Impact of thiotepa dose in haploidentical allogeneic stem cell transplantation with thiotepa-busulfan-fludarabine conditioning for Acute Myeloid Leukemia in remission: A retrospective analysis of the EBMT acute leukemia working party

Abstract Background - Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) has become an increasingly adopted curative strategy for patients with acute myeloid leukemia (AML), especially in the absence of a fully matched donor. Among conditioning regimens, thiotepa-busulfan-fludarabine (TBF) has emerged as one of the most widely used. The intensity of this regimen can be modulated by adjusting the doses of thiotepa and busulfan, potentially balancing toxicity and antileukemic efficacy. However, evidence on the optimal doses of these alkylating agents on transplant outcomes remains limited.Methods – This is a retrospective analysis based on data from the European Society for Blood and Marrow Transplantation (EBMT) registry. From 2010 to 2022, AML patients receiving a first haplo-HSCT in first or second complete remission (CR1, CR2) with a TBF-based conditioning and post-transplant cyclophosphamide (PTCy) as GVHD prophylaxis were identified. Based on busulfan dose, conditioning intensity was defined as myeloablative (TBF-MAC, ≥8.8 mg/kg busulfan) or reduced-intensity (TBF-RIC, <8.8 mg/kg). In both groups, thiotepa was considered high-dose when ≥8 mg/kg.Results – A total of 2393 patients (mean age 53.2 years) were included in the analysis, mostly transplanted in CR1 (77.6%, n=1856), with a predominance of intermediate risk AML (70.6%, n=1201). At 2-years, overall survival (OS), relapse incidence (RI), non-relapse mortality (NRM) were 65.7%, 18.7% and 21.5%, respectively. Acute graft versus host disease (aGVHD) ≥3 and extensive chronic GVHD (cGVHD) occurred in 9.1% and 10.5% of cases.TBF-RIC conditioning was administered to 1255 patients, while 1138 were treated with TBF-MAC. OS rates were 71.1% and 59.3% with NRM rates of 17.7% and 26% in the TBF-MAC and TBF-RIC group, respectively.In multivariate analysis within TBF-RIC group, high-dose thiotepa was associated with increased risk of grade ≥3 aGVHD (HR 1.95, 95% CI: 1.17-3.24, p=0.01) and cGVHD (HR 1.49, 95% CI: 1.07-2.08, p=0.018), without impact on extensive cGVHD, RI and OS.In the TBF-MAC group, high-dose thiotepa was significantly associated with increased mortality risk (HR 1.62, 95% CI: 1.05-2.52, p=0.03), primarily driven by higher NRM (HR 1.83, 95% CI: 1.01-3.33, p=0.046). Additionnaly, transplant in CR2 correlated with worse survival (HR 1.39, 95% CI: 1.04-1.86, p=0.026) and higher RI (HR 1.27, 95% CI: 1.01-1.60, p=0.045).Peripheral blood as graft source was associated with higher grade ≥2 aGVHD (HR: 1.47, 95% CI: 1.03-2.09, p=0.033) in TBF-RIC group, and grade ≥3 aGVHD (HR:1.74, 95% CI: 1.09-2.79, p=0.021) in TBF-MAC group, but did not affect OS and RI.Older age and lower Karnofsky score were significantly associated with poorer OS and higher NRM across both TBF-MAC and TBF-RIC.Conclusions – In this large retrospective analysis on patients with AML undergoing haplo-HSCT in CR1-CR2 with a TBF conditioning, thiotepa dose significantly influenced outcomes in both RIC and MAC setting. High-dose thiotepa was associated with increased risk of severe aGVHD and cGVHD when combined with RIC busulfan doses, and with higher NRM and poorer survival, when used in combination with MAC busulfan doses. Across both TBF-RIC and TBF-MAC cohorts, no advantage in terms of RI was observed with high-dose thiotepa, suggesting no added benefit in disease control. These findings indicate that lower thiotepa doses (<8 mg/kg) may be preferable, regardless of conditioning intensity. Additionnaly, among TBF-MAC treated patients, those transplanted in CR2 had higher risk of relapse and mortality. Irrespective of TBF regimen, the use of peripheral blood stem cell grafts increased aGVHD risk without affecting OS or RI, and older age and lower Karnofsky performance scores remain adverse prognostic factors for survival.