Rapid, Efficient and Durable Fetal Hemoglobin Production Following CS-101 Treatment in Transfusion-Dependent β-Thalassemia Participants: An Autologous, Ex Vivo Edited CD34+ Stem Cell Product Using the Innovative Transformer Base Editor (tBE)

Background: β-hemoglobinopathies, such as β-thalassemia and sickle cell disease (SCD), result from mutations in the β-globin gene (HBB). Hereditary persistence of fetal hemoglobin (HPFH) is a natural condition where high levels of fetal hemoglobin (HbF) continue into adulthood, mitigating the severity of these disorders, highlighting the therapeutic potential of increasing HbF levels as a treatment strategy. We use transformer Base Editor (tBE) to edit the transcription repressor BCL11A binding site at the promoter region of the γ-globin genes (HBG1/2) in chromosome 11, installing the HPFH single nucleotide variants (SNVs). tBE-mediated editing reactivates HbF production in patients, offering a potential curative strategy for β-hemoglobinopathies.

Methods: Participants aged 6-35 years with transfusion-dependent β-thalassemia (TDT) with a history of ≥ 8 units packed RBC transfusions in the past 12 months prior to the screening were eligible to participate in 1 of the 2 ongoing CS-101 clinical trials (NCT06024876, NCT06291961). After enrollment, participants undergo collection of autologous hematopoietic stem cells (HSCs), ex vivo base editing, busulfan conditioning to prepare the bone marrow, infusion of CS-101 product (the edited HCSs), and subsequent monitoring for safety, efficacy, and clinical outcomes. Key safety endpoints include adverse events, engraftment. Key efficacy endpoints include total hemoglobin and fetal hemoglobin levels and proportion of participants achieving transfusion independence (defined as maintaining a weighted average Hb ≥ 90 g/L without RBC transfusion) for at least 6 consecutive months.

Results: As of the data cut-off (22 Jul 2024), 6 participants (including 3 with severe genotype β⁰/β⁰, 1 with a history of splenectomy) received CS-101. The average transfusion volume within the past 12 months before screening is 1.2 u/kg. The mean follow-up duration is 4.9 months. After infusion, all participants engrafted neutrophils and platelets (median 16 and 28 days, respectively). The average time for the last RBC transfusion is day 16 post-infusion. All participants remained transfusion-free after the last RBC transfusion and the mean duration of transfusion freedom was 4.3 months. For the 3 participants followed for over 3 months, mean total Hb was 130.6 g/L at Month 3. The mean HbF level in the first three months were 23.6 g/L, 92.2 g/L and 118.0 g/L, respectively, and the mean F-cell percentage were 30.5%, 76.2% and 94.6%, respectively. The allele base editing efficiency in bone marrow and peripheral blood nucleated cells was stable over time.

All participants had ≥1 adverse event (AE) of Grade 3 or 4 severity (including laboratory-based AEs). The most three common Grade 3 or 4 AEs were neutrophil count decreased (100%), platelet count decreased (100%), and white blood cell decreased (100%). Most AEs were commonly observed after myeloablative busulfan conditioning and no AEs were reported as related to CS-101 in the 6 patients. No SAEs, discontinuations, or malignancies were reported.

As the follow-up period for these participants has not reached the time for clinical endpoint analysis, clinical endpoint data are not yet available.

Conclusion: The data demonstrate clinically significant increases in both total Hb and HbF, prompt and durable engraftment, and therapeutic benefits for all the participants received CS-101 infusion. The treatment prompted an early and substantial elevation in HbF level, pancellular distribution, and stable editing efficiency. CS-101 represents the first reported clinical base editing therapy for β-hemoglobinopathies and has the potential to become the best-in-class gene editing therapy for TDT.

Submitted on behalf of the CS-101 Investigators.