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Background: Blinatumomab has demonstrated encouraging activity in relapsed/refractory (r/r) and minimal residual disease–positive (MRD+) acute lymphoblastic leukemia (ALL). Extramedullary disease (EMD) relapse or relapse with CD19– disease has been observed after blinatumomab therapy in patients with r/r or MRD+ ALL. However, the pathophysiology and risk factors of treatment failure are not fully understood. Methods: This study retrospectively reviewed the outcomes of adult patients with B‐cell ALL treated with blinatumomab (n = 132) for either r/r (n = 103) or MRD+ disease (n = 29) at the authors' center (2013‐2021) and analyzed factors associated with treatment response and EMD failure. Results: The overall response rate was 64%. A lower marrow blast burden before blinatumomab (P =.049) and no history of previous EMD (P =.019) were significantly associated with a higher response. Among the patients who responded to blinatumomab, 56% underwent consolidation with allogeneic transplantation. Blinatumomab failure was observed in 89 patients; 43% of these patients (n = 38) either progressed or relapsed at extramedullary sites. A history of extramedullary involvement (53% vs 24%; P =.005) and retention of CD19 expression at the time of relapse/progression (97% vs 74%; P =.012) were associated with a higher risk for extramedullary failure. Central nervous system (CNS) failure after blinatumomab was encountered in 39% of the patients with EMD. Conclusions: A history of EMD predicted an inferior response to blinatumomab therapy with a higher risk for relapse/progression at extramedullary sites (particularly CNS). Consolidation with allogenic transplantation in patients who primarily responded to blinatumomab did not abrogate the risk of extramedullary relapse. The incorporation of extramedullary assessment and the intensification of CNS prophylaxis may help in addressing extramedullary failure. Lay Summary: Extramedullary failure is common during blinatumomab therapy for relapsed/refractory acute lymphoblastic leukemia.A history of extramedullary disease predicts an inferior response to blinatumomab therapy and a higher risk for relapse/progression at extramedullary sites.Most extramedullary failure cases retain CD19 expression. Extramedullary failure is common during blinatumomab therapy for relapsed/refractory acute lymphoblastic leukemia. A history of extramedullary disease predicts an inferior response to blinatumomab therapy and a higher risk for relapse/progression at extramedullary sites. [ABSTRACT FROM AUTHOR]

• HCT outcomes were analyzed in r/r ALL patients, after salvage with blinatumomab. • Pre-HCT blinatumomab was given for morphologic (n = 24) or MRD-positive disease (n = 11). • No unusual toxicities or delayed engraftment was observed post-HCT. • Subgroup analysis (MRD/cytogenetics) showed no significant difference in survival. • Blinatumomab may be considered as a safe and effective salvage therapy pre-HCT. Historically, outcomes of adult patients with relapsed acute lymphoblastic leukemia (ALL) who fail to enter remission with conventional chemotherapy are very poor. Blinatumomab, a bispecific CD3/CD19 antibody, has shown remarkable activity in relapsed/refractory (r/r) ALL. Although allogeneic hematopoietic cell transplant (HCT) is the recommended consolidation therapy for patients with r/r ALL who respond to salvage therapy, HCT and toxicity outcomes for those who received blinatumomab salvage and HCT remain largely unknown. We treated 89 patients with r/r ALL with blinatumomab, of whom 43 patients (48%) achieved remission. Here we describe our single-center experience in the subset of patients who responded to blinatumomab salvage therapy for eradication of either gross (n = 24) or minimal residual disease (n = 11) before HCT. Overall survival at 1 and 2 years after allogeneic HCT was 77% and 52%, respectively. Leukemia-free survival at 1 and 2 years were 65% and 40%, respectively. Additionally, with blinatumomab administration pre-HCT, no unusual toxicities such as delayed neutrophil/platelet engraftment or graft failure were observed. Acute grades II to IV graft-versus-host disease (GVHD) at day +100 post-HCT was at 43% and 2-year chronic GVHD was 36%, both comparable with historic control subjects. Finally, results of our subset analysis based on pre-HCT minimal residual disease (MRD) status indicated no significant difference in survival outcomes among patients undergoing transplant in MRD-negative status and the entire cohort. In conclusion, based on results of this study, blinatumomab may be considered as a safe and effective agent for r/r ALL patients before HCT. [ABSTRACT FROM AUTHOR]