PARIS -(Dow Jones)- French pharmaceutical giant Sanofi-Aventis SA (SAN.FR) Friday said two patients treated with its Multaq atrial fibrillation drug have suffered liver failure and require transplants, though no causal link has been established between taking the drug and the disease.

Sanofi sent a letter to healthcare providers in the U.S. about the case, company spokesman Jean-Marc Podvin told Dow Jones Newswires Friday. "Patient safety is Sanofi's priority," Podvin said. The company is also "working with European health authorities and in the countries where Multaq is approved and those where it is submitted for review."

In a separate statement on its website Friday, the U.S. Food and Drug Administration said it was "alerting healthcare professionals and patients about cases of rare, but severe liver injury, including two cases of acute liver failure leading to liver transplant in patients treated with the heart medication dronedarone (Multaq)."

The company wouldn't disclose in which country the two cases of liver failure happened. On its Multaq website, Sanofi-Aventis cautions that patients with "severe liver problems" should not take the drug.

Sanofi-Aventis's Podvin said about 200,000 patients have been prescribed Multaq - chemical name dronedarone - since its launch in July 2009.

The FDA said that "since dronedarone's approval in July 2009 through October 2010, around 492,000 dronedarone prescriptions were dispensed and around 147,000 patients filled dronedarone prescriptions at outpatient retail pharmacies in the United States."

The agency said it is continuing to review reports of possible adverse events and drug interactions with Multaq submitted to its Adverse Event Reporting System.

Multaq was first approved for use in 2009. It is sold in France, the U.K., Germany, the U.S., Denmark, Norway, Ireland and Canada, among other countries. In draft guidance last year, the U.K. health institute said Multaq should only be considered as a treatment for patients who haven't benefited from other medicines and are at greater risk of having a heart attack because of conditions like diabetes or high blood pressure.

In August, Multaq was granted a class I recommendation in the European Society of Cardiology's new guidelines for the management of atrial fibrillation.

Over the first nine months of last year, Multaq brought EUR109 million in revenue to Sanofi, the company's spokesman said. Like most big drug companies, Sanofi-Aventis is counting on recently launched drugs to bolster revenue as older drugs reach patent expiration and face competition from generic drug makers. Multaq was with launched with hopes for hundreds of millions of euros of sales.

Sanofi-Aventis shares ended lower on the Euronext Paris exchange after the news. The stock closed down EUR0.60, or 1.2%, form the previous close to EUR51.20.

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