Tacrolimus and Sirolimus without Methotrexate as Acute GVHD Prophylaxis after Matched Related Donor Reduced Intensity Conditioning (RIC) Stem Cell Transplantation (SCT).

We have previously shown that the combination of tacrolimus and sirolimus (TAC/SIR) without methotrexate (MTX) is a safe and effective regimen for GVHD prophylaxis after myeloablative SCT. However, TAC/SIR has not been investigated in the RIC setting. We hereby report a prospective phase II trial testing TAC/SIR as GVHD prophylaxis after RIC SCT from matched related donors (MRD). All patients received fludarabine (FLU) 30 mg/m² IV and intravenous busulfan (BU) 0.8 mg/kg IV daily × 4 days (day -5 thru day -2) as conditioning, followed by transplantation of filgrastim mobilized peripheral blood stem cells. Filgrastim 5 mcg/kg SC QD was started on day +1 until neutrophil engraftment. Tacrolimus and sirolimus were started on day-3, and doses were adjusted to maintain target serum trough levels 5–10 ng/ml and 3–12 ng/ml, respectively. Twenty-six patients have been transplanted, with a median age of 52 years (range 29–64 yrs). Diagnoses include NHL (8), AML (6), HD (5), CLL/SLL (3), CML (2), MDS (1), MM (1). Median CD34+ cells infused was 8.06 × 10⁶ cells/kg (range: 2.96–38.0 × 10⁶ cells/kg). All patients had sustained hematologic engraftment. Only 4 (15%) patients developed neutropenia below ANC 500, and 6 (23%) had platelet counts below 20K. One patient had late graft failure at 7 months post transplant. Grade II-IV acute GVHD incidence was 20%, with grade III-IV incidence of 12%. No hepatic VOD or thrombotic microangiopathy was observed. Day +100 transplant related mortality (TRM) was 0%. The cumulative incidences of TRM and relapse at 1 year were 4% and 38%, respectively. The cumulative incidence of chronic GVHD at 1 year was 74%. A high level (>90%) of donor-derived hematopoiesis was achieved in 68% by 1 month post transplant. Median donor chimerism at between day 20–50 post transplant was 93.5% (range 32%-99%), and 94% (range 22%-100%) at day 85–115. With a median follow-up of 13.5 months among survivors (range 5.5–19 months), progression-free survival (PFS) and overall survival (OS) at 1 year were 58% and 79%, respectively. Compared to a historical cohort of 47 MRD transplant recipients treated with the same FLU/BU conditioning regimen and using TAC/SIR ± mini-MTX (5 mg/m² IV day +1,3,6) as GVHD prophylaxis, there was no statistical difference between TAC/SIR vs. TAC/SIR/MTX in the incidence of grade II-IV acute GVHD (20% vs 11%, p= 0.48), cumulative incidence of relapse at year (38% vs. 57%, p= 0.25), 1-yr PFS (58% vs. 41%, p= 0.33), or 1-yr OS (79% vs. 73%, p= 0.79). These results demonstrate that omission of mini-MTX is permissible, and that the combination of TAC/SIR alone as GVHD prophylaxis following reduced intensity conditioning with FLU/BU is associated with low rates of acute GVHD, TRM, and high levels of donor chimerism after MRD SCT.