Efimosfermin

Efimosfermin alfa (formerly LLF580 under Novartis origination, subsequently designated BOS-580 under Boston Pharmaceuticals development, and now GSK-6519754 following acquisition by GSK plc) is a genetically engineered, long-acting analog of human fibroblast growth factor 21 (FGF21), a hepatokine and adipokine that regulates hepatic lipid oxidation, adipose tissue glucose uptake, triglyceride metabolism, and inflammatory signaling through formation of a ternary complex with the transmembrane tyrosine kinase FGF receptor 1c (FGFR1c) and the obligate co-receptor beta-klotho (KLB). The molecule is a homodimeric fusion protein in which two copies of a stabilized FGF21 variant are fused at their N-termini to the crystallizable fragment (Fc) of human immunoglobulin G1 (IgG1). The FGF21 domain is further stabilized by the introduction of a non-native disulfide bond and point mutations that increase proteolytic resistance, together producing a pharmacokinetic half-life of approximately 21 days in humans and enabling subcutaneous administration in the published literature. This dosing interval distinguishes efimosfermin from the FGF21 analogs efruxifermin (Akero Therapeutics) and pegozafermin (89bio), which require subcutaneous injection in the published literature.

The compound entered clinical development as LLF580 in a Novartis-sponsored Phase 1/2 trial (CLLF580X2102) in obese adults with modest hypertriglyceridemia, where 300 mg subcutaneously in the published literature for 12 weeks produced a 54 percent reduction in serum triglycerides, a 36 percent increase in HDL cholesterol, a 52 percent reduction in hepatic fat fraction by magnetic resonance imaging-estimated proton density fat fraction (MRI-PDFF), a 24 percent reduction in pro-peptide type III collagen (Pro-C3, a circulating fibrosis biomarker), and improvements in insulin sensitivity (38 percent reduction in fasting insulin, 29 percent reduction in C-peptide, and a 103 percent increase in adiponectin), all without significant body weight change. The compound was subsequently licensed to Boston Pharmaceuticals, renamed BOS-580, and advanced into a Phase 2a multicenter randomized double-blind placebo-controlled trial in participants with phenotypic metabolic dysfunction-associated steatohepatitis (MASH), followed by a Phase 2b trial (NCT04880031) in 84 patients with biopsy-confirmed MASH and F2 or F3 fibrosis, where 300 mg for 24 weeks produced fibrosis improvement of at least one stage without MASH worsening in 45.2 percent of treated patients versus 20.6 percent on placebo (p = 0.038), MASH resolution without fibrosis worsening in 67.7 percent versus 29.4 percent on placebo (p < 0.01), and clinically meaningful improvements in glycemic control markers including glycated hemoglobin.

In May 2025, GSK plc completed acquisition of efimosfermin from Boston Pharmaceuticals for 1.2 billion United States dollars upfront and up to 800 million dollars in milestone payments, with tiered royalties owed to Novartis Pharma AG. The compound has received United States Food and Drug Administration Breakthrough Therapy Designation and European Medicines Agency Priority Medicines (PRIME) Designation for the treatment of MASH. Phase 3 development (the ZENITH program) commenced in December 2025, with a potential first launch projected for 2029. GSK has indicated development plans in both MASH (including compensated cirrhosis) and alcohol-related liver disease (ALD), with potential for combination therapy with GSK'990, a complementary hepatology pipeline candidate. This monograph reviews the chemistry, molecular design, and receptor pharmacology of efimosfermin; the FGF21 signaling pathway; the comprehensive preclinical and clinical pharmacokinetic record; the clinical evidence base across Phase 1/2 and Phase 2 trials; the sourcing and quality considerations for this biologic investigational agent; reconstitution and handling; stack interactions and combinations; adverse events and safety signals; and a comparative assessment of five FGF21-pathway therapeutic candidates against efimosfermin on five competency standards.