Drugmaker Alexion Pharmaceuticals said it would buy Synageva BioPharma for $8.4 billion to boost its rare drug pipeline.
Alexion's cash-and-stock offer values Synageva at $225.92 per share—more than double of Synageva closing price of $95.87 on Tuesday.
Synageva's shares were trading at $216 before the bell on Wednesday, while Alexion was down 4 percent at $162.
Makers of drugs that treat rare diseases are attractive because they typically charge hefty premiums for their products.
The deal will give Alexion access to Kanuma—Synageva's treatment for a rare disease called Lysosomal Acid Lipase Deficiency, where build-up of fatty material in the blood and liver causes unexpected complications and some times early death.
Kanuma's marketing application is being reviewed in the United States and Europe.
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The deal is the latest in a string of M&As in the health care sector this year.
Recent deals involving rare disease companies include Teva Pharmaceutical's $3.5 billion purchase of Auspex Pharmaceuticals in March and Shire's acquisition of NPS Pharmaceuticals for $5.2 billion in January.
Alexion is offering $115 in cash and 0.6581 of its shares for each share of Synageva.
The deal is expected to add to Alexion's 2018 profit, while allowing $150 million in cost savings in 2017, the companies said in a statement.
Alexion has received committed financing of $3.5 billion from Bank of America/Merrill Lynch and JPMorgan Chase for the deal.
Lazard and JPMorgan were Alexion's financial advisers, while Goldman Sachs advised Synageva.
Wachtell Lipton Rosen & Katz is Alexion's legal counsel, while Sullivan & Cromwell and Ropes & Gray are Synageva's legal counsel.