Biogen Idec expects the FDA to rule by March’s end on whether to approve new labeling for its multiple sclerosis drug Tysabri warning that its use has been linked to a rare but deadly brain disease called PML, executives from the firm said in a presentation to investors yesterday.Biogen CEO Jim Mullen said the new label would dictate that Tysabri should only be used “with great caution,” by patients with compromised immune systems, according to a Dow Jones Newswires report.The three cases of PML, two of which were fatal, led Biogen Idec and Elan to withdraw Tysabri from the U.S. market last February and suspend clinical trials of the drug. Scientists still don’t fully understand how Tysabri may have enhanced those patients’ risk for contracting the disease.Mullen reiterated that the company expects to be able to have Tysabri back on the market by mid-2006, according to the published report.Mullen said the firm expects to have the FDA hold a special advisory panel meeting on the matter, with a major topic likely being the sort of language that should be included on the label about diagnosing and treating PML in its earliest stages. However, the FDA has yet to contact Mullen about such a meeting, he explained.