According to Bloomberg, Biogen says another patient was confirmed to have progressive multifocal leukoencephalopathy (PML), bringing the number of infections to eight in the past year.

PML is a rare and usually fatal disease of viral origin – typically JC virus or John Cunningham virus (JCV). The disease is characterized by progressive damage or inflammation of the white matter of the brainat multiple locations and occurs almost exclusively in people with severe immune deficiency (e.g. transplant patients on immunosuppressive medications) or AIDS.

As everyone remembers, Biogen and Elan pulled Tysabri from the market in 2005 after three patients were reported to have developed PML, two of which died. The drug was re-introduced in 2006 after the FDA said the benefits of preventing MS relapses outweighed the risks of developing PML.

More than 56,000 patients have been treated with Tysabri and according to the drug’s label. The likelihood of contracting PML is 1:1000.

PML has received a lot of press recently due to the Tysabri infections, and to increased surveillance and press coverage. In April, Novartis reported that one patient on Fingolimod (FTY720) came down with PML.

Recently, Roche/Genentech’s Raptiva, which is indicated for psoriasis, was been completely pulled from the U.S. and the EU after three patients were diagnosed with PML, two of which died.

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