Nonmyeloablative conditioning combined with the anti-CD117 antibody briquilimab in older adults with high-risk AML and MDS

ABSTRACT: Briquilimab is a monoclonal antibody inhibiting stem cell factor (SCF) binding to CD117 (c-Kit). Based on preclinical data demonstrating the antibody clears hematopoietic stem and progenitor cells (HSPC) and myeloid malignant cells, we conducted a phase 1 trial examining briquilimab plus nonmyeloablative fludarabine (Flu) and total body irradiation (TBI) as conditioning for older adults with acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) undergoing matched donor allogeneic hematopoietic cell transplantation (HCT). Briquilimab was infused 10 to 14 days before transplant day (TD) 0; Flu 30 mg/m2 and TBI 2 to 3 Gy were administered on TD -4 to -2 and TD0, respectively. Graft-versus-host disease prophylaxis consisted of tacrolimus, sirolimus, and mycophenolate mofetil. Thirty-two patients enrolled (n = 13 AML in complete remission [CR]; n = 3 AML in relapse; n = 16 MDS). Median age was 70 years, and most had detectable measurable residual disease (MRD) at screening. There were no briquilimab infusion reactions, dose-limiting toxicities, or primary graft failure events; briquilimab clearance was predictable across patients. Among the AML in CR cohort, 1-year event-free survival (EFS) was 69.2%; 1-year overall survival (OS) was 75%. Among the MDS cohort, 1-year EFS was 53.8%; 1-year OS was 76.4%. One of 3 patients with AML in relapse experienced a transient response. Marrow samples obtained before and after briquilimab and before Flu/TBI demonstrated AML/MDS HSPC depletion (mean, 62.4% ± 22.7%) with resultant threefold increase in serum SCF. In summary, we demonstrate the feasibility, safety, and proof of concept of CD117 targeting with briquilimab as HCT conditioning for AML/MDS. The trial was registered at www.clinicaltrials.gov as NCT04429191.