A Phase II Clinical Trial of Post-Transplant Cyclophosphamide for Graft Versus Host Disease Prevention Following Myeloablative Peripheral Blood Stem Cell Matched Unrelated Donor Hematopoietic Stem Cell Transplantation

Background: The use of post-transplant cyclophosphamide (PT-Cy) in combination with other immunosuppressive agents for graft versus host disease (GVHD) prophylaxis in matched unrelated donor (MUD) allogeneic hematopoietic stem cell transplantation (HSCT) is not well defined. PT-Cy mitigates GVHD after T-cell replete HLA haploidentical (Haplo) bone marrow transplant. Extrapolating from the success of PT-Cy in haplo transplants, we investigated the benefit of PT-Cy (at a lower dose than that used in haplo HSCT) in preventing GVHD following myeloablative peripheral blood stem cell (PBSC) MUD HSCT.

Methods: We conducted a phase II clinical trial between September 2013 and June 2018 of PT-Cy for GVHD prophylaxis following myeloablative MUD HSCT. GVHD prophylaxis consisted of 1 dose of PT-Cy 50mg/kg on day +3, mycophenolate mofetil starting day +5 till day +35 and tacrolimus starting on day +5 with taper starting at day +100. The primary endpoint of the study was to determine the incidence of grade II-IV acute GVHD. Secondary endpoints included overall survival (OS), disease free survival (DFS), non-relapse mortlity (NRM) and time to engraftment. Approval for the study was obtained from the Institutional Review Board.

Results: There were 39 patients enrolled in the study (Table 1). The mean age of the study population was 47.36 years (SD 13.41). There were 23 females (59%) and majority of the patients were white (85%). Thirty six donors were 8/8 HLA MUD and 3 were 7/8 matched unrelated. The stem cells were collected from peripheral blood in all cases. Indications for HSCT included AML/MDS (62%), ALL (15%), myelofibrosis (10%), NHL/HL (10%) and CML (3%). Based on disease risk index (DRI) assessment, 6 (15%) patients were DRI high, 32 (82%) were intermediate and 1 (3%) was low. The most commonly used myeloablative conditioning regimen was Busulfan and Fludarabine (74%). All 6 patients with ALL received TBI (12 Gy) based conditioning. Three patients with NHL received Fludarabine and Melphalan.

There was 1 death within the first 30 days before engraftment. The remaining 38 patients (97%) successfully engrafted. The median time to ANC engraftment was 12 days (range 9-14 days). The incidence of day 100 acute GVHD maximum grade was 36% for grade I/II and 5% for grade III/IV (Table 2). The overall incidence of limited and extensive chronic GVHD was 10% and 36% respectively. There were 8 (21%) confirmed relapses within the first year after HSCT. Twenty four patients were alive at the 1-year mark after transplant and 17 of them were in complete remission. The 1-year and 2-year OS rates were 61.5% and 51.2% respectively. The median OS for the entire cohort was 21.2 months with a median follow up of 50 months (Figure 1). The day 100, 1-year and overall NRM rates were 10%, 28% and 33% respectively with infectious complications being the most common cause of death.

Conclusion: We report a low incidence of acute severe GVHD with the combination of one dose of PT-Cy in combination with MMF and tacrolimus following myeloablative PBSC MUD HSCT. The single dose of PT-Cy may explain the modest control over chronic GVHD with this regimen. We also report favorable survival outcomes along with acceptable levels of NRM for the entire cohort. The use of PT-Cy in combination with other immunosuppressant agents for GVHD prevention appears to be a promising strategy in MUD HSCT and may play a vital role in mismatched unrelated donor transplants as well.

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