TYSABRI - MULTIPLE SCLEROSIS THERAPY

Tysabri is considered a highly effective therapy for multiple sclerosis, but its growth has been slower than originally hoped due to concerns about the risk of PML that led to the drug's temporary withdrawal beginning in 2005. The infection re-emerged among Tysabri patients in mid-2008, and Biogen provides monthly updates about the number cases.

The overall global PML rate is about 0.90 per 1,000 patients, the company said, which falls within the 1-in-1,000 rate previously seen in clinical trials and implied on the drug's label.
As of June 30, 52,700 patients were using the drug around the world. In total, about 71,400 patients have used the drug since its launch.

Regulators have said that they watch the cases, but have concluded that the benefits of the medicine to MS patients outweigh the risks. Tysabri is usually used in MS patients who do not respond to earlier therapy or in patients with aggressive cases of the disease, and is distributed under a strict risk-management plan that monitors patients for PML.

The risk of the infection increases with the number of monthly infusions that a patient receives. The incidence rate appears to drop after 30 months of use, but Biogen views the drop as inconclusive, because there aren't enough patients to have confidence in that finding.