A Phase II Trial of Post-Transplant Cyclophosphamide As Graft-Versus-Host Disease Prophylaxis in HLA-Mismatched Unrelated Donor Hematopoietic Cell Transplantation

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.