The new study found no new cases of progressive multifocal leukoencephalopathy (PML) and confirmed the three previously identified cases of PML associated with use of the drug. One fatal and one nonfatal case of PML occurred in a trial using natalizumab as a multiple sclerosis treatment; a second fatality happened in a trial that used the drug to treat patients with Crohn's disease, an inflammatory bowel disorder.

"Our analysis suggests about one in every1,000 people who took natalizumab contracted this disease; however, there weren't enough patients exposed to the drug to allow us to precisely estimate the risk, which could be as low as one in 5,000 or as high as one in 300," says senior author David Clifford, M.D., the Melba and Forest Seay Professor of Clinical Neuropharmacology in Neurology at Washington University School of Medicine in St. Louis.

The results of the study, along with two separate studies of natalizumab's effectiveness as an MS treatment, are published in this week's issue of The New England Journal of Medicine.

The brand name of natalizumab, which was jointly developed by Biogen and Elan Pharmaceuticals, is Tysabri. The drug is a monoclonal antibody that binds to inflammatory immune T cells and prevents them from crossing membranes that protect the brain and the central nervous system. Prior to the studies that were halted last year, earlier studies showed a 66 percent reduction in the rate of relapses in MS patients treated with the natalizumab, which has to be injected on a monthly basis.

Clifford predicts that his group's report and the other studies of natalizumab will be important elements in "lively" discussions to be held by the Food and Drug Administration reg
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Contact: Michael C. Purdy
purdym@wustl.edu
314-286-0122
Washington University School of Medicine
1-Mar-2006