Follistatin-344

Follistatin is an endogenous, high-affinity trap for several TGF-beta superfamily ligands. By wrapping around myostatin (GDF-8) and the activins, it prevents them from engaging ActRIIA/ActRIIB and therefore shuts down the Smad2/3 signalling that normally restrains muscle protein synthesis and satellite cell activity. FST-344 is the precursor form; after the 29-residue signal peptide is cleaved it becomes FST-315, the circulating isoform, so injected FST-344 and FST-315 converge on the same active molecule. The muscle-growth potential is genuine and dramatic in animals - follistatin gene therapy produced large, durable hypertrophy in mice and non-human primates. What is not established is that a short course of injected recombinant protein, with a circulating half-life measured in hours and broad activin-blocking activity outside muscle, reproduces any of that.

Targets: Myostatin (GDF-8), Activin A, Activin B, GDF-11, Activin type II receptors (indirectly)

• · 100 mcg per day for 10-30 consecutive days is the near-universal forum protocol. It has no experimental basis whatsoever - it is a number that propagated through vendor marketing.
• · Higher doses raise cost dramatically without any evidence of extra effect, and the counterfeiting problem means you are frequently paying more for more of nothing.

Work out your exact syringe units →

• very commonReceiving a counterfeit or inactive product— The single most likely outcome. Genuine recombinant follistatin is expensive to make; most vials sold at consumer prices cannot plausibly contain it.
• commonInjection-site reaction
• uncommonDisruption of activin-dependent physiology— Activins regulate FSH secretion, wound healing, inflammation and reproductive function; a systemic activin trap does not act only on muscle.
• uncommonImmunogenicity to a recombinant protein— Antibody formation is a standard risk with unregulated recombinant proteins and would be undetectable without testing.
• rareTheoretical tumour-growth risk— Activin signalling is tumour-suppressive in several tissues, so blocking it chronically is not obviously benign.

• Active or suspected malignancy - activin is a growth-inhibitory signal in several cancers.
• Pregnancy and breastfeeding - activin and follistatin are central to reproductive physiology.
• Fertility treatment or active attempts to conceive.

• redundantace-031 — Both block the myostatin/activin axis. Stacking them multiplies off-target activin blockade without a demonstrated benefit.
• redundantfollistatin-315 — FST-344 becomes FST-315 after signal peptide cleavage. These are effectively the same product.
• synergyigf-1-lr3 — Different mechanisms - removing the brake versus pressing the accelerator - and a common theoretical stack.

• · There is no validated marker. Serum myostatin assays are unreliable and not clinically actionable.
• · Track lean mass by DEXA if you want an objective answer about whether anything happened.
• · Verify product identity with third-party mass spectrometry before spending significant money.

Not approved for human use anywhere. Sold as a research chemical; myostatin-pathway inhibitors are prohibited at all times in sport by WADA.

• Kota et al. 2009, Science Translational Medicine - follistatin gene delivery produces durable muscle enlargement in mice and non-human primates (preclinical)
• Reviews of follistatin, activin and myostatin signalling in skeletal muscle (review)
• WADA Prohibited List, myostatin-function-modifying agents (guideline)

The first thing to fix is the naming, because the market treats FST-344 and FST-315 as two products and they are essentially one. UniProt P19883 is a 344-residue precursor with a 29-residue signal peptide; cleave it and you get the 315-residue secreted protein. FST344 is the transcript and the construct - it is what Mendell's gene therapy trial delivered as AAV1.CMV.FS344 - and FS315 is what the cell actually secretes. The genuinely different isoform is FS288, produced by alternative splicing, which lacks the acidic C-terminal tail and therefore binds heparan sulfate proteoglycans and stays anchored at the cell surface. That distinction matters more than the one people pay for: FS288 acts locally at the muscle it is expressed in, FS315 circulates. Mechanistically follistatin is a promiscuous ligand trap, not a receptor antagonist. It wraps around TGF-beta superfamily dimers - myostatin, activin A, GDF11, and several BMPs - and occludes the type II receptor binding site so the ligand can never reach ActRIIA or ActRIIB. No receptor engagement means no ALK4/ALK5 recruitment, no Smad2/3 phosphorylation, no Smad4 complex in the nucleus, and so no transcription of the atrophy programme. Simultaneously, relieving Smad3 lifts its inhibition of Akt, so mTORC1 activity rises. UniProt's curated function statement also notes an essential role in muscle fibre formation through SMAD3/AKT/mTOR signalling that is independent of myostatin, which fits the observation that follistatin overexpression produces more hypertrophy than myostatin knockout alone. That promiscuity is also the safety problem. Activin A is not a spare part: it regulates FSH secretion from the pituitary, and follistatin was originally isolated as an FSH-suppressing protein. Systemic follistatin therefore suppresses FSH and disturbs gonadal function, and it interferes with BMP signalling in bone and vasculature. That is a very different risk profile from a selective anti-myostatin antibody.

