Stoke Therapeutics and Biogen Announce Publication of Two-Year Natural History Data Demonstrating the Severity of Dravet Syndrome, Including Frequent Seizures and Significant Cognitive and Behavioral Impairments

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 the publication of final data from the BUTTERFLY study, a prospective, two-year natural history study in people with Dravet syndrome. Dravet syndrome is a severe developmental and epileptic encephalopathy (DEE) characterized by recurrent seizures and significant cognitive and behavioral impairments. There are currently no approved disease-modifying medicines to treat Dravet syndrome.

The BUTTERFLY study evaluated the impact of Dravet syndrome on adaptive functioning and neurodevelopment over two years in children and adolescents ages 2 to 18 years old. Major motor seizure frequency was evaluated as a secondary outcome measure. In the study, patients were treated with standard-of-care, including anti-seizure medicines (ASMs). Highlights from the research, published November 14, 2025, in Neurology, ¹ the medical journal of the American Academy of Neurology, include:

• Regardless of the age at which patients with Dravet syndrome entered the study, neurodevelopment plateaued at the developmental age of approximately two years old. This created a gap that widened over time versus what would be expected for children with typical development.
• Results showed that over the course of the two-year study, patients experienced minimal changes in cognition and behavior, including communication, motor skills and personal skills, compared to typical neurodevelopment expected for children of the same age.
  • Assessments of cognition and behavior used in BUTTERFLY were pre-determined endpoints that were incorporated into this natural history study to support use in future clinical studies of Dravet syndrome. They included the Vineland Adaptive Behavior Scale, Third Edition (Vineland-3); Bayley Scales of Infant Development, Third Edition (BSID-III); and Wechsler Preschool and Primary Scale of Intelligence, Fourth Edition (WPPSI-IV). These assessments, as well as other outcome measures used in BUTTERFLY, are consistent with the primary and secondary endpoints of the pivotal, global Phase 3 EMPEROR study evaluating zorevunersen as a potential disease-modifying medicine for the treatment of Dravet syndrome.
• Major motor seizure frequency increased by 10.6 percent over two years (average of 14.3 seizures/28 days at baseline; p=0.63; n=23).

“These natural history data confirm that the life-altering effects of Dravet syndrome extend well beyond seizures, resulting in significant deficits in patients’ functioning and development despite treatment with the best available medicines,” said Joseph Sullivan, M.D., FAES, lead author of the Neurology publication and Professor of Neurology and Pediatrics at the University of California San Francisco. “The findings make clear that as children with Dravet syndrome grow up, they experience a widening gap in their cognitive and behavioral development compared to a neurotypical child of the same age. By giving us a baseline characterization of what current treatments can do for patients, these data become increasingly important in helping us understand how potential disease-modifying medicines might change the trajectory of Dravet syndrome. I’d like to extend my deep appreciation to the families who participated in BUTTERFLY and made significant sacrifices to help us more fully comprehend the debilitating effects of this disease.”