Incorporation of Posttransplant Cyclophosphamide (PCy) As Part of Standard Immunoprophylaxis (IP) for All Allogeneic Transplants: A Retrospective, Single Institution Study

Background: The incorporation of PCy in IP has allowed transplantation of stem cells from haploidentical (HI) family members such that nearly all patients have a potential donor. Thus far, HI stem cell transplantation with PCy appears to yield comparable results to matched unrelated (MUD) and matched sibling donors (MSD) who have been treated with conventional GVHD regimens, but with less chronic GVHD (cGVHD). Particularly in light of the low incidence of cGVHD, which has not been achieved with other IP strategies after T cell-replete products, PCy is being investigated after MUD and MSD transplantation where complications from cGVHD remain the major cause of non-relapse mortality. A recent study from the BMTCTN showed that in patients conditioned with reduced intensity regimens and who received MSD and MUD stem cells, the addition of PCy to standard IP (SIP) was superior to either bortezomib or maravoric in the composite endpoint of graft-versus-host disease-free, relapse-free survival (GRFS). However, this study did not include patients who received ablative conditioning regimens and did not report on the percentage of patients who were disease-free and off immunosuppression (DFOI) at 1 year after transplant. In the present study, we have compared our experience with the addition of PCy for essentially all allogeneic stem cell transplants treated over a 2 year period with the results of patients treated with SIP in the prior two year span. Outcomes of interest included one-year overall survival (OS) and one-year GRFS as well as the percentage of patients DFOI at one year.

Methods: With the exception of patients receiving umbilical cord blood transplants, beginning in April 2016, all but two patients who received allogeneic transplants were given mobilized peripheral blood stem cells and then treated with PCy on days +3 and +4 followed by tacrolimus and mycophenolate on day 5. In the absence of GVHD, mycophenolate was stopped at days +35-50 and tacrolimus was tapered beginning after day +100 unless there was low donor chimerism or a suspicion of relapse in which case tacrolimus could be tapered sooner. In order to have at least one-year follow-up, the last patient included in the study was treated before April 2018. During this time period, MSD were prioritized over MUD which in turn were chosen over haploidentical donors. For comparison, we looked at the prior 2 year period (2014-2016) in which patients were treated with SIP (including ATG in patients who received MUD stem cells). Because of a higher percentage of patients with an advanced disease risk index (DRI) in the years 2014-2016, we restricted our analysis in the SIP cohort to those patients with low and intermediate risk disease but included all patients in the more recent period who received PCy.

Results: There were 68 patients treated in the PCy group, including 2 patients who received PCy after HI transplants in the years 2014 and 2015. After eliminating patients with high DRI there were 40 patients in the earlier SIP cohort of patients. The resulting patient groups were similar with respect to median age (53) and diagnosis (approximately 80% of patients with AML and ALL). There was a slightly higher percentage of patients in the SIP group with hematopoietic cell transplantation-comorbidity index scores of 3 or more (52.5 vs 48.5). In the PCy group the number of patients with early, intermediate and advanced DRI were 2, 53 and 13, whereas in the (modified) SIP category 2 patients had a low DRI and 38 had intermediate DRI. In the PCy group, HI donors comprised 26.5% of the total compared to 19.1% MSD and 54.4% MUD donors. In the SIP group, MSD and MUD donors accounted for 30% and 70% of the donors. One-year percentages of OS, GRFS and DFOI were 79.4, 47.1 and 44.1 in the PCy group compared to 72, 45 and 35 in the SIP cohort. If the analysis of the PCy group is limited to the 50 patients with MSD and MUD donors (as in the SIP cohort), the one-year OS, GRFS and DFOI are 88, 52 and 52.

Conclusions: PCy in combination with SIP resulted in at least comparable results as SIP despite the inclusion of 19% of patients with a high DRI and 26.5% HI donors. The results with the addition of PCy are excellent in patients with MSD and MUD donors with more than half of the patients GRFS and DFOI at one year. Future studies on GVHD prophylaxis should report DFOI as the latter status may be the best platform for posttransplant strategies aimed at eliminating minimal residual disease and for improving QOL.