Semax vs N-Acetyl Semax Amidate: A Structural and Regulatory Comparison

Introduction

Semax and N-Acetyl Semax Amidate share every amino acid. Both are the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro, a synthetic analog of a fragment of adrenocorticotropic hormone (ACTH) designed in the Soviet Union in the early 1980s. What differs is the two ends of the chain: N-Acetyl Semax Amidate carries an acetyl group on its N-terminal amine and a primary amide in place of its C-terminal carboxylate. Those two small modifications change the molecule's terminal charge, its mass, and, on the rationale published for peptides generally, its exposure to exopeptidase cleavage. They also separate the compounds sharply on regulatory status, since Semax is a registered drug in the Russian Federation and its terminally modified variant is registered nowhere. This article compares the two on structure, origin, published research record, and regulatory standing, reporting what investigators and regulators have published without drawing conclusions about the suitability of either compound for any purpose. Research-grade Semax and N-Acetyl Semax Amidate reference materials are cataloged with per-batch certificates of analysis.

Semax: The ACTH(4-7)-Pro-Gly-Pro Heptapeptide

Semax (Met-Glu-His-Phe-Pro-Gly-Pro; one-letter code MEHFPGP) has a calculated mass of roughly 814 daltons. Its design is described in a 2013 review by Kolomin, Shadrina, Slominsky, Limborska, and Myasoedov: the group of Academicians Igor Ashmarin and Nikolai Myasoedov at the Institute of Molecular Genetics, Russian Academy of Sciences, took the tetrapeptide core of ACTH(4-7), Met-Glu-His-Phe, and appended the tripeptide Pro-Gly-Pro to the C-terminus [1]. The same review reports that the Pro-Gly-Pro extension was chosen to resist carboxypeptidase-mediated cleavage, which limited the usefulness of the natural ACTH(4-10) fragment in earlier animal studies [1]. Semax is therefore sometimes written as ACTH(4-7)-PGP, a shorthand that captures its two-part construction.

Even with that extension, the molecule is degraded in biological fluids. Inozemtseva and colleagues reported in Peptides in 1993 that rat serum enzymes cleaved both ACTH(4-10) and Semax, and used enzyme inhibitors to characterize which classes of protease were responsible [2]. Shevchenko and colleagues reported in the Russian Journal of Bioorganic Chemistry in 2006 that, following administration to rats, intact Semax declined rapidly while the tripeptide Pro-Gly-Pro accumulated as a principal metabolite [3]. Findings from research models do not establish safety or efficacy in humans. Sparta Labs makes no claims about the use of this compound.

The mechanistic literature centers on the melanocortin receptor family and on neurotrophin expression. Shadrina and colleagues reported in Neuroscience Letters in 2001 that Semax rapidly induced neurotrophin mRNAs in rat glial cell cultures [4], and Dolotov and colleagues reported in the Journal of Neurochemistry in 2006 that Semax bound specifically in rat basal forebrain preparations and was associated with changes in brain-derived neurotrophic factor protein levels [5]. Tabbì and colleagues reported in the Journal of Inorganic Biochemistry in 2015 that Semax binds copper(II) ions with high affinity, a property that turns out to be relevant to its acetylated variant [6]. The Semax research overview and the Semax mechanism of action article cover this work in depth, and the Selank vs Semax comparison places Semax among its Russian regulatory-peptide relatives.

On regulatory status, Semax received registration from the Russian Ministry of Health in 1994 following phase I and II trials conducted between 1990 and 1996, and the approved formulation is a nasal solution [1]. Deigin and colleagues, reviewing Russian peptide biopharmaceuticals in Pharmaceutics in 2022, listed Semax among registered products and noted its inclusion on the Russian List of Vital and Essential Drugs [7]. Semax has not been approved by the FDA or by the European Medicines Agency for any indication. In the United States, Semax was nominated for consideration as a bulk drug substance under section 503A of the Federal Food, Drug, and Cosmetic Act and was placed on the agenda of the FDA Pharmacy Compounding Advisory Committee meeting of July 23-24, 2026 [8]. The outcome of that proceeding is not described here; an advisory committee review is not an approval, and no approval had been granted at the time of the sources cited.

N-Acetyl Semax Amidate: The Terminally Capped Variant

N-Acetyl Semax Amidate is Ac-Met-Glu-His-Phe-Pro-Gly-Pro-NH2. The backbone is identical to Semax; the changes are confined to the two termini, and each has a specific chemical meaning.

