Educational information only. Not medical advice.

Research Memos

Semax vs Dihexa

Semax is a seven-amino-acid peptide with small human neurological and brain-imaging reports. Dihexa is an angiotensin-IV-derived peptidomimetic with rodent research, a formal concern on its early pharmacology report and a retracted mechanism paper. Their separate studies do not establish which improves human memory more, and neither record provides a dependable human half-life comparison.

Research reviewed October 15, 2026. Educational information only.

Their peptide ancestry leads to different molecules

Semax is the seven-amino-acid sequence Met-Glu-His-Phe-Pro-Gly-Pro, developed from a segment of adrenocorticotropic hormone, or ACTH. Dihexa, also called PNB-0408, is N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide, a synthetic molecule retaining selected peptide features, called a peptidomimetic derived from an angiotensin IV analogue. Shared interest in cognition does not give these compounds the same chemical identity or a shared established therapeutic effect.

Form also matters within each name. FDA’s 2026 Semax evaluation distinguishes free base and acetate and notes that clinical reports often do not identify which was used. FDA’s Dihexa entry specifically names acetate. A result from one preparation cannot establish absorption or clinical equivalence for another merely because the shorter compound name is the same.

The main differences

What are the actual molecules?

The parent structures and the acetate forms are distinct pieces of the evidence. Clinical reports do not always specify the chemical form.

Semax

Semax is the seven-amino-acid sequence Met-Glu-His-Phe-Pro-Gly-Pro, developed from an ACTH fragment. FDA evaluates the free base and acetate separately and notes unclear form reporting in clinical papers.

Sources

Dihexa

Dihexa / PNB-0408 is the defined angiotensin-IV-derived peptidomimetic N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide. FDA’s safety entry specifically names Dihexa acetate.

Sources

What does the proposed biology rest on?

A protein or signaling measurement explains a biological hypothesis. It does not place the two compounds on a common human-benefit or potency scale.

Semax

A rat hippocampus study measured more brain-derived neurotrophic factor (BDNF), a protein supporting nerve-cell function, and greater activation of its TrkB receptor after Semax. These are nerve-support signaling measurements, not a measured human memory improvement.

Sources

Dihexa

The prominent 2014 HGF/c-Met mechanism paper was retracted in 2025 for falsified or fabricated data in specified figures and an erratum submission. Later mouse signaling and rat antagonist experiments are separate observations; they do not restore those withdrawn measurements.

Sources

What evidence concerns actual human function?

No direct Semax-versus-Dihexa trial was identified. The human and animal studies below test different populations and measurements.

Semax

An older comparison of 30 Semax-treated stroke patients and 80 conventional-treatment controls reported faster neurological improvement, but random allocation, blinded assessment and a dependable treatment-effect estimate are not established. A separate 24-person study measured resting brain-network changes, without demonstrating a memory benefit.

Sources

Dihexa

Dihexa-treated APP/PS1 mice found a hidden swimming-maze platform faster than mice with the same genetic changes given preparation without Dihexa. Head-injured rats given Dihexa directly into a brain ventricle made more correct maze choices than injured rats given buffered salt solution (DPBS). A separate 40-rat toxin study found no protection of measured movement, learning or weight gain. Human cognitive benefit remains unestablished.

Sources

Can measured exposure establish which lasts longer in people?

Routes, species and analytes matter. A tissue radioactivity signal, plasma concentration and behavioral result cannot be substituted for one another.

Semax

FDA found no human pharmacokinetic study of free base or acetate by any route. Its rat review describes labeled-peptide distribution and rapid breakdown in blood and brain; label measurements need separation of parent peptide from metabolites.

Sources

Dihexa

A 2013 rat report with a Notice of Concern calculated terminal plasma half-lives of 12.68 days intravenously and 8.83 ± 2.41 days intraperitoneally (mean ± standard error), with four rats in the latter analysis. These do not establish human exposure or effect duration. FDA found no human exposure data for Dihexa acetate.

Sources

What remains uncertain about harms?

The sources do not provide a shared clinical safety comparison. An absence of sufficiently studied harms cannot establish safety.

Semax

FDA found insufficient clinical safety information and no safety data for the proposed subcutaneous route. It raised possible bleeding and immune-reaction concerns without establishing reliable event rates. Russian nasal-drop registration does not establish safety for every form or route.

Sources

Dihexa

FDA’s acetate entry states that important human safety information is missing, including whether administration would cause harm. The reviewed rodent studies cannot quantify uncommon events, medicine interactions or long-term human risks.

