Scientists Find Clue to Dangerous Side Effect of MS Drug

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"We don't have a very good handle right now on how to determine who's at higher risk for PML," O'Looney stated. "There's no way to monitor patients and no way to predict who will be susceptible. It's important to find some marker or indication of the presence of these viruses either in the urine or the blood."

In this latest study, 19 patients with relapsing-remitting multiple sclerosis who were taking Tysabri underwent lab work at three, six, 12 and 18 months after starting treatment.

After 12 months, measurements of JC virus in the urine rose from 19 percent (about normal) of samples to 63 percent at 12 months. At this time, only one patient showed JC virus in their blood.

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By 18 months, the virus had infiltrated plasma samples in 20 percent of patients and blood cells in 60 percent of patients. And the virus type in question here had undergone a rearrangement that made it more adept at crossing into the brain. "The changes are usually only found in the brains of patients with PML [and] we think the virus acquired the [changes] during Tysabri treatment," Koranik said.

Immune system responses associated with higher levels of JC virus dropped after six and 12 months of treatment. "This was unexpected," said Koralnik, who is director of the HIV/Neurology Center at Beth Israel Deaconess Medical Center in Boston. "It dampened the immune response as measured in blood samples and that was associated with the appearance of the virus in the urine."

None of the patients developed PML or any indication that they might develop the infection.

"The next question is: does that mean that all those who have virus in the blood or rearrangement in the urine will develop PML?" Koralnik said. "This is something that obviously we can't answer with this pilot study. The epidemiology indicates that the development of PML is still a rare event in Tysabri-treated patients, but we hope that if we follow the appearance of the virus in the blood or urine and follow changes [in the actual virus], in the future we will be able to detect better those at risk of developing PML while in treatment."

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