FDA: MS Drug Tysabri is Riskiest in Third Year of Treatment

Biogen Idec’s multiple sclerosis drug Tysabri poses the highest risk of brain infection during the third year of treatment, U.S. health officials warned on April 22. An infection called progressive multiform leukoencephalopathy (PML), occurs in an estimated 1.5 per 1,000 patients treated with Tysabri during months 25 to 36, the Food and Drug Administration said.

The FDA reported that the PML risk was 0.3 per 1,000 patients during the first two years of treatment. After three years, the rate was 0.9 per 1,000. Limited data is available beyond four years. The drug, which is given through a monthly transfusion, is expected to reach annual sales of nearly $1.5 billion by 2015.

The drug was taken off the market in 2005 amid fears of PML, but returned with restrictions in 2006. The FDA plans to update the prescribing instructions of Tysabri to reflect the new research, although the agency believes that “the benefits of taking Tysabri outweigh the potential risks.”

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