Ex Vivo Lentiviral Hematopoietic Stem Cell (HSC) Gene Therapy May Represent a Curative Therapy for the Life-Threatening Inborn Error of Immunity Severe Combined Immunodeficiency Due to Adenosine Deaminase (ADA) Deficiency (ADA-SCID)

Introduction: Ex Vivo Lentiviral Hematopoietic Stem Cell (HSC) Gene Therapy May Represent a Curative Therapy for the Life-Threatening Inborn Error of Immunity Severe Combined Immunodeficiency Due to Adenosine Deaminase (ADA) Deficiency (ADA-SCID).

Methods: We present long-term results from an integrated analysis of 62 patients with ADA-SCID treated in the U.S. and U.K. with autologous ex vivo hematopoietic stem cell gene therapy between 2012-2019, representing a cumulative 473 patient-years of follow-up. Patients received busulfan reduced-intensity conditioning followed by transplantation with autologous CD34+ hematopoietic stem cells transduced ex vivo with a lentiviral vector encoding human ADA (“UCL/A-ADA”). ADA enzyme replace therapy (ERT) was withdrawn 30 days post-transplant. Results: At a data cut-off of Dec 31, 2023, the median length of follow-up was 7.5 years (range 5-11.2). Overall survival was 100% and event-free survival (defined as survival without reinstatement of ERT or rescue allogeneic HSCT) was 95% (59/62). The most common adverse events (AEs) were consistent with the known safety profile of busulfan. Among all treated patients, no events of monoclonal expansion, leukoproliferative complications, emergence of replication-competent lentivirus, or deaths have been noted. 59 of 62 treated patients demonstrated sustained multilineage engraftment of gene-corrected cells. The median granulocyte vector copy number (VCN) has remained stable at ~0.2, indicating durable modification of ~20% of engrafted HSCs. In PBMCs, which exhibit a selective advantage for gene-corrected cells, median VCN increased from ~0.5 at month 3 to ~1.0 at month 36 and has been maintained at stable levels of VCN 0.2-2.0 through last assessment. Normalization of ADA activity levels and metabolic detoxification in red blood cells was observed by 3 months post-treatment, with results sustained through last follow-up visit. T cell counts nadired at month 3 following conditioning and withdrawal of ERT and increased starting at 6 months, with maximal counts achieved by 24 months. By 24 months, T cell counts in most patients had reached the expected normal ranges for age; T cell counts have remained stable up to and beyond 11 years post-gene therapy. Similarly, CD19+ B-cell and CD56+/CD16+ natural killer cell counts decreased to below baseline after treatment, attributable to conditioning and cessation of ERT; over time, the counts returned to approximately the baseline levels observed when previously on ERT. Restoration of B cell function was reflected by increased serum IgM and IgA levels to within normal limits over time. Among the 59 patients with successful transplants, 57 discontinued immunoglobulin-replacement therapy at a median of 12.4 months with maintenance of IgG levels in normal range after cessation. 24 of 24 (100%) of evaluable patients had protective (>0.15 IU/mL) anti-tetanus titers. Integration site analysis at latest follow-up indicated polyclonal engraftment in all treated patients, with no single clone accounting for more than 2.5% of total measured integrations. There were no vector or cell product-related adverse events.

Conclusions: Autologous gene therapy with UCL/A-ADA demonstrates safe and sustained restoration of immune function in patients with ADA-SCID.

These studies were funded by National Institute of Allergy and Infectious Diseases, National Institutes of Health award U01 AI100801 and the California Institute for Regenerative Medicine (CIRM) award CLIN2-09339. The National Gene Vector Biorepository (NGVB) (https://ngvbcc.org/) is funded with federal funds from the National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health (NIH), Department of Health and Human Services (HHS), under Contract no. 75N92019D00018.