Biogen says Tysabri goals "difficult" to achieve

BOSTON (Reuters) - Biogen Idec Inc backed off its previous forecast that 100,000 patients would be on its multiple sclerosis drug Tysabri by the end of 2010 and reported a new case of the brain disease PML.

Biogen Chief Executive Jim Mullen said on a conference call with investors that it will be "difficult" to achieve its original 100,000 projection. The company also announced a new case of the potentially deadly brain infection progressive multifocal leukoencephalopathy, or PML.

Concerns over PML have curbed growth in sales of Tysabri.

(Reporting by Toni Clarke; Editing by Derek Caney)