Pf784 comparison of alemtuzumab, anti-thymocyte globulin and post-transplant cyclophosphamide for acute graft-vs-host disease and graft-vs-leukemia effect in xenogeneic hematopoietic cell transplantation

Background: Hematopoietic stem cell transplantation is a curable treatment for refractory hematopoietic malignancies. However, the alloimmune response provokes graft‐versus host disease (GVHD). Prophylaxis against GVHD in HLA‐mismatched transplantation includes immunosuppressive drugs, such as anti‐thymocyte globulin (ATG), alemtuzumab, or post‐transplant cyclophosphamide (PTCY). Yet, attenuation of graft versus leukemia (GVL) effect is a major problem associated with these immunosuppressants. It is difficult to compare these immunosuppressants in human clinical trials Aims: We sought to clarify whether ATG, alemtuzumab or PTCY could suppress GVHD without reducing GVL using xenogeneic GVHD/GVL models. Methods: In the xenogeneic GVHD model, human peripheral blood mononuclear cells (hPBMC) were injected into irradiated NOG mice on the day of transplantation. To prevent xenogeneic GVHD, each group of mice was treated with ATG (5 mg/kg for 2 days), alemtuzumab (0.25 mg/kg for 2 days) or PTCY (50 mg/kg for 2 days). For the xenogeneic GVL model, A20 murine B‐cell leukemia cells were co‐transplantation with hPBMC in the xenogeneic GVHD model. Tumor growth were detected using IVIS imaging system. Results: All mice in the control GVHD group died within 25 days after transplantation, whereas none in the TBI‐alone group died during this time period (Figure 1). Alemtuzumab treatment prolonged survival despite considerable number of invaded human cells and the high level of serum IFN‐γ. Moreover, alemtuzumab group showed an apparent decrease of Tregs. The numbers of T and B cells and serum IFN‐γ level were markedly reduced in the PTCY groups compared to the GVHD group. PTCY treatment did not affected the proportion of Tregs. However, the improvement of survival with PTCY treatment remained minimal. ATG treatment diminished GVHD symptoms, completely depleted infiltration of inflammatory cells in the tissues, and led to prolonged survival. Moreover, the serum IFN‐γ level was also significantly decreased in the ATG group. However, these findings might have strongly depended on the doses of the drugs rather than their properties, and the doses used clinically for humans might not be appropriate in this mouse model. Therefore, we determined the optimal dosage of immunosuppressants for this mice model. The optimal dose of immunosuppressant is the dose at which GVHD is prevented without reducing GVL. We used xenogeneic GVL model bearing luciferase‐transfected A20 leukemia cells. Without co‐infusion of hPBMC, A20 leukemia mice died about 30 days after transplantation with a marked increase in luciferase activity, whereas A20 leukemia mice co‐transplanted with hPBMC (GVL model) exhibited the complete eradiation of A20 cells, but died with GVHD (Figure 2). We allocated the GVL mice into several groups with different treatment dosages of alemtuzumab, ATG and PTCY. Eventually, we found that ATG at 1.5 to 2.0 mg/kg and alemtuzumab at 0.6 to 0.9 mg/kg reduced GVHD with minimal loss of GVL effect. Mice treated with PTCY at 400 mg/kg did not develop either GVHD or leukemia, but it was difficult to evaluate the GVL effect due to the sensitivity of A20 cells to cyclophosphamide. Summary/Conclusion: The therapeutic windows for ATG and alemtuzumab appeared to be very narrow. PTCY 400 mg group showed long survival without leukemia or GVHD, but this result does not mean that PTCY at 400 mg/kg prevents GVHD while maintaining a GVL effect because of the cytotoxicity of cyclophosphamide. Further studies are warranted to maximize the benefit of each immunosuppressant. image