Semax: Structure, Mechanism and Research Context

Research Use Only. This article is written for laboratory and research professionals. The compounds discussed are supplied strictly for in-vitro laboratory research. They are not drugs, dietary supplements, or foods. They are not for human or veterinary use, and not for diagnostic or therapeutic use. Nothing here is medical, legal, or regulatory advice.

The short answer

Semax is a synthetic heptapeptide with the sequence Met-Glu-His-Phe-Pro-Gly-Pro. Its first four residues reproduce positions 4 to 7 of adrenocorticotropic hormone; the last three are a Pro-Gly-Pro tripeptide added to slow enzymatic breakdown. The molecular formula is C₃₇H₅₁N₉O₁₀S and the average mass is 813.9 g/mol. The literature, most of it from a small cluster of Moscow laboratories, examines neurotrophin transcript and protein levels in glial and hippocampal preparations, monoamine turnover in rodent striatum, transcriptomic and proteomic responses in rat vessel-occlusion models, and inhibition of enkephalin-degrading peptidases in human serum in vitro. It is registered as a medicine in the Russian Federation and has never been approved by FDA.

This article covers structure, mechanism and the quality of the evidence. It is not a guide to use of any kind.

What Semax is, chemically

The heptapeptide sequence

The peptide is written Met-Glu-His-Phe-Pro-Gly-Pro, or MEHFPGP in single-letter code, read from the free amino terminus to the free carboxyl terminus. Seven residues, six peptide bonds. In full notation it is L-methionyl-L-glutamyl-L-histidyl-L-phenylalanyl-L-prolylglycyl-L-proline. Catalogues also list it as ACTH(4-7)PGP, the more informative name because it states the construction.

What each region contributes:

• Methionine at position one carries a thioether side chain. That sulfur is the most oxidation-prone feature of the molecule and converts readily to methionine sulfoxide, which appears in mass spectra 16 Da heavier than the parent.
• Glutamate at position two contributes a side-chain carboxylate, negatively charged around neutral pH.
• Histidine at position three carries an imidazole with a side-chain pKa near 6, so at physiological pH the population is partly protonated. This residue and the bulky aromatic phenylalanine that follows it are the surviving fragment of the melanocortin recognition core.
• Pro-Gly-Pro at positions five through seven is the synthetic extension, added for chemical rather than pharmacological reasons.

Acidic glutamate and basic histidine roughly cancel, leaving no strong net charge at neutral pH. The molecule is very polar: PubChem gives a computed XLogP of -2.8 and a topological polar surface area of 312 Ų, which predicts good aqueous solubility and poor passive diffusion across lipid membranes.

Molecular formula and mass

Semax is C₃₇H₅₁N₉O₁₀S, average molecular mass 813.9 g/mol, monoisotopic mass 813.348 Da, CAS number 80714-61-0, catalogued as PubChem CID 9811102. The nine nitrogens are one backbone amide per residue plus the two ring nitrogens of the histidine imidazole; the single sulfur is the methionine thioether. Supplied material is normally a salt, commonly acetate or trifluoroacetate, so gross vial weight overstates peptide mass. Net peptide content is the meaningful number on a certificate of analysis.

The parent molecule, and why Semax lacks corticotropic activity

Adrenocorticotropic hormone is a 39-residue peptide cut from the precursor protein proopiomelanocortin. In the human sequence catalogued as UniProt P01189, the ACTH segment begins at residue 138 of the precursor and reads SYSMEHFRWGKPVGKKRRPVKVYPNGAEDESAEAFPLEF. Counting from the start of that segment, positions 4 through 10 are Met-Glu-His-Phe-Arg-Trp-Gly, the fragment written ACTH(4-10).

	Sequence
ACTH(4-10)	Met-Glu-His-Phe-Arg-Trp-Gly
Semax	Met-Glu-His-Phe-Pro-Gly-Pro

Two structural facts follow. The melanocortin peptides (ACTH, and the alpha, beta and gamma forms of melanocyte-stimulating hormone) share a conserved His-Phe-Arg-Trp message sequence at ACTH positions 6 through 9, and that tetrapeptide is the element the melanocortin receptors MC1R through MC5R recognise. Semax retains His and Phe from it; the arginine and tryptophan are replaced by proline and glycine. Semax is therefore melanocortin-derived by lineage while carrying only half of the receptor pharmacophore.

