Cadwalader, Wickersham & Taft and Ropes & Gray are advising on an agreement between biotechnology giants Biogen Idec and Elan that will see the former spend at least $3.25 billion to acquire full rights to the drug Tysabri, which is used to treat multiple sclerosis and Crohn’s disease.

Tysabri, which generated $1.6 billion in sales in 2012, has been Biogen’s best-selling drug, according to Bloomberg. The company has been splitting profits from Tysabri equally with Elan, an Irish drugmaker that has now essentially become an investment vehicle after selling its sole remaining asset. While the market reaction has been harsh to Elan’s decision to sell its rights to Tysabri for $3.25 billion in cash plus future royalties, Reuters reports that the Dublin–based company will use the proceeds to acquire new drugs.

Cadwalader corporate partners Christopher Cox and William Mills III and finance partners Ira Schacter and Braden McCurrach are leading a team from the firm advising Elan on the deal. (Irish firm A&L Goodbody, which opened an office in Silicon Valley a little over a year ago, is also advising Elan.)

Cox joined Cadwalader in January 2012 from Cahill Gordon & Reindel, where he had previously handled major transactions for Elan. Among those deals were the $960 million sale of Elan’s drug delivery business to U.S. biotech Alkermes in 2011 and the $1 billion sale of an 18.4 percent stake in Elan itself to pharmaceutical giant Johnson & Johnson in 2009.

The Johnson & Johnson deal led to litigation between Elan and Biogen over a breached agreement for the development of Tysabri that eventually led Johnson & Johnson to pay a reduced price of $885 million for the same 18.4 percent stake in Elan, according to our previous reports. The following year, activist shareholders upset with Elan’s direction lambasted the company over the independence of an internal report released in September 2010 by McKenna Long & Aldridge, prompting at least two members of Elan’s board of directors to resign.

Fabiana Lacerca-Allen became Elan’s chief compliance officer in June 2010. Elan’s former general counsel, John Moriarty Jr., left the company in December and is now the top in-house attorney at Cheshire, Connecticut–based Alexion Pharmaceuticals.

Suburban Boston-based Biogen, meanwhile, is being advised on its agreement with Elan by a team of lawyers from Ropes & Gray led by life sciences partner Michael Sexton, antitrust partner Michael McFalls and counsel Deidre Johnson, tax partner David Saltzman, and associates Max Pakaluk and Lindsay Tunney. Biogen, the product of a $6.8 billion merger in 2003 with San Diego–based Idec, has been a longtime Ropes client.

Susan Alexander, a former partner at Hinckley, Allen & Snyder and now-defunct Boston firm Fine & Ambrogne, has served as Biogen’s general counsel and corporate secretary since 2006. Lynn Schenk, an attorney, consultant, and former congressman, is an independent member of Biogen’s board.