Highlights
- Efimosfermin alfa (BOS-580), a long-acting FGF21 analogue, was evaluated with a convenient once-monthly (Q4W) subcutaneous dosing regimen.
- In this 24-week Phase 2 trial, the primary safety and tolerability endpoints were met in participants with biopsy-confirmed MASH and F2/F3 fibrosis.
- Gastrointestinal adverse events were the most frequent but remained transient and mostly mild to moderate in severity.
- No clinically significant signals regarding vital signs or grade 3+ laboratory abnormalities were observed, supporting the drug’s safety profile for further clinical progression.
The Evolving Landscape of MASH Therapeutics
Metabolic dysfunction-associated steatohepatitis (MASH), formerly known as non-alcoholic steatohepatitis (NASH), remains a critical global health challenge. Characterized by hepatic steatosis, inflammation, and ballooning, MASH is the leading driver of progressive liver fibrosis, which can culminate in cirrhosis and hepatocellular carcinoma. Despite the recent approval of the first MASH-specific therapy, there remains a substantial unmet need for treatments that not only address metabolic dysfunction but also directly target the fibrotic pathways that determine long-term clinical outcomes.
Fibroblast Growth Factor 21 (FGF21) has emerged as a high-priority therapeutic target due to its pleiotropic effects. Endogenous FGF21 is a metabolic hormone that regulates lipid and glucose metabolism, improves insulin sensitivity, and exerts anti-inflammatory and anti-fibrotic effects directly on the liver. However, the short half-life of native FGF21 necessitates the development of stabilized analogues. Efimosfermin alfa (BOS-580) is an engineered FGF21 analogue designed for extended stability, potentially offering a more patient-friendly monthly dosing schedule compared to weekly alternatives.
Study Design and Methodology
This 24-week, randomized, double-blind, placebo-controlled, Phase 2 trial (NCT04880031) was conducted across 34 clinical sites in the United States. The study specifically targeted a high-risk population: adults aged 18–75 with a body-mass index (BMI) of at least 27 kg/m2 and biopsy-confirmed MASH with moderate (F2) or advanced (F3) fibrosis.
Participant Selection and Randomization
Inclusion required a Nonalcoholic Fatty Liver Disease Activity Score (NAS) of at least 4, with a minimum of 1 point in each component (steatosis, ballooning, and inflammation). Participants were randomized 1:1 to receive either 300 mg of efimosfermin alfa or a placebo via subcutaneous injection every four weeks (Q4W). Stratification was performed based on the fibrosis stage (F2 vs. F3) to ensure balance between the treatment arms.
Primary and Secondary Objectives
The primary endpoint focused on safety and tolerability, assessed through treatment-emergent adverse events (TEAEs), changes in vital signs (blood pressure and heart rate), and the incidence of grade 3 or 4 laboratory abnormalities. Secondary outcomes, though exploratory in this 24-week safety-focused analysis, included histological changes and metabolic biomarkers.
Key Findings: Safety and Tolerability Profile
Between May 2023 and March 2024, 1,171 individuals were screened, resulting in 84 randomized participants (43 in the efimosfermin group and 41 in the placebo group). The cohort was balanced by sex (52% female) and fibrosis severity (57% F2, 43% F3).
Adverse Events and Patient Experience
The safety analysis revealed that efimosfermin was generally well-tolerated. Adverse events were reported in 67% of the efimosfermin group compared to 55% of the placebo group. Crucially, the majority of these events were categorized as mild (56% for efimosfermin vs. 38% for placebo) or moderate (42% vs. 35%).
Gastrointestinal (GI) symptoms were the most common side effects associated with the treatment. These included nausea and diarrhea, which are frequently observed with FGF21 analogues. However, these GI events were transient, typically occurring within the first few weeks of treatment initiation and resolving without the need for discontinuation.
Laboratory and Physiological Monitoring
One of the concerns with metabolic modulators is the potential for off-target cardiovascular effects. In this study, there were no clinically meaningful changes in blood pressure or heart rate between the treatment and placebo groups. Furthermore, no clinically significant grade 3 or higher laboratory abnormalities were detected, and no deaths occurred during the 24-week period.
Clinical Interpretation and Mechanistic Context
The results of this Phase 2 trial provide essential evidence for the safety of efimosfermin alfa in a monthly dosing regimen. From a clinical perspective, the Q4W schedule is a significant differentiator. Most current FGF21 analogues in development require weekly injections. A monthly regimen could significantly enhance long-term treatment adherence, which is vital for managing a chronic, asymptomatic condition like MASH-related fibrosis.
Mechanistically, the lack of significant safety signals—particularly the absence of adverse cardiovascular or severe laboratory changes—suggests that the engineered stability of efimosfermin does not lead to toxic accumulation or deleterious systemic signaling within the 24-week window. The concentration of adverse events in the GI tract suggests a localized or receptor-mediated response that the body adapts to over time.
Expert Commentary
While the primary focus was safety, the inclusion of patients with F2 and F3 fibrosis is strategically important. These stages represent the “sweet spot” for therapeutic intervention, where the risk of progression to cirrhosis is high, but the potential for fibrosis reversal remains significant. Experts in the field note that while safety is the first hurdle, the true test for efimosfermin will be its ability to demonstrate significant fibrosis improvement and MASH resolution in larger, longer-term Phase 2b and Phase 3 trials.
The study’s limitations include the relatively small sample size and the short duration (24 weeks), which is sufficient for safety but limited for assessing definitive histological outcomes. However, the high rate of evaluable week-24 biopsies (65 out of 84) provides a solid foundation for the next phase of research.
Conclusion
The Phase 2 trial of efimosfermin alfa (BOS-580) confirms that a 300 mg monthly dose is safe and well-tolerated in patients with MASH and F2/F3 fibrosis. By meeting its primary safety endpoints and demonstrating a manageable GI side-effect profile, the study supports the continued development of this FGF21 analogue. If subsequent trials confirm histological efficacy, efimosfermin’s monthly dosing could offer a competitive advantage in the expanding therapeutic arsenal against MASH.
Funding and Trial Registration
This study was funded by Boston Pharmaceuticals and GSK. ClinicalTrials.gov Identifier: NCT04880031.
References
Noureddin M, Kowdley KV, Odrljin T, et al. Efimosfermin alfa (BOS-580) once per month in people with metabolic dysfunction-associated steatohepatitis with F2 or F3 fibrosis: results from a 24-week, randomised, double-blind, placebo-controlled, phase 2 trial. Lancet. 2026;S0140-6736(25)02276-7.