BEDFORD, Mass.– Stoke Therapeutics and Biogen have reported long-term clinical data showing sustained reductions in seizures and continued improvements in cognition and behavior among patients with Dravet syndrome treated with the investigational therapy zorevunersen.
The companies presented the findings at the 16th European Epilepsy Congress, being held Sept. 5-9 in Athens, Greece.
Zorevunersen is being developed as a potential disease-modifying treatment for Dravet syndrome, a severe developmental and epileptic encephalopathy characterized by recurrent seizures and significant cognitive and behavioral impairments.
The data include more than five years of clinical experience from two Phase 1/2a studies and ongoing open-label extension studies. Four-year extension results showed durable seizure reductions and continued improvements in cognition and behavior among patients receiving zorevunersen in addition to standard anti-seizure medicines.
An exploratory analysis also found substantial reductions in generalized tonic-clonic and focal-to-bilateral tonic-clonic seizures, which are among the seizure types most strongly associated with sudden unexpected death in epilepsy, or SUDEP.
The companies said SUDEP is the leading cause of premature death in Dravet syndrome and that as many as 20% of children and adolescents with the condition die before reaching adulthood.
Another analysis showed improvements in quality of life through 28 months of treatment, based on the EuroQol Visual Analog Scale.
“Seizures are the most acute symptom of Dravet syndrome but the disease affects nearly every aspect of a child’s development, from their ability to communicate with loved ones to skills like dressing and feeding themselves,” said Helen Cross, MB ChB, Ph.D., Professor, The Prince of Wales’s Chair of Childhood Epilepsy and Director of University College London Great Ormond Street Institute of Child Health, and Honorary Consultant in Paediatric Neurology at Great Ormond Street Hospital.
“The continuing improvements in cognition and behavior shown in these studies suggest zorevunersen has the potential to narrow the developmental gap between these children and their neurotypical peers, helping them gain more independence and participate in experiences that many thought might never be possible,” Cross said.
Among patients eligible to enter the open-label extensions after the Phase 1/2a studies, 93%, or 75 of 81 patients, continued treatment. At the four-year data cutoff, 58 of those 75 patients remained in the studies.
The companies said statistically significant improvements in cognition and behavior were observed at one, two, three and four years compared with the open-label extension baseline.
Zorevunersen was generally well tolerated, with some patients receiving treatment for more than five years. As of July 31, 2026, more than 930 doses had been administered.
Elevated cerebrospinal fluid protein levels were reported in about 94% of patients, with 59% classified as treatment-emergent adverse events. The companies said no serious or severe clinical manifestations were associated with the elevations and no cases of hydrocephalus had been reported.
“Up to 20% of children and adolescents with Dravet syndrome die before reaching adulthood, and SUDEP is the primary cause,” said Barry Ticho, M.D., Ph.D., Chief Medical Officer of Stoke Therapeutics. “These data are especially meaningful because they show substantial reductions in the severe seizures most strongly correlated with SUDEP and demonstrate continuing improvements in the debilitating neurodevelopmental aspects of the disease.”
Stephanie Fradette, Pharm.D., Head of the Rare Neurology Development Unit at Biogen, said the ongoing extension studies will continue to provide information about zorevunersen’s long-term safety and its potential to address the genetic cause of Dravet syndrome.
The global pivotal Phase 3 EMPEROR study is evaluating the efficacy and safety of zorevunersen. Enrollment has been completed in the planned primary analysis population of 162 patients in the United States, United Kingdom and Japan, who will receive either zorevunersen or a sham procedure administered by lumbar puncture.
Enrollment in Europe has also been completed with 34 participants.
Stoke and Biogen expect Phase 3 data in the third quarter of 2027. The results are expected to support completion of a planned rolling New Drug Application submission to the U.S. Food and Drug Administration in the second half of 2027.
