Semax

Overview

Semax (ACTH 4-7-PGP) is a synthetic heptapeptide with the sequence Met-Glu-His-Phe-Pro-Gly-Pro. It was designed by modifying the ACTH(4-7) fragment with a C-terminal Pro-Gly-Pro (PGP) tripeptide to enhance enzymatic stability.[2] This modification renders the compound approximately 20 times more resistant to peptidase degradation compared to the native ACTH(4-10) fragment, extending its experimental duration from minutes to 20–24 hours.[3]

The compound is completely devoid of the hormonal corticotropic activity associated with full-length ACTH, meaning it does not stimulate adrenal cortisol production.[4] Regulatory records in the Russian Federation cite Semax's registration as a pharmaceutical on the "Vital and Essential Drugs" list for investigations related to stroke, cognitive disorders, and optic nerve conditions.[5] It is not registered by the U.S. FDA and has been flagged for immunogenicity concerns in compounding contexts.[6]

Semax was originally developed by the Institute of Molecular Genetics of the Russian Academy of Sciences (IMG RAS).[7] Upon enzymatic degradation, the PGP tripeptide fragment possesses its own independent experimental activity, including neuroprotective and anti-ulcer effects, contributing to the overall profile of the compound.[8]

Mechanism of Action

Receptor Targets and Binding

Specific binding sites for Semax have been identified in basal forebrain membranes. Binding is reversible, specific, and time-dependent, with a dissociation constant (K_d) of 2.4 ± 1.0 nM and maximal binding capacity (B_max) of 33.5 ± 7.9 fmol/mg protein.[9] This binding strictly requires calcium ions (Ca²⁺) and is blocked by manganese ions (Mn²⁺), characteristic of G-protein-coupled receptor interactions.[10]

In receptor assays, Semax acts as a competitive antagonist of α-melanocyte-stimulating hormone (α-MSH) at the MC4 and MC5 melanocortin receptors. No antagonism was observed at MC3.[11] Additionally, Semax inhibits enzymes responsible for enkephalin degradation (IC₅₀ = 10 μM).[12]

BDNF/TrkB Signaling Cascade

Semax stimulates tyrosine phosphorylation of TrkB receptors (the high-affinity receptor for BDNF), producing a 1.5–1.6-fold increase in TrkB phosphorylation in the hippocampus within 3 hours of administration.[9] In glial cell cultures, BDNF mRNA increased 8-fold and NGF mRNA 5-fold within 30 minutes.[13] In vivo, Semax increased hippocampal BDNF protein by 1.4-fold and exon III BDNF mRNA by 3-fold.[14]

Neurotransmitter Modulation

Semax activates the dopaminergic and serotonergic systems. It increases extracellular levels of 5-HIAA (a serotonin metabolite) in the striatum by approximately 25%.[1] While it does not alter basal dopamine levels alone, it significantly potentiates dopamine release induced by D-amphetamine.[1] Semax also regulates intracellular calcium homeostasis, preventing Ca²⁺ deregulation under glutamate excitotoxicity conditions.[10]

Gene Expression and Transcription

Genome-wide transcriptional analysis following ischemia-reperfusion demonstrated that Semax modulates 394 differentially expressed genes. It upregulates neurotransmission-related genes including Gpr6, Drd2, Hes5, and Gpr88, while downregulating pro-inflammatory genes such as Il1b, Il6, and Ccl6.[15][16]

Pharmacokinetic Paradox

Despite a relatively short plasma elimination half-life of approximately 1–2 hours, the experimental effects of a single intranasal dose persist for 20–24 hours.[5] Intranasal bioavailability is reported at 60–70%, with rapid CNS penetration across the blood-brain barrier.[17]

Research Applications

In laboratory research, Semax is utilized in multiple experimental paradigms:

1. Ischemic Stroke and TIA Models — Registered in the Russian Federation for investigations related to acute ischemic stroke. Experimental readouts involve suppression of inflammatory genes and activation of neurotransmitter gene networks in the penumbra zone.[15][16]
2. Cognitive Enhancement / Nootropic Paradigms — Studied for effects on memory consolidation and selective attention. In experimental protocols, a single intranasal dose produced measurable cognitive effects lasting 20–24 hours.[18]
3. Optic Nerve Investigations — Experimental readouts involving optic nerve atrophy and glaucomatous neuropathy models demonstrate improved visual acuity, expanded visual field, and increased optic nerve electric sensitivity.[19]
4. Neuroprotection and Oxidative Stress — In neuronal cultures, Semax demonstrated anti-apoptotic effects, protecting against glutamate neurotoxicity and oxidative stress. It contributes to mitochondrial stability and upregulates neurotrophin expression (BDNF, NGF).[9][14]
5. Stress and Anxiety Models — In rodent models of chronic stress, Semax exhibited anxiolytic effects linked to modulation of dopaminergic and serotonergic systems and normalization of hippocampal BDNF levels. Corticosterone levels decreased 28–34%.[20]
6. Alzheimer's Disease Models — In APP/PS1 transgenic mice, intranasal Semax reduced amyloid plaque count in cortex by 2.8-fold and hippocampus by 2.6-fold (p<0.0001), with restored cognitive function.[21]
7. Spinal Cord Injury Models — Improved functional recovery scores via μ-opioid receptor (Oprm1) / USP18 / FTO pathway, inhibiting lysosomal membrane permeabilization and pyroptosis.[22]
8. Gastric Ulcer Investigations — In a comparative study, ulcer healing was observed in 89.5% of subjects receiving Semax vs. 30.8% in the control group at day 14.[23]
9. Immune System Modulation — Transcriptional analyses indicate Semax influences immunoglobulin and chemokine gene expression, with potential antiviral activity observed in experimental influenza models.[16]

Preclinical Research Summary

Animal Studies

Clinical Studies / Human Data

Pharmacokinetic Parameters

Comparison with Related Compounds