Myostatin inhibitor / activin-binding protein · Also known as FS-344, FST-344, Follistatin
A recombinant form of follistatin, a naturally occurring glycoprotein that binds and neutralizes myostatin, the hormone that limits muscle growth. By blocking myostatin signaling, it allows muscles to grow beyond their normal genetic limits.
Follistatin gained attention after dramatic muscle-growth results in animal studies, including doubled muscle mass in mice and significant gains in primates via gene therapy. A Phase 1/2a gene therapy trial in Becker muscular dystrophy patients showed improved walking distance and reduced fibrosis. The injectable peptide form has very limited clinical data and a short half-life of roughly 90 minutes. WADA banned follistatin-related substances in 2019.
Follistatin works primarily by binding to myostatin (GDF-8) and other TGF-beta superfamily members like activin A, preventing them from reaching their receptors on muscle cells. Myostatin normally acts as a brake on muscle growth, so when follistatin neutralizes it, the brake is released and muscle fibers can proliferate and enlarge beyond normal capacity.
The 344 isoform was specifically chosen for research because it has lower affinity for heparin sulfate proteoglycans compared to the 317 isoform, which means it circulates more freely rather than binding to cell surfaces. This also means it has less impact on reproductive hormones like FSH. However, the injectable peptide has a half-life of about 90 minutes, which is why most clinical research uses AAV gene therapy vectors to provide sustained expression over months.
No clinically validated injectable dose
N/ANo clinically validated injectable dose
N/AMost side effects tend to improve as your body adjusts.