Selank and Semax: A Structural and Experimental Comparison

Selank and Semax get mentioned in the same breath so often that people assume they are siblings. Structurally they are not. Selank is a tuftsin analogue, Semax is an ACTH(4-10) analogue, and the only thing they genuinely share is a C-terminal Pro-Gly-Pro tail bolted onto a short parent sequence. Both came out of the Institute of Molecular Genetics in Moscow, which is also why most of the primary literature is Russian and why the quality varies a lot from paper to paper. Below is the structural comparison, then what the published experiments actually measured.

Sequences first

Semax is MEHFPGP. The MEHF part is residues 4 to 7 of adrenocorticotropic hormone, so it belongs to the melanocortin peptide family by lineage, and the Pro-Gly-Pro is a synthetic addition [3]. You will see it written as ACTH(4-10) analogue in some papers and ACTH(4-7)PGP in others [4]. Same molecule, two naming conventions, which is worth knowing when you search.

Selank is TKPRPGP. TKPR is tuftsin, a tetrapeptide released from the Fc region of immunoglobulin G, so its parent molecule sits in immunology rather than endocrinology [2]. Again, Pro-Gly-Pro on the end.

That shared tail is the design element people miss. Both molecules are short natural fragments with the same stabilising extension, which makes them chemically parallel without making them pharmacologically similar.

	Selank	Semax
Sequence	Thr-Lys-Pro-Arg-Pro-Gly-Pro	Met-Glu-His-Phe-Pro-Gly-Pro
Length	Heptapeptide	Heptapeptide
Parent fragment	Tuftsin (TKPR), from IgG Fc [2]	ACTH(4-7), melanocortin family [4]
Added motif	Pro-Gly-Pro	Pro-Gly-Pro
Most cited measurement type	Monoamine and BDNF content, behavioural test scores [2][5]	Transcriptome and neurotrophin gene expression [1][3][4]
Named receptor	None established	None established

Neither has an established receptor

This is the honest headline and it rarely gets said. Across the papers we read, neither peptide has a cloned, characterised receptor with binding data the way GHS-R1a or the oxytocin receptor do. Selank is frequently described as acting through GABAergic mechanisms, and a 2021 pharmacology review groups it with other GABA-receptor-related agents while describing it plainly as poorly studied [7]. We would treat that grouping as a working hypothesis, not a measured affinity.

What exists instead is downstream readouts: gene expression, neurotransmitter and neurotrophin content, imaging, behavioural scores. Useful data, but a different kind of evidence, and one that cannot tell you where the molecule binds.

What the Semax experiments measured

The cleanest single result is Agapova and colleagues in 2007. Wistar rats, one hour of exposure at 50 micrograms per kilogram, then real-time PCR on brain regions. Bdnf and Ngf expression rose in hippocampus, Bdnf also rose in brainstem and cerebellum, and Ngf fell in frontal cortex [1]. Note the direction flip by region. Anyone who summarises this as “Semax increases neurotrophins” is dropping half the finding.

The larger studies are transcriptome-wide and run in ischaemia models. Medvedeva and colleagues compared Semax-treated and untreated rat cortex at 3 and 24 hours after permanent middle cerebral artery occlusion. By 24 hours, genes tied to the immune response made up more than half of the transcripts with altered expression, with immunoglobulin and chemokine genes the most prominent groups, and 24 vascular-system genes were altered at 3 hours versus 12 at 24 hours [3]. Filippenkov’s group later used RNA-Seq in a transient occlusion model and found 394 differentially expressed genes at 24 hours, with inflammation-related transcripts suppressed and neurotransmission-related transcripts raised relative to saline [4].

Two caveats we would keep attached to those numbers. Both are injury models, so the comparison is against damaged tissue, not normal tissue. And both come from the same institute that developed the peptide, which is not disqualifying but is worth logging.

There is also a line of in vitro chemistry that has nothing to do with gene expression. Semax forms a stable complex with Cu2+, and in artificial membrane models it interfered with copper-induced amyloid beta aggregation using spectrofluorometric, calorimetric and MTT assays [8]. That is a metal-coordination result in a cell-free and model-membrane system.

What the Selank experiments measured

Selank’s literature leans on rodent behavioural models with biochemical endpoints attached. Kolik and colleagues gave outbred rats 0.3 mg/kg daily for 7 days after 30 weeks of ethanol as the sole fluid source, ran object recognition testing, and measured BDNF content ex vivo: Selank prevented the ethanol-associated rise in BDNF in hippocampus and frontal cortex [2]. Prevented a rise, note. Not produced one.

