Enhancement of the Sensitivity of Human Colon Cancer Cells to Growth Inhibition by Acivicin Achieved through Inhibition of Nucleic Acid Precursor Salvage by Dipyridamole

This study was undertaken to determine if salvage of nucleic acid precursors might constitute a mechanism of resistance to acivicin in human colon cancer cells and, if so, to establish whether dipyridamole, an inhibitor of nucleoside and nucleobase transport, can block the salvage process and restore sensitivity to acivicin. Acivicin inhibited the replication of human colon cancer cells (VACO 5) in vitro in a dose- and time-dependent fashion. In addition, marked cell lysis was evident after a 24-hr exposure to acivicin at concentrations greater than 1 μg/ml. The primary metabolic effect of acivicin was depletion of the cytidine triphosphate and guanosine triphosphate pools. Adenosine triphosphate levels were also reduced, but apparently as a consequence of the guanosine triphosphate depletion. VACO 5 cells exposed to acivicin (3 μg/ml) efficiently salvaged low levels (1 μm) of cytidine, guanosine, and guanine and could, therefore, restore the depleted nucleotide pools. The combination of cytidine and guanosine, but not either nucleoside alone, provided significant protection against the growth-inhibitory properties of acivicin. Dipyridamole, at a noncytotoxic concentration (5 μM), blocked repletion of the cytidine triphosphate and guanosine triphosphate pools in cells exposed to acivicin and the nucleic acid precursors. As a result, the growth-inhibitory effects of acivicin were maintained. The salvage of cytidine was particularly sensitive to inhibition by dipyridamole, and no restoration of cytidine triphosphate pools was evident. The cellular uptake of a variety of nucleic acid precursors was differentially sensitive to inhibition by dipyridamole. The 50% inhibitory dose values ranged from 0.01 to 2.5 μM for cytidine and uridine, respectively. The results of this study indicate that, although the replication of VACO 5 cells was inhibited by acivicin, low levels of nucleosides and nucleo bases can circumvent the cytotoxicity. Dipyridamole effectively blocked the salvage pathways and restored the sensitivity of the cancer cells to the antiproliferative actions of acivicin.