According to the U.S. Food and Drug Administration, there have been 13 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. Although the absolute risk for PML in patients treated with Tysabri cannot be precisely determined, the frequency to date remains less than the one-in-one thousand risk that was estimated at the time of Tysabri’s re-approval in 2006. The latest post-marketing safety warnings provided by the FDA on Tysabri can be found here.
Early Detection Important It appears that when PML is detected and treated early, it generally improves outcomes. It is important that individuals taking this drug and their doctors be vigilant in monitoring for any occurrence of new, unusual symptoms that might indicate PML.
Typical symptoms associated with PML progress over days to weeks, and can include:
clumsiness and progressive weakness on one side of the body,
disturbances of vision, and
changes in thinking, memory, and orientation leading to confusion and personality changes.
If individuals taking Tysabri experience new, unusual symptoms, they should contact their prescribing physician immediately. Physicians who need guidelines on the protocol to follow when they have a patient on Tysabri who experiences unusual symptoms should contact Biogen Idec.
There is no specific therapy to treat PML, but the best hope is to reconstitute a person’s immune responses. Based on small-scale studies supported by Biogen Idec, plasma exchange, a blood-cleansing treatment, has been used to clear the bloodstream of Tysabri. There is insufficient evidence to determine whether plasma exchange can reduce PML symptoms.
Tysabri is a registered trademark of Biogen Idec and Elan.