The European Medicines Agency said it was concerned about the medicine's safety, noting that a high proportion of patients stopped taking it within two years, mainly due to side effects such as flu-like symptoms, injection site reactions and liver toxicity.

The European rebuff contrasts with a green light for the drug, known generically as mipomersen, by a U.S. advisory panel in October.

Sanofi's Genzyme unit said it was disappointed by the decision and planned to request a re-examination.

A Sanofi spokeswoman said the company expected feedback from the regulator in the second quarter of 2013.

Isis founder and Chief Executive Stanley Crooke said on a conference call with analysts he was still optimistic on securing approvals in both the United States and Europe.

"Even though I cannot address the specific concerns of the (EU) committee ... I can say that each issue raised by the (committee) was thoroughly reviewed by the FDA," Crooke said. "We do not believe that today's opinion should have a negative impact elsewhere."

At 1514 GMT Sanofi shares were trading 0.1 percent lower at 71.72 euros, in line with the CAC 40 index, while Isis was up 0.3 percent at $9.87 in New York.

California-based Isis is entitled to 30 percent of total sales of Kynamro once the drug reaches the market, rising to 50 percent for sales over $2 billion.

(Reporting by Ben Hirschler; Additional reporting by Elena Berton in Paris and Zeba Siddiqui in Bangalore; Editing by David Cowell and David Holmes)

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