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 T2-hypertense MRI activity by 83%, reduced new active contrast-enhancing lesions by 92% and slowed the likelihood of progression of disability by 42%. Based on an MRI scan at 2 years, 97% of patients were free of active contrast-enhanced lesions.
Risks include severe allergic infections and serious infections. A rare and untreatable viral brain infection called progressive multifocal leukoencephalopathy (PML) has been associated with Tysabri use.Previous exposure to the JC virus is a very important risk factor for an increased risk of developing PML. Greater than 50% of people 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. Approximately 1/4 of the MS patients who developed 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), cyclophosphamide (Cytoxan), methotrexate, and mitoxantrone (Novantrone).
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