Influence of dose and duration of exposure on the cytotoxic effect of cytarabine toward human hematopoietic clonogenic cells.

The cytotoxic effect of cytarabine (ara-C) on the clonogenic potential of human normal and leukemic hematopoietic cells was investigated. Cells were exposed to ara-C at different concentrations and for periods of one to 120 hours and continuously. Colony growth (CFU-GM/CFU-E/BFU-E) in semisolid culture was assessed after seven and 14 days. The exposure time to ara-C appeared much more important in colony growth inhibition than the drug concentration. For instance, one-hour exposure to 10(-5) mol/L ara-C appeared not cytotoxic, while cocultivation in the presence of 10(-7) mol/L ara-C resulted in a total inhibition of colony growth. Cell fractionation with use of counterflow centrifugation allowed enrichment in subfractions for cells with different amounts of DNA. The inhibition of the clonogenic potential by ara-C was most pronounced in the cell fractions with the highest proliferation activity. Colony-forming cells, assessed after 14 days of culturing appeared less sensitive for ara-C cytotoxicity than seven-day CFUs. This means that ara-C is more cytotoxic to cycling than noncycling cells because compared to the seven-day colonies, 14-day colonies originate from the more resting, more primitive progenitor cells. The cytotoxicity of ara-C to clonogenic cells appears to be related to the proliferating and the cycling state of the progenitor cells and increases with the time of exposure. This phenomenon may be explained by the fact that ara-C is a cycle-specific drug and by the fact that the probability of cells entering the cell cycle is a time-dependent process. The results of this study indicate that development of more effective antileukemic therapy should not aim so much at very high drug levels but rather at prolonged administration of ara-C.