Semax: A Research Overview of the ACTH(4-7) Analog in 2026

Semax is the second well-characterized product of the synthetic regulatory peptide program run at the Institute of Molecular Genetics of the Russian Academy of Sciences. Like Selank, it takes a short fragment of a larger endogenous molecule and appends a Pro-Gly-Pro cap to protect it from peptidase degradation. Unlike Selank, its parent molecule is a pituitary hormone, and the design decision behind the fragment selection is the most interesting thing about the compound.

For in-vitro laboratory research use only. Not for human consumption.

Separating Two Activities in One Hormone

Adrenocorticotropic hormone is a 39-amino-acid peptide released from the anterior pituitary. Its best-known function is endocrine: it acts at the melanocortin-2 receptor on the adrenal cortex to stimulate glucocorticoid synthesis and release. That is the hypothalamic-pituitary-adrenal axis function that appears in every physiology textbook.

ACTH also has effects in the central nervous system that are not explained by adrenal stimulation. Work through the 1970s and 1980s established that ACTH and fragments of it altered learning and memory performance in rodent models, affected attention and arousal measures, and produced neurotrophic effects in cultured neurons — and that these effects persisted in adrenalectomized animals, where the endocrine pathway is unavailable. The behavioral and neurotrophic activity was therefore not downstream of cortisol.

That observation set up a specific structure-activity problem: could the neurotropic activity be separated from the corticotropic activity? If a fragment retained the CNS effects while losing the ability to stimulate the adrenal cortex, it would be a considerably cleaner experimental tool than the parent hormone.

Systematic fragment work identified the region around residues 4–10 as carrying the behavioral activity, and the corticotropic activity as requiring the basic residues at positions 15–18 that engage the melanocortin-2 receptor. The tetrapeptide ACTH(4-7) — Met-Glu-His-Phe — retained neurotropic effects in the assays used and had no measurable corticotropic activity. It also had a functional half-life of well under a minute.

Semax is ACTH(4-7) with the Pro-Gly-Pro stabilizing extension.

Structure

Semax is the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro, with a molecular weight near 814 g/mol. The first four residues are the ACTH fragment; the last three are the cap shared with Selank.

Structural features that matter for experimental work:

• Methionine at the N-terminus. The thioether side chain of methionine is the most readily oxidized functional group in the molecule. Oxidation to methionine sulfoxide adds 16 Da and is the single most common degradation product observed in Semax preparations. It occurs on exposure to air, to trace metal ions, and to peroxides, and it accumulates over time in improperly stored material.
• Histidine. The imidazole side chain has a pKa near 6.0, meaning its protonation state changes across the physiological pH range. This makes the molecule's net charge pH-sensitive in a way that most short peptides are not, which matters for ion-exchange behavior and for buffer selection.
• Glutamate and the two basic-adjacent residues. The peptide carries both acidic and basic character, and its isoelectric point falls in a range where solubility can be reduced. Buffer pH selection should account for this.
• Phenylalanine. Provides weak but usable UV absorbance around 257 nm; combined with histidine, the molecule has more chromophore than Selank does, though still far less than a tryptophan-containing peptide.
• Three prolines. As with Selank, the high proline content imposes polyproline-II character and produces cis-trans isomerization behavior visible in chromatography.

Mechanistic Research Domains

Neurotrophin Expression

The most substantial body of Semax research concerns brain-derived neurotrophic factor and nerve growth factor. Dolotov and colleagues (2006) reported that Semax administration increased BDNF expression in rat hippocampus, with the transcript response detectable within a short window after administration and protein following.

Subsequent work extended this to NGF and to the receptor side of the system — TrkA and TrkB expression — suggesting that the effect is not limited to ligand availability but includes the signaling apparatus that receives it. Studies in cultured neurons have reported effects on neurite outgrowth and on survival under conditions of trophic factor withdrawal, which is the in-vitro correlate of the in-vivo neurotrophic claims.

The neurotrophin hypothesis is attractive because it provides a mechanistic bridge between a short peptide with no obvious receptor and the durable functional changes reported in behavioral studies. BDNF-TrkB signaling drives structural synaptic plasticity, and changes at that level persist long after the initiating stimulus has cleared.

