South San Francisco -- Amgen, a Thousand Oaks-based biotechnology firm, said on Tuesday it will pay $315 million in cash to acquire South San Francisco-based KAI Pharmaceuticals, a pharmaceutical firm developing drugs for patients with chronic kidney disease. KAI will become a subsidiary of Amgen. As part of the deal, Amgen has provided a loan to enable KAI to continue developing its lead drug candidate, codenamed KAI-4169. "Amgen is ideally positioned to bring KAI-4169 to market and to patients, given the company's decades of experience in developing and delivering therapies for patients with chronic kidney disease," said Steve James, the president and CEO of KAI. KAI has raised $63 million in two rounds of funding from InterWest Partners, Skyline Ventures, Intersouth Partners, Aberdare Ventures, Investor Growth Capital, Kearny Venture Partners, Lumira Capital and Delphi Ventures.