THOUSAND OAKS and SOUTH SAN FRANCISCO, Calif. — Amgen plans to acquire oncology specialist Onyx Pharmaceuticals Inc. in a deal valued at $10.4 billion overall. Under the acquisition agreement, announced Sunday evening, Amgen will purchase all of the outstanding shares of Onyx at $125 per share in cash. Net of estimated Onyx cash, the purchase price is $9.7 billion. Amgen said it will effect the transaction through a tender offer and expects to close the deal at the start the fourth quarter, pending regulatory clearance and customary closing conditions. Onyx has a growing multiple myeloma franchise, with Kyprolis for Injection (carfilzomib) already approved in the United States. In addition, Onyx has three partnered oncology assets: Nexavar (sorafenib) tablets (an Onyx and Bayer HealthCare Pharmaceuticals Inc. compound), Stivarga (regorafenib) tablets (a Bayer compound), and palbociclib (a Pfizer Inc. compound).