The FDA announced earlier this fall that it had confirmed 13 cases of progressive multifocal leukoencephalopathy (PML) in patients receiving natalizumab (Tysabri) monotherapy for multiple sclerosis. There were no reported confirmed cases in patients receiving natalizumab for Crohn's disease, an indication the FDA approved in January 2008. However, according to the FDA, less than 2% of natalizumab use in the U.S. has been for Crohn's patients.
The first reports of PML from Tysabri surfaced in 2005. Because of the risks of PML, Tysabri is now only prescribed through a program developed by the FDA and the drug's manufacturer called The TOUCH Prescribing Program.
The FDA said the risk for developing PML appears to increase with the number of natalizumab infusions received. The 13 patients who developed PML received an average of 12 to 35 monthly infusions per patient. The average number of infusions received before the diagnosis of PML was 25.
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