We conducted this study to compare transplant outcomes between FLAMSA-FB and standard conditioning regimens in previously untreated MDS.

In this retrospective study, we compared allo-SCT outcomes between FLAMSA-FB and standard conditioning regimens.

This study was approved by the Ethics Committee of the University Medical Center Hamburg-Eppendorf (UKE) (reference number: 2022-100940-BO-ff). The study was performed in accordance with the Declaration of Helsinki. Informed consent was obtained from all participants.

All patients who underwent allo-SCT for untreated MDS (with 5-20% bone marrow blasts) at the University Medical Center Hamburg between 2006 and 2024 were included. Myeloablative conditioning regimens were defined according to the working group definition [26]. The FLAMSA-FB regimen included fludarabine (30 mg/m; total dose 120 mg/m), amsacrine (100 mg/m; total dose 400 mg/m), and cytarabine (1 g/m; total dose 4 g/m) from days -11 to -8, followed by a 3-day interval without therapy followed by Busulfan (total dose 6.4 mg/kg) from days -4 to -3, and Fludarabine (30 mg/m; total dose 60 mg/m) on days -4 and -3.

The TB regimen consisted of Thiotepa (5 mg/kg per day; total dose 10 mg/kg) on days -6 and -5, and Busulfan (3.2 mg/kg per day; total dose 6.4 mg/kg) on days -4 and -3.

The Treo-Flu regimen consisted of Treosulfan (12 g/m; total dose 36 g/m) on days -6 to -4, and Fludarabine (30 mg/m; total dose 150 mg/m) on days -6 to -2.

The FB regimen consisted of Busulfan (total dose 6.4 mg/kg) from days -7 to -5, and Fludarabine (30 mg/m; total dose 150 mg/m) on days -7 to -3.

Anti-T lymphocyte globulin (ATLG) (Grafalon®, Neovii, Switzerland) was administered with a test dose of 200 mg on day -4, and the remaining doses were fractionated between days -3 and -1. Thymoglobulin (ATG) doses were fractionated between days -3 and -1. Post-transplant cyclophosphamide (PTCy) was administered as 50 mg/kg/day on days +3 and +4. Post-transplant GVHD prophylaxis involved either Ciclosporine A (CSA) for recipients of matched donors or Tacrolimus (Tac) for recipients of mismatched donors, with Methotrexate 10 mg/m administered on days +1, +3, and +6. Alternatively, CSA/Tac was combined with Mycophenolate Mofetil from day +1 to day +28 for recipients of matched donors and from day +1 to day +35 for recipients of mismatched donor.

All outcomes were measured from the time of allo-SCT. Our Endpoints were progression-free survival (PFS), overall survival (OS), non-relapse mortality (NRM), and cumulative incidence of relapse (CIR). acute graft versus host disease (aGVHD) and chronic graft versus host disease (cGVHD). aGVHD was graded according to the EBMT definition [27]. cGVHD was graded according to National Institute of Health criteria routinely at every visit after allo-SCT [28]. Neutrophil engraftment was defined as the first three consecutive days with a measure of an absolute neutrophil count >0.5 × 10/L. Platelet engraftment was defined as the first consecutive days with a platelet count >20 × 10/L without transfusion support.

PFS was defined as the duration of survival without relapse or progression, with censoring for patients without these events at last follow-up. OS was defined as death from any cause, while NRM was defined as death without evidence of relapse.

For statistical analysis, Kaplan-Meier methods were employed to estimate probabilities for PFS and OS, with group differences assessed via the log-rank test. Cumulative incidence functions were used to estimate engraftment, CIR, NRM, aGVHD, and cGVHD in a competing risk framework. Specifically, CIR and NRM were treated as competing events, and death without the respective event was considered the competing risk for aGVHD, cGVHD, and engraftment.

For univariate analyses (UVA), continuous variables were categorized. Univariate comparisons were performed using the log-rank test and Gray's test for cumulative incidences. Multivariate analysis (MVA) was conducted using a Cox proportional hazards model to calculate adjusted hazard ratios and 95% confidence intervals. A p value of less than 0.05 was considered statistically significant. MVA using Fine and Gray's competing risks regression model was performed to identify independent prognostic factors for CIR, GVHD, and engraftment. Variables significant in univariate analysis or clinically relevant were included.

To reduce confounding and improve comparability between treatment groups, we performed propensity score matching (PSM) using the MatchIt package in R. Patients were restricted to those with an Eastern Cooperative Oncology Group performance status of 0 or 1, and haploidentical transplants were excluded to enhance matching quality. A nearest-neighbor matching algorithm with a caliper of 0.25 was applied without replacement. The propensity score was estimated using a logistic regression model, incorporating the following covariates: IPSS score, donor sex, transplant year, donor relationship (related vs. unrelated), and HLA matching status (matched vs. mismatched).

Balance between groups before and after matching was assessed using standardized mean differences, with a threshold of <0.1 considered acceptable. A Love plot and propensity score density plots were generated to visually assess balance improvement.

All statistical analyses were conducted using R (version 3.0, R Development Core Team, Vienna, Austria; https://www.r-project.org/). The following R packages were used: survival, cmprsk, ggplot2, dplyr, survminer, mstate, tableone, forestplot, matchit, and cobalt.