Sources

What the biological measurements actually show

Dolotov and colleagues (2006) gave Semax to rats and measured an increase in the amount of brain-derived neurotrophic factor, or BDNF, in the hippocampus, a region involved in learning. The investigators also measured more activation of its TrkB receptor and changes in RNA messages used to make these proteins. BDNF helps support nerve-cell function. An increase in this protein is a biological observation; it does not measure how much a person’s memory or stroke recovery improves.

Dihexa’s best-known mechanism claim involved hepatocyte growth factor, or HGF, and its MET receptor. The 2014 paper supporting that claim was retracted in 2025. The notice reports falsified or fabricated data in Figures 1B and 2A/C and a subsequent erratum submission after a Washington State University investigation. The withdrawn measurements cannot establish binding strength, receptor activation or the proposed explanation of improved learning.

The later APP/PS1 mouse study linked Dihexa treatment to changes in PI3K/AKT cell-survival signaling and to better maze performance. The 2025 rat head-injury experiment reported that a MET antagonist blocked Dihexa-associated improvement after direct brain administration. Those experiments have their own methods and limitations. They do not restore the retracted data or provide a common scale on which to compare Semax and Dihexa potency.

Semax’s human reports measure different things

Gusev and colleagues (1997) compared 30 Semax-treated patients with 80 conventionally treated controls and reported faster improvement in neurological deficits, especially movement problems. Random allocation, blinded outcome assessment and a reliable numerical treatment effect are not established. Treatment-group differences, other care and assessment methods could therefore influence the reported recovery.

A separate Lebedeva et al. (2018) imaging report compared intranasal Semax with placebo before and within 20 minutes after administration. It reported a larger medial-frontal component of the default mode network, a pattern of coordinated brain activity measured at rest. This is a functional-imaging result, not evidence that brain tissue grew over minutes. It also does not establish better memory, attention or daily functioning.

The current FDA assessment found insufficient information to characterize Semax’s clinical safety and insufficient effectiveness evidence for the uses it evaluated. These reports show that human investigation occurred. They do not support a general cognitive-enhancement claim or a dependable clinical comparison with Dihexa.

Dihexa’s later animal findings include an unfavorable result

The early Dihexa analogue report described learning and exposure findings in rats and cell experiments. It carries a 2021 Notice of Concern, distinct from the subsequent retraction of the 2014 mechanism paper.

Later work tested different neurological problems. In APP/PS1 mice, Dihexa treatment was associated with faster hidden-platform learning and more crossings of the platform’s former location. Administration was described inconsistently in the paper, limiting a firm oral-delivery claim. In rats after repeated mild head injury, direct infusion into a fluid-filled cavity in the brain (ventricle) improved delayed maze choices. Conversely, 40 rats exposed to a toxin that disrupts energy metabolism did not show protection of the measured learning, movement or weight-gain outcomes with Dihexa over five weeks. The size and precision of any remaining effect are unclear.

These experiments do not test the same condition as Semax’s stroke reports or the same outcome as its human imaging study. Animal maze performance cannot be translated into a relative human memory benefit by comparing verbal descriptions of the results.

The pharmacokinetic gaps prevent a human duration ranking

FDA found no human pharmacokinetic study of Semax free base or acetate by any route. Its review describes rat experiments using labeled peptide after intranasal or intravenous administration, alongside rapid breakdown in blood and brain tissue. Radioactivity associated with brain tissue needs to be separated into intact peptide and breakdown products before it can establish exposure to the parent compound. A nasal route and a short-term imaging change do not supply a measured human concentration curve.

The Dihexa rat report under a formal concern calculated a 12.68-day terminal half-life after intravenous administration and 8.83 ± 2.41 days (mean ± standard error) after intraperitoneal administration, with four rats in the latter analysis. These values are neither human estimates nor a duration of memory improvement. FDA’s Dihexa acetate assessment reports no identified human exposure data. A claim that one compound lasts longer in people would therefore run ahead of the measurements available for both.

Neither evidence base supplies a sound personal-use comparison

FDA’s Semax assessment found insufficient clinical safety information, no human pharmacokinetic studies and no safety data for the proposed subcutaneous route. It also raised possible bleeding concerns from reported antithrombotic activity and concerns about immune reactions to aggregates or peptide-related impurities. These are identified uncertainties and possible hazards, not measured rates of harm. The review describes a registered Russian nasal-drop medicine; that country-specific history cannot establish the safety of a different preparation or route.

