Encoded Therapeutics reports 76% seizure reduction

Encoded Therapeutics said on May 13 that updated Phase 1/2 data for its experimental gene therapy ETX101 showed a roughly 76% median reduction in monthly countable seizure frequency in a small Dravet syndrome cohort followed through 52 weeks. The South San Francisco company presented the data at the American Society of Gene & Cell Therapy’s 2026 meeting as it pushes the program into pivotal-stage testing. The result that circulated on social media on Friday came from the third dose level of the ongoing POLARIS development program, where Encoded said three patients were included in that seizure analysis. The company also reported no treatment-related serious adverse events across four dose levels. ### Where did the 76% number come from? Encoded said the 76% figure refers to the median reduction in monthly countable seizure frequency, or MCSF, at dose level 3 from week 5 through week 52 after a single dose of ETX101. The company said the dose-level-3 analysis included three patients. (encoded.com) April 10, 2026 was the data cutoff for the ASGCT presentation, according to the company. Encoded described the antiseizure effect as dose-dependent and durable through 52 weeks of observation. (encoded.com) ### How broad is the dataset behind this update? CGTLive reported that the updated ASGCT dataset covered 21 treated participants across the ENDEAVOR, WAYFINDER and EXPEDITION studies in the broader POLARIS program. The same report said follow-up extended up to 91 weeks as of January 26, 2026. (encoded.com) Encoded’s May 13 release highlighted additional patients, early data from the top dose level, called DL4, and longer-term outcomes in children ages 6 months to 7 years. The company did not disclose sample sizes for every subgroup in the press release summary that was widely shared online. (cgtlive.com) ### What else did Encoded say improved besides seizures? Encoded said clinically meaningful improvements in adaptive behavior were observed across the age range studied. The company also said children treated before age 2 showed cognitive trajectories that were generally consistent with neurotypical development at 52 weeks, comparing them with expected developmental stagnation from natural history. (encoded.com) Mary Anne Meskis, chief executive of the Dravet Syndrome Foundation, said in the company release that seeing seizure reductions paired with developmental gains was “profoundly encouraging.” That characterization was hers, not an independent study conclusion. (encoded.com) ### What is ETX101 designed to do? ETX101 is an AAV9-based gene regulation therapy designed to selectively increase SCN1A expression in GABAergic inhibitory interneurons, according to Encoded and CGTLive. The program targets SCN1A-positive Dravet syndrome, a severe developmental and epileptic encephalopathy commonly linked to SCN1A haploinsufficiency. (encoded.com) Dravet syndrome is typically treated with antiseizure medicines that can lower seizure burden but do not directly correct the underlying genetic deficit, CGTLive reported. Encoded has positioned ETX101 as a one-time treatment intended to address that mechanism. ### Why are investors and families watching the next study? (encoded.com) May 6 marked the dosing of the first patient in ENDEAVOR Part 2, the pivotal study of ETX101, Encoded said. The company said that trial will enroll 30 infants and young children ages 6 months to 4 years across sites in the United States, United Kingdom and Australia. (cgtlive.com) Encoded said enrollment in ENDEAVOR Part 2 is expected to finish by the end of 2026, with initial data anticipated by the end of 2027. The company also said it has started dosing in ENDEAVOR Part 1B, an expansion study in patients ages 4 to 18 years, with initial data expected in the fourth quarter of 2026. (businesswire.com)