Outcomes of Gene Therapy for Severe Sickle Disease and Beta-Thalassemia Major Via Transplantation of Autologous Hematopoietic Stem Cells Transduced Ex Vivo with a Lentiviral Beta AT87Q-Globin Vector

Background: In patients with hemoglobinopathies, hematopoietic stem cell (HSC) gene therapy has the potential to induce production of functional β-globin in the red blood cell lineage with the aim of reducing or eliminating the symptoms of disease. Previous results from 1 subject with severe sickle cell disease (SCD; 6 months follow-up) and 2 subjects with β⁰/β^E-thalassemia major (up to 15 months follow-up) treated in clinical study HGB-205 suggested that transplantation with autologous CD34⁺ cells transduced with the LentiGlobin BB305 lentiviral vector containing an engineered β^A-T87Q-globin gene (LentiGlobin BB305 Drug Product) resulted in near-normal levels of total hemoglobin (Hb) and rapid clinical improvement. Here we provide data on a new subject enrolled and additional follow-up data on the 3 subjects previously presented in Study HGB-205.

Subjects and Methods: Subjects with severe SCD underwent HSC collection via bone marrow harvest, while subjects with β-thalassemia major underwent HSC collection via peripheral blood apheresis following mobilization. CD34⁺ cells were selected and transduced with LentiGlobin BB305 lentiviral vector to produce the drug product. Subjects underwent myeloablation with intravenous busulfan, followed by infusion of drug product. Subjects were monitored for hematological engraftment, vector copy number, β^A-T87Q-globin expression, adverse events and transfusion requirements. Integration site analysis (ISA) and replication-competent lentivirus (RCL) assays were performed. Prophylactic RBC transfusions were continued in subjects with SCD who were on chronic transfusion pre-transplant to maintain HbS <30%, followed by gradual taper over time.

Results: As of 31 July 2015, 1 subject with severe SCD (Subject 1204, β^S/β^S with multiple vaso-occlusive crises, silent infarct, acute chest syndrome, and on prophylactic transfusions) and 3 subjects withβ-thalassemia major (Subjects 1201, 1202 and 1203) have been infused with the LentiGlobin BB305 Drug Product. The outcome of these subjects to date is shown in Table 1. No subject has experienced a drug product-related adverse event, and ISA analyses demonstrate highly polyclonal reconstitution without clonal dominance. The subject with severe SCD is producing approximately 51.5% of anti-sickling hemoglobin (48% HbA^T87Q, 1.8% HbF, 1.7% HbA₂) at 9 months post-infusion. This subject has not had a post-infusion hospitalization for a SCD-related event despite stopping chronic transfusions at Day +88. Both subjects with β⁰/β^E-thalassemia major have remained transfusion-free for at least 15 months post-infusion, with a consistent expression of β^A-T87Q-globin; the subject with β⁰/β⁰-thalassemia major has only had 1 month follow-up post-drug product infusion to date.

Conclusion: The subject with severe SCD is producing approximately 51.5% anti-sickling globins with HbS of 48.5% and remains free of SCD-related events despite stopping chronic transfusion therapy. Two subjects with β⁰/β^E-thalassemia major remain transfusion-free for at least 15 months post infusion of LentiGlobin BB305 Drug Product. Gene therapy using autologous HSC transduced with LentiGlobin BB305 lentiviral vector is a promising approach for the treatment of patients with hemoglobinopathies.