Semax (molecular formula C37H51N9O10S, monoisotopic molecular weight 813.92 Da) is a synthetic linear heptapeptide with the sequence: Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP-OH). It is structurally based on the N-terminal core fragment of adrenocorticotropic hormone, ACTH(4-10), which had long been recognized to exert positive modulatory actions on attention, visual memory, and learning in animal models.

However, native ACTH(4-10) exhibits an in vivo half-life of only several minutes due to rapid enzymatic degradation by circulating and tissue peptidases. To overcome this limitation, Russian researchers appended the tripeptide sequence Pro-Gly-Pro (PGP) to the C-terminus of ACTH(4-7).

The terminal PGP motif alters the spatial conformation of the peptide, sterically shielding the adjacent Phe-Pro bond from endopeptidases and preventing carboxypeptidase degradation. As a result, Semax exhibits a 20-to-30-fold increase in metabolic half-life in biological fluids and achieves high bioavailability across mucosal and blood-brain barriers.

The primary cellular mechanism driving Semax's cognitive and neuroprotective efficacy is the robust stimulation of neurotrophin expression. Dolotov et al. (Neurochem Res 2006) demonstrated that Semax administration in rats produces a rapid and dramatic increase in brain-derived neurotrophic factor (BDNF) protein and mRNA levels within the hippocampus and frontal cortex.

Within 3 hours of administration, BDNF mRNA expression increases by 200% to 300% in the CA1 and dentate gyrus regions of the hippocampus. Concurrently, Semax upregulates the mRNA and protein expression of tropomyosin receptor kinase B (TrkB), the high-affinity receptor for BDNF.

Binding of BDNF to TrkB stimulates receptor dimerization and autophosphorylation of intracellular tyrosine residues, activating three classical downstream neuroplasticity cascades: (1) the MAPK/ERK pathway, which promotes synaptic plasticity and long-term potentiation (LTP); (2) the PI3K/Akt pathway, which promotes neuronal survival and inhibits apoptotic caspases; and (3) the PLC-gamma/IP3 pathway, which mobilizes intracellular calcium to enhance neurotransmitter release.

In addition to neurotrophic induction, Semax exerts neuromodulatory actions on central monoamine neurotransmission. In microdialysis studies in awake rodents, Semax stimulates the release and turnover of dopamine and serotonin (5-HT) in the striatum and prefrontal cortex, enhancing executive function, working memory, and sustained vigilance.

In models of focal cerebral ischemia (middle cerebral artery occlusion, MCAO), Semax administration reduces total ischemic infarct volume by 30% to 50% compared to untreated controls. Genome-wide expression profiling in ischemic brain tissue revealed that Semax modulates the expression of over 100 genes involved in the inflammatory and vascular response.

Specifically, Semax downregulates pro-inflammatory cytokines (IL-1beta, TNF-alpha, IL-6), prevents leukocyte adhesion to cerebral microvessels, and promotes collateral re-vascularization by inducing vascular endothelial growth factor (VEGF) synthesis in the ischemic penumbra.

Pure Semax is supplied as a white lyophilized crystalline cake (molecular weight 813.92 Da). Reconstitution protocol: for a standard 10.0 mg vial, adding 2.0 mL of Bacteriostatic Water USP (0.9% benzyl alcohol) yields a stock concentration of 5.0 mg/mL (5,000 mcg/mL). Adding 5.0 mL yields 2.0 mg/mL (2,000 mcg/mL).

Syringe calibration on a standard U-100 syringe (100 units = 1.0 mL; 1 unit = 0.01 mL = 50 mcg Semax at 5 mg/mL): a common research dose of 500 mcg corresponds to 10 units (0.10 mL); a 1,000 mcg (1.0 mg) dose corresponds to 20 units (0.20 mL). At 2.0 mg/mL: 500 mcg corresponds to 25 units (0.25 mL).

Solvation instructions: introduce diluent gently down the inside glass wall. Swirl slowly; complete optical dissolution occurs within 30 seconds. Storage parameters: Solid lyophilate is stable at -20°C for up to 36 months, or 2°C–8°C for 24 months. Reconstituted aqueous solution with 0.9% benzyl alcohol must be stored refrigerated at 2°C–8°C away from light and consumed within 28 days.

Semax displays an exemplary clinical safety and tolerability profile, validated through extensive human use in Eastern Europe where it is approved as a prescription medication for acute ischemic stroke, transient ischemic attacks, optic nerve disease, and cognitive recovery.

Crucially, because Semax lacks the steroidogenic ACTH(1-3) sequence, it does not bind to melanocortin MC2 receptors in the adrenal cortex and produces no elevation in circulating ACTH, cortisol, or corticosterone, completely avoiding the hypertension, hyperglycemia, and adrenal suppression associated with systemic corticosteroids.

Clinical trials and post-marketing surveillance involving thousands of patients have reported an absence of serious adverse events. Mild nasal mucosa irritation can occur with intranasal formulations, while parenteral administration produces no systemic cardiovascular, renal, or hepatic toxicity.