GSK has announced the acquisition of efimosfermin alfa, a Phase III-ready fibroblast growth factor 21 (FGF21) analog from Boston Pharmaceuticals. According to the company, efimosfermin is a potential best-in-class, once-monthly treatment aimed at halting and reversing the advancement of steatotic liver disease (SLD), including metabolic dysfunction-associated steatohepatitis (MASH) and alcohol-related liver disease (ALD).

What are the Financial Terms for GSK’s Acquisition of Boston Pharmaceuticals’ Novel Therapy?

Under terms of the deal, GSK will acquire BP Asset IX, Inc., a subsidiary of Boston Pharmaceuticals, to obtain rights to efimosfermin alfa. GSK is expected to pay $1.2 billion upfront and up to an additional $800 million in milestone payments.¹

“The FGF21 class has shown some of the most exciting data in MASH including first-in-disease evidence of cirrhosis reversal, and efimosfermin has the potential to define a new standard-of-care with its monthly dosing and tolerability profile,” said Tony Wood, chief scientific officer, GSK, in a press release. “Efimosfermin will significantly expand our hepatology pipeline and provide us the opportunity to develop a new potential best-in-class medicine with first launch expected in 2029. It complements GSK‘990, also in development for ALD and MASH, offering GSK options to develop both monotherapy and potential combinations to improve patient outcomes.”

Efimosfermin Shows Positive Phase II Results in MASH

In November 2024, Boston Pharmaceuticals presented full data from a Phase II study of efimosfermin at the American Association for the Study of Liver Diseases Liver Meeting.1 The randomized, double-blind, placebo-controlled study assessed the efficacy and safety of once-monthly efimosfermin 300 mg in 84 patients with biopsy-confirmed stage F2/F3 fibrosis due to MASH over 24 weeks. The dual primary endpoints of the study included fibrosis improvement ≥1 stage without worsening of MASH and MASH resolution without worsening of fibrosis. The secondary endpoint was achievement of both fibrosis improvement and MASH resolution.

Results found that 45.2% of patients achieved ≥1 stage fibrosis improvement without worsening of MASH, compared to 20.6% on placebo. Additionally, 67.7% achieved MASH resolution without worsening of fibrosis, compared to 29.4% on placebo. Further, 38.7% of patients in the efimosfermin group achieved both fibrosis improvement and MASH resolution compared to 17.6% in the placebo group.

During the 24-week treatment period, there was a low discontinuation rate due to adverse events (AEs) as well as an overall low incidence of gastrointestinal AEs and injection site reactions.²

According to a study published by the National Center for Biotechnology Information, MASH has a global estimated prevalence of 5.27%, with a 5% prevalence in North America alone.³

Global Impact and Next Steps for Efimosfermin

“I am very proud of today’s agreement with GSK, a company I know and admire with proven expertise in liver disease, and the outstanding work of the Boston Pharmaceuticals team,” said Elias Zerhouni, MD, board chair, Boston Pharmaceuticals, in the press release. “This would not have been possible without the impressive, sustained and long-term strategic commitment to leading science and biotechnology ventures from the Bertarelli family, and the expertise of Ernesto Bertarelli, which led to the development of efimosfermin as a potential best-in-class therapy. We are delighted that GSK, a global leader, recognized efimosfermin’s potential to address a growing global public health concern and unmet medical need. Together, we look forward to efimosfermin’s ongoing journey to become a best-in-class treatment for patients with SLD.”

Closing of the transaction is dependent upon customary conditions, such as applicable regulatory agency clearances under the Hart-Scott-Rodino Act.¹

References

1. GSK to acquire efimosfermin, a phase III-ready potential best-in-class specialty medicine to treat and prevent progression of steatotic liver disease (SLD). GSK. May 14, 2025. Accessed May 14, 2025. https://www.gsk.com/en-gb/media/press-releases/gsk-to-acquire-efimosfermin-a-phase-iii-ready-potential-best-in-class-specialty-medicine-to-treat-and-prevent-progression-of-steatotic-liver-disease-sld/

2. Boston Pharmaceuticals to Announce Positive Phase 2 Data on Efimosfermin Alfa (BOS-580) in F2/F3 MASH During Late-Breaking Oral Presentation at AASLD 2024, The Liver Meeting. BusinessWire. November 15, 2025. Accessed May 14, 2025. https://www.businesswire.com/news/home/20241115839453/en/Boston-Pharmaceuticals-to-Announce-Positive-Phase-2-Data-on-Efimosfermin-Alfa-BOS-580-in-F2F3-MASH-During-Late-Breaking-Oral-Presentation-at-AASLD-2024-The-Liver-Meeting

3. Metabolic Dysfunction-Associated Steatohepatitis and Progression to Hepatocellular Carcinoma: A Literature Review. PubMed. Accessed May 14, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC10969013/#:~:text=The%20estimated%20global%20incidence%20of,in%20North%20America%20%5B9%5D.