Semax vs Selank: ACTH(4-10) vs Tuftsin Peptide Scaffolds
Semax and Selank are both seven residues long and both end in the same three amino acids, which is why they get discussed as a pair. The difference lives in their front halves: Semax carries a fragment of a pituitary hormone, Selank carries a fragment of an antibody. We trace both scaffolds back to their parent peptides, explain what the shared Pro-Gly-Pro tail contributes to stability, and set the two structures side by side.
by Research Assistant·
Semax and Selank get discussed as a pair almost reflexively, and the reason isn't arbitrary. Both run exactly seven amino acids long, both emerged from the same Russian structural-biology tradition, and both finish with the identical three residues. Both are also supplied strictly for research use only — no human or animal consumption of any kind. What separates them sits entirely in the front half of each molecule. Semax carries a fragment of a pituitary hormone; Selank carries a fragment of an antibody. That one difference in provenance explains why the laboratory literature on the two compounds pulls in such different directions. Below we walk the shared blueprint, trace each scaffold to its parent peptide, sort out what the common Pro-Gly-Pro tail actually contributes, and set the two structures side by side.
One Blueprint, Two Borrowed Motifs
The short version: seven residues each, and the last three are the same.
The design pattern
Both compounds follow one recipe — take a short bioactive motif from an endogenous human peptide, then bolt the tripeptide Pro-Gly-Pro onto the C-terminus. Semax is the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro, usually abbreviated MEHFPGP. Selank is the heptapeptide Thr-Lys-Pro-Arg-Pro-Gly-Pro, abbreviated TKPRPGP. Line the two sequences up and the final three positions match exactly. The first four share no residues at all.
This is a recognisable move in peptide chemistry. Reworking the terminus of a natural fragment to buy stability is the same logic behind the differences in N-terminal chemistry in analog design among growth-hormone-releasing hormone analogs. A short natural sequence is rarely usable as-is, so the chemistry happens at the ends.
Why anyone builds a peptide this way
A bare four-residue fragment is a hard thing to study. Peptidases in plasma and tissue chew inward from both termini, and a tetrapeptide can be cleared before anything measurable happens. So the engineering question isn't how to make the motif more active — it's how to keep the motif intact long enough to observe it in a model system. Pro-Gly-Pro is the answer both molecules reached for.
Hold onto that as you read the rest of this comparison. The shared tail is exactly why these two compounds keep landing in the same conversations, even though their active ends come from unrelated corners of human biology.
Semax's Scaffold: ACTH(4-10) and the Melanocortin Family
Semax's working end is a four-residue piece of a pituitary hormone.
What ACTH(4-10) actually denotes
ACTH(4-10) means residues four through ten of adrenocorticotropic hormone — a seven-residue window cut out of a much longer hormone. Semax gets described as an ACTH(4-10) analogue routinely, but the more literal description is narrower. The molecule keeps ACTH(4-7), the tetrapeptide Met-Glu-His-Phe, and swaps Pro-Gly-Pro in for everything after it. Which is why some papers label the compound ACTH(4-7)PGP rather than an ACTH(4-10) analog. Both labels point at the same molecule. The chemical particulars: formula C37H51N9O10S, molar mass 813.93 g/mol, CAS number 80714-61-0.
The alpha-MSH overlap
Here's where provenance starts to matter. The retained Met-Glu-His-Phe site coincides with the same site in alpha-melanocyte-stimulating hormone, which puts Semax squarely in the melanocortin derivative family rather than in some isolated structural category of its own. Researchers have reported that in both in-vitro and in-vivo experimental conditions, Semax competitively antagonises alpha-MSH at the MC4 and MC5 melanocortin receptors. If you've been following the melanocortin thread elsewhere on this site, our write-up on linear and cyclic alpha-MSH analogs covers the structural end of that same receptor family.
What the ischemia literature looked at
The most detailed structural-pharmacology work on Semax comes out of rodent cerebral ischemia models. In a rat transient middle cerebral artery occlusion study, three proteins elevated by ischemia-reperfusion — phosphorylated JNK, MMP-9 and c-Fos — came down by more than 1.5-fold in cerebral cortex in the peptide-exposed group, while phosphorylated CREB in subcortical tissue rose by more than 1.5-fold. The pattern was region-dependent: in subcortex the peptide lowered pJNK and lifted pCREB without moving MMP-9 or c-Fos at all. The authors frame the overall result as compensation for expression profiles disturbed by ischemia, and note that CREB activation and JNK inhibition are already implicated in how melanocortins behave in brain inflammation models.
Selank's Scaffold: Tuftsin from the Fc Fragment of IgG
Selank's working end comes from an antibody, not a hormone.
What tuftsin is
Tuftsin is the tetrapeptide Thr-Lys-Pro-Arg, and it isn't synthesised as a standalone molecule. It's released from the Fc fragment of immunoglobulin G by two specific enzymes — an endocarboxypeptidase in the spleen, and a leukokininase sitting on the outer membrane of neutrophilic leukocytes. Put plainly: the body cuts tuftsin out of its own antibodies.
Its documented activity profile is immunological through and through. Tuftsin binds polymorphonuclear leukocytes and monocyte-macrophages, and in animal models it has been associated with stimulation of phagocytosis, macrophage killing of tumour cells, delayed hypersensitivity, cytolytic T-cell activity, antibody production, antibody-dependent cell-mediated cytotoxicity, and natural killer cell activity. Reported functions span phagocytosis, cell motility, immunogenic response, and bactericidal and tumoricidal activity.
