MID-35: Myostatin Inhibitory D-Peptide-35 — Structure, Pharmacology, and Preclinical Record

Myostatin inhibition has produced more disappointment than approved medicine. Antibodies, receptor traps, and ligand scavengers have all moved muscle mass in animals; almost none have converted that into durable functional benefit in humans. Against that backdrop, MID-35 is interesting for a narrower reason than the marketing usually suggests. It is not a better drug candidate than the antibodies. It is a demonstration that a 16-residue synthetic peptide, built entirely from D-amino acids, can hold the myostatin-blocking activity of a much larger endogenous protein domain and survive proteolysis long enough to act.

This article covers what the primary literature actually establishes about the MID-35 peptide: where the sequence came from, what its potency numbers mean, where its selectivity breaks down, and which analytical questions matter most when characterizing a lot.

What MID-35 Is

MID-35 (myostatin inhibitory D-peptide-35) is a 16-mer retro-inverso peptide first reported by Takayama and colleagues in 2022 in ACS Medicinal Chemistry Letters. Its published sequence is lrxkrwirxkiwriyw-amide, where lowercase letters denote D-configuration residues and x denotes D-cyclohexylglycine (D-Chg), an unnatural residue occupying positions 3 and 9. The chain carries a free N-terminus and a C-terminal amide.

Calculated from that published sequence, the free base is:

Two details in that table are worth pausing on, because vendor listings across this category get both wrong routinely. First, several sources circulate a molecular weight near 2349.5; recomputing residue-by-residue from the published sequence with a C-terminal amide gives 2350.99. Second, the compound has no assigned CAS number — any listing that supplies one is supplying something invented.

The Biology: Myostatin as a Brake

Myostatin, also called growth differentiation factor 8 (GDF-8), is a TGF-β superfamily member and the best-characterized negative regulator of skeletal muscle mass. McPherron, Lawler, and Lee established the phenotype in 1997: mice lacking the gene develop dramatically enlarged muscles.

Mature myostatin signals by binding activin type II receptors (ACVR2 and ACVR2B), which recruit type I receptors (ALK4/ALK5) and phosphorylate Smad2/3. The activated complex translocates to the nucleus and suppresses the transcriptional program supporting muscle growth, while promoting the atrophy-associated E3 ubiquitin ligases atrogin-1 (Fbxo32) and MuRF-1 (Trim63). Blocking that axis is the shared logic behind every agent in the class — including the ACE-031 decoy receptor, which intercepts ligands at the receptor rather than at the ligand surface.

Myostatin is synthesized as a latent precursor. The prodomain remains associated with the mature dimer and keeps it inactive until proteolytic release. That prodomain is the natural inhibitor — and it is the sequence source MID-35 was carved from.

Design Lineage: From a Protein Domain to 16 Residues

MID-35 did not appear fully formed. It is the endpoint of roughly a decade of chain-shortening work.

2015 — the minimum prodomain fragment. Takayama et al. identified minimum peptides of 24 and 23 residues from the mouse myostatin prodomain capable of inhibiting human myostatin. The 23-mer was a genuine but weak inhibitor, with a reported IC₅₀ near 3.56 µM. Because murine and human myostatin are identical in mature sequence, the mouse-derived fragment was a legitimate starting point for a human target.

2019 — chain shortening to a 16-mer. Structure–activity work produced peptide 8a, later designated MIPE-1686: a 16-mer with three unnatural residues and a β-sheet propensity, roughly twice as potent as the 22-mer it descended from, with a reported IC₅₀ near 0.13 µM. Intramuscular injection into mdx mice — a Duchenne muscular dystrophy model — increased tibialis anterior mass by roughly 14% over saline controls at day 42 and improved hindlimb grip strength.

2020 — the stability problem. MIPE-1686 held up well against aminopeptidase N, chymotrypsin C, and trypsin 3 in isolated-enzyme work, but a linear L-peptide with an unprotected N-terminus remained a liability for in vivo development.

2022 — the retro-inverso solution. The group synthesized a panel of retro-inverso versions of the 16-mer. The retro-inverso strategy reverses sequence direction and inverts every stereocenter to D; the result approximately reproduces the parent’s side-chain topology while presenting a backbone that mammalian proteases are poorly equipped to cleave. MID-35 emerged as the arginine-containing member of that panel, carrying four D-Arg residues and two D-Chg residues.

In the 2022 head-to-head, MID-35 was reported at an IC₅₀ near 0.19 µM against myostatin, against roughly 0.26 µM for MIPE-1686 measured alongside it. Related analogs (MID-36, MID-39) mapped the size floor: below roughly 15 residues, potency in the 0.2–0.3 µM band was not reachable.

A caution worth stating plainly, because it gets mangled constantly in secondary write-ups: the 0.13 µM figure for MIPE-1686 from 2019 and the 0.26 µM figure from 2022 are the same peptide in different assay runs. Cross-paper IC₅₀ comparisons in this series are not meaningful. Only the within-paper comparison is.