Separately, adrenal steroidogenesis requires more of the ACTH molecule than the message sequence. The basic cluster Lys-Lys-Arg-Arg at positions 15 through 18 acts as an address region, and ACTH(1-24) is the shortest fragment reproducing full steroidogenic potency. ACTH(4-10) contains neither that region nor the N-terminal residues, and neither does Semax. The absence of corticotropic activity is a consequence of what was left out, not an unexplained property.

Why Pro-Gly-Pro is on the C-terminus

Short linear peptides do not last long in biological fluids. Aminopeptidases work inward from the free N-terminus, carboxypeptidases from the free C-terminus, endopeptidases internally at sequence-specific sites. Native ACTH(4-10) is degraded quickly by all three routes.

Proline interrupts this. Its side chain loops back and bonds to its own backbone nitrogen, forming a five-membered pyrrolidine ring. The amide nitrogen carries no hydrogen, backbone rotation is constrained, and most peptidases cannot productively bind an adjacent bond. Proline residues act as protease stop signs.

Placing prolines at positions five and seven does two jobs. The one at position five sits immediately after the ACTH-derived tetrapeptide, shielding that junction; the one at position seven occupies the free C-terminus, where carboxypeptidases would otherwise start. Glycine between them is the smallest possible spacer. The same three residues were appended to Selank, a peptide built on an unrelated parent, which is why the two look superficially similar. That convergence is examined in the structural comparison of Semax and Selank.

One complication is frequently omitted: Pro-Gly-Pro is not an inert tag. Dmitrieva and colleagues ran Semax and free PGP side by side in rat cortex after permanent middle cerebral artery occlusion and reported that both altered transcription of neurotrophins and their receptor genes, with partially overlapping profiles. Their reading was that the Semax effect was selective to ischemic cortex while the PGP effect was largely non-selective. Either way, a change observed under Semax cannot be assigned to the ACTH-derived portion without a PGP control arm.

What the published literature examines

A caveat belongs at the top rather than buried at the end. Most primary work on Semax comes from one network of institutions, principally the Institute of Molecular Genetics of the Russian Academy of Sciences, now within the National Research Centre Kurchatov Institute, often with Moscow State University and the Zakusov Institute of Pharmacology. Much of it appears in Russian-language journals with English abstracts only, and independent replication outside that network is scarce.

Neurotrophin transcript and protein levels

Shadrina and colleagues treated glial cell cultures prepared from the basal forebrain of newborn rats and measured NGF and BDNF messenger RNA, reporting maxima at 30 minutes with BDNF transcript about eightfold and NGF transcript about fivefold above control. Dolotov and colleagues moved to rat hippocampus in vivo and measured transcript and protein together, reporting roughly a 1.4-fold rise in BDNF protein, a 1.6-fold rise in trkB tyrosine phosphorylation, and threefold and twofold rises in exon III BDNF and trkB messenger RNA.

The gap between those numbers is the interesting part. Eightfold transcript changes in isolated glia become 1.4-fold protein changes in whole hippocampal tissue, a normal consequence of dilution across cell types, translational regulation and protein turnover. A transcript measurement is not a protein measurement, and neither is a functional measurement.

Monoaminergic signalling

Eremin and colleagues examined dopaminergic and serotonergic parameters in rodent striatum and reported a roughly 25% rise in tissue 5-hydroxyindoleacetic acid, the principal serotonin metabolite, two hours after administration, with extracellular 5-HIAA rising progressively over one to four hours.

The dopamine result is often summarised inaccurately. Semax alone did not change tissue or extracellular dopamine or its metabolites in that study. What the authors observed was that Semax given before D-amphetamine amplified the amphetamine-driven rise in extracellular dopamine and the accompanying locomotor response. That is a modulatory interaction with a releasing agent, not a direct dopaminergic action.

Rodent models of experimental vessel occlusion

Permanent or transient middle cerebral artery occlusion in rats is the standard surgical lesion model here and accounts for much of the Semax literature. Medvedeva and colleagues ran a genome-wide transcriptional analysis of rat brain after focal ischemia and reported that the largest coordinated changes fell in immune-response and vascular gene sets rather than classical neurotransmission genes. A later group extended the approach to the protein level, profiling brain protein expression in a rat ischemia-reperfusion model. These are descriptive omics readouts in an acute surgical preparation, and the model corresponds only loosely to events in any other species.