N-terminal acetylation. In unmodified Semax, the N-terminal methionine carries a free alpha-amino group. At physiological pH that amine is largely protonated (NH3+) and contributes a positive charge. Acetylation forms an amide bond between the amine and an acetyl group (CH3CO-), consuming the lone pair that would otherwise be protonated. The result is a neutral N-terminus that structurally resembles an internal peptide bond. The modification adds 42 daltons.

C-terminal amidation. In unmodified Semax, the C-terminal proline carries a free carboxyl group, deprotonated to a carboxylate (COO-) at physiological pH and contributing a negative charge. Amidation replaces the hydroxyl of that carboxyl with an amino group, giving a primary carboxamide (CONH2) that is neutral. The modification lowers the mass by about one dalton, so the dual-modified molecule has a calculated mass of roughly 855 daltons. C-terminal amidation also occurs naturally in many endogenous peptide hormones.

The rationale for terminal capping is documented for peptides in general rather than for this compound in particular. Fosgerau and Hoffmann, reviewing peptide therapeutics in Drug Discovery Today in 2015, described N-terminal acetylation and C-terminal amidation among the standard structural modifications used to slow degradation by exopeptidases, the enzymes that trim peptides from either end and that require a free terminal amine (aminopeptidases) or a free terminal carboxylate (carboxypeptidases) to act [9]. Whether and to what extent this general rationale translates into a measurable stability difference for N-Acetyl Semax Amidate specifically has not been established in the sources cited here.

Two published studies bear directly on the acetylated form. Magrì and colleagues reported in the Journal of Inorganic Biochemistry in 2016 that N-terminal acetylation changed how Semax coordinates copper(II) and zinc(II): the copper center shifted from the CuN4 arrangement observed with free Semax, in which the N-terminal amine participates, to a distorted CuN3O arrangement with the acetylated peptide [10]. This is a direct consequence of removing the amine as a metal-binding donor. Levitskaya and colleagues reported in Biology Bulletin in 2005 on a series of Semax analogs modified at the N-terminal region, tested in rat behavioral models; they reported that the methionine at position 1 was critical for activity and that certain acyl modifications of its alpha-amino group altered rather than abolished the observed effects [11]. Neither study examined C-terminal amidation, and neither compared the dual-modified amidate against the parent compound in the same experiment. The N-Acetyl Semax Amidate research overview discusses what the sparse dedicated literature does and does not show.

N-Acetyl Semax Amidate holds no regulatory approval in any jurisdiction. It is not separately named in the FDA 503A proceeding that concerns Semax [8], and the Russian registration of Semax does not extend to it. In the United States it is supplied strictly as a research-use-only material.

Structural Comparison

• Sequence. Identical: Met-Glu-His-Phe-Pro-Gly-Pro in both compounds.
• N-terminus. Semax: free alpha-amine, positively charged at physiological pH. N-Acetyl Semax Amidate: acetylated, neutral.
• C-terminus. Semax: free carboxylate, negatively charged at physiological pH. N-Acetyl Semax Amidate: primary amide, neutral.
• Mass. Roughly 814 daltons for Semax against roughly 855 daltons for the acetylated amidate, a difference of about 41 daltons accounted for entirely by the two terminal groups.
• Net charge. Approximately the same. The glutamate side chain contributes a negative charge in both; Semax's terminal charges (+1 and -1) cancel, and the amidate has none. What changes is the location of charge, not its sum: the capped molecule loses the zwitterionic ends.
• Metal coordination. Different, as reported by Magrì and colleagues: the free N-terminal amine of Semax participates in copper(II) binding, and acetylation removes that donor [6, 10].
• Exopeptidase exposure. On the general rationale reviewed by Fosgerau and Hoffmann, the capped termini present no free amine or carboxylate for aminopeptidases or carboxypeptidases to recognize [9]; the internal Pro-Gly-Pro motif is shared by both.
• Origin. Semax: Institute of Molecular Genetics, Russian Academy of Sciences, early 1980s [1]. N-Acetyl Semax Amidate: a later synthetic derivative without a single canonical discovery publication.
• Regulatory status. Semax: registered drug in the Russian Federation, not FDA- or EMA-approved [1, 7]. N-Acetyl Semax Amidate: not approved anywhere.

Pharmacological Class Context

Both compounds are classified as synthetic melanocortin peptides, specifically noncorticotropic ACTH-fragment analogs. The melanocortin system comprises five G-protein-coupled receptor subtypes (MC1R through MC5R) whose natural ligands derive from proopiomelanocortin; ACTH(4-10) is regarded as a minimal pharmacophore within that system [1]. The steroidogenic activity of ACTH requires a much longer sequence, ACTH(1-24) or greater, so neither heptapeptide engages the adrenal axis that defines the parent hormone [1]. In this respect the class is distinct from the other ACTH-derived research peptides in the library, and from unrelated peptides that happen to carry the same terminal modifications.

