A Study to Evaluate XEN1101 as Adjunctive Therapy in Primary Generalized Tonic-Clonic Seizures

This is a Phase 3, multicenter, randomized, double-blind, placebo-controlled study to evaluate the clinical efficacy, safety, and tolerability of XEN1101 administered as adjunctive treatment in primary generalized tonic-clonic seizures (PGTCS).

Key Inclusion Criteria: 1. Subject is properly informed of the nature and risks of the study and gives informed consent in writing prior to entering the study (for adult subjects) and for adolescent subjects parent/legal guardian and subject gives informed consent or assent in writing prior to entering the study. 2. Subject is ≥12 years of age with a BMI ≤40 kg/m2 at Visit 1. 3. Subject must have had adequate trials of at least 2 ASMs, which were given (and tolerated) at adequate therapeutic doses, without achieving sustained seizure freedom. 4. Subject has probable or possible PGTCS (with or without other subtypes of generalized seizures) for ≥1 year, in the setting of generalized epilepsy according to the International League Against Epilepsy 2017 classification criteria, and subject is approved by The Epilepsy Study Consortium (TESC). 5. Subject is on a stable dose of 1 to 3 allowable current ASMs for at least 3 months prior to the planned randomization (Visit 2), during screening/baseline, and throughout the DBP. 6. Subject is able to keep accurate seizure diaries. Key Exclusion Criteria: 1. Subject has had status epilepticus within the 12 months prior to Visit 1. 2. Subject has history of repetitive seizures within the 12-month period preceding Visit 1 where the individual seizures cannot be counted. 3. Subject has a history of non-epileptic psychogenic seizures within 10 years prior to Visit 1. 4. Subject has a concomitant diagnosis of focal-onset seizures (FOS). 5. Subject has presence or history of a developmental and epileptic encephalopathy, including Lennox-Gastaut syndrome. 6. Subject has seizures secondary to drug or alcohol use, ongoing infection, neoplasia, demyelinating disease, degenerative neurological disease, metabolic illness, progressive structural lesion, encephalopathy, or progressive central nervous system (CNS) disease. 7. Subject has history of neurosurgery for seizures \45 of 56 days, if no retrospective data prior to Visit 1 are used). 3. Subject did not change dose of, stop, or initiate any new ASM(s) during the 3 months that preceded randomization and plans on maintaining a stable dose of ASM(s) during the DBP.