Biogen reports 10th case of PML brain infection

BOSTON, June 29 (Reuters) - A 10th patient taking Biogen Idec Inc's (BIIB.O) multiple sclerosis drug Tysabri has developed a potentially deadly brain infection since it was reintroduced to the market in July 2006.

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(ELN.I), 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The drug was temporarily withdrawn from the market in 2005 after it was linked with a brain infection known as progressive multifocal leukoencephalopathy, or PML.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.

Sales of the drug have failed to live up to expectations. It generated $227 million in the first quarter, less than the $246 million analysts had expected.

Biogen has recently begun taking a more aggressively upbeat tone in marketing the drug, insisting physicians are becoming more comfortable with risk of PML. Even so, some analysts believe doctors may take patients off the drug for certain periods of time.

"We view increased adoption of drug holidays as a strong possibility," said Eun Yang, an analyst at Jefferies & Co. He said he is not changing his 2009 sales estimate for Tysabri of $1 billion. (Reporting by Toni Clarke; Editing by Steve Orlofsky)