(HealthDay News) –  Multiple sclerosis patients taking natalizumab are at higher risk of developing progressive multifocal leukoencephalopathy (PML) if they are positive for the anti-JC virus antibodies, have been treated with immunosuppressants, and have been treated with natalizumab for longer periods, according to a study published in the May 17 issue of the New England Journal of Medicine.

Gary Bloomgren, MD, and colleagues from Biogen Idec in Weston, MA, estimated the risk of PML based on the presence of anti-JC virus antibodies, use of immunosuppressants, and treatment length in 212 confirmed cases of PML from a cohort of 99,571 patients treated with natalizumab (2.1 cases per 1,000 patients).

The researchers found that 54 patients who had samples available before diagnosis were all positive for anti-JC antibodies. The PML risk was lowest in patients negative for anti-JC virus antibodies (0.09 cases or less per 1,000 patients). The risk was highest in patients positive for anti-JC virus antibodies who had taken immunosuppressants before starting treatment and were treated for 25–48 months (11.1 cases per 1,000 patients).

“Positive status with respect to anti-JC virus antibodies, prior use of immunosuppressants, and increased duration of natalizumab treatment, alone or in combination, were associated with distinct levels of PML risk in natalizumab-treated patients with multiple sclerosis,” Bloomgren and colleagues conclude.

The study was funded by Biogen Idec and Elan Pharmaceuticals. All of the authors disclosed financial ties to Biogen Idec.

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