Preliminary Results of a Phase 1/2 Clinical Study of Zinc Finger Nuclease-Mediated Editing of BCL11A in Autologous Hematopoietic Stem Cells for Transfusion-Dependent Beta Thalassemia

Methods: The Thales trial (NCT03432364) is a Phase I/II study of the safety, tolerability and efficacy of ST-400 in adult patients with TDT, defined as undergoing ≥8 annual packed red blood cell transfusion episodes for at least 2 consecutive years before enrollment. After routine leukapheresis following mobilization with G-CSF and plerixafor, autologous collections are enriched for CD34+ cells and transfected with mRNA encoding ZFNs with binding sites flanking the GATA-binding region of BCL11A ESE. ST-400 product is infused following myeloablative busulfan conditioning. The trial will enroll 6 patients who are monitored for safety and efficacy for 3 years post-infusion. Results: Three patients have completed ST-400 manufacturing, and two have been infused. Patient 1 (β0/β0 genotype) received an ST-400 dose of 6.1 x 106 cells/kg. The patient experienced a serious adverse event (SAE) of hypersensitivity during ST-400 infusion considered to be related to the product cryoprotectant, DMSO, that resolved by the end of infusion. The patient had prompt hematopoietic reconstitution (ANC recovery day +14; platelet recovery day +24), with increasing HbF fraction that contributed to stable total hemoglobin. After being free from PRBC transfusions for 6 weeks, the patient has since required intermittent PRBC transfusions. At last follow-up, on-target DNA insertions-deletions (indels) at BCL11A ESE were present in peripheral blood mononuclear cells (PBMCs), and HbF levels remain elevated at 6 months post-infusion. Patient 2 (homozygous for the severe β+ IVS-I-5 G>C mutation) received an ST-400 dose of 4.5 x 106 cells/kg. There was prompt hematopoietic reconstitution (ANC recovery day +15; platelet recovery day +29) with on-target indels detected in PBMCs at last follow-up, and rising HbF levels observed through 90 days post-infusion. Longer follow-up will be required to assess the clinical significance of these early results. Patient 3 (β0/β+ genotype including the severe IVS-II-654 C>T mutation) has completed ST-400 manufacturing. Besides the SAE reported for Patient 1, no other SAEs related to ST-400 have been reported and other AEs have been consistent with myeloablation. No clonal hematopoiesis has been observed. Conclusions: ST-400 is an ex vivo, ZFN-edited autologous HSC product for increased erythroid HbF expression in TDT. Two infused patients had rapid hematopoietic reconstitution following myeloablative conditioning, and both have elevated HbF levels following HSCGT. These data are preliminary, and additional patients and longer follow-up will be required to understand the safety and efficacy of this therapy.