Elan Corporation, plc and Biogen Idec expect the safety evaluation of Tysabri (natalizumab) in Crohn's disease and rheumatoid arthritis will be completed soon, and expected to be submitted a supplemental Biologics License Application for Tysabri in multiple sclerosis (MS) to the US Food and Drug Administration.

On August 9, 2005, the companies had completed the safety evaluation of Tysabri in MS, which resulted in no new confirmed cases of progressive multifocal leukoencephalopathy (PML). The companies have previously reported three confirmed cases of PML, two of which were fatal.

On February 28, 2005, Biogen Idec and Elan voluntarily suspended Tysabri from the US market and all ongoing clinical trials based on reports of PML, a rare and potentially fatal, demyelinating disease of the central nervous system.