Monoaminergic Modulation

Semax has been reported to affect dopaminergic and serotonergic transmission, with studies describing changes in dopamine turnover in striatal tissue and in serotonin metabolism in cortical and hippocampal regions. As with Selank, the direction of these effects varies with region and with the baseline state of the system, and the pattern is more consistent with modulation of existing tone than with unidirectional drive.

Some work has examined interactions with the endogenous opioid system, reporting effects on enkephalin-degrading enzyme activity analogous to those described for Selank. The shared Pro-Gly-Pro cap is a plausible common factor here, and this is one of the cases where running both peptides in parallel is informative.

Neuroprotection in Ischemia Models

A significant portion of the Semax literature uses experimental models of cerebral ischemia — middle cerebral artery occlusion in rodents being the most common. Reported endpoints include infarct volume, neurological deficit scores, and markers of inflammation and oxidative damage in peri-infarct tissue.

Medvedeva and colleagues (2014) approached this with transcriptome profiling, characterizing gene expression changes in rat brain following ischemic insult with and without Semax administration. The reported result described effects on immune and inflammatory gene clusters rather than on a small number of individually large changes — the same distributed pattern seen in the Selank transcriptome work, and a pattern that argues for gene-set-level analysis in future studies of this class.

Attention and Cognitive Performance Models

Rodent studies have examined Semax in learning and memory paradigms, including passive avoidance, active avoidance, and maze-based spatial tasks. This line of work is a direct descendant of the original ACTH fragment behavioral studies from the 1970s and 1980s, and it uses many of the same paradigms.

Methodologically these studies carry the usual caveats of rodent cognition research: performance in avoidance paradigms is sensitive to shock intensity, handling, and stress state, and improvements in a proxy measure do not straightforwardly map onto a cognitive construct. Studies that include locomotor and nociceptive controls are considerably more interpretable than those that do not.

Semax and Selank Compared

The shared cap means shared handling characteristics and a shared set of analytical challenges from the proline content. The active fragments engage entirely different systems.

Analytical Verification

Purity by reverse-phase HPLC. The research-grade floor is ≥99%. As with any proline-rich peptide, cis-trans isomerization can broaden peaks; elevated column temperature or orthogonal MS confirmation distinguishes conformational broadening from genuine heterogeneity.

The oxidation peak. This is the analysis point specific to Semax. Methionine sulfoxide is more polar than methionine and elutes earlier on reverse-phase columns, typically as a shoulder or a resolved peak just ahead of the main product. A certificate showing a small early-eluting peak in a Semax chromatogram is very likely showing partial oxidation. Mass spectrometry confirms it: the oxidized species is exactly 16 Da heavier. A supplier that reports only a purity percentage, without the chromatogram, makes this invisible to the buyer.

Identity by mass spectrometry. Theoretical and observed masses should both be stated. Beyond the +16 oxidation signature, the informative differences are deletion sequences from incomplete proline couplings.

UV detection wavelength. Phenylalanine and histidine give weak absorbance near 257 nm, but HPLC detection for this peptide is normally at 214–220 nm on the peptide bond. Quantification by absorbance is not reliable; amino acid analysis or corrected gravimetry should be used.

Counterion and net peptide content. TFA counterions from reverse-phase purification contribute meaningfully to gross weight. Molar concentration calculations should use net peptide content.

A note on why storage discipline matters more for this compound than for most: methionine oxidation is a slow, cumulative, and entirely invisible process. There is no colour change, no precipitation, and no odour to signal it. A vial that has spent months in a frost-free freezer — where the automatic defrost cycle repeatedly warms the contents — can carry a substantially higher sulfoxide fraction than the certificate of analysis describes, while looking identical to a properly stored vial. Laboratories running long studies with a single lot should therefore treat re-verification partway through as routine rather than exceptional, and should record the storage unit type in their methods. Where a manual-defrost freezer is available, it is preferable to a frost-free unit for this class of material, and a temperature logger on the unit converts an assumption about storage conditions into a documented fact.

Handling and Storage

Lyophilized Semax should be stored at -20°C, protected from light and moisture, and kept lyophilized until required.

The oxidation liability drives most of the handling guidance:

• Minimize headspace and air exposure. Opening a vial repeatedly introduces oxygen each time; a single aliquoting operation at first reconstitution is preferable to repeated access.
• Avoid trace metal contamination. Copper and iron ions catalyze methionine oxidation. Chelator-treated buffers and metal-free labware reduce the rate substantially.
• Avoid peroxide-containing solvents. Ethers in particular accumulate peroxides on storage and should not be used with methionine-containing peptides.
• Protect from light and keep cold during any working session.
• Do not freeze-thaw cycle prepared solutions.

