Selank: Structure, Mechanism and Research Context

Research Use Only. This article is written for laboratory and research professionals. The compounds discussed are supplied strictly for in-vitro laboratory research. They are not drugs, dietary supplements, or foods. They are not for human or veterinary use, and not for diagnostic or therapeutic use. Nothing here is medical, legal, or regulatory advice.

The short answer

Selank is a synthetic heptapeptide with the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro. Its first four residues are tuftsin, a naturally occurring tetrapeptide released by proteolysis of the immunoglobulin G heavy chain; the last three are a Pro-Gly-Pro extension added to slow enzymatic breakdown. The molecular formula is C₃₃H₅₇N₁₁O₉ and the average mass is 751.9 g/mol. The published literature examines GABA radioligand binding in isolated brain membranes, expression of neurotransmission-related genes in rat frontal cortex and in a human neuroblastoma line, monoamine turnover, BDNF messenger RNA in rat hippocampus, and inflammation-related gene and cytokine measurements in rodent spleen and serum. It is registered as a medicine in the Russian Federation and has never been approved by FDA.

This article covers structure, mechanism and the quality of the evidence. It is not a guide to use of any kind.

What Selank is, chemically

The heptapeptide sequence

The peptide is written Thr-Lys-Pro-Arg-Pro-Gly-Pro, or TKPRPGP in single-letter code, read from the free amino terminus to the free carboxyl terminus. Seven residues, six peptide bonds, no cysteine and so no disulfide. Catalogues also list it as TP-7 and as L-threonyl-L-lysyl-L-prolyl-L-arginyl-L-prolylglycyl-L-proline.

What each region contributes:

• Threonine at position one carries a hydroxyl side chain and a free N-terminal amine.
• Lysine at position two ends in a primary amine with a side-chain pKa near 10.5, fully protonated at physiological pH.
• Proline at position three is the first of four prolines and puts a conformational constraint inside the tuftsin motif.
• Arginine at position four carries a guanidinium group with a side-chain pKa near 12.5, the most persistently charged side chain in the set.
• Pro-Gly-Pro at positions five through seven is the synthetic extension.

Two consequences follow. First, the molecule carries two permanently protonated basic side chains and no acidic ones, so it is strongly cationic at neutral pH, and it is extremely hydrophilic: PubChem gives a computed XLogP of -6.1 and a topological polar surface area of 322 Ų. Passive membrane permeability at that polarity is poor.

Second, there is no tryptophan, tyrosine or phenylalanine in the sequence. Selank has no aromatic chromophore, so it is effectively invisible to ultraviolet detection at 280 nm. Reversed-phase HPLC assays are run at low wavelength, typically around 214 nm, where the peptide bond itself absorbs. Check that a certificate of analysis states the detection wavelength, because a 280 nm purity trace on this molecule means very little.

Molecular formula and mass

Selank is C₃₃H₅₇N₁₁O₉, average molecular mass 751.9 g/mol, monoisotopic mass 751.434 Da, CAS number 129954-34-3, catalogued as PubChem CID 11765600. The eleven nitrogens are one backbone amide per residue, the lysine side-chain amine, and three in the arginine guanidinium. There is no sulfur. Supplied material is usually an acetate or trifluoroacetate salt, so net peptide content rather than gross vial weight is the meaningful figure on a certificate of analysis.

Tuftsin: the parent tetrapeptide

Tuftsin is Thr-Lys-Pro-Arg, formula C₂₁H₄₀N₈O₆, average mass 500.6 g/mol, CAS 9063-57-4, catalogued as PubChem CID 156080. It was described by Najjar and Nishioka at Tufts University and reported in Nature in 1970 under the title that gave it its name, one of the earliest characterised immunologically active peptides.

Its origin is the interesting part. The sequence has no dedicated gene. It sits inside the constant region of the immunoglobulin G heavy chain, at residues 289 to 292 of the CH2 domain, and is liberated by proteolysis of that parent protein. In the human IgG1 heavy chain constant region catalogued as UniProt P01857, the Thr-Lys-Pro-Arg motif appears within the CH2 sequence WYVDGVEVHNAKTKPREEQYNSTYRVVS. Cutting an antibody in a specific place releases a signalling tetrapeptide.

