Follistatin 344 Research

Follistatin 344 research rests on an unusual foundation. The name refers to a 344-amino-acid protein precursor encoded by the human FST gene, so this is a secreted glycoprotein rather than a peptide. No CAS number or molecular formula applies to it, because it is not a small molecule, and PubChem's compound database returns no record for it. The authoritative identifier is the UniProt protein accession P19883. ClinicalTrials.gov lists five interventional studies that use follistatin itself as the intervention. Each of the five delivered a gene in a vector or plasmid rather than an injected protein.

What Follistatin 344 Is

Shimasaki and colleagues sequenced eight human testis cDNA clones in 1988, of which three predicted a precursor of 344 amino acids and five a precursor of 317 ^[1]. Both are precursor lengths and both include a 29-residue signal peptide. Once that peptide is cleaved, the FST344 transcript yields the mature 315-residue protein FS-315, and FST317 yields mature FS-288. FS-344 and FS-315 are the same gene product at two stages of processing, not two competing versions a researcher chooses between. Measured with an FS315-specific immunoassay, FS-315 was the dominant follistatin form in human serum and was undetectable in follicular fluid ^[2]. FSTL1 and FSTL3 are separate genes, and much of the recent literature a follistatin search returns concerns those two instead.

Mechanism and Isoform Biology

Follistatin binds and neutralizes activin A, and it also binds myostatin. Sidis and colleagues compared recombinant FST288, FST303, FST315 and FSTL3 directly in vitro ^[3]. FST303 is an intermediate form of the protein, identified in human follicular fluid ^[2]. Activin-binding affinities were comparable across the isoforms, while cell-surface binding differed markedly, ranking FST288 above FST303, above FST315. Inhibition of endogenous activin bioactivity tracked cell-surface binding rather than affinity. In a TT-cell bioassay, FST288 suppressed activin-dependent proliferation while FST315 enhanced it. FS-315 is therefore the weakest of the three isoforms at inhibiting endogenous activin in cell-based assays. The 344 designation marks the isoform selected for lower off-target cell-surface activity, not for higher potency.

Activin antagonism is intrinsic to this mechanism rather than an incidental side effect. A narrative review from the trial group states that follistatin interacts with the pituitary activin-inhibin axis and suppresses secretion of follicle-stimulating hormone ^[4]. That review names this effect on the human reproductive axis as the reason caution was called for in clinical use. It also names it as the reason the FS344 variant was chosen. The review's authors are the investigators who ran the muscular dystrophy and myositis gene therapy trials. In an animal model, mice expressing only FST288 survived to adulthood but showed fertility defects including reduced litter size ^[5]. Mice lacking follistatin entirely were growth-retarded, had reduced diaphragm and intercostal muscle mass, and died within hours of birth ^[6].

Why the Human Evidence Does Not Transfer

ClinicalTrials.gov lists five interventional studies using follistatin as the intervention, and each one delivers the FST344 open reading frame in a viral vector or plasmid ^[7]. No study in that census administers follistatin as an injected protein. Under gene delivery the recipient's own muscle transcribes, translates, glycosylates and secretes the protein locally across months. Injecting recombinant protein produces a different exposure profile entirely, so efficacy findings from those trials do not support claims about an injected protein.

What the Gene Therapy Trials Reported

In a nonhuman primate study, AAV1-FS344 injected into macaque quadriceps increased muscle size and strength ^[8]. PubMed carries an erratum for that paper, published in August 2026 ^[9]. The erratum's content could not be retrieved from the publisher, from PubMed Central or from any indexed summary. Its significance is therefore unknown.

In an open-label, uncontrolled human phase 1/2a trial, the vector was injected into the quadriceps of six Becker muscular dystrophy patients ^[10]. Four improved on the six-minute walk test and two showed no change. The trial had no control group.

