The acquisition will help diversify Alexion's portfolio by bringing on Portola's coagulation factor Xa drug Andexxa.
At last check shares of Portola, South San Francisco, had more than doubled to $17.72. They'd closed Monday trading at $7.76. Alexion shares were off 4.8% to $98.81.
Andexxa is marketed as Ondexxya in Europe and is the only approved Factor Xa inhibitor reversal agent.
"The acquisition of Portola represents an important next step in our strategy to diversify beyond C5," Alexion Chief Executive Ludwig Hantson said in a statement.
"Andexxa is a strategic fit with our existing portfolio of transformative medicines and is well-aligned with our demonstrated expertise in hematology, neurology and critical care."
Alexion, Boston, said Andexxa has the potential to become the "global standard of care" for patients who experience life-threatening bleeding while taking Factor Xa inhibitors like apixaban and rivaroxaban.
"By leveraging Alexion’s strong operational and sales infrastructure and deep relationships in hospital channels, we are well positioned to expand the number of patients helped by Andexxa, while also driving value for shareholders," Hantson said.
Andexxa, which was approved in the U.S. in 2018, brought in sales of $111.5 million in 2019.
Alexion said it would fund the transaction with cash on hand.
The deal is subject to conditions including approval by holders of a majority of Portola shares outstanding and clearance by regulators, including U.S. antitrust authorities.
The companies expect to close the transaction in the third quarter.