Tysabri is approved as single therapy for the treatment of people with relapsing forms of multiple sclerosis (MS) to reduce the frequency of flare-ups and delay progression of physical disability.

After the FDA initially approved Tysabri, the drug's manufacturer took the drug off the U.S. market due to reports of a rare, but serious brain infection called PML (progressive multifocal leukoencephalopathy). But the drug returned to the market after the drugmaker included a risk-minimization program with mandatory patient registration and periodic follow-up to identify any possible cases of PML as soon as possible.

The risk for PML increases with the number of doses received. Data that's been collected by the FDA indicates the risk is 0.3 per 1,000 patients treated for up to 24 months, 1.5 per 1,000 patients treated for 25-26 months, and 0.9 per 1,000 patients treated for 37-48 months.

Risk also is higher among patients treated with immune-suppressing therapy before using Tysabri. Because of this risk, Tysabri is usually recommended for patients who cannot tolerate or have not responded to other MS treatments.

Tysabri is unique among MS drugs in that it works by binding to a protein found on the surface of immune cells (white blood cells), which are believed to play a role in the damaging effects of MS. Tysabri, a monoclonal antibody, curbs the ability of white blood cells to enter the brain and spinal cord.

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