Alexion Pharmaceuticals, Inc. (NASDAQ:ALXN) is buying fellow ultra orphan drug developer Synageva Biopharma Corp (NASDAQ:GEVA) for $8.4 billion.

Alexion will pay $115 in cash and 0.6581 Alexion shares for each share of Synageva, implying a per share value of $230 – a premium of over 135% to Tuesday’s close.

Alexion is paying a hefty premium for access to Synageva’s pipeline and only marketed product, Kanuma (sebelipase alfa), for LAL Deficiency. At some 10 times Kanuma’s estimated peak sales (the biologic is not yet approved), Alexion is betting that Synageva’s pipeline holds just as much promise as Kanuma.

Alexion already markets Soliris, which once carried the title of most expensive drug in the world. It has since been displaced, but Alexion’s focus on ultra rare disease indications hasn’t waned. The combined company will have a pipeline of 7 clinical-stage assets targeting a myriad of orphan indications.

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The acquisition is likely to jump-start – at least temporarily – what has been a lagging biotech sector for the last few weeks. It’s also the second expensive acquisition in biotech this year: AbbVie Inc (NYSE:ABBV)’s $21 billion purchase of Pharmacyclics, Inc. (NASDAQ:PCYC) in March shocked many on the Street given the high sales multiple that AbbVie paid for Imbruvica, PCYC’s lead asset.

One or more of PropThink’s contributors are long ALXN.