Salvage pathways as targets of chemotherapy

Abstract

This paper discussed the significance of the activities of purine and pyrimidine salvage enzymes in cancer cells and the targeting against them of chemotherapy. 1. 1.|The activities of salvage enzymes in the rat liver were orders of magnitude higher than those of the rate-limiting enzymes of de novo biosynthesis. A similar relationship was observed in rat hepatomas of different growth rates and in primary colon carcinoma in human. 2. 2.|The concentrations of nucleosides and nucleobases were measured in plasma, liver and hepatoma 3924A in the rat. The freeze-clamp method was required to determine the concentrations of these precursors in rat liver and hepatoma in a reliable and precise fashion because ischemia markedly altered the concentrations of nucleosides, nucleobases and, as shown earlier, nucleotides in these tissues. The results indicated that the liver markedly concentrated the purine precursors, hypoxanthine, guanine and adenine, but not thymidine, which was one-third that of the plasma. Uridine and deoxycytidine occurred in the same concentration as in plasma, but cytidine was 3-fold higher in liver. In the hepatoma in comparison to the liver the concentrations of the nucleosides and bases were altered and for some of the changes the enzymic differences between liver and hepatoma appeared to be accountable. 3. 3.|Kinetic parameters for purine and pyrimidine synthetic enzymes and for the substrates and co-factors were determined in liver and hepatoma 3924A. When enzymic activities were calculated at the tissue steady-state concentrations of the various ligands, the activities of the salvage enzymes were markedly higher than those of the rate-limiting enzymes. 4. 4.|Hepatoma cells were highly sensitive to the action of the transport inhibitor, dipyridamole, in lag and log phases. However, plateau phase cells lost their sensitivity to dipyridamole. 5. 5.|Amphotericin B rendered plateau phase cells sensitive to the inhibitory action of dipyridamole for the incorporation of thymidine. 6. 6.|Amphotericin B enhanced cytotoxicity of dipyridamole in hepatoma and human colon cancer HT-29 cells. 7. 7.|In these studies we discovered the decreased responsiveness to dipyridamole of plateau phase cells and the ability of amphotericin B to restore the sensitivity. Moreover, dipyridamole and amphotericin B were synergistic in their cytotoxic action in rat hepatoma cells and human colon cancer cells. Because human solid tumors have a high concentration of non-growing cells, the action of amphotericin B in rendering cancer cells sensitive to dipyridamole action in combination with inhibitors of de novo purine and pyrimidine biosynthesis is potentially useful in the chemotherapy of human solid tumors.