JC Virus Activation Common with Tysabri


While the Tysabri (natalizumab) helps in the treatment of multiple sclerosis, it also commonly activates the JC virus.

Yiping Chen, M.D and colleagues published their research in the Sept. 10 New England Journal of Medicine. They reported the absence of the JC virus in peripheral blood mononuclear cells at the start of Tysabri treatment in all patients, but found the virus in 60% of blood samples after 18 months.

The study involved 19 multiple sclerosis patients just starting Tysabri and followed them for 18 months. In addition to being found in the blood, the JC virus increased was also found in urine sample after just one year of treatment with Tysabri.


The JC virus is responsible for progressive multifocal leukoencephalopathy (PML). None of the 19 patients in this study developed any symptoms or brain lesions suggestive of PML. There were reports of two new cases of PML published by other researchers in the NEJM this week, bringing the total to 14 in multiple sclerosis patients since natalizumab was introduced in 2004.

In an accompanying editorial, Eugene Major, PhD, of the National Institute of Neurological Disorders and Stroke in Bethesda, Md., noted that JC viruria and viremia are not uncommon in the general population. He wrote, "It is generally accepted that many people (up to 40% of the population) excrete JC virus DNA in urine with no pathologic significance, which makes viruria an unlikely candidate for a predictive marker."

It is clear from the report that Tysabri increases the presence of JC virus in the urine and blood, but as Chen and colleagues point out, it remains unclear how the virus in the blood can be transmitted into the central nervous system to cause PML.

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Chen Y, et al "Asymptomatic reactivation of JC virus in patients treated with natalizumab" N Engl J Med 2009; 361: 1067-74.
Major E "Reemergence of PML in natalizumab-treated patients -- New cases, same concerns" N Engl J Med 2009; 361: 1041-42.
Wenning W, et al "Treatment of progressive multifocal leukoencephalopathy associated with natalizumab" N Engl J Med 2009; 361: 1075-80.