This site does not publish reconstitution protocols, dosing guidance, or administration procedures. Those determinations belong to the researcher operating under an approved protocol.

Experimental Readouts Used in Semax Studies

BDNF and NGF quantification. ELISA for protein in tissue homogenate, quantitative PCR for transcript. Measuring both is important because the two respond on different timescales and neurotrophin expression is heavily post-transcriptionally regulated.

Receptor expression. TrkA, TrkB, and p75NTR levels by immunoblot or qPCR. A change in ligand without a change in receptor implies a different mechanistic picture than coordinated change in both.

Neurite outgrowth in culture. Length, branching, and the proportion of neurite-bearing cells in primary neuronal or PC12 cultures, quantified by automated image analysis. This is the workhorse in-vitro neurotrophic assay and provides a cell-autonomous readout independent of systemic effects.

Trophic withdrawal survival assays. Culturing neurons in the absence of their required trophic factor and measuring survival with and without the test compound isolates a protective effect from a growth-promoting effect.

Infarct volume in ischemia models. Triphenyltetrazolium chloride staining of serial brain sections, with volumes corrected for edema. Correction is essential — uncorrected infarct volumes systematically overestimate damage because swelling inflates the measured area.

Neurological deficit scoring. Composite scales assessing forelimb flexion, circling behavior, and reflex responses. These require blinded scoring; the effect sizes reported in unblinded studies in this literature are systematically larger.

Monoamine and metabolite quantification. HPLC with electrochemical detection on dissected regions, or microdialysis in freely moving animals.

Transcriptome profiling. Gene-set-level analysis is generally more informative than individual-gene analysis for this compound class, given the distributed pattern of modest changes.

Common Pitfalls in Semax Experimental Design

Not checking oxidation state before use. This is the pitfall specific to Semax. Material that was ≥99% pure at manufacture can accumulate a meaningful fraction of methionine sulfoxide over months of imperfect storage. Two laboratories using the same nominal compound from the same original lot can be working with materially different mixtures. Where a study spans a long period, re-verification partway through is prudent.

Assuming corticotropic activity is fully absent. The fragment was selected because it lacked measurable adrenal stimulation in the assays used at the time. In a study where HPA axis activation is a possible confound, measuring corticosterone directly is a small addition that removes the ambiguity.

Comparing across administration routes. Much of the published work uses intranasal administration. Exposure profiles differ substantially between intranasal, intraperitoneal, and intravenous routes for a charged heptapeptide, and effect sizes do not transfer.

Treating BDNF transcript change as BDNF protein change. They are different measurements on different timescales, and the inference from one to the other is reasonable but should be stated as an inference rather than a result.

Omitting behavioral controls. Cognitive paradigms in rodents are sensitive to arousal, locomotion, and nociception. Without controls for these, an apparent cognitive effect cannot be distinguished from a change in motivation or sensitivity.

Grouping with Selank by provenance. The two compounds share a design origin and a stabilizing cap, not a mechanism. Literature reviews that treat them as one class are grouping by history rather than pharmacology.

Search-term incompleteness. As with Selank, much of the primary literature originated in Russian-language journals with variable indexing. Searching only "Semax" in English-language databases returns an incomplete slice of the field; searching the ACTH(4-7) descriptors recovers additional work, including the older fragment studies that established the structure-activity basis for the compound.

Reading the Semax Literature Critically

The Semax literature has a distinctive shape, and knowing that shape helps in weighting individual studies.

It has three eras. The earliest layer is the ACTH fragment structure-activity work of the 1970s and 1980s, which predates Semax itself and establishes why the 4-7 fragment was chosen. The middle layer is the 1990s and 2000s Russian-language mechanistic and clinical work that characterized the stabilized compound. The most recent layer is international work, often using modern molecular tools, that revisits the mechanistic claims. Studies from different eras use different methods and different reporting standards, and a citation from one era does not validate a claim made in another.

