We report that coincubation of 647V cells for one cell cycle with low concentrations (30 μM) of 5′-amino-5′-deoxythymidine increased IdUrd DNA incorporation and radiosensitivity at low extracellular pH <tex-math>$({\rm pH}_{{\rm e}}\ 6.8)$</tex-math> in a fashion similar to treatment at normal pH e. IdUrd DNA incorporation is inhibited by high (300 μM) 5′-AdThd concentrations at both normal and low pH e (7.4 and 6.8), resulting in no significant radiosensitization. These results at low pH e were not anticipated based on previously published studies of 5′-AdThd modulation of thymidine kinase (TK) activity and nucleoside cellular uptake. Our results suggest that regulation of intracellular pH ( pH i) during the course of one cell cycle negates the 5′-AdThd dose-dependent modulation of TK activity demonstrated previously. Flow cytometric measurement of pH i in 647V cells showed that normal pH i (pH 7.4) was maintained in 647V cells over a 12- to 24-h exposure to low pH e (pH 6.8). Thus the concomitant use of IdUrd and high concentrations of 5′-AdThd (>30 μM) is unlikely to result in selective in vivo radiosensitization of human tumors under conditions which are intermittently or chronically acidic. However, low concentrations of 5′-AdThd may prove to be an effective in vivo modulator of IdUrd radiosensitization of human tumors under both normal and acidic conditions.

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