A Study to Evaluate XEN1101 as Adjunctive Therapy in Focal Epilepsy

Key Inclusion Criteria: * Be properly informed of the nature and risks of the study and give informed consent in writing, prior to entering the study * BMI ≤40 kg/m2 * Diagnosis (≥2 years) of focal epilepsy according to the International League Against Epilepsy \[ILAE\] Classification of Epilepsy (2017) * Prior neuroimaging within the last 10 years and documentation is available * Treatment with a stable dose of 1 to 3 allowable current AEDs for at least one month prior to screening, during baseline, and throughout the duration of the DBP * Must be willing to comply with the contraception requirements * Males must agree not to donate sperm from the time of the first administration of study drug until 6 months after the last dose. Females must agree not to donate ova from the time of the first administration of study drug until 6 months after the last dose. * Able to keep accurate seizure diaries Key Exclusion Criteria: * History of pseudoseizures, psychogenic seizures, primary generalized seizure, or focal aware non-motor seizures only * Presence or previous history of Lennox-Gastaut syndrome * Seizures secondary to other diseases or conditions * History of repetitive seizures within the last 12 months where the individual seizures cannot be counted * History of neurosurgery for seizures \<1 year prior to enrollment, or radiosurgery \<2 years prior to enrollment * Schizophrenia and other psychotic disorders, or active suicidal plan/intent in the past 6 months, or a history of suicide attempt in the last 2 years, or more than 1 lifetime suicide attempt * History or presence of any significant medical or surgical condition or uncontrolled medical illness at screening, or history of cancer within the past 2 years, with the exception of appropriately treated basal cell or squamous cell carcinoma * Any clinically significant abnormalities on pre-study physical examination, vital signs, laboratory values or ECG indicating a medical problem that would preclude study participation including but not limited to: 1. History of presence of long QT syndrome; QTcF \> 450 msec at baseline; family history of sudden death of unknown cause 2. History of skin or retinal pigment epithelium abnormalities caused by ezogabine * Past use of vigabatrin without stable visual fields tested twice over the 12 months after the last dose of vigabatrin (concomitant use of vigabatrin is not allowed) * If felbamate is used as a concomitant AED, patients must be taking it for at least 2 years, with a stable dose for 2 months (or no less than 49 days) and acceptable hematology and LFT history and values prior to Screening. If received in the past, felbamate must have been discontinued 2 months (or no less than 49 days) prior to Screening. * Have had multiple drug allergies or a severe drug reaction to an AED(s), including dermatological (e.g., Stevens-Johnson syndrome), hematological, or organ toxicity reactions * Current use of a ketogenic diet