Peptidase inhibition

One of the few mechanisms measured directly rather than inferred. Kost and colleagues found that Semax and Selank both inhibited enkephalin-degrading enzymes in human serum in vitro, with half-maximal inhibitory concentrations near 10 µM for Semax and 20 µM for Selank; pentapeptide fragments retained activity while shorter fragments did not. Note the scale: micromolar inhibition of a serum peptidase is a real observation but a high concentration relative to circulating levels of endogenous regulatory peptides, and the paper presents it as one candidate contributor rather than a complete mechanism.

The intranasal formulation as it appears in the literature

Searches for "semax nasal spray" reflect how the compound is presented in its country of registration. The factual position, stated as a description of the published record and not as instruction:

The product registered in the Russian Federation is an aqueous solution presented for nasal application, and rodent studies commonly used intranasal delivery. The stated reason in those papers is pharmacokinetic: the molecule is small, extremely polar and rapidly hydrolysed in the gastrointestinal tract, so the nasal route was investigated on the hypothesis of transport along olfactory and trigeminal pathways. That hypothesis has a substantial general literature and remains contested for any specific molecule.

None of that is guidance. Research-grade Semax is a lyophilized powder. It is not a formulated solution, not sterile, carries no excipients or delivery device, and is not characterised for administration to a human or an animal.

Regulatory status

Semax was developed from the 1980s onward at the Institute of Molecular Genetics of the Russian Academy of Sciences, associated principally with the groups of I. P. Ashmarin and N. F. Myasoedov, and later registered as a medicine in the Russian Federation.

The United States position is straightforward:

• Semax has never been approved by FDA. There is no US marketing authorisation for it in any form.
• It does not appear on the binding 503A bulks list at 21 CFR 216.23, which contains six substances and no peptides, unchanged since 2019.
• It is not among the Category 1 entries, which include VIP and GHK-Cu except injectable.
• The Pharmacy Compounding Advisory Committee meeting of 23 to 24 July 2026 produced a non-binding recommendation. No proposed rule and no final rule has followed. Nothing about the legal status of any peptide changed.

Russian-language sources classify Semax within a drug category translated as "nootropic." That describes a registration category in another jurisdiction with its own evidentiary standards. It is not an FDA determination and it does not transfer. On how the compound actually circulates, an analytical study in Drug Testing and Analysis characterised Semax and related peptides in seized pharmaceutical preparations.

What the evidence base does and does not support

Authorship is concentrated. A single institutional network appears on most primary papers. That is not misconduct, but a literature dominated by one group with near-uniformly positive findings warrants more caution than the same number of papers from independent laboratories.

Language and indexing limit scrutiny. Many reports exist in Russian with English abstracts only. Methods, statistical detail and negative controls are inaccessible to most Western readers, and some journals are not indexed in the databases used for systematic review.

Sample sizes are small. Randomisation, blinding and pre-specification are inconsistently described in the older literature by current standards.

No receptor has been identified. There is no characterised high-affinity binding site for Semax. Proposed mechanisms are indirect: allosteric influence on other receptor systems, peptidase inhibition, or activity attributable partly to the PGP fragment.

Model-to-model inference does not hold. Transcript changes in cultured glia do not establish protein changes in tissue; protein changes in rat hippocampus do not establish a physiological consequence; an omics signature in an occluded rat brain does not establish anything about an intact organism.

The honest summary: Semax has a real and internally consistent experimental literature describing effects on neurotrophin gene expression, monoamine metabolites and injury-model transcriptomes in rodents, produced largely by one research network, with the molecular target unresolved.

Handling Semax as a laboratory material

Methionine at position one makes the molecule oxidation-sensitive, so exposure to air, peroxides and trace transition metals matters, and a +16 Da satellite in the mass spectrum is the diagnostic sign. The lyophilizate is hygroscopic. Cold, dry, dark storage is materially more stable than solution, and aliquoting avoids repeated freeze-thaw.

Documentation carries the rest: identity by mass spectrometry against a monoisotopic mass of 813.35 Da, purity by reversed-phase HPLC with column and gradient stated, counterion identified, water content and net peptide content, all tied to a named lot with the issuing laboratory, method and date.

