Dihexa

01 · What it is

The paper that gave Dihexa its mechanism was retracted in April 2025.

Dihexa is not a peptide. It is a peptidomimetic — N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide, also designated PNB-0408 ¹². Only two of its three units are amino acids. The compound was built by taking the three N-terminal residues of the angiotensin IV analogue norleucine¹-angiotensin IV, which an earlier study had identified as carrying the procognitive activity, then replacing the terminal residues with hexanoic acid and 6-aminohexanoic acid: aliphatic chains chosen to raise hydrophobicity and cut hydrogen bonding ¹. Those substitutions are what make it orally active and blood-brain-barrier permeant, and they are also what make it not a peptide. Guides that group it with Semax and Selank are placing it in the wrong chemical category.

What the retraction was. In 2014 a group at Washington State University published, in the Journal of Pharmacology and Experimental Therapeutics, the paper establishing that Dihexa’s procognitive and synaptogenic effects depend on activating the hepatocyte growth factor/c-Met system — reporting that Dihexa binds HGF with high affinity and that Dihexa and its parent compound induce c-Met phosphorylation in the presence of subthreshold HGF concentrations ². In April 2025 the journal published a retraction notice for it ¹³. The notice carries no abstract, so the grounds for retraction are not available from the indexed record, and this guide does not speculate about them. Three of the nine authors — Kawas, Wright and Harding — list M3 Biotechnology, Inc., Seattle among their affiliations on both the original paper and the retraction notice ^2,13.

What survives it. The retraction removes the paper, not the compound. Chemical synthesis and behavioural results were published separately a year earlier and are not retracted ¹. An independent zebrafish study found protection of lateral line hair cells that was attenuated by an HGF antagonist and by inhibitors of Akt, TOR and MEK, which is HGF-pathway evidence from a different lab, a different species and a different endpoint ³. Three unrelated stem-cell groups use Dihexa as a working HGF-agonist reagent to replace growth factors in hepatocyte differentiation protocols ^6,7,11 — a functional corroboration that comes from people with no interest in the cognitive claim.

And what does not survive scrutiny. The one test of Dihexa in a disease model by a group unconnected to its developers was negative: in 40 rats given the mitochondrial toxin 3-nitropropionic acid to model Huntington’s disease, the compound did not protect against the induced deficits ¹². No human has ever been given Dihexa in a published study.

The evidence for Dihexa is thin, entirely preclinical, largely produced by one laboratory with a commercial interest, and its central mechanistic paper has been withdrawn.

02 · Evidence at a glance

Evidence grade

Preclinical — no published human study of any kind

Chemical class

Peptidomimetic, not a peptide. N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide ^1,12

Also designated

PNB-0408 ¹²

Derived from

The N-terminal tripeptide of norleucine¹-angiotensin IV ¹

Records matching the compound name

Roughly 12 of 18 PubMed hits; the remainder are unrelated dyes and lipids

Mechanistic foundation

Retracted. J Pharmacol Exp Ther 2014;351:390–402, retracted April 2025 ^2,13

Grounds for retraction

Not stated in the indexed record — the notice has no abstract ¹³

Developer affiliation on that paper

Three authors list M3 Biotechnology, Inc., Seattle ^2,13

Independent HGF-pathway evidence

Zebrafish lateral line, protection attenuated by HGF antagonist 6-AH and by Akt/TOR/MEK inhibitors ³

Independent use as an HGF-agonist reagent

Three stem-cell laboratories, hepatocyte differentiation ^6,7,11

Independent disease-model test

Negative — no protection in a 3-nitropropionic acid rat model of Huntington’s disease, n = 40 ¹²

Independent efficacy result

Cognitive rescue in APP/PS1 mice, attributed to AngIV/PI3K/AKT rather than HGF/c-Met ¹⁰

Human trials

None

Human safety data

None

Toxicology, published

None located

US regulatory status

Not an approved drug; not a dietary supplement ingredient

03 · Mechanism of action

The HGF/c-Met claim, and its current standing

Dihexa’s mechanistic identity is as an HGF mimetic. The proposal is that it acts allosterically: as a small molecule it forms a functional ligand by dimerising with endogenous hepatocyte growth factor, activating the c-Met receptor and its downstream cascades, rather than binding c-Met itself ³.

