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Despite of the continuous increase in the number of volunteer donors available through the registry, many patients who require an allogeneic hematopoietic cell transplantation (HCT) cannot find a fully-matched donor. While a mismatched unrelated donor (MMUD) is frequently available, it is associated with inferior outcomes and increased risk of graft-versus-host disease (GvHD). Post-transplant cyclophosphamide (PTCy) has been effective in haploidentical HCT, and increasingly used in matched donor HCTs. However, limited data exist in MMUD setting. We conducted a prospective single center trial (NCT 03128359) of PTCy for MMUD HCT with the primary objective of estimating 1-year GvHD-free relapse/progression-free survival (GRFS). As of October 2019, all planned 39 patients have been enrolled with a median follow up of 11 months (range: 1-23). Here we present the preliminary estimate of 1-year GRFS and other HCT outcomes in two strata; myeloablative conditioning (n=19) using Fludarabine (90 mg/m2) and FTBI (1200 cGy) or reduced intensity conditioning (n=19) using Fludarabine (100 mg/m2) and Melphalan (140 mg/m2 or 100 mg/m2 if >60 years old). Patients between 0 to 75 years of age and KPS of ≥70% with hematologic malignancies undergoing HCT from a 7/8 HLA-matched (A-, B-, C-, and DR-) donor were eligible. Patients with donor specific antibodies to the mismatched HLA-locus were excluded. All patient received PBMCs (3-5 × 106/kg) followed by GVHD prophylaxis consisting of PTCy (50 mg/kg for 2 days), Tacrolimus (1 mg), and mycophenolate mofetil (1 gr 3 × a day). Median age at the time of HCT was 53 years (range: 21-72), and 50% of patients were male. Disease risk was low in 47% (n=18), intermediate in 37% (n=14), and high in 16% of the patients (n=6). At transplant, 29 patients were in complete remission, and 9 had active disease. HCT-CI was 0 in six (16%) and 1-2 in 15 (39%) and >2 in 17 (45%) patients. Donors' median age was 32 years (range: 19-53) and donors were mismatched at HLA-A (n=14), -B (n=12), -C (n=8), or DR-loci (n=5). Median number of mismatches was 2 of 12 (range: 1-4). Female to male donor HCT was in 11% of recipients. Neutrophil engraftment occurred in all patients (median time to engraft: 16 days; range 13-35). One-year overall survival (OS) and GRFS were 92% (95% CI: 70-98) and 70% (95% CI: 51-83), respectively. Non-relapse mortality and relapse rate at 1 year were at 8% (95% CI: 2-29) and 13% (95% CI: 5-34), respectively. Cumulative incidence of day 100 acute GvHD grade 2-4 was 50% (95% CI: 35-71) and 1-year chronic GvHD was 56% (95% CI: 39-81). No severe chronic GvHD by the NIH criteria was observed. In conclusion, the data from our phase II trial of PTCy showed highly promising OS/GRFS in patients receiving 7/8 MMUD HCT, and that PTCy in MMUD setting offers an alternative and effective HCT approach for patients who do not have an available matched donor. [ABSTRACT FROM AUTHOR]