PE-22-28

PE-22-28 is a seven-residue peptide that came out of an unusually clean piece of academic pharmacology. In 2010, a French group publishing in PLoS Biology described spadin, a 17-amino-acid peptide released during the maturation of sortilin, and showed that it selectively blocks TREK-1, a two-pore-domain background potassium channel [1]. TREK-1 had already attracted interest because mice lacking the channel show a depression-resistant phenotype. Blocking it pharmacologically was therefore a direct test of a target rather than another serotonergic variation on existing antidepressants.

Spadin worked in the rodent behavioural batteries, and it worked quickly. The effects appeared after roughly four days of treatment rather than the two to four weeks typical of SSRIs, and were accompanied by increased hippocampal neurogenesis and enhanced synaptic markers. A 2012 Neuropharmacology study specifically examined whether blocking a channel expressed in heart and gut as well as brain produced the predictable off-target problems, looking at cardiac function and seizure threshold, and reported that the expected TREK-1-related side effects did not materialise at the doses used [3].

The limitation of spadin itself was stability: a 17-residue peptide is degraded rapidly in plasma. That is where PE-22-28 comes in. A 2017 study in Frontiers in Pharmacology screened shortened analogues and reported that several of them, including the seven-residue PE-22-28, gave stronger TREK-1 inhibition, longer in vivo persistence, and antidepressant-like activity at lower doses than the parent peptide [2]. That paper is the primary reference for the molecule now sold under this name. A 2019 review in Pharmacology and Therapeutics summarises the whole TREK-1 blocker programme and its prospects [4], and a 2019 Neuropharmacology paper extended sortilin-derived peptides into models of stroke recovery and post-stroke depression, reporting protective effects [5].

One point of correction is worth making, because it appears repeatedly in vendor descriptions. PE-22-28 is frequently marketed as derived from pigment epithelium-derived factor, PEDF. The primary literature does not support that origin. Spadin, and therefore PE-22-28, is a fragment of the sortilin propeptide, sometimes called the neurotensin receptor 3 propeptide. The confusion may stem from a superficial similarity in abbreviations. It matters because the neurotrophic story attached to PEDF is different from the ion-channel pharmacology that actually underlies this peptide's reported effects.

The reported profile, then, is a fast-acting antidepressant-like effect mediated by TREK-1 blockade, with downstream increases in neurogenesis and synaptic density. Those are the endpoints that have made the compound attractive to people looking for cognitive or mood interventions outside the prescription system, and they are also the reason to be careful: rapid mood-altering effects in rodents are a starting point, not a demonstration of clinical value.

There are no human trials. Nothing has been registered on ClinicalTrials.gov for spadin or its analogues, and no phase I safety or pharmacokinetic study has been published, despite the target being described as clinically promising since 2010. Sixteen years of enthusiasm without a registered human study is itself a data point, usually reflecting some combination of formulation difficulty, funding, and the general difficulty of taking peptide central nervous system drugs into humans. It should temper any assumption that the rodent effects will simply carry over.

Safety in humans is unknown. The rodent side-effect work is reassuring as far as it goes, but TREK-1 is expressed in cardiac tissue, in smooth muscle and in the gastrointestinal tract, and it also has a documented role in anaesthetic and neuroprotective responses. Chronic systemic blockade of a background potassium channel is not a mechanistically trivial intervention, and no one has looked at what it does in a person over months. There is no interaction data with SSRIs, SNRIs, ketamine, or anything else someone with a mood disorder might already be taking, and the possibility of pharmacodynamic interaction with those agents is not theoretical.

Dosing quoted in consumer contexts is extrapolated from rodent microgram-per-kilogram intravenous or intranasal studies. Those conversions are unreliable in the absence of human pharmacokinetics, and intranasal delivery in particular has very different bioavailability between species.

Our position: PE-22-28 rests on genuine, published, target-directed pharmacology from academic laboratories, which puts it well above the vendor-literature tier. It also has zero human evidence, an origin story that is commonly misstated in marketing, and a mechanism that touches organs outside the brain. Tier 4 is the correct placement, and anyone using it should understand they are the first human data point in a programme that has never run a clinical trial.