The paper that established this in the compound’s own field was retracted in 2025 ^2,13. That is not the same as the claim being disproved — a retraction may follow from figure problems, data irregularities, authorship disputes or institutional findings, and this notice states none of them. But it means that the primary evidence a reader would be sent to no longer stands, and that any downstream review resting on it inherits the problem.

What remains is indirect and, for that reason, more interesting than it looks.

The evidence that does not depend on the retracted paper

In larval zebrafish, Dihexa at 1 μM gave optimal protection of lateral line hair cells against both neomycin and gentamicin ³. Three findings in that study bear on mechanism. Pretreatment did not reduce the amount of fluorescently tagged gentamicin entering hair cells, so the protection is intracellular rather than a matter of blocking uptake. Co-treatment with the HGF antagonist 6-AH attenuated it, and so did inhibitors of the downstream HGF targets Akt, TOR and MEK. And adding an amino group to the N-terminus attenuated protection — a single-atom-scale change altering activity, which argues for a specific target rather than a general effect ³.

Separately, three stem-cell laboratories with no connection to the cognitive programme use Dihexa as a substitute for growth factors when differentiating human pluripotent stem cells into hepatocytes ^6,7,11. One protocol pairs it with dexamethasone explicitly as a hepatocyte growth factor agonist ⁷; another combines vitamin C, Dihexa and forskolin to replace growth factors entirely ¹¹.

People who need a working HGF agonist for an unrelated purpose reach for this compound and it works for them. That is a form of replication no review article can supply, and it is the strongest thing in this file.

A competing mechanism, from an independent group

A 2021 study in APP/PS1 transgenic mice attributes the cognitive effect to a different pathway entirely. It reports that angiotensin IV levels in mouse tissue rose after oral Dihexa, that the compound activated PI3K/AKT signalling, and that the PI3K inhibitor wortmannin reversed its anti-inflammatory and anti-apoptotic effects ¹⁰. The authors frame their conclusion around a brain AngIV/PI3K/AKT axis. HGF and c-Met do not appear in their account.

Two mechanisms, from two labs, for one compound — and the one that does not invoke HGF/c-Met comes from the group with no stake in the HGF/c-Met story. Neither has been tested against the other.

An unresolved problem with the class

A systematic review of experimental studies of angiotensin IV and angiotensin-(1-7) found that eight of nine studies in cognitive-deficit models reported that AngIV and its analogues — including Dihexa — improved spatial working memory and passive avoidance performance ⁸. Its conclusion adds a condition: brain renin-angiotensin peptides appear most effective when administered intracerebroventricularly, and close to the time of learning acquisition or retention testing ⁸.

Delivered directly into the ventricles, and timed to the learning event. Dihexa was designed specifically to escape the first of those constraints, and the review does not report that it escapes the second.

04 · Key research findings

Chemistry and behaviour, 2013. The founding paper set out to fix the physicochemical liabilities of AngIV-related peptides — susceptibility to metabolic degradation and impermeability to gut and blood-brain barriers. N- and C-terminal modifications produced dramatic stability improvements while retaining the ability to reverse scopolamine-induced deficits in Morris water maze performance and to augment hippocampal synaptogenesis. Further modifications for hydrophobicity and reduced hydrogen bonding yielded Dihexa, reported as orally active, blood-barrier permeant and metabolically stabilised, with activity in the scopolamine and aged-rat models ¹.

This is a medicinal chemistry paper that succeeded at a medicinal chemistry problem. Its behavioural endpoints are scopolamine reversal and water maze performance in rats — screening assays, not disease models.