Clinical literature exists but is jurisdiction-specific. Semax has a regulatory history in the Russian Federation that it does not have elsewhere. Clinical reports arising from that history are part of the published record and are frequently cited, but they were conducted under a regulatory framework different from FDA or EMA requirements, with correspondingly different trial design and reporting conventions. They are informative about the compound and are not equivalent to registration-standard clinical evidence. Nothing on this site should be read as a clinical claim; the compound is supplied for laboratory research only.

Ischemia model heterogeneity. Middle cerebral artery occlusion can be permanent or transient, filament-based or embolic, and each variant produces a different lesion with different time courses of injury and inflammation. Infarct volume reductions are not comparable across variants. Studies should state the model precisely, and reviews that pool across variants are averaging over meaningfully different biology.

Effect sizes and blinding. In the neurological deficit scoring literature generally, unblinded studies report larger effects than blinded ones. Where a Semax study does not state its blinding procedure, the reported effect size should be treated as an upper bound.

Route again. As noted, intranasal administration dominates the published work. A study using a systemic route is not directly comparable and should not be cited as replication of an intranasal result.

The Pro-Gly-Pro Cap as an Experimental Variable

The C-terminal extension shared with Selank was added for pharmacokinetic reasons, but it is not pharmacologically silent and should not be described as if it were.

The cap roughly doubles the molecular weight of the active fragment, imposes polyproline-II conformational character on a chain that would otherwise be flexible, and changes the hydrodynamic and charge properties of the molecule. Several consequences follow.

Semax is not simply "stable ACTH(4-7)." Its activity profile overlaps that of the free fragment but is not identical. Studies that cite free-fragment data as evidence for a Semax mechanism are making an assumption the structural change does not license. Where the distinction matters, both compounds should be run.

The Pro-Gly-Pro tripeptide has described activities of its own. Whether it is released intact during metabolism of Semax, and whether any released fragment contributes to observed effects, is a question most studies do not address. It should be held open as an explanation for unexpected results.

Rigidity constrains induced fit. A polyproline-II rod loses less conformational entropy on binding than a flexible chain, which generally favors association, but it also cannot adopt bent conformations that a flexible peptide could. Where the ACTH(4-7) core would need to flex to engage a site, the capped version may not be able to.

The cap is the basis for the Selank comparison. Because both compounds carry it, effects common to both are candidates for cap-attributable or handling-attributable phenomena, and effects unique to one are candidates for active-fragment-attributable mechanism. This is the strongest reason to run the two together rather than separately.

Why Synthesis Quality Matters More Than Usual Here

Semax combines two independent manufacturing liabilities in a seven-residue chain, which is unusual and which makes analytical documentation more valuable than it would be for a routine peptide.

The oxidation-sensitive residue sits at the N-terminus. Methionine at position one is exposed throughout synthesis, cleavage, purification, and lyophilization. Cleavage cocktails for methionine-containing peptides require appropriate scavengers; omitting them, or using an aged cocktail, produces material that is partially oxidized before it is ever bottled. Because methionine sulfoxide is only 16 Da heavier and only modestly more polar, a steep gradient will bury it under the main peak and a summary purity number will not show it.

The proline-rich C-terminus is coupling-hostile. Three prolines mean multiple sterically hindered couplings in a short chain. Incomplete coupling produces deletion sequences 97 Da lighter than target. These are readily detected by mass spectrometry and less reliably by chromatography alone.

Histidine racemization risk. Histidine is among the residues most prone to racemization during activation, particularly under prolonged coupling conditions or with certain activator chemistries. The D-histidine-containing epimer has identical mass and near-identical retention behavior on achiral columns, so it will not be detected by the standard analytical package at all. Manufacturers who control this use appropriate side-chain protection and minimize activation time; buyers cannot verify it from a routine certificate, which is a reason to weight manufacturer track record alongside documentation.

C-terminal chemistry. The specification must be met exactly, since resin choice and cleavage conditions determine whether the product terminates in an amide or a free acid — a one-Dalton, one-charge difference that changes electrostatic interactions.

The practical conclusion is the same as for other structurally demanding peptides: a percentage is not documentation. The chromatogram shape, the numeric mass comparison, and the lot match all carry information the summary number does not.

Where Semax Fits Among Research Tools

Semax is most useful experimentally as a probe for neurotrophin-mediated plasticity — a short, stable, synthetically accessible molecule that reproducibly modulates BDNF and NGF expression in rodent models. Compounds that do this without being neurotrophins themselves are scarce, and that scarcity is what gives Semax its niche.

