Follistatin 344

Overview

Follistatin 344 is a recombinant form of the naturally-occurring glycoprotein (344-residue splice variant) that inhibits activin and myostatin signaling. Clinical validation exists ONLY through small AAV gene-therapy phase 1/2 trials in Becker muscular dystrophy; recombinant injectable protein sold on the grey market has no clinical evidence. NOT FDA-approved. WADA-prohibited (myostatin inhibitor) and banned in competitive sport.

Mechanism of Action

The 344-amino-acid isoform binds and neutralizes activin A, activin B, and myostatin (GDF-8), preventing ActRIIB activation and downstream Smad2/3 phosphorylation. This removes the myostatin brake on satellite cell proliferation and myofiber hypertrophy. The C-terminal acidic tail confers broader tissue distribution than FS-288/FS-303 isoforms. Because myostatin also restrains cardiomyocyte growth, systemic follistatin elevation carries a theoretical cardiac hypertrophy risk.

Research Summary

AAV1-FS344 intramuscular gene therapy Phase 1/2a in Becker muscular dystrophy (n=6) reported improved 6-minute walk test and quadriceps volume changes (Mendell et al., Molecular Therapy, 2015). Preclinical mdx mouse and cynomolgus primate studies showed durable muscle mass and strength increases (Kota et al., Science Translational Medicine, 2009). Recombinant Follistatin-344 protein sold as an injectable research chemical has NO validated human clinical data — all positive human evidence is from AAV gene therapy, not peripheral protein injection. Not FDA-approved. WADA-prohibited at all times.

Dosing Information

Reported Side Effects

• No validated human safety data for injected recombinant protein
• Theoretical cardiac hypertrophy risk from systemic myostatin inhibition (myostatin normally restrains cardiomyocyte growth)
• Potential suppression of FSH and effects on fertility (follistatin is a physiologic FSH inhibitor)
• Injection site reactions (erythema, swelling)
• Possible off-target effects from activin-pathway inhibition (wound healing, hematopoiesis)
• Theoretical promotion of occult tumor growth via myostatin blockade

Research References

• A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy (2015)
• Follistatin gene delivery enhances muscle growth and strength in nonhuman primates (2009)