Announcement • Sep 04

Stoke Therapeutics and Biogen Present Long-Term Clinical Data Supporting Disease-Modifying Potential of Zorevunersen for Dravet Syndrome

Stoke Therapeutics and Biogen presented data at the 16th European Epilepsy Congress (EEC) that support the potential of zorevunersen as a first-in-class disease-modifying treatment for Dravet syndrome. Data presented at EEC represent more than 5 years of clinical experience with zorevunersen in patients with Dravet syndrome, including two Phase 1/2a and ongoing open-label extension studies (OLEs). Four-year OLE results showed substantial and durable reductions in seizures and continuing improvements in cognition and behavior. A new exploratory sub-analysis also showed substantial reductions in the most severe seizure types, which are the leading risk factor for sudden unexpected death in epilepsy (SUDEP). An additional sub-analysis presented at EEC demonstrated substantial improvements in quality of life through 28 months in the OLEs. Zorevunersen continues to be generally well tolerated in the OLEs. The global, pivotal Phase 3 EMPEROR study is underway to evaluate the efficacy and safety of zorevunersen. Enrollment has completed in the planned primary analysis population, which will evaluate zorevunersen compared to sham administered via lumbar puncture (LP) in 162 patients enrolled in the U.S., U.K. and Japan. A Phase 3 data readout is anticipated in the third quarter of 2027 to complete the planned rolling New Drug Application (NDA) to the U.S. Food and Drug Administration (FDA) in the second half of 2027. Enrollment in Europe has also completed with 34 participants enrolled. 4-year OLE data: Following treatment in the Phase 1/2a studies, 93% (75/81) of eligible patients continued treatment in the OLEs. As of the 4-year data cutoff, 77% (58/75) of patients remained in these studies. Patients receiving zorevunersen on top of standard anti-seizure medicines (ASMs) continued to experience durable reductions in seizures and ongoing improvements in cognition and behavior. Statistically significant improvements in cognition and behavior were demonstrated at 1, 2, 3 and 4 years of treatment compared to OLE baseline. Severe seizure analysis: Patients with Dravet syndrome experience frequent, prolonged and refractory seizures. Seizures are classified based on severity, with generalized tonic-clonic (GTC), focal-to-bilateral tonic-clonic (focal-to-BTC) and tonic-clonic seizures of unknown origin considered the most severe and correlated with the highest morbidity and mortality in people with epilepsy. Substantial reductions in GTC and focal-to-BTC seizures were demonstrated through 3 years of the OLEs, compared to Phase 1/2a baseline, in patients treated with zorevunersen on top of standard of care ASMs. Quality of life analysis: Patients experienced substantial improvements in quality of life through 28 months in the OLEs, compared to Phase 1/2a baseline, as measured by EuroQol Visual Analog Scale (EQ-VAS, a component of the Euro-Qol-5D Youth). EQ-VAS is a validated measure of overall health status ranging from 0 to 100 (worst to best imaginable health) and provides insight into the real-world impact of zorevunersen on patients with Dravet syndrome and their families. Zorevunersen continues to be generally well tolerated, with some patients treated for more than 5 years in the Phase 1/2a and ongoing OLE studies. As of July 31, 2026, more than 930 doses have been administered. Elevated CSF protein lab values occurred in approximately 94% of patients, of which 59% have been classified as a treatment-emergent adverse event. Importantly, no serious or severe clinical manifestations have been associated with CSF protein elevations. There have been no reports of hydrocephalus. Zorevunersen is an investigational antisense oligonucleotide that is designed to treat the underlying cause of Dravet syndrome by increasing functional NaV1.1 protein production in brain cells from the unaffected (wild-type) copy of the SCN1A gene. This highly differentiated mechanism of action aims to reduce seizure frequency beyond what has been achieved with anti-seizure medicines and to improve neurodevelopment, cognition and behavior. Zorevunersen has demonstrated the potential for disease modification and has been granted orphan drug designation by the FDA and the EMA. The FDA has also granted zorevunersen rare pediatric disease designation and Breakthrough Therapy Designation for the treatment of Dravet syndrome with a confirmed mutation not associated with gain-of-function in the SCN1A gene, and China’s Center for Drug Evaluation has granted zorevunersen Breakthrough Therapy Designation. Stoke has a strategic collaboration with Biogen to develop and commercialize zorevunersen for Dravet syndrome. Under the collaboration, Stoke retains exclusive rights for zorevunersen in the United States, Canada, and Mexico; Biogen receives exclusive rest of world commercialization rights. Zorevunersen is currently in clinical development, and its safety and efficacy have not been evaluated by any regulatory authority. Two Phase 1/2a open-label, multicenter studies evaluated the effects of zorevunersen in patients with highly refractory Dravet syndrome ages 2 to 18 years (N=81). Primary endpoints were the safety profile, plasma pharmacokinetics (PK) and exposure in cerebrospinal fluid (CSF) of single and multiple doses of zorevunersen. Secondary endpoints included percentage change from baseline in major motor seizure frequency, overall clinical status (a measure of patients’ overall functioning) and quality of life. The ADMIRAL Phase 1/2a study included an exploratory endpoint to evaluate changes in neurodevelopmental status (cognition & behavior) as measured by Vineland Adaptive Behavior Scales, Third Edition (Vineland-3). The Phase 1/2a studies were completed in November 2023. Following treatment in the Phase 1/2a studies, eligible patients continued treatment with zorevunersen every four months in one of two OLEs. There was at least a 6-month gap between the last dose administered in the Phase 1/2a studies and the first dose administered in the OLEs. The primary endpoints are the safety profile of multiple doses of zorevunersen. Secondary endpoints include PK parameters, percentage change from baseline in major motor seizure frequency, change in overall clinical status, and change from baseline in quality of life. Exploratory endpoints include changes in neurodevelopment status as measured by Vineland-3. Results from the Phase 1/2a and OLE studies were published in The New England Journal of Medicine in March 2026. The OLE studies are ongoing. The Phase 3 EMPEROR Study (NCT06872125) is a global, double-blind, sham-controlled study evaluating the efficacy, safety and tolerability of zorevunersen in children ages 2 to.

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