Outcomes of older matched sibling donors compared to younger matched unrelated donors in allogeneic hematopoietic cell transplantation for AML in first complete remission.

e18558 Background: Matched sibling donors (MSDs) have long been the preferred choice for allogeneic HCT. However, recent studies suggest younger donors may lead to better outcomes. This has led to analyses exploring whether younger matched unrelated donors (MUDs) might be better than older MSDs. Although results remain inconclusive, some believe younger MUDs may be preferable, especially for high-risk patients, due to lower relapse rates. In this study, we compared survival outcomes between older MSDs and younger MUDs in AML transplants at our center. Methods: In this retrospective analysis, younger MUDs were defined as donors aged ≤35, while older MSDs were defined as those aged ≥50. We included patients with AML in first complete remission (CR1) who underwent HCT at our center between 2020 and 2024. High-risk AML was defined based on ELN2022 or the presence of positive measurable residual disease (MRD) prior to HCT. Results: A total of 31 and 119 AML patients underwent HCT from a MSD and a MUD, respectively, between 2020 and 2024 (Table). The proportion of high-risk patients was 52% in the MSD group and 66% in the MUD group. Among these high-risk patients, 37.5% in the MSD group and 53.8% in the MUD group had positive MRD prior to HCT. For all patients, relapse-free survival (RFS) was comparable between the two groups (hazard ratio [HR] MSD vs MUD 1.52, 95% confidence interval [CI] 0.63–3.66), as was overall survival (OS) (HR 1.69, 95% CI 0.65–4.42). The 1-year survival rates were 93% for the MSD group and 85% for the MUD group. Altogether, 11 and 66 high-risk AML patients underwent HCT from an older MSD and a younger MUD, respectively. The median patient age was 64 years in the MSD group and 61 years in the MUD group, while the median (Q1, Q3) donor age was 59 (54, 62) and 24 (21, 27) years, respectively. Both groups were well-matched in terms of conditioning regimen intensity and graft-versus-host disease prophylaxis. There were no significant differences between the groups in RFS (HR 0.87, 95% CI 0.25–3.03) or OS (HR 0.76, 95% CI 0.21–2.70). For these high-risk patients, the 1-year survival rates were 91% in the MSD group and 89% in the MUD group. Conclusions: This study is limited by a small sample size, but it suggests that AML patients in CR1 may achieve comparable outcomes whether they receive HCT from an older MSD or a younger MUD. Baseline Characteristics Matched sibling donors (n=31) Matched unrelated donors (n=119) p-value Patient age, median (Q1, Q3) 56 (45, 64) 60 (51, 67) 0.07 Donor age, median (Q1, Q3) 53 (41, 59) 25 (22, 31) <0.001 Disease risk, n (%) Favorable Intermediate Adverse 6 (19)9 (29)16 (52) 9 (7.5)32 (27)78 (66) 0.12 Conditioning intensity, n (%) Myeloablative Reduced intensity 16 (52)15 (48) 39 (33)80 (67) 0.06 GVHD prophylaxis regimen, n (%) PTCy/MMF/tacrolimus Tacrolimus/MTX Tacrolimus/sirolimus 6 (19)23 (74)2 (6.5) 19 (16)99 (83)1 (0.8) 0.2