The same group treated six sporadic inclusion body myositis patients, comparing them against eight untreated subjects matched for age, gender and baseline ^[11]. That comparison was neither randomized nor blinded. Four of the six treated subjects improved by 58 to 153 meters on the six-minute walk test. The other two improved by only 5 to 23 meters. Changes of that size sit within measurement noise for the six-minute walk test. An independent commentary in the same journal argued the functional claims were unfounded ^[12]. It stated that participants received at least four potentially therapeutic interventions: the gene therapy, roughly 60 days of high-dose prednisone, a prescribed exercise program, and placebo effects. It also noted that the comparison rested on aggregate clinic data rather than a control group enrolled alongside the treated subjects. It identified an outcome switch as well, and the registry confirms the registered primary outcome was safety ^[7]. The investigators replied in the same issue ^[13].

A third human trial, in Duchenne muscular dystrophy, enrolled three of six planned patients ^[14]. Its posted registry results record zero of three subjects with increased six-minute-walk distance. North Star Ambulatory Assessment improved in one of three, and dose-limiting toxicities were zero. The other-events table lists increased muscle weakness in one of three and compression fracture in one of three. A serious adverse event of head injury from a fall was also recorded. No peer-reviewed publication of this trial was located on PubMed.

Conflicts of interest inside this literature are concentrated. Every registered follistatin gene therapy trial in a muscular dystrophy or myositis population ran at one center under one investigator, and both positive papers come from that group ^[10][11]. That group's vector had a commercial sponsor, which held the relevant FDA orphan drug designations. Orphan designation is granted on rarity and plausibility, and carries no efficacy finding and no marketing authorization. No independent group has replicated the positive follistatin gene therapy findings. Three further follistatin gene therapy studies are registered by commercial sponsors rather than by independent investigators ^[7]. Two are run by a company that sells follistatin gene therapy directly to consumers. The third is registered under the condition age-related muscle decline, not a rare muscle disease. One of the company studies is a 43-participant plasmid study with no posted results and no indexed publication. Its circulating outcome figures appear only on the sponsor's own commercial pages, making them marketing claims rather than findings. Registration on ClinicalTrials.gov is self-submitted, and it is not a regulatory review or an endorsement of any registered study. That plasmid study's own registry record declares that it is not an FDA-regulated drug study ^[7].

Injected Follistatin Protein

One preclinical study characterized native FS-315 directly as a candidate injectable ^[15]. The authors concluded that its intrinsic pharmacokinetic and pharmacodynamic properties are poorly suited to parenteral administration with broad systemic effects. Fusing FST315 to a murine IgG1 Fc and removing its heparan-sulfate binding gave roughly 100-fold longer terminal half-life. Only that engineered variant produced an effect in mouse muscle atrophy models, and native FST315 did not. In rats, intravenous follistatin left the circulation biexponentially and accumulated in liver ^[16]. The initial serum half-life was 4.0 minutes and the terminal serum half-life was 130.8 minutes ^[16].

ACE-083 is the closest analogue to an injected follistatin protein ever given to humans. It is a follistatin-based Fc fusion rather than follistatin itself. In a randomized, placebo-controlled human phase 1 in 58 healthy postmenopausal women, intramuscular ACE-083 raised rectus femoris volume by up to 14.5% ^[17]. No significant changes in mean muscle strength were observed. In a randomized human phase 2 in facioscapulohumeral muscular dystrophy, total muscle volume rose 16.4% against placebo in the biceps group ^[18]. There were no consistent improvements in functional or patient-reported measures. In a randomized human phase 2 in Charcot-Marie-Tooth disease, muscle volume rose significantly and ankle dorsiflexion strength differed from placebo ^[19]. Fat fraction and all other functional outcomes were not significantly improved. The authors concluded that the Charcot-Marie-Tooth patients treated in the tibialis anterior did not show greater functional improvement than those receiving placebo ^[19].

