Conditioning Regimens in Pediatric Myeloid Malignancies Undergoing Allogeneic HSCT: A Comparative Single-Center Study

Andra Daniela Marcu^1,2Cristina Georgiana Jercan^1,2*Ana Maria Bica^1,2Andreea Nicoleta Serbanica^1,2Letitia Elena Radu^1,2Irina Avramescu^1,2Anda Mocanu^1,2Oana Otilia Niculita^1,2Delia Codruta Popa^1,2Cerasela Jardan^1,2Silvia Mihaela Dragomir²Andrei Colita^1,3Alina Daniela Tanase^1,2Anca Colita^1,2

• ¹Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
• ²Fundeni Clinical Institute, Bucharest, Romania
• ³Coltea Clinical Hospital, Bucharest, Romania

Optimal conditioning regimen for pediatric myeloid malignancies is still subject for debate. This single-center retrospective study compares the efficacy and toxicity profiles of three conditioning strategies, myeloablative conditioning (MAC), reduced-toxicity conditioning (RTC), and reducedintensity conditioning (RIC), in 59 pediatric patients with myeloid malignancies undergoing allogeneic hematopoietic stem cell transplantation (HSCT). Primary objectives evaluated graft versus host disease (GvHD), relapse, overall survival (OS), disease-free survival (DFS), and mortality causes. Secondary endpoints assessed early complications such as mucositis, engraftment kinetics, viral reactivation, and hospitalization duration. A subgroup analysis compared fludarabine-and clofarabine-based RTC regimens. Results: RTC was associated with significantly lower transfusion needs, faster platelet engraftment and shorter hospitalization. Viral reactivations were more common in RTC and RIC, yet viral control, particularly CMV clearance, was superior seemed more effective in RTC. While oneyear OS and DFS were generally comparable across regimens, RTC showed a numerically higher OS, with a specific possible negative influence on relapse rate for children under 10 years old. Severe acute GvHD was similar across groups, but chronic GvHD was more frequent tended to occur more frequently in RIC. CR status strongly appeared to influenced relapse and mortality patterns, withwhile AML patients transplanted in CR1 experiencinghad significantly better OS and DFS. Subgroup analysis within RTC (clofarabine vs. fludarabine) revealed promising trends toward improved OS, lower acute GvHD, and reduced relapse-related mortality when using clofarabine. Conclusions: These findings support the use of individualized conditioning strategies in pediatric myeloid malignancies, with RTC emerging as a potentially balanced approach for selected cases.