Tysabri (natalizumab), a drug used to treat multiple sclerosis (MS) or Crohn's disease, was found to increase the risk of developing progressive multifocal leukoencephalopathy (PML) in some patients. PML is "a rare but serious brain infection," according to FDA MedWatch.

The FDA notified healthcare professionals that risk factors for PML are anti-JC virus (JCV) antibodies, which can be detected by the Stratify JCV Antibody ElLISA test2 (a detection test that was approved by the FDA on January 20, 2012). If a patient tests positive for anti-JCV antibodies, that means that the patient has been exposed to JCV in the past.

According to Medwatch, "Tysabri is in a class of medications called immunomodulators [which work] by stopping certain cells of the immune system from causing damage to the body."

Since November of 2004, Tysabri has been approved for the treatment of relapsing forms of multiple sclerosis, and was approved for the treatment of moderately to severely active Crohn's disease in January of 2008. While the FDA is not recalling this drug, they are recommending that patients who test positive for anti-JCV antibodies, and have one of the other two risk factors, should be closely monitored when taking Tysabri. Patients with all 3 risk factors have about an 11/1000 chance of developing PML.