Onyx Pharmaceuticals has rejected a takeover bid by biotechnology company Amgen.

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.

Related: Brutal June clouds stocks outlook for 2nd half

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.