Selectivity: Where the “Selective Myostatin Inhibitor” Label Fails

MID-35 is frequently described as a selective myostatin inhibitor. The primary literature does not support that description.

In the cachexia work published by Hanada and colleagues in Cancer Science (2022), HepG2 cells carrying an (SBE)₄-luc Smad-responsive reporter were stimulated with myostatin, GDF-11, activin A, or TGF-β after preincubation with MID-35 or the ALK4/5/7 kinase inhibitor SB-431542. MID-35 suppressed TGF-β- and GDF-11-induced reporter activity in addition to myostatin-induced activity. SB-431542 suppressed all ligands tested.

Two implications follow. Structurally, this is unsurprising: GDF-11 shares roughly 90% mature-domain identity with myostatin and uses the same receptor machinery, so a peptide binding the myostatin ligand surface has an obvious route to cross-reactivity. Practically, any experimental design attributing an MID-35 effect specifically to myostatin blockade — rather than to broader Smad2/3 pathway suppression — needs its own selectivity controls.

A second nuance from the same paper deserves attention. When Smad2 phosphorylation was measured directly in C2C12 cells, MID-35 produced only a marginal decrease, while SB-431542 abolished it. A ligand-binding peptide and a receptor-kinase inhibitor are not interchangeable tools, and reporter-level inhibition does not automatically translate to proportional suppression of the immediate phosphorylation event.

Note that several secondary sources — including an earlier version of this page — describe MID-35 as acting on “GDF-11 and activin A.” The primary result names TGF-β and GDF-11. That correction matters for anyone designing around the compound.

Preclinical In Vivo Record

All published in vivo work is rodent, and essentially all of it is local intramuscular administration rather than systemic dosing.

Single-injection hypertrophy. In the 2022 report, a single intramuscular injection into the mouse tibialis anterior (30 nmol) produced roughly a 1.3-fold increase in muscle weight at 28 days, with in vivo potency significantly exceeding MIPE-1686 — a separation the authors attributed to prolonged myostatin inactivation from the protease-resistant backbone rather than to greater intrinsic potency.

Kinetics and durability. The most informative pharmacology arrived in 2026, when Morito and colleagues published a detailed time-course in ACS Pharmacology & Translational Science across young, adult, and aged mice. Differentiation markers (Pax7, Myod1, Myog) rose and atrophy markers (Trim63, Fbxo32) fell robustly within 3 days. Measurable weight gain did not appear until day 14. Most notably, the hypertrophy was sustained for 12 weeks after treatment. Increased centralized nuclei and Pax7-positive signal indicated satellite-cell-associated regeneration rather than simple fiber swelling.

The sphingolipid finding — and the aging signal. The same study tracked sphingosine 1-phosphate (S1P), a bioactive sphingolipid implicated in muscle mass regulation. S1P rose significantly at day 3 in young and adult mice, consistent with a role in satellite cell activation. In aged mice, that increase did not occur. Since sarcopenia is the therapeutic rationale most often attached to this compound, a mechanism that partially fails in aged tissue is the single most consequential open question in the file.

Cancer cachexia. In tumor-bearing mice, intramuscular MID-35 alleviated skeletal muscle atrophy and outperformed the earlier prodomain-derived peptide-2. Combined with anamorelin — a ghrelin receptor agonist, mechanistically adjacent to growth hormone secretagogues such as ipamorelin — the pairing increased food intake, maximized grip strength, and extended survival relative to either agent alone. Appetite/GH-axis stimulation and direct ligand blockade appear to address non-overlapping arms of the wasting phenotype.

Delivery research. Because intramuscular injection is invasive, Michiue and colleagues tested iontophoresis — transdermal delivery driven by weak electric current (0.34 mA/cm²) — in Pharmaceuticals (2023). Fluorescently labeled peptide reached at least 1000 µm into muscle from the skin surface. Three applications of MID-35 (75 nmol, days 0/7/14) produced a statistically significant 1.25-fold tibialis anterior mass increase at day 42. The comparison is honest in the source: the iontophoresis dose was substantially higher than the 30 nmol used for direct injection to reach a similar effect size. Gastrocnemius weight did not change significantly.

Analytical Considerations: Purity Is Not Content

This is where most peptide characterization in this category quietly falls apart, and where MID-35 is a particularly good illustration.

Purity and content are different measurements. HPLC purity reports the fraction of peptide-related material represented by the main peak. It says nothing about how much peptide is actually in the vial. A lot can be 98% pure by HPLC and still be substantially less than 98% peptide by mass, because the remainder is counterion, residual water, and residual solvent.

