Selank and Semax are both synthetic heptapeptides from the Institute of Molecular Genetics of the Russian Academy of Sciences, and both end in the same Pro-Gly-Pro tripeptide. They differ in where the other four residues come from: Selank is built on tuftsin (Thr-Lys-Pro-Arg), a fragment of the immunoglobulin G heavy chain, while Semax is built on ACTH(4-7) (Met-Glu-His-Phe), a fragment of adrenocorticotropic hormone. Both are used as medicines in Russia; neither is approved by the FDA.
That shared design is the reason the two are so often discussed together. The comparison below keeps to what can be checked: sequence, molecular data, parent molecule, the systems each has been studied in, regulatory status and how each behaves at the bench.
Selank and Semax at a glance
| Selank | Semax | |
|---|---|---|
| Sequence | Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP) | Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP) |
| Length | 7 residues | 7 residues |
| Parent molecule | Tuftsin (TKPR), from the IgG heavy chain | ACTH(4-7) (MEHF), from adrenocorticotropic hormone |
| Molecular formula | C33H57N11O9 | C37H51N9O10S |
| Molecular weight | 751.9 g/mol | 813.9 g/mol |
| CAS number | 129954-34-3 | 80714-61-0 |
| PubChem CID | 11765600 | 9811102 |
| Other names | TP-7 | ACTH(4-7)PGP, ACTH(4-10) analog |
| Net character at neutral pH | Basic (Lys, Arg) | Mildly acidic (Glu), with His |
| Oxidation-prone residues | None of note | Met (position 1) |
| Developer | Institute of Molecular Genetics, RAS, with the Zakusov Institute of Pharmacology | Institute of Molecular Genetics, RAS |
| US status | Not FDA approved | Not FDA approved |
Formula, weight, CAS and CID values are taken from the PubChem compound records for each peptide. Catalog material is described on the Selank and Semax pages.
The shared design: a C-terminal Pro-Gly-Pro
Short regulatory peptides such as tuftsin and ACTH fragments are cleared quickly by peptidases. The group at the Institute of Molecular Genetics took the same approach with both molecules: keep a biologically active fragment and extend it at the C terminus with Pro-Gly-Pro. For Selank, the authors describe tuftsin "elongated at the C terminus via the addition of three natural L-amino acids (Pro-Gly-Pro)," with the stated aim of improving metabolic stability and lengthening the duration of action.
Pro-Gly-Pro belongs to a family the same group calls glyprolines. Proline residues restrict backbone rotation and are poor substrates for many common peptidases, which is the usual reasoning behind proline-rich caps on short peptides. The outcome is two heptapeptides of almost identical size (751.9 versus 813.9 g/mol) with the same last three residues and entirely different first four.
For background on how residues join and why sequence order matters, see what peptides are.
Selank: a tuftsin analog
Tuftsin is the tetrapeptide Thr-Lys-Pro-Arg, a short sequence in the heavy chain of immunoglobulin G. It is studied as an immunomodulatory peptide, and a large family of tuftsin analogs has since been made for immunology and other research.
Selank adds Pro-Gly-Pro to that tetrapeptide, giving Thr-Lys-Pro-Arg-Pro-Gly-Pro. With a lysine and an arginine it carries a net positive charge at neutral pH. It was designed and produced at the Institute of Molecular Genetics in cooperation with the V. V. Zakusov Research Institute of Pharmacology, and it is registered in Russia as an anxiolytic medicine. It has no FDA approval.
What Selank has been studied in
- Radioligand binding. In rat brain membrane preparations, Selank changed the specific binding of [3H]GABA in a way the authors describe as positive allosteric modulation at GABA-A receptors (Vyunova and colleagues, 2018).
- Enzyme inhibition. An earlier study reported that Selank inhibits enzymes that break down enkephalins in human plasma in vitro, with an IC50 of about 15 µM.
- Gene expression in rat brain. A 2016 study from the developing institute measured changes in mRNA for genes involved in GABAergic signaling in rat frontal cortex, one and three hours after exposure to Selank or GABA.
- Cell culture. In the human neuroblastoma line IMR-32, which expresses functional GABA-A receptors, Selank alone did not change expression of GABAergic genes, while its combination with GABA almost completely suppressed the expression changes seen with GABA alone (2017).
Most of this literature comes from Russian groups, much of it from the developing institute. That is worth stating plainly when weighing it.
Semax: an ACTH(4-10) analog
Adrenocorticotropic hormone is a 39-residue peptide. Its 4-10 region (Met-Glu-His-Phe-Arg-Trp-Gly) carries activity studied separately from the hormone's corticotropic role. Semax keeps the first four residues of that region, Met-Glu-His-Phe, and replaces Arg-Trp-Gly with Pro-Gly-Pro. The literature therefore calls it both ACTH(4-7)PGP and an ACTH(4-10) analog; the two names describe the same molecule.
Its developers state that Semax lacks the hormonal (corticotropic) activity of ACTH. Semax is included on the Russian Federation's List of Vital and Essential Drugs for Medical Application (Government Decree No. 2406-r, 2019). It has no FDA approval.
