Efficacy and safety of a new cladribine-based conditioning regimen for allogeneic hematopoietic stem cell transplantation in children with relapsed or refractory acute myeloid leukemia.

Objectives: Cladribine, a synthetic analog of deoxyadenosine, exhibits potent activity against hematological malignancies. While cladribine-containing regimens combined with allogeneic hematopoietic stem cell transplantation (allo-HSCT) have been proposed as a potential strategy to improve outcomes in relapsed or refractory (R/R) acute myeloid leukemia (AML), additional clinical evidence is needed, particularly in pediatric populations. This single-center retrospective study aimed to describe the efficacy and safety of a cladribine-based conditioning regimen for allo-HSCT in children with R/R AML. Materials and methods: Clinical data of 16 children with R/R AML who underwent allo-HSCT following a cladribine-based conditioning regimen at our hospital from October 2020 to June 2024 were analyzed retrospectively. Key outcomes included hematopoietic reconstruction, regimen-related toxicity (RRT), cumulative incidence of graft-versus-host disease (GVHD), infection profiles, overall survival (OS), disease-free survival (DFS), relapse rate, and non-relapse mortality (NRM). Flow-cytometry minimal residual disease (MRD) results before HSCT were collected and analyzed as an additional key baseline index. Results: All 16 patients attained hematopoietic reconstruction, with pre-transplant flow-cytometry MRD negative in 13 cases (81.25%) and positive (MRD ≥0.01%) in three cases (18.75%). The median time of neutrophil and platelet engraftment was 12 (10-16) days and 15 (10-25) days, respectively. The incidence of I/II grade RRT was 31.3% (oral cavity: three cases, liver: two cases), with no III/IV grade RRT observed. The cumulative incidence of acute GVHD (aGVHD) was 50.0% (grade I/II skin: five cases, grade IIIl: three cases). Among 15 evaluable patients, the cumulative incidence of chronic GVHD (cGVHD) was 26.7% (local skin: three cases, ocular keratoconjunctivitis sicca: one case). Post-transplant infections occurred in 31.3% of patients, predominantly viral pathogens: one case of BK virus-associated hemorrhagic cystitis, one case of BK virus combined with bacterial infection, and three cases of cytomegalovirus (CMV) DNAemia. The median follow-up time was 28.03 (11.67-55.34) months (follow-up cutoff: 30 June 2024). Using the Kaplan-Meier method, the 1-year OS rate was 87.5% (95% CI: 65.2%-96.4%), and the 1-year DFS rate was 87.4% (95% CI: 64.9%-96.3%). The relapse rate and NRM were both 6.3% (95% CI: 0.8%-29.1%); the NRM case was confirmed as bronchiolitis obliterans syndrome (BOS) induced by pulmonary cGVHD. Conclusion: In this small single-center retrospective series, the cladribine-based conditioning regimen was associated with favorable hematopoietic reconstruction, mild RRT, and promising survival outcomes in children with R/R AML, even in partial patients with pre-transplant MRD positivity. However, due to the limited sample size, single-arm design, and lack of a control group, conclusions regarding superiority (e.g., improved OS or reduced relapse) cannot be drawn. Larger prospective multi-center studies are required to validate these preliminary findings.