FDA: Tysabri risks greatest during year 3

The FDA has updated its warning on Biogen Idec's multiple sclerosis drug Tysabri. According to the agency, the risk of a potentially fatal brain infection appears to be greatest during the third year of treatment.

Since Tysabri was reintroduced in 2006, Biogen and the FDA have been keeping close tabs on cases of progressive multifocal leukoencephalopathy, or PML. Biogen regularly updates case counts, and the FDA has been estimating the cumulative risk posed by the drug. This is the first time the agency has calculated PML risk according to specific time intervals. The newly sliced data is designed to help doctors "better assess risk based on duration of treatment," the agency said.

PML risk amounts to 0.3 cases per 1,000 during the first two years of treatment, the agency said. After three years, the rate was 0.9 per 1,000. The highest risk was found in patients treated with Tysabri for 25 to 36 months; PML occurred in an estimated 1.5 per 1,000 patients during that time interval. The agency said it believes the benefits of using Tysabri outweigh its risks.

Biogen spokeswoman Kate Weiss said the FDA's figures reinforced the company's view that PML risk increased after two years of treatment, according to Reuters. The total number of cases worldwide came to 111 as of April 1, she said.

- see the news from Reuters