STOCKHOLM (Reuters) -Swedish drugmaker Orphan Biovitrum (SOBI) has agreed an $8 billion takeover by U.S. private equity firm Advent International and Singapore’s sovereign wealth fund, boosting its shares on speculation of a higher offer. SOBI’s stock, already up 15% this year, soared as much as 28% to 245.40 crowns - above the 235 crowns per share offer, which the rare disease drugmaker said represented a premium of 34.5% to its Aug. 25 closing price of 174.70 crowns. SOBI said on Thursday its board had unanimously recommended that shareholders accept the offer from Advent and Aurora, which is a nominated investment vehicle of GIC Special Investments Pte Ltd, a subsidiary of Singapore’s GIC Pte Ltd. Investor AB and Fjärde AP-Fonden, with SOBI stakes of about 36.45% and 6.96% respectively, have separately agreed to accept the 69.4 billion Swedish crowns ($8 billion) offer, SOBI said. Pareto Securities said in a morning briefing the bid was too low. “Our models indicate a potential value of well over 300 Swedish crowns per share,” the investment bank said. Other private equity suitors were beaten out by Advent in securing approval from the target’s management and owners, said one person close to a rival investor. SOBI’s cash-generating distribution network and a relatively modest development budget had high appeal with private equity firms, the source added. SOBI, which sells drugs within haemophilia, autoimmune diseases, metabolic diseases and cancer supportive care, said 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 recognise 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. Its best-selling drug, established haemophilia A treatment Elocta, with 4.6 billion crowns in 2020 revenues, is facing competition from Roche’s Hemlibra. SOBI’s drug development prospects have improved with positive trial results of its pegcetacoplan drug against rare blood disorders PNH and of nirsevimab, in which Sanofi and AstraZeneca hold certain rights, to prevent a common respiratory infection in infants. SOBI, which employs around 1,500 people worldwide and had revenues of 15.3 billion Swedish crowns in 2020, has suffered from weakened demand for its drugs as healthcare has focused on COVID-19 during the pandemic. Europe’s medicines regulator is evaluating SOBI’s arthritis drug Kineret for the treatment of COVID-19 patients with pneumonia. ($1=8.6178 Swedish crowns) Additional reporting by Pamela Barbaglia in London and Terje Solsvik in OsloEditing by Alexander Smith and Mark Potter Our Standards: The Thomson Reuters Trust Principles.