Safe mobilization of CD34+ cells in adults with β-thalassemia and validation of effective globin gene transfer for clinical investigation

Safe mobilization of CD34+ cells in adults with β-thalassemia and validation of effective globin gene transfer for clinical investigation

We conducted a pilot trial to investigate the safety and effectiveness of mobilizing CD34+ hematopoietic progenitor cells (HPCs) in adults with β-thalassemia major. We further assessed whether thalassemia patient CD34+ HPCs could be transduced with a globin lentiviral vector under clinical conditions at levels sufficient for therapeutic implementation. All patients tolerated granulocyte colony-stimulating factor well with minimal side effects. All cell collections exceeded 8 × 106 CD34+ cells/kg. Using clinical grade TNS9.3.55 vector, we demonstrated globin gene transfer averaging 0.53 in 3 validation runs performed under current good manufacturing practice conditions. Normalized to vector copy, the vector-encoded β-chain was expressed at a level approximating normal hemizygous protein output. Importantly, stable vector copy number (0.2-0.6) and undiminished vector expression were obtained in NSG mice 6 months posttransplant. Thus, we validated a safe and effective procedure for β-globin gene transfer in thalassemia patient CD34+ HPCs, which we will implement in the first US trial in patients with severe inherited globin disorders. This trial is registered at www.clinicaltrials.gov as #NCT01639690.

The β-thalassemias are hereditary anemias caused by the deficient production of the β-chain of hemoglobin.1 The standard of care for patients with β-thalassemia major consists in lifelong transfusion therapy combined with pharmacologic iron chelation. The only curative treatment is allogeneic bone marrow transplantation from a matched, related donor. Most patients, however, lack such matched donor. The goal of therapeutic globin gene transfer is to stably insert a functional globin gene into the patient’s own hematopoietic progenitor cells (HPCs) to achieve transfusion independence. We previously demonstrated successful globin gene therapy in murine thalassemia models, using a lentiviral vector that includes the human β-globin promoter and arrayed regulatory elements uniquely combined to achieve high level and erythroid-specific globin expression. The vector termed TNS9 increased hemoglobin levels by an average 4 to 6 g/dL per vector copy.8-10 Several groups have confirmed and extended these results in models of thalassemia and sickle cell disease, using variant vectors encoding β-, γ-, or mutated β-globin genes. For the past decade, the inability to transduce patient CD34+ HPCs at potentially therapeutic levels under clinically relevant conditions has precluded effective implementation of this therapy.

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