Calcineurin Inhibitor-Free GVHD Prophylaxis with Post-Transplant Cyclophosphamide and Brief-Course Sirolimus Results in Low Rates of Non-Relapse Mortality and Chronic GVHD Following Matched Related and Unrelated Donor Peripheral Blood Stem Cell Transplantation (PBSCT)

Calcineurin inhibitors (CNIs) form the foundation of current GVHD prophylaxis regimens. Although advances in immunosuppressive regimens have had a significant impact on the incidence and severity of acute GVHD, it is noteworthy that CNIs have had little impact on chronic GVHD. We hypothesized that a CNI-free regimen consisting of post-transplant cyclophosphamide (Cy) and brief-course sirolimus would decrease the risk of chronic GVHD and non-relapse mortality and improve outcomes following reduced intensity allogeneic PBSCT. Twenty-seven patients with high risk hematologic malignancies were enrolled in the study: median age 61 years (25-73). All patients had a 10/10 locus matched donor; MRD=18, MUD=9. Conditioning consisted of fludarabine 30mg/m2 on days -9 to -6, IV busulfan 130 mg/m2 on days -5 to -4, and Cy 14.5 mg/kg on days -3 and -2 followed by unmanipulated PBSCT. Post-grafting immunosuppression consisted of Cy 50 mg/kg/day on days 3 and 4 and sirolimus starting day +5 and completing day+90 in the absence of GVHD. Donor engraftment occurred in all patients with a median time to neutrophil and platelet recovery of 15 and 30 days, respectively. The median day +90 donor T cell and myeloid chimerism was 94% (40-100%) and 100% (11-100%) respectively. Three patients received donor lymphocyte infusions for incomplete donor T cell chimerism. The cumulative incidence of grade II-IV acute GVHD, grade III-IV acute GVHD, all chronic GVHD, and severe chronic GVHD was 41%, 15%, and 32%, and 12% respectively. Non-relapse mortality (NRM) and relapse incidence at 2 years was 4% and 17% respectively. With a median follow-up of 18 months, the estimated 2 year overall and disease-free survival was 71% and 80% respectively for the whole cohort, while it was 87% and 89%, respectively in the subgroup of 18 patients receiving MRD transplants. Good immune reconstitution was evidenced by low cytomegalovirus reactivation rates, occurring in only 4 of 19 at-risk patients (21%). Transplant-related toxicity included BK virus-associated cystitis in 33% of patients and a non-fatal hepatotoxicity syndrome in three patients consisting of transaminase elevation and ascites, with resolution following discontinuation of sirolimus. CNI-free GVHD prophylaxis with post-transplant Cy and brief-course sirolimus achieves consistent donor engraftment, low rates of GVHD and NRM, and excellent outcomes in recipients of HLA-identical donor allogeneic PBSCT.