Hematopoietic Recovery in Fanconi Anemia Patients Utilizing a Novel Anti-CD117 Antibody-Containing, Irradiation/Busulfan-Free Haploidentical Hematopoietic Stem Cell Transplant Regimen

Fanconi Anemia (FA) is caused by inherited mutations in various FANC genes, leading to impairment in DNA repair that compromises genomic integrity of all tissues. Nearly all FA patients develop bone marrow failure (BMF) in early childhood, necessitating allogeneic hematopoietic stem cell transplantation (allo-HSCT). While allo-HSCT often alleviates severe cytopenias , current protocols utilize genotoxic chemotherapy and/or irradiation conditioning, which has short and long-term toxicities that are exacerbated in these fragile patients. We have launched a novel Phase 1b/2a clinical trial (NCT#04784052) using an anti-CD117 antibody briquilimab in place of conventional busulfan or total body irradiation (TBI) conditioning which is combined with transplantation of TCRαβ+ T-cell/CD19+ B-cell haploidentical grafts (Fig 1). We investigated the safety and efficacy of this treatment through clinical assessment and detailed analysis of BM recovery at various timepoints post HSCT. All patients promptly engrafted (n=8, neutrophils by day 9-14, platelets by day 11-15) with 99-100% donor CD34+ chimerism at last follow-up (Fig 1). Minimal toxicities were observed, with no clinically significant acute GvHD , thereby meeting the primary outcome and safety/tolerability goals of the study. Detailed assessments of recovering hematopoiesis revealed no difference in colony-forming capacity of BM hematopoietic stem and progenitor cells (HSPCs) by 24 weeks post HSCT compared to age-matched controls (n=6; P=0.71) with development of resistance to mitomycin C (MMC) DNA-damage (Fig 2). Single-cell RNA-seq analysis of BM HSPCs indicated an increasing G1/G0 cell-cycle ratio from 4 weeks, peaking at 8 weeks, and subsequently declining to levels comparable to healthy controls from 12 to 52 weeks post-HSCT. Gene Set Enrichment Analysis at 4-8 weeks post-HSCT revealed upregulation of multiple pathways, including ECM-receptor interaction and cAMP signaling, with a positive trend in cell-cycle and JAK-STAT signaling compared to healthy controls. Hematopoiesis-related cytokines, including EPO, KITLG, FLT3LG, ANGPT1, and CSF2, which were abnormal at screening, normalized in BM plasma comparable to healthy samples. Our results highlight the successful development of a busulfan and TBI-free protocol for FA patients using anti-CD117 mAb-containing conditioning and haploidentical grafts. Moreover, our findings reveal the dynamics of hematopoietic recovery and ongoing changes in signaling pathways during hematopoietic reconstitution. While additional patient treatment and analyses with longer follow-up are ongoing, these results underscore the exciting potential of reduced-intensity conditioning for FA patients and describe potential pathways that support proper hematopoiesis and long-term survival of donor HSPCs in an FA environment.