Efimosfermin alfa
Efimosfermin alfa (BOS-580)
Also known as: BOS-580
GSK's once-monthly FGF21 analog, acquired for $1.2 billion upfront, with FDA Breakthrough Therapy designation for MASH and Phase 2 results published in 2026.
Overview
Efimosfermin is an engineered FGF21 variant with a half-life long enough for monthly injection, the only monthly FGF21 in development. Its Phase 2 in F2/F3 MASH showed histologic benefit and good tolerability over 24 weeks, earning Breakthrough Therapy status from the FDA and PRIME from the EMA. GSK is preparing Phase 3.
Mechanism of action
Long-acting FGF21 receptor agonism that reduces liver fat, inflammation and fibrosis.
Key studies & citations
- Human2026
Efimosfermin alfa in MASH with F2/F3 fibrosis: Phase 2
Histologic improvement and good tolerability over 24 weeks.
PubMed - Human2025
Once-monthly FGF21 analog efimosfermin: pharmacology and early clinical data
Supports monthly dosing.
PubMed - Human2025
GSK's efimosfermin receives FDA Breakthrough Therapy designation
Regulatory acceleration for MASH.
GSK press release
Frequently asked questions
Is efimosfermin a weight-loss drug?
No. Like other FGF21 analogs it targets liver disease and metabolic health; weight effects are modest.