The research records differ mainly in depth. Semax has decades of Russian-language and English-language literature, a clinical development program, and a national registration [1, 7]. N-Acetyl Semax Amidate has a small number of chemistry and structure-activity papers, most of which characterize the acetylation alone [10, 11]. No study cited here compared the two compounds under the same conditions, and this article accordingly makes no comparative statement about activity or efficacy.

Research-grade Semax and N-Acetyl Semax Amidate sold by chemical suppliers are laboratory reference materials, not pharmaceutical products, and are strictly for research use only.

References

1. Kolomin T, Shadrina M, Slominsky P, Limborska S, Myasoedov N. A new generation of drugs: synthetic peptides based on natural regulatory peptides. Neuroscience & Medicine. 2013;4:223-252. DOI: 10.4236/nm.2013.44035
2. Inozemtseva LS, Dolotov OV, Zolotarev YA, Dolotova OS, Andreeva LA, Myasoedov NF. Degradation of ACTH/MSH(4-10) and its synthetic analog semax by rat serum enzymes: an inhibitor study. Peptides. 1993;14(4):745-750. PMID: 8392718. DOI: 10.1016/0196-9781(93)90104-U
3. Shevchenko KV, Nagaev IY, Alfeeva LY, Andreeva LA, Kamenskii AA, Levitskaia NG, et al. Kinetics of Semax penetration into the brain and blood of rats after its intranasal administration. Russ J Bioorg Chem. 2006;32(1):57-62. DOI: 10.1134/S1068162006010055
4. Shadrina MI, Dolotov OV, Grivennikov IA, Slominsky PA, Andreeva LA, Inozemtseva LS, Limborska SA, Myasoedov NF. Rapid induction of neurotrophin mRNAs in rat glial cell cultures by Semax, an adrenocorticotropic hormone analog. Neurosci Lett. 2001;308(2):115-118. PMID: 11457573. DOI: 10.1016/S0304-3940(01)01994-2
5. Dolotov OV, Karpenko EA, Inozemtseva LS, Seredenina TS, Levitskaya NG, Zolotarev YA, Kamensky AA, Grivennikov IA, Engele J, Myasoedov NF. Semax, an analogue of adrenocorticotropin (4-10), binds specifically and increases levels of brain-derived neurotrophic factor protein in rat basal forebrain. J Neurochem. 2006;97 Suppl 1:82-86. PMID: 16635254. DOI: 10.1111/j.1471-4159.2006.03658.x
6. Tabbì G, Magrì A, Giuffrida A, Lanza V, Pappalardo G, Naletova I, Nicoletti VG, Attanasio F, Rizzarelli E. Semax, an ACTH4-10 peptide analog with high affinity for copper(II) ion and protective ability against metal induced cell toxicity. J Inorg Biochem. 2015;142:39-46. PMID: 25310602. DOI: 10.1016/j.jinorgbio.2014.09.014
7. Deigin VI, Poluektova EA, Beniashvili AG, Kozin SA, Poluektov YM. Development of peptide biopharmaceuticals in Russia. Pharmaceutics. 2022;14(4):716. PMCID: PMC9030433. DOI: 10.3390/pharmaceutics14040716
8. US Food and Drug Administration. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory Committee. Advisory committee calendar. FDA advisory committee calendar
9. Fosgerau K, Hoffmann T. Peptide therapeutics: current status and future directions. Drug Discov Today. 2015;20(1):122-128. PMID: 25450771. DOI: 10.1016/j.drudis.2014.10.003
10. Magrì A, Munzone A, Peana M, Medici S, Zoroddu MA, Hansson Ö, et al. Influence of the N-terminus acetylation of Semax, a synthetic analog of ACTH(4-10), on copper(II) and zinc(II) coordination and biological properties. J Inorg Biochem. 2016;164:59-69. PMID: 27586814. DOI: 10.1016/j.jinorgbio.2016.08.013
11. Levitskaya NG, Sebentsova EA, Andreeva LA, Alfeeva LY, Kamenskiy AA, Myasoedov NF. Effect of modification of the N-terminal region of Semax on the expression of nootropic effect of Semax analogs. Biol Bull. 2005;32(4):381-386. DOI: 10.1007/s10525-005-0116-0