WASHINGTON (Reuters) - The U.S. Food and Drug Administration appears skeptical that data submitted by Johnson & Johnson proves its anticoagulant Xarelto is effective in reducing the risk of further heart problems in patients who have recently suffered a heart attack. The agency questioned the way in which J&J analyzed clinical trial data, and said there was no convincing proof the drug confers significant benefit or fills an unmet medical need, given that there are other therapies on the market. Xarelto is already used to treat and prevent deep vein thrombosis and pulmonary embolisms and to reduce the risk of stroke and blood clots in patients with an irregular heart beat that is not caused by heart problems. Now the company is hoping it will also be approved for patients with acute coronary syndrome (ACS), an umbrella term covering any condition brought on by a sudden, reduced blood flow to the heart, including heart attack and chest pain. Some analysts are doubtful. "Based on our review of this material, we continue to have low expectation of approval," Larry Biegelsen, an analyst at Wells Fargo Securities, said in a research note. However, he expects sales of the drug to rise to $1.2 billion in 2015 from an estimated $703 million in 2013 based on the indications for which it is approved. "We expect the ACS potential to be modest even if approved," he said. J&J originally filed for approval of Xarelto in ACS at the end of 2011. The FDA rejected the application, saying efficacy data was not strong enough to support approval. The company provided additional information but the FDA once again declined to approve the drug, prompting J&J to appeal the decision. The FDA denied the appeal but said limiting the duration of use to one month might be a pathway forward because efficacy was more evident and the risk of bleeding, a side effect of the drug, was lower during this period. J&J filed a new application seeking a treatment duration of 90 days. The FDA's latest review suggests the agency remains skeptical. Dr. Paul Burton, vice president of clinical development at Janssen Research and Development, a J&J unit, defended the drug, saying the company believes that when added to standard treatments it "delivers a strong incremental benefit by significantly reducing the risk of cardiovascular events, including death, at a time when patients are at the highest risk." The FDA's review also questioned whether the benefit of the drug outweighs the heightened risk of bleeding since two other drugs, Eli Lilly & Co's Effient and AstraZeneca Plc's Brilinta, are currently approved for ACS. J&J's proposed prescribing information would warn that treatment in combination with the Effient, known also as prasugrel, and Brilinta, also known as ticagrelor, has not been studied and is not recommended because of the risk of bleeding. Xarelto would therefore only be available as an add-on to Bristol-Myers Squibb Co's antiplatelet Plavix, or clopidogrel. "There are no data demonstrating that ACS patients treated with clopidogrel plus rivaroxaban will have superior outcomes compared to treatment with prasugrel or ticagrelor," the review said. So rivaroxaban does not provide therapy for an unmet medical need. And while treatment for a limited duration "has an intuitive appeal," the review said, "the task for the analyses of the effect of rivaroxaban over time is not to pick a time period in which the benefit-risk is acceptable." Reporting by Toni Clarke in Washington; Editing by Nick Zieminski, Chizu Nomiyama and Bernard Orr