The purpose of this study is to evaluate the efficacy and safety of ION582 in children and adults with Angelman syndrome caused by a deletion or mutation of the UBE3A gene.

This is a Phase 3, randomized, double-blind, placebo-controlled study in people with Angelman syndrome. The study will consist of 4 periods: a screening period of up to 28 days, an approximate 60-week double blind, placebo-controlled treatment period, followed by an approximate 25-month Long-Term Extension (LTE) treatment period, and an approximate 8-month Post-LTE follow-up period. The study will be comprised of 2 cohorts. Cohort 1 will include pediatric participants, aged 2 to less than (<)18 years old and serve as the population for evaluation of primary and secondary outcome measures; Cohort 2 will include adult participants, aged 18 to ≤50 years old. Participants will be randomized 1:1 to 80 mg ION582 or placebo during the double-blind placebo-controlled treatment period. Participants from both cohorts completing the placebo-controlled treatment period will be eligible to transition into the LTE Treatment Period wherein all trial participants will receive ION582. Participant, Caregiver, Investigator and Sponsor will remain blinded to the ION582 dose administered to participants during the LTE.

The study initiated in 2024 with three dosing groups (40 mg ION582, 80 mg ION582, and placebo). Following additional review of data from the ongoing Phase 1/2 trial of ION582 (HALOS), REVEAL was amended in December 2025 to its current form as a two-arm study: 80 mg ION582 and placebo. Following the amendment, participants who were randomized to receive 40 mg ION582 will be transitioned to receive 80 mg ION582.

Key Inclusion Criteria:

1. The participants caregiver(s)/ legally authorized representative must have given written informed consent and any authorizations required by local law and be able to comply with all study requirements.
2. Medically stable and can undergo sedation and/or general anesthesia without intubation.
3. Male or female between 2 and lesser than or equal to (≤)50 years of age, depending on specific cohort, at the time of the in-clinic Screening visit.
4. Participant has a clinical diagnosis of Angelman syndrome (AS) with molecular confirmation of either Ubiquitin-protein ligase E3A (UBE3A) deletion or UBE3A mutation.
5. Currently receiving stable doses of concomitant medications typically prescribed for AS, such as anti-epileptic medication, behavioral management medications, sleep medications, gabapentin, cannabidiol, and special diets, supplements, or nutritional support for at least 8 weeks prior to the Baseline visit.
6. Legally authorized representative/caregiver(s) agree(s) not to post any of the participant's personal medical data or information related to the study on any website or social media site (e.g., Facebook, Instagram, X (formerly Twitter), YouTube, TikTok, etc.) from the time of enrollment until they are notified that the study is completed.

Key Exclusion Criteria:

1. Must not have any clinically significant abnormalities in medical history (e.g., major surgery within 3 months of screening), or on physical examination for which treatment with an antisense oligonucleotide (ASO) would be contraindicated or which, in the opinion of the Principal Investigator (PI), could confound the results of this study.
2. Known brain or spinal disease that would interfere with the lumbar puncture (LP) procedure, cerebrospinal fluid (CSF) circulation, or presence of other factors would affect the safety of the LP procedure.
3. Must not have any other conditions, which, in the opinion of the Investigator, would make the participant unsuitable for inclusion or could interfere with the participant participating in or completing the study.
4. Must not have any laboratory abnormalities or any other clinically significant abnormalities that would, as assessed by the Investigator, at screening or Baseline, render a participant unsuitable for inclusion.
5. Previous treatment with an oligonucleotide (including small interfering ribonucleic acid (RNA) [siRNA], ASOs) gene therapy or gene editing. This exclusion criterion does not apply to approved nucleic acid-based vaccines, including mRNA vaccines, which are allowed.
6. Has molecular confirmation of AS due to paternal uniparental disomy, imprinting center defect, or mosaic findings.

Other inclusion/exclusion criteria may apply.

Ionis may share anonymized individual participant data, aggregated clinical data, and other types of data that support the results in this study. Data requests from qualified researchers will be considered once all three of the following criteria are met: (1) 12 months from marketing approval of the study drug in both the United States and European Union; (2) 18 months from conclusion of the study; and (3) 6 months from publication of study article. Access would be via a secure environment and is contingent upon approval of a research proposal and entry into an appropriate data use agreement. Requests to access data can be submitted via the website https://vivli.org/ourmember/ionis/.