Three-Year Outcomes in Recipients of Mismatched Unrelated Bone Marrow Donor Transplants Using Post-Transplantation Cyclophosphamide: Follow-Up from a National Marrow Donor Program-Sponsored Prospective Clinical Trial

The use of Post-Transplant Cyclophosphamide (PTCy) as Graft Versus Host Disease (GVHD) prophylaxis has resulted in reductions in GVHD and improved outcomes in allogeneic hematopoietic cell transplant (HCT) using HLA-mismatched related donors. We report the 3-year outcomes of the first multi-center prospective clinical trial using PTCy in the setting of mismatched unrelated donor (MMUD) bone marrow HCT. The study enrolled 80 patients (Either myeloablative (MAC) (N=40) or reduced intensity conditioning (RIC) (N=40)) with the primary endpoint of 1-year overall survival (OS). The median follow-up for this report is 34 months (range 12-46) in RIC and 36 months (range 18-49) in MAC. Three-year OS and non-relapse mortality (NRM) were 70% and 15%, and 62% and 10% in the RIC and MAC strata, respectively. No GVHD was reported after 1 year. Relapse incidence was 29% and 51% in RIC and MAC strata. OS did not differ based on HLA match grade (63% in the 7/8 strata and 71% in the 4-6/8 strata). These encouraging outcomes, sustained 3 years post-HCT, support the continued exploration of MMUD HCT using a PTCy platform. Important future areas to address include relapse reduction and furthering our understanding of optimal donor selection based on HLA and non-HLA factors.