Study was terminated due to data from another study showing no efficacy. There were no safety issues of concern.

This study will evaluate the safety, tolerability, efficacy, and pharmacokinetic and pharmacodynamic activity of 3 different dosages of RO5186582 compared with placebo. A total of approximately 46 participants will be enrolled, in order to have at least 32 evaluable, and will be randomly assigned to 1 of 4 treatments in a 1:1:1:1 ratio, with 9 children per treatment arm. The target ratio between 6-8 years and 9-11 years age groups is approximately 1:1 in each treatment arm, with a minimum of 3 children per age group in each treatment arm.

Inclusion Criteria:

• Diagnosis of Down syndrome, except for mosaic Down syndrome
• Available parent or caregiver to attend clinic visits and provide information about the participant's behavior and symptoms

Exclusion Criteria:

• Any primary psychiatric comorbid disorder
• History of infantile spasms, West syndrome, Lennox-Gastaut syndrome, early infantile epileptic encephalopathy, treatment-refractory epilepsy with cognitive/developmental regression, severe head trauma, or central nervous system (CNS) infection
• Seizure event of any type within 12 months prior to Screening or relevant changes in anti-epileptic drugs 6 weeks prior to enrollment
• Significant sleep disruption
• Significant gastrointestinal, renal, hepatic, endocrine, or cardiovascular disease
• New-onset or ongoing hematologic/oncologic disorder
• Severe lactose intolerance
• Participation in another clinical study within 1 month or 6 half-lives prior to first dose, or any extent of participation in Study BP29589 (NCT02451657)