Shares in Amylin Pharmaceuticals have soared on reports that the company has recently turned down a $3.50 billion unsolicited takeover offer from Bristol-Myers Squibb.

The news agency Bloomberg, quoting two unnamed people with knowledge of the matter, says that B-MS offered $22 a share in a letter to Amylin last month. The latter's board apparently turned down the offer as it is more interested in finding a marketing partner outside the USA for the diabetes drug Byetta (exenatide) and its once-weekly successor Bydureon; it is in the process of ending its long-time collaboration with Eli Lilly.

The news sent Amylin's stock up 54.5% to close at $23.77 and sent analysts scrambling to make sense of it all. Joshua Schimmer at Leerink Swann issued an investor note saying "we generally believe these stories are 'leaks' that serve a purpose. In this case, the purpose of the announcement, we believe, is to tie Amylin's hands and prevent them from doing an ex-US deal instead of a sale".

He added that "at this valuation and with this information in the public domain, management will have a hard time justifying an extreme level of downside to shares, which would almost certainly come from an ex-US partnership". Mr Schimmer also claimed that B-MS "may not be the strongest bidder given its weaker presence in the diabetes space in Europe and emerging markets".

AstraZeneca mentioned as possible bidder

He believes, as do many other analysts, that AstraZeneca, which co-markets the diabetes drug Onglyza (saxagliptin) with B-MS may be tempted to make Amylin an offer. Bernstein Research issued a note saying it continues to believe the Anglo-Swedish drugmaker "is actively pursuing mid-sized target companies in the biopharmaceutical space" to help it "fill in the gaps left open by a lagging pipeline and ongoing future patent expiries".