Amgen Inc. has reached an agreement to purchase South San Francisco-based KAI Pharmaceuticals—a clinical stage biopharmaceutical company—for $315 million, the companies said Tuesday.

With the deal, Thousand Oaks-based Amgen will acquire the worldwide rights, except in Japan, to KAI’s promising drug KAI-4169, which is being studied to treat secondary hyperparathyroidism in individuals with chronic kidney disease who are undergoing dialysis. In a joint news release, Amgen said it has provided a loan to KAI for phase 3 clinical development planning.

“We are excited about acquiring KAI, as well as the opportunity to potentially deliver a novel therapy for chronic kidney disease patients on dialysis suffering from secondary hyperparathyroidism,” Sean E. Harper, executive vice president of research and development at Amgen, said in a statement.

The deal has been approved by both companies’ boards of directors and KAI’s stockholders, the news release said. The agreement is still subject to regulatory approvals.

As part of the deal, KAI will become a wholly-owned subsidiary of Amgen.

Amgen’s stock closed at $66.19 down 1.24 percent from a day earlier.

Andrew Khouri