This is a prospective, single-center, observational study to evaluate the efficacy and safety of salvage haploidentical allogeneic hematopoietic stem cell transplantation (haplo-HSCT) combined with post-transplant relapse prevention strategies in patients with relapsed/refractory acute myeloid leukemia (R/R AML).

Eligible patients are adults aged 18-65 years with active AML (bone marrow blasts >5% or extramedullary disease) and HCT-CI score ≤5. All patients will receive a uniform conditioning regimen consisting of fludarabine, busulfan, and MECCNU, with addition of targeted agents (such as sorafenib, midostaurin, or venetoclax) according to mutation status. Graft-versus-host disease (GVHD) prophylaxis includes reduced-dose ATG (6 mg/kg), FK506, MMF, and basiliximab. Post-transplant maintenance with targeted therapy or azacitidine and prophylactic donor lymphocyte infusion (DLI) will be administered to reduce relapse risk.

The primary endpoints are cumulative incidence of relapse (CIR), overall survival (OS), and progression-free survival (PFS). Secondary endpoints include incidence of acute and chronic GVHD, CMV/EBV reactivation, non-relapse mortality (NRM), and GVHD-free, relapse-free survival. Patients will be followed for 24 months after transplantation. This study aims to explore an optimized transplant strategy to improve long-term survival in this high-risk population.

Inclusion Criteria:

• Signed and dated informed consent Willing and able to comply with all study procedures and follow-up Adults aged 18 to 65 years Diagnosis of acute myeloid leukemia (AML) Active disease before transplantation, defined as bone marrow blasts >5% or presence of extramedullary disease HCT-CI (Hematopoietic Cell Transplantation-Comorbidity Index) score ≤5

Exclusion Criteria:

• Bone marrow blasts ≤5% without extramedullary disease before transplantation Age <18 years or >65 years HCT-CI score >5 Patients with other diagnoses besides AML

This is a single-center prospective observational study enrolling adult patients (18-65 years) with relapsed/refractory acute myeloid leukemia (R/R AML) who have active disease (bone marrow blasts >5% or extramedullary involvement) and are scheduled to undergo salvage haploidentical allogeneic hematopoietic stem cell transplantation (haplo-HSCT) at Union Hospital, Tongji Medical College, Huazhong University of Science and Technology. The study population consists of high-risk AML patients with HCT-CI ≤5 who are eligible for intensive transplant therapy according to institutional practice.