C-8. Immunological and Metabolic Correction After Lentiviral Vector Gene Therapy for ADA Deficiency

Background: Adenosine deaminase deficiency leads to severe combined immunodeficiency. Autologous haematopoietic stem cell gene therapy may offer a curative therapy. We developed a self inactivating lentiviral vector in which the human ADA gene is driven by an internal EFS promoter. Parallel trials using this vector were conducted in UCL, London and UCLA, Los AngelesObjective: A Phase I/II trial to assess the safety and efficacy of EFS-ADA lentiviral vector mediated gene modification of autologous CD34+ cells from ADA-deficient individuals.Methods: 20 patients (12 male; 8 female) aged between 0.4-6.5yrs were treated. All had been on enzyme replacement prior to treatment. CD34+ cells were collected either by BM harvest or peripheral blood stem cell mobilisation. Busulfan i.v. at a single dose of 4-5mg/kg was given as conditioning. CD34+ cells were stimulated with cytokines for 24hrs before transduction with vector for a further 18hrs. The dose of CD34+ cells returned ranged from 3-17 × 10e6/kg with a vector copy number (VCN) in the transduced population of between 0.25-6.3 vector copies per cell.Results: The procedure was well tolerated by all patients with no adverse events related to Busulfan conditioning or the vector. The follow up in all patients ranges from 1-35 months. All 20 patients remain off PEG ADA enzyme replacement therapy. There is evidence of immunological and metabolic recovery in all 16 patients treated for longer than 6 months. All patients have shown a rise in total T cell counts and at last follow-up the following levels were seen; total T cells, range 220-3,370; CD4+ cells 190-1509. Mitogen responses to PHA have increased in all patients and there is naive T cell recovery with an increase in CD45RA+CD4+CD27+ subpopulations and the number of TRECs. Immunoglobulin replacement therapy has been stopped in 4 patients so far and the one patient who has completed vaccinations shows normal vaccine specific responses. Gene marking is detectable in the periphery with highest marking in T cells but significant marking in B cells, NK cells and myeloid cells. Intracellular red blood cell ADA activity was negligible prior to gene therapy but at latest follow up is detectable at near or above normal levels in all patients. Integration site analysis shows occasional expansions but no persistence of expanded clones. There is no detection of expansion of clones with genes previously associated with insertional mutagenesis. All 20 patients are clinically well and the earliest treated patients are free of social restriction.Summary: Lentiviral vector mediated gene therapy for ADA deficiency is well tolerated and allows effective recovery of immunological and metabolic parameters with no evidence for mutagenesis thus far. These trials of 20 patients in two centres demonstrates the high efficacy and safety of this treatment strategy.