Autologous Ex Vivo Lentiviral Gene Therapy for Adenosine Deaminase Deficiency

Abstract

Background: Severe combined immunodeficiency due to adenosine deaminase (ADA) deficiency (ADA-SCID) is a rare and life-threatening primary immunodeficiency.

Methods: We treated 50 patients with ADA-SCID (30 in the United States and 20 in the United Kingdom) with an investigational gene therapy composed of autologous CD34+ hematopoietic stem and progenitor cells (HSPCs) transduced ex vivo with a self-inactivating lentiviral vector encoding human ADA. Data from the two U.S. studies (in which fresh and cryopreserved formulations were used) at 24 months of follow-up were analyzed alongside data from the U.K. study (in which a fresh formulation was used) at 36 months of follow-up.

Results: Overall survival was 100% in all studies up to 24 and 36 months. Event-free survival (in the absence of reinitiation of enzyme-replacement therapy or rescue allogeneic hematopoietic stem-cell transplantation) was 97% (U.S. studies) and 100% (U.K. study) at 12 months; 97% and 95%, respectively, at 24 months; and 95% (U.K. study) at 36 months. Engraftment of genetically modified HSPCs persisted in 29 of 30 patients in the U.S. studies and in 19 of 20 patients in the U.K. study. Patients had sustained metabolic detoxification and normalization of ADA activity levels. Immune reconstitution was robust, with 90% of the patients in the U.S. studies and 100% of those in the U.K. study discontinuing immunoglobulin-replacement therapy by 24 months and 36 months, respectively. No evidence of monoclonal expansion, leukoproliferative complications, or emergence of replication-competent lentivirus was noted, and no events of autoimmunity or graft-versus-host disease occurred. Most adverse events were of low grade.

Conclusions: Treatment of ADA-SCID with ex vivo lentiviral HSPC gene therapy resulted in high overall and event-free survival with sustained ADA expression, metabolic correction, and functional immune reconstitution. (Funded by the National Institutes of Health and others; ClinicalTrials.gov numbers, NCT01852071, NCT02999984, and NCT01380990.).

Figure 1.. Kaplan–Meier Curves for Event-free Survival.

Event-free survival over 24 months (U.S. studies) and 36 months (U.K. study) is shown. Events were defined as death, rescue allogeneic hematopoietic stem-cell transplantation, or reinitiation of pegylated adenosine deaminase enzyme-replacement therapy. At 12 months, event-free survival was 97% among the patients in the U.S. studies and 100% among the patients in the U.K. study; it remained at 97% among the patients in the U.S. studies at 24 months and was 95% among the patients in the U.K. study at 24 and 36 months. Circles indicate censored data.

Figure 2.. Median Vector Gene Marking in Granulocytes and PBMCs.

Vector copy number was determined by quantitative polymerase chain reaction (real-time or digital) with DNA extracted from fractionated peripheral blood with the use of primers and a probe specific for the human adenosine deaminase complementary DNA spanning exons 6 and 7. Panel A shows the median vector copy number in a granulocyte fraction enriched through immunomagnetic depletion with antibodies to CD3, CD19, and CD56. Panel B shows the median vector copy number in peripheral blood mononuclear cells (PBMCs). In both panels, I bars indicate interquartile ranges. Values of 0 are plotted as 0.001.

Figure 3.. Median dAXP and dATP Levels in Red Cells.

Levels of deoxyadenosine metabolites measured in red cells by high-pressure liquid chromatography are shown. Panel A shows median total deoxyadenosine nucleotide (dAXP) (including deoxyadenosine monophosphate, deoxyadenosine diphosphate, and deoxyadenosine triphosphate [dATP]) levels, and Panel B shows median dATP purine metabolite levels. In both panels, I bars indicate interquartile ranges. The dashed line indicates the threshold below which patients are considered to have adequate detoxification.

Figure 4 (facing page).. Median Absolute Lymphocyte Counts as Determined by Flow Cytometry.

Median absolute lymphocyte counts over 24 or 36 months are shown. In all panels, I bars indicate interquartile ranges. NK denotes natural killer.