Effects of dose and duration of exposure on 5-aza-2'-deoxycytidine cytotoxicity for L1210 leukemia in vitro.

We have investigated the effects of 5-aza-2'-deoxycytidine (DAC) on the growth and clonogenic potential of L1210 leukemia in vitro. Cells were exposed to DAC (0.001-100 micrograms/ml) for periods of 1-120 hrs. Following drug removal, cell growth in suspension culture was measured for up to 7 days, and cell survival was estimated by a colony-formation assay. When cell clonogenicity was plotted against DAC concentration in log-log axis, curves for each exposure time were linear between 0.01 and 0.5 micrograms/ml of DAC, but survival leveled off to a constant percentage for drug concentrations greater than 0.5 micrograms/ml. Percent survival decreased as exposure time increased up to 24 hrs; however, increases in exposure time greater than 24 hrs did not consistently decrease survival any further. At the lower concentrations this leveling of cytotoxicity is due to the spontaneous decomposition of DAC and to the lack of cytotoxicity of the breakdown products. At the higher concentrations the cause of the leveling remains uncertain. Incubation of L1210 with DAC at concentrations greater than 0.5 micrograms/ml for greater than or equal to 24 hrs resulted in total inhibition of measurable cell growth for 72-96 hrs following drug removal. Sequential colony-formation assays at various intervals following drug removal demonstrated a time-dependent increase in cell clonogenicity at a rate approximating the growth rate of untreated L1210 cells. This suggests that despite total cytostasis of the major population of cells, a small fraction of cells is capable of dividing at a near normal rate if removed from the drug environment. Implications of these results for in vivo applications of DAC are discussed.