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

Inclusion Criteria:

1. Subject is properly informed of the nature and risks of the study and gives informedconsent in writing prior to entering the study (for adult subjects) and foradolescent subjects parent/legal guardian and subject gives informed consent orassent 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 (andtolerated) at adequate therapeutic doses, without achieving sustained seizurefreedom.

4. Subject has probable or possible PGTCS (with or without other subtypes ofgeneralized seizures) for ≥1 year, in the setting of generalized epilepsy accordingto the International League Against Epilepsy 2017 classification criteria, andsubject 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 1 monthprior to screening (Visit 1), during screening/baseline, and throughout the DBP.

6. Subject is able to keep accurate seizure diaries.

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 precedingVisit 1 where the individual seizures cannot be counted.

3. Subject has a history of non-epileptic psychogenic seizures.

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 <1 year prior to Visit 1, orradiosurgery <2 years prior to Visit 1.

8. Subject has schizophrenia and other psychotic disorders (eg, schizophreniformdisorder, schizoaffective disorder, psychosis not otherwise specified), bipolardisorder, obsessive-compulsive disorder, or another serious mental health disorder.Subject has uncontrolled unipolar major depression where changes in pharmacotherapyare needed or anticipated during the study.

9. Subject has any clinically significant laboratory abnormalities or clinicallysignificant abnormalities on prestudy physical examination, vital signs, or ECGthat, in the judgment of the investigator, indicate a medical problem that wouldpreclude study participation, including but not limited to: a. History or presence of long QT syndrome; QTcF >450 msec at baseline; familyhistory of sudden death of unknown cause.

10. Any personal circumstance that, in the opinion of the investigator, preventsadherence to the protocol.

The criteria to be eligible for randomization are:

1. During the last 56 days of the baseline period that preceded the randomization visit (Visit 2), subject must have had a sufficient documented seizure frequency of PGTCS,including ≥1 PGTCS during each of the first and second 4-week periods precedingrandomization.

2. Seizure diary was completed a minimum of 80% of all days (ie, ≥45 days) during thelast 56 days of the baseline period that preceded randomization as evidence ofadequate compliance.

3. Subject did not change dose of, stop, or initiate any new ASM(s) during the baselineperiod and plans on maintaining a stable dose of ASM(s) during the DBP.