Dihexa

Mechanism of action

Dihexa (PNB-0408, N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide) is a synthetic hexapeptide developed by Joseph Harding's research group at Washington State University as a cognitive enhancer derived from angiotensin IV through systematic structure-activity optimisation. The molecular design started from angiotensin IV (Val-Tyr-Ile-His-Pro-Phe), the hexapeptide product of further hydrolysis of angiotensin II that was characterised in the 1990s as a regulator of central nervous system functions including learning and memory, with the goal of producing a stable, orally-bioavailable analogue retaining the cognitive-enhancing activity while addressing the rapid peptidase degradation of native angiotensin IV. The result was a markedly modified analogue retaining only the core Tyr-Ile recognition element of native angiotensin IV with N-terminal hexanoyl-modification for protease resistance and C-terminal aminohexanoic acid extension for receptor binding optimisation. The molecular mechanism of Dihexa is distinct from the BDNF/NGF-upregulating mechanisms of Semax and the multi-pathway pharmacology of cerebrolysin. Dihexa is a potent positive allosteric modulator of hepatocyte growth factor (HGF) / c-Met receptor signalling — binding HGF directly (with an apparent KD in the femtomolar range, ~10⁻¹⁵ M) and stabilising the HGF-c-Met complex to potentiate downstream c-Met receptor activation. HGF is a multifunctional growth factor with established roles in tissue regeneration, including a substantial role in central nervous system development and adult hippocampal neurogenesis through c-Met expression on hippocampal neurons and neural progenitor cells. The femtomolar potency of Dihexa as an HGF/c-Met potentiator places it approximately seven orders of magnitude more potent than BDNF on equivalent endpoint readouts (dendritic-spine formation in hippocampal slice preparations) — an extraordinary potency that has driven the substantial pre-clinical research interest in the compound. The mechanistic distinction from BDNF/NGF-targeted cognitive peptides is that Dihexa addresses an entirely separate growth-factor signalling axis (HGF/c-Met) rather than the more conventional neurotrophin/Trk-receptor pathway. The result is a complementary pharmacological mechanism that could in principle be combined with BDNF-upregulating compounds (Semax) for additive cognitive enhancement, though this combination has not been clinically characterised. Downstream of HGF/c-Met receptor potentiation, Dihexa produces dendritic-spine formation, enhancement of long-term potentiation at hippocampal CA3-CA1 synapses, and improvements in standard rodent learning and memory behavioural endpoints. The structural correlates include increased dendritic spine density in CA1 pyramidal neurons (measured by Golgi staining and two-photon imaging) and increased synaptic protein expression in the hippocampus. The effects on adult hippocampal neurogenesis are less consistently documented but are reported in some studies. A practical pharmacological feature distinguishing Dihexa from other peptide cognitive enhancers is the oral bioavailability. The hexanoyl N-terminal modification provides both protease resistance and improved oral absorption — most peptides have negligible oral bioavailability due to gastric and intestinal peptidase degradation, but Dihexa's hexapeptide architecture with the hexanoyl modification achieves meaningful oral absorption (~10-20% in rodent models). This is the principal practical advantage of Dihexa over Semax and Selank, which require intranasal delivery to bypass blood-brain barrier limitations. Whether Dihexa's plasma-to-CSF crossing is sufficient for clinically meaningful central effects after oral administration in humans is an open question pending human pharmacokinetic studies. In pre-clinical scopolamine-induced cognitive impairment models — a standard paradigm for cognitive-enhancement screening — orally administered Dihexa produces dose-dependent reversal of scopolamine-induced memory deficits with effect sizes that compare favourably to donepezil and other clinically established cognitive enhancers. In aged-rodent models, Dihexa restores spatial learning and memory performance toward young-rodent baseline levels. These pre-clinical efficacy data have established the compound's substantial research interest despite the absence of human clinical-trial data.

Dihexa potentiates HGF/c-Met receptor signalling at femtomolar concentrations — approximately seven orders of magnitude more potent than BDNF on equivalent dendritic-spine-formation endpoints in hippocampal slice preparations (McCoy et al., J Pharmacol Exp Ther 2013) — establishing HGF/c-Met as a tractable cognitive-enhancement target distinct from the BDNF/NGF axis that dominates conventional cognitive-peptide pharmacology.

