Elan has announced a restructuring of its MS drug Tysabri collaboration with Biogen Idec.

The deal will see Elan sell its interest in the drug for $3.25 billion to Biogen to give the US firm full ownership and control of the drug.

Elan has co-marketed the drug with the larger US company for 12 years. 

It said it would receive a royalty of 12% of Tysabri global net sales for the first 12 months after the restructuring closes.

A tiered royalty structure will kick in after that, it said, with Elan receiving 18% on up to $2 billion of global net sales and 25% on any sales over that amount.

Sales of Tysabri, which provides Elan with almost all of its revenue, rose 8% to $1.6 billion last year. The number of patients using the drug rose by 12%.

A filing last month for approval to sell the drug as a first-line treatment for could boost sales further.

Elan chief executive Kelly Martin said the Tysabri move provides for a meaningful de-risking of the business.

"The restructuring of this business collaboration provides Elan with significant strategic flexibility. The risk of one asset and a single collaborator was not ideal," Mr Martin said in a statement.

"We are enthusiastic about the market opportunities around the globe and remain flexible and creative about the manner in which we would participate in those opportunities,'' he added.

See how Elan shares are doing in Dublin trade.

The company today reported total revenues of $1.202 billion for the year to the end of December, up 13% on the previous year. It said that adjusted EBITDA rose by 31% to $220m from $168.6m mainly due to the continued growth of Tsabri.

It said it expects to complete the Tysabri deal in the first half of this year. It said it expects Tysabri in-market sales to rise by about 15% in 2013 over the $1.6 billion seen in 2012.