WEDNESDAY, May 16 (HealthDay News) -- Drug company researchers say they've determined which multiple sclerosis patients are most vulnerable to developing a rare brain infection while taking a powerful drug called Tysabri (natalizumab).
Even the multiple sclerosis patients at highest risk of developing the brain infection face low odds of getting it while on Tysabri, but the infection can be deadly.
"Now we can identify which risk group each patient is in. This will help doctors and patients make better decisions," said Dr. Gary Bloomgren, lead author of a study released Wednesday and vice president of drug safety at Biogen Idec, which makes the drug.
About 100,000 patients have taken Tysabri at least once. The drug, which is used mainly to delay the progression of multiple sclerosis, has been on the market since 2006. It originally went on the market in 2005 but was removed due to medical problems that some patients encountered, Bloomgren said.
Tysabri is expensive, costing about $40,000 a year. It dampens the immune system, potentially reducing the inflammation that is a major part of multiple sclerosis. Dampening the immune system, however, can allow an infection known as the JC virus to wreak havoc on the brain and cause a condition called progressive multifocal leukoencephalopathy.
The JC virus is extremely common, affecting about half of adults by middle age, but the immune system normally keeps it under control so it causes no symptoms. If unleashed, however, progressive multifocal leukoencephalopathy can cause cognitive problems, such as unusual behavior, paralysis and problems with vision, speech and balance, said study co-author Dr. Sandra Richman, senior medical director at Biogen Idec.
Once the symptoms appear, doctors usually advise patients to stop the treatment, she said. Progressive multifocal leukoencephalopathy often causes permanent disabi
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