Update on the safety and efficacy of retroviral gene therapy for immunodeficiency due to adenosine deaminase deficiency

Abstract

Adenosine deaminase (ADA) deficiency is a rare, autosomal-recessive systemic metabolic disease characterized by severe combined immunodeficiency (SCID). The treatment of choice for ADA-deficient SCID (ADA-SCID) is hematopoietic stem cell transplant from an HLA-matched sibling donor, although <25% of patients have such a donor available. Enzyme replacement therapy (ERT) partially and temporarily relieves immunodeficiency. We investigated the medium-term outcome of gene therapy (GT) in 18 patients with ADA-SCID for whom an HLA-identical family donor was not available; most were not responding well to ERT. Patients were treated with an autologous CD34(+)-enriched cell fraction that contained CD34(+) cells transduced with a retroviral vector encoding the human ADA complementary DNA sequence (GSK2696273) as part of single-arm, open-label studies or compassionate use programs. Overall survival was 100% over 2.3 to 13.4 years (median, 6.9 years). Gene-modified cells were stably present in multiple lineages throughout follow up. GT resulted in a sustained reduction in the severe infection rate from 1.17 events per person-year to 0.17 events per person-year (n = 17, patient 1 data not available). Immune reconstitution was demonstrated by normalization of T-cell subsets (CD3(+), CD4(+), and CD8(+)), evidence of thymopoiesis, and sustained T-cell proliferative capacity. B-cell function was evidenced by immunoglobulin production, decreased intravenous immunoglobulin use, and antibody response after vaccination. All 18 patients reported infections as adverse events; infections of respiratory and gastrointestinal tracts were reported most frequently. No events indicative of leukemic transformation were reported. Trial details were registered at www.clinicaltrials.gov as #NCT00598481.

Figure 1

GT resulted in 100% survival, with few patients requiring intervention. No deaths have occurred in this patient population. Intervention-free survival was defined as survival without post-GT SCT or PEG-ADA use for a continuous period of ≥3 months. Three patients resumed PEG-ADA therapy following GT; 2 of those patients later received a matched-sibling SCT (previously unavailable before GT). Intervention-free survival represents a sensitivity analysis of the overall survival rate.

Figure 2

Transduced cells persist long-term and proliferate. All values shown are medians; black dotted lines represent minimum and maximum values. The number of patients contributing to each data point is indicated on the graph. Day 0 indicates baseline (pre-GT) values. (A) CD34⁺ cells were purified from bone marrow aspirate. (B-K) CD15⁺ cells (B), CD3⁺ cells (C-D), CD19⁺ cells (E-F), CD16⁺CD56⁺ cells (G-H), CD3⁺CD4⁺ cells (I), CD3⁺CD8⁺ cells (J), and CD4⁺CD45RA⁺ cells (K) were purified from peripheral venous blood. (A-C,E,G) Genotype was determined by qRT-PCR. (D,F,H-K) Absolute cell counts are also shown; 10 was set as the minimum value for graphs using a logarithmic scale. The shaded blue and purple regions represent median and fifth percentile values, respectively, in normal children. The top edges correspond to levels in children ages 2 to 5 years; bottom edges correspond to levels in children ages 10 to 16 years. Values for children ages 5 to 10 typically fall within the shaded areas. The green and orange shaded areas represent median and 10th percentile values, respectively, in normal children. Top edges correspond to levels in children ages 2 to 6 years; bottom edges correspond to levels in children ages 12 to 18 years. Values for children ages 6 to 12 years fall within the shaded areas.

Figure 3

GT supports ADA activity and lymphocyte function. All values shown are medians; black dotted lines represent minimum and maximum values. The number of patients contributing to each data point is indicated on the graph. (A) Lymphocyte ADA levels were measured in peripheral venous blood. ADA activity of ≥210 nmol/h per mg (blue line) represents 10% of normal and was considered as a threshold for adequate activity in subjects with ADA-SCID following treatment. (B) RBC dAXP levels were measured in peripheral venous blood. The blue line denotes dAXP levels found in successfully allogeneic transplanted patients (≤100 nmol/mL).^, (C-D) Proliferative capacity was assessed following challenge with (C) anti-CD3 antibody and (D) phytohemagglutinin (PHA). Blue lines indicate normal response in healthy children. (E) TREC abundance in whole blood was determined by qRT-PCR.

Figure 4

The rate of severe infections declined after GT. Severe infections were defined as infections leading to, or prolonging, hospitalization. The number of patients contributing to each data point and the number of severe infections reported are indicated on the graph. Severe infections in the 3 months immediately following GT were not included in the analysis. Severe infections after the reintroduction of long-term PEG-ADA treatment (patients 2, 8, and 17) were excluded from the analysis.