Risk Factors Influencing Transplant Outcomes of Adults with Acute Lymphoblastic Leukemia in First Complete Remission: A Retrospective Analysis from the ALWP of the EBMT

Background: Allogeneic hematopoietic stem cell transplantation (allo-HSCT) in first complete remission (CR1) remains a standard of care for adult patients with acute lymphoblastic leukemia (ALL) at high risk of relapse, i.e. high-risk features at time of diagnosis (ex. Philadelphia (Ph) positive ALL), or detectable minimal residual disease (MRD+). The aim of our study was to identify risk factors influencing transplant outcomes in ALL patients transplanted between 2015 and 2021.

Methods: We included adults undergoing allo-HSCT for ALL in CR1 with available data of ALL phenotype, Ph-status, and MRD pre-transplant. All donor types and stem cell sources were eligible, except cord blood. Ex-vivo T-cell depletion (TCD) represented an exclusion criterion.

Results: We identified 2349 transplanted patients including 870 from HLA-matched sibling (MSD), 1150 from unrelated (79% 10/10 and 21% 9/10 UD), and 329 from haploidentical donors (Haplo). Median age was 42.6 (range 18-76.1) years. Median follow-up was 23.5 (95% CI 21.8-24.2) months. The entire cohort was composed of 667 patients (28.4%) with Ph-negative B-ALL, 1325 (56.4%) with Ph-positive B-ALL and 357 (15.2%) with T-ALL. Median time from diagnosis to allo-HSCT was 5.8 (range 1.3-18) months. Stem cell source was peripheral blood (PB) in 85.3% of cases. Conditioning regimen was reduced-intensity (RIC) in 21% and myeloablative (MAC) in 79% of cases. Total body irradiation (TBI) was used in 65.8% of patients, mostly as a MAC (93.9%). Pre-transplant MRD+ was reported in 35.4% while in-vivo TCD was used in 51.5% of cases.

At 180 days, cumulative incidence of grade II-IV and grade III-IV acute graft-versus-host disease (aGVHD) were 29.2% (95% CI 27.3-31.1) and 10.1% (95% CI 8.9-11.4), respectively. Two-years overall and extensive chronic GVHD (cGVHD) were 34% (95% CI 31.7-36.3) and 15% (95% CI 13.2-16.8), respectively. At two years, relapse incidence (RI), non-relapse mortality (NRM), leukemia-free survival (LFS), overall survival (OS) and GVHD-free/relapse free survival (GRFS) were 22.2% (95% CI 20.3-24.2), 15.4% (95% CI 13.8-17.1), 62.3% (95% CI 60-64.6) 73.9% (95% CI 71.7-76) and 48% (95% CI 45.6-50.3), respectively.

By multivariate analysis, MSD (reference group) was associated to higher RI compared to 9/10 UD (HR 1.47, 95% CI 1.01-2.12; p<0.05) and Haplo (HR 1.64, 95% CI 1.15-2.27; p<0.01) but lower NRM and grade II-IV aGVHD compared to both 9/10 and 10/10 UD (for NRM HR 0.54, 95% CI 0.36-0.81; p<0.01 and HR 0.65, 95% CI 0.48-0.88; p<0.01; for aGVHD: HR 0.58, 95% CI 0.44-0.78; p<0.01 and HR 0.70, 95% CI 0.56-0.87; p<0.01). Compared to Ph-negative ALL (reference group), Ph-positive ALL was associated with a lower RI (HR=0.61, 95% CI 0.49-0.77; p<0.01) and higher LFS (HR=0.71, 95% CI 0.60-0.85; p<0.01), OS (HR=0.59, 95% CI 0.48-0.72; p<0.01) and GRFS (HR=0.81, 95% CI 0.70-0.94; p<0.01), while T-ALL was associated to lower NRM (HR 0.58, 95% CI 0.37-0.91: p<0.02) and cGVHD (HR 0.73, 95% CI 0.54-0.98; p<0.04). MRD+ group faced higher RI (HR=1.74, 95% CI 1.42-2.12; p<0.01), lower LFS (HR=1.41, 95% CI 1.21-1.65; p<0.01), OS (HR=1.31, 95% CI 1.08-1.58; p<0.01) and GRFS (HR=1.31, 95% CI 1.14-1.49; p<0.01). Use of TBI enabled lower RI (HR 0.78, 95% CI 0.61-0.99; p=0.04), higher LFS (HR=0.80, 95% CI 0.67-0.96; p<0.02) but higher grade II-IV aGVHD (HR 1.54, 95% CI 1.23-1.92; p<0.01) and cGVHD (HR 1.47, 95% CI 1.16-1.87; p<0.01) compared to chemotherapy. Patients receiving a RIC had a higher RI (HR=1.52, 95% CI 1.17-1.98; p<0.01) and a lower NRM (HR=0.55, 95% CI 0.39-0.77; p<0.01). In vivo TCD allowed lower risk of cGVHD (HR=0.69, 95% CI 0.55-0.85; p<0.01) and higher GRFS (HR=0.78, 95% CI 0.66-0.91; p<0.01). PB stem cells and female-donor to male-recipient were independently associated to higher risk of cGVHD. Elderly patients faced lower LFS, OS and GRFS.

Conclusions: Our study reported overall favorable outcomes in the population of recently allografted patients with ALL in CR1 (LFS> 60% et OS> 70% at 2 years). Prognostic role of pre-transplant MRD is confirmed. Results in Ph-positive ALL are not inferior compared to Ph-negative ALL, most probably due to the use of tyrosine kinase inhibitors. Whether incorporation of novel agents will change the role of allo-HSCT in Ph-positive ALL is under investigation. Use of TBI and HLA mismatches are both associated with lower RI despite a higher risk of GVHD, highlighting the importance of tailoring GVHD prophylaxis in these settings.