As part of an effort to expand its portfolio of rare disease treatments, Ireland-based drug developer Horizon Pharma plans to acquire Raptor Pharmaceutical for about $800 million. Horizon said the deal will strengthen its business in the US and provide a platform to expand in Europe and other international markets.

The Raptor portfolio includes Procysbi, a treatment for a rare metabolic disorder, and Quinsair, which has been approved for managing a chronic pulmonary infection in adult patients with cystic fibrosis. Terms of the transaction, expected to close in this year’s fourth quarter, foresee Horizon paying $9 in cash for each share of the Novato, California-based company.

In recent years, drugmakers have been shifting more money to developing rare disease treatments as mass-market drugs lose patent protection.