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Brain & sleep

NNZ-2591

Also called cyclic glycine-proline

EvidenceLow research

ClassIGF-1-related peptide

NNZ-2591 is an investigational oral peptide studied for rare neurodevelopmental disorders. Learn how it works and what its early trial data show.

Reviewed by: William Maish, MD MBA MPH, Clinical Product Lead
Published: October 5, 2026
Last updated: October 5, 2026

Key takeaway

NNZ-2591 is an investigational oral peptide related to a natural fragment of IGF-1, developed by Neuren Pharmaceuticals for several rare genetic neurodevelopmental disorders including Phelan-McDermid, Rett, and Angelman syndromes. Its published human evidence is an open-label (uncontrolled) Phase 2 trial reporting broad improvement, and a placebo-controlled Phase 3 is under way. As of 2026 it is not approved.

At a glance

FDA status	Investigational peptide, not approved by the FDA
What it is	Oral analog of cyclic glycine-proline, related to the IGF-1 system
Under study	Several rare neurodevelopmental syndromes
Research	Published human data from an open-label Phase 2; a placebo-controlled Phase 3 is ongoing
Evidence strength	Low research

What is NNZ-2591?

NNZ-2591 is an experimental drug based on cyclic glycine-proline, a naturally occurring breakdown product of insulin-like growth factor-1 (IGF-1). It is developed by Neuren Pharmaceuticals, taken by mouth, and studied across several genetic neurodevelopmental disorders, including Phelan-McDermid, Rett, Angelman, and Pitt-Hopkins syndromes. It is investigational and not approved.

How does it work?

As an analog of cyclic glycine-proline, NNZ-2591 is proposed to modulate the IGF-1 signaling¹ system, including helping to normalize the availability of IGF-1, which supports the formation and function of connections between nerve cells. The rationale is that correcting IGF-1-related signaling could help the brain circuits affected in these developmental disorders. This describes the proposed mechanism; the human efficacy data so far are uncontrolled.

Is it FDA-approved?

No. NNZ-2591 is investigational and not approved. Phase 2 trials have been completed in several indications, and a placebo-controlled Phase 3 trial in Phelan-McDermid syndrome is enrolling.

What does the research show?

The published human evidence is open-label. According to PubMed, a single-group, open-label Phase 2 trial¹ in children and adolescents with Phelan-McDermid syndrome reported that the drug was generally well tolerated and that most efficacy measures improved from baseline over 13 weeks. Because this study had no placebo group, improvement cannot be separated from natural variation or expectation effects.

Results in Rett, Angelman, and Pitt-Hopkins syndromes have been reported mainly through company announcements. A placebo-controlled Phase 3 is intended to test efficacy properly; until then, the benefit is promising but unproven.

More for brain & sleep

Sources: the open-label Phase 2 trial in Phelan-McDermid syndrome (Neumeyer et al., Neurology Genetics, 2025, DOI: 10.1212/NXG.0000000000200338¹; erratum DOI: 10.1212/NXG.0000000000200405²), indexed on PubMed; and Neuren Pharmaceuticals development updates. This document is educational, is not medical advice or a health claim, and is not an offer to sell any product.