Pacira BioSciences said on Monday it would acquire Flexion Therapeutics Inc for about $427.5 million, gaining access to several promising pain drugs and bolstering growth prospects.

Pacira will buy Flexion's outstanding shares for $8.50 each, a 47% premium to their last closing price. Flexion's stock soared past the offer price in premarket trade, rising 70% to $9.81.

Flexion shareholders will also receive an additional payment of up to $8 per share based on the company's knee pain drug, Zilretta, achieving certain sales milestones and other experimental treatments receiving regulatory clearances.

Zilretta is a non-opiod drug for chronic osteoarthritis knee pain launched in 2017, and recorded sales of $85.6 million last year.

Pacira will also get access to two drugs under development, FX201 and FX301, which are aimed at treating musculoskeletal pain and postsurgical pain respectively.

The company, which expects the deal to close by the end of the year and improve its earnings from 2022 onwards, said it would pay Flexion shareholders $1 per share following U.S. FDA approval of each of the two drugs in development and up to $3 per share for certain net sales milestones for Zilretta.

Currently Pacira's drug portfolio comprises of only Exparel, a local non-opiod anesthetic administered at the time of surgery.

Pacira recorded $413.3 million in sales of Exparel in 2020, and $121.9 million in the third quarter alone, but now faces challenges from generic manufacturers.

The company last week received a notice that eVenus Pharmaceutical Laboratories had filed an application with the U.S. FDA for manufacturing and marketing a generic of Exparel.

(Reporting by Dania Nadeem and Leroy Leo in Bengaluru; Editing by Saumyadeb Chakrabarty and Vinay Dwivedi) ((Dania.Nadeem@thomsonreuters.com;))