Lentiviral Gene Therapy with Autologous Hematopoietic Stem and Progenitor Cells (HSPCs) for the Treatment of Severe Combined Immune Deficiency Due to Adenosine Deaminase Deficiency (ADA-SCID): Results in an Expanded Cohort

Background: Severe combined immunodeficiency due to adenosine deaminase deficiency (ADA-SCID) is a rare disorder caused by ADA gene mutations, leading to lymphotoxic build-up of purine metabolites and profound immunodeficiency. Historically, enzyme replacement therapy (ERT) has been used as a bridge therapy until patients can receive an allogeneic hematopoietic stem cell transplantation (HSCT), ideally from a matched related donor (MRD) or, if none is identified, a non-matched and/or unrelated donor. We developed a self-inactivating lentiviral vector (LV), denoted EFS-ADA LV, encoding the human ADA cDNA sequence under the control of a shortened human elongation factor 1α gene promoter. A fresh or cryopreserved formulation of a drug product (OTL-101), composed of autologous hematopoietic stem and progenitor cells (HSPCs) transduced ex vivo with EFS-ADA LV, was evaluated in 2 prospective, non-randomized Phase I/II clinical trials at 2 USA centers. We report on safety and efficacy of OTL-101 in 30 ADA-SCID pediatric gene therapy (GT) subjects treated from 2013-2017 with a median follow up (FU) of 24 months (mo; range 12-26 mo), compared to a historical cohort of 26 ADA-SCID patients treated with HSCT.Methods: UCLA Fresh Study (NCT01852071): Autologous CD34+ HSPCs were isolated from bone marrow and pre-stimulated with cytokines before transduction with EFS-ADA LV to yield OTL-101, which was infused as a fresh formulation in 20 subjects (9 male, 11 female; aged 4 mo-4.3 yrs). Single dose busulfan (4 mg/kg) was administered prior to infusion of OTL-101. Subjects were followed for 24 mo.UCLA Cryo Study (NCT02999984): 10 subjects (4 male, 6 female; aged 5-15 mo) received a cryopreserved formulation of OTL-101, which allowed for an extended shelf-life and full quality control prior to infusion. Busulfan was administered in 2 doses, the first at 3 mg/kg and the second adjusted to target a total area under the curve of 4,900 µM*min (20 ng/mL*hr). At the time of analysis, all subjects reached 12 mo FU (except 1 subject who was withdrawn from the study due to lack of engraftment); 7 subjects reached 18 mo of FU.Historical Control Group: 26 patients (aged 0.2 mo-9.8 yrs) were treated with allogeneic HSCT (MRDs n=12, non-MRDs n=14) at Great Ormond Street Hospital, UK (n=16) or Duke University Children's Hospital, USA (n=10) from 2000-2016.Results: Sustained engraftment of genetically modified HSPCs was observed in 29/30 GT subjects by 6-8 mo and persisted through FU in both studies, based on vector gene marking in granulocytes and CD3+ T cell reconstitution (Figure). Subjects who engrafted maintained long-term metabolic detoxification from deoxyadenosine nucleotides after stopping ERT approximately 1 mo post-GT.At last FU (median 24 mo; range 12-24 mo) in the GT group, overall survival (OS) was 30/30 (100%) and event-free survival (survival in the absence of ERT reinstitution or rescue allogeneic HSCT; EvFS) was 29/30 (97%). OS and EvFS were higher in the GT group at last FU compared with HSCT controls (with or without an MRD) at 2 years (Table). One of 30 OTL-101 subjects (3%) did not engraft and was restarted on ERT; the subject was withdrawn from the study at 5.9 mo and subsequently received a rescue HSCT, whereas 42% of HSCT patients required rescue HSCT, PEG-ADA ERT or died.Among the 20 OTL-101 subjects in the UCLA Fresh Study who reached 2 years FU, 18 (90%) stopped immunoglobin replacement therapy (IgRT), compared to 52% of HSCT patients. Preliminary results were observed in 5/7 (71%) OTL-101 subjects in the UCLA Cryo Study with more limited (18 mo) FU.Twelve OTL-101 subjects experienced one or more serious adverse events, most frequently infections and gastrointestinal events; only 1 of which was considered treatment-related (bacteremia due to product contamination). In the GT group, there were no events of autoimmunity with ≤24 mo FU. Due to the autologous nature of OTL-101, there was no incidence of graft vs host disease (GvHD); in contrast, 8 HSCT patients experienced GvHD events (5 acute, 3 chronic events), 1 of which resulted in death.Conclusions: Based on sustained gene correction and restoration of immune function in all subjects who engrafted, treatment of ADA-SCID with OTL-101 has a favorable benefit-risk profile. Key correlates of engraftment were consistent across the expanded cohort. Importantly, higher rates of OS and EvFS compared with HSCT (with or without an MRD) were observed.

Duke Scholars

Author Rebecca Hatcher Buckley Pediatrics, Allergy and Immunology