GSK has signed an agreement to acquire efimosfermin alfa, a phase III-ready investigational therapy for steatotic liver disease (SLD), from Boston Pharmaceuticals in a deal valued at up to $2 billion.

The transaction includes a $1.2 billion upfront payment and up to $800 million in milestone-based payouts. The London-based pharmaceutical company expects the deal to expand its pipeline of specialty treatments targeting liver fibrosis driven by steatotic and viral conditions.

Efimosfermin is a long-acting analog of fibroblast growth factor 21 (FGF21), administered via monthly subcutaneous injection. It is being developed for metabolic dysfunction-associated steatohepatitis (MASH) and alcohol-related liver disease (ALD), both classified as forms of SLD. In a recent phase II trial, efimosfermin demonstrated significant reversal of liver fibrosis in patients with moderate-to-advanced MASH, including those with cirrhosis. The treatment also showed potential to improve triglyceride levels and glycemic control, with tolerability described as manageable.
“The FGF21 class has shown some of the most exciting data in MASH including first-in-disease evidence of cirrhosis reversal,” said Tony Wood, GSK’s chief scientific officer. “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.”

Efimosfermin’s mechanism of action, combined with GSK’s internal RNA therapeutic (GSK’990), positions the company to pursue both monotherapy and combination treatment strategies for SLD. The acquisition aligns with GSK’s broader R&D focus on immune-mediated diseases and fibrosis.

Boston Pharmaceuticals will divest its subsidiary BP Asset IX Inc., which holds the rights to efimosfermin. Novartis retains tiered royalty rights on future sales. GSK will account for the transaction as a business combination, pending regulatory approvals under the Hart-Scott-Rodino Act.

The companies were advised by Evercore and Centerview Partners, respectively.

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