Outcomes of Allogeneic Transplantation with Clofarabine and Post-Transplant Cyclophosphamide-Based Conditioning in Patients with Very High-Risk Acute Myeloid Leukemia

Introduction: Patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT) with active disease exhibit relapse rates exceeding 50% and an estimated survival of less than 25%. Among those with very adverse genetics, particularly acute myeloid leukemia (AML) with complex karyotypes and/or bi-allelic mutations in TP53, the relapse rate exceeds 80%. Clofarabine (Clo) directly induces apoptosis through caspase activation and interacts with the mitochondrial membrane, conferring superior anti-leukemic activity compared to other nucleoside analogs. Its use as part of the conditioning regimen for HSCT in AML and high-risk or active disease has been explored, particularly in the context of sequential transplantation and in some HSCT platforms utilizing matched donors with conventional prophylaxis for graft-versus-host disease (GVHD). The objective of this study was to analyze our center's experience with patients at very high risk of relapse who received Clo-based conditioning and post-transplant cyclophosphamide (PT-CY) as GVHD prophylaxis. Patients and Methods: A unicentric retrospective study was conducted, including adult patients who underwent allogeneic HSCT with Clo-based conditioning.The criteria for using this conditioning regimen was visible-phase AML at the time of transplantation, multi-hit TP53 mutation, and/or very high-risk cytogenetics (complex/monosomal karyotye). The intermediate-intensity regimen (TCI 2.5) consisted of Clo 30 mg/m² (for 5 days) plus Melphalan 140 mg/m² on day -2, while the high-intensity regimen (TCI 4) consisted of Clo 40 mg/m² (for 5 days) plus Busulfan 3.2 mg/kg/day (for 4 days). All patients received PT-CY on days +3 and +4, followed by a calcineurin inhibitor (CNI) and mycophenolate mofetil (MMF) from day +5. No patient received antithymocyte globulin (ATG). Results: A total of 20 HSCTs were identified between 2018 and 2024, with a median follow-up of 37.6 months (range 27-53). The ge-adjusted HCT-CI score was ≥ 3 in 60%; 25% had previously undergone an allogeneic HSCT. Disease risk index was high or very high in 90%, 4% were primary refractory, 10% presented with visible extramedullary disease at the time of transplantation, only 4 patients underwent transplantation with a negative measurable residual disease (in these patients, it was their second bone marrow transplant), and 15 of the 20 patients had very high-risk genetic alterations at diagnosis. 80% of patients received the intermediate-intensity conditioning. Stem cell source was in all cases, and 9 (45%) patients received a haploidentical HSCT. There were no cases of graft failure, with cumulative incidences of neutrophil and platelet engraftment at 28 days of 90% and 50%, respectively (median of 17 and 27 days). The most frequent complications were Grade ≥ 2 mucositis (55%), reactivation of CMV (55%), hepatotoxicity Grade 2-4 (35%), hemorrhagic cystitis (15%), cardiotoxicity (5%), clofarabine-related skin rash (15%), endotheliolopathy (10%), neuropathy (10%), and hepatic veno-occlusive disease (5%). Cumulative incidence of acute GVHD II-IV at 180 days was 16% (Grade III-IV GVHD at 5%). Cumulative incidence of chronic GVHD was 20%, with moderate-severe chronic GVHD of 11%. The combined objective of relapse and GVHD-free survival (GRFS) was 47% at 3 years, with a median of 33 months (95% CI 19-47). At 3 years, overall survival was 52%, with a median of 40 months (95% CI 27-57), and event-free survival was 49% with a median of 37 months (95% CI 24-52). Treatment-related mortality (TRM) at 2 years was 15%; there were two late events, resulting in a TRM of 26% at 3 years. The relapse incidence at three years was 26%, with no differences in subgroup analysis for higher risk (active disease at transplant and TP53 mutation). Conclusions: In our experience, the incorporation of Clo to the conditioning regime in patients with very high relapse-risk AML, including visible disease at transplantation, using peripheral blood and post-transplant cyclophosphamide GVHD prophylaxis with both matched and haploidentical donors, offers favorable outcomes for this population, which has historically presented adverse results.