Low Dose Rabbit Antithymocyte Globulin Plus Low-dose Post-transplantation Cyclophosphamide as GVHD in Hapl-HSCT for Patients With Hematologic Malignancies: a Phase II Trial

A novel regimen, which is composed of a low dose of ATG (5 mg/kg) and low-dose PTCy (one dose of PTCy, 50mg/kg) for GVHD prophylaxis in haplo-HSCT for patients with hematologic malignancies, is going to be evaluated in the prospective, randomized control, phase II clinical trial. It is theoretically feasible because the mechanisms of ATG and Cyclophosphamide on T lymphocyte are different. ATG plays the immunosuppressive activity on the depletion of peripheral T lymphocytes through complement dependent lysis or activation-associated apoptosis. ATG also modulates cell-surface expression of surface adhesion molecules or chemokine receptors. In addition, ATG can also affect or interfere with the function of different immune cells such as B lymphocytes, regulatory T lymphocytes (Treg), natural killer (NK)-T lymphocytes and dendritic cells (DC).Cyclophosphamide is nontoxic to hematopoietic stem cells and can selectively deplete the alloreactive T cells. Therefore, we hypothesis that ATG followed by PTCy have the synergistic effect on GVHD prophylaxis. (2) Luznik et al also showed that there was no difference in the incidence of severe acute GVHD between one or two doses of PTCy. Furthermore, there was a trend toward a lower incidence of extensive cGVHD among patients of two doses of PTCy compared with one dose PTCy. One dose of PTCy might preserve the GVL effect without influencing the incidence of the severe aGVHD. (3) Y Wang et al reported a randomized clinical trial comparing two different doses of ATG (6 and 10mg/kg) as GVHD prophylaxis for Haplo-HSCT. There was no difference in the median myeloid and platelet engraftment time and the rate of graft failure. The results showed that the incidence of grade III-IV acute GVHD was higher in the ATG-6 group than in the ATG-10 group. But the EBV reactivation occurred more frequently in the ATG-10 group than in the ATG-6 group. The higher rate of infection and NRM may influence the transplant outcomes in this GVHD prophylaxis strategy. We speculated that low dose of ATG (5 mg/kg) will ensure the engraftment and decrease infection frequency. But the optimal timing of ATG administration still needed to be considered. The immunosuppressive activity of ATG is not only dose-dependent but also rely on the timing of drug administration, especially when the lower dose of ATG was used as GVHD Prophylaxis. So we designed a randomized control phase II study to evaluate the efficacy and toxicity with low dose ATG followed by low dose PTCy as GVHD prophylaxis .