Mechanism, 2014, retracted 2025. The HGF/c-Met paper ² and its retraction notice ¹³.

Eleven years standing, then withdrawn. Any guide, review or product page still citing it as the mechanism is citing a retracted paper.

Ototoxicity protection in zebrafish, 2015. Dose–response protection of lateral line hair cells against neomycin and gentamicin, optimal at 1 μM, attenuated by an HGF antagonist and by Akt, TOR and MEK inhibitors, unaffected by aminoglycoside uptake, and abolished by an N-terminal amino group ³.

Independent, mechanistically probed, and in a species where the relevant hair cells are homologous to those of the mammalian inner ear. It is also an endpoint no one selling this compound talks about.

Use as a laboratory reagent, 2015–2022. Small-molecule-driven hepatocyte differentiation of human pluripotent stem cells ⁶; a published protocol pairing Dihexa with dexamethasone to differentiate hepatoblasts into hepatocyte-like cells ⁷; and a complete small-molecule strategy combining vitamin C, Dihexa and forskolin in place of growth factors ¹¹.

Three groups, none of them studying cognition, treating the compound as a functioning HGF agonist. This is the quiet corroboration in the file.

Systematic review of the class, 2018. Eight of nine studies in cognitive-deficit models found improvement with AngIV and its analogues; the reviewers note that the route producing those effects was generally intracerebroventricular, and that timing relative to learning mattered ⁸.

A synthesis of the class’s animal literature, and its qualifications are more informative than its headline.

Peripheral nerve repair in rats, 2021. A study of mesenchymal stem cells, granulocyte colony- stimulating factor and/or Dihexa on limb function recovery after sciatic nerve transection and repair ⁹. Harding is a co-author.

A combination design in which Dihexa is one of three interventions, from a group including the compound’s originator.

Alzheimer’s model in mice, 2021. Oral Dihexa in APP/PS1 mice restored spatial learning and cognitive function on the Morris water maze, increased neuronal cell counts and synaptophysin expression on Nissl staining, reduced astrocyte and microglial activation, lowered IL-1β and TNF-α, raised IL-10, and activated PI3K/AKT — with wortmannin reversing the anti-inflammatory and anti-apoptotic effects ¹⁰.

The most substantial independent efficacy result for the compound, in a genetic Alzheimer’s model rather than a scopolamine screen. It proposes a mechanism that does not involve HGF.

Huntington’s disease model in rats, 2024 — negative. Forty male Wistar rats randomised into vehicle, 3-nitropropionic acid, and 3-NP plus PNB-0408 groups, with body weight, motor function and cognition measured over five weeks before euthanasia and histopathology. The toxin reduced weight gain, impaired spatial learning and memory consolidation and produced marked motor dysfunction. Dihexa did not protect against any of it. The authors conclude the compound may not be an efficacious strategy in this model ¹².

A randomised, vehicle-controlled, adequately described animal study by investigators at Whitworth University, Oregon Health and Science University and the University of Washington — none of them the originating laboratory — reporting that it did not work. It is the only such test in the file, and its result is negative.

A 2026 review still states the original mechanism. A January 2026 review of therapeutic peptides in orthopaedics groups Dihexa with Selank and Semax as neuroactive peptides that enhance brain-derived neurotrophic factor and HGF/c-Met pathways ¹⁴.

Published nine months after the retraction, describing a peptidomimetic as a peptide and asserting a mechanism whose primary paper has been withdrawn. Reviews propagate; retractions do not.

05 · Evidence overview

06 · Safety profile

Animal data. No dedicated toxicology study appears in the indexed literature. Safety information is limited to what efficacy studies incidentally report: the APP/PS1 study administered different doses orally without reporting adverse findings ¹⁰, and the Huntington’s study dosed 40 rats for five weeks alongside a toxin, reporting no protective effect and describing histopathological analysis without a separate toxicity finding ¹².

Human data. None. No published study of any design has administered Dihexa to a person.

