GM-CSF enhances sensitivity of leukemic clonogenic cells to long-term low dose cytosine arabinoside with sparing of the normal clonogenic cells.

Leukemic clonogenic cells (CFU-L) and normal myeloid progenitor cells (CFU-GM) were exposed to Ara-C in the presence of crude CSF obtained from placentas (HPCM) or recombinant human GM-CSF for varying periods. The cytotoxicity of Ara-C to CFU-L increased considerably when the exposure time to Ara-C in the presence of HPCM was extended from 20 hours to 10 days. The ID50 of the CFU-L was 1.5 +/- 2.2 x 10(-8) M Ara-C compared to 5.5 +/- 2.9 x 10(-8) M Ara-C for the CFU-GM after an exposure to Ara-C for 10 days (p less than 0.05). Replacement of crude CSF from placenta conditioned medium by rh GM-CSF resulted in identical observations. Interesting was the observation that secondary leukemic colony forming cells were more or at least equally sensitive to Ara-C in the presence of GM-CSF when compared to the primary leukemic clonogenic cells. This contrasted the secondary normal CFU-GM, which were less sensitive to Ara-C than the primary CFU-GM. This indicates that GM-CSF induces leukemic clonogenic cells with selfrenewal capacity into proliferation, and in doing so, it may enhance the cytotoxicity of a cell cycle specific drug like Ara-C with sparing of the normal clonogenic cells.