[211At]Astatine-Based Conditioning with a Humanized CD45 Antibody for Autologous Hematopoietic Stem Cell Gene Therapy

Abstract

Successful transplantation of autologous gene-modified hematopoietic stem/progenitor cells (HSPCs) requires efficient ablation of resident hematopoietic stem cells. Since conventional myeloablative conditioning regimens are associated with non-hematologic toxicities, we evaluated CD45-directed radioimmunotherapy (RIT) using the a-emitter astatine-211 (211At) before transplantation of ex vivo gene-edited autologous HSPCs as an alternative in nonhuman primates. We humanized the CD45 antibody, BC8 (HuBC8), and labeled it with 211At. As a model, mobilized CD34+ HSPCs were multiplex gene-edited using an adenine base editor, modifying the HBG promoter to reactivate fetal hemoglobin (HbF) and deleting CD33. Two animals each received 300 or 400 µCi/kg of 211At. In contrast to historic controls conditioned with total body irradiation, CD45-RIT animals did not show any noticeable non-hematopoietic toxicities and were almost entirely transfusion independent with rapid recovery of neutrophils and platelets. 211At enabled dose-dependent engraftment of gene-edited cells. A new single cell sequencing assay revealed up to 70% combined mono- and bi-allelic gene-editing efficiency in the blood, consistent with complete replacement of the bone marrow stem cell compartment. Assessment by bulk analysis underestimated the frequency of gene-edited cells, highlighting the importance of a single cell readout. Single cell sequencing further confirmed stable and unbiased contribution of multiplex-edited HSPCs to all mature lineages in the blood, providing high-resolution data assuring successful replacement upon autologous HSPC gene therapy. Levels of edited cells remained stable for the entire follow-up of >18 months. Together, these studies identify 211At-CD45 RIT as a targeted alternative for myeloablative conditioning for autologous gene therapy.