Active infection at the time of CD34+ selected stem cell boost is associated with treatment failure and poor survival

• CD34+ selected stem cell boost can restore hematopoiesis following poor graft function after allogeneic hematopoietic stem cell transplant

• An active infection at the time of CD34+ selected stem cell boost is the strongest predictor of treatment failure and poor overall survival

Background: The use of CD34+ selected stem cell boost (SCB) post allogeneic hematopoietic cell transplant (alloHCT) has been increasing. Predictors of treatment failure following SCB, both in the context of poor graft function (PGF) or other settings, are not well-characterized. We report among the largest single center retrospective experiences of the use of SCB and evaluate potential predictors of response and outcomes. Methods: 58 patients who underwent HCT between 2015 and 2022 and who received SCB were identified. The indication for SCB was predominantly PGF, defined as the presence of 2 or more cytopenias for at least two consecutive weeks beyond day +14 after alloHCT in the presence of ≤ 30% bone marrow cellularity and ≥ 90% donor myeloid chimerism in the absence of morphological disease. Results: The median dose of infused CD34+ selected SCB products was 3.88 x 106 CD34+ cells/kg (range: 0.99-9.92). The median 2-year OS and NRM following SCB was 47% and 38%, respectively. The cumulative incidences of 6-month grade III-IV acute and 2-year moderate-severe chronic GVHD following SCB were 3.4% and 12%, respectively. Overall response (CR + PR) was attained in 36/58 (62%) patients, and in 69% with PGF. On multivariable analysis, an active infection at the time of SCB was the greatest predictor of poor response and survival (p=0.013) following SCB. Conclusion: SCB can restore hematopoiesis in the majority of patients, particularly for those with poor graft function in whom there is no active infection at infusion.