Biogen Idec Inc. is planning the first clinical trial to use controversial multiple sclerosis treatment Tysabri whose generic name is natalizumab and is sold with Elan Corp. The long-term trial, will measure the effectiveness of Tysabri in patients with active MS that have switched from either Teva Pharmaceutical Industries Ltd.'s Copaxone or Rebif, sold by Pfizer Inc. and Germany's Merck KGaA.

Tysabri was temporarily withdrawn from the market in 2005 because of its link to PML. The PML rate for patients given at least one infusion is 0.5 cases per 1,000 patients. Tysabri clinical benefits overweigh the risks as it is considered a highly effective therapy for MS. Measures are been taken to direct phyicians to use Tysabri when patients aren't responding to their current therapy. The company is signing up sites for the trial to enroll the first of an estimated 1,800 patients. If successful, Biogen can update Tysabri's label and allow it to market.

Biogen's shares were down 9 cents at $53.01 in afternoon Nasdaq trading, while the NYSE Arc Biotech Index fell 1.2 percent. Elan shares were flat at $7.14 on the New York Stock Exchange. The FDA had said in September the PML rate appeared to increase with the number of infusions.