Irish drug maker Shire (SHPG) - Get Reportis acquiring the biotech firmDyax (DYAX) for $5.9 billion to expand its portfolio of drugs treating a rare swelling disorder known as hereditary angioedema, the companies said Monday.

The deal values Dyax at $37.30 per share, or a 35% premium over the $27.53 closing price on Friday.

Dyax shareholders will also receive a nontradable contingent value right that will pay another $4 per share if DX-2930, a late-stage drug candidate for the treatment of hereditary angioedema, secures regulatory approval. The total value of the DX-2930 CVR is $646 million.

Shire grew into the business of selling drugs for hereditary angioedema (HAE) through the November 2013 acquisition ofViropharma for $4.2 billion. That deal brought Shire the HAE drug Cinryze.

Monday's acquisition bolsters Shire's HAE portfolio with Dyax's approved HAE drug Kalbitor and the still-experimental DX-2930. A phase III study of DX-2930 is expected to start before the end of the year. If approved, Shire believes DX-2930 could deliver up to $2 billion in peak annual sales.

"This highly complementary transaction aligns with and accelerates our strategy to build a global leading biotechnology company focused on rare diseases and specialty conditions," said Shire CEO Flemming Ornskov in a statement. "It adds to our portfolio of best-in-class therapies addressing unmet needs in our core therapeutic areas, expanding and extending our leadership position in HAE. We have closely followed DX-2930's progress in the evolving HAE landscape for some time, and we admire the work of the Dyax team in moving this next-generation therapy forward."

HAE is a rare, genetic inflammatory disease which causes episodes of swelling in the face, limbs and gastrointestinal tract. The disease can be deadly if the swelling closes off a patient's airway. Some drugs targeting HAE work by reducing the swelling after a patient has an attack, but other drugs are designed to prevent HAE attacks from happening.

The proposed deal is expected to be slightly dilutive to Shire earnings in 2016 and 2017, but turn accretive in 2018, assuming approval of DX-2930, the company said.

Shire shares

closed Friday at $227.05.

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