U.S. private equity firm Advent International and Aurora Investment, an affiliate of capital markets group GIC, has offered to buy the Swedish rare disease drugmaker Orphan Biovitrum (SOBI), the Swedish company said Thursday. The offer values the drugmaker at about 69.4 billion Swedish crowns ($8 billion).

SOBI's stock, already up 15% so far this year, rose 24.2% to 238 crowns after news of the 235 Swedish crowns per share offer, which SOBI said represented a premium of about 34.5% to its Aug. 25 closing price of 174.70 crowns on Nasdaq Stockholm.

Investor AB and Fjarde AP-Fonden, with SOBI stakes of about 36.45% and 6.96% respectively, have separately agreed to accept the offer, SOBI said, adding that its board had unanimously recommended that shareholders accept.

SOBI, which sells drugs within hemophilia, autoimmune diseases, metabolic diseases and cancer supportive care, added that Danske Bank had said the offer was "fair to SOBI's shareholders from a financial point of view."

"The board believes that the terms of the offer recognize SOBI's long-term growth prospects, as well as the risks associated with those prospects, and provide certainty, in cash, to shareholders," SOBI said in a statement.

SOBI shares rose as much as 22% on Aug. 26, after Bloomberg reported, citing people with knowledge of the matter, that Advent International was considering a bid.

During the pandemic, SOBI has suffered from weakened demand for its drugs as health care has focused on COVID-19.

Europe's medicines regulator is evaluating SOBI's arthritis drug Kineret, with the active substance anakinra, for the treatment of COVID-19 in adults with pneumonia who are at risk of severe respiratory failure.