Marrow sensitization to 5-fluorouracil using the ligands for Flt-3 and c-Kit

In vivo administration of c-kit ligand (KL) expands early hemopoietic progenitors and stem cells and sensitizes clonogenic progenitors to 5-FU-mediated cell death. Studies were performed to determine whether the in vivo administration of Flk-2/Flt-3 ligand (FL) is also capable of sensitizing progenitors to 5-FU. Mice were treated with FL (100 micrograms/kg every 12 hours for a total of 3 doses), KL (50 micrograms/kg, same schedule) or both, either alone or in combination with 5-FU (a single 125 mg/kg injection 3 hours before the last dose of cytokine). Femurs and spleens were harvested 48 hours following the last dose of cytokine, and the total numbers of mononuclear cells and colony forming unit cells (CFU-C) per femur and spleen were determined. Statistically significant increases in the number of CFU-C per femur were observed in response to FL, KL and FL + KL. In the spleen, statistically significant increases in CFU-C were observed only with the FL + KL combination. 5-FU alone produced marked reductions in CFU-C both in the femur and in the spleen. In the femur, 5-FU-mediated reductions in CFU-C were enhanced 3- to 30-fold in the presence of concomitant KL, FL or KL + FL administration. Surprisingly, the combination of KL + FL was no more effective in sensitizing marrow CFU-C to 5-FU than was KL alone, suggesting that CFU-C that are capable of surviving the KL/5-FU combination cannot be driven into cell cycle by FL. The effects of concomitant cytokine/5-FU administration in the spleen contrasted sharply with those observed in the femur, as FL, KL and FL + KL all failed to enhance 5-FU-mediated reductions in CFU-C. The ability of FL + KL to stimulate CFU-C expansion in the spleen combined with the inability of this cytokine combination to augment 5-FU-mediated progenitor toxicity in the spleen supports the contention that cytokine-mobilized progenitors are not in cycle. FL's capacity to specifically sensitize marrow to the effects of cytotoxic drugs may have applications in bone marrow transplant conditioning regimens.