Outcomes of Total Body Irradiation- Versus Chemotherapy-Based Myeloablative Conditioning Regimen in Haploidentical Hematopoietic Cell Transplantation with Post-Transplant Cyclophosphamide for Acute Lymphoblastic Leukemia: ALWP of the EBMT Study

Background The intensity of a conditioning regimen has significant impact on outcomes of allogeneic hematopoietic cell transplantation in acute leukemia. In acute lymphoblastic leukemia (ALL) patients undergoing matched donor transplant, total body irradiation (TBI)- based myeloablative conditioning (MAC) regimen was associated with improved leukemia free survival (LFS) compared to chemotherapy (CT)-based MAC (Eder S. et al. 2017). Haploidentical hematopoietic cell transplantation(haplo-HCT) with post-transplant cyclophosphamide (PTCy) has emerged as a safe alternative in absence of a matched donor. The optimal MAC in haplo-HCT setting is yet to be defined. We studied the outcomes of TBI- vs. CT-based MAC in ALL patients undergoing haplo-HCT and reported to the Acute Leukemia Working Party of the EBMT. Methods The study included 427 ALL (B-ALL-75%) patients, that underwent haplo-HCT with PTCy during the years 2010-2018, following TBI- (n=188, 44%) or CT- (n=239, 56%) based MAC. Regimen intensity was defined by EBMT criteria and cases with busulfan dose Results In univariate analysis, day 100 incidences of acute GVHD II-IV and III-VI were 38% vs. 30% (p-0.07) and 19% vs. 13% (p-0.14) for TBI and CT cohort, respectively. Two-year overall and severe chronic GVHD incidences were 34% vs. 30% (p-0.51) and 17% vs. 12% (p-0.18) for TBI and CT cohort, respectively. Graft failure was reported in 6 (3%) and 19 (8%)(p-0.09) patients who received TBI and CT-based MAC, respectively. Death from veno-occlusive disease was reported in 4 (5%) TBI patients and 8 (7%) CT patients. There was no difference in reported deaths due to infection (28%) or interstitial pneumonitis (4%) among study cohorts. In multivariate analysis, TBI was associated with significant improvement in nonrelapse mortality (NRM) [HR=0.51, 95% CI:0.32-0.83, p In a subgroup univariate analysis of patients Conclusions TBI based MAC resulted in significant reduction of NRM, translating into a better LFS without impacting aGVHD III-IV, chronic GVHD, RI or OS when compared to CT based MAC. Non-significant difference in OS between TBI and CT cohorts might be related to lower number of events, short follow-up and/or salvage therapies after relapse. These novel findings based on a large cohort of ALL patients, support the use of TBI based MAC and bone marrow graft in haplo-HCT with PTCy. Disclosures Dholaria: Celgene: Honoraria. Labopin:Jazz Pharmaceuticals: Honoraria. Angelucci:Novatis: Honoraria, Other: Chair Steering Committee TELESTO protocol; Celgene: Honoraria, Other: Participation in DMC; BlueBirdBio: Other: Local advisory board; Jazz Pharmaceuticals: Other: Local advisory board; Roche: Other: Local advisory board; Vertex Pharmaceuticals Incorp., and CRISPR Therapeutics: Other: Participation in DMC. Apperley:Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Socie:Alexion: Consultancy. Mohty:Jazz Pharmaceuticals: Honoraria, Research Funding.