Dihexa

What it is

Dihexa, originally designated PNB-0408 and chemically described as N-hexanoic-Tyr-Ile-(6) aminohexanoic amide, is a small modified-peptide molecule designed at Washington State University as a metabolically stabilized analog of angiotensin IV. Angiotensin IV (a hexapeptide derived from angiotensin II) had been observed to enhance cognition in rodent models but was too rapidly degraded in plasma to be a practical research tool, much less a drug. Dihexa was engineered with non-natural protecting groups at both termini, producing a small molecule that retains the procognitive activity of the parent peptide while resisting peptidase clearance and being orally bioavailable.

The molecule is sometimes described as a "peptide-derived small molecule" rather than a true peptide; it weighs about 477 Da and behaves pharmacokinetically more like a conventional drug than like a typical injectable peptide.

History

The Dihexa program emerged from the Harding lab at Washington State University, which had spent years dissecting the cognitive effects of angiotensin IV and its analogs. McCoy and colleagues published the medicinal chemistry program in JPET in 2013, and a follow-up paper by Benoist and colleagues in 2014 proposed that the procognitive and synaptogenic effects of these compounds depended on activation of hepatocyte growth factor (HGF) signaling through its receptor c-Met. That 2014 paper was retracted in 2025 after the journal raised concerns about data integrity, though the broader HGF/c-Met hypothesis remains part of the published account in the 2015 review.

A spin-out company, M3 Biotechnology (later Athira Pharma), was founded around the c-Met agonist platform. Athira's subsequent clinical programs (notably the ATH-1017 / fosgonimeton small-molecule HGF positive modulator) were structurally distinct from Dihexa; the original Dihexa molecule itself has not progressed to human trials.

Regulatory status

Dihexa has no approved medical use anywhere in the world. There are no registered Phase I trials at ClinicalTrials.gov for Dihexa under any of its designations. It is sold only through research-chemical suppliers, with not-for-human-use labeling, and there is no published Investigational New Drug application.

The retraction of the 2014 Benoist paper has narrowed the citable evidence base. The 2013 McCoy paper and the 2015 review remain in the literature and continue to be the standard references for the molecule's preclinical profile.

How researchers describe its action

The published mechanism account, prior to the 2025 retraction, described Dihexa as acting through the hepatocyte growth factor / c-Met receptor system, which is involved in neuronal survival, synaptogenesis, and dendritic-spine formation. The proposed effects, characterized in cell culture and in rodents:

1. HGF/c-Met activation. Dihexa was reported to act as a positive modulator of HGF signaling, enhancing c-Met phosphorylation in neuronal cell lines.
2. Synaptogenesis. Cultured hippocampal neurons exposed to nanomolar Dihexa showed increased dendritic-spine density.
3. Procognitive activity in rodent models. Aged rats and scopolamine-impaired rodents showed improved performance in spatial learning tasks after oral Dihexa.

The 2025 retraction of one of the key mechanistic papers means the strength of the published evidence base is now lower than it appeared a decade ago. Independent replication outside the original lab is limited.

Half-life and dosing intervals

The pharmacokinetic data underlying Dihexa are limited and were generated almost entirely in rodents. The published half-life is unusually long for a peptide-derived molecule — estimates range from 9 to 13 days in plasma, attributable to its small size, lipophilicity, and resistance to peptidase degradation. The molecule is orally bioavailable, again unusual for a peptide-derived structure.

In rodent studies, oral doses on the order of 1.25 mg/kg produced procognitive effects in aged rats. There is no human dose. The doses commonly cited in informal observational use (a few milligrams orally, typically once daily or every other day) have no clinical-trial basis.

Reconstitution example

Research-grade Dihexa is sold either as a lyophilized powder or as a solid suitable for oral capsule preparation, typically in 10 mg or 20 mg quantities. If reconstituted for injection (an unusual route given oral bioavailability), a 10 mg vial dissolved in 1 mL of bacteriostatic water yields 10 mg/mL — but the standard research route is oral and many lots are sold for oral preparation rather than injection.

Vial's calculator can convert vial mass and water volume to a per-unit dose if reconstitution is used. Because Dihexa is highly lipophilic, suppliers typically provide vehicle guidance; DMSO or ethanol stock solutions are common in cell-culture work.

What to know

• No human trials. Despite a decade of preclinical interest, no Phase I trial of Dihexa has been registered or published.
• One key paper retracted. The 2014 Benoist et al. paper describing the HGF/c-Met mechanism was retracted in 2025, narrowing the citable evidence base.
• Long half-life. Unlike most peptides, Dihexa persists in plasma for over a week, so accumulation with repeated dosing is a real consideration in any research protocol.
• Theoretical safety concerns. Sustained HGF/c-Met activation has been linked to tumor biology in unrelated literature; long-term safety has not been characterized in any controlled study.
• Storage. Lyophilized Dihexa is stable refrigerated and most stable frozen. Stock solutions should be aliquoted and frozen to avoid repeated freeze-thaw cycles.
• Not technically a peptide by strict definition — it is a small peptide-derived molecule that is orally active and behaves more like a conventional drug than a typical injectable peptide.