Stoke Therapeutics and Biogen Present 4-Year Zorevunersen Data Supporting Potential Disease-Modifying Treatment for Dravet Syndrome

BEDFORD, Mass. and CAMBRIDGE, Mass. — Stoke Therapeutics, Inc. (Nasdaq: STOK), a biotechnology company dedicated to restoring protein expression by harnessing the body’s potential with RNA medicine, and Biogen Inc. (Nasdaq: BIIB) today announced presentations of data at the 16th European Epilepsy Congress (EEC), taking place September 5–9 in Athens, Greece. These data support the potential of zorevunersen as a first-in-class disease-modifying treatment for Dravet syndrome. The global, pivotal Phase 3 EMPEROR study is underway to evaluate the safety and efficacy of zorevunersen, and results are anticipated in the third quarter of 2027.

Data to be shared at EEC include the first medical meeting presentation of 4-year results from the ongoing Phase 1/2a open-label extension (OLE) studies of zorevunersen in patients with Dravet syndrome. Improvements in cognition and behavior were sustained through 4 years, in addition to durable seizure reductions and a generally well-tolerated safety profile. New exploratory sub-analyses show effects on the most severe seizure types, which are the leading risk factor for sudden unexpected death in epilepsy (SUDEP)¹, as well as improvements in quality of life for patients treated with zorevunersen.

“Together with the Phase 1/2a results, the ongoing OLE studies provide more than 5 years of clinical data that continue to deepen our understanding of zorevunersen’s long-term safety and benefits,” said Barry Ticho, M.D., Ph.D., Chief Medical Officer of Stoke Therapeutics. “These data have consistently shown substantial and durable seizure reductions with continued treatment. For the first time at EEC, we will share an additional analysis showing zorevunersen’s effects on the most severe seizure types. Alongside continuing improvements in cognition and behavior over time, these new findings support the potential of zorevunersen to change the course of Dravet syndrome by targeting the underlying genetic cause of the disease.”

“Dravet syndrome is a devastating neurodevelopmental disease that impacts nearly every aspect of daily life for patients and those who care for them,” said Stephanie Fradette, Pharm.D., Head of the Rare Neurology Development Unit at Biogen. “The data shared at EEC provide continued support for the potential of zorevunersen to give people living with Dravet syndrome the possibility of more neurotypical development. We look forward to discussing these data with the epilepsy community at EEC.”

Details of the EEC presentations are as follows:

• Title: Zorevunersen Demonstrates Disease-Modifying Potential in Patients with Dravet Syndrome with Improvements in Seizure Burden, Quality of Life, and Overall Clinical Status
  Oral Presentation Date & Time: Sunday, September 6, 12:53-1:02 PM EEST/ UTC+3 (5:53-6:02 AM ET)Oral Presenter: J. Helen Cross, MB ChB, Ph.D., Professor, The Prince of Wales’s Chair of Childhood Epilepsy and Head of the Developmental Neuroscience Programme at University College London Great Ormond Street Institute of Child Health, Honorary Consultant in Paediatric Neurology and Past President of the International League Against Epilepsy

• Title: Zorevunersen Demonstrates Potential as a Disease-Modifying Therapy in Patients with Dravet Syndrome: A Matching-Adjusted Indirect Comparison Between Natural History and Patients Receiving Zorevunersen
  Poster Presentation Date & Time: Monday, September 7, 2:00-3:00 PM EEST/ UTC+3 (7:00-8:00 AM ET)Poster Presenter: Andreas Brunklaus, MD, Consultant Paediatric Neurologist at the Royal Hospital for Children in Glasgow, Honorary Professor at the University of Glasgow
  Poster Number: P0803

• Title: Zorevunersen Demonstrates Potential as a Disease-Modifying Therapy in Patients with Dravet Syndrome Through Durable Seizure Reduction and Improvements in Cognition, Behavior, and Quality of Life Through 48 Months of Treatment in Open-Label Extension Studies
  Poster Presentation Date & Time: Tuesday, September 8, 2:00-3:00 PM EEST/ UTC+3 (7:00-8:00 AM ET)Poster Presenter: Elaine Wirrell​, MD, Director of Pediatric Epilepsy at Mayo Clinic, Director of the Child and Adolescent Neurology Residency Training Program at Mayo Clinic
  Poster Number: P0797

 

Summary of Zorevunersen Safety Data

• Following treatment in the Phase 1/2a studies, 93% (75/81) patients continued treatment in one of two OLE studies. As of the 4-year data cutoff, 77% (58/75) patients remained in these studies.
• 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.