— Notable finding

Research history

Dihexa was developed by Joseph Harding's research group at Washington State University as part of a research programme exploring angiotensin IV's central nervous system effects and the development of optimised analogues. The foundational work characterising angiotensin IV as a regulator of learning and memory was published across the 1990s and 2000s. The molecular optimisation that produced Dihexa (PNB-0408) was reported in 2013 (McCoy et al., J Pharmacol Exp Ther), with subsequent publications characterising the HGF/c-Met potentiation mechanism, the oral bioavailability, and the pre-clinical efficacy across cognitive-impairment models. M3 Biotechnology (subsequently known as Athira Pharma) was founded by Joseph Harding and colleagues to advance Dihexa and related HGF/c-Met-modulating compounds toward clinical development. The company's lead compound advanced through pre-clinical development with the goal of treating Alzheimer's disease through HGF/c-Met-mediated synaptic restoration. Phase I and Phase II clinical development was initiated for ATH-1017 (a related but not identical HGF/c-Met-modulating compound — fosgonimeton) but Dihexa itself has not advanced to formal clinical trials. The clinical-development focus shifted to the related but distinct fosgonimeton, which has progressed through Phase II Alzheimer's-disease trials with mixed results. Dihexa exists in academic research and research-chemical-community channels as a pre-clinical research tool. The compound has gained moderate visibility in nootropic research-chemical contexts driven by the dramatic potency claims (~10⁷-fold more potent than BDNF on equivalent endpoints) and the oral bioavailability that distinguishes it from other cognitive peptides requiring parenteral or intranasal administration. Research-chemical supply is available but quality varies more than for the more established research peptides given the synthesis complexity. No formal commercial Phase III development programme is currently announced for Dihexa itself. The compound's research relevance is principally as a mechanistic tool for characterising HGF/c-Met-mediated synaptic plasticity and as a template for the broader HGF/c-Met-modulating drug-development space being explored by Athira Pharma and other groups.

Reported research-model dose ranges

The ranges below are taken from published pre-clinical literature. They do not constitute a dosing recommendation for human use.

Summarised studies

Safety profile

Dihexa's safety dataset is limited to pre-clinical work — there are no published human clinical-trial data. The pre-clinical safety record across rodent cognitive and pharmacology studies has been favourable: oral and parenteral doses over the research dose range have produced no overt toxicity, no significant changes in routine haematology or hepatic enzymes, and no organ-specific lesions in standard histopathology. Acute toxicology has not identified dose-limiting adverse effects at doses substantially above the cognitive-efficacy range. Theoretical safety considerations centre on the HGF/c-Met pathway's broader biological roles. HGF/c-Met signalling is critical in tissue regeneration but is also implicated in tumour growth and metastasis in several cancer contexts — HGF/c-Met inhibitors are oncology development targets (cabozantinib, tepotinib, capmatinib), reflecting the role of c-Met activation in cancer biology. Pharmacological potentiation of HGF/c-Met signalling could theoretically promote tumour growth in subjects with occult or recent malignancy. This theoretical concern has not been substantiated in pre-clinical Dihexa studies, but the chronic-dosing oncology safety profile in humans is uncharacterised. Additional theoretical considerations include the broader effects of HGF/c-Met activation on non-CNS tissues (liver regeneration is the original HGF context — the protein was first characterised as a hepatic regeneration factor). Chronic HGF/c-Met potentiation could theoretically affect hepatic, renal, and pulmonary tissue dynamics in ways not characterised at research-dose tiers. The peptide's hexapeptide architecture with N-terminal hexanoyl modification carries low immunogenicity risk. Anti-drug antibodies have not been a documented issue in the pre-clinical work. No serious adverse events have been reported in any published Dihexa research. The acute and sub-chronic safety profile in research animals is favourable; the chronic-dosing and human safety record is absent.

UK regulatory status

Dihexa is not authorised as a medicinal product by the UK Medicines and Healthcare products Regulatory Agency (MHRA) and holds no marketing authorisation in any jurisdiction. It is not a controlled substance under the Misuse of Drugs Act 1971. Research-grade material is available from research-chemical suppliers for legitimate pre-clinical and academic research use; possession for bona fide laboratory research is generally unrestricted in the UK. Supply or administration of Dihexa to humans outside an authorised clinical-trial framework engages the Human Medicines Regulations 2012 and is generally an offence. Dihexa is not currently on the WADA Prohibited List. The compound's mechanism (HGF/c-Met potentiation, cognitive enhancement) does not currently fall within any WADA category, though athletes should verify the current annual Prohibited List as classifications can change with emerging compounds. For animal research under ASPA, Dihexa work in vertebrates requires standard project and personal licences from the Home Office Drugs and Firearms Licensing Unit. There is no current UK clinical-research infrastructure for Dihexa. Research-chemical-community use exists but lacks the pharmacokinetic, toxicology, and efficacy framework that would support evidence-based human dosing.

Frequently asked questions