David Meeker, president of Sanofi's Genzyme subsidiary, maker of the drug Lemtrada, said he "strongly" disagreed with the decision by the U.S. Food and Drug Administration and will appeal the ruling, Radio France Internationale reported Monday.
Lemtrada is already approved in the European Union, Canada and Australia.
Sanofi had planned to release Lemtrada in the United States March 31.
The FDA's decision that "one or more" clinical trials were necessary before the drug could be approved would probably delay approval past that date, Sanofi said.
"We are extremely disappointed with the outcome of the review and the implications for patients in the U.S. suffering with multiple sclerosis who remain in need of alternative therapies to manage a devastating disease," Meeker said in a statement.
In rejecting the drug, the FDA said the drug maker had not submitted sufficient evidence from research studies "that demonstrate the benefits of Lemtrada outweigh its serious adverse effects."
Sanofi says Lemtrada's most common side effects include infections of the upper respiratory and urinary tracts; a reduction in the number of lymphocytes, a type of white blood cells, and complications from autoimmune conditions.
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