Two research compounds chase the same target and share almost nothing else. MID-35 is a 16-residue synthetic peptide of about 2.4 kDa. FLGR242 is an engineered follistatin protein. One carries a decade of peer-reviewed academic work behind it and no clinical data at all. The other carries a set of manufacturer specifications and no peer-reviewed record. Comparing them fairly means being clear about which claims rest on what.
This page sets out what each molecule is, how each blocks myostatin, where the two diverge on selectivity, delivery and evidence quality, and what neither one has. Kimera supplies MID-35 for laboratory research and does not sell FLGR242, which appears here for comparison only.
The Shared Target: Myostatin
Myostatin, also called growth differentiation factor 8 (GDF-8), belongs to the TGF-β superfamily and functions as the best-characterized negative regulator of skeletal muscle mass. It binds activin type II receptors (ACVR2 and ACVR2B), which recruit type I receptors (ALK4/ALK5) and phosphorylate Smad2/3. The activated complex suppresses the growth program and switches on the atrophy-associated ubiquitin ligases Trim63 (MuRF-1) and Fbxo32 (atrogin-1). Animals carrying loss-of-function myostatin mutations show the double-muscled phenotype.
One species fact governs how far any rodent result travels. Circulating myostatin runs 4- to 18-fold lower in monkeys, rats and humans than in mice, while activin A runs 3- to 4-fold higher in primates. Blocking myostatin alone grew mouse tibialis anterior by 20.1%; blocking activin A alone produced a non-significant 5.0%. In cynomolgus monkeys, neither antibody alone did much and only the combination worked. Mice run on myostatin. Primates run on both. Hold that in mind for the selectivity section, because it is the axis on which these two compounds differ most consequentially.
MID-35: A Minimal Synthetic Peptide
MID-35 stands for myostatin inhibitory D-peptide-35. It is a 16-mer of 2350.99 g/mol with the sequence lrxkrwirxkiwriyw-amide, where lowercase marks D-configuration residues and x marks D-cyclohexylglycine at positions 3 and 9. Every residue is D-form and the sequence runs reversed relative to its parent. That is a retro-inverso design: reversing the order and inverting every stereocenter approximately reproduces the parent’s side-chain arrangement in space while presenting a backbone mammalian proteases cleave poorly.
The molecule is the endpoint of roughly a decade of chain-shortening work that began with a 23-residue fragment of the mouse myostatin prodomain and an IC₅₀ near 3.56 µM. Structure-activity work produced a 16-mer designated MIPE-1686, and retro-inversion of that peptide plus two arginine substitutions produced MID-35 (Takayama et al., 2022). The program originated at Tokyo University of Pharmacy and Life Sciences, and the scaffold patent names Hayashi, Takayama and Negishi as inventors.
Mechanistically, MID-35 binds the ligand rather than the receptor, docking at the surface mature myostatin uses to engage its type I receptor. In mice, a single intramuscular injection into the tibialis anterior produced a treated muscle 133 ± 10% of the contralateral saline-injected muscle at day 28. Follow-up work in 2026 traced the timeline: differentiation markers rose and atrophy markers fell within three days, measurable weight gain appeared around day 14, and the effect was sustained to day 84 (Morito et al., 2026).
The growth signature is regenerative rather than simple swelling. Centrally nucleated fibers, the marker of tissue that has been broken down and rebuilt, reached roughly 35% in treated young muscle at day 84 against roughly 5% in the saline limb. Pax7-positive satellite cell signal rose. Worth stating plainly: a sustained effect after one injection does not mean the peptide persisted for three months. The compound triggers remodeling and the remodeled tissue stays remodeled. No study has measured how long MID-35 itself lasts anywhere.
Additional published work covers a cancer cachexia model, where MID-35 combined with the ghrelin receptor agonist anamorelin improved food intake, grip strength and survival beyond either agent alone, and a delivery study using iontophoresis to drive the peptide transdermally into muscle (Michiue et al., Pharmaceuticals, 2023). The full picture is set out in the MID-35 research profile.
