Semax and Selank: Same Lab, Same Trick, Two Different Targets

Semax and Selank get grouped together constantly — both are heptapeptides, both came out of the same Russian research programme, and both end in the same three amino acids. That shared ending is real and it matters. Almost everything else about them is different, starting with the molecules they were derived from. This guide sets Semax vs Selank out in full: where the two genuinely overlap, and where treating them as interchangeable will cost you an experiment.

Semax vs Selank: two peptides, one design principle

Both compounds came out of the Institute of Molecular Genetics of the Russian Academy of Sciences, from a line of work built on a specific idea: that short fragments of larger endogenous peptides often carry a subset of the parent molecule’s activity, and that you can keep the part you want while discarding the part you do not.

That idea has an obvious problem. Short peptides are fragile. Free in plasma, a seven-residue chain is a substrate for a long list of aminopeptidases and carboxypeptidases, and its half-life is measured in minutes. The programme’s answer was to bolt a Pro-Gly-Pro tripeptide onto the C-terminus of the active fragment. Proline residues are poor substrates for most exopeptidases, so the tail acts as a chemical bumper.

So the family resemblance between Semax and Selank is not a shared mechanism. It is a shared stabilisation strategy applied to two unrelated parent molecules.

Semax: an ACTH fragment with the hormone removed

Adrenocorticotropic hormone is 39 residues long, and its job is to drive cortisol release from the adrenal cortex. But the corticotropic activity lives at the N-terminal end of the molecule; a mid-chain fragment, ACTH(4–10), retains behavioural and neurotrophic activity in animal models without meaningfully stimulating the adrenal axis.

Semax is that fragment — Met-Glu-His-Phe — with Pro-Gly-Pro appended. The result is a non-corticotropic peptide, which is the whole point: it was designed to act on the brain without dragging the endocrine consequences of ACTH along with it.

The mechanistic work most often cited concerns neurotrophins. Dolotov and colleagues reported that Semax administration altered BDNF and trkB expression in the rat hippocampus, and later work found that both Semax and the bare Pro-Gly-Pro fragment influenced transcription of neurotrophin and neurotrophin-receptor genes in a rat cerebral ischaemia model. That second finding is worth pausing on: the stabilising tail is not necessarily inert.

Selank: a tuftsin analogue from the immune system

Tuftsin is an endogenous tetrapeptide — Thr-Lys-Pro-Arg — released from the CH2 domain of the IgG heavy chain. It was first characterised for its effects on phagocytes, which places its origins squarely in immunology rather than neuroscience.

Selank is tuftsin plus the same Pro-Gly-Pro tail, giving Thr-Lys-Pro-Arg-Pro-Gly-Pro. Despite the immunological parentage, the preclinical literature on Selank is dominated by anxiety-related models. Volkova and colleagues reported in Frontiers in Pharmacology that Selank administration altered expression of genes involved in GABAergic neurotransmission in rat brain tissue, and follow-up work examined the same class of effects in IMR-32 neuroblastoma cells alongside GABA and olanzapine.

A separate and older line of work looked at enzymes rather than genes: Zozulya and colleagues reported that Selank inhibits enkephalin-degrading enzymes in human serum, proposing that a slower breakdown of endogenous enkephalins could contribute to the behavioural profile seen in animal models.

Semax vs Selank side by side

Reduced to the differences that actually change an experiment, Semax vs Selank looks like this.

What the Pro-Gly-Pro tail actually does

It is tempting to treat the tail as packaging — a delivery convenience with no pharmacology of its own. The literature does not entirely support that reading. Pro-Gly-Pro is itself a described regulatory peptide, and the ischaemia study mentioned above found transcriptional effects from the tripeptide alone.

For anyone designing an in-vitro experiment, that has a practical consequence: if you are comparing Semax or Selank against a vehicle control, a Pro-Gly-Pro arm is a more informative comparator than vehicle alone, because it separates the contribution of the active fragment from the contribution of the stabiliser.

Where Semax and Selank genuinely overlap

Two places. First, the enkephalin-degrading enzyme work covers both compounds — the inhibitory effect on human serum enzymes has been reported for Semax as well as Selank. Second, both were developed for intranasal administration in their original research context, which reflects a shared problem: getting a hydrophilic seven-residue peptide past the blood–brain barrier is not straightforward, and nasal delivery sidesteps first-pass metabolism.

Outside of those two overlaps, treating them as interchangeable is a category error. They come from different parent molecules, engage different literatures, and are studied in different model systems.

Handling in the laboratory

• Both are supplied as lyophilised powder and should be kept at −20 °C, protected from light, until reconstitution.
• Reconstitute with bacteriostatic water where repeat withdrawals from the same vial are planned; the benzyl alcohol preservative is what makes multiple entries defensible.
• Add diluent slowly down the vial wall rather than directly onto the cake. Swirl; do not shake. Shear and foaming denature peptides.
• Label the vial with the reconstitution date. Track it — peptide solutions are far less stable than the lyophilised powder.
• Avoid freeze–thaw cycling. Aliquot once, then work from aliquots.

Our full protocol is in the peptide reconstitution guide.

Semax vs Selank: frequently asked questions

Related products

For a compound from the same era whose receptor was never found at all, see our guide to the DSIP peptide and what the sleep research actually shows.

References

The primary literature below is indexed on PubMed, and compound records are held at PubChem.

1. Dolotov OV, et al. Semax, an analog of ACTH(4-10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus. Brain Research, 2006. PMID 16996037.
2. Stavchansky VV, et al. Semax and Pro-Gly-Pro activate the transcription of neurotrophins and their receptor genes after cerebral ischemia. PMID 19633950.
3. Volkova A, et al. Selank administration affects the expression of some genes involved in GABAergic neurotransmission. Frontiers in Pharmacology, 2016;7:31.
4. Kolomin T, et al. GABA, Selank and olanzapine affect the expression of genes involved in GABAergic neurotransmission in IMR-32 cells. Frontiers in Pharmacology, 2017;8:89.
5. Zozulya AA, et al. The inhibitory effect of Selank on enkephalin-degrading enzymes as a possible mechanism of its anxiolytic activity. Bulletin of Experimental Biology and Medicine, 2001. PMID 11550013.