The purpose of this study is to determine whether NNZ-2566 is safe and well tolerated in the treatment of Rett syndrome in children and adolescents.

Rett syndrome is a neurodevelopmental disorder primarily affecting females. The disorder is characterized by apparent normal development in early infancy (6-18 months), followed by a period of regression with onset of systemic and neurological signs. The CNS symptoms of Rett syndrome include learning disability, autism symptomatology and epilepsy and these can be severe and highly debilitating. Affected individuals also show signs of autonomic dysfunction, reflected in cardiovascular and respiratory abnormalities. There is no currently effective treatment for Rett syndrome.

This study will investigate the safety, tolerability and blood pharmacokinetics of treatment with oral administration of NNZ-2566 at 50 mg/kg, 100 mg/kg, 200 mg/kg BID, or placebo BID, in children and adolescent females with Rett syndrome. The study also will also investigate measures of efficacy and biomarkers during treatment.

Inclusion Criteria:

• Diagnosis of classic/typical Rett syndrome with a documented mutation of the MeCP2 gene.
• Age 5 - 15 years.
• Weight at Screening and Baseline between 15.0 kg-100.0 kg (at least 15.0 kg and no greater than 100.0 kg).
• Each subject must be able to swallow the study medication provided as a liquid solution, or via gastrostomy tube.

Exclusion Criteria:

• Actively undergoing neurological regression
• Abnormal QT interval, prolongation or significant cardiovascular history.
• Current treatment with insulin.
• Anti-convulsants with liver enzyme inducing effects.
• Unstable seizure profile.
• Excluded concomitant medications.
• Current clinically significant (as determined by the investigator). cardiovascular, renal, hepatic, or respiratory disease.
• Gastrointestinal disease which may interfere with the absorption, distribution, metabolism or excretion of the study medication.
• History of, or current cerebrovascular disease or brain trauma.
• History of, or current clinically significant endocrine disorder, e.g. hypo- or hyperthyroidism, or diabetes mellitus.
• History of, or current, malignancy.
• Significant hearing and/or visual impairments that may affect ability to complete the test procedures.
• Allergy to strawberry.