New York-based Neurogene (Nasdaq: NGNE) reported updated Phase I/II data for NGN-401, an AAV9 gene therapy delivering a full-length human MECP2 transgene, showing that all 10 treated participants gained developmental milestones through 30 months of follow-up with no plateau or milestone loss — results that, if reproduced in the registrational Embolden trial, could position the therapy as the first disease-modifying intervention in a condition where the only approved treatment addresses symptoms rather than underlying cause.
The data carry particular weight because Rett syndrome, caused by loss-of-function mutations in MECP2, has no approved genetic correction. Acadia Pharmaceuticals' Daybue (trofinetide), approved by the US FDA in 2023, provides symptomatic benefit through synaptic pathway modulation but requires twice-daily lifelong dosing and does not restore MeCP2 protein function. NGN-401 is designed as a one-time intracerebroventricular administration, and the durability signal now extending to 30 months is the central question any gene therapy in this space must answer.
Across the 10 evaluable participants, 47 total developmental milestones were gained — an average of 4.7 per participant — spanning hand function, gross motor function, and communication. All 10 met the composite endpoint of Clinical Global Impression-Improvement (CGI-I) improvement plus gain of at least one developmental milestone, the same composite used in the Embolden registrational trial. All participants also gained at least one additional milestone within the most recent 12 months of the observation window, indicating continued accumulation rather than stabilization.
The trajectory of milestone acquisition is arguably the most clinically relevant detail in the dataset. Milestone gains increased 95% between the six- and 12-month timepoints and 147% between six months and the latest assessment at or beyond 12 months. Seven of 10 participants gained two or more milestones across two or more core Rett syndrome functional domains, and this multi-domain pattern was observed in both pediatric and adolescent or adult participants — an important finding given that older patients with more established regression are generally considered less amenable to neurological recovery.
Median time to first clinical improvement was reported at two months post-treatment, which is rapid for a gene therapy approach in a neurodevelopmental condition. Improvements on the Rett Syndrome Gross Motor Scale and Rett Syndrome Hand Function Scale reached statistical significance at p<0.001. The principal investigator, Bernhard Suter of Texas Children's Hospital, noted that the sequential, developmentally ordered pattern of milestone acquisition resembles normal developmental progression — a pattern absent from the natural history of Rett syndrome.
Safety in the 35 participants treated at the 1×10¹⁵ vg dose remained consistent with prior reports. All treatment-related adverse events were Grade 1 or Grade 2, and no new serious adverse events have been reported since the October 2025 data cutoff. In the Embolden registrational trial, where dosing is now complete, no treatment-related serious adverse events or dose-limiting toxicities have been observed as of the June 16, 2026 data cutoff.