FLGR242: An Engineered Follistatin Construct
FLGR242 starts from the opposite end. Rather than shrinking a natural inhibitor to its minimum, it takes a natural inhibitor and engineers around it.
Follistatin is a secreted glycoprotein that binds and neutralizes several TGF-β superfamily ligands, myostatin among them, by sequestering the growth factor so it never reaches its receptor. The gene produces two principal isoforms. FST-317 lacks a heparin-binding domain and circulates. FST-344 is processed to FST-315 and carries the heparin-binding region, which tethers it to cell surfaces. That distinction matters for any construct claiming a defined circulating profile, because the parent isoform’s natural behavior is tissue association rather than long circulation.
According to its developer, FLGR242 is a fragmented, modified version of FST-344 carrying two engineered features. First, it is altered so that it no longer binds activin. Natural follistatin grabs activin as well as myostatin, and activin binding brings broader effects. Second, the protein carries an albumin-binding element, described as a hydrophilic glycine-serine linker gripping serum albumin at sub-20 nM affinity, with a claimed resulting half-life of up to roughly 19 days.
Two points about the evidence deserve emphasis. Follistatin’s general biology as a myostatin antagonist rests on solid ground and decades of independent work. FLGR242’s specific engineering claims, meaning the activin-sparing selectivity and the 19-day half-life, come from its manufacturer. No independent peer-reviewed characterization has confirmed either. The compound reached the research market only in late 2025.
The Follistatin Precedent Most Comparisons Omit
A follistatin-based myostatin trap has already been through two randomized placebo-controlled phase 2 trials in humans. Anyone weighing FLGR242 should know how that went.
ACE-083 is a modified human follistatin engineered as a ligand trap for myostatin, activin A and related negative regulators, designed for local intramuscular action. In a phase 1 study in healthy volunteers, injected muscle volume rose while strength did not, and serum drug was detectable in only 14 of 252 samples. Two phase 2 trials followed, one in facioscapulohumeral muscular dystrophy and one in Charcot-Marie-Tooth disease.
Both hit their muscle-volume endpoints convincingly. In the muscular dystrophy trial, treated muscle volume rose 14.20% against 5.46% for placebo in the lower limb and 20.41% against 1.07% in the upper arm, with isolated biceps strength rising 32.58% against a 3.54% decline. In the neuropathy trial, total muscle volume rose 13.5% over placebo and contractile volume 23.3%. Then every functional endpoint failed in both trials: walking distance, walk and run time, stair climb, upper-limb performance, quality of life, and fall risk. Anti-drug antibodies appeared in 39.3% and 47.8% of treated patients across the two studies. Both programs were terminated (Statland et al., 2022; Thomas et al., 2022).
ACE-083 is not FLGR242. It is a different construct with different engineering and a local rather than systemic design. It is, however, the closest thing to a clinical readout that follistatin-based myostatin trapping has produced, and the readout was muscle without function plus substantial immunogenicity. A recombinant protein carrying engineered non-native elements is an immunogenicity question by construction, and that is the specific risk a synthetic 16-mer does not share in the same form.
Mechanism: Cap the Ligand or Mop It Up
Both compounds stop myostatin from signaling, and neither touches the receptor. MID-35 caps myostatin at its type I receptor interface, a direct competitive block at a defined surface with six myostatin residues reported as critical to the interaction. FLGR242 behaves as follistatin does, wrapping and neutralizing the ligand so it never reaches ActRIIB at all.
Neither binds ActRIIB directly, which matters for one specific safety mechanism. ActRIIB also binds BMP9 and BMP10 and regulates vascular function through Smad1/5/8, and that cross-inhibition is the mechanism to which the ACE-031 bleeding events in Duchenne trials have been attributed. Agents acting at the ligand rather than the receptor should not engage it. That is a mechanistic argument for both compounds, not a safety dataset for either.
Selectivity: The Sharpest Difference on Paper
The two compounds make opposite claims here, and only one side has published numbers.
