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.
You may not be if
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)