The original literature characterised tuftsin as a stimulator of phagocytosis by neutrophils and macrophages, and later work examined receptor binding on phagocytic cells and effects on motility and oxidative burst in vitro. That pedigree is why a peptide built on tuftsin is routinely tested in immune assays as well as neural ones, and why the Selank literature contains spleen gene-expression panels alongside brain measurements.

Why Pro-Gly-Pro is on the C-terminus

Free tuftsin is short-lived. Aminopeptidases attack the free N-terminus, carboxypeptidases the free C-terminus, endopeptidases internally. A tetrapeptide with two free termini has little protection.

Proline supplies that protection. Its side chain loops back and bonds to its own backbone nitrogen, forming a five-membered pyrrolidine ring, so the amide nitrogen carries no hydrogen and the backbone is conformationally restricted. Most peptidases cannot productively bind a bond adjacent to a proline. Appending Pro-Gly-Pro places a proline immediately after the tuftsin motif and another at the free C-terminus, where carboxypeptidases would otherwise begin, with glycine as a minimal spacer between them.

The identical three residues were appended to Semax, which derives from an ACTH fragment. Two unrelated starting sequences received the same C-terminal treatment because both came from the same research programme and both faced the same degradation problem. That convergence is examined in the comparison of the two compounds.

A caveat carries over from the Semax literature: Pro-Gly-Pro and its shorter fragments are not inert. In the Selank gene-expression work discussed below, the dipeptide Gly-Pro reproduced much of the parent peptide's profile, so an effect measured under Selank cannot be attributed to the tuftsin portion without running the fragment as a separate arm.

What the published literature examines

Almost all primary work on Selank comes from one institutional network, principally the Institute of Molecular Genetics of the Russian Academy of Sciences with collaborating Russian institutes. A substantial share appears in Russian-language journals with English abstracts only.

GABAergic signalling

This is the most-developed mechanistic thread and the one with a directly measured biochemical readout. A review from the originating group describes radioligand binding experiments on isolated brain cell plasma membranes in which Selank altered [³H]GABA binding in a manner the authors interpret as positive allosteric modulation, with joint application of Selank and benzodiazepines producing non-additive effects that differed from either compound alone.

Gene-expression work runs alongside it, and the two published results do not point the same way. Volkova and colleagues measured 84 neurotransmission-related genes in rat frontal cortex and reported significant changes in 45 of them at one hour and 22 at three hours, with the Selank and GABA profiles positively correlated.

A companion study by the same laboratory tested the same 84-gene panel in cultured IMR-32 human neuroblastoma cells and found no changes in messenger RNA levels under Selank alone. Combining GABA with Selank nearly abolished the changes GABA produced by itself, while Selank plus olanzapine altered more genes than olanzapine alone.

That negative in-vitro result is usually omitted from summaries. Taken together, the two papers argue against Selank acting directly on GABAergic gene transcription in an isolated neuronal line, and in favour of something that modifies how other ligands act at the receptor. That is a narrower claim than "Selank affects the GABA system," and it is the one the data support.

Monoamine turnover and BDNF expression

Rodent work has measured serotonin metabolism, including comparisons of Selank against free tuftsin in animals pretreated with the tryptophan hydroxylase inhibitor PCPA. Inozemtseva and colleagues separately measured BDNF expression in rat hippocampus. The neurotrophin thread is thinner for Selank than for Semax, resting on fewer measurements.

Immunomodulatory measures

Given tuftsin's origin, immune readouts appear throughout. Kolomin and colleagues examined chemokine, cytokine and receptor gene expression in mouse spleen 6 and 24 hours after administration and reported changes across much of the panel, with the dipeptide Gly-Pro producing largely similar changes.

A later study from the same group followed four specific genes over time in mouse spleen and reported a roughly threefold fall in complement C3 messenger RNA within 30 minutes, a fluctuating caspase-1 profile, early changes in the interleukin-2 receptor gamma chain gene Il2rg, and a reduction in the chemokine receptor gene Xcr1 at 90 minutes. Again the Gly-Pro fragment tracked the parent peptide closely.

Protein-level measurements exist as well. A rat study using a sensory-contact confrontation paradigm measured serum IL-1β, IL-6, IL-4, TGF-β1 and TNF-α by ELISA and reported that the paradigm raised IL-1β, IL-6 and TGF-β1 and that peptide administration moved those values back toward control. That published abstract also switches between naming Selank and naming Semax partway through its results, an error that is itself a fair illustration of the care this literature requires.

