A Phase 1 Feasibility Trial of BE-CAR33, an “Off-the-shelf” Base-Edited CAR33 T Cell Therapy for Acute Myeloid Leukemia

Chimeric antigen receptor (CAR) T cell therapy for acute myeloid leukemia (AML) is constrained by antigen heterogeneity and shared expression with healthy compartments, and there are often challenges in obtaining autologous T cells from heavily pretreated patients. To address these challenges, we developed universal donor-derived, base-edited, anti-CD33 CAR T cells (BE-CAR33) that used precise multiplexed cytidine deamination to simultaneously disrupt the TRAC, CD52, and CD7 loci to prevent graft-versus-host disease and evade immunotherapy effects. An open-label, nonrandomized, single-center phase 1 study (ISRCTN14430213) evaluated the safety, feasibility, and activity of BE-CAR33 cell therapy ahead of allogeneic stem cell transplantation (allo-SCT) for patients with AML. Eligible participants were aged less than 16 years with relapsed/refractory AML. Five patients were screened, and three were enrolled; one additional adult received BE-CAR33 through compassionate access. Participants received fludarabine, cyclophosphamide, and alemtuzumab followed by 1.2 to 1.8 × 106 BE-CAR33 cells per kilogram. Treatment-emergent adverse events included cytokine release syndrome (grade ≤2), neurotoxicity (grade 3), cytopenias (grade 4), and transient rashes. Two patients demonstrated reduced minimal residual disease and proceeded to allo-SCT. Serial flow cytometry, chimerism quantification, and vector copy number analyses tracked BE-CAR33 T cells until elimination during transplant. Differentially expressed genes included editing signatures and switched from manufacturing-related toward postexpansion effector and exhaustion profiles. Although primary end points were not met, this first-in-human study demonstrated the feasibility of an "off-the-shelf" base-edited CAR T cell approach and informs future multiantigen strategies against AML.