Biogen Idec has agreed to purchase Elan’s interest in Tysabri and will gain full commercial rights to the drug. The companies’ previous collaboration agreement, under which Tysabri profits were split 50/50, will be terminated.   Biogen will make a payment of $3.25 billion upon closing and will make future contingent payments equal to 12% of global sales of Tysabri for the first 12 months, and will continue to make payments of 18% on the first $2.0 billion in annual global sales and 25% on annual...