Biogen Idec, Elan report two new cases of PML in patients taking Tysabri; shares drop

Biogen Idec and Elan saw company shares drop late Thursday by as much as 16 percent and 36 percent, respectively, after announcing that two cases of progressive multifocal leukoencephalopathy (PML) were confirmed this week in patients with multiple sclerosis treated with Tysabri (natalizumab). Both cases occurred in the EU, the companies stated in a filing with the US Securities and Exchange Commission.

The first case of PML involves a treatment-naive patient who had been taking Tysabri as monotherapy for around 17 months, the companies noted, adding that "the patient remains clinically stable." In the second case, the drugmakers explained that the patient, who is currently hospitalised, was taking Tysabri as monotherapy for about 14 months but had previously taken other therapies for multiple sclerosis.

These reports of PML are the first since the companies voluntarily suspended sales of Tysabri in the US in 2005 after two cases of the potentially fatal brain infection were reported, and a third case was later confirmed. The drug was approved by the FDA for re-introduction in 2006, and Tysabri was approved in Europe during the same year.

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