Peptidase inhibition

Kost and colleagues found that Selank and Semax both inhibited enkephalin-degrading enzymes in human serum in vitro, reporting a half-maximal inhibitory concentration near 20 µM for Selank and 10 µM for Semax, with pentapeptide fragments retaining activity while shorter fragments did not. Micromolar inhibition of a serum peptidase is a real measurement, though high relative to circulating concentrations of endogenous regulatory peptides.

The intranasal formulation as it appears in the literature

Searches for "selank nasal spray" reflect how the compound is presented in its country of registration. The factual position, stated as a description of the published record and not as instruction:

The product registered in the Russian Federation is an aqueous solution presented for nasal application, and rodent studies in the literature frequently used intranasal delivery, including the hippocampal BDNF work cited above. The stated rationale in those papers is pharmacokinetic. Selank is small, doubly cationic, extremely hydrophilic and rapidly hydrolysed in the gastrointestinal tract, so the nasal route was investigated on the hypothesis of transport along olfactory and trigeminal pathways. That hypothesis has a substantial general literature and remains contested for any specific molecule.

None of that is guidance. Research-grade Selank is a lyophilized powder. It is not a formulated solution, not sterile, carries no excipients or delivery device, and is not characterised for administration to a human or an animal.

Regulatory status

Selank was developed at the Institute of Molecular Genetics of the Russian Academy of Sciences with the Zakusov Institute of Pharmacology, and later registered as a medicine in the Russian Federation.

The United States position:

• Selank has never been approved by FDA and has no US marketing authorisation in any form.
• It does not appear on the binding 503A bulks list at 21 CFR 216.23, which contains six substances and no peptides, unchanged since 2019.
• It is not among the Category 1 entries, which include VIP and GHK-Cu except injectable.
• The Pharmacy Compounding Advisory Committee meeting of 23 to 24 July 2026 produced a non-binding recommendation. No proposed rule and no final rule has followed, and nothing about the legal status of any peptide changed.

Russian-language sources classify Selank within a drug category translated as "anxiolytic." That describes a registration category in another jurisdiction operating under its own evidentiary standards. It is not an FDA determination, it does not transfer, and it says nothing about material supplied for in-vitro research.

What the evidence base does and does not support

Authorship is concentrated. One institutional network produced most of the primary literature. A body of work dominated by a single group warrants more caution than the same volume from independent laboratories.

Language and indexing limit scrutiny. Many reports exist only in Russian with English abstracts. Methods, statistics and control arms are inaccessible to most readers, and several journals sit outside the databases used for systematic review.

Reporting quality is uneven. The cytokine study noted above names the wrong compound in its own results text. Older reports frequently do not describe randomisation, blinding or pre-specified endpoints.

Results are not uniformly positive. The IMR-32 study found nothing under Selank alone. Reporting that alongside the frontal-cortex results is the accurate summary; citing only the positive one is not.

The Gly-Pro problem is unresolved. Across several gene-expression datasets the dipeptide fragment reproduced much of the parent peptide's profile, so attribution to the tuftsin sequence is not established.

No receptor has been cloned or characterised for Selank. The best-supported mechanistic account is allosteric modulation at GABA receptors inferred from radioligand binding in membrane preparations, plus peptidase inhibition. Both are indirect.

The defensible summary: Selank has a coherent but narrow experimental literature, produced largely by one network, describing allosteric effects on GABA radioligand binding, mixed gene-expression findings, and immune-panel changes shared with a dipeptide fragment.

Handling Selank as a laboratory material

With no cysteine and no methionine, Selank has no oxidation-labile side chain, which makes it more forgiving than Semax on that axis. The dominant liabilities are moisture uptake by a hygroscopic lyophilizate, adsorption of a doubly cationic peptide to glass and some plastics at low concentration, and hydrolysis in solution. Cold, dry, dark storage of the powder is materially more stable than solution, and aliquoting avoids repeated freeze-thaw.

Documentation carries the rest: identity by mass spectrometry against a monoisotopic mass of 751.43 Da, purity by reversed-phase HPLC with column, gradient and detection wavelength stated, counterion identified, water content and net peptide content, all tied to a named lot with the issuing laboratory, method and date.

