Selank 10 mg

Compound profile

Selank: compound profile, literature landscape and handling notes

Where Selank came from

Selank sits at the junction of two unrelated research histories, and it is much easier to read once they are separated.

The first is tuftsin. In 1970 Najjar's group at Tufts University described a tetrapeptide, threonine-lysine-proline-arginine, that can be liberated enzymatically from the CH2 domain of the immunoglobulin G heavy chain and that stimulated phagocytic uptake by neutrophils and macrophages. The peptide carries the name of the institution. Everything about that original work is immunological: the parent molecule is an antibody, the readout was phagocytosis, and the proposed physiology was that a fragment cut out of a circulating antibody could act as a signal in its own right. Tuftsin was worked on in that frame for years, and a large share of the literature bearing the name is about macrophage and neutrophil activation rather than about anything neural.

The second history is the Russian glyproline program. From the 1980s onward the Institute of Molecular Genetics of the Russian Academy of Sciences pursued a general strategy for making short, rapidly cleared regulatory peptides survive long enough to be studied in a whole animal: extend the C-terminus with the tripeptide proline-glycine-proline, a motif borrowed from the endogenous glyprolines. The best known product of that strategy is Semax, an adrenocorticotropic hormone fragment carrying the same tail. Selank is that same strategy applied to tuftsin, so its design lineage runs through Semax and the glyproline work, not through any immunology program.

The practical consequence for a reader is worth stating bluntly. Most primary work on Selank appeared in Russian-language journals, some of which are not indexed in the databases an English-speaking researcher searches by default and few of which have full-text English versions. The English-language secondary literature has filled that gap with review articles, and those reviews are then cited by later reviews. A citation chain that looks broad frequently narrows to a small number of laboratories and a small number of original reports. Selank also has a regulatory history inside Russia with no counterpart elsewhere; that history is a fact about a jurisdiction, not evidence about the molecule, and none of it transfers to bench work.

Reading the structure of Selank

The sequence, formula and mass for this listing are printed in the specification table further down this page and are not restated here. What is worth doing instead is reading the seven residues for what each part is doing, because this is a compound where the structure is the entire design argument.

Residues one through four, threonine-lysine-proline-arginine, are tuftsin unchanged. Residues five through seven, proline-glycine-proline, are the addition. There is no substitution anywhere, no non-proteinogenic residue, no acylation, no amidation of the C-terminus in the usual commercial form, and no disulfide. Selank is therefore a plain L-amino-acid peptide, which is unusual for a designed analog and means that everything interesting about it comes from residue choice and order rather than from chemical modification.

Three of the seven residues are proline. That has consequences that go well beyond composition. Proline is a secondary amine locked into a ring, so it cannot donate a backbone hydrogen bond and it sharply restricts the phi angle of the chain. A heptapeptide with prolines at positions three, five and seven has no capacity to form a regular helix or sheet; it is a short, conformationally constrained, solvent-exposed chain. Proline also interconverts between cis and trans amide configurations slowly on the timescale of a chromatographic separation, which is an analytical fact you will meet again later on this page.

The most elegant structural detail is that both basic residues are immediately followed by proline. Trypsin cleaves after lysine and arginine but not when proline occupies the next position, so the two sites that would ordinarily make this peptide a trypsin substrate are blocked by the residue that follows them. The same proline placement narrows the set of other endopeptidases that can act on the chain interior.

The proline-glycine-proline tail is where the design intent lives. It is not there for activity; it is there because exopeptidases that trim from the C-terminus handle a proline-rich terminus poorly, so the tail acts as a sacrificial and slow-to-remove cap. It is important to be precise about what that buys. The extension protects the carboxy end. It does not protect the amino end, where threonine-lysine remains an ordinary aminopeptidase substrate, and it does not confer resistance to prolyl-specific peptidases, which are the enzymes most suited to a sequence like this one. The stabilization is real and it is asymmetric.

The target and the pathway in more detail

The honest starting point is that Selank has no confirmed high-affinity receptor. There is no cloned target, no accepted radioligand binding curve for the intact heptapeptide, and no antagonist that reverses its reported effects in a way that would establish receptor mediation. Secondary sources routinely describe it as a modulator of one system or another, and it is worth noticing that the word modulator is doing the work an identified target would otherwise do.

What the tuftsin half brings is a partial exception. Tuftsin itself has a longer target literature than Selank does, including reports of binding to receptors on phagocytes and later work identifying neuropilin-1 as a binding partner in macrophage and microglial systems. Whether the intact heptapeptide retains that interaction is a separate empirical question, and the C-terminal extension sits exactly where a receptor engaging the tuftsin C-terminus would be affected. Treating tuftsin target data as Selank target data is one of the most common errors made about this compound.

