Outcomes of Haploidentical Salvage Transplantation Using Post-Transplant Cyclophosphamide for Graft Failure Following Allogeneic Hematopoietic Stem Cell Transplantation

Introduction: Graft failure (GF) is a fatal complication after allogeneic stem cell transplantation (SCT). Although salvage SCT is the only curative therapy for GF, optimal donors and strategies for this procedure have not yet been established. Although in the last decade haploidentical donors have emerged as alternative donors, only limited data are available regarding the outcomes after haploidentical salvage SCT using post-transplant cyclophosphamide (PTCy). Therefore, this nationwide retrospective study aimed to evaluate the transplant outcomes and risk factors for survival after haploidentical salvage SCT using PTCy on behalf of the Transplant Complications Working Group of the Japan Society for Transplantation and Cellular Therapy. Methods: Clinical data were provided by the nationwide database of the Japanese Data Center for Hematopoietic Cell Transplantation. Patients who were diagnosed with GF and underwent a second or higher allogeneic SCT from the haploidentical related donor (≥2 antigen-mismatch), using PTCy as graft-versus-host disease (GVHD) prophylaxis, between 2011 and 2019 were included. Organ failure was defined as either ejection fraction ≤50%, serum creatinine ≥2 mg/dL, bilirubin ≥1.5 × upper limit of normal, or aspartate aminotransferase/alanine aminotransferase ≥2.5 × upper limit of normal. Overall survival (OS) probabilities were estimated using the Kaplan-Meier method and differences among groups were analyzed using the log-rank test. The multivariate analysis for OS was performed using the Cox proportional hazard regression model. Factors from the univariate analysis that demonstrated significance with P values < 0.1 were included in the multivariate analysis. Results: A total of 33 patients were included in the study. The median age was 34 years (range, 2-67), while performance status (PS) was 0-1 in 21 patients (64%). At salvage transplantation, 12 (36%) were receiving treatment for active infection, and 5 (15%) had organ failure. The median interval from SCT to salvage SCT was 49 days (range, 26-1,468), and 21 patients (68%) underwent salvage SCT within 100 days after previous SCT. Conditioning regimens consisted of fludarabine (Flu)/ cyclophosphamide (Cy)-based in 10 (31%), Flu/melphalan (Mel)-based in 10 (31%), and Flu/busulfan (Bu)-based in 7 (21%). The total dose of PTCy was 75-100 mg/kg in 26 patients (84%) and 40-50 mg/kg in 5 patients (16%). Most patients (84%) received tacrolimus plus mycophenolate mofetil as GVHD prophy