To investigate the effects of 1-β-d-arabinofuranosylcytosine (ara-C) at high doses as applied in human acute leukemia, the cytotoxic effect of high-dose ara-C was studied in L1210 leukemia cells grown in tissue culture or as tumors in syngeneic mice. Exponentially growing cells displayed the expected S-phase specificity and dose saturation properties of drug action. In contrast, in stationary cultures, progressively more cells were killed by increasing the concentration of the drug. Moreover, the fraction of cells killed at high doses exceeded by 2- to 3-fold the number of cells in drug-sensitive S phase detectable by flow cytometry or [3H]thymidine radioautography.

To identify these apparent non-S targets of ara-C at high doses, L1210 cells were separated according to cell cycle position by velocity sedimentation at unit gravity. Large fractions of cells, accumulating at the G1-S boundary by nutrient starvation, were detected in stationary tissue culture cells as well as in ascites or solid tumor cells. The cells located in this cell cycle compartment (termed S1 cells) were sensitive to the cytotoxic effect of high-dose ara-C. The putative presence of similar S1 fractions in advanced human acute nonlymphocytic leukemia may explain in part the clinical efficacy of a high-dose application of the drug.

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