Alexion Pharmaceuticals will acquire Synageva BioPharma Corporation for $8.4 billion in cash and stock to develop in the field of rare-disease treatments. The company said in a statement that Alexion Pharmaceuticals will offer $115 in cash and 0.6581 Alexion shares for each share of Synageva.

It also said that it indicates a share value of $230 based on the nine day volume-weighted average closing price of Alexion stock through May 5, 2015. The boards of both the companies have supported the transaction.

A medicine called Soliris was developed by Alexion, which led to a sale of $2 billion last year. The medicine was developed for patients with two rare but life-threatening disorders. Synageva also looks forward to developing such drugs together with a medicine called Kanuma.

David Hallal, Chief Executive Officer of Alexion, said in the statement, "Synageva is an ideal strategic and operational fit for Alexion that aligns with what we know well and do well -- providing life-transforming therapies to an increasing number of patients with devastating and rare diseases".

Synageva shares rose by over 125% in pre-market trade on Wednesday. Alexion's shares declined by almost 3 per cent.