The long partnership between Elan Corp and Biogen Idec is turning sour after the the former announced it is suing the US firm over its bid to prevent the Irish drugmaker completing its proposed $1 billion link-up with Johnson & Johnson.

The problem stems from a deal signed last month which saw Elan sell an 18.4% stake in the firm and the rights to its portfolio of Alzheimer’s disease investigational drugs to J&J in return for $1 billion. That agreement also gives the latter an option to acquire the 50% stake held in the multiple sclerosis drug Tysabri (natalizumab) by Biogen if control of that company changes hands.

However last week Elan received a letter from Biogen which claimed that the Dublin-headquartered company was in “material breach” of their collaboration concerning Elan’s ability to obtain financing from a J&J affiliate of Johnson & Johnson to potentially purchase its Tysabri rights, if Biogen undergoes a change of control. In response, Elan has filed suit in a New York court seeking “declaratory and injunctive relief” and permanently preventing Biogen from terminating their Tysabri agreement.

Elan is arguing that is in compliance in all respects with the pact with Biogen which has not actually seen the details of the J&J transaction even though it was offered the opportunity to do so. The firms have been partners on Tysabri since 2000 and “nothing about that relationship has changed”, Elan said.

The firm added that “this is the same agreement we have been operating under for the last nine years. It is unfortunate that, because of Biogen’s actions, Elan was left with no alternative but to seek court intervention to protect its interest.” It concluded by noting that Elan is “committed to aggressively and proactively protecting its science, property and assets”.

Tysabri has become a big earner and the drug had second-quarter sales of $253.8 million; Elan’s share was $173.7 million. Biogen was reported to be reviewing its legal options