 

About Dravet Syndrome
Dravet syndrome is a severe developmental and epileptic encephalopathy (DEE) characterized by recurrent seizures as well as significant cognitive and behavioral impairments. Most cases of Dravet are caused by mutations in one copy of the SCN1A gene, leading to insufficient levels of NaV1.1 protein in neuronal cells in the brain. Even when treated with the best available anti-seizure medicines (ASMs), up to 57 percent of patients with Dravet syndrome do not achieve ≥50 percent reduction in seizure frequency. Complications of the disease often contribute to a poor quality of life for patients and their caregivers. Developmental and cognitive impairments often include intellectual disability, developmental delays, movement and balance issues, language and speech disturbances, growth defects, sleep abnormalities, disruptions of the autonomic nervous system and mood disorders. Compared with the general epilepsy population, people living with Dravet syndrome have a higher risk of sudden unexpected death in epilepsy, or SUDEP; up to 20 percent of children and adolescents with Dravet syndrome die before adulthood due to SUDEP, prolonged seizures, seizure-related accidents or infections ². Dravet syndrome occurs globally and is not concentrated in a particular geographic area or ethnic group. Currently, it is estimated that up to 38,000 people are living with Dravet syndrome in the U.S. (~16,000), UK, EU-4 and Japan ³. There are no approved disease-modifying therapies for people living with Dravet syndrome.

 

About Zorevunersen
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 (Nasdaq: BIIB) 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.

 

About the Phase 1/2a and Open-Label Extension Studies
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 (NEJM) in March 2026. The OLE studies are ongoing.

 

About Stoke Therapeutics
Stoke Therapeutics (Nasdaq: STOK), is a biotechnology company dedicated to restoring protein expression by harnessing the body’s potential with RNA medicine. Using Stoke’s proprietary TANGO (Targeted Augmentation of Nuclear Gene Output) approach, Stoke is developing antisense oligonucleotides (ASOs) to selectively restore naturally-occurring protein levels. Stoke’s first medicine in development, zorevunersen, has demonstrated the potential for disease modification in patients with Dravet syndrome and is currently being evaluated in a Phase 3 study. Stoke’s initial focus are diseases of the central nervous system and the eye that are caused by a loss of ~50% of normal protein levels (haploinsufficiency). Proof of concept has been demonstrated in other organs, tissues, and systems, supporting broad potential for Stoke’s proprietary approach. Stoke is headquartered in Bedford, Massachusetts. For more information, visit https://www.stoketherapeutics.com/ or follow us on LinkedIn.

 

About Biogen
Founded in 1978, Biogen is a leading biotechnology company that pioneers innovative science to deliver new medicines to transform patients’ lives and to create value for shareholders and our communities. We apply deep understanding of human biology and leverage different modalities to advance first-in-class treatments or therapies that deliver superior outcomes. Our approach is to take bold risks, balanced with return on investment to deliver long-term growth. We routinely post information that may be important to investors on our website at www.biogen.com. Follow us on social media – Facebook, Instagram, LinkedIn, X, YouTube.

 

Biogen Digital Media Disclosure
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References

1. Beniczky, S. et al. Updated classification of epileptic seizures: Position paper of the International League Against Epilepsy. Epilepsia. 2025; 1804–1823.
2. Symonds, J. et al. Early childhood epilepsies: epidemiology, classification, aetiology, and socio-economic determinants. Brain. 2021;144(9):2879-2891.
3. Based on Stoke Therapeutics’ preliminary estimates, which scaled annual incidence to prevalence using country-specific live birth rates over the past 85 years and adjusted for Dravet-specific mortality. The estimate is based on incidence rates published by Wu et al., Pediatrics, 2015.