Low Incidence of Chronic Graft-Versus-Host Disease in Myeloablative Allogeneic Hematopoietic Cell Transplantation with Post-Transplant Cyclophosphamide Using Matched Related or Unrelated Donors: Phase II Study Interim Analysis

Introduction Allogeneic hematopoietic cell transplantation (HCT) is the only potentially curative treatment for many high-risk hematologic malignancies. Myeloablative conditioning is currently the standard of care for young and fit patients; however, graft-versus-host disease (GVHD) continues to be a major limitation to the success of HCT, increasing post-transplant morbidity and mortality. An ideal HCT is one combining strategies that reduce incidence and severity of GVHD, without compromising graft-versus-tumor effect. We hypothesized that GVHD prophylaxis regimen consisting of post-transplant cyclophosphamide (PTCy), tacrolimus (Tac) and mycophenolate mofetil (MMF) will reduce the incidence of chronic GVHD in patients receiving a standard hematopoietic myeloablative HCT without an increase in risk of malignant relapse. Methods This is an interim analysis of a phase II study using a myeloablative preparative regimen of either: 1. total body irradiation (TBI, total dose 1320 cGy administered twice a day from days -4 to -1) or 2. Busulfan 3.2mg/kg daily (cumulative AUC 19,000 - 21,000 µmol/min/L) plus fludarabine 160mg/m 2 days -5 to -2 for patients unable to receive further radiation, followed by a GVHD prophylaxis regimen of PTCy (50mg/kg days +3 and +4), Tac and MMF (beginning day +5). The primary endpoint is cumulative incidence of chronic GVHD requiring systemic immunosuppressive treatment at 1 year post-transplant. Patient and disease characteristics are detailed in Table 1. Eligibility included: age ≤ 60 years, malignant or non-malignant diagnosis, matched related (MRD) or unrelated (MUD) donor with either a bone marrow (BM) or filgrastim-mobilized peripheral blood (PB) graft. Results Through October 2020 we treated 63 patients with a median follow up of 502 days post-transplant. Of those, 48% were female and n=11 (17%) younger than 18 with median age at HCT of 36 years (range, 2-55; Interquartile range [IQR], 20-48). Donor source was 8/8 MRD in 44 patients (70%), 8/8 MUD in 18 (29%), and one with 7/8 MUD. Graft source was BM in n=28 (44%) and PB in n=35 (56%). Preparative regimen was TBI in 94% of patients. All patients achieved primary neutrophil engraftment by 42 days, median 16 days (range, 13-27). Overall, 94% achieved platelet engraftment by 6 months, median 25 days (range, 16-98). At day 100, 48 patients (86%) achieved full donor bone marrow chimerism (>95% donor DNA); 29 (52%) and 52 (95%) achieved full donor peripheral blood CD3 and CD33 ch