Effect of altering administration order of busulphan and cyclophosphamide on the myeloablative and immunosuppressive properties of the conditioning regimen in mice.

Objective: The present study was designed to investigate the influence of the administration sequence of busulphan (Bu) and cyclophosphamide (Cy) during conditioning regimen on myeloablative and immunosuppressive effects and on engraftment.

Methods: Female Balb/C mice were treated with either Bu-Cy or Cy-Bu (assigned order of administration). Bu was administered as 8.75 mg/kg/day x 4 and Cy as 100 mg/kg/day x 2. The control consisted of untreated animals. Bone marrow and spleen were harvested during the conditioning regimen and for up to 19 days after treatment. Colony-forming unit granulocyte macrophage assay was performed on marrow cells. Immunological analyses were performed using spleen cells. Liver status was determined using aspartate amino transferase (AST), alanine amino transferase (ALT), and bilirubin. Animals assigned for engraftment study were conditioned as above and transplanted using sca-1 cells from male Balb/C donors. Engraftment was followed using fluorescence in situ hybridization up to 30 days posttransplantation.

Results: No significant difference in myeloablative effect was observed between treatments. Immunosuppressive activity expressed as CD3+/CD19+ and CD4+/CD8+ was also similar. Levels of cytokines interleukin 2, tumor necrosis factor alpha, and interferon gamma at the end of the conditioning regimen were lower in the Cy-Bu group, while liver enzymes were higher after the Bu-Cy regimen. Engraftment in bone marrow was reached faster within the first 20 days after conditioning with Cy-Bu compared to Bu-Cy. However, no difference in chimerism was observed at 30 days.

Conclusion: Cy-Bu treatment resulted in lower levels of cytokines, faster bone marrow engraftment, and lower values of liver enzymes compared to Bu-Cy regimen, which may benefit stem cell transplantation outcomes.