ANAVEX2-73 Study in Pediatric Patients With Rett Syndrome

Inclusion Criteria: * Aged ≥ 5 years to 17 (inclusive). * Diagnosis of classic RTT, according to 2010 criteria, and a MECP2 mutation. * Post-regression stage, defined as ≥ 6 months since last loss of spoken language or motor (fine or gross) skills. * Clinical Global Impression - Severity (CGI-S) score of 4 or greater at Screening. * Current pharmacological treatment regimen, including supplements, has been stable for at least 4 weeks. * If on AEDs, 1-4 AEDs allowed. Treatment must be stable (drug, dose, interval of administration) for 30 days prior to enrollment. * If the subject is already receiving stable non-pharmacologic educational, behavioral, and/or dietary interventions, participation in these programs must have been continuous during the 90 days prior to the screening visit and subjects or their parent/caregiver/LAR will not electively initiate new or modify ongoing interventions for the duration of the study. * The subject's caregiver/LAR is English-speaking and has sufficient language skills to complete the caregiver assessments and has the ability to keep accurate seizure diaries. * If participant is a woman of childbearing potential (WOCBP#), a negative urine or serum pregnancy test is required to confirm she is not pregnant. * Prior to the conduct of study-specific procedures, the subject's parent/caregiver/LAR must provide written informed consent. If applicable, the research team must attempt to obtain consent from both parents. Exclusion Criteria: * Patients who have a progressive medical or neurological condition that in the opinion of the Investigator would interfere with the conduct of the study. * Current clinically significant systemic illness that is likely to result in deterioration of the patient's condition or affect the patient's safety during the study. * History or clinically evident neurologic (e.g., head trauma with loss of consciousness) or psychiatric condition that the Investigator deems may interfere with interpretability of data. * Indication of liver disease, defined by serum levels of ALT (SGPT), AST (SGOT), or alkaline phosphatase above 3x upper limit of normal (ULN) as determined during screening. * Treatment with immunosuppressive medications (e.g., systemic corticosteroids) within the last 90 days (topical and nasal corticosteroids and inhaled corticosteroids for asthma are permitted) or chemotherapeutic agents for malignancy within the last 3 years. * Other clinically significant abnormality on physical, neurological, laboratory, or electrocardiogram (ECG) examination (e.g., long QT) that could compromise the study or be detrimental to the participant. * Any known hypersensitivity to any of the excipients contained in the study drug or placebo formulation. * Other co-morbid or chronic illness beyond that known to be associated with RTT. * Subjects who plan to initiate or change pharmacologic or nonpharmacologic intervention during the course of the study. * Subjects taking another investigational drug currently or within the last 30 days. * Any other criteria (such as a clinically significant screening blood test result), which in the opinion of the Investigator could interfere with the study conduct or outcome. * Treatment with strong inhibitors or inducers of CYP3A4 or CYP2C19 is not stable (drug, dose) for 30 days prior to screening. Although these medications are not excluded, caution is advised when enrolling participants on potent CYP3A4 or CYP2C19 inducers or inhibitors (see respective section). * Patients with hepatic and renal impairment.