Dipyridamole is a potent inhibitor of membrane nucleoside transport into mammalian cells. Since the membrane transporter mediates both the influx and the efflux of nucleosides, dipyridamole should be able to block nucleoside efflux from cells as well. In human ovarian carcinoma cells (2008) and promyelocytic leukemia cells (HL60), we observed that sequential treatment with 20 microM dipyridamole 2 h after their initial exposure to varying concentrations of 1-beta-D-arabinofuranosylcytosine (ara-C) increased the cytotoxicity of this nucleoside analogue by 100 to 300% at all drug concentrations tested. In washout experiments in which cells were exposed to radiolabeled ara-C for 2 h and reincubated in fresh medium, the presence of 20 microM dipyridamole in the reincubation medium resulted in significantly elevated levels of intracellular radioactivity at the end of a 24-h period. High performance liquid chromatography analyses of cellular nucleotide pools during this 24-h period revealed that cells treated with the sequential ara-C/dipyridamole regimen have 2-to 3-fold higher levels of ara-CTP at all time points studied. Using alkaline elution assays, we measured a 30% increase in DNA strand breaks in cells treated with ara-C followed by dipyridamole when compared to cells treated with ara-C alone, while dipyridamole alone did not produce DNA lesions. ara-C resistance in tumor cells is associated with either the natural substrates competing with ara-C for phosphorylation and incorporation into macromolecules or increased catabolism of the parent drug. Sequential exposure regimens may overcome such tumor resistance by increasing the cellular pools of ara-C and its metabolites. A second advantage to the sequential regimen is that the prolonged retention of ara-C in non-S-phase cells may improve its efficacy. The applicability of such regimens in treating human cancer awaits the results from preclinical efficacy and toxicity trials.