Follistatin-315 monograph

Follistatin-315

Overview

Follistatin-315 is a 315–amino acid splice isoform of follistatin discussed mainly as an investigational way to block myostatin signaling and other TGF-beta family ligands to increase muscle size and strength. The best human-facing evidence is not from bodybuilding use but from preclinical gene-therapy work and biomarker studies; there is no established clinical dosing regimen for enhancement use. Most practical bodybuilding claims are extrapolated from animal data or community anecdotes, so accuracy and caution matter here. PubMed review, NCBI Gene

Pharmacokinetics & Mechanism

• Class: Endogenous glycoprotein / myostatin and activin-binding antagonist; investigational biologic/gene-therapy target rather than approved drug
• Mechanism of action: Follistatin binds and neutralizes several members of the TGF-beta superfamily, with strong emphasis on antagonizing myostatin/activin signaling, thereby removing restraint on muscle growth. In the muscle-gene-therapy literature, the FS344 transgene product is described as a secreted 315-amino-acid peptide that circulates in serum and avoids cell-surface binding sites. That mechanism is preclinical/experimental for hypertrophy purposes; human efficacy and safety for performance enhancement are not established. PubMed review, PubMed review
• Target(s): Primary targets: myostatin (GDF8) and activins within the TGF-beta family; broader binding to related ligands is reported in the follistatin literature. Pathway-level effect: reduced SMAD2/3-mediated signaling downstream of activin/myostatin receptor systems. No unique receptor agonism is established for FS315 itself. PubMed review, NCBI Gene FST, NCBI Gene MSTN
• Route(s) of administration: Not well established in humans for enhancement use. In research settings and the muscle-disease gene-therapy literature, delivery has been by adeno-associated virus to muscle; community use discussions typically assume injection-based peptide use, but that is not an established clinical route and should not be treated as validated therapy. PubMed review
• Onset: Not well established in humans. Preclinical gene-delivery studies imply delayed onset over days to weeks as transgene expression develops, not an immediate pharmacologic effect. PubMed review
• Half-life: Not well established for purified Follistatin-315 in humans. The review describes the FS344 transgene product as a secreted 315-amino-acid peptide circulating in serum, but does not provide a validated human half-life; no reliable bodybuilding-grade half-life estimate was found in authoritative sources. PubMed review, NCBI Gene
• Metabolism / clearance: Not well established in humans. As an endogenous secreted glycoprotein, clearance is expected to be proteolytic/catabolic rather than via a single defined hepatic pathway, but I did not find authoritative human PK data for FS315. PubMed review