PE-22-28 vs NNZ-2591
Comprehensive side-by-side comparison of mechanisms, dosing, side effects, and research
Also: PE22-28, Spadin Analog
PE-22-28 is a seven-amino-acid peptide that blocks a potassium channel in the brain called TREK-1, the same target tied to depression. It is a shortened, more potent and more stable version of spadin, a natural fragment cut from the sortilin propeptide, and it was built to act like a fast antidepressant. All evidence is from rodents. There are no human clinical trials.
Also: Cyclo-L-glycyl-L-2-allylproline, Cyclic glycine-proline analog
NNZ-2591 is Neuren Pharmaceuticals' second drug, a synthetic analog of cyclic glycine-proline (cGP), a small ring-shaped molecule the body makes when it breaks down IGF-1. It is taken by mouth and is being developed for rare childhood neurodevelopmental disorders: Phelan-McDermid syndrome is furthest along, with the first-ever phase 3 trial in that disease started in late 2025, and Angelman and Pitt Hopkins syndromes behind it. Phase 2 results looked encouraging, but they were small open-label studies, so nobody knows yet whether it truly works.
Key Comparison Insights
- Both peptides belong to the Cognitive category, suggesting similar primary applications.
- NNZ-2591 has stronger research evidence (Human Trials) compared to PE-22-28 (Animal Studies).
Detailed Comparison
| Attribute | PE-22-28 | NNZ-2591 |
|---|---|---|
| Category | Cognitive | Cognitive |
| FDA Status | Not FDA Approved | Not FDA Approved |
| Clinical Status | Pre I II III IV FDA | Pre I II III IV FDA |
| Mechanism of Action | PE-22-28 inhibits TREK-1, a two-pore-domain potassium channel that normally lets potassium leak out of neurons and quiets them down. Block that leak and serotonin-producing neurons in mood-related regions become easier to fire, which is the proposed antidepressant mechanism. This is the same pathway implicated in why TREK-1 knockout mice show a depression-resistant phenotype. Compared to the parent peptide spadin, PE-22-28 binds TREK-1 far more tightly and survives much longer in the body, which is why it was developed. Whether TREK-1 blockade translates into a real antidepressant effect in people remains unproven. | Cyclic glycine-proline is thought to fine-tune IGF-1 activity in the brain by changing how IGF-1 binds to its carrier protein IGFBP-3. A 2014 Scientific Reports study proposed that cGP nudges IGF-1 activity up when it is low and down when it is high, a normalizing effect rather than a simple boost. NNZ-2591 adds an allyl group to the proline ring, making it resistant to enzymes and able to reach the brain after oral dosing. Trial protocols also describe reduced neuroinflammation and support for synapse function in animal models. It is a close cousin of trofinetide in spirit (both are small IGF-1-derived molecules from the same company), but the exact way it helps in these genetic syndromes has not been pinned down in humans. |
| Common Dosing | 100-300 mcg Once daily | 12 mg/kg twice daily (phase 2 target trial dose) Twice daily |
| Administration | Intranasal or subcutaneous injection | Oral liquid |
| Typical Duration | Research protocols vary, often 2-4 weeks | 13 weeks in phase 2 and phase 3 trials, with 12-month open-label extensions |
| Best Time to Take | Morning | Twice daily (trial schedule) |
Possible Side Effects May vary by individual |
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| Research Summary | The core data come from a single 2017 paper by Djillani and colleagues in Frontiers in Pharmacology, which screened shortened spadin analogs and found PE-22-28 to be the standout: dramatically stronger TREK-1 inhibition, longer in vivo stability, and antidepressant-like behavior in mice using standard tests like the forced swim and novelty-suppressed feeding paradigms. The same line of work reported that short treatment courses increased hippocampal neurogenesis in rodents, which the authors framed as a possible mechanism for faster mood effects than conventional SSRIs. The broader TREK-1 and spadin story traces back to Mazella et al. in 2010, who first described spadin as a natural antidepressant-type TREK-1 blocker. Importantly, every result is preclinical and in animals. There are no completed or published human efficacy or safety trials of PE-22-28, and it is not an approved drug anywhere. Claims that it treats depression, anxiety, or cognitive decline in people go well beyond what the evidence supports. | The human data so far come from three small, open-label phase 2 trials of 13 weeks each, all with twice-daily oral dosing titrated up to a target of 12 mg/kg. In Phelan-McDermid syndrome (18 children, published in Neurology Genetics in 2026), 10 of 14 efficacy measures improved significantly from baseline, including clinician and caregiver global ratings, behavior, sleep and communication, and the drug was well tolerated with mostly mild to moderate side effects. Angelman syndrome (reported August 2024) showed a mean CGI-I of 3.0, with 11 of 13 children rated improved by clinicians. Pitt Hopkins syndrome (reported May 2024) showed improvement in 6 of 11 children on clinician-rated severity. The big caveat: none of these had a placebo group, and rare-disease trials with caregiver-rated outcomes are especially prone to placebo effects. A planned Prader-Willi trial was withdrawn before enrolling anyone. The real test is Koala, a randomized, double-blind, placebo-controlled phase 3 trial in about 160 children aged 3 to 12 with Phelan-McDermid syndrome (first patient dosed February 2026; estimated primary completion late 2027), with an open-label extension started in 2026. The FDA has given the program Fast Track, Rare Pediatric Disease and Orphan Drug designations. Neuren has also started work in neonatal hypoxic-ischemic encephalopathy, which is still preclinical. |
Frequently Asked Questions: PE-22-28 vs NNZ-2591
What is the difference between PE-22-28 and NNZ-2591?
PE-22-28 is a cognitive peptide that pe-22-28 is a seven-amino-acid peptide that blocks a potassium channel in the brain called trek-1, the same target tied to depression. it is a shortened, more potent and more stable version of spadin, a natural fragment cut from the sortilin propeptide, and it was built to act like a fast antidepressant. all evidence is from rodents. there are no human clinical trials. NNZ-2591 is a cognitive peptide that nnz-2591 is neuren pharmaceuticals' second drug, a synthetic analog of cyclic glycine-proline (cgp), a small ring-shaped molecule the body makes when it breaks down igf-1. it is taken by mouth and is being developed for rare childhood neurodevelopmental disorders: phelan-mcdermid syndrome is furthest along, with the first-ever phase 3 trial in that disease started in late 2025, and angelman and pitt hopkins syndromes behind it. phase 2 results looked encouraging, but they were small open-label studies, so nobody knows yet whether it truly works. The main differences lie in their mechanisms of action and clinical applications.
Which is better, PE-22-28 or NNZ-2591?
Neither is universally "better" - the choice depends on your specific goals. PE-22-28 is typically used for cognitive purposes, while NNZ-2591 is used for cognitive. Always consult with a healthcare provider to determine which may be appropriate for your situation.
Can PE-22-28 and NNZ-2591 be used together?
Some peptide protocols combine multiple compounds for synergistic effects. However, using PE-22-28 and NNZ-2591 together should only be considered under medical supervision, as both compounds have their own side effect profiles and potential interactions. Research on their combined use may be limited.