Onyx (ONXX), which develops cancer drugs and treatments, said Monday that Amgen's takeover bid of $120 a share undervalued the company. Onyx stock closed Friday at $86.82.
"We are actively exploring the potential to combine Onyx with another company as an option to create additional value for Onyx shareholders," said Onyx CEO N. Anthony Coles.
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Shares of Onyx rose more than 6% last week on news of Amgen's offer. Overall, the stock has increased nearly 36% since last year.
Amgen (AMGN) is a major player in biopharmaceuticals, and manufactures drugs for many conditions, including ones that counteract the effects of cancer treatments.