The natural pairing is with Selank. The two share the Pro-Gly-Pro cap, the handling profile, and the synthesis challenges from proline content, while engaging unrelated systems through their active fragments. Running both allows a study to separate scaffold and handling effects from active-fragment effects, which is a control that cannot be constructed with either compound alone. Researchers working on mitochondrial contributions to neuronal function sometimes extend the panel with SS-31, which addresses bioenergetic capacity rather than trophic signaling.

Sourcing Considerations

Semax has a manufacturing profile that rewards careful supplier selection.

The methionine problem is a manufacturing problem before it is a storage problem. Oxidation can occur during synthesis, during the cleavage step, during purification, and during lyophilization. A manufacturer working under nitrogen with degassed solvents and appropriate scavengers in the cleavage cocktail produces material with a low starting sulfoxide fraction; one that does not produces material that is already partially oxidized on the day it is bottled. The purity number alone will not reveal this if the oxidized species is not resolved from the main peak, which is why the chromatogram matters.

Proline coupling deletions. Three prolines in seven residues means multiple sterically hindered couplings. Under compressed schedules, incomplete coupling produces truncated sequences differing by 97 Da — clearly resolved by MS, less reliably resolved by a steep HPLC gradient.

Free acid versus amide. The C-terminal chemistry must match the specification. Resin choice and cleavage conditions determine it, and the difference is one Dalton plus a charge.

What to require from a supplier:

• Independent third-party analysis from a laboratory with no stake in the result.
• Both HPLC and MS on the shipped lot, with theoretical and observed masses stated numerically.
• The chromatogram itself, so the region just ahead of the main peak can be inspected for the oxidation shoulder.
• Lot-matched documentation — certificate identifier and vial identifier must agree.
• Counterion disclosure, with acetate exchange available where the application requires it.
• Controlled shipping, since sustained heat accelerates oxidation as well as general degradation.

Excalibur Peptides supplies Semax at ≥99% HPLC-verified purity with an independent, batch-level certificate of analysis, shipped from a United States facility. Material that does not meet the ≥99% floor is not released.

Frequently Asked Questions

What is Semax?

Semax is a synthetic heptapeptide (Met-Glu-His-Phe-Pro-Gly-Pro) consisting of the ACTH(4-7) fragment with a Pro-Gly-Pro stabilizing extension. It is studied in preclinical models for effects on BDNF and NGF expression, monoamine modulation, and neuroprotection. It is supplied strictly for in-vitro laboratory research and is not for human consumption.

What is Semax derived from?

Semax is based on residues 4 through 7 of adrenocorticotropic hormone. That fragment retains neurotropic activity described for the parent hormone while lacking the corticotropic activity, which requires basic residues located further along the ACTH sequence.

How is Semax different from Selank?

Both share the Pro-Gly-Pro stabilizing cap and originated in the same research program, but their active fragments are unrelated. Semax is built on ACTH(4-7) and is studied mainly for neurotrophic effects on BDNF and NGF; Selank is built on tuftsin and is studied mainly for GABAergic and immunomodulatory effects.

Why is methionine oxidation a concern with Semax?

The N-terminal methionine residue oxidizes readily to methionine sulfoxide, adding 16 Da and producing an earlier-eluting species on reverse-phase HPLC. This can occur during manufacture or during storage, so both the chromatogram and appropriate cold, light-protected, low-oxygen storage matter for this compound.

What purity should research-grade Semax meet?

The research-grade floor is ≥99% by reverse-phase HPLC with identity confirmed by mass spectrometry against the theoretical molecular weight, both reported on a lot-specific certificate of analysis from an independent laboratory.

Is Semax approved for human use?

No. Semax is sold strictly for in-vitro laboratory research and identification purposes. It is not approved by the FDA for human therapeutic use, is not intended for veterinary administration, and is not intended for any clinical application.

View the Semax product page · View the Selank product page · Browse the research catalog · View the COA database

All compounds referenced in this article are sold for in-vitro laboratory research purposes only. They are not drugs, supplements, or medical products, and are not intended for human use, veterinary use, or consumption of any kind. Always consult applicable regulations regarding peptide research in your jurisdiction.

Last updated: July 2026