Factors modifying the synergistic toxicity of deoxycytidine in combination with thymidine plus 5-fluorouracil in HeLa cells.

We reported previously that deoxycytidine (CdR) enhances the cytotoxic effects of the drug combination thymidine (TdR) plus 5-fluorouracil (FUra) against HeLa S-3 cells. We have now examined the relationships between the concentration of CdR and its cytotoxic and cytokinetic effects, and have also investigated the role of certain other components of the culture medium in this phenomenon. Cell survival was determined by a colony-forming assay; cytokinetic effects were monitored by flow cytometry. In the initial experiments, cells were grown in Ham's F12 medium and exposed for 22 hr to 4 mM TdR, 0 X 025 mM FUra, and dCyd ranging from 1 microM to 4 X 0 mM. The individual drugs were at most only slightly toxic under these conditions; for TdR plus FUra, the survival decreased to 50% (in 5% FCS), and in the three-drug combination it varied from 8% at 1 microM CdR to 28% at 0 X 10 mM and back to a low of 3% at 4 X 0 mM CdR. Results from flow cytometry appeared correlated with the survival data, in that cells accumulated in the S phase to a greater extent in the region around 0 X 10 mM CdR than at higher or lower concentrations. When cells were exposed to the drugs in MEM medium in place of F12, their sensitivity to FUra and the TdR-FUra combination was enhanced, although the additional synergistic effect of CdR was reduced. We found that hypoxanthine, present in F12 but not in MEM, was the principal compound responsible for the observed differences between media.