The core problem is that what is sold as injectable follistatin-344 has no relationship to the one human study anyone cites. That study was AAV gene transfer directly into quadriceps muscle, producing sustained local expression, in six patients. What is being sold is a recombinant protein in a vial with a suggested daily subcutaneous dose invented by a vendor. A short-lived circulating protein dosed daily will not reproduce sustained intramuscular expression, and the pharmacokinetics that would tell you what dose might are unmeasured. The second problem is the endocrine cost people do not price in: follistatin was discovered as an FSH-suppressing protein, and systemic administration does the thing it was named for. Men running it for a muscle effect are also suppressing spermatogenesis without checking. The third is protein quality - follistatin is a 315-residue, disulfide-rich, normally glycosylated protein, which is at the hard end of what grey-market synthesis produces, and misfolded protein is both inactive and more immunogenic.

• Myostatin (GDF-8) — High-affinity direct binding, confirmed by structural and interaction studies

  Sequesters the mature dimer so it cannot engage ActRIIB. Removes Smad2/3-driven suppression of muscle protein synthesis and satellite cell activity.

• Activin A (INHBA) — High affinity - follistatin was originally purified on the basis of activin binding and FSH suppression

  This is the second half of the muscle effect and the whole of the endocrine problem. Blocking activin A adds hypertrophy beyond myostatin blockade alone, and suppresses pituitary FSH.

• GDF11 — Direct binding confirmed

  GDF11 is highly homologous to myostatin and shares receptors. Its physiological roles include axial skeletal patterning, so blocking it is not a free action.

• Bone morphogenetic proteins (BMP2, BMP4, BMP6, BMP7) — Lower than for activin, but real

  BMP antagonism. Relevant to bone turnover and to neural differentiation, and a reason systemic follistatin is not a clean muscle drug.

• AAV1.CMV.FS344 phase 1/2a gene therapy trial in Becker muscular dystrophy Phase 1/2a · n=6 · 2015

  Safety of bilateral intramuscular quadriceps AAV1.CMV.FS344 at 3 x 10^11 and 6 x 10^11 vg/kg/leg, with six-minute walk distance as the functional readout. No adverse effects were encountered. Four of six patients improved on the six-minute walk (+58 m, +125 m, +108 m, +29 m) and two did not change. Biopsies showed reduced endomysial fibrosis, reduced central nucleation and muscle hypertrophy, most clearly at the high dose.

• NCT02354781 - Clinical Intramuscular Gene Transfer of rAAV1.CMV.huFollistatin344 to Patients With Duchenne Muscular Dystrophy Phase 1/2 · n=3

  Safety and tolerability of intramuscular AAV-delivered follistatin-344 in Duchenne muscular dystrophy. Listed as completed on ClinicalTrials.gov.

Nothing acute. In the gene therapy trial, functional change on the six-minute walk was assessed over months, and the biopsy changes reflect sustained local expression. For injected recombinant protein there is no established timeline because there is no established effect. Endocrine changes, if the protein is real and active, would be measurable in FSH within two to four weeks - which is why that is the test to run.

Follistatin is stacked with GH and IGF-1 analogues on the theory that you are removing a brake while pressing an accelerator, and the theory is mechanistically sound - Smad2/3 suppression and PI3K/Akt activation converge on the same translational machinery from opposite directions. Nothing has tested it in a human. The combination to think hard about is follistatin with anything else that suppresses the gonadal axis, because activin A blockade lowers FSH on its own; adding exogenous androgens on top gives you two independent routes to a shut-down axis and a longer restart. Follistatin also overlaps almost completely with ACE-031, bimagrumab and the anti-myostatin antibodies, so running it alongside any of them is stacking the same mechanism twice with additive off-target risk and no additive rationale.

