175. A clinical trial in Japan of retroviral-mediated gene transfer to bone marrow CD34+ cells as a treatment of adenosine deaminase (ADA)-deficiency

For ADA-deficiency, a form of severe combined immunodeficiency, retroviral-mediated gene correction of peripheral T cells or hematopoietic stem/progenitor cells (HSCs) is a treatment of choice alternative to bone marrow (BM) transplantation. Recently, two new such clinical trials aiming for the correction of patients' HSCs were conducted in US and Italy, resulting in remarkably different outcomes: Italian patients showed immune reconstitution whereas the US trial led to no clinical improvement. There are two major differences between their protocols, either or both of which may explain these consequences. 1) In Italy, patients were mildly myeloablated, while not in the US trial. 2) US patients were on the concomitant enzyme replacement therapy (PEG-ADA) that may lessen in vivo selective advantages for gene-corrected cells and thus the efficacy of gene therapy. Italian group has not given the enzyme throughout the studies. We thus have proposed a trial that differs from these two to see If only the absence of PEG-ADA is enough to benefit patients, or if any kind of myeloablation is absolutely necessary. Two ADA-deficiency patients (#1, a 4 yr-old girl; #2, a 13 yr-old boy) were treated under the approval by the Japanese authorities. First, PEG-ADA was discontinued prior to the treatment. Rapid decrease in peripheral counts of lymphocytes and of neutrophils, liver enzyme abnormalities, and mild anorexia became evident, suggesting that the generalized toxicity due to ADA-deficiency had deteriorated. Nevertheless, both patients well tolerated the first several weeks with no serious problems. Good purity CD34+ BM cells (>80%) were transduced with the retroviral vector GCsapM-ADA/PG13. The use of serum-free medium, a cocktail of cytokines (SCF, TPO, flt3-L, IL-6, and soluble IL-6 receptor) and the fibronectin fragments resulted in ~50–70% transduction efficiencies and normal levels of ADA activities in the infused cell populations. No immediate adverse reactions were noted upon cell infusion (1.3 and 0.9 × 106 CD34+ cells/kg for #1 and #2, respectively). Patients did not receive any myeloablative conditionings. Although still preliminary in the limited time frames after treatment (6 weeks for #1 and several days for #2), we have observed normalization of liver enzyme abnormalities in #1 with gradual recovering from anorexia. Moreover, her neutrophil counts, not lymphocytes' though, have also returned to normal by week 6, suggesting that gene-corrected HSCs have engrafted and started expanding at least towards the myeloid lineage, while providing the patient with significant amount of ADA enough to detoxify the affected organs. In light of the serious adverse events in the French gene therapy trial for XSCID, careful follow-up of the patients is mandatory and thus underway, including monitoring a clonal expansion of any type of hematopoietic cells. More detailed outcomes will be presented.