Poor Engraftment After Posttransplant Cyclophosphamide Graft‐Versus‐Host Disease Prophylaxis in Patients With Myelofibrosis

Patients with myelofibrosis (MF) often have impaired engraftment after allogeneic hematopoietic cell transplantation (alloHCT). To determine whether posttransplant cyclophosphamide (PTCY) exerts a detrimental impact on engraftment, we compared clinical outcomes of all patients receiving PTCY (N = 49) and no-PTCY (N = 89) regimens as GVHD prophylaxis after first alloHCT for MF from 2016 to 2023. Median age was 64 in both groups, and baseline characteristics were balanced except that the PTCY group received more haploidentical transplants and were more likely to have secondary MF. Among engrafted individuals, median time to neutrophil engraftment was D + 18 for PTCY and D + 15 for no-PTCY (p < 0.0001). Median time to platelet recovery was D + 33.5 and D + 26 for PTCY and no-PTCY, respectively (p = 0.0006). The graft failure rate was higher for PTCY but did not reach statistical significance (27% for PTCY and 15% for non-PTCY, p = 0.095). The need for donor lymphocyte infusion, CD34 boost, or second transplant was significantly higher for PTCY (33% vs. 13% at 1-year, respectively, p = 0.004). Most cases of graft failure in both cohorts were due to prolonged cytopenias in the absence of relapse. While T-cell chimerism at D + 30 and D + 100 was robust after PTCY, the proportion of patients with granulocyte chimerism ≥ 90% at Day + 100 was significantly lower in PTCY (76% vs. 98.2%, respectively, p = 0.0028). All other outcomes were similar except for a significantly lower chronic GVHD rate in PTCY compared with no-PTCY (p = 0.0002). GVHD prophylaxis with PTCY in patients with MF is associated with delayed engraftment, lower donor granulocyte chimerism, and increased need for additional donor cell infusions after transplant without compromising survival.