Amgen Inc. plans to lay off 300 workers at its Onyx subsidiary in South San Francisco, and transfer others to its Thousand Oaks headquarters.

Amgen bought Onyx Pharmaceuticals Inc., a South San Francisco company, in 2013 for $10.4 billion – largely to acquire its Kyprolis treatment for multiple myeloma blood cancer.

The company has about 750 workers, with the layoffs occurring at the end of this year. Amgen will close the Onyx site.

In a memo to employees on Monday, Chief Executive Robert Bradway said that Amgen’s cancer team would be consolidated, with some Onyx employees moving to the company’s Thousand Oaks headquarters and others to another Amgen research facility in South San Francisco.

Amgen “will consolidate our commercial and development oncology presence in Thousand Oaks,” the memo said.

As part of the restructuring, Onyx U.S. field staff, primarily its sales and medical personnel, will become part of Amgen’s field organization. But company spokeswoman Kristen Davis said it’s unclear where individual employees will end up.

“(It) is still being decided,” Davis said.

The new follows the announcement of clinical study results earlier this month that found patients with relapsed multiple myeloma who took Kyprolis and the steroid dexamethasone lived twice as long without their disease worsening compared to patients who took Velcade, the established treatment for the cancer from Johnson & Johnson in New Brunswick, N.J.

Kyprolis had sales of about $330 million last year, and analysts have predicted sales of $570 million next year, but that number could grow if more doctors recommend the drug. Velcade had sales last year of about $3 billion.

The news, reported by the Business Journal late yesterday, sent shares tumbling $1.43 by Tuesday's closing bell and in after-hours trading. They were down another 88 cents on Wednesday to close at $152.70 on the Nasdaq.