MID-35 is often labeled a selective myostatin inhibitor. Its own primary literature does not support that. Dose-response data from a single assay run give myostatin 0.19 µM, GDF-11 0.63 µM, activin A 0.89 µM and TGF-β1 1.6 µM. The four values span roughly eightfold. A compound whose weakest measured target sits within an order of magnitude of its strongest is a broad TGF-β family inhibitor with a preference.
The activin A figure is contested. A separate 2022 paper, using a Smad-responsive reporter in HepG2 cells at a single 3 µM concentration, reported that MID-35 did not inhibit activin A. That concentration sits more than threefold above the 0.89 µM IC₅₀ reported elsewhere, so the assay should have detected inhibition. The author groups overlap and no third measurement exists. Different cell backgrounds are the only proposed explanation and nobody has tested it.
FLGR242’s central design claim is the mirror image: engineered specifically not to bind activin. No published dose-response data support it. The claim is plausible, since follistatin’s activin and myostatin binding surfaces are distinguishable and separating them is a recognized engineering goal, but plausible is not measured.
Read against the primate species data above, the two positions carry different implications. Activin-sparing selectivity is attractive on safety grounds and unattractive on efficacy grounds in primates, where activin A does much of the work. Broad blockade is the reverse. Neither compound has the data to settle which trade its design actually makes in a non-rodent species.
Local Versus Systemic: The Practical Divide
This is the difference most likely to matter in an actual experiment, and it is rarely stated.
Every published MID-35 in vivo result comes from local delivery into a specific muscle, either by direct intramuscular injection or by iontophoresis into the tissue underneath. No intravenous, subcutaneous, intraperitoneal or oral study has ever been published. Every study uses the same internal control, compound into the left tibialis anterior and saline into the right in the same animal, and across young, adult and aged mice out to 84 days the saline limb held flat on every readout. The compound acts where it is put, and there is no published evidence it does anything anywhere else.
FLGR242’s entire design premise is the opposite. An albumin-binding element exists to keep a molecule in circulation, and a 19-day half-life claim is a claim about systemic exposure. The two compounds are therefore not substitutes for one another in an experimental design. One is a local tool with a local evidence base. The other is positioned as a systemic agent whose systemic behavior has not been independently measured.
Stability: Two Different Tricks, One Common Error
MID-35 resists degradation through chemistry. Reported figures are 99% and 97% intact after 400 minutes in bovine trypsin and α-chymotrypsin, and related work on the parent 16-mer series measured 14 days intact in human serum. FLGR242 is designed to endure through pharmacokinetics instead, with albumin binding slowing clearance.
The common error is treating either mechanism as a half-life. Protease resistance measured in an isolated enzyme solution is a stability figure. The class literature contains documented cases where D-substitution delivered no in vivo benefit at all: dermorphin analogues with extra D-residues were cleaved faster than the parent, and D-substituted GRF(1-29)-amide analogues showed no significant plasma half-life difference in rats. The clearest payoff of the strategy in an approved drug is octreotide, which went from minutes to roughly 1.5 hours. Hours, not days.
On the other side, an albumin-binding half-life claim of 19 days is a specific quantitative assertion that would normally require published pharmacokinetic data in a named species by a named route. That data has not been published for FLGR242. Neither compound has a measured half-life in the peer-reviewed record.
Size, Production and Analytical Characterization
The scale gap drives everything downstream. MID-35 is made by solid-phase peptide synthesis, which specifies every residue and every stereocenter exactly. FLGR242 is a recombinant protein grown in an expression system and then modified, which means batch-to-batch glycosylation, host-cell protein carryover and higher-order structure all become quality attributes.
For MID-35, the characterization trap is that purity and content are different measurements. HPLC purity reports the fraction of peptide-related material in the main peak and says nothing about how much peptide is in the vial. MID-35 carries seven basic sites, four D-arginines, two D-lysines and a free N-terminus, and peptides purified in trifluoroacetic acid systems isolate as TFA salts with counterion loading tracking basic site count. At typical loading, net peptide content lands near 80 to 87% of vial mass. A further trap is stereochemistry: L to D substitution is mass-silent, so mass spectrometry alone cannot confirm that a D-peptide was synthesized correctly.
