Sanofi's (NYSE: SNY) Multaq has stumbled again. The company stopped a trial of the atrial fibrillation drug in patients with a permanent version of AF after researchers flagged some serious cardiovascular side effects. Approved to treat a temporary form of the abnormal heart rhythm, Multaq might have gained a large new target market had it aced the study.

Sanofi didn't say much about why it stopped the study, known as PALLAS, the Los Angeles Times reports. It only mentioned a "significant increase in cardiovascular events" in the trial and emphasized that Multaq is still considered safe for patients with non-permanent AF. "We remain committed to Multaq as an essential treatment option for non-permanent atrial fibrillation patients," CMO Jean-Pierre Lehner said in a statement.

But the assurance comes after two of those non-permanent AF patients developed acute liver failure--and after France decided that Multaq might not be worth paying for. Liver problems didn't arise in PALLAS, Sanofi says. In the wake of the reports of liver damage, French authorities said Multaq is "insufficient," meaning that the government could stop reimbursing for its use.

The PALLAS news is not good for Sanofi, which has been counting on Multaq to help fill in the gaps as it loses patent protection on some key products. The drug has been expected to reach some $1.3 billion in revenues, but the failure of PALLAS might dampen those expectations. Analysts might "reduce their sales estimates for Multaq and the contribution that the drug will make to the profits of the group," Oddo Securities' Jean-Jacques Le Fur told Reuters.

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