First-in-Human Study of Autologous HBA2-Edited CD34+ Hematopoietic Stem and Progenitor Cells in Alpha-Thalassemia with Constant Spring Mutation

Background: Hemoglobin H Constant Spring (HbH-CS) (–/αCSα) is the most common non-deletion α-thalassemia that can lead to severe health problems. The clinical manifestations of HbH-CS patients are widely variable and a considerable proportion of patients with HbH-CS need regular or irregular transfusions to support normal growth, development and life. Currently, there is no specific therapy for transfusion dependent α-thalassemia, and allogeneic hematopoietic stem cell transplantation is the only available curative option. Because patients with HbH-CS has only a single base mutation in the Hemoglobin Subunit Alpha 2 (HBA2: c.427T>C) compared to that of asymptomatic carriers (–/αα), we speculate that repairing of the mutation by base editing may provide a potential cure for this type of disease. Aims: To repair the CS mutation of HBA2 gene and increase the expression of normal α-globin chain in erythrocytes, we used ex vivo Cytosine Base Editor (CBE)-based gene editing to modify HBA2 gene in hematopoietic stem and progenitor cells (HSPCs), producing RM-004. NCT06107400 is an investigator initiated first-in-human study of edited patient cells (RM-004) in transfusion-dependent HbH-CS. Here, we present the initial results from the first patient treated with RM-004. Methods: Patients (12-35 y of age) with HbH-CS receiving packed red blood cell (pRBC) transfusions of ≥100 mL/kg/y or ≥10 units/y in the previous 2 y were eligible. Peripheral CD34+ HSPCs were collected by apheresis after mobilization with G-CSF and plerixafor. CD34+ cells were edited with CBEs using a guide RNA specific for the CS mutation. Prior to RM-004 infusion, patient received myeloablative conditioning with busulfan form day-7 to day-3. Patient were monitored for stem cell engraftment/hematopoietic recovery, adverse events (AEs), Hb production, HbA/HbH/Hb-CS expression and packed red blood cell (pRBC) transfusion requirements. Results: The first subject was a 14-y female patient with HbH-CS, who received an annualized 29 units/y pRBC transfusions before enrollment. She received a single dose of RM-004 cells, achieved neutrophil and platelet engraftment on Day14 and Day20 post-RM-004 infusion, respectively. She received the last pRBC transfusion on Day13 after RM-004 treatment, and has been transfusion-free over 3 months. The total Hb stably over 9 g/dL since Day26 and maintained between 10~11 g/dL currently. The proportion of HbH and Hb-CS were 3.8% and 1.1% respectively before enrollment (with regular transfusions), both dropped to 0.3% at 3-month after RM-004 infusion; the proportion of HbA1 has increased to 97.2%. The safety profile was generally consistent with busulfan myeloablation and autologous hematopoietic stem cell transplantation. No serious adverse event reported. Conclusion: The first patient treated with RM-004 demonstrated successful engraftment and clinical meaningful increase in Hb level. Although the follow-up time is relatively short, available data have shown that the patient has entered a transfusion-independence state. This is the first-ever clinical report of patients with α-thalassemia treated successfully with gene editing and indicates that RM-004 is a promising approach for the treatment of HbH-CS. Data will be updated for the presentation.