Post-transplant cyclophosphamide-based graft-versus-host disease prophylaxis following mismatched unrelated donor peripheral blood stem cell (PBSC) transplantation.

6503 Background: Our prior prospective multi-center clinical trial (15-MMUD; NCT02793544) estimated 79% overall survival (OS) at one year in adult patients with hematological malignancies undergoing reduced intensity conditioning (RIC), receiving bone marrow (BM) grafts from HLA-mismatched unrelated donors (MMUD), and post-transplant cyclophosphamide (PTCy)-based graft- versus-host disease (GVHD) prophylaxis (Shaw et al, J Clin Oncol 2021). We sought to determine whether OS in adults receiving mobilized PBSC from MMUD would be comparable. Methods: We conducted a prospective, multi-center Phase II study (ACCESS; NCT04904588) to assess the impact of PTCy-based GVHD prophylaxis on OS following MMUD transplantation in adults and children with advanced hematological malignancies. The study included three strata: two for adults based on conditioning regimen intensity myeloablative (MAC) or RIC using PBSC, and one pediatric cohort using MAC and BM. Eligible donors were matched at 4-7/8 HLA-loci (HLA -A, -B, -C, and -DRB1) and < 35 years. The primary endpoint was OS at one year following transplantation. We report the results of a planned interim analysis of the first 70 adult patients enrolled on the RIC PBSC stratum. Results: Thirteen sites enrolled 70 patients with the following demographics: median age of 65 years (range 24-77); 50% male; diagnoses: AML (53%), MDS (26%), ALL (7%), other heme malignancies (14%); and patient race/ethnicity: non-Hispanic white (48%). Donor characteristics included: median age 25y (range 18-35); male (44%); and HLA match level: 7/8: 67%; 6/8: 27%; 5/8: 6%. Conditioning regimens included fludarabine (flu) and melphalan (63%), flu/busulfan (20%), other (17%). OS at one year post HCT was 79% (95% confidence interval (CI): 68-87%). Secondary endpoints are provided in the table. Conclusions: Encouraging OS was observed at one year following MMUD PBSC in patients receiving RIC and PTCy. Notably, half of enrolled patients were people of color. OS was similar to our prior study using BM grafts. Rates of GVHD and other complications appear comparable to those in HLA-matched donor recipients, suggesting MMUD HCT could expand access to a potentially life-saving therapy. Accrual to both adult strata of ACCESS is complete, with over 200 patients in follow-up. Accrual to the pediatric stratum continues. Clinical trial information: NCT04904588 . Table: see text