Moving to cement its position as the market leader for multiple sclerosis therapies, Biogen Idec Inc. said Wednesday that it has agreed to pay $3.2 billion and future contingent payments to acquire full rights to the blockbuster MS drug Tysabri.

The biotechnology company, based in Weston, said it would buy out the 50 percent share of Tysabri owned by its Irish partner, Elan Corporation PLC, which developed the treatment and sold half of the drug program to Biogen Idec in 2000.