Horizon Pharma plc (Nasdaq: HZNP) agreed to buy Raptor Pharmaceutical Corp. (Nasdaq: RPTP) for about $800 million to bolster its rare-disease treatment business and expand internationally.

Pharmaceuticals have been picking up in the middle-market as companies look to replenish their pipelines. Bristol-Myers Squibb Co. (NYSE: BMY) bought Cormorant Pharmaceuticals; Pfizer Inc. (NYSE: PFE) agreed to buy Medivation Inc. (Nasdaq: MDVN); and Sunovion Pharmaceuticals Inc. is acquiring Cynapsus Therapeutics Inc. (Nasdaq: CYNA) (TSX: CTH).

Horizon will pay $9 a share in cash, about 21 percent more than Novato, California –based Raptor’s closing price on Sept. 9 of $7.45, the companies said in a statement. The transaction is expected to close in the fourth quarter of 2016, and should add to earnings in 2017, they said.

As part of the deal, Horizon gains Procysbi, a treatment for nephropathic cystinosis, a rare metabolic disorder, as well as Quinsair, which is given to cystic fibrosis patients. Raptor’s previously disclosed total net sales guidance for full-year 2016 is $125 million to $135 million, which includes both Procysbi and Quinsair. Horizon is domiciled in Dublin, Ireland, and has corporate offices in Lake Forest, Illinois.

Horizon will fund the purchase through $675 million of debt along with cash on hand, according to the statement. MTS Health Partners LP and Citigroup Inc. (NYSE: C) were Horizon Pharma’s main advisers on the transaction. Horizon Pharma’s legal advisers were Cooley LLP and McCann FitzGerald. Centerview Partners LLC and Leerink Partners LLC advised Raptor, while Latham & Watkins LLP provided legal advice.

--Additional reporting by Mergers & Acquisitions' Demitri Diakantonis