FDA just approved a myostatin antibody for muscle loss. Every follistatin muscle trial pepmg found delivered a gene, not the protein.

The approval

An antibody for spinal muscular atrophy, with a fracture warning

Isembyld is a fully human IgG4 monoclonal antibody that binds promyostatin and latent myostatin and inhibits myostatin activation, according to its label.[2] FDA approved it for spinal muscular atrophy in adults and children 2 and older who already receive an SMN2-targeted treatment, as an intravenous infusion of 10 mg/kg every 4 weeks.[1][2] Drugs@FDA lists it as BLA 761463, a new molecular entity approved under priority review, held by Scholar Rock.[3] An FDA notice on public inspection, scheduled for the October 26 Federal Register, awards Scholar Rock a rare pediatric disease priority review voucher for it.[4]

The evidence is one phase 3 trial, SAPPHIRE, which enrolled 188 patients aged 2 to 21 who could not walk; 156 aged 2 to 12 formed the primary analysis.[5] The published paper's two primary comparisons split: both doses combined beat placebo by 1.8 points on the 66-point Hammersmith motor scale (95% CI 0.30 to 3.32, p = 0.019), but 20 mg/kg alone did not (1.4 points, 95% CI −0.34 to 3.13, p = 0.11).[5][2] The label reports the 10 mg/kg arm instead: scores rose 1.0 point on the drug and fell 1.2 on placebo, a 2.2-point difference (95% CI 0.49 to 3.95) that the label calls "nominally statistically significant" (p = 0.0121), and 34.2% versus 13.5% of patients gained 3 points or more.[2] The label does not recommend 20 mg/kg because it "did not demonstrate additional efficacy."[2]

The label's only warning is fractures. In SAPPHIRE, 5 of 53 patients (9%) on 10 mg/kg had fractures, as did 3 of 53 (6%) on 20 mg/kg and 1 patient (2%) on placebo; three were femur fractures. In an ongoing open-label extension at 20 mg/kg, 22 patients (9%) had fractures, including 5 serious femur fractures. Most patients with fractures had a history of low bone density, earlier fractures or contractures, and some fractures had no clear trigger such as a fall.[2] In rats, the drug damaged the femoral head at every dose tested and impaired fertility at exposures below the human dose.[2]

Scholar Rock has also tested apitegromab alongside GLP2 TZ . In its phase 2 EMBRAZE trial, 102 adults with overweight or obesity took tirzepatide with apitegromab 10 mg/kg or with placebo; at 24 weeks the apitegromab group had lost 1.9 kg less lean mass, reported with an 80% confidence interval of 1.2 to 2.7 kg (p = 0.001), with similar total weight loss.[6] Eight of its 13 authors are or were Scholar Rock employees.[6] That use is not approved; the label covers spinal muscular atrophy only.[2]

The other myostatin blocker

Follistatin is a protein, and its human trials delivered a gene

Follistatin is a secreted protein that binds myostatin and inhibits the myostatin pathway, increasing skeletal muscle mass in preclinical studies.[7][22] It also acts on the pituitary activin-inhibin axis and can suppress follicle-stimulating hormone, which the Ohio trial team said "called for caution." They chose the FS344 variant, which is processed into FS315, a circulating form with 10-fold lower activin affinity than FS288.[7]

In the first trial, published in 2015, researchers at Nationwide Children's Hospital in Columbus, Ohio, injected an AAV1 virus carrying the FS344 gene into both quadriceps of six men with Becker muscular dystrophy, at two doses, with no control group. Four walked farther in six minutes, by 58, 125, 108 and 29 meters; two did not improve. The authors reported no adverse effects. Patients took prednisone from a month before injection to about a month after, to blunt an immune response to the virus.[8]

