Trofinetide (predecessor programs)
IGF-1-derived peptide
Also NNZ-2566, NNZ2566, Daybue, Glycyl-L-2-methylprolyl-L-glutamate
- Evidence
- BSome human evidence
- FDA status
- FDA approvedas of 2026-10-26
- Route
- Oral
- Studies cited
- 2 human1 animal or cell
Before its approval for Rett syndrome, this drug was tried for fragile X and brain injury. Fragile X had one 72-person trial with a positive signal; brain injury work was in rats.
Sold for
Before Rett syndrome, trofinetide was pitched as a brain-repair drug for concussion, fragile X and other neurodevelopmental conditions.
Studies say
The fragile X program produced one small 28-day randomized trial with a positive signal at the higher dose and the brain injury work is in rats, so the predecessor programs grade B; only the Rett indication reached phase 3.
Why grade B. This page covers trofinetide's development outside Rett syndrome. The fragile X program produced one randomized placebo-controlled phase 2 trial in 72 adolescent and adult males, which reported an efficacy signal at the higher trial dose over 28 days on a novel composite analysis; the published brain injury work is in rats. A single small, short randomized trial makes the fragile X use B; the Rett syndrome use, graded on its own page, has a 187-patient phase 3 trial. 125
FDA. Trofinetide is approved by the FDA as Daybue (March 2023) for Rett syndrome in adults and children 2 years and older. The earlier programs covered here, in fragile X syndrome and traumatic brain injury, were investigational and did not lead to approval. 34
Written from 5 cited sources. Fact-checked 2026-10-27: 13 claims checked, 5 corrected. Last refreshed 2026-10-27. Not physician reviewed.
What this means for you
- What people take it for
- Fragile X syndrome and brain injury.
- Can a US doctor prescribe it?
- No. It is approved only for Rett syndrome. Fragile X and brain injury uses are not approved.
- Properly tested in people?
- Partly: some human trials, with gaps: one 72-person fragile X trial for 28 days; brain injury work is in rats.
Evidence checklist
- Randomised controlled trial Phase 2 fragile X and TBI trialsyes
- Human study over 100 people Non-Rett trials were smallno
- More than one research group Sponsor programs; limited outside datano information
- Clinical outcomes, not surrogate markers Exploratory measures; programs droppedno
- Human safety data beyond case reports Short trials; diarrhea common in Rett phase 3yes
Drawn from the cited studies below. A dash means the sources give no information.
What it is
Trofinetide is a synthetic analog of glycine-proline-glutamate, the three-amino-acid fragment that the body cuts from the end of insulin-like growth factor 1 (IGF-1) in the brain. The natural tripeptide is unstable; trofinetide adds a methyl group to the proline, which lets it survive in the blood long enough to be taken by mouth. Neuren Pharmaceuticals in New Zealand developed it under the code NNZ-2566. 1
Neuren ran a phase 2 trial of NNZ-2566 in fragile X syndrome, an inherited form of intellectual disability, and the compound was studied in rat models of brain injury. Neither led to an approval. The Rett syndrome program, licensed to Acadia Pharmaceuticals, is the one that did. 1235
How it works
The IGF-1 fragment and its analog act on supporting cells in the brain and on synapses, the connections between neurons. In animal models they reduce neuroinflammation, normalize overactive signaling, and help immature synapses mature. Rett syndrome and fragile X both involve synapses that fail to mature normally, which is why the same molecule was tried in both; the fragile X trial reported a signal at the higher dose and the Rett phase 3 trial met its primary endpoints. 125
What human research shows
The fragile X phase 2 trial randomized 72 adolescent and adult males to one of two trofinetide doses or placebo for 28 days. Both doses were well tolerated, and the higher dose met the trial's prespecified efficacy criteria on a novel analysis combining several symptom domains, with improvement on three fragile X-specific measures. Trofinetide has not been approved for fragile X. 13
The published brain injury work is in animals: in rats with a penetrating ballistic-like brain injury, NNZ-2566 reduced neuroinflammation and pro-inflammatory cytokine expression. Trofinetide has no approval for brain injury. 53
The work that did progress was in Rett syndrome: earlier phase 2 trials and the LAVENDER phase 3 trial of 187 girls and women, which showed improvement on caregiver-rated behavior and clinician-rated global impression scales versus placebo and led to approval. 23
| Study | Model | Finding |
|---|---|---|
| Fragile X phase 2 trial (Berry-Kravis 2020) | Humans, n=72 adolescent and adult males with fragile X syndrome, randomized double-blind placebo-controlled, 28 days | Both doses were well tolerated; the higher dose met prespecified efficacy criteria on a composite analysis. 1 |
| LAVENDER (Nature Medicine 2023) | Humans, n=187 females with Rett syndrome, randomized placebo-controlled, 12 weeks | Trofinetide improved both co-primary endpoints versus placebo, with diarrhea in 81 percent of treated patients. 2 |
| Rat brain injury model (Wei 2009) | Rats, penetrating ballistic-like brain injury | NNZ-2566 reduced neuroinflammation and pro-inflammatory cytokine expression. 5 |
Safety
Across the development program the dominant adverse effects were gastrointestinal. In LAVENDER, diarrhea affected 81 percent of trofinetide patients and vomiting 27 percent, and these are the main warnings in the approved label along with weight loss. In the 28-day fragile X trial both doses were well tolerated. 124
Questions for your clinician
- Is trofinetide approved for any condition other than Rett syndrome?
- Is there any controlled trial for fragile X or brain injury?
- How can the diarrhea and weight loss be managed?
Questions people ask
What were trofinetide's predecessor programs?
Neuren Pharmaceuticals first tested the molecule, then called NNZ-2566, in a phase 2 trial in fragile X syndrome, and it was studied in rat models of brain injury. Neither led to approval. The Rett syndrome program, run with Acadia, led to FDA approval in 2023. 135
Is trofinetide approved for fragile X or concussion?
No. Its only approved indication is Rett syndrome in patients aged 2 and older. Any use for fragile X, brain injury or other conditions is off-label; the fragile X evidence is a single 28-day phase 2 trial. 34
Why did Rett syndrome reach approval and fragile X did not?
The published record does not give a reason. The fragile X trial was a 28-day phase 2 study that reported a signal at the higher dose on a novel composite analysis; the Rett program went on to a 12-week phase 3 trial that met both co-primary endpoints. The molecule's effect on synaptic maturation is the shared rationale. 12
Citations
- Berry-Kravis 2020, Pediatric Neurology: randomized placebo-controlled phase 2 trial of trofinetide in fragile X syndrome doi.org
- Neul 2023, Nature Medicine: Trofinetide for the treatment of Rett syndrome, phase 3 LAVENDER trial doi.org
- FDA 2023: Daybue (trofinetide) approval label accessdata.fda.gov
- DailyMed: Daybue (trofinetide) current prescribing information dailymed.nlm.nih.gov
- Wei 2009, J Neuroinflammation: NNZ-2566 inhibits neuroinflammation after penetrating ballistic-like brain injury in rats pubmed.ncbi.nlm.nih.gov