For a recombinant protein the questions are entirely different, and identity confirmation by peptide mapping or intact mass with glycan analysis is not comparable work. A certificate for one tells you very little about what a certificate for the other should contain. Every Kimera lot ships with third-party COA verification and lot-specific documentation is archived in the public COA archive, to be read off the certificate itself rather than from a catalog-level claim.
Anti-Doping Status Is Not Symmetric
Both compounds fall under the 2026 WADA Prohibited List, but by different routes, and the difference is worth knowing.
Section S4.3 covers agents preventing activin receptor IIB activation, and its myostatin examples name myostatin-binding proteins including follistatin explicitly. A follistatin-derived construct is therefore squarely inside the named examples. MID-35 is not named anywhere on the List, and as a synthetic 16-mer peptide it matches none of the three listed example categories literally. It is caught by the non-exhaustive phrasing of S4.3 and unambiguously by S0, Non-Approved Substances. Both are prohibited. Only one is named.
Doping control laboratories have published detection methodology covering the MID-35 peptide series, and that work documented substantial analytical losses from nonspecific binding across this peptide class. Method-development researchers use MID-35 as a reference analyte.
Comparison at a Glance
| Attribute | MID-35 | FLGR242 |
|---|---|---|
| Modality | Synthetic 16-mer retro-inverso D-peptide | Recombinant engineered follistatin construct |
| Size | 2350.99 g/mol | Protein scale, developer-defined |
| Parent | Mouse myostatin prodomain, via MIPE-1686 | Follistatin-344 |
| Mechanism | Caps myostatin at its type I receptor interface | Sequesters the ligand, as follistatin does |
| Selectivity | Measured. Broad: GDF-11, activin A and TGF-β1 all within roughly eightfold. Activin A result contested | Claimed activin-sparing. No published dose-response data |
| Stability strategy | D-configuration and reversed backbone resist proteolysis | Albumin binding slows clearance |
| Measured half-life | None published | None published; 19 days claimed by developer |
| Delivery in the evidence base | Local intramuscular and iontophoresis only | Designed for systemic exposure |
| Production | Solid-phase synthesis | Recombinant expression plus modification |
| Immunogenicity profile | No data. Small synthetic peptide | No data. Recombinant protein with non-native elements |
| Evidence base | Five indexed papers naming the compound, peer reviewed, cells and rodents | Manufacturer characterization |
| Named on the WADA List | No, caught by non-exhaustive language and S0 | Follistatin-binding proteins named explicitly under S4.3 |
| Human data | None | None |
What Neither Compound Has
The similarities in the gaps are as informative as the differences in the designs.
- No human data. Neither compound has been administered to a human in any published or registered study.
- No published pharmacokinetics. No half-life, clearance or bioavailability in any species by any route for either molecule.
- No toxicology. No LD50, acute, repeat-dose, genotoxicity or safety pharmacology study for either. The only non-muscle tissue ever measured in a MID-35 study is heart weight relative to body weight, which was unchanged.
- No immunogenicity data for either compound in any system.
- No validated bioanalytical assay for either in serum, plasma or urine.
- No approval anywhere. The only agent in the myostatin pathway approved in any jurisdiction is luspatercept, and it is approved for anaemia rather than for muscle.
The class record is worth reading alongside both. Across myostatin-pathway agents that reached clinical trials, muscle mass has repeatedly increased without proportional functional benefit, and the majority of programs were discontinued. Follistatin biology carries its own open question on bone, since follistatin overexpression has been associated with fracture and the GDF-11 that these agents also block promotes osteogenesis. Neither compound has been evaluated on that endpoint.
Adjacent compound profiles are collected in the peptides research library, and the receptor-side comparator in this pathway is ACE-031.
Frequently Asked Questions
What is the main difference between MID-35 and FLGR242?
Size and evidence. MID-35 is a 2.35 kDa synthetic D-peptide with five peer-reviewed papers naming it. FLGR242 is a recombinant engineered follistatin protein characterized by its manufacturer. They also differ in intent: MID-35’s entire evidence base is local intramuscular delivery, while FLGR242 is designed for systemic circulation.
