Rate of PML With Tysabri Inches Upward

As of February 2nd, the total number of PML cases associated with Tysabri (natalizumab) use equals 95 (since the relaunch of the drug in 2007). Risk data on the monoclonal antibody, which is a highly effective disease-modifying agent for relapsing-remitting MS, comes by way of comarketer Biogen Idec, which recently disclosed to news sources that 10 patients developed the life-threating brain infection last month. Four of these patients may have died, according to Reuters, bringing the total number of Tysabri-related cases of fatal PML to 20.

The overall rate of PML with Tysabri use is now calculated at 1.6 per 1000 patients. Unfortunately there is no reported information on the duration of treatment in the newly affected patients with PML. It is a vital piece of missing data, given that the risk of PML with Tysabri use increases substantially after 18 months of treatment: from 0.1 per 1000 after 12 monthly infusions to 1.64 per 1000 after 25 monthly infusions.

According to Biogen Idec's annual report for 2010, approximately 56,600 patients were receiving Tysabri worldwide as of December 31st. Last year's product revenues for Biogen Idec, which shares Tysabri income with developer Elan, were $900 million—a 16% increase over 2009 revenues.

PML = progressive multifocal leukoencephalopathy.