In a naloxone-precipitated morphine withdrawal model, a single 0.3 mg/kg exposure lowered the total withdrawal index by 39.6 percent and raised the tactile sensitivity threshold ninefold against active control, with diazepam at 2 mg/kg somewhat ahead on both [5].

The one modern human dataset we found is imaging rather than clinical. Panikratova and colleagues scanned 52 healthy participants with resting-state fMRI three times, before and at 5 and 20 minutes after Selank, Semax or placebo, and reported differences in functional connectivity between the right amygdala and a right temporal region, with effects they describe as partly shared and partly specific to each peptide [6]. Small, short, exploratory, and the first of its kind, which is how the authors frame it.

Reading the two side by side

If you want a one-line separation: Semax is studied mostly through gene expression in injury models, Selank mostly through behavioural pharmacology with neurochemical endpoints. Semax has also been examined in a developmental model, where it altered behavioural scores and brain monoamine levels in rats previously exposed to fluvoxamine [9], so the two literatures do overlap at the edges.

What neither literature supports is a potency ranking between them. There is no head-to-head concentration-response experiment with a shared endpoint that we could find, and without one, “Semax is stronger” or “Selank is stronger” are just opinions wearing lab coats. The 2020 fMRI study is the closest thing to a direct comparison, and it measured connectivity in healthy volunteers, not potency [6].

Characterisation and purity

Both peptides are short, proline-rich and easy to get wrong in synthesis, so analytical characterisation is not a formality. Reversed phase HPLC gives the chromatographic purity figure for a specific batch on a specific run, and mass spectrometry confirms the molecular mass against the expected sequence, which for these two differ enough that identity confusion is detectable. Any purity percentage should arrive with its method and batch attached. Ours are published at wholesalepeptide.com/coas, with every batch tested by a U.S.-based third-party laboratory.

Frequently asked questions

Are Selank and Semax chemically related?

Only through the shared Pro-Gly-Pro tail. Selank derives from tuftsin [2] and Semax from the ACTH(4-7) fragment [4], which are unrelated parent sequences.

Do Selank or Semax have known receptors?

No characterised receptor is established for either in the literature we reviewed. Selank is often associated with GABAergic mechanisms, though a 2021 review describes it as poorly studied [7].

What kind of studies dominate each literature?

Semax appears mainly in rodent gene expression and transcriptome work, often in ischaemia models [1][3][4]. Selank appears mainly in rodent behavioural models with neurochemical measurements [2][5].

References

1. Agapova TY, Agniullin YV, Shadrina MI, et al. Neurotrophin gene expression in rat brain under the action of Semax, an analogue of ACTH 4-10. Neurosci Lett. 2007. PMID 17353092
2. Kolik LG, Nadorova AV, Antipova TA, et al. Selank, peptide analogue of tuftsin, protects against ethanol-induced memory impairment by regulating of BDNF content in the hippocampus and prefrontal cortex in rats. Bull Exp Biol Med. 2019. PMID 31625062
3. Medvedeva EV, Dmitrieva VG, Povarova OV, et al. The peptide semax affects the expression of genes related to the immune and vascular systems in rat brain focal ischemia. BMC Genomics. 2014. PMID 24661604
4. Filippenkov IB, Stavchansky VV, Denisova AE, et al. Novel insights into the protective properties of ACTH(4-7)PGP (Semax) peptide at the transcriptome level following cerebral ischaemia-reperfusion in rats. Genes. 2020. PMID 32580520
5. Konstantinopolsky MA, Chernyakova IV, Kolik LG. Selank, a peptide analog of tuftsin, attenuates aversive signs of morphine withdrawal in rats. Bull Exp Biol Med. 2022. PMID 36322304
6. Panikratova YR, Lebedeva IS, Sokolov OY, et al. Functional connectomic approach to studying Selank and Semax effects. Dokl Biol Sci. 2020. PMID 32342318
7. Doyno CR, White CM. Sedative-hypnotic agents that impact gamma-aminobutyric acid receptors: focus on flunitrazepam, gamma-hydroxybutyric acid, phenibut, and selank. J Clin Pharmacol. 2021. PMID 34396551
8. Sciacca MFM, Naletova I, Giuffrida ML, Attanasio F. Semax, a synthetic regulatory peptide, affects copper-induced abeta aggregation and amyloid formation in artificial membrane models. ACS Chem Neurosci. 2022. PMID 35080861
9. Glazova NY, Manchenko DM, Volodina MA, et al. Semax, synthetic ACTH(4-10) analogue, attenuates behavioural and neurochemical alterations following early-life fluvoxamine exposure in white rats. Neuropeptides. 2021. PMID 33418449