Intrabone hematopoietic stem cell gene therapy for adult and pediatric patients affected by transfusion-dependent β-thalassemia

beta-thalassemia is caused by beta-globin gene mutations resulting in reduced (beta(+)) or absent (beta(0)) hemoglobin production. Patient life expectancy has recently increased, but the need for chronic transfusions in transfusion-dependent thalassemia (TDT) and iron chelation impairs quality of life(1). Allogeneic hematopoietic stem cell (HSC) transplantation represents the curative treatment, with thalassemia-free survival exceeding 80%. However, it is available to a minority of patients and is associated with morbidity, rejection and graft-versus-host disease(2). Gene therapy with autologous HSCs modified to express beta-globin represents a potential therapeutic option. We treated three adults and six children with beta(0) or severe beta(+) mutations in a phase 1/2 trial (NCT02453477) with an intrabone administration of HSCs transduced with the lentiviral vector GLOBE. Rapid hematopoietic recovery with polyclonal multilineage engraftment of vector-marked cells was achieved, with a median of 37.5% (range 12.6-76.4%) in hematopoietic progenitors and a vector copy number per cell (VCN) of 0.58 (range 0.10-1.97) in erythroid precursors at 1 year, in absence of clonal dominance. Transfusion requirement was reduced in the adults. Three out of four evaluable pediatric participants discontinued transfusions after gene therapy and were transfusion independent at the last followup. Younger age and persistence of higher VCN in the repopulating hematopoietic cells are associated with better outcome.