News Feature | April 2, 2014

Study Discovers Why MS Drug Tysabri Increases Risk Of PML

By Marcus Johnson

Researchers at the US National Institute of Neurological Disorders and Stroke (NINDS) believe that patients taking the MS drug Tysabri face a higher risk of developing the brain disease multifocal leukoencephalopathy (PML). PML is rare and can be fatal. The drug Tysabri mobilizes certain blood cells containing the JC virus that causes PML. Researchers believe that because the virus can hide in blood cells, current tests can miss detection and delay diagnosis.

Eugene Major, who authored the study at the U.S. National Institute of Neurological Disorders, said the risk for PML could be as high as one in 75 patients who have taken Tysabri for more than two years, along with other immune-suppressing drugs, and that have tested positive for the antibodies to the virus. According to Major, "We need to be able to understand why this therapy puts patients at risk. As we further define that, we'll be able to develop better tests and better treatment decisions can be made.”

Researchers at NINDS claim that 30 to 50 percent of all patients diagnosed with PML die a few months after their diagnosis. Normally, the JC virus is “harmless,” however in PML, this “JC Virus” attacks the white matter of the brain, eliminating the nerve cell insulation that enables the nerve cells to effectively carry brain signals. PML usually only affects individuals with extremely suppressed immune function, including patients with AIDS, or those taking immune-suppressing drugs, like Tysabri. According to Major, “Turns out in these MS patients treated with [Tysabri], the number of these blood stem cells is three- to 10-fold higher than you'd see normally under normal physiologic conditions. JC virus is able to infect these blood stem cells as they become a B lymphocyte.”

Tysabri has faced scrutiny in years past. Tsyabri was first approved by the FDA in 2004, but was pulled off the market three months later after PML occurred in ongoing clinical trials. The drug was brought back into the market in 2006 with strict prescribing instructions. However, since its re-release, there have been 440 cases of PML reported in patients using the drug.  In 2010, the FDA altered the drug’s label to include warnings about the heightened risk of PML