* Biogen reports 11th case of Tysabri brain infection
* Tysabri seen as key growth driver for Biogen
* Company says won't report new cases on website
BOSTON, July 24 (Reuters) - An 11th patient taking Biogen Idec Inc'smultiple sclerosis drug Tysabri has developed a potentially deadly brain infection since July 2006, when it was reintroduced to the market.
The Cambridge, Massachusetts-based biotech company released the news on its website late on Friday.
Tysabri, which Biogen sells in conjunction with Irish drugmaker Elan Corp Plc, is considered critical to the future growth of both companies.
The drug was temporarily withdrawn from the market in 2005 after it was linked with a brain infection known as progressive multifocal leukoencephalopathy, or PML. It was brought back in 2006 with stricter safety warnings.
Biogen has recently adopted a more aggressively upbeat tone in marketing the drug, insisting physicians are becoming more comfortable with risk of PML.
That approach appears to be working. Sales of Tysabri accelerated in the second quarter, rising to $254 million from $200 million a year ago. The company said the drug is on track to generate some $1 billion in sales this year.
Even so, some analysts believe doctors may take patients off the drug for certain periods of time.
At the end of June, about 43,300 patients were on Tysabri, compared with 40,000 at the end of March.
Biogen and Elan originally predicted 100,000 patients would be taking the drug by the end of 2010. The companies still believe they will reach that figure, but not within that time frame.
This is the last case of PML that the company plans to announce on its website. Biogen said on its earnings conference call earlier this month that it would in future communicate new cases by word of mouth to physicians and patient advocacy groups. (Reporting by Toni Clarke, editing by Matthew Lewis)
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