Post-transplant Cyclophosphamide (PTCy) for HLA-Matched Hematopoietic Stem Cell Transplant (HCT) in Pediatric and Young Adult Patients with Hematologic Malignancies.

High-dose post-transplant cyclophosphamide (PTCy) for graft versus host disease (GVHD) prophylaxis after HLA-matched allogeneic hematopoietic cell transplantation (HCT) in adults with hematologic malignancies is standard of care when combined with tacrolimus and mycophenolate mofetil (MMF) after reduced intensity conditioning (RIC). However, use of PTCy after HLA-matched HCT in pediatrics is not well described. To show that PTCy allows for successful engraftment, survival, and low rates of GVHD, a retrospective analysis of patients ≤21 years treated at Johns Hopkins (n = 22) who underwent HLA-matched related (MSD, 73%) or unrelated HCT (MUD, 27%) between 2013 and 2023 was performed. GVHD prophylaxis included PTCy alone (n = 15) or a priori with tacrolimus and MMF for peripheral blood stem cell grafts (n = 5) and/or RIC (Flu/Cy/200cGy TBI, n = 4). Median patient age was 16.5 years and diagnoses included AML [CR1 = 4, CR≥2 = 4; minimal residual disease (MRD) = 2], ALL [CR1 = 4, CR≥2 = 6; MRD = 2], and MDS [1 treated, 3 untreated]. Evaluable patients had full donor chimerism at day +60. Median time to neutrophil and platelet recovery was 18 and 19.5 days. Two developed acute GVHD (aGVHD) Gr IV after myeloablative MUD bone marrow HCT with PTCy alone and both died; no chronic GVHD (cGVHD). Cumulative incidence of relapse at 3y was 48%, 38% for MRD- pre-HCT. With a median follow-up of 1883 days, actuarial OS is 67%, EFS is 43%, and GRFS is 43% at 3 years. Our observations add to limited published pediatric data that PTCy is feasible for HLA-matched HCT with 100% engraftment, no aGVHD with MSD, and no cGVHD. Additional prospective data needed.