Efimosfermin Alfa

Efimosfermin alfa is the newest and least far along of this site's three FGF21-analog MASH drugs — a once-monthly candidate GSK paid up to $2 billion for in 2025, which then received FDA Breakthrough Therapy and EMA PRIME designations in April 2026 on the strength of a real, peer-reviewed Phase 2 result. It has never been tested in a completed Phase 3 trial.

What it is, and where it came from

Efimosfermin alfa is a long-acting, engineered variant of fibroblast growth factor 21 (FGF21) dosed once monthly by subcutaneous injection — less frequently than either efruxifermin (weekly) or pegozafermin (weekly or every two weeks), also covered on this site. The molecule originated at Novartis and was acquired, under the development code BOS-580, by Boston Pharmaceuticals in 2020. GSK then acquired Boston Pharmaceuticals' rights to the drug in a deal announced 13 May 2025 and closed 7 July 2025, for up to $2 billion (a $1.2 billion upfront payment plus up to $800 million in milestones), with royalties still owed to Novartis under the original license.

The Phase 2 evidence

An earlier Phase 2a trial established dosing and tolerability in phenotypic MASH without biopsy confirmation of efficacy (Loomba R, Kowdley KV, Rodriguez J, Kim NJ, Alvarez AM, Morrow L, Jeglinski B, Clawson A, Chowdhury S, Bain G, Odrljin T, Lancet Gastroenterol Hepatol 2025;10(8):734-745, DOI 10.1016/S2468-1253(25)00067-6, PMID 40484014). The more clinically relevant result is a separate 24-week, randomized, double-blind, placebo-controlled Phase 2 trial in 84 patients with biopsy-confirmed F2/F3 MASH, first presented as a late-breaking abstract at AASLD's The Liver Meeting in November 2024 and later published in full (Noureddin M, Kowdley KV, Odrljin T, Bain G, Zhao J, Jeglinski B, Koziel MJ, Loomba R, Lancet 2026;407(10530):794-804, DOI 10.1016/S0140-6736(25)02276-7, PMID 41655584). Among the 65 patients with evaluable week-24 biopsies, efimosfermin 300mg monthly produced statistically significant improvement over placebo on both endpoints tested: fibrosis improvement of at least one stage without worsening of MASH (45% vs. 21%) and MASH resolution without worsening of fibrosis (68% vs. 29%).

Breakthrough Therapy, PRIME, and Phase 3

On 27 April 2026, GSK announced that efimosfermin had received FDA Breakthrough Therapy designation and EMA PRIME (Priority Medicines) designation for MASH, based on the Phase 2 result above — both are process designations meant to speed up FDA/EMA interaction during development, not approvals in themselves. GSK is now running the Phase 3 ZENITH-1 and ZENITH-2 trials in F2/F3 MASH, including a pivotal study registered as NCT07221227, with a further Phase 3 trial in F4 cirrhosis reported as planned; GSK has stated a potential first launch in 2029 if the program succeeds.

What's not yet resolved

Efimosfermin has the smallest completed efficacy trial of the three FGF21 analogs covered on this site — 84 randomized patients, versus 219 for pegozafermin's ENLIVEN trial and 182 for efruxifermin's SYMMETRY trial — and no completed Phase 3 result exists for any endpoint yet. The acquisition price and the two regulatory designations are real and specific facts, but they are signals of institutional confidence, not evidence of efficacy on their own; anyone relying on this page for current status should check GSK's own disclosures and ClinicalTrials.gov directly once the Phase 3 trials begin reporting.