During the study period, a total of 86 patients with high-risk leukemia underwent hematopoietic stem cell transplantation (HCT). Nine patients were excluded due to active disease or significant dysplasia, leaving a cohort of 77 patients with acute leukemia who had achieved morphological remission. Among these, 39 were diagnosed with acute lymphoblastic leukemia (ALL) (29 B-cell precursor ALL [BCP-ALL], 10 high-risk T-cell ALL), and 38 with acute myeloid leukemia (AML). Patients were stratified into two groups based on their pre-transplant measurable residual disease (MRD) status: 35 MRD+ (23 AML, 12 ALL) and 42 MRD- (27 ALL, 15 AML).
Post-transplant MRD assessments were conducted at key time points: day 30 (n = 30, 38.9%), day 60 (n = 27, 35.0%), and day 100 (n = 60, 77.9%). The progression of each subgroup is illustrated in Figure 1.
Baseline Characteristics
The study included 77 patients with a median age of 36.4 years (range 2.0–62.5 years), with a male predominance (55.8%). Disease status at the time of transplantation revealed that the majority (68.8%) were in their first complete remission (CR1), while 28.6% were in CR2 and 5.6% in CR3. A higher proportion of CR1 patients were in the MRD- group (83.3% vs. 51.4%, p = 0.003). Among the ALL patients, molecular analysis identified BCR::ABL1 fusion in 14 cases (35.9% of the ALL group), one with hyperdiploidy, and two cases of KMT2A pro-B ALL.
Transplant Characteristics
Transplantation details included donor type (38 matched-related, 27 unrelated, 12 haploidentical), cell source (44 bone marrow, 33 peripheral blood), and conditioning regimens. The majority received myeloablative regimens, with 42.9% undergoing total body irradiation (TBI) and cyclophosphamide (Cy/TBI), 41.8% receiving Busulfan and cyclophosphamide (Bu/Cy) ± rabbit anti-thymocyte globulin (ATG), and 9.1% on fludarabine-based (Flu/TBI) regimens. TBI use was more frequent in the MRD- group (66.7% vs. 37.1%, p = 0.012). Graft-versus-host disease (GvHD) prophylaxis consisted of methotrexate (MTX) and cyclosporine A (CSA with 77.9%) or post-transplant cyclophosphamide (PTCy) plus CSA and mycophenolate mofetil (22.1%). There were no significant differences in patient characteristics between MRD+ and MRD- groups.
Engraftment and Early Complications
Neutrophil engraftment occurred at a median of 19 days (range 15–23), and platelet engraftment at 21 days (range 13–28). One patient required a second transplant for engraftment failure, while six succumbed without engraftment. Infections were reported in 54.5% of patients within the first 30 days, with 22.1% requiring intensive care. Mucositis grade III-IV was observed in 75.3% of patients, with no difference between MRD groups. Acute GvHD grades III-IV at day 100 occurred in 28.6%, and chronic GvHD in 49.3%. Chimerism analysis showed complete chimerism in 80.4% and incomplete in 19.6% at day 100.
Non-relapse mortality (NRM) was higher in the MRD+ group (11.9% vs. 34.3%, p = 0.019). Relapse occurred in 11 patients, 8 from the MRD+ and 3 from the MRD- group (22.9% vs. 7.1%, p = 0.02).
MRD Analysis and Sensitivity
A total of 257 bone marrow samples, including 77 pre-transplant and 180 post-transplant samples, were analyzed. Using high-sensitivity flow cytometry, the limits of detection and quantification were 0.002% and 0.005% for AML and 0.0002% and 0.0005% for ALL, respectively. In the ALL group, eight patients had MRD >0.01%, with four showing low-level MRD. Phenotypic analysis revealed specific markers in BCP-ALL and high-risk T-cell ALL patients.
MRD Kinetics and Relapse
Post-transplant MRD was assessed at multiple time points: d30 (n = 30), d60 (n = 27), d100 (n = 60), d180 (n = 19), and d360 (n = 18). Relapses occurred in 8 MRD+ and 3 MRD- patients. Pre-transplant MRD+ was associated with positivity at d30 (p = 0.02) and d100 (p = 0.038), predictive of relapse (p = 0.009).
Survival and Follow-Up
With a median follow-up of 2.96 years, overall survival (OS) was significantly higher in the MRD- group (87.9% vs. 54.0%, p = 0.0001). Similarly, event-free survival (EFS) favored the MRD- group (85.3% vs. 51.1%, p = 0.0004). The cumulative incidence of relapse (CIR) was 2.6% in the MRD- group vs. 17.5% MRD+ group (p = 0.04). NRM was also higher in the MRD+ group (31.4% vs. 12.1%, p = 0.019).
Prognostic Impact
MRD positivity pre-transplant predicted lower OS and EFS, with significant differences observed in both ALL and AML subgroups. A higher proportion of MRD- patients were transplanted in CR1 (p = 0.0009). TBI use was associated with lower relapse rates (p = 0.001). Post-transplant MRD positivity at d100 had a high positive predictive value (PPV) for relapse (100%).
Clinical Sensitivity and Specificity
Pre-transplant MRD had a high negative predictive value (NPV) for relapse (91.4%) but a low positive predictive value (PPV, 19.1%). However, persistence of MRD post-transplant (at d100) showed high specificity and PPV for predicting relapse (100%).
Conclusion
Pre-transplant MRD status significantly impacts survival outcomes in patients undergoing HCT for acute leukemia. MRD negativity predicts superior OS, EFS, and lower relapse rates, while persistent MRD post-transplant is a strong predictor of relapse.
