The acquisition also creates the most robust rare disease pipeline in the biotech industry across a range of therapeutic modalities.
The deal was completed through a merger of Synageva with and into a direct, wholly owned subsidiary of Alexion.
Alexion chief executive officer David Hallal said: "As we complete this acquisition, the combination of our two companies provides us the exciting opportunity to build upon our collective strengths and talents to firmly establish Alexion as the global leader in serving patients with devastating and rare diseases.
"With Soliris, and the anticipated approvals of Strensiq and Kanuma, Alexion is poised to have three innovative products serving patients with four severe diseases in 2015 while also advancing the deepest and broadest pipeline in our history."
Currently, Kanuma is under priority review with the US Food and Drug Administration (FDA) and has been granted accelerated assessment of its marketing authorization application (MAA) by the European Medicines Agency (EMA).
The company said that regulatory decisions in the US and Europe are expected in the second half of this year.
Additionally, a new drug application (NDA) for Kanuma was submitted to Japan’s Ministry of Health, Labour and Welfare (MHLW).
Currently, Alexion has eight product candidates in clinical trials for 11 indications, including SBC-103, an investigational enzyme replacement therapy in an ongoing Phase I/II trial for patients with mucopolysaccharidosis IIIB (MPS IIIB), a genetic and progressive rare metabolic disease.