For Dihexa acetate, FDA states that important safety information is missing, including whether administration would cause harm. Neither the questioned long rat half-life nor favorable maze experiments resolve human risks. No direct trial identified in this review establishes which compound is more effective or safer. The compounds’ different preparations, routes and outcomes leave their relative clinical effects unknown.

Frequently asked questions

Has Dihexa been shown to be more potent than Semax?

No shared human study or suitable common assay in this review establishes that ranking. A BDNF measurement in Semax-treated rats and a Dihexa maze result concern different measurements. The retracted Dihexa mechanism paper cannot supply a reliable extreme-potency claim.

Does a larger brain-network component mean new brain tissue formed?

The Semax imaging report described the spatial pattern of coordinated resting brain activity. It did not demonstrate new anatomical tissue or better cognitive performance. A change measured within minutes needs to be interpreted as the functional-imaging observation actually made.

Why is the Dihexa publication warning repeated beside results?

The 2013 report carrying a Notice of Concern supplies the frequently quoted early exposure measurements. The 2014 mechanism paper was separately retracted. The exposure estimates therefore remain observations from a report under a formal journal concern.

Does more human investigation establish that Semax works?

The small clinical and imaging reports show that people were studied. Unclear allocation, limited numerical outcomes and imaging rather than functional endpoints prevent a general cognitive-benefit conclusion. FDA’s current evaluation found insufficient evidence for the uses it evaluated and inadequate clinical safety information.

Does either source establish a human half-life suitable for comparison?

FDA found no human pharmacokinetic study of Semax free base or acetate and no identified human exposure data for Dihexa acetate. Dihexa’s long estimates are rat results from a report with a formal concern. No dependable human duration ranking or administration schedule follows.

Semax — full profile · Selected studies

Dihexa — full profile · Selected studies

Compare other compounds · Explore the guide previews

References

  1. U.S. Food and Drug Administration, Center for Drug Evaluation and Research. FDA Briefing Document: Semax-Related Bulk Drug Substances (Semax free base and Semax acetate). Pharmacy Compounding Advisory Committee, 23–24 July 2026.

    Complete official 69-page FDA PDF accessed October 15, 2026. Relevant sections freshly read; this is a regulatory assessment, not a treatment trial. PDF pages 8–9 identity; nonclinical PK assessment; pages 37 and 42 human PK/safety; clinical stroke-study assessment.

  2. Dolotov OV, Karpenko EA, Inozemtseva LS, Seredenina TS, Levitskaya NG, Rozyczka J, Dubynina EV, Novosadova EV, Andreeva LA, Alfeeva LY, Kamensky AA, Grivennikov IA, Myasoedov NF, Engele J. Semax, an analog of ACTH(4-10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus. Brain Research. 2006;1117(1):54–60. doi:10.1016/j.brainres.2006.07.108.

    Complete original PubMed abstract accessed October 15, 2026; full methods not acquired. Abstract: sequence, rat hippocampus BDNF/TrkB measurements and conditioned avoidance.

  3. Gusev EI, Skvortsova VI, Miasoedov NF, Nezavibat'ko VN, Zhuravleva EIu, Vanichkin AV. [Effectiveness of semax in acute period of hemispheric ischemic stroke (a clinical and electrophysiological study)]. Zhurnal nevrologii i psikhiatrii imeni S.S. Korsakova. 1997;97(6):26–34. PMID:11517472.

    Verified baseline bibliography retained October 15, 2026. Direct original PubMed retrieval was incomplete in this pass; FDA 2026 evaluation freshly accessed. This is retained access, not a claim of new full-text acquisition. Original abstract and current FDA clinical review; study identity retained from 0.10.5.

  4. Lebedeva IS, Panikratova YR, Sokolov OY, Kupriyanov DA, Rumshiskaya AD, Kost NV, Myasoedov NF. Effects of Semax on the Default Mode Network of the Brain. Bulletin of experimental biology and medicine. 2018;165(5):653–656. doi:10.1007/s10517-018-4234-3. PMID:30225715.

    Verified baseline bibliography retained October 15, 2026. Direct original PubMed retrieval was incomplete in this pass; FDA 2026 evaluation freshly accessed. This is retained access, not a claim of new full-text acquisition. Original abstract and current FDA clinical review; study identity retained from 0.10.5.

