Onyx Pharmaceuticals Inc. on Sunday confirmed that it received an unsolicited takeover bid from Amgen Inc. for $120 per share. But the drug developer says it rejected the offer, because it “significantly undervalued” the company.

Onyx also said that other companies have expressed interest in a buyout, and that its board authorized its financial adviser, Centerview Partners, to contact potential suitors.

Onyx reported net loss of $187.8 million, or $2.88 per diluted share, for 2012, on revenue of $362.2 million. That reversed a 2011 profit of $76.1 million, or $1.19 per share, on $447.2 million.

In the 2013 first quarter, the company posted a net loss of $33.7 million, or 47 cents per share, on revenue of $145.5 million. That was a smaller loss than the $56.2 million, or 88 cents per share, posted for the 2012 first quarter, as revenue more than doubled from $72 million.

Onyx’s tablet medication Nexavar, which treats liver and kidney cancer, is approved in more than 100 countries. It brought in $70.3 million in revenue in the first quarter, a slight drop from a year ago.

It also received approval last year from the FDA for an injection medication called Kyprolis, which is for treatment of multiple myeloma, a cancer of plasma cells which accumulate in bone marrow. In the first quarter, sales of the drug reached $64 million.

Stivarga, a pill to treat colorectal cancer pill developed by Onyx and Bayer HealthCare, also got the OK from the FDA last year. Onyx received royalty revenue of $9.2 million in the first quarter for Stivarga.

Onyx CEO N. Anthony Coles said in a statement that the company is “actively exploring the potential to combine Onyx with another company as an option to create additional value for Onyx shareholders.”

Reports of the offer late Friday caused Onyx shares to jump 26 percent in after-hours trading to $109.01. The stock closed the regular session at $86.82, up 15 percent for the first half of the year, but down from its peak of $101.57, reached in April.