What is genuinely unknown. Nearly everything relevant to human exposure. There is no published human pharmacokinetics, no dose-finding, no repeat-dose toxicology, no genotoxicity or carcinogenicity assessment, and no reproductive toxicity data.

Two specific unknowns deserve naming. The first is that c-Met is a proto-oncogene. Sustained activation of the HGF/c-Met axis is a well-established feature of several cancers, and a compound whose entire proposed rationale is amplifying that axis has, in this file, no published carcinogenicity assessment whatever. The second is that the compound’s identity as an HGF-pathway activator now rests on indirect evidence ^3,6,7,11 because the direct evidence was retracted ^2,13 — which means the magnitude and duration of c-Met activation achieved in a living mammal are not established anywhere this guide could find.

Beyond those: whether the compound accumulates on repeat dosing; whether an off-target profile has ever been screened; and what any material sold under this name actually contains, on which nothing is published.

07 · US regulatory status

Current as of 6 September 2026. Dihexa is not an approved drug in the United States. It has no marketing authorisation in any jurisdiction identified here, is not a controlled substance, and is not a lawful dietary supplement ingredient.

It carries the developmental designation PNB-0408 ¹² and was associated with M3 Biotechnology, Inc. of Seattle through the affiliations of three authors on the retracted paper ^2,13. This guide has not consulted corporate filings, patent records or clinical trial registries, and makes no statement about the compound’s development history or current ownership beyond what those affiliation lines show.

Under the World Anti-Doping Code, Dihexa does not appear as a named prohibited substance in the classes reviewed for this guide. Competitors should consult the current Prohibited List directly rather than rely on secondary summaries, including this one.

08 · Limitations of the evidence

1. The primary mechanistic paper has been retracted, and the reason is not public. The 2014 HGF/c-Met paper was withdrawn in April 2025 ^2,13. The notice has no abstract. A reader cannot tell from the indexed record whether the retraction reflects an error in one figure or a problem with the dataset, and this guide will not guess. What is certain is that the paper can no longer be cited as evidence, and that a great deal of secondary literature does exactly that ¹⁴.
2. No human has ever been given this compound in a published study. No trials, no pharmacokinetics, no dose-finding, no safety cohort. The grade is Preclinical and it is not near the boundary.
3. The one independent disease-model test was negative. Forty rats, randomised, vehicle-controlled, five weeks, and no protection against 3-nitropropionic acid toxicity ¹². Negative animal studies are published less often than positive ones, which makes this one disproportionately informative.
4. Animal-to-human translation is entirely unattempted here, and the models are weak. Scopolamine reversal and Morris water maze performance in rats ¹ are screening assays for procognitive activity, not models of a human disease. The APP/PS1 mouse ¹⁰ is a genetic amyloid model whose record of predicting human Alzheimer’s outcomes is poor across the whole field.
5. Research concentration is severe. The founding chemistry, the retracted mechanism paper, two reviews and a nerve-repair study all involve the same Washington State University group ^1,2,4,5,9,13, with three of its members also affiliated to the commercial developer. The genuinely independent work is one zebrafish study ³, one mouse study ¹⁰, one negative rat study ¹² and three stem-cell protocols that use the compound as a reagent rather than study it ^6,7,11.
6. The two proposed mechanisms have never been tested against each other. HGF/c-Met ³ and AngIV/PI3K/AKT ¹⁰ are different pathways proposed by different groups. No experiment in this file discriminates between them.
7. Publication bias is likely and structurally invisible. A compound developed inside a company generates unpublished data by default; nothing here indicates how many animal experiments were run. The single negative study reaching print ¹² came from outside that structure.
8. The oral-activity claim sits awkwardly against the class review. Dihexa is presented as orally active ^1,10 while the systematic review of AngIV analogues reports that the class works best given intracerebroventricularly and timed to the learning event ⁸. This guide has not read the full texts that would reconcile that.
9. c-Met is a proto-oncogene and no carcinogenicity assessment exists. This is stated as an absence of data, not as evidence of harm. It is the single largest gap in the file.
10. The compound is routinely misclassified, including in the peer-reviewed literature. It is a peptidomimetic with two amino acids and two aliphatic chains, and a 2026 review calls it a neuroactive peptide ¹⁴. Category errors of this kind travel into product descriptions.
11. Nothing is known about material sold under this name. No published analysis has examined the identity, purity or composition of any Dihexa-labelled product.