The mechanisms actually reported for Selank are downstream and indirect. Several are enzymatic rather than receptor-based. The most mechanistically interesting is the reported inhibition of enkephalin degradation in blood: rather than binding an opioid receptor, the peptide is described as slowing the enzymes that clear leucine-enkephalin, which would prolong the lifetime of an endogenous ligand rather than substituting for it. That is a genuinely different class of claim from receptor agonism, and it is testable in a cell-free system, which is a point in its favor.

The other reported mechanisms are expression-level. Published work describes changes in the expression of gamma-aminobutyric acid type A receptor subunits in rodent brain regions, changes in brain-derived neurotrophic factor expression, effects on monoamine turnover, and changes in the expression of cytokine and interferon-related genes. That last category is the one most consistent with the tuftsin heritage and is often overlooked by sources that present Selank purely as a nervous-system compound.

Expression-level readouts are informative but they are several steps removed from a target. A change in subunit messenger RNA hours after exposure is compatible with direct receptor engagement, with an indirect signaling cascade, with a stress response, and with a systemic effect fed back into the brain. Distinguishing those requires binding data that does not currently exist in a form a careful reader can rely on.

What the published literature on Selank actually measures

The published record on Selank divides into five recognizable groups, and identifying which one a given citation belongs to tells you most of what you need to know about how much weight it carries.

The largest by volume is rodent behavioral pharmacology from Russian laboratories. These are conflict paradigms, elevated plus maze and open field work, and comparisons against benzodiazepine reference compounds run in the same experiments. What these papers measure are behavioral endpoints in animals under defined conditions. Many predate the current conventions on reporting randomization, blinding and sample-size justification, and a substantial fraction were published in journals whose peer review process is not documented in English. They are evidence, but they are evidence of a kind that requires the reader to know its provenance.

The second group is gene-expression work from the originating institute, and it is the part of the record most accessible in English. Microarray and quantitative polymerase chain reaction studies in rat hippocampus, other brain regions and peripheral blood cells report expression changes across immune, neurotransmitter and neurotrophic gene sets. These papers are useful precisely because their readouts are concrete and the methods are reproducible in principle by anyone with the same platform.

The third is peptide chemistry and stability: comparisons of how quickly Selank and tuftsin disappear in plasma or blood, identification of the fragments generated, and characterization of which enzymes are responsible. This is the literature that supports the design rationale for the tail, and it is the literature a bench researcher planning an incubation should read first.

The fourth is the enzymology around enkephalin-degrading activity, a small but focused set of reports that describe an effect measurable outside a cell.

The fifth is clinical reporting in Russian psychiatric and neurological journals. Whatever its merits in its own context, it is of no use as a bench reference. It does not characterize the molecule, and it should not appear in the methods section of an in vitro protocol.

A sixth category deserves naming because it is so often mistaken for the literature itself: the English-language narrative review that summarizes the Russian work without reproducing its data. Those reviews are where most online descriptions of Selank actually come from.

Where the Selank literature is thin or frequently misread

The Selank literature has a specific and unusual weakness, which is not that it is small but that it is hard to audit.

The language barrier is the first and most consequential issue. A researcher who wants to check a claim about Selank frequently cannot reach the original report, and the intermediate source is a review that paraphrases it. Paraphrase loses the experimental conditions, and experimental conditions are exactly what determines whether two reports agree. This is not an argument that the underlying work is wrong. It is an argument that a claim about Selank should be cited to whatever the reader has actually read, and that a review should be cited as a review rather than dressed up as primary data.

The second is concentration of authorship. A large share of the mechanistic work traces back to one institute and its immediate collaborators. Single-group findings are not invalid, but they have not been subjected to the independent replication that would let a reader treat them as settled, and the review literature does not usually flag this.

The third is the missing target. Because there is no receptor to anchor the pharmacology, effects reported at the expression level cannot be ordered into a causal chain, and the resulting descriptions tend toward lists of systems the compound is said to influence. A list of affected systems is a weaker claim than it looks.

The fourth is the tuftsin transfer error already mentioned: immunological data generated on the tetrapeptide is regularly presented as though it characterized the heptapeptide.