MID-35 makes this gap unusually wide. The molecule carries seven basic sites — four D-Arg, two D-Lys, and a free N-terminus. Peptides purified by reverse-phase HPLC in trifluoroacetic acid systems isolate as TFA salts, with counterion loading tracking basic site count. At a typical 3–5 TFA equivalents, net peptide content lands somewhere near 80–87% of vial mass. A 10 mg nominal vial characterized only by HPLC purity may contain closer to 8–8.7 mg of actual peptide. For a quantitative inhibition assay, that is the difference between a clean IC₅₀ and an unexplained rightward shift.

Orthogonal methods resolve it. HPLC establishes chromatographic purity. Mass spectrometry confirms identity against the calculated free base of 2350.99 Da — and note that observed salt-form mass will read higher, which is expected rather than a red flag. Elemental analysis provides an independent quantification anchor for net peptide content that chromatography cannot supply. Sequence-level confirmation matters especially here, since D-amino acid content and the two unnatural D-Chg positions are not verifiable by mass alone; L/D substitution is mass-silent, and a peptide synthesized with the wrong stereochemistry weighs exactly the same as the right one while being pharmacologically inert against a protease-rich environment.

Every Kimera lot ships with third-party COA verification, and lot-specific documentation is archived in the public COA archive. Purity figures should always be read against the specific lot rather than a catalog-level claim.

Handling. Treat MID-35 as a standard lyophilized research peptide: store the dry solid frozen, protect from moisture and light, and minimize freeze–thaw cycles. Reconstitution and storage protocols should be validated in-house against the intended assay.

Anti-Doping and Regulatory Context

Agents that block the activin receptor signaling pathway are prohibited in sport. WADA’s Prohibited List addresses them under S4.3, “Agents preventing activin receptor IIB activation,” which explicitly covers myostatin-binding proteins and myostatin-neutralizing antibodies, and which — like most List categories — is written with non-exhaustive example language.

Doping-control laboratories have responded accordingly. Walpurgis and colleagues published detection methodology for myostatin inhibitory peptides in Drug Testing and Analysis (2023), with the Takayama peptide series explicitly in scope. For method-development researchers, MID-35 functions as a reference analyte; the same work documented significant analytical losses from nonspecific binding to serum proteins and sample tubes across this peptide class, which is itself a useful methodological warning for anyone building quantitative assays.

What the Evidence Does Not Show

Stating the limits precisely is more useful than repeating the positive findings:

• No human data of any kind. There are no clinical trials of MID-35. The entire record is cell-based assays and rodent models.
• Local, not systemic. Efficacy has been demonstrated by direct intramuscular injection and by transdermal iontophoresis into the tissue underneath. No published work establishes systemic exposure producing whole-body muscle effects.
• Class-level translation has been poor. Myostatin pathway inhibitors have repeatedly increased lean mass in humans without delivering proportional functional benefit; the anti-myostatin antibody LY2495655 and the broader dystrophy program are the standard cautionary references. Muscle mass and muscle function are not the same endpoint.
• Selectivity is incomplete, as described above.
• The aged-tissue signal is unresolved. The S1P response that accompanies hypertrophy in young and adult animals was absent in aged mice.

Researchers comparing modalities in this space may find the broader peptides research library useful for adjacent compound profiles.

Research Use Only

MID-35 is supplied 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 must not be administered to humans or animals. 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

Ojima C, et al. Peptide-2 from mouse myostatin precursor protein alleviates muscle wasting in cancer-associated cachexia. Cancer Sci. 2020;111(8):2954–2964. doi:10.1111/cas.14520

Takayama K, Hitachi K, Okamoto H, Saitoh M, Odagiri M, Ohfusa R, Shimada T, Taguchi A, Taniguchi A, Tsuchida K, Hayashi Y. Development of myostatin inhibitory D-peptides to enhance the potency, increasing skeletal muscle mass in mice. ACS Med Chem Lett. 2022;13(3):492–498. doi:10.1021/acsmedchemlett.1c00705

Morito K, Nishikawa N, Hitachi K, Tamaki R, Nishikawa M, Hayashi Y, Tsuchida K, Takayama K. 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, Imai S, Takayama K, Hirai H, Ohfusa R, Hayashi Y, Itoh F. 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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McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-β superfamily member. Nature. 1997;387(6628):83–90. doi:10.1038/387083a0

Walker RG, McCoy JC, Czepnik M, et al. Molecular characterization of latent GDF8 reveals mechanisms of activation. Proc Natl Acad Sci USA. 2018;115(5):E866–E875. doi:10.1073/pnas.1714622115

Walpurgis K, et al. Myostatin inhibitory peptides in sports drug testing. Drug Test Anal. 2023. doi:10.1002/dta.3473

World Anti-Doping Agency. The Prohibited List — S4. Hormone and Metabolic Modulators. wada-ama.org/en/prohibited-list

Golan T, Geva R, Richards D, et al. LY2495655, an antimyostatin antibody, in pancreatic cancer: a randomized, phase 2 trial. J Cachexia Sarcopenia Muscle. 2018;9(5):871–879. doi:10.1002/jcsm.12331

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