DihexaWhat the studies show

The paper that introduced dihexa, in the Journal of Pharmacology and Experimental Therapeutics, had two parts. In cultured rat hippocampal neurons, dihexa increased the number of dendritic spines, the small protrusions on which synapses form, at very low concentrations, and the new spines carried the proteins of working synapses ¹. In live rats, the researchers first impaired memory with scopolamine, a drug that blocks the cholinergic system and is a standard way to mimic a dementia-like deficit, then tested the animals in a water maze, where they must learn the position of a hidden platform. Rats given dihexa, whether by mouth or by injection, found the platform about as well as rats that had never received scopolamine; rats given scopolamine alone did not ¹. Old rats with naturally poor water-maze performance also did better after the compound ¹.

Two points about that design. Scopolamine amnesia is a chemical model, not a disease, and improvement in it is common for many compounds that later do nothing in people. And the water maze measures one kind of spatial memory in one species.

A 2014 follow-up from the same group asked what the molecule actually binds. The answer was not the angiotensin system at all but hepatocyte growth factor (HGF) and its receptor c-Met, a signalling pair known for driving cell growth, movement and repair in many tissues. Dihexa bound HGF and made it a stronger activator of c-Met; blocking c-Met removed both the spine growth in culture and the memory effect in rats ². An earlier 2011 paper had shown the parent angiotensin IV analogues increasing hippocampal synapses and spatial memory in rats by what turned out to be the same route ³. Harding and colleagues laid out the case for c-Met as a target in Alzheimer's disease in a 2015 review ⁴.

That is a real mechanism, with a receptor, a ligand and a blocking experiment, which puts dihexa ahead of many research peptides on the question of how it might work. It is also a mechanism that cuts both ways, as the next section explains.

Nothing is known about dihexa in people: no pharmacokinetics, no safety data, no efficacy data, no trial. The mechanism is also the reason for caution. c-Met is a growth receptor that is over-active in a range of cancers, and drugs that block it are used in cancer treatment; a molecule that makes c-Met signalling stronger would need long-term safety work before anyone could say what it does over months or years in a person. The rat findings come from one laboratory, and no independent group has published a replication. What exists is a well-worked-out rat story with a clear mechanism and an open question about what that mechanism would do in a human over time.