Shares in Elan Corp are on the rise this morning after the Irish drugmaker announced a restructuring of its pact with Biogen Idec for their multiple sclerosis blockbuster Tysabri.

Biogen is getting full ownership and control of Tysabri (natalizumab), which the firms have collaborated on for 12 years. Under the terms of the restructuring, Elan will get $3.25 billion upfront and a 12% royalty on global net sales for the first year after the restructuring closes, sometime in the first half.

After that, a tiered royalty structure will see Elan receive 18% on up to $2 billion of global net sales and 25% on any revenues over that amount. Kelly Martin, chief executive of the Dublin-based group, said the restructuring provides Elan with "significant strategic flexibility", adding that “our motivation was to diversify and de-risk the company to move forward and for the patients to continue to benefit from the profound efficacy of Tysabri. The risk of one asset and a single collaborator was not ideal”.

Following the cash boost, Mr Martin said "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 that "we look forward to participating in Tysabri’s success", noting that "additional life cycle opportunities in secondary progressive MS and/or other non-MS indications may represent further advancements for…this unique therapeutic asset".

The announcement came as Elan posted fourth-quarter turnover of $319.8 million, up 18%. This was driven by a 14% increase in Tysabri global in-market net sales to $432.8 million; pretty much all of Elan's revenues come from the drug. Net income came in at $152.8 million, compared to a loss of  $134.7 million in the like, year-earlier period.

Last week, the company sold its remaining shareholding in Alkermes, bringing the total proceeds from the sale of its Elan’s Drug Technology (EDT) business in 2011 to $1.05 billion.