Initial Results from the Northstar Study (HGB-204): A Phase 1/2 Study of Gene Therapy for β-Thalassemia Major Via Transplantation of Autologous Hematopoietic Stem Cells Transduced Ex Vivo with a Lentiviral β^Α-T87Q -Globin Vector (LentiGlobin BB305 Drug Product)

Background: Hematopoietic stem cell (HSC) gene therapy has the potential to induce globin production and mitigate the need for blood transfusions in β-thalassemia major. Promising early results for 2 subjects with β⁰/β^E -thalassemia major in the ongoing HGB-205 study suggested that transplantation with autologous CD34⁺ cells transduced with a replication-defective, self-inactivating LentiGlobin BB305 lentiviral vector containing an engineered β-globin gene (β^A-T87Q) can be safe and yield robust production of β^A-T87Qglobin resulting in rapid transfusion independence. The Northstar study (HGB-204), which uses the same lentivirus vector and analogous study design as study HGB-205, is multi-center and multi-national, and centralizes drug product manufacturing. Herein, we provide the initial data on subjects enrolled and treated in this study.

Subjects and Methods: Transfusion-dependent subjects with β-thalassemia major undergo HSC collection via mobilized peripheral blood apheresis and CD34⁺ cells are selected. Estimation of the mean ex-vivo vector copy number (VCN) is obtained by quantitative PCR performed on pooled colony-forming progenitors. Subjects undergo myeloablation with intravenous busulfan, followed by infusion of transduced CD34⁺ cells. Subjects are monitored for hematologic engraftment, β^A-T87Q -globin expression (by high performance liquid chromatography) and transfusion requirements. Integration site analysis (ISA, by linear amplification-mediated PCR and high-throughput sequencing on nucleated cells) and replication-competent lentivirus (RCL) assays are performed for safety monitoring.

Results: As of 31 July 2014, 3 subjects have undergone HSC collection and ex-vivo LentiGlobin BB305 gene transfer. One subject (Subject 1102) has undergone myeloablation and drug product infusion. Outcomes data are shown in Table 1. The initial safety profile is consistent with myeloablation, without serious adverse events or gene therapy-related adverse events. This subject has increasing production of β^A-T87Q-globin: the proportion of β^A-T87Qglobin was 1.5%, 10.9% and 19.5% of total Hb at 1, 2 and 3 months post-infusion, respectively. This subject received pRBCs on Day +14 following drug product infusion and required no further transfusions until a single unit of pRBC was transfused on Day +96 for a Hb of 8.6 g/dL and fatigue. Two additional subjects have undergone drug product manufacture and are awaiting transplantation. Safety data related to ISA and RCL assays are pending.

Conclusion: The first subject treated on the Northstar study has safely undergone drug product infusion with autologous HSCs transduced with LentiGlobin BB305 lentiviral vector and is producing steadily increasing amounts of β^A-T87Q-globin. Additional follow-up of this subject plus data on additional subjects who undergo drug product infusion will be presented at the meeting. Ex-vivo gene transfer of β^A-T87Q-globin to autologous HSCs is a promising approach for the treatment of patients with β-thalassemia major.