What the immune inheritance implies for research interest
Because the parent fragment is immunological, Selank inherits a class of reported activity that Semax's scaffold has no obvious route to. Laboratory work has described modulation of interleukin-6 expression and shifts in the balance of T-helper cell cytokines — findings that sit naturally next to tuftsin's phagocytosis profile and awkwardly next to a melanocortin story. If you're researching this compound, two research areas generate most of the interest: neuropeptide and anxiolytic pharmacology on one side, immunomodulation on the other. Those are the reasons the compound attracts study. They aren't statements about what it does for anyone.
Why Pro-Gly-Pro Is Doing More Than Protecting a Tail
The identical back half is a stability device. It's also, less obviously, a second active element.
This is the detail most side-by-side comparisons skip. The glyproline literature frames Pro-Gly-Pro as a regulatory peptide in its own right, not as a protecting group — the tripeptide on its own has been reported to alter rat brain gene expression in focal ischemia. That cuts both ways for interpretation. Any measured difference between Semax and Selank can't be pinned on the tail, because the tail is held constant across both molecules. But the tail can't be waved off as pharmacologically silent background either, which means a Semax result is never cleanly a result about ACTH(4-7) alone.
Where the Two Scaffolds Diverge — and the One Mechanism They Share
Different parent peptides, different reported mechanisms, one notable overlap.
On the Selank side, the mechanism work points at GABA. Radioligand-receptor analysis found that Selank affects [3H]GABA binding as a positive allosteric modulator — subtype-selective, concentration-dependent, and with an interaction at benzodiazepine binding that was not simply additive. A gene-expression study backs this up from a different angle. After intranasal application in rats, 45 of 84 neurotransmission-related genes shifted within one hour, with Gabre and Gabrq moving around 20-fold and Hcrt up 128.29-fold by the three-hour mark. The one-hour pattern correlated with GABA's own profile (r = 0.86; p ≤ 0.05), yet 16 genes changed only after GABA — which the authors read as evidence that the peptide acts allosterically rather than at the GABA binding site itself. For the receptor-selectivity concepts underlying this kind of comparison, see what melanocortin receptor selectivity actually means.
The shared mechanism
One overlap is well documented. Both compounds have been reported to inhibit the enzymes that degrade enkephalins. For Selank, inhibition of plasma enkephalin hydrolysis was concentration-dependent with an IC50 of 15 μM — more potent in that assay than the peptidase inhibitors bacitracin and puromycin. Semax has a reported IC50 of 10 μM against the same enzyme class. Both have also been associated with elevated BDNF expression in rodent hippocampus despite their unrelated parent fragments. That's an empirical convergence worth noting, and it isn't the same thing as evidence of a shared pathway.
Semax vs Selank: Side-by-Side Structure
With the chemistry laid out, the comparison compresses into a short table.
Read down the rows and one thing stands out. Most of the differences are differences of degree — a few atoms here, sixty g/mol there. Only one row differs in kind, and it's the row naming the parent system.
Frequently Asked Questions
Are Semax and Selank the same kind of peptide?
They share an architecture, not a lineage. Both are heptapeptides assembled from a short bioactive motif borrowed from a human peptide plus the tripeptide Pro-Gly-Pro at the C-terminus. The borrowed motifs come from different biological systems: Semax's four N-terminal residues (Met-Glu-His-Phe) are the ACTH(4-7) fragment of a pituitary hormone, while Selank's (Thr-Lys-Pro-Arg) are tuftsin, a fragment released from the Fc region of immunoglobulin G. So the design pattern is shared and the parent chemistry is not.
What does ACTH(4-10) mean in the context of Semax?
ACTH(4-10) refers to residues four through ten of adrenocorticotropic hormone — a seven-residue window cut out of a much longer hormone. Semax keeps the first four of those residues (Met-Glu-His-Phe) and replaces the rest with Pro-Gly-Pro, which is why some papers describe it as ACTH(4-7)PGP rather than an ACTH(4-10) analog. Both labels point at the same molecule; the second is the more literal description of what was built.
Why do both molecules end in Pro-Gly-Pro?
Short peptides are cleared quickly by peptidases that chew in from the ends. Pro-Gly-Pro is a proline-rich tripeptide whose structure resists that cleavage, and review work on glyproline peptides reports enzymatic stability comparable to established pharmacological preparations. Appending it to a short bioactive motif is a stability strategy. Notably, Pro-Gly-Pro is not inert — it has its own documented activity in laboratory models, so both molecules carry two active structural elements rather than one active head and one protective tail.
Is there any mechanism the two scaffolds actually share?
One well-documented overlap: both have been reported to inhibit enzymes that degrade enkephalins, with published IC50 values in the low-micromolar range. Both have also been associated with elevated BDNF expression in rodent hippocampus despite their unrelated parent fragments. Outside that overlap, the laboratory literature points in different directions — melanocortin receptor interactions for Semax, GABA receptor allosteric modulation and cytokine effects for Selank.
Are Semax or Selank FDA approved?
No. Neither compound holds FDA approval. Both are registered pharmaceuticals in Russia (Selank is also dispensed in Ukraine) and are generally unscheduled in other jurisdictions. Research-grade material sold for laboratory work is not equivalent to, and should not be confused with, any approved pharmaceutical product bearing the same name. Material supplied by Optides is for research use only.
What the Comparison Actually Tells You
The Semax-versus-Selank question is a comparison of borrowed motifs, not of whole molecules. The stabilising tails are identical. The parent fragments come from different systems — endocrine for one, immune for the other — and the reported mechanisms overlap at exactly one point: inhibition of enkephalin-degrading enzymes. Everything else in the two literatures diverges because the front halves diverge.
That's the practical takeaway for anyone reading across these papers. When two heptapeptides look superficially similar, structural provenance is the most reliable way to keep them straight. For adjacent reading, our pieces on alpha-MSH analog structure and on melanocortin receptor selectivity go deeper into the family Semax belongs to.
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