May 17, 2012 — A new study published in the New England Journal of Medicine has identified risk factors for Progressive Multifocal Leukoencephalopathy (PML), a deadly brain infection that is a severe side effect of Tysabri (natalizumab). In some patients, Tysabri triggered a latent JC-virus to re-activate, causing PML.
The researchers discovered that patients who tested negative for the JC-virus had almost zero risk for PML. They also found that PML was most likely to occur in patients who had the JC-virus, used Tysabri for long periods of time, and had a history of immunosuppressant therapy.
The researchers were able to analyze the pre-treatment antibody status of 54 patients who had developed PML on Tysabri. All 54 patients tested positive for the JC-virus.
As of February 29, 2012, a total of 212 people taking Tysabri have developed PML, and 99,571 people have taken Tysabri.
Experts first realized that PML was a side effect of Tysabri soon after the drug was launched in 2004. In 2005, the FDA recalled Tysabri from the U.S. market. After the manufacturers created strict criteria for determining risk factors for developing PML, the FDA allowed Tysabri to be re-introduced on the U.S. marketplace.
Early in 2012, a test for the existence of the JC-virus became available. The researchers highlighted the need for periodic testing to determine whether a patient contracted the virus during Tysabri treatment. As an example, one Tysabri patient tested negative for the JC-virus before treatment, but later tested positive for the virus during treatment and subsequently suffered from PML.
Even among patients who test positive for the JC-virus, however, the risk of developing PML is still small. It is approximately 1 per 1,000 patients with no other risk factors, but increases to 11 per 1,000 patients who have a long history of Tysabri use (longer than 24 months) and a past history of immunosuppressant therapy.