An Open-Label Phase II Feasibility Study for the Use of Ublituximab in Adults With Down Syndrome Regression Disorder

The goal of this clinical trial is to evaluate whether the monoclonal antibody ublituximab can treat Down Syndrome Regression Disorder (DSRD) by assessing its safety, tolerability, and preliminary efficacy in affected individuals. This study is conducted in adults aged 18-40 years with Down syndrome who have DSRD and have had an inadequate or partial response to first-line therapies. The main questions it aims to answer are: * Does ublituximab demonstrate acceptable safety and tolerability, as measured by treatment-emergent adverse events from baseline through Week 24? * Does ublituximab lead to improvement in cognitive, neuropsychiatric, motor, and functional outcomes from baseline to Weeks 12 and 24? This is a single-arm study, so there is no comparison group. Participants will: * Receive two intravenous infusions of ublituximab (150 mg on Day 1 and 450 mg on Day 15) * Attend study visits at Screening, Baseline (Day 1) , Week 2 (Day 15), Week 3, Week 6, Week 12, and Week 24 * Undergo clinical assessments, including neurological and physical exams and safety monitoring labs throughout the study * Complete cognitive, behavioral, and functional evaluations at Baseline, Week 12, and Week 24 * Provide blood samples for safety monitoring and research biomarker analyses