Sanofi (SNY -2.27%) announced today that the Food and Drug Administration accepted its supplemental Biologics License Application requesting permission to market Lemtrada for the treatment of relapsing multiple sclerosis.

The active ingredient in Lemtrada was originally approved as Campath to treat leukemia, but Sanofi pulled Campath off the market, with some speculating that this was because the difference between the leukemia dose and the multiple sclerosis dose would have made the drug extremely cheap for the multiple sclerosis patients.

The marketing application is based on two phase 3 studies comparing Lemtrada to Pfizer (PFE -0.19%) and Merck KGaA's Rebif. One study compared the drugs in patients new to treatment while the other tested patients who had relapsed while on a prior therapy.

Sanofi expects a decision from the FDA in the second half of the year. A decision from the Committee for Medicinal Products for Human Use in the EU is expected in the second quarter.

Separately, Sanofi gave investors an update on the launch of its recently approved oral multiple sclerosis drug, Aubagio. More than 80% of multiple sclerosis specialists in the U.S. have prescribed Aubagio, according to the company, and four out of five patients are switching from another medication with more than half of those coming from Teva Pharmaceutical's Copaxone or Biogen Idec's Avonex, which both have to be injected.

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