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    New Clues to Link Between MS Drug Tysabri and Rare Brain Disease

    Researchers report drug mobilizes a kind of cell easily infected by a virus that can attack the brain

    continued...

    Since Tysabri was allowed back on the U.S. market in 2006 with strict prescribing conditions, more than 440 cases of PML have been reported in patients taking the drug, according to the study background. In 2010, the FDA added a warning about the heightened risk of PML to the drug's labeling.

    To find out why the drug carries such a high risk of PML, researchers collected blood samples from two groups of MS patients -- those just starting treatment with Tysabri, and those who had been on the drug for more than two years. They compared those samples to blood taken from healthy volunteers.

    The investigators were looking for a particular type of cell in the blood -- a kind of stem cell that turns into white blood cells called B-cell lymphocytes.

    "Turns out in these MS patients treated with [Tysabri], the number of these blood stem cells is three- to 10-fold higher than you'd see normally under normal physiologic conditions," Major said.

    "JC virus is able to infect these blood stem cells as they become a B lymphocyte," he explained. His working theory has been that these infected B lymphocytes then carry the infection into the brain.

    To test that theory, the researchers wanted to see if they could find traces of the JC virus in circulating blood stem cells.

    And they did. Of 26 patients who were just starting treatment with Tysabri, 50 percent had traces of the JC virus in their circulating blood stem cells. Of 23 patients who had taken the drug for more than two years, 44 percent had JC virus DNA in more than one kind of blood stem cell type. In contrast, only 17 percent of the 18 healthy volunteers had signs of the JC virus in those cells.

    "It was somewhat surprising to us that quite a high percentage of individuals had detectable viral DNA in these blood stem cells," Major said.

    But what isn't exactly clear is how this could affect their risk of developing PML. Most patients who tested positive for JC virus had only a few copies of the virus, suggesting that they were still at low risk of infection. Patients who had taken the drug for more than two years had higher virus counts than those who were just starting treatment.

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