Outcomes after stem cell boost for poor engraftment following allogeneic hematopoietic stem cell transplantation for patients with primary and secondary myelofibrosis.

e18561 Background: The introduction of JAK2 inhibitors has improved symptom control for patients with myelofibrosis (MF), but allogeneic hematopoietic stem cell transplant (HSCT) remains the only curative option. A significant challenge for patients with MF undergoing HSCT is poor or delayed hematopoietic engraftment (PGF), presumably reflecting the challenge of establishing a stable hematopoietic niche in a fibrotic marrow space. Methods: We retrospectively studied a cohort of 41 consecutive patients with primary MF and post-ET/PV MF who received an HSCT from 2012 to 2023 at our institution. We examined patient and HSCT-specific variables influencing poor engraftment necessitating a CD 34+ stem cell boost (SCB) for poor graft function following transplantation. We used a logistic regression model to analyze various factors, including age, graft source, donor type, match grade, CMV seropositivity, acute and chronic graft vs host disease (GVHD), conditioning regimens, GVHD prophylaxis, DIPSS, HCT-CI score, and mutations in JAK2, MPL, and CALR. Results: Six of the 41 patients (15%) received a SCB because of poor engraftment. This rate is significantly higher compared to the 1.4% of acute myeloid leukemia/acute lymphoblastic leukemia (AML/ALL) patients and 3.8% of myelodysplastic syndrome (MDS) patients who required an SCB at our institution. The most common reason for SCB was for pancytopenia in 5 patients and bicytopenia (anemia and thrombocytopenia) in 1 patient. All six MF patients receiving SCB had originally received a peripheral blood stem cell graft, and all had received a JAK2 inhibitor prior to transplant. The median time from transplant to SCB was 112 days. However, we did not identify any statistically significant predictors of the need for an SCB. Two year overall survival was 83% in the SCB group vs 54% in the non-SCB group (p=NS). Infection was the primary cause of non-relapse mortality among patients receiving SCB. We also found that the SCB group had numerically lower rates of acute GVHD but had slightly higher rates of cGVHD (p=NS). Conclusions: We acknowledge that our analysis is limited by the small sample size. However, despite the challenges of poor graft function in MF patients undergoing HSCT, our findings suggest that favorable outcomes are possible if hematopoietic and immune reconstitution is successfully achieved following SCB.