STOK Zorevunersen: 4-Year Durability in Dravet Syndrome

Stoke Therapeutics (STOK) and Biogen reported that zorevunersen, their antisense drug for Dravet syndrome, sustained cognition and behavior improvements through four years of treatment, alongside durable seizure reductions and a clean safety profile. The data, from the Phase 1/2a open-label extension (OLE), will be presented at the 16th European Epilepsy Congress in Athens this week.

The new element is a set of exploratory sub-analyses showing effects on the most severe seizure types, the leading risk factor for SUDEP (sudden unexpected death in epilepsy). Up to 20% of children with Dravet syndrome die before adulthood, and no drug yet treats the disease’s underlying cause, a mutated SCN1A gene that leaves the brain short of the NaV1.1 sodium channel. Zorevunersen is an antisense oligonucleotide that nudges the healthy copy of SCN1A to make more of that protein, rather than simply suppressing seizures the way Epidiolex and Fintepla do.

The biology is specific: most Dravet cases come from a loss-of-function mutation in one copy of SCN1A, leaving the brain with half the normal NaV1.1 channel, a protein that calms overexcited neurons. By boosting the healthy allele, zorevunersen aims to restore that channel instead of just damping seizures. Dravet affects roughly 16,000 people in the US and about 38,000 across the US, UK, EU-4, and Japan, a small but high-need population where a disease-modifying drug would command premium pricing.

STOK closed at $30.89 on September 1, valuing the company near $1.9 billion.

The honest read is that this is long-term durability data, not the registrational result. It de-risks the question that hangs over central nervous system antisense programs: whether the benefit fades. The SUDEP-linked sub-analysis matters because severe seizures, not overall seizure counts, are what kill these children. But it is exploratory, and the companies did not quantify the effect.

The binary event remains the Phase 3 EMPEROR Dravet study, a randomized sham-controlled trial that is fully enrolled, with a readout expected in Q3 2027. The 162-patient study runs a 52-week treatment period followed by an open-label extension, with key secondary endpoints tracking major motor seizure frequency and cognition and behavior on the Vineland-3 scales. Stoke retains US, Canada, and Mexico rights; Biogen holds the rest of the world. The FDA has granted zorevunersen Breakthrough Therapy, orphan drug, and rare pediatric disease designations.

What to watch: the full four-year dataset at EEC and any update on EMPEROR’s data-cut date. A miss there would cut the $1.9 billion valuation sharply, since zorevunersen is Stoke’s lead asset.