Tysabri (natalizumab) is a new, highly effective drug for treatment of multiple sclerosis. A rare, life threatening viral infection, progressive multifocal leukoencephalopathy (PML), can occur during treatment with Tysabri.
Among the risk factors identified so far are: 1)length of therapeutic exposition to Tysabri – majority of PML cases has appeared after the first 24 months of treatment, 2)previous therapy with immunosuppressant, and 3) persistence of JCVirus in the patient’s body, which can be identify by a two-step ELISA diagnostic test.