death, compared with 11.7 percent in the Plavix group.

Plavix and Brilinta work by stopping platelets from sticking together and forming clots that can cause heart attacks and strokes. Both drugs carry a risk of bleeding.