What Semax has been studied in
- Rat models of cerebral ischemia. Genome-wide transcription studies in rats with permanent middle cerebral artery occlusion measured changes in immune-related and vascular-related gene expression in brain tissue after Semax exposure (Medvedeva and colleagues, 2014). Later work used a rat ischemia-reperfusion model and profiled brain proteins.
- Copper and amyloid chemistry. Biophysical studies have examined Semax as a copper(II) chelator and its influence on copper-induced aggregation of amyloid-beta in membrane models, entirely in vitro.
- Transgenic mouse models. A 2025 study compared Semax with a derivative heptapeptide (Met-Glu-Asp-Arg-Pro-Gly-Pro) in a transgenic mouse model used in amyloid research, measuring amyloid inclusions in brain tissue.
As with Selank, the bulk of the work on Semax comes from Russian institutes, including the Institute of Molecular Genetics itself.
Semax vs Selank: the pathway each is tied to
Online sources group Semax and Selank together as "nootropic peptides". That is a marketing label, not a chemical class or a mechanism of action. The primary literature ties each synthetic peptide to a different pathway.
For Semax it is neurotrophic signaling. Dolotov and colleagues (2006) measured brain-derived neurotrophic factor (BDNF) and its receptor, trkB, in rat hippocampus after exposure to Semax, at both the protein and the mRNA level, and proposed the BDNF/trkB system as part of how the peptide acts. For Selank it is the GABA-A receptor and enkephalin-degrading enzymes, as set out above, which is the molecular background to its registration in Russia as an anxiolytic.
Where they differ, point by point
| Question | Selank | Semax |
|---|---|---|
| Which natural peptide does the active part come from? | Tuftsin, an immunoglobulin fragment | ACTH, a pituitary hormone |
| Main molecular targets described | GABA-A receptor binding (allosteric), enkephalin-degrading enzymes | Gene-expression programs in ischemia models; copper binding |
| Typical research systems | Rat brain membranes, IMR-32 cells, rat stress models | Rat cerebral ischemia models, in-vitro amyloid and copper assays, transgenic mice |
| Charge at neutral pH | Net positive | Near neutral to mildly negative |
| Chemical liabilities | Few; no Met, Cys or Trp | Met can oxidize to the sulfoxide (+16 Da) |
| Regulatory status | Registered in Russia; not FDA approved | On Russia's essential drugs list; not FDA approved |
The practical point for a lab is that the molecules are not interchangeable. They share a tail, but the receptor systems, the parent-molecule literature and the chemistry of the N-terminal half are all different.
Laboratory handling
Both peptides are supplied by Anhydrolabs as lyophilized powder in vacuum-sealed vials, and also as 10-vial kits. Each vial is labeled by net content (for example, 5 mg).
Dissolving. Both are short and hydrophilic, and both dissolve readily in water. Selank, being basic, dissolves well in sterile water or dilute acetic acid. Semax dissolves in water or neutral buffer. Stock solutions are usually prepared at a stated concentration, such as 1 mg/mL, and diluted into assay buffer. Our general method is in how to reconstitute peptides.
Oxidation. Semax's N-terminal methionine is the main stability concern for either molecule. Methionine oxidizes to methionine sulfoxide, which appears in mass spectra as a +16 Da species and often elutes slightly earlier in reversed-phase HPLC. Degassed solvents, minimal headspace and cold storage limit it.
Storage. Lyophilized vials are best kept at -20 °C, dry and away from light. Once in solution, split the stock into single-use aliquots and freeze them, which avoids repeated freeze-thaw. More detail is on the storage and handling page.
Identity and purity. Mass spectrometry distinguishes the two instantly: the monoisotopic masses differ by about 62 Da. Purity is reported by HPLC as the area of the principal peak relative to all integrated signals. The peptide purity article explains what that number does and does not tell you.
Reading the Selank and Semax literature
Three habits make the published work on these two peptides easier to use.
Check the sequence in every paper. Several derivatives circulate under related names. The 2025 Acta Naturae study, for example, compares Semax with Met-Glu-Asp-Arg-Pro-Gly-Pro, a different heptapeptide. Acetylated and amidated variants of Selank are also sold under modified names, and N-acetyl Semax amidate has its own PubChem record (CID 172638603, C39H54N10O10S, 855.0 g/mol), about 41 Da heavier than Semax. A paper's methods section should state the exact sequence and salt form; if it does not, treat comparisons across papers with caution.
Note the salt form. Selank is often described in the literature as a diacetate salt. Counter-ions add mass, so the peptide content of a vial of a salt is lower than its gross weight. Certificates of analysis report this as net peptide content; our guide to reading a certificate of analysis covers how.
Separate the language of the source from the data. Russian papers often use the registered-medicine framing ("the drug Semax") because both are marketed there. The data themselves are usually receptor binding, gene expression or tissue measurements in rodents and cell lines, and that is the level at which they can be compared.
Related compounds
Both peptides sit in a wider group of short regulatory peptides studied in Russia. For a very different example, a porcine-brain peptide mixture rather than a defined sequence, see our note on Cerebrolysin. All Anhydrolabs material is supplied for in-vitro laboratory research only, as set out on the research use page.
