A Single-arm, Prospective Study of TBI + BUMEL As a Conditioning Regimen for Salvage HSCT in Patients with R/R AML

Acute myeloid leukemia (AML) is one of the hematologic malignancies, for which patients typically undergo chemotherapy to achieve complete remission. However, approximately 30% of patients fail to respond to initial treatment, and many experience relapse after achieving remission. For patients with relapsed or refractory AML, allogeneic hematopoietic stem cell transplantation (HSCT) offers a potentially curative option. Sibling-matched HSCT has demonstrated a disease-free survival rate of 20-30%, while unrelated donor transplants yield an overall survival rate of approximately 22%. Haploidentical transplantation is a viable alternative for patients lacking a sibling donor. A 2019 study involving 1,693 patients with relapsed/refractory (R/R) AML revealed that haploidentical transplants yielded outcomes comparable to other transplant modalities, including HLA-matched and 9/10 matched unrelated donor transplants, thus supporting haploidentical transplantation as a viable therapeutic option. The conditioning regimen is a critical component of the transplantation. In China, the modified BU/CY conditioning regimen, which combines busulfan (BU) and cyclophosphamide (CTX), is widely utilized for tumor cytoreduction and immunosuppression. Some centers also employ post-transplant cyclophosphamide (PTCy) to mitigate the risk of graft-versus-host disease (GVHD). Despite advances, relapse remains a significant challenge. Optimizing conditioning regimens to enhance tumor cell targeting and achieve deeper remission is crucial. Additionally, many patients are unfit due to prior chemotherapy, infections, and organ dysfunction, which may make them unable to tolerate high-intensity conditioning. Recent studies suggest that melphalan (MEL)-based conditioning regimens may offer advantages over CTX-based protocols. While total body irradiation (TBI) has been traditionally used in conditioning for HSCT, it is associated with considerable organ toxicity. A low-dose TBI regimen combined with BU+MEL represents a promising conditioning regimen for R/R AML. In preliminary studies, 7 patients treated with this regimen successfully achieved hematopoietic stem cell engraftment. Building on these results, a clinical study is planned to evaluate further the safety and efficacy of the TBI+BUMEL (IBM) conditioning regimen in relapsed/refractory AML, with a focus on improving engraftment rates, reducing relapse rates, minimizing GVHD incidence, and enhancing overall survival outcomes.