A Study of RG1662 in Adults and Adolescents With Down Syndrome (CLEMATIS)

Inclusion Criteria: * Individuals aged 12-30 years of age inclusive * Clinical diagnosis of Down syndrome (trisomy 21) confirmed by chromosomal analysis (karyotyping) * Males, or non-pregnant, non-lactating females. For females of childbearing potential, strict contraceptive prevention is required. * Body-mass Index (BMI) 18-42 and 15-30 kg/m2 inclusive for adults and adolescents respectively * Ability to complete the Clinical Evaluation of Language Fundamentals (CELF)-preschool 2 word classes task * Subjects must have a parent, or other reliable caregiver who agrees to accompany the subject to all clinic visits, provide information about the subject as required by the protocol, and ensure compliance with the medication schedule * Study participants must have sufficient language, vision and hearing to participate in study evaluations, as judged clinically by investigator Exclusion Criteria: * Subjects with a current DSM 5 diagnosis of any primary psychiatric diagnosis (including ASD or MDD) * Subjects with a history of infantile spasms, of West syndrome, Lennox-Gastaut syndrome, Early Infantile Epileptic Encephalopathy or any treatment-refractory epilepsy associated with cognitive or developmental regression, of severe head trauma or CNS infections (e.g. meningitis) * Subjects with a known or suspected clinical seizure event of any type within 24 months prior to screening * Clinically relevant ECG abnormalities at screening or baseline; QTcF above 450 ms; personal or family history (first degree relatives) of congenital long QT syndrome * Inadequate renal or hepatic function