Do MID-35 and FLGR242 work the same way?
Both prevent myostatin from reaching its receptors, and neither binds the receptor itself. MID-35 caps myostatin’s type I receptor interface. FLGR242 sequesters the whole ligand the way natural follistatin does.
Which one is more selective?
FLGR242 claims to be, and MID-35 measurably is not. MID-35’s published data show inhibition of GDF-11, activin A and TGF-β1 within roughly eightfold of its myostatin value, though the activin A result is disputed between two papers. FLGR242’s activin-sparing claim has no published dose-response data behind it.
Why does MID-35 use D-amino acids?
Because mammalian proteases cleave D-configured backbones poorly. A retro-inverso design reverses the sequence and inverts every stereocenter, which approximately preserves the side-chain arrangement while defeating proteolysis. Piecemeal D-substitution was tried first in this program and mostly reduced activity, which is why full retro-inversion was used.
Has anyone independently confirmed FLGR242’s 19-day half-life?
No. That figure comes from the developer and reflects its albumin-binding design rather than an independent peer-reviewed pharmacokinetic study.
Has a follistatin-based myostatin trap ever been tested in humans?
Yes. ACE-083, a modified human follistatin ligand trap, completed two randomized placebo-controlled phase 2 trials. Both significantly increased injected-muscle volume and both failed every functional endpoint, with anti-drug antibodies in 39.3% and 47.8% of treated patients. Both programs were terminated. ACE-083 is a different construct from FLGR242, but it is the closest clinical readout the approach has produced.
Research Use Only
Kimera supplies MID-35 strictly for laboratory research and analytical use. It is not a drug, food, cosmetic, or dietary supplement, it is not for human or veterinary use, and it must not be administered to humans or animals. FLGR242 is discussed here for comparison and is not sold by Kimera. No statement here has been evaluated by the FDA, and nothing here describes or implies a therapeutic use. Sales are limited to qualified research professionals and institutions.
References
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Morito K, Nishikawa N, Hitachi K, et al. Myostatin inhibitory D-peptides induce skeletal muscle hypertrophy along with alteration of bioactive sphingolipid metabolism. ACS Pharmacol Transl Sci. 2026;9(6):1544–1553. doi:10.1021/acsptsci.6c00124
Hanada K, Fukasawa K, Hinata H, et al. Combination therapy with anamorelin and a myostatin inhibitor is advantageous for cancer cachexia in a mouse model. Cancer Sci. 2022;113(10):3547–3557. doi:10.1111/cas.15491
Michiue K, Takayama K, Taniguchi A, Hayashi Y, Kogure K. Increasing skeletal muscle mass in mice by non-invasive intramuscular delivery of myostatin inhibitory peptide by iontophoresis. Pharmaceuticals. 2023;16(3):397. doi:10.3390/ph16030397
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Pearsall RS, Davies MV, Cannell M, et al. Follistatin-based ligand trap ACE-083 induces localized hypertrophy of skeletal muscle with functional improvement in models of neuromuscular disease. Sci Rep. 2019;9(1):11392. doi:10.1038/s41598-019-47818-w
Glasser CE, Gartner MR, Wilson D, Miller B, Sherman ML, Attie KM. Locally acting ACE-083 increases muscle volume in healthy volunteers. Muscle Nerve. 2018;57(6):921–926. doi:10.1002/mus.26113
Statland J, Campbell C, Desai U, et al. Randomized phase 2 study of ACE-083, a muscle-promoting agent, in facioscapulohumeral muscular dystrophy. Muscle Nerve. 2022;66(1):50–62. doi:10.1002/mus.27558
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Suh J, Lee YS. Myostatin inhibitors: panacea or predicament for musculoskeletal disorders? J Bone Metab. 2020;27(3):151–165. doi:10.11005/jbm.2020.27.3.151
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World Anti-Doping Agency. The Prohibited List 2026, S4. Hormone and Metabolic Modulators. wada-ama.org/en/prohibited-list