09 · References

1. 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. J Pharmacol Exp Ther. 2013 Jan;344(1):141–154.

   PMID 23055539 ↗

2. Benoist CC, Kawas LH, Zhu M, Tyson KA, Stillmaker L, Appleyard SM, Wright JW, Wayman GA, Harding JW. 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. 2014 Nov;351(2):390–402. RETRACTED — see reference 13..

   PMID 25187433 ↗

3. Uribe PM, Kawas LH, Harding JW, Coffin AB. Hepatocyte growth factor mimetic protects lateral line hair cells from aminoglycoside exposure. Front Cell Neurosci. 2015 Jan 28;9:3.

   PMID 25674052 ↗

4. Wright JW, Kawas LH, Harding JW. The development of small molecule angiotensin IV analogs to treat Alzheimer’s and Parkinson’s diseases. Prog Neurobiol. 2015 Feb;125:26–46.

   PMID 25455861 ↗

5. Wright JW, Harding JW. The brain hepatocyte growth factor/c-Met receptor system: a new target for the treatment of Alzheimer’s disease. J Alzheimers Dis. 2015;45(4):985–1000.

   PMID 25649658 ↗

6. Siller R, Greenhough S, Naumovska E, Sullivan GJ. Small-molecule-driven hepatocyte differentiation of human pluripotent stem cells. Stem Cell Reports. 2015 May 12;4(5):939–952.

   PMID 25937370 ↗

7. Mathapati S, Siller R, Impellizzeri AA, Lycke M, Vegheim K, Almaas R, Sullivan GJ. Small-molecule-directed hepatocyte-like cell differentiation of human pluripotent stem cells. Curr Protoc Stem Cell Biol. 2016 Aug 17;38:1G.6.1–1G.6.18.

   PMID 27532814 ↗

8. Ho JK, Nation DA. Cognitive benefits of angiotensin IV and angiotensin-(1-7): a systematic review of experimental studies. Neurosci Biobehav Rev. 2018 Sep;92:209–225.

   PMID 29733881 ↗

9. Weiss JB, Phillips CJ, Malin EW, Gorantla VS, Harding JW, Salgar SK. Stem cell, granulocyte-colony stimulating factor and/or Dihexa to promote limb function recovery in a rat sciatic nerve damage-repair model: experimental animal studies. Ann Med Surg (Lond). 2021 Oct 8;71:102917.

   PMID 34703584 ↗

10. 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 Sci. 2021 Nov 11;11(11):1487.

    PMID 34827486 ↗

11. Pan T, Wang N, Zhang J, Yang F, Chen Y, Zhuang Y, Xu Y, Fang J, You K, Lin X, Li Y, Li S, Liang K, Li YX, Gao Y. Efficiently generate functional hepatic cells from human pluripotent stem cells by complete small-molecule strategy. Stem Cell Res Ther. 2022 Apr 11;13(1):159.

    PMID 35410439 ↗

12. 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. J Huntingtons Dis. 2024;13(1):55–66.

    PMID 38489193 ↗

13. 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]. J Pharmacol Exp Ther. 2025 Apr;392(4):103567.

    PMID 40312093 ↗

14. Rahman OF, Lee SJ, Seeds WA. Therapeutic peptides in orthopaedics: applications, challenges, and future directions. J Am Acad Orthop Surg Glob Res Rev. 2026 Jan 2;10(1):e25.00236.

    PMID 41490200 ↗