Aside from his battles with Amylin, billionaire investor Carl Icahn has been suggesting that Biogen Idec, with whom he lost a proxy fight last year, should be split in two.

Mr Icahn, who is trying again to get four directors on the Biogen board at the firm’s annual general meeting on June 3, has filed a 52-page presentation with the US Securities and Exchange Commission. In it, he says that a split, with one firm focusing on neurology and the other on cancer.

One of the slides proclaims that "separating Biogen assets will enhance shareholder value as management focus should improve and disparate assets appeal to different buyers". Mr Icahn argues that a neurology-focused company would have had $2.93 billion in sales last year, driven by the multiple sclerosis blockbusters Avonex (interferon beta-1a) and Tysabri (natalizumab). It would also have a decent pipeline such as BG-12 (dimethyl fumarate), an oral compound for patients with relapsing-remitting MS, lixivaptan for hyponatremia and the heart failure drug Adentri.

The oncology and autoimmune-focused company would have had sales of $1.17 billion, thanks to the rheumatoid arthritis and cancer drug Rituxan (rituximab) and the psoriasis treatment Fumaderm (fumaric acid). Its pipeline would include galixibam, lumiliximab and ocrelizumab.

Mr Icahn compares the situation at Biogen to ImClone Systems which he sold to Eli Lilly for $6.5 billion. He adds that the nominees he had on the board there helped "ImClone shares dramatically outperform” and also “recharged” partner relations and optimised cost structure.

Biogen has urged shareholders to reject Mr Icahn’s slate of nominees for the board. Speaking before the SEC filing, the company said that “Carl Icahn has offered no ideas to enhance shareholder value in the ten months since last year's annual meeting”.

The AGM at Biogen will take place after a meeting at Amylin where Mr Icahn and fellow major shareholder Eastbourne Capital are looking to take control of the board. Amylin repeated its request to shareholders to support the present board but Mr Icahn’s campaign received a boost with the news that a former chief executive of the company, Howard Greene, has backed his and Eastbourne’s nominees.