An experimental gene therapy cut seizures 83% in children with a severe epilepsy syndrome
ETX101 doesn't replace the faulty gene behind Dravet syndrome — it turns up the healthy copy that's still there. New data from 21 children show the effect holding for a year.
Encoded Therapeutics presented updated results from its experimental Dravet syndrome gene therapy, ETX101, in the Presidential Symposium at the 2026 annual meeting of the American Society of Gene & Cell Therapy — the conference's most visible slot — reporting a median 83.5% reduction in monthly seizure frequency among children who received the therapy's higher doses [s1].
What the therapy actually does
Dravet syndrome is a severe, drug-resistant childhood epilepsy caused by a mutation in one copy of the SCN1A gene, which normally helps regulate electrical signaling in inhibitory brain cells. Because patients still carry one working copy of the gene, ETX101 does not attempt to deliver a replacement gene, the way many gene therapies do. Instead, it is designed to selectively boost expression of the patient's own existing healthy SCN1A copy specifically within GABAergic inhibitory interneurons, using an AAV9 viral vector for delivery [s1]. Encoded Therapeutics describes this mechanism — increasing output from an endogenous gene rather than inserting a new one — as the first clinical proof of cell-type-selective AAV gene regulation [s1].
The data presented
The updated results combine outcomes from 21 treated participants across three studies that make up the broader POLARIS clinical program: ENDEAVOR, WAYFINDER and EXPEDITION [s1]. Among participants treated at the higher dose levels (DL3/DL4) who received prednisolone without sirolimus as part of their treatment regimen, the median reduction in monthly countable seizure frequency was 83.5% through week 28, layered on top of standard-of-care antiseizure medications the children continued taking [s1]. Participants who also received sirolimus showed a smaller median reduction of 45.4%, bringing the aggregate median reduction across all high-dose participants to 75.7% [s1]. The company reports the antiseizure effect was dose-dependent and durable through 52 weeks of observation following a single dose [s1].
Beyond seizure counts, Encoded Therapeutics reported effects on the developmental decline that typically accompanies Dravet syndrome. Children treated before age 2 showed cognitive trajectories diverging from the developmental stagnation expected under the condition's natural history, with outcomes the company describes as "generally consistent with neurotypical development" [s1].
Why the comparator group matters
Dravet syndrome's natural history — progressive developmental regression alongside frequent, treatment-resistant seizures — is well documented enough that Encoded Therapeutics is comparing treated outcomes against that expected trajectory rather than against a randomized placebo arm. That is standard practice at this stage for a rare pediatric disease where withholding an experimental therapy from a comparison group raises significant ethical questions, but it also means the 83.5% figure describes a difference from an external historical benchmark rather than a concurrently randomized control — a distinction that matters when interpreting how large an effect the therapy is really producing.
What is not yet established
This is a Phase 1/2 dataset, not a completed pivotal trial: 21 treated participants across three separate protocol names is a small number for firm conclusions about a therapy's typical effect. The company's own release frames the durability finding as "on the strength of these results," noting Encoded Therapeutics has now launched a pivotal clinical trial — the step before an FDA approval submission — rather than describing regulatory approval as imminent [s1]. Long-term safety beyond the 52-week observation window reported here, the durability of effect years rather than months out, and how ETX101 performs in a larger and more diverse patient population all remain open questions that only the pivotal trial can answer.
What to watch
Whether the pivotal trial replicates the roughly 84% seizure reduction seen in this small, more carefully selected group at the highest doses, and whether the sirolimus-versus-no-sirolimus difference in response — 45.4% versus 83.5% — holds up and gets explained, since it suggests dosing regimen and concomitant medication choices meaningfully affect outcomes with this therapy. Also worth tracking: whether cognitive outcomes in children treated younger than 2 remain distinct from those treated later, which would strengthen the case for early intervention if the therapy eventually reaches approval.
Sources
- [s1] Encoded Therapeutics, Inc., "Encoded Therapeutics Presents New Clinical Data from POLARIS Phase 1/2 Trials of ETX101 Gene Therapy in Dravet Syndrome at the ASGCT 2026 Presidential Symposium," 13 May 2026. https://www.businesswire.com/news/home/20260513482347/en/Encoded-Therapeutics-Presents-New-Clinical-Data-from-POLARIS-Phase-12-Trials-of-ETX101-Gene-Therapy-in-Dravet-Syndrome-at-the-ASGCT-2026-Presidential-Symposium
Sources
- Encoded Therapeutics Presents New Clinical Data from POLARIS Phase 1/2 Trials of ETX101 Gene Therapy in Dravet Syndrome at the ASGCT 2026 Presidential Symposium — Encoded Therapeutics, Inc. , May 13, 2026
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