Search either compound and you will find the other. Selank and Semax are catalogued together, discussed together, and described in nearly identical language — two Russian heptapeptides, two "neuropeptides," two entries on the cognitive-mood shelf. The pairing is not arbitrary. They came out of the same institute, they were built on the same structural idea, and they are exactly the same length.
That is also the least interesting thing about them.
Strip off the three residues they share and what remains are fragments of two molecules with nothing to do with each other: one cut from an antibody, one cut from a pituitary hormone. The shared part is engineering. The unshared part is the pharmacology. This piece separates the two.
One Chassis, Two Messages
Both peptides follow a message–address architecture: a biologically meaningful N-terminal fragment welded to a C-terminal tripeptide whose job is to keep the molecule intact long enough to matter.
That tripeptide is Pro-Gly-Pro (PGP), one of the glyprolines — short proline-containing peptides notable for resisting peptidase attack. Proline's ring constrains the backbone in ways that many proteases handle poorly, and the published glyproline literature — including review work on natural and hybrid, or "chimeric," stable regulatory glyproline peptides — describes PGP-containing oligopeptides as unusually stable for molecules of that size. Bolting PGP onto a short, fragile bioactive fragment was the platform. Selank and Semax are two products of it.
Selank is Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP). The first four residues are tuftsin — an endogenous tetrapeptide released by enzymatic cleavage from the Fc region of immunoglobulin G, historically characterized for immunomodulatory and phagocyte-stimulating activity.
Semax is Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP). The first four residues are ACTH(4–7) — the core of the adrenocorticotropic hormone fragment ACTH(4–10), which comes from the POMC precursor described in our melanocortin system explainer. Native ACTH(4–10) is Met-Glu-His-Phe-Arg-Trp-Gly; Semax deletes Arg-Trp-Gly and substitutes PGP.
So the two compounds are not variants of one another. They are two unrelated messages sent on the same carrier.
| Selank | Semax | |
|---|---|---|
| Sequence | TKPRPGP | MEHFPGP |
| Message fragment | Tuftsin (IgG-derived) | ACTH(4–7) (POMC-derived) |
| Parent family | Immunoglobulin fragment | Melanocortin |
| Dominant research strand | GABAergic / enkephalin / immune | Neurotrophin (BDNF, NGF) |
| Approximate MW | ~751.9 Da | ~813.9 Da |
Selank's Strand: GABA Binding and the Enkephalin Clock
The Selank literature clusters around three threads, none of which is a clean single-receptor story.
GABAergic involvement. Radioligand work has reported that Selank alters GABA binding, and gene-expression studies have found broad shifts across GABA-system genes. Volkova and colleagues (Frontiers in Pharmacology, 2016) profiled 84 neuroreception genes in rat frontal cortex after Selank or GABA administration and reported significant changes in 45 genes at one hour and 22 at three hours, with the Selank and GABA expression patterns positively correlated — the authors framed allosteric modulation of the GABAergic system as one possible mechanism. A 2017 follow-up in the same journal extended the comparison to IMR-32 cells alongside GABA and olanzapine. Note the framing: "affects the expression of genes involved in GABAergic neurotransmission" is a different and weaker claim than "is a positive allosteric modulator of the GABA-A receptor." The distinction gets flattened constantly in secondary sources.
The enkephalin thread. A paper titled Semax and Selank Inhibit the Enkephalin-Degrading Enzymes of Human Serum reported that both peptides slow enzymatic breakdown of enkephalins in serum at low micromolar concentrations. The proposal is indirect: rather than binding an opioid receptor, the peptide extends the working life of endogenous enkephalins. This thread is unusual in that it links directly to a human endpoint — the 2008 Zozulia and Neznamov trial (Zhurnal Nevrologii i Psikhiatrii, 108(4):38–48) enrolled 62 participants with generalized anxiety disorder and neurasthenia, 30 receiving Selank and 32 receiving the benzodiazepine medazepam, and reported an increase in serum leu-enkephalin half-life that correlated with reduction in anxiety ratings. Whatever one makes of the trial, it is one of the few places where a proposed biochemical mechanism and a clinical measurement were recorded in the same study.
The immune inheritance. Selank has not shed its tuftsin ancestry. Published work reports altered expression across chemokine and cytokine genes, induction of interferon-alpha gene expression, and reduced IL-6 in stimulated human peripheral blood mononuclear cells. This is the part of Selank that has no counterpart in Semax at all.
Semax's Strand: Neurotrophins, Not a Characterized Receptor
Semax's most consistently replicated finding is upregulation of neurotrophic factors. Dolotov and colleagues (Brain Research, 2006) reported that a single intranasal administration in rats produced roughly a 1.4-fold rise in hippocampal BDNF protein and a 1.6-fold rise in TrkB tyrosine phosphorylation, with about a 3-fold increase in exon III BDNF mRNA and a 2-fold increase in TrkB mRNA. Subsequent work has described effects on NGF and on dopaminergic and serotonergic systems in rodents.
