This multi-center, randomized double-blind, placebo-controlled study will assess the safety and tolerability of RG1662 in individuals with Down Syndrome. Eligible subjects will be randomized in cohorts to receive either multiple oral doses of RG1662 or placebo. Anticipated time on study treatment is 38 days.

Inclusion Criteria:

• Adult individuals, 18 to 30 years of age, with diagnosis of Down Syndrome (confirmed by karyotype)
• Males and non-pregnant non-lactating females
• Parent or legal guardian/representative and caregiver willing to give written informed consent
• Subject willing and assenting or consenting to participate

Exclusion Criteria:

• Major depressive disorder not adequately controlled by a selective serotonin reuptake inhibitor or a serotonin norepinephrine reuptake inhibitor
• Subjects meeting clinical diagnostic criteria for autistic spectrum disorder, attention deficit or hyperactivity disorder unlikely to cooperate and take part successfully in the study assessments
• Subjects with other primary psychiatric diagnosis
• Subjects with evidence or meeting clinical diagnosis of dementia
• Subjects with personal history of cardiac abnormalities; patients that have previously undergone congenital heart surgery can be enrolled provided they have no residual complication requiring intervention (e.g. pace maker)
• Subjects with clinically significant obstructive pulmonary disease or asthma that is not adequately treated (oral steroids are not allowed)
• Subjects with thyroid dysfunction that is not adequately controlled and stabilized on treatment for at least 8 weeks
• Subjects with sustained elevated supine blood pressure (SBP/DBP) at rest exceeding 140/90 mmHg or notable resting tachycardia (mean HR > 60 bpm) or blood pressure below 90/40 mmHg
• Subjects who have taken any other investigational medications within 3 months
• Body mass index (BMI) > 40 kg/m2