Shares of Onyx Pharmaceuticals (ONXX) climbed more than 51% on Monday after the drug giant turned down a buyout offer from Amgen (AMGN) and said late Sunday it was actively pursuing bids from other potential suitors.

Onyx said Friday that Amgen’s unsolicited offer to buy all of its outstanding shares for $120 each in cash “significantly undervalued” Onyx and its prospects and was “not in the best interest” of shareholders.

Two days later, Onyx said it was “actively exploring” deals with other companies and confirmed that it has received interest from third parties.

“Onyx has tremendous momentum and, with the expansion of our pipeline and two successful product launches, the company and our talented employees have created significant value for Onyx shareholders,” Onyx CEO Anthony Coles said in a statement.

BMO raised its price target on Onyx to $144 from $132 on Monday but maintained its “outperform” rating. Its shares in recent trade were up 51% to an all-time high of $131.52.

The South San Francisco-based company, which hired Centerview Partners as a financial adviser and Goodwin Procter as a legal advisor, said it would not comment further regarding the Amgen proposal or the hunt for a suitor.

Thousand Oaks, Calif.-based Amgen did not immediately respond to a request for comment. Its shares recently increased 2% to $100.57. 

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