A Study of RG1662 in Individuals With Down Syndrome

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