The 2017 trial treated both legs of six patients with sporadic inclusion body myositis, who also followed an exercise program and took prednisone 60 mg a day for about two months around the injection.[9] Annualized, their median six-minute walk distance rose 56.0 meters a year, against a 25.8 meter yearly decline in 8 untreated patients followed in the same clinic (p = 0.01); four improved by 58 to 153 meters and two by 5 to 23 meters. The authors found no adverse events related to the gene therapy; serious events included a concussion and torn hamstring from a fall in one patient and a biking accident and a fall in another.[9] The registry record lists 15 patients enrolled across both diseases, including three given a lower dose in one leg, and names safety, not walking distance, as its primary outcome.[12]

Steven Greenberg, a neurologist at Brigham and Women's Hospital and Harvard Medical School, called the functional claims "unfounded" in the same journal. He wrote that treated patients received "at least 4 potentially therapeutic interventions," including about 60 days of high-dose prednisone, a monitored exercise program and the placebo effect of taking part in a trial; that the comparator was aggregate walk-test data from 8 clinic patients; and that the "annualized 6MWT" was created post hoc.[10] Jerry Mendell, the lead author, replied that historical controls suit small rare-disease trials, that the controls were matched for age and severity, that prednisone was a safety-board request, and that exercise alone was unlikely to explain gains of more than 100 meters.[11]

A third trial gave a larger dose to the legs of three patients with Duchenne muscular dystrophy, of six planned. Its ClinicalTrials.gov results, posted in 2020, show no dose-limiting toxicity, one serious head injury from a fall, and none of the three walking farther at two years; one of three improved on the North Star ambulatory scale.[13] pepmg found no journal publication of it on PubMed.[18]

Three newer registrations deliver the gene by other means, all sponsored by companies and none with posted results. Minicircle reports finishing a 43-person single-arm study of an injected follistatin plasmid in healthy adults in Honduras in 2023, and a 14-person klotho-plus-follistatin plasmid pilot in April 2026.[14][15] Unlimited Biotechnology began a trial of AAV9-follistatin, alone or with a VEGF plasmid, in Honduras in June 2026, aiming to enroll 12 adults aged 35 to 75.[16]

The searches behind this note, run October 25, 2026, covered ClinicalTrials.gov (15 records with follistatin as an intervention term) and PubMed (34 records with follistatin in the title indexed as clinical trials). Apart from the gene trials, the records are mostly exercise, diet, drug or observational studies that measured follistatin in blood, plus one 2011 placebo-controlled hair-regrowth trial, run by the company that made the product, in which 26 people with male pattern baldness received a single intradermal injection of a cell-derived mixture that contained follistatin, KGF and VEGF.[17][18][19] pepmg found no trial that injected follistatin protein on its own into people.

Status

No approval, no compounding category, banned in sport

pepmg found no follistatin product in Drugs@FDA or FDA's NDC directory, both searched through openFDA on October 25, 2026.[27] Under section 503A, a pharmacy may compound from a bulk substance with a USP or NF monograph, one that is a component of an FDA-approved drug, or one on FDA's 503A bulks list.[24] That list has six entries, none of them follistatin, and follistatin is in none of FDA's three interim-policy categories in the list updated May 14, 2026, nor on FDA's safety-risk page, which also lists withdrawn nominations.[25][23][26] pepmg did not check USP monograph status.

WADA's 2026 Prohibited List names follistatin among myostatin-binding proteins under S4.3, prohibited at all times, and lists apitegromab among myostatin-neutralizing antibodies in the same section.[22]

The market

Vials labeled with a gene variant's name

In pepmg's index generated October 25, 2026, 11 of the 101 vendors with listings carried follistatin, 16 listings in all. Nine are 1 mg vials. Eight use the name "Follistatin 344", the variant whose gene the Ohio trials delivered, and six use "FLGR242", a name that returned no PubMed or ClinicalTrials.gov record when searched October 25, 2026.[28]

Doping-control researchers in Austria and Germany tested 17 black-market products, most labeled follistatin 344, and found follistatin in only nine; some of the others held growth-promoting peptides such as MGF and GHRP-2 . All nine follistatin products carried His purification tags, which the authors say distinguishes them from the body's own follistatin.[20] The same lab later found a product sold as ACE-031 that contained black-market follistatin 344 instead.[21] pepmg makes no claim about what is in any vendor's vial.

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