According to information released yesterday by Biogen Idec, there have been 24 confirmed cases of progressive multifocal leukoencephalopathy (PML, a viral infection of the brain that usually leads to death or severe disability) among people who have used Tysabri® (natalizumab, Biogen Idec and Elan Pharmaceuticals) after it became available for prescription in July 2006.
As of the end of September 2009, 60,700 people have used Tysabri worldwide. Although the absolute risk for PML in patients treated with Tysabri cannot be precisely determined, the sponsor has now released data suggesting that the risk increases with increasing time on therapy, starting out lower than the one-in-one thousand level that was estimated at the time of Tysabri’s re-approval in 2006, and rising after two years of infusions to about one in one thousand. There is insufficient information to determine the risk of PML in those who have been on therapy for three years or more. Right now only 2,000 people have been on the therapy for over three years.
This release followed an October 23 announcement from the EMEA, the European equivalent of the U.S. FDA, indicating that one of its advisory committees was launching a review of the risks and benefits of Tysabri in light of the increasing number of new cases of PML.