Biogen Idec ( BIIB) agreed to acquire privately held Syntonix Pharmaceuticals, a drug developer working on treatments for chronic diseases.

Under the deal, Biogen will pay Syntonix $40 million at the deal's closing, as well as milestone payments of up to $80 million if certain development goals are achieved. The acquisition, which has already been approved by both companies' boards of directors, is subject to customary closing conditions.

Through the deal, Biogen will gain Syntonix's experimental long-acting drugs designed to improve existing treatment regimens for lifelong diseases like hemophilia. This includes the company's lead product, FIX:Fc, which Biogen says has the potential to reduce the frequency of injections needed to treat patients with hemophilia B. This year, Syntonix expects to request approval from the Food and Drug Administration to test the drug in humans.

"With this transaction, we continue to deliver on our strategic initiative to enhance our pipeline and expand into additional specialized markets," said Biogen CEO James Mullen. "Syntonix brings promising science and technology to areas of unmet medical need such as hemophilia, and we look forward to building on their expertise with Biogen Idec's global development, manufacturing, and commercial capabilities."

Biogen's shares dipped 2 cents to $49.31 Thursday morning.