35. Generalized Detoxification Associated with Engraftment of Gene-Corrected Repopulating Cells Achieved in ADA-SCID Patients by Stem Cell Gene Therapy without Myelopreparative Pre-Conditioning

Severe combined immunodeficiency due to genetic defects of adenosine deaminase (ADA-SCID) has been a target disease of retroviral-mediated gene therapy. Recently, great success was reported in one of such clinical trials conducted in Italy aiming for the correction of patients' hematopoietic stem cells (HSC). In contrast, the US trial which is similar to, but differs significantly from that of Italy has not lead to visible clinical improvement. Considering the differences in protocol between two, absence of concomitant enzyme replacement therapy (PEG-ADA) and/or mild pre-conditioning seem to be critical for the efficacy of stem cell gene therapy (SCGT) for ADA-SCID. However, since immune reconstitution has been achievable by SCGT for untreated XSCID patients, it is still unknown whether pre-conditioning is absolutely necessary for ADA-SCID.We have treated two ADA-SCID patients (pt 1, a 4 year-old girl; pt 2, a 13 year-old boy) by SCGT in a way that differs from the above two trials; PEG-ADA had been discontinued before treatment and patients were not pre-conditioned in any way. Following the cessation of PEG-ADA, decrease of peripheral leukocytes, liver enzyme abnormalities, and mild anorexia became evident, reflecting generalized accumulation of toxic metabolites due to ADA-deficiency. BM CD34 + cells successfully transduced with the retroviral vector GCsapM-ADA were infused back to patients without serious problems. Starting from as early as one week after cell infusion, liver enzyme abnormalities and anorexia had started improving and eventually been resolved. Consistent with these, lowered levels of toxic metabolites has been reached and maintained, indicating continuous production of ADA in patients. T cell counts had started increasing from ∼6 mo post treatment, reaching the values higher than pre-SCGT levels. Vector sequences were detected in both peripheral lymphocytes and bone marrow progenitors. ADA enzyme activities in lymphocytes have clearly improved in both patients. At present (13.5 and 12 mo post treatment for pts 1 and 2, respectively), no serious adverse event has been noted. Both patients are doing well with pt 1 at home and pt 2 enjoying back in his school life. Most importantly, although some prophylactic measures are still necessary including immunoglobulin replacement, both patients have been off PEG-ADA for longer than 1 year with no life-threatening infections. These results suggest that SCGT for ADA-SCID, even without pre-conditio