Guanfacine for Hyperactivity in Children With Down Syndrome (HYPEbeGONE_DS) (HYP01)

This is a randomized, double-blind, placebo-controlled flexibly dosed trial of guanfacine immediate release (GIR) in children with Down syndrome (DS) and symptoms of hyperactivity, inattention, and impulsivity. Participants will undergo a screening period of up to 29 days. Eligible participants meeting study criteria will be randomized 2:1 GIR or placebo. There are a total of up to 4 in person visits (screening, baseline, at Week 4, and at Week 8). Participants will receive GIR or placebo for up to 8 weeks. Weekly dose escalation will be determined via a telephone assessment at Weeks 1-3 and Weeks 4-7. Unmasking of participant and site staff will occur at the week 8, in-person visit. After unmasking, participants who were randomized to receive GIR will be given the option to 1) remain on GIR and to transition to open-label GIR per standard of care or 2) taper off of GIR. A Telephone Safety Assessment will be conducted for all participants, at 5 (+2) days after final study product administration. Blood specimens will be collected at the Week 4 and Week 8 visits for Pharmacokinetic (PK) analyses and lab assessments. Participant data will be collected via a daily study diary and study measures completed at screening/baseline, Week 4 and Week 8. Parents/Caregivers will need to complete the Study Diary during the bridge/taper period for those who are in the GIR arm.