Dravet syndrome shrugs off most drugs. Encoded Therapeutics wants to fix it with one injection. The obstacle is the immune suppressant that keeps that injection safe.
Phase 1/2 POLARIS data show a median 76% reduction in monthly countable seizures sustained through 52 weeks after a single dose.
The unresolved question is durability. A gene therapy delivered once by virus cannot easily be re-dosed if it fades, and the drug used to manage the immune response may blunt the therapy itself.
Dravet syndrome is the most common developmental and epileptic encephalopathy. More than 85% of cases trace to loss-of-function variants in the SCN1A gene, which encodes a sodium channel that inhibitory neurons depend on. Children get seizures that resist medication, and their development stalls.
The company is clinical-stage, based in South San Francisco. Its lead candidate, ETX101, is an adeno-associated virus (AAV) therapy. It carries a transcription factor that raises expression of the patient's own SCN1A gene in GABAergic inhibitory neurons, the cells that act as the brain's brakes.
Why one shot might be enough
ETX101 is given once. The POLARIS program runs three open-label, dose-escalation trials: ENDEAVOR in the United States, EXPEDITION in the United Kingdom, and WAYFINDER in Australia. Enrolled children are between six months and seven years old.
The dataset has now survived two public readouts this year. At the American Society of Gene and Cell Therapy in May, it reported a dose-dependent antiseizure effect, with roughly a 76% median reduction in monthly countable seizures from week five through week 52. In September, at the European Epilepsy Congress, the company said those reductions held through 52 weeks. Children treated before age two tracked inside the neurotypical range on cognitive measures for up to 76 weeks. Adaptive behavior improved across the full age range. No treatment-related serious adverse events appeared in up to 117 weeks of follow-up.
Seizure reduction at 52 weeks
Median monthly countable seizure reduction from week five through week 52 after a single ETX101 dose. · Encoded Therapeutics, 2026
Regulators have noticed. ETX101 carries a Regenerative Medicine Advanced Therapy (RMAT) designation, a Breakthrough Therapy designation granted in January 2026, Fast Track, Rare Pediatric, and Orphan Drug status in the United States, and Orphan designation in Europe. It has begun producing doses for pivotal trials at its own facility in North Carolina.
One dose, durable through at least a year, with a safety record clean to 117 weeks. If that holds in the pivotal trial, ETX101 addresses the cause of Dravet rather than its symptoms.
The durability problem the round does not solve
Gene therapy has a math problem that small molecules do not. The immune system learns the AAV capsid after the first exposure, which is why repeat dosing is hard. If the effect wanes at year five, the fallback is thin.
Managing that immune response adds its own cost. Patients in POLARIS receive immunosuppression, and STAT News reported in May that the drug meant to make the gene therapy safer may also make it less effective. It has not published a definitive answer on the tradeoff.
While early, the emerging clinical impact of ETX101 underscores its potential as a one-time, disease-modifying medicine for Dravet syndrome.— Kartik Ramamoorthi, Chief Executive Officer, Encoded Therapeutics
The commercial race sharpens the stakes. Stoke Therapeutics is developing zorevunersen, an antisense oligonucleotide for Dravet that requires chronic dosing. Biogen signed a deal worth up to $550 million for rights to the asset outside Canada, the United States, and Mexico.
One-time therapy and chronic therapy are different businesses. A single dose can command a high price if it lasts. A chronic drug earns revenue every month, but it asks patients to keep showing up.
| Parameter | ETX101 (Encoded) | Zorevunersen (Stoke) |
|---|---|---|
| Mechanism | AAV9 gene regulation, upregulates SCN1A | Antisense oligonucleotide |
| Dosing | ✔ One time | ✗ Chronic |
| Redosing | ✗ Limited by capsid immunity | ✔ Repeatable |
| Partner deal | ◐ None announced | Biogen, up to $550M ex-US |
Manufacturing is half the bet
A gene therapy is only as good as the factory behind it. Making AAV vectors at commercial scale is slow, expensive, and prone to low yields, which is why the round explicitly funds a build-out rather than just a trial. The company started producing ETX101 for pivotal trials at its own North Carolina facility in 2025, and the Series F pays to take that line to commercial volume.
That choice carries risk. Owning manufacturing ties up capital for years before a single patient is billed, and the process has to scale without changing the product. The upside is control: no contract manufacturer sets the timeline, and no third party captures the margin on a therapy that may only ever be sold once per patient.
What $275 million actually buys
The Series F was co-led by GV, Alphabet's venture arm, and an undisclosed healthcare fund. ARCH Venture Partners, Braidwell, Farallon Capital Management, Illumina Ventures, Invus, Janus Henderson Investors, Matrix Capital Management, Nolan Capital, RTW Investments, SoftBank Vision Fund 2, and Venrock also joined.
Series F, September 2026
Co-led by GV and an undisclosed healthcare fund, with return backers including ARCH and SoftBank Vision Fund 2. · BioSpace, 2026
The round buys three things. It funds pivotal and expansion studies for ETX101. It pays for commercial-scale buildout of its in-house manufacturing, run under good manufacturing practice (GMP). And it moves ETX301 toward a 2027 Investigational New Drug (IND) filing. ETX301 is a vectorized microRNA therapy aimed at post-amputation neuroma pain.
The round did not land in isolation. In the same week, BrainChild Bio raised $116 million for a glioma drug, Moonwalk Biosciences brought in $70 million for an RNAi obesity therapy, Caspian Therapeutics spun out with $50 million, and Solstice Oncology launched with $225 million. Capital is moving back into clinical-stage biotech after a long freeze.
Encoded has made this bet before, with less room. In February 2025 it cut 29% of its staff to keep the Phase 1/2 program running. The bet appears to have worked: by the end of last year the trial showed a 78% reduction in seizures.
The pivotal ETX101 readout in infants and young children, first dosed in May 2026.
Any update on the sirolimus tradeoff between immune suppression and therapy effect.
Whether the 52-week seizure reduction holds beyond two years.
How payers price a one-time cure against a monthly antisense drug.
The next milestone is the pivotal readout. If the 52-week effect holds in a larger trial, it becomes the first to market a one-time therapy for Dravet. If it fades, the honest comparison is an antisense drug with a simpler dose and a lower ceiling.
As we wrote in September, when Ionis' Zanvastro became the first approved treatment for Alexander disease, gene-targeted therapy reached the clinic for rare neurological disease. Zanvastro is given repeatedly. ETX101 is the opposite wager: a single dose, carried by virus, expected to last a childhood.