The fifth concerns the fragments. It is often implied that the tail simply makes the molecule last longer and that whatever is cleaved off is inert. Proline-glycine-proline and its N-acetylated form have their own reported activity in the neutrophil chemotaxis literature, and other glyprolines have been studied as active peptides in their own right. A degradation product with reported activity is a confound, not a footnote, and any experiment long enough for meaningful cleavage should account for it.

The sixth is over-reading of stability. The tail slows exopeptidase trimming from one end. It is frequently described as though it made the peptide robust in general, which it does not.

How Selank behaves in solution

Selank is at the opposite end of the solubility spectrum from the acylated peptides elsewhere in this catalog, and most of its handling behavior follows from that.

It is a small, strongly hydrophilic peptide with two basic side chains and a free alpha-amino group, so it carries a net positive charge across the ordinary working range of pH and dissolves in water readily and completely. There is no lipophilic chain, no self-association driven by a hydrophobic tail and no tendency to form the higher-order assemblies that complicate work with amphiphilic analogs. Lyophilized material of this kind is typically hygroscopic, and a cake that has taken up atmospheric moisture will often look slightly collapsed or glassy before anything is wrong with the peptide itself.

Adsorption still occurs, but the mechanism is electrostatic rather than hydrophobic. A cationic peptide binds to the negatively charged silanol surface of untreated glass and to some plastics, and the loss is worst at low concentration and high surface-to-volume ratio, which is exactly where a working dilution lives. Low-binding labware, a modest concentration of a competing salt or a carrier protein, and fewer transfer steps all reduce it, and any of those choices belongs in the record because they change the matrix.

The chemical degradation picture is unusually favorable and worth appreciating. There is no methionine, no cysteine and no tryptophan, so oxidation is not a dominant route. There is no asparagine or glutamine, so the deamidation chemistry that dominates the stability of many peptides is simply unavailable here. What remains is hydrolysis of the backbone and one specific route worth knowing: a sequence with proline in the third position is prone to intramolecular attack that releases the N-terminal two residues as a diketopiperazine, and Selank has proline in exactly that position.

Microbial growth in an unpreserved aqueous solution is a more realistic threat to this material than chemical breakdown over short storage. And the trap specific to Selank is that a degraded solution usually still looks perfectly clear. An aggregating peptide announces its failure visually; this one does not, so appearance is close to worthless as an integrity check. The storage statement in the specification table on this page is the product record for the material, and the documentation supplied with the lot is what governs.

Analytical notes specific to Selank

Selank presents two analytical problems at once: it is invisible where peptides are usually detected, and it does not stay on the column.

The detection problem comes first. There is no tryptophan, tyrosine, phenylalanine, histidine or cystine anywhere in the sequence, so the peptide has no aromatic chromophore at all and no usable absorbance near 280 nanometers. Detection has to sit at low ultraviolet, around 214 nanometers, where the signal comes from the peptide bond itself. Two things follow. Response there scales roughly with the number of amide bonds, so a short impurity gives a peak far smaller than its molar share, and an area-percent figure from a single low-ultraviolet wavelength is a biased estimate rather than a composition. Low ultraviolet also responds to plenty of non-peptide species, including residual solvent and some buffer components.

The retention problem compounds it. A seven-residue peptide with two basic residues and no hydrophobic face has almost nothing to hold onto a C18 stationary phase, so without ion-pairing it elutes at or near the void volume. That is precisely where salts, free amino acids, short truncation and deletion sequences and small process residues also elute. Trifluoroacetic acid as an ion-pairing additive improves retention and peak shape substantially, and a mixed-mode or hydrophilic-interaction separation is a genuinely orthogonal second method rather than a variation on the first. If a purity figure for a peptide this polar comes from one reversed-phase run, the near-void population is what it is least able to see.

The three prolines add a third effect. Cis-trans isomerization on the separation timescale broadens peaks and can split them into partially resolved pairs. A raised, tightly controlled column temperature usually coalesces them. A split peak here is a conformational artifact more often than a second species, and it is misread as an impurity often enough to be worth stating.

Mass spectrometry is comparatively kind here. At seven residues, losing a single amino acid is a large fractional change in mass and stands out clearly, unlike a forty-residue peptide where a small deletion hides inside the isotope envelope. Tandem mass spectrometry producing a sequence ladder is what distinguishes Selank from a rearranged or truncated peptide of identical composition, because intact mass cannot order residues. One method note: amino acid analysis using ortho-phthalaldehyde derivatization does not react with secondary amines and misses proline, and this molecule is three-sevenths proline, so the derivatization chemistry matters here.

Compounds researchers confuse with Selank

Questions specific to Selank