New case of PML will not bother Tysabri, says Zacks

USA-based Biogen Idec says there has been a sixth case of a rare brain disorder associated with the multiple sclerosis treatment Tysabri  (natalizumab; Marketletters passim).

Zacks Investment Research notes that: the latest patient to develop the  disease did so after 31 months; all patients are taking Tysabri as a  monotherapy, with the minimum therapy before noted disease of 12 months;  the average length of time for the sixth patient's dosing is 19 months;  and five of the six reported cases were located outside the USA.

Zacks continues to expect more cases of PML over the next several  quarters, but does not believe this will be a significant deterrent to  new patient growth. Worldwide sales of Tysabri were $227.0 million in  the first quarter and Zacks expects revenue to grow to $1.05 billion  worldwide in 2009.