Sirolimus, Tacrolimus and Reduced-Dose Methotrexate as Graft Versus Host Disease (GVHD)Prophylaxis after Non-Myeloablative Stem Cell Transplantation: Low Incidence of Acute GVHD Compared with Tacrolimus/Methotrexate or Cyclosporine/Prednisone.

Sirolimus (rapamycin) is a macrocyclic lactone similar in structure to tacrolimus and cyclosporine (CSA) but with a distinct mechanism of action. Sirolimus binds to both FKBP12 and mTOR and inhibits signal transduction and cell cycle progression. The drug is synergistic with tacrolimus but has a distinct toxicity profile, thereby allowing their use in combination. We report results of a phase II trial combining sirolimus with tacrolimus and low-dose methotrexate (MTX) as GVHD prophylaxis in matched related and unrelated donor NST. Results of this trial were compared with patients who had previously undergone NST receiving tacrolimus/MTX alone or CSA/prednisone(pred). All patients received fludarabine (30 mg/m2/d x 4days) and intravenous busulfan (0.8mg/kg/d x 4 days) as conditioning. All patients received G-CSF mobilized peripheral blood stem cells with a targeted cell dose of 1 x 10⁷ CD34+ cells/kg. G-CSF 5 mcg/kg was started on day 1. Sirolimus containing GVHD prophylaxis included sirolimus 12 mg loading dose on day −3 and then 4 mg po qd targeting a serum level of 3–12 ng/ml. Tacrolimus was initiated at 0.05 mg/kg po b. i.d. beginning day −3 with a targeted serum level of 5–10 ng/ml. MTX (5 mg/m2) was given days, 1, 3 and 6. Planned taper of the GVHD medications was ~30% at days 60, and 120 with discontinuation by day 180. The median follow up is 14 months for patients receiving sirolimus and all evaluable patients have been followed for >100 days. 40 patients have been transplanted, 20 from related and 20 from unrelated donors. The median age was 57 years (range 20–69). Diseases included: NHL (9), MDS (7), Hodgkin’s disease (6), CLL (6), AML(5), CML (5), MM (1) and CMML(1). 18 patients (45%) had received prior myeloablative transplantation. 31 patients (78 %) had advanced disease at the time of transplantation. Patients receiving tacrolimus/MTX (n=36) and CSA/pred (n=49)had similar characteristics. Sirolimus was well tolerated and no severe adverse events related to the drug were noted. Acute grade 2–4 GVHD was significantly reduced in patients receiving sirolimus/tacrolimus/MTX, 8% vs 18% in patients receiving tacrolimus/MTX and 37% in those receiving CSA/Pred (p=0.003). Time to neutrophil engraftment was slower in methotrexate containing regimens (13 days vs 9 days, p=0.01), but there was no difference between sirolimus/tacrolimus/MTX and tacrolimus/MTX alone. Median donor derived hematopoiesis, measured 1–2 months after transplant, was high in all groups (sirolimus/tacrolimus/MTX 91%, tacrolimus/MTX 95% and CSA/pred 90%, p=0.91). The 1 year overall survival was sirolimus/tacrolimus/MTX 71%, tacrolimus/MTX 48% and CSA/pred 45% (p=0.17). 1-year progression free survival was 49%, 27% and 37%, respectively (p=0.11). The addition of sirolimus to tacrolimus and low dose MTX is well tolerated and is associated with a low incidence of acute GVHD. The addition of sirolimus does not delay engraftment compared with tacrolimus/MTX and results in a similar high level of donor derived hematopoiesis. Further patient accrual and longer follow-up is needed to yield information on the incidence of chronic GVHD and overall outcome.