A Study to Evaluate the Safety, Tolerability and Efficacy of XEN1101 in Major Depressive Disorder

Key Inclusion Criteria: * Be properly informed of the nature and risks of the study and given written informed consent. * Male or female, aged 18 through 65 years (inclusive) with a body mass index (BMI) ≤35 kg/m². * Subject must meet the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for current MDD and currently in a moderate to severe major depressive episode (MDE), confirmed using the Mini International Neuropsychiatric Interview (MINI). * Current MDE duration ≥2 months and \<24 months at the time of screening. * Current illness severity that is at least moderate, defined as a score of ≥20 on the HAM-D17 at screening and on Day 1. * Score ≥20 on the SHAPS at screening and on Day1. * Must be willing to comply with the study protocol for the full term of the study. Key Exclusion Criteria: * A primary psychiatric diagnosis other than MDD as defined by DSM-5 (comorbid anxiety disorders \[including agoraphobia, generalized anxiety disorder, social anxiety disorder, post-traumatic stress disorder (PTSD), and panic disorder\] are allowed). * Concomitant use of antidepressants and/or other disallowed pharmacotherapy (including benzodiazepines). * History of schizophrenia or other psychotic disorder, MDD with psychotic features, bipolar I or II disorder, or MDD with mixed features. * History of non-response to \>1 antidepressant drug due to lack of efficacy in the current MDE. * Failing \>3 antidepressant drug trials, for any reason, in the current MDE. * History of non-response to electroconvulsive therapy (ECT) in the past 10 years. * Active suicidal plan/intent in the past 6 months, or more than 1 lifetime suicide attempt. * Females who are pregnant, breastfeeding, or planning to become pregnant during the first administration of study drug until 3 months after the last dose of study drug. * Meets criteria for a substance use disorder within the past 12 months, with the exception of tobacco use, and/or has a positive urine toxicology screen for drugs of abuse. * Any medical condition or personal circumstance that, in the opinion of the investigator, exposes the subject to unacceptable risk by participating in the study or prevents adherence to the protocol.