Research-grade Selank sits alongside the other compounds in the neuro research collection. The companion article on Semax covers the other heptapeptide built on the same Pro-Gly-Pro design.

Working with this material

The certificate of analysis for Selank, from Horizon Analytical, is on the product page and on Lab results. Our own lot records appear at the lot lookup as lots are released. Supplied for laboratory research use only. Not for human or veterinary use.

Compliance notice. Nothing in this article should be read as guidance for administering any compound to a human or animal subject. "Research Use Only" describes the conditions under which a material is supplied; it is not a determination of legal status and confers no exemption from the Federal Food, Drug, and Cosmetic Act. Researchers are responsible for compliance with all applicable federal, state, and institutional requirements, and for obtaining any approvals their work requires.

Frequently asked questions

What is Selank?

Selank is a synthetic heptapeptide with the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro. Its first four residues are the natural tetrapeptide tuftsin and the last three are an added Pro-Gly-Pro extension. Its molecular formula is C₃₃H₅₇N₁₁O₉, its average mass is 751.9 g/mol, and its CAS number is 129954-34-3.

What is Selank derived from?

From tuftsin, Thr-Lys-Pro-Arg, a tetrapeptide first described in Nature in 1970. Tuftsin is not encoded by its own gene. It sits at residues 289 to 292 of the CH2 domain of the immunoglobulin G heavy chain and is released by proteolysis of that antibody. The original literature characterised it as a stimulator of phagocytosis by neutrophils and macrophages, which is why Selank is examined in immune assays as well as neural ones.

What does the research literature on Selank examine?

Radioligand binding of [³H]GABA in isolated brain cell plasma membranes, expression of neurotransmission-related genes in rat frontal cortex and in cultured IMR-32 human neuroblastoma cells, monoamine turnover in rodents, BDNF messenger RNA in rat hippocampus, chemokine, cytokine and complement gene expression in mouse spleen, serum cytokine concentrations by ELISA in rat stress paradigms, and inhibition of enkephalin-degrading peptidases in human serum in vitro.

What is Selank nasal spray?

The product registered in the Russian Federation is an aqueous solution presented for nasal application, and rodent studies frequently used intranasal delivery because the peptide is doubly cationic, extremely hydrophilic and degraded in the gastrointestinal tract. That is a description of the published record. Research-grade Selank is a lyophilized powder, not a formulated, sterile or characterised preparation, and nothing here is administration guidance.

Is Selank FDA-approved?

No. Selank has never been approved by FDA and has no United States marketing authorisation. It does not appear on the binding 503A bulks list at 21 CFR 216.23, which contains six substances and no peptides, and it is not among the Category 1 entries. The non-binding advisory committee recommendation of July 2026 changed no legal status.

How reliable is the Selank research base?

Narrow and uneven. Most primary work comes from one Russian institutional network, much of it published in Russian with English abstracts only, and reporting quality varies to the point that one published cytokine study names the wrong compound in its own results text. No receptor has been characterised for Selank, a dipeptide fragment reproduces much of its gene-expression profile, and one in-vitro study found no effect at all in a human neuronal line. The findings are best described as reported in specific models rather than as established.

Primary sources

The remaining literature cited above, for direct verification:

• describes radioligand binding experiments on isolated brain cell plasma membranes: https://pubmed.ncbi.nlm.nih.gov/30255741/
• reported significant changes in 45 of them at one hour and 22 at three hours: https://pmc.ncbi.nlm.nih.gov/articles/PMC4757669/
• found no changes in messenger RNA levels under Selank alone: https://pmc.ncbi.nlm.nih.gov/articles/PMC5328971/
• measured BDNF expression in rat hippocampus: https://pubmed.ncbi.nlm.nih.gov/18841804/
• reported changes across much of the panel: https://pubmed.ncbi.nlm.nih.gov/21786679/
• reported a roughly threefold fall in complement C3 messenger RNA within 30 minutes: https://pubmed.ncbi.nlm.nih.gov/24291245/
• measured serum IL-1β, IL-6, IL-4, TGF-β1 and TNF-α by ELISA: https://pubmed.ncbi.nlm.nih.gov/32621722/
• inhibited enkephalin-degrading enzymes in human serum in vitro: https://pubmed.ncbi.nlm.nih.gov/11443939/