Against follistatin-315: these are the same molecule at different stages - FST344 is the precursor and construct designation, FS315 is the secreted mature protein. Vendors selling both as distinct products are selling a naming convention. The genuinely different isoform is FS288, which is heparin-binding and stays local. Against ACE-031 and bimagrumab: those hit the receptor rather than the ligands, so they block everything signalling through ActRII including ligands follistatin does not bind - broader still, and demonstrably more effective in humans, with 3.3% lean mass at 29 days for ACE-031 and 1.7 kg at 48 weeks for bimagrumab. Against apitegromab, which binds only pro- and latent myostatin: that is the selective end of the spectrum, with no activin blockade and therefore no FSH suppression, and it is the design the field moved toward precisely because follistatin-like promiscuity causes trouble. Follistatin sits at the broadest, least selective end with the least human evidence.

$200–$700/month. 1 mg vials commonly 80-200 USD grey market, and vendor protocols of 100-200 mcg daily consume 3-6 mg a month. It is one of the more expensive research proteins in this class, and among the least likely to be what the label says.

• ?No pharmacokinetic data exist for injected recombinant follistatin in humans. Route, dose and frequency in circulating protocols are unanchored.
• ?It is not established that subcutaneously injected follistatin protein reaches muscle at concentrations sufficient to sequester myostatin locally.
• ?The magnitude and reversibility of FSH suppression from exogenous follistatin in humans is uncharacterised, though the mechanism is certain.
• ?Whether grey-market follistatin is correctly folded, correctly disulfide-bonded and glycosylated is unknown, and all three affect activity and clearance.
• ?Immunogenicity of repeated recombinant follistatin dosing has never been assessed.
• ?Blood-brain barrier penetration is marked no on molecular-size grounds rather than measurement.
• ?No molecular weight is given because it depends on the expression system and glycosylation state; the unglycosylated mature protein is roughly 34-35 kDa but this was not resolved to an authoritative value.

• A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy Mendell JR, Sahenk Z, Malik V, Gomez AM, Flanigan KM, Lowes LP, Alfano LN, Berry K, Meadows E, Lewis S, Braun L, Shontz K, Rouhana M, Clark KR, Rosales XQ, Al-Zaidy S, Govoni A, Rodino-Klapac LR, Hogan MJ, Kaspar BK, Molecular Therapy, 2015 · PMID 25322757

  The only human trial of follistatin-344 in any form. Note what it actually was: AAV gene transfer directly into quadriceps, six patients, not injected recombinant protein.

• Characterization of follistatin-type domains and their contribution to myostatin and activin A antagonism Cash JN, Angerman EB, Kattamuri C, Nolan K, Zhao H, Sidis Y, Keutmann HT, Thompson TB, Molecular Endocrinology, 2012 · PMID 22593183

  Domain-level dissection of how follistatin antagonises myostatin versus activin A - the structural basis for why it cannot be selective for one of them.

• Follistatin-288-Fc Fusion Protein Promotes Localized Growth of Skeletal Muscle Castonguay R, Camper M, Sako D, Kumar R, Elliott J, Schneyer AL, Liharska K, Davies MV, Cannell M, Grinberg AV, Liu H, Barron R, Buonopane M, Pearsall RS, Kumar R, Lachey J, Journal of Pharmacology and Experimental Therapeutics, 2019 · PMID 30563942

  Demonstrates the FS288 versus FS315 distinction in practice - the heparin-binding isoform produces localised rather than systemic muscle growth.

• UniProtKB P19883 (FST_HUMAN) - Follistatin UniProt Consortium, UniProt Knowledgebase

  Source of the 344-residue precursor sequence, the 29-residue signal peptide, the FS315/FS288 isoform annotation, and the curated function statement covering activin, myostatin, GDF11 and BMP antagonism plus the myostatin-independent SMAD3/AKT/mTOR role.