Selank and Semax: Two Peptides Derived From Endogenous Sequences

Selank and Semax are frequently grouped together because they share a development lineage and a common design strategy, not because they act on the same target. Both are synthetic extensions of naturally occurring peptide fragments.

The shared design strategy

Both compounds append a C-terminal Pro-Gly-Pro tripeptide to a biologically derived parent sequence. That extension is the deliberate part of the design: proline-rich termini resist exopeptidase cleavage, and adding Pro-Gly-Pro substantially slows degradation of an otherwise short-lived fragment. The same strategy appears across several peptides developed by the same research programme.

Selank

Selank is built on tuftsin, a tetrapeptide (Thr-Lys-Pro-Arg) that occurs as a fragment of the immunoglobulin G heavy chain. Tuftsin itself has a long literature in phagocyte biology dating to the 1970s. Selank is tuftsin extended with Pro-Gly-Pro, giving a heptapeptide. Literature: PubMed and tuftsin.

Semax

Semax derives from ACTH(4-10), a fragment of adrenocorticotropic hormone that lacks the steroidogenic activity of the full hormone — the portion of ACTH responsible for adrenal stimulation is not present in the fragment. Semax is ACTH(4-7) extended with Pro-Gly-Pro. Because the melanocortin core motif is partially retained, its relationship to melanocortin receptor pharmacology is worth considering alongside the melanocortin receptor subtypes. Literature: PubMed.

Evidence considerations

As with several compounds developed within the same research tradition, much of the primary literature is Russian-language and incompletely indexed internationally, and independent replication outside the originating groups is limited. Mechanistic proposals in the published work vary considerably between papers. A researcher should treat mechanism as an open question rather than settled, and design experiments that test rather than assume it.

Why Pro-Gly-Pro works

The extension is not arbitrary. Proline is conformationally restricted — its side chain loops back to the backbone nitrogen, removing the amide hydrogen and locking the phi angle. Two consequences follow, and both are exploited here.

First, exopeptidases that trim peptides from the C-terminus struggle with a proline-containing terminus. Carboxypeptidases in particular are markedly slowed by proline at or adjacent to the cleaved bond. A short peptide that would otherwise be degraded within minutes gains a substantially longer window.

Second, prolines break helices and impose turns, so a Pro-Gly-Pro tail keeps the parent fragment’s recognition surface presented rather than allowing the whole molecule to collapse into an ensemble of unproductive conformations. The glycine between the two prolines supplies the flexibility that a Pro-Pro pair would not.

The trade-off is that the extension is itself a structural change, and a heptapeptide is not a tetrapeptide with a longer half-life — it is a different molecule. Any assumption that Selank reproduces tuftsin pharmacology, or Semax reproduces ACTH(4-10) pharmacology, is a hypothesis rather than a given, and it is one the published literature does not fully resolve.

The parent fragments in more detail

Tuftsin

Thr-Lys-Pro-Arg, residues 289–292 of the immunoglobulin G heavy chain, released by proteolysis. Its literature runs from the early 1970s and centres on phagocyte function — binding to receptors on macrophages and neutrophils, with reported effects on phagocytosis and chemotaxis. It is worth noting that the receptor identification in that older literature is not as clean as the citation frequency implies, and proposed targets have shifted over time.

ACTH(4-10)

The heptapeptide Met-Glu-His-Phe-Arg-Trp-Gly, a fragment of adrenocorticotropic hormone. It contains the His-Phe-Arg-Trp melanocortin core motif, which is the reason its relationship to melanocortin receptor pharmacology cannot simply be waved away. What it lacks is the C-terminal region of ACTH responsible for adrenal steroidogenesis, so the fragment does not carry that activity. Semax truncates further to ACTH(4-7) before appending Pro-Gly-Pro, which removes part of the core motif — a structural detail that materially affects what melanocortin receptor engagement can be expected, and one that is frequently glossed over.

Reading a literature with limited independent replication

This is the honest difficulty with both compounds, and it is worth being explicit rather than diplomatic about it.

