Early Outcome of a Phase I/II Clinical Trial (NCT03538899) of Gene-Corrected Autologous CD34+ Hematopoietic Cells and Low-Exposure Busulfan in Newly Diagnosed Patients with Artemis-Deficient Severe Combined Immunodeficiency (ART-SCID)

Abstract

<h3>Background</h3> ART-SCID represents ∼3% of all SCID, but occurs in 1/2000 births in Athabascan-speaking Native Americans. Artemis protein, encoded by <i>DCLRE1C</i>, is essential for repairing DNA double-stranded breaks, including those generated during V(D)J recombination essential for T & B cell development. Artemis-deficiency causes not only T-B-NK+ SCID, but also increased sensitivity to alkylating drugs and radiation. ART-SCID is the most difficult type of SCID to treat with allogeneic hematopoietic cell transplantation due to high rates of rejection and GVHD, and incomplete immune reconstitution, combined with increased toxicity following exposure to intensive conditioning regimens. Thus, we developed a self-inactivating lentiviral vector containing the human Artemis promoter and cDNA (AProArt) and are evaluating its toxicity and efficacy in a Phase I/II trial in ART-SCID patients. <h3>Methods</h3> Newly diagnosed infants with ART-SCID needed to be at least 2 m/o with acceptable organ function and no matched sibling donor. CD34+ cells were isolated from bone marrow, cultured with cytokines, transduced x2 with AProArt, and cryopreserved. Patients received 2 daily doses of busulfan (PK targeted for a cumulative exposure (cAUC) of 20mg*hr/L) followed the next day by infusion of thawed cells. <h3>Results</h3> Five newly diagnosed infants have been enrolled and treated at a median age of 2.6m (range 2.3-3.7), all diagnosed by newborn screening for SCID, with a median follow-up of 10.5m (range 0-15.6). The mean (SD) Bu cAUC was 18.8±0.9 mg*hr/L. Infants received 6.6±2.4 × 10⁶ AProArt-transduced CD34+ cells/kg with average vector copy number (VCN) in the grafts of 2.1±1.0 copies/cell and transduction efficiency 75±9%. There were no serious busulfan side effects. Four evaluable patients (≥4w post infusion) had transduced blood cells by 4w, and 3/3 evaluable patients (≥8w) developed multilineage gene marking (T, B, NK and myeloid cells) (Fig. 1). Gene-corrected CD3, CD4, CD4+45RA+CCR7+, CD8 and CD19 cells have appeared in the 3/3 evaluable patients (≥8w) (Fig. 2) with normalization of lymphocyte proliferation to mitogen in all 3 (Fig. 3). All 3 are now off isolation and managed as outpatients, although 2 developed autoimmune hemolytic anemia (AIHA) that resolved in PT001 by 18m of age and did not require therapy in PT002. Infections included rhinovirus at presentation in PT001 that resolved with T cell reconstitution. After discharge PT002 acquired and recovered from CMV and rotavirus. Analyses of insertion sites and T cell receptor diversity are pending. <h3>Conclusion</h3> Infusion of AProArt-transduced autologous CD34+ cells into ART-SCID infants pretreated with very low exposure busulfan has resulted in multilineage engraftment of transduced cells with reconstitution of T cell immunity and evidence for B cell immune development. AIHA, the only complication, has resolved upon development of T cell immunity.