Alexion Pharma (ALXN) will buy Achillion Pharma (ACHN) for the initial value of $930 million, or $6.30 per share in cash plus added contingent considerations for a total potential value of $8.30 per share.

Alexion will also be acquiring the cash currently on Achillion's balance sheet and as of Sept. 30, this was approximately $230 million.

"Alexion has demonstrated the transformative impact that inhibiting C5 can have on multiple rare and devastating diseases. However, we believe this is just the beginning of what's possible with complement inhibition," said Ludwig Hantson, CEO of Alexion.

Achillion Pharma shares were trading higher by 85% at $6.75 in Wednesday’s pre-market session.