Can an immune-targeting antibody reverse sudden losses in down syndrome?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Age 18 to 40 years, inclusive at time of consent * Diagnosis of Down syndrome (trisomy 21 or translocation type) * Diagnosis of possible or probable Down Syndrome Regression Disorder (DSRD) based on 2022 International Consensus Criteria * History of partial response or non-response to first-line treatment (e.g., lorazepam, SSRIs, corticosteroids, and/or IVIg). Partial response = 10-50% improvement, Non-response = \<10% improvement, Based on BFCRS or NPI-Q after 3 months of treatment. * Ability to attend all study visits with a study partner or legally authorized representative * Study partner willing to comply with all study procedures * Willingness to complete required washout periods for medications that may interfere with the study * Highly effective contraception for reproductive participants and partners of childbearing potential. * Study partner/LAR sufficiently proficient in English to complete assessments. Exclusion Criteria: * Mosaic Down syndrome * Weight \< 40 kg at screening * Pregnancy or breastfeeding * Current or past tobacco smoking * Poor venous access or inability to comply with IV procedures * Use of IVIg within 8 weeks prior to baseline * Use of immunosuppressive drugs within 12 weeks prior to baseline * Prior treatment with anti-CD20 or B-cell therapies within required washout unless B-cell recovery criteria are met * Other immunosuppressive biologics within 24 months. * Any prior use of chemotherapeutic agents (e.g., methotrexate, cyclophosphamide) * History of solid organ transplant * Recent receipt of blood or plasma products (≤30 days) * Clinically significant cardiac disease, clotting disorder, or other serious medical condition * Active or chronic infection of clinical significance (including HBV, HCV, TB, HIV per protocol criteria) * History of malignancy * Abnormal screening labs indicating unacceptable risk * Receipt of live or live-attenuated vaccines within 4 weeks prior to treatment * Untreated thyroid disease (hypo- or hyperthyroidism) * History of moyamoya syndrome or stroke * Wards of the state * Employees or students of Children's Hospital Los Angeles (CHLA) * Prior neurosurgical intervention (exceptions for longstanding shunts) * Current investigational therapy/prohibited medications * Detailed HBV, HCV, and TB criteria * Requirement for B-cell recovery \>50 cells/μL after prior anti-CD20 therapy.