Tysabri label updated to reflect link between PML and number of infusions

In documents posted to its website, the FDA warned that patients taking Biogen Idec and Elan's Tysabri (natalizumab) increase their risk of developing progressive multifocal leukoencephalopathy (PML) as the number of infusions of the drug increases. The new safety data, which is based on reports of 31 confirmed cases of PML received as of January 21, will now be included in the drug's label.

The FDA noted that since Tysabri's reintroduction to the market in 2006, "there have been no reports [of PML] in patients treated for less than 12 months," while in patients treated with 24 to 36 infusions, the rate of infections worldwide and in the US is similar to the rate of 1 case per 1000 patients seen during clinical trials. Outside the US, the rate rises to about 2 cases per 1000 patients, the agency said, adding that "the reasons for this difference are unknown.

Commenting on the matter, Biogen Idec spokeswoman Naomi Aoki remarked that the drugmaker thinks "that heavy use of prior immunosuppressants is a risk factor… That's one thing we've seen as a difference that seems like it might be contributing, but we're not really sure." The US regulator stated that it continues to believe that the benefits of the medicine outweigh its risks.

In November, the drug's label was updated in the US to include information on an increased risk of PML with longer usage.

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