Elan Corp and partner Biogen Idec have made changes to the US label on Tysabri to reflect the increased risk of a rare and potentially fatal brain disease when the multiple sclerosis blockbuster is taken over a longer period of time.

The companies said that effective immediately, they are updating the label on Tysabri (natalizumab) following consultation with the US Food and Drug Administration. Specifically it notes that the “risk of developing progressive multifocal leukoencephalopathy increases with longer treatment duration, and for patients treated for 24 to 36 months is generally similar to the rates seen in clinical trials. There is limited experience beyond three years of treatment”.

Ian Hunter, an analyst at Irish broker Goodbody said that from a regulatory point of view, “this is a fairly benign label update for the drug as it does not change the risk rate (one in 1,000), apply a quantitative time-based weighting to the risk of PML or require/suggest that patients should take a drug holiday after a certain period”.

The changes comes just a couple of weeks after the European Medicines Agency’s Committee for Medicinal Products for Human Use announced that it has begun a review of the benefits and risks of Tysabri in view of reports of 23 cases of progressive multifocal leukoencephalopathy worldwide since the drug was reintroduced in July 2006. Mr Hunter added that “it has still to be seen, however, if the sudden increase in PML cases will drive the EMEA or individual authorities within the European Union to make changes to their prescribing and use requirements for the drug”.

He went on to say that the commercial impact of the label change will only be quantified in the next results releases from Elan and Biogen, which are not due until February 2010. As when the first cases of PML appeared last year, “we believe the current ramp up in cases could see Tysabri progress slowed as patients/physicians new to the drug take a more cautious approach to initiating treatment,” Mr Hunter concluded.