Here is the point most write-ups get wrong: no receptor has been convincingly established as Semax's target. The effects are reported downstream — at the level of gene transcription and neurotrophin signalling — not from a characterized binding interaction. Semax is grouped with the melanocortins by lineage, because ACTH(4–7) is a melanocortin fragment, but membership in a family by sequence is not the same as demonstrated activity at that family's receptors. Claims that Semax is a weak MC3R/MC4R agonist circulate widely and rest on thin, inconsistent sourcing; treat them as unsettled.
What is structurally clear is the absence of corticotropic activity. The residues required for ACTH's action at MC2R are not present in Semax, which is the structural basis for it not driving the steroidogenic HPA-axis branch its parent hormone controls.
The Shared Tail Is Not Inert
This is the caveat that almost never appears in side-by-side comparisons, and it complicates both stories.
PGP was chosen as a stabilizer, but PGP is itself a bioactive regulatory peptide — and it is the dominant Semax degradation product in plasma and rat brain, still detectable an hour after administration. A separate line of research has examined PGP on its own: work published in Molecular Biology (2014) profiled the effect of Pro-Gly-Pro on rat brain transcriptome during focal ischemia, and a related study is titled Semax and Pro-Gly-Pro Activate the Transcription of Neurotrophins and Their Receptor Genes after Cerebral Ischemia. Comparative work has also reported anticoagulant activity across proline-containing regulatory oligopeptides including glyprolines, Semax, and Selank.
The implication is uncomfortable and worth stating plainly: when an effect is measured after administering Semax or Selank, some fraction of what is being observed may belong to the shared tail or its liberation, not to the tuftsin or ACTH message. The "address" has its own pharmacology. Any attribution of an outcome cleanly to the N-terminal fragment is an assumption, not a finding — and it is an assumption the two compounds share equally.
Where the Two Research Records Diverge — and Where They Don't
The mechanistic stories differ. The evidence architecture barely does.
Semax carries the larger human record, concentrated in acute ischemic stroke, where it is registered in Russia and has been used in acute neurology settings. The stroke work traces to the Gusev group, whose 1997 clinical and electrophysiological study compared patients receiving Semax alongside combined intensive therapy against patients on conventional therapy alone — note that the comparator was standard care, not placebo. Later reports in this literature describe improved neurological deficit scores and functional recovery, and separately describe elevated plasma BDNF in treated patients.
Selank is registered in Russia in the anxiety indication, supported by the 2008 comparative trial above and related Russian work.
Neither compound is approved by the FDA or EMA for any indication. And both sit under the same structural limitation that we have flagged for Epithalon and Thymalin: a single national research lineage. The great majority of the clinical literature originates from a small set of Russian institutions, is published largely in Russian-language journals, and has not been reproduced by independent groups outside that lineage under modern trial standards. That is not an accusation of error. It is a statement about what replication has and has not been done — and it applies to both compounds identically, which is why choosing between them on "strength of evidence" is not really a coherent exercise. Our cognitive-mood class primer situates both within the wider shelf.
Identity Testing: Sixty-Two Daltons Apart
For laboratory handling, the useful fact is that these two are easy to tell apart by mass and easy to confuse by appearance.
Selank sits near 751.9 Da and Semax near 813.9 Da — about 62 Da apart, a gap any competent mass spectrometer resolves. Exact-mass confirmation should be unambiguous.
But both are short, both are proline-rich, and both terminate in the same tripeptide. A partial or low-quality identity check that only establishes "a small proline-containing peptide is present" distinguishes nothing. That is the case for MS/MS sequence confirmation rather than intact mass alone, as covered in our piece on HPLC vs mass spec. The same logic applies to truncation impurities: a synthesis that drops a residue produces something that still looks broadly right on a chromatogram.
Both are typically supplied as lyophilized powder and as a salt — see TFA vs acetate for why the counterion line on a certificate of analysis is not a formality. General expectations for documentation are on our quality page, and the full compound index is in the library.
FAQ
Are Selank and Semax chemically related? Only in their C-terminal Pro-Gly-Pro tripeptide and their overall length. Their N-terminal fragments derive from immunoglobulin G (tuftsin) and the POMC-derived ACTH sequence respectively — unrelated parent molecules with unrelated biology.
Does Semax act on melanocortin receptors? It has not been convincingly established that it does. Semax is grouped with the melanocortins because ACTH(4–7) is a melanocortin fragment, but its documented effects are described downstream at the level of neurotrophin gene expression rather than from a characterized receptor interaction. Reports of weak MC3R/MC4R activity are inconsistently sourced and should be treated as unsettled.
Why is so much of the literature Russian? Both peptides were developed at the Institute of Molecular Genetics of the Russian Academy of Sciences and progressed through the Russian regulatory system. Development, trials, and publication have stayed largely within that research lineage, which is the central limitation on interpreting either compound's record.
This article is educational and for the laboratory research community. Trulogic Labs products are sold for laboratory and research use only and are not for human consumption.