Amgen Inc. said Friday that it plans to buy Avidia, a small privately held biotech drug developer in Northern California, in a $290 million deal.

In the transaction, Thousand Oaks-based Amgen will gain Mountain View-based Avidia’s lead product candidate, an experimental drup that’s in early clinical trials to treat inflammation and autoimmune diseases. Amgen Ventures, its venture capital investment unit, already owns a 4 percent stake in the company.

“The Avimer technology is among the most attractive protein-based technologies currently under development,” said Roger Perlmutter, Amgen’s executive vice president for research and development. “Avimers may have several advantages as therapeutic products in terms of biological activity, tissue distribution, reduced immunogenicity and improved manufacturing efficiencies.”

Both companies’ boards have approved the sale, which Amgen said includes up to an additional $90 million in potential milestone payments. The transaction, which will make Avidia a wholly owned subsidiary of Amgen, is expected to close in the fourth quarter.