• Language and indexing. A substantial fraction of the primary work is Russian-language and incompletely indexed in the databases most researchers search. Absence from a PubMed search is not absence of literature, but it does mean the accessible record is a biased sample.
• Concentrated authorship. Much of the work originates from a small number of connected groups. That is not a criticism of the work; it is a reason independent replication carries extra weight when it exists, and a reason to note when it does not.
• Mechanistic proposals vary. Different papers advance different mechanisms for the same compound. Where proposed mechanisms are mutually inconsistent, at most one can be right, and a study design that assumes any of them is fragile.
• Model systems. Much of the in vivo work uses behavioural endpoints in rodents, which are sensitive to handling, housing and experimenter effects, and which have a well-documented replication problem across the whole field, not just here.

The constructive position is to treat mechanism as the object of study rather than as background. An experiment that measures a proposed target directly is worth more here than one that assumes the target and measures a downstream phenotype.

Designing a study on these compounds

• Include the parent fragment as a comparator. Tuftsin alongside Selank, ACTH(4-10) or ACTH(4-7) alongside Semax. Without it, there is no way to attribute an effect to the extension rather than to the parent, and that attribution is the central open question.
• Include Pro-Gly-Pro alone. Cheap, and it rules out the extension itself being active — a control that is almost never run and would be informative if it were.
• Measure stability under your own conditions. The half-life advantage is the design rationale; confirming it in your buffer and serum concentration takes one LC-MS time course and turns an assumption into a measurement.
• For Semax, address melanocortin receptors explicitly. Given the partial core motif, a melanocortin receptor panel is a reasonable inclusion rather than an exotic one.
• Blind and randomise behavioural work. Given the replication record in this literature, unblinded behavioural endpoints will not persuade anyone.

Analytical characterisation

• Mass confirmation against the stated sequence. Both are short, so the calculated mass is unambiguous and the check is strong. Confirm whether the C-terminus is free acid or amide — an 18 Da or 1 Da difference that changes the molecule.
• RP-HPLC purity with gradient, column and wavelength stated. Short hydrophilic peptides elute early, where resolution is poorest and where buffer and injection artefacts appear; the chromatogram is worth looking at rather than the number alone. See reading an HPLC chromatogram.
• Net peptide content. Both are highly charged for their length, so counterion mass is proportionally large and gross vial mass overstates peptide mass appreciably. See net peptide content explained.
• Oxidation check for Semax. Methionine and tryptophan both oxidise; a plus-16 satellite on the MS trace is the signature. See deletion, truncation and oxidation impurities.
• Counterion identity. Residual trifluoroacetate is active in some cell assays, which matters here because much of the relevant work is cell-based. See counterions and salt form.

Handling in practice

• Both dissolve readily in water or a near-neutral buffer. Neither contains cysteine, so disulfide scrambling is not a concern and reducing agents in the buffer are not a hazard.
• Semax needs light and oxygen discipline. Amber vials or foil, minimal headspace, minimal bench time. Selank has neither methionine nor tryptophan and is correspondingly forgiving.
• Aliquot on reconstitution. Short peptides are not immune to freeze–thaw losses, and adsorption to plastic at low working concentrations is a real and invisible loss for charged sequences.
• Track solution age, not vial age. Documentation practice is in keeping a peptide inventory; conditions in storage and stability and degradation by light, oxygen and temperature.

Terms used here

• Tuftsin — Thr-Lys-Pro-Arg; immunoglobulin G heavy chain fragment, parent of Selank.
• ACTH(4-10) / ACTH(4-7) — adrenocorticotropic hormone fragments; parent region of Semax.
• Pro-Gly-Pro — C-terminal tripeptide extension conferring exopeptidase resistance.
• Exopeptidase — enzyme cleaving residues from a peptide terminus, as distinct from an endopeptidase cleaving internally.
• HFRW motif — the melanocortin receptor recognition core, partially present in ACTH fragments.
• Free acid vs amide C-terminus — a small mass difference with real effects on charge, stability and receptor engagement.

Handling notes

Both peptides are short, highly soluble in water and contain no cysteine, so disulfide scrambling is not a concern. Semax contains methionine and tryptophan, both oxidation-prone; Selank contains neither, and is correspondingly more forgiving in storage. For Semax, minimising headspace oxygen and light exposure is worthwhile.

Products: Selank 5mg, Selank 10mg, Semax 10mg, listed under neuropeptides.

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