Research-grade Semax sits with the other compounds in the neuro research collection, alongside the companion article on Selank. Researchers sourcing comparable material should work from the documentation rather than the product name.

Working with this material

The certificate of analysis for Semax, from Horizon Analytical, is on the product page and on Lab results. Our own lot records appear at the lot lookup as lots are released. Supplied for laboratory research use only. Not for human or veterinary use.

Compliance notice. Nothing in this article should be read as guidance for administering any compound to a human or animal subject. "Research Use Only" describes the conditions under which a material is supplied; it is not a determination of legal status and confers no exemption from the Federal Food, Drug, and Cosmetic Act. Researchers are responsible for compliance with all applicable federal, state, and institutional requirements, and for obtaining any approvals their work requires.

Frequently asked questions

What is the Semax peptide?

Semax is a synthetic heptapeptide with the sequence Met-Glu-His-Phe-Pro-Gly-Pro. The first four residues reproduce positions 4 to 7 of adrenocorticotropic hormone and the last three are an added Pro-Gly-Pro extension. Its molecular formula is C₃₇H₅₁N₉O₁₀S, its average mass is 813.9 g/mol, and its CAS number is 80714-61-0.

What is the Semax peptide used for?

That framing does not fit the material. Semax supplied for research is an in-vitro laboratory reagent with no approved use of any kind. What can be described is what the literature has examined: neurotrophin transcript and protein levels in rodent glial and hippocampal preparations, monoamine metabolite concentrations in rat striatum, transcriptomic and proteomic responses in rodent vessel-occlusion models, and inhibition of enkephalin-degrading serum peptidases in vitro.

What is Semax derived from?

It is derived from the ACTH(4-10) fragment of adrenocorticotropic hormone. Semax keeps residues 4 to 7 of that fragment, Met-Glu-His-Phe, and replaces the remaining Arg-Trp-Gly with Pro-Gly-Pro. Because the melanocortin recognition sequence His-Phe-Arg-Trp is only half retained and the ACTH address region is absent, Semax is melanocortin-derived by lineage but lacks corticotropic activity.

Why does Semax have Pro-Gly-Pro on the end?

For enzymatic stability. Proline's side chain bonds back to its own backbone nitrogen, so most peptidases cannot cleave adjacent bonds. A proline immediately after the ACTH-derived tetrapeptide shields that junction from endopeptidases, and a proline at the free C-terminus blocks carboxypeptidase attack. The same extension was added to Selank, which comes from an unrelated parent molecule.

What is Semax nasal spray?

The product registered in the Russian Federation is an aqueous solution presented for nasal application, and rodent studies commonly used intranasal delivery because the peptide is highly polar and degraded in the gastrointestinal tract. That is a description of the published record. Research-grade Semax is a lyophilized powder, not a formulated, sterile or characterised preparation, and nothing here is administration guidance.

Is Semax FDA-approved?

No. Semax has never been approved by FDA and has no United States marketing authorisation. It is not on the binding 503A bulks list at 21 CFR 216.23, which contains six substances and no peptides, and it is not among the Category 1 entries. The non-binding advisory committee recommendation of July 2026 changed no legal status.

How strong is the research evidence on Semax?

Narrow. Most primary work comes from one Russian institutional network, much of it published in Russian-language journals with English abstracts only, with small sample sizes and inconsistent reporting of randomisation and blinding by current standards. No high-affinity receptor for Semax has been identified, so proposed mechanisms remain indirect. The observations are best described as reported in specific models by specific groups rather than as established.

Primary sources

The remaining literature cited above, for direct verification:

• measured transcript and protein together: https://pubmed.ncbi.nlm.nih.gov/16996037/
• reported a roughly 25% rise in tissue 5-hydroxyindoleacetic acid: https://pubmed.ncbi.nlm.nih.gov/16362768/
• genome-wide transcriptional analysis of rat brain after focal ischemia: https://pmc.ncbi.nlm.nih.gov/articles/PMC3987924/
• profiling brain protein expression in a rat ischemia-reperfusion model: https://pmc.ncbi.nlm.nih.gov/articles/PMC8226508/
• inhibited enkephalin-degrading enzymes in human serum in vitro: https://pubmed.ncbi.nlm.nih.gov/11443939/
• characterised Semax and related peptides in seized pharmaceutical preparations: https://pubmed.ncbi.nlm.nih.gov/31667971/