Another case of a potentially fatal brain infection has been reported in a patient taking the multiple sclerosis drug Tysabri, the biotech who developed the drug announced yesterday.

This is the fourth case of infection, called progressive multifocal leukoencephalopathy (PML) this year. They are the only cases reported since the drug was taken off the market in 2005 because of three cases of infection. The FDA allowed Tysabri back on the market in 2006 with restrictions and stronger warnings. Regulators have still not been able to understand the link between the drug -- a monoclonal antibody -- and the brain infections.

A spokeswoman for the drug's manufacturer, Biogen Idec Inc, Naomi Aoki, said the drug's effectiveness at preventing relapses of multiple sclerosis symptoms justifies its use, despite the relatively low risk of PML, according to Reuters. About 35,000 patients worldwide are taking Tysabri.

PML is a viral disease that infects and damages the white matter of the brain, usually affecting people who are immunosuppressed.


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