Dihexa

What is Dihexa?#

Dihexa (PNB-0408) is a synthetic oligopeptide derived from angiotensin IV (AngIV; Val-Tyr-Ile-His-Pro-Phe), developed by Joseph Harding and colleagues at Washington State University. Unlike its parent compound, dihexa was engineered for metabolic stability, blood-brain barrier permeability, and oral bioavailability, addressing the pharmacokinetic limitations that prevented earlier angiotensin IV analogs from being therapeutically viable.

The compound has a molecular weight of 504.66 Da (C27H44N4O5), making it substantially smaller than most neuropeptides. Its structure consists of an N-hexanoic acid cap linked to a tyrosine-isoleucine dipeptide core with a C-terminal 6-aminohexanoic amide modification. This design provides resistance to enzymatic degradation while preserving the neurotrophic activity of the angiotensin IV family.

Mechanism of Action#

Dihexa exerts its neurotrophic effects through potentiation of the hepatocyte growth factor (HGF)/c-Met signaling system rather than through traditional angiotensin receptors.

HGF/c-Met Potentiation#

Dihexa binds to HGF with high affinity and augments HGF's capacity to activate the c-Met receptor tyrosine kinase at subthreshold concentrations. Specifically, dihexa at picomolar to nanomolar concentrations induces c-Met phosphorylation in the presence of low HGF concentrations that would otherwise be insufficient to activate the receptor. At higher concentrations (1 micromolar), dihexa inhibits HGF dimerization, suggesting a concentration-dependent modulation of HGF signaling.

Downstream Signaling#

Activation of c-Met by the HGF/dihexa complex triggers several downstream pathways:

• PI3K/AKT pathway: Promotes neuronal survival and reduces apoptosis. Studies in APP/PS1 Alzheimer's model mice demonstrated that dihexa activated PI3K/AKT signaling, and the PI3K inhibitor wortmannin reversed its anti-inflammatory and anti-apoptotic effects.
• Synaptogenesis and spinogenesis: Dihexa and its parent compound Nle1-AngIV induce hippocampal spinogenesis and synaptogenesis at picomolar concentrations, similar to HGF itself.
• Neuroinflammation reduction: In APP/PS1 mice, dihexa decreased activation of astrocytes and microglia, reduced pro-inflammatory cytokines (IL-1beta, TNF-alpha), and increased anti-inflammatory IL-10.

Neurotrophic Potency#

In an assay measuring neurotrophic activity, dihexa was reported to be seven orders of magnitude (10 million-fold) more potent than brain-derived neurotrophic factor (BDNF), though this comparison reflects a specific in vitro assay and should be interpreted cautiously.

Research Overview#

All published dihexa research is preclinical. The evidence base relies primarily on two groups of studies, one of which has been partially compromised by a retraction.

Key Preclinical Findings#

McCoy et al. (2013) published the foundational characterization of dihexa as a metabolically stable, orally active, BBB-permeant angiotensin IV analog. In aged (24-month-old) Sprague-Dawley rats, oral dihexa (2 mg/kg per day) significantly improved Morris water maze performance compared to vehicle-treated controls. The compound also reversed scopolamine-induced cognitive deficits in younger rats.

A 2021 study in APP/PS1 transgenic mice (a model of Alzheimer's disease) demonstrated that dihexa restored spatial learning in the Morris water maze, increased neuronal cell density, reduced neuroinflammation, and activated the PI3K/AKT signaling pathway.

Data Integrity Concerns#

A 2014 follow-up study by Benoist et al. in the Journal of Pharmacology and Experimental Therapeutics, which provided key mechanistic evidence for dihexa's HGF/c-Met-dependent synaptogenic effects, was retracted following a Washington State University investigation. The investigation found that figures and data contained falsified and/or fabricated elements, with Leen H. Kawas and Joseph W. Harding identified as solely responsible. This retraction affects the strength of the evidence linking dihexa's cognitive effects specifically to HGF/c-Met activation, though the independent 2021 APP/PS1 mouse study provides some corroborating evidence through a different research group.

Important Considerations#

• Dihexa remains exclusively preclinical with no human safety or efficacy data
• The c-Met/HGF pathway is involved in cell proliferation and is implicated in cancer biology, raising theoretical oncogenic risk concerns
• A key mechanistic study was retracted due to data fabrication, weakening the evidence base
• No regulatory agency has approved or authorized dihexa for any therapeutic use
• Long-term safety in any species has not been established

Key Research Findings#

Evaluation of Metabolically Stabilized Angiotensin IV Analogs as Procognitive/Antidementia Agents, published in Journal of Pharmacology and Experimental Therapeutics (McCoy AT et al., 2013; PMID: 23055539):

Foundational study characterizing dihexa as a metabolically stable, orally active, BBB-permeant angiotensin IV analog. Demonstrated cognitive enhancement in aged rats and scopolamine-treated rats.

• Oral dihexa (2 mg/kg/day) improved Morris water maze performance in 24-month-old rats
• Dihexa reversed scopolamine-induced cognitive deficits
• Compound demonstrated oral bioavailability and BBB permeability

AngIV-Analog Dihexa Rescues Cognitive Impairment and Recovers Memory in the APP/PS1 Mouse via the PI3K/AKT Signaling Pathway, published in Brain Sciences (Sun X et al., 2021; PMID: 34827486):

Independent study demonstrating that dihexa rescues cognitive impairment in APP/PS1 Alzheimer's model mice through PI3K/AKT pathway activation, anti-inflammatory effects, and anti-apoptotic mechanisms.

• Dihexa restored spatial learning and memory in Morris water maze
• Increased neuronal cell density and SYP protein expression
• Reduced activation of astrocytes and microglia

Related Reading#

• Dihexa research studies and evidence
• Dihexa dosing protocols
• Dihexa side effects profile
• Semax research guide
• Selank research guide