68: Gene Therapy for Adenosine Deaminase (ADA)-Deficient Severe Combined Immune Deficiency (SCID): Comparative Results with or without PEG-ADA Withdrawal and Myelosuppressive Chemotherapy

In 2001–2002, we treated 4 ADA-SCID patients in a gene therapy (GTx) clinical trial evaluating the efficacy of 2 different retroviral vectors while continuing enzyme replacement with pegylated bovine ADA (PEG-ADA). No cytoreductive conditioning was used. All patients have been monitored for 6 years. No treatment-related SAE occurred. A mild transient elevation in absolute lymphocyte count (ALC) was seen in 2 patients early post-treatment, however, no durable immunologic changes were observed. Low levels (0.1–0.7%) of vector-marked PBMC persist for at least five years in 2 patients treated at the age of 4–5 years. All patients remain on PEG-ADA, prophylactic antibiotics and IVIg. In 2004, we revised the protocol to facilitate engraftment and selective advantage of gene-corrected cells by withdrawing PEG-ADA and giving busulfan (75 mg/m2) before GTx. In November 2005, a 1st patient was treated who developed unexpected prolonged bone marrow (BM) aplasia. Cytogenetics revealed trisomy 8 mosaicism, present on a BM specimen obtained pre-GTx (Blood 2007; 109:503). Our 2nd patient enrolled in January 2007. He received 5 × 106 CD34+ cells/kg that showed 40–200U of ADA activity after transduction (normal range 58–128). Over 6 months, this patient showed a slow increase in ALC (up to 750/mcL) and lymphocyte function. PBMC ADA activity has been up to 50U. The dAXP level has decreased to below 10%, levels typically seen after allogeneic BM transplant. A 3rd patient was treated in May 2007 and received 2 × 106 CD34+ cells/kg with ADA activity of 7–87U. Over three months, lymphocyte function has increased over pretreatment level. PBMC ADA activity has been up to 24U and the dAXP levels have decreased to less than 10%. Both patients have been off PEG-ADA since 2 weeks pre-GTx and had mild post-GTx courses, with no documented infection, and no need for blood products. AST and ALT rose to 200 U/L, and then resolved, an effect attributed to PEG-ADA withdrawal or busulfan administration. Without myeloid growth factors, times to neutrophil count above 500/mcL were 36 and 45 days. These data are consistent with the positive results of GTx for ADA-SCID obtained in Milan and London and directly show that PEG-ADA withdrawal and reduced conditioning improve the outcome of GTx for ADA-SCID. Longer follow-up should allow us to study engraftment of cells containing vector-specific sequences and to conclude if either vector contributes more to recovery of lymphoid immunity.