Alexion

Alexion Pharmaceuticals (ALXN - Get Report)  investors were jilted Monday as speculation about a potential acquisition by Amgen (AMGN - Get Report) came to naught, with the biotech giant instead disclosing a deal involving another company.

Alexion's stock price fell as much as 5% on news that Amgen agreed to shell out $13.4 billion cash for Celgene's (CELG - Get Report) Otezla psoriasis treatment.

Shares of Alexion recently were trading down 2% at $113.

The Boston maker of drugs for rare diseases had long been the subject of acquisition speculation involving Amgen.

But a Thursday report that the Thousand Oaks, Calif., biotech might be preparing to make an offer sent ALXN's shares up sharply.

Mizuho analyst Salim Syed had published a note calling feasible the report by Spain's Intereconomia, which, citing unnamed sources, declared Amgen to be "in the final stretch to buy Alexion" for $200 a share. 

Shares of Alexion shot up as high as $125 on Thursday, then pulled back.

Amgen sees Otezla helping bolster its lineup of treatments, which currently includes Enbrel, another psoriasis drug.

Shares of Celgene, the Summit, N.J., biopharma company, advanced 3.1% to $96.92 on news of the deal with Amgen. Shares of Amgen rose 3% to $204.96.