The Best-Controlled Tests of the Mechanism Are Negative on Function

Bimagrumab blocks activin receptor IIB, the axis follistatin acts on. In a randomized human trial in COPD with low muscle mass, thigh muscle volume rose significantly against placebo at every timepoint measured ^[20]. Six-minute-walk distance did not increase significantly in either group. Domagrozumab, an anti-myostatin antibody, was tested in a randomized, placebo-controlled human trial in 120 boys with Duchenne muscular dystrophy ^[21]. The difference in mean change in four-stair-climb time at week 49 was 0.27 seconds, p = 0.94. There were no significant between-group differences in any secondary clinical endpoint. Increases in muscle volume were non-significant in that trial as well. Taldefgrobep alfa, an anti-myostatin adnectin, raised thigh muscle volume in healthy adult volunteers, and its Duchenne program delivered no functional benefit and was discontinued ^[22]. All three programs were industry-sponsored and reported null functional results against their sponsors' own commercial interest. That provenance makes them more reliable rather than less. The same split appears in animal work. In Pompe mice, AAV8-follistatin raised muscle mass and grip strength only in mice treated before 10 months of age ^[23]. Maximal strength gains in those mice required concurrent GAA enzyme therapy to clear glycogen first ^[23]. Follistatin treatment failed to improve rotarod performance in those mice regardless of age. Expression of the myostatin pathway is downregulated in human muscle-wasting disease, with myostatin and activin receptor decreased and follistatin itself increased ^[24]. The authors suggest this may explain the poor clinical efficacy of anti-myostatin approaches.

Product Identity and Regulatory Status

Reichel and colleagues analyzed 17 black-market products labeled follistatin 344, and only nine contained follistatin at all ^[25]. Some of the others held different growth-promoting peptides, with MGF and GHRP-2 named. All nine follistatin-containing products carried His-tagged FS344 with a high degree of its oligomers. Tagged, aggregated material is not the molecule any published study administered. Aggregated protein carrying a foreign polyhistidine tag also raises an immunogenicity concern. Overexpressing wild-type follistatin in mice greatly increased muscle mass, whereas an N-terminus-deleted construct had no effect on muscle mass in that same animal work ^[26].

Follistatin is not an approved drug anywhere. The FDA's public approval and labeling databases hold no record for it. Its highest recorded stage of development is phase 1/2. The World Anti-Doping Agency's 2026 Prohibited List covers it under section S4.3, "Agents Preventing Activin Receptor IIB Activation", which names follistatin among prohibited myostatin-binding proteins. It is prohibited at all times, in and out of competition.

Limitations and Conclusion

The follistatin evidence base is thin in a specific way. The five ClinicalTrials.gov studies using follistatin itself as the intervention tested a gene rather than a protein. The investigator-led trials were run in rare muscle disease and delivered by intramuscular viral vector. The best-controlled human trials of the pathway it acts on are negative on function. The one preclinical study characterizing native follistatin as a candidate injectable called the molecule poorly suited to that purpose ^[15]. Muscle volume and muscle function have diverged repeatedly across this field, so a mass measurement is not a functional result. Researchers sourcing Follistatin 344 should treat isoform designation and material identity as open questions.

Frequently Asked Questions

Is Follistatin 344 a peptide?

No. Follistatin is a secreted glycoprotein encoded by the human FST gene, and the authoritative identifier is UniProt accession P19883. PubChem's compound database returns no record for it, so no CAS number applies.

What is the difference between FS-344 and FS-315?

The number 344 is a precursor length that includes a 29-residue signal peptide ^[1]. Once that peptide is cleaved, the FST344 transcript yields the mature 315-residue protein FS-315. They are one gene product at two stages of processing, not two competing versions.

Has follistatin protein itself been given to humans in a registered trial?

None of the five ClinicalTrials.gov studies using follistatin itself as the intervention administers it as an injected protein, since each delivers the FST344 gene ^[7]. The closest human test of an injected follistatin-derived protein is ACE-083, an Fc fusion rather than follistatin ^[17][18][19]. It raised muscle volume in randomized placebo-controlled human trials without consistent functional improvement.

Have the positive gene therapy results been independently replicated?

No independent group has replicated the positive follistatin gene therapy findings. Both positive human papers come from a single center and investigator group ^[10][11]. An independent commentary in the same journal argued the inclusion body myositis functional claims were unfounded ^[12].

References

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