This multi-center, randomized, double-blind, 3-arm, parallel-group, placebo-controlled study will evaluate the efficacy and safety of RG1662 in adults and adolescents with Down syndrome. Subjects will be randomized to receive RG1662 either at low or high dose or placebo orally twice daily for 26 weeks.

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