  5. McCoy AT, Benoist CC, Wright JW, Kawas LH, Bule-Ghogare JM, Zhu M, Appleyard SM, Wayman GA, Harding JW. Evaluation of Metabolically Stabilized Angiotensin IV Analogs as Procognitive/Antidementia Agents. Journal of Pharmacology and Experimental Therapeutics. 2013;344(1):141–154. doi:10.1124/jpet.112.199497.

    Original PMC indexed full-text sections and PubMed figure legends accessed October 15, 2026. Direct PMC open returned a browser challenge. Read together with the 2021 Notice of Concern; full notice reasons were not acquired. Chemical design; Figure 2 tracer distribution; Figure 3 water maze; Figure 5 and Table 3 rat pharmacokinetics; Discussion.

  6. Journal of Pharmacology and Experimental Therapeutics. Notice of Concern: McCoy AT, Benoist CC, Wright JW, Kawas LH, Bule-Ghogare JM, Zhu M, Appleyard SM, Wayman GA, and Harding JW (2013) Evaluation of Metabolically Stabilized Angiotensin IV Analogs as Procognitive/Antidementia Agents, J Pharmacol Exp Ther, 344: 141-154; DOI: https://doi.org/10.1124/jpet.112.199497. 2021;378(3):313. doi:10.1124/jpet.112.199497concern.

    Original PubMed bibliographic notice opened October 15, 2026. No abstract or full notice text was available; no specific misconduct reason is assigned to this notice. PubMed title, notice linkage, date and DOI.

  7. Benoist CC, Kawas LH, Zhu M, Tyson KA, Stillmaker L, Appleyard SM, Wright JW, Wayman GA, Harding JW. Retraction notice to “The Procognitive and Synaptogenic Effects of Angiotensin IV–Derived Peptides Are Dependent on Activation of the Hepatocyte Growth Factor/c-Met System” [J Pharmacol Exp Ther 351 (2014) 390-402]. Journal of Pharmacology and Experimental Therapeutics. 2025;392(4):103567. doi:10.1016/j.jpet.2025.103567.

    Complete original PMC and publisher retraction statement accessed October 15, 2026. Publisher URL: https://www.sciencedirect.com/science/article/abs/pii/S0022356525397806 . Full retraction statement: Figures 1B, 2A/C, subsequent erratum data; responsibility statement.

  8. Sun X, Deng Y, Fu X, Wang S, Duan R, Zhang Y. AngIV-Analog Dihexa Rescues Cognitive Impairment and Recovers Memory in the APP/PS1 Mouse via the PI3K/AKT Signaling Pathway. Brain Sciences. 2021;11(11):1487. doi:10.3390/brainsci11111487.

    Original PubMed abstract and indexed original PMC Methods, Results and Discussion accessed October 15, 2026. Direct publisher retrieval failed; no separate correction/retraction was found in the checked original/indexed records and title/identifier searches. Methods 2.2; Results 3.1–3.2; Figure 2; PI3K inhibition experiments; Discussion route wording.

  9. Wells RG, Azzam AF, Hiller AL, Sardinia MF. Effects of an Angiotensin IV Analog on 3-Nitropropionic Acid-Induced Huntington’s Disease-Like Symptoms in Rats. Journal of Huntington’s Disease. 2024;13(1):55–66. doi:10.3233/JHD-231507.

    Original publisher page and original IOS Press indexed PDF abstract accessed October 15, 2026. Full methods, group denominators and outcome tables were not extracted. No correction/retraction was found in the checked publisher record or title search. Original abstract: 40 male Wistar rats, three randomized groups, five weeks and failure to protect.

  10. Martino KA, Nakhre A, Demarest RM, Devilbiss DM. Hepatocyte Growth Factor/MET Activator Rescues Working Memory Deficits after Repeated Mild Traumatic Brain Injury. Neurotrauma Reports. 2025;6(1):991–998. doi:10.1177/2689288X251392851.

    Original eight-page publisher PDF and indexed PubMed record accessed October 15, 2026. Supplementary methods were not acquired. No correction/retraction was found in the checked original records and title/identifier searches. PDF pages 2–5; Animals and surgery; Drug administration; Figure 1 delayed alternation and trial time; Figure 2 E-cadherin.

  11. U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Dihexa acetate entry, substances nominated but withdrawn. Accessed October 15, 2026.

    Current official FDA page opened and Dihexa row read October 15, 2026. The entry concerns acetate; it does not establish tested equivalence with the parent research material. Dihexa acetate row under substances nominated but withdrawn.