Tysabri (natalizumab) is an antibody treatment given in the vein (I.V.) every 4 weeks. Tysabri blocks lymphocytes, a type of white blood cell, from crossing the blood vessel wall to enter the brain or spinal cord. In clinical trial, Tysabri prevented 68% of relapses, reduced new MRI activity by 83%, and slowed the likelihood of progression of disability by 42%. Risks include severe allergic infections and serious infections.

Out of approximately 115,000 people that have been treated with Tysabri, 377 total cases of a rare and untreatable viral brain infection called progressive multifocal leukoencephalopathy (PML) have been associated with Tysabri use.  Twenty-three percent of the PML patients have died. The risk is greater when treated for more than 2 years on Tysabri or previously treated with an immunosupressant medication such as azathioprine (Imuran), cylcophosphamide (Cytoxan), methotrexate, and mitoxantrone (Novantrone).  Previous exposure to the JC virus is a very important risk factor for the risk of developing PML.  Fifty to sixty percent of MS patients have been exposed to the JC virus.  Testing is available free of charge through Quest Diagnostics with an order from your healthcare provider.  Testing for the JC virus every 6 months is recommended unless already known to be positive.