Irish-based Horizon Pharma said it would buy US drugmaker Hyperion Therapeutics for about $1.1bn to add to its line-up of drugs that treat rare diseases.

Horizon will pay $46 per share, representing a premium of 7.6% to Hyperion's Friday closing price.

Hyperion's shares, which have nearly doubled in the last three months, rose 6% in premarket trading today. Horizon's shares rose about 7.4%.

Hyperion has two drugs on the market - Ravicti and Buphenyl - that are used to treat urea cycle disorders, a collection of inherited metabolic diseases affecting about 2,100 people in the United States.

The transaction, expected to close in the second quarter, will immediately add to Horizon's adjusted earnings.

The contribution from Ravicti and Buphenyl is expected to add $100m to Horizon's adjusted EBITDA in 2016 and result in savings of more than $50m, Chief Executive Timothy Walbert said in a statement.

Jefferies LLC, Citigroup Global Markets and Cowen and Co are financial advisers to Horizon, while its legal advisers are Cooley LLP and McCann FitzGerald.

Centerview Partners LLC and Houlihan Lokey Capital Inc were financial advisers to Hyperion, and Shearman & Sterling LLP acted as its legal adviser.