A San Francisco pharmaceutical company has rejected a nearly $10 billion takeover bid from Amgen Inc.

Officials from Onyx Pharmaceuticals Inc., which sells three cancer drugs, said in a statement Sunday that it felt the offer from the Thousand Oaks biopharmaceutical giant undervalued the company.

“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,” said Anthony Coles, chairman and chief executive of Onyx, in a statement.

Onyx indicated that Amgen’s offer was unsolicited, but that the company is now actively exploring other merger or takeover options as a result. It has an existing relationship with pharmaceutical firm Bayer SA with licensing agreements between the two involving two cancer drugs; and Pfizer Inc., which is developing a breast cancer drug that Onyx will receive royalties from.

Amgen’s offer of $120 a share was a 38 percent premium to Onyx’s Friday closing price.

Shares of Amgen lost $1.17, or more than 1 percent, to close at $97.49 on the Nasdaq.