Uridylate-trapping sugar analogs in combination with inhibitors of uridylate synthesis de novo and 5-fluorouridine

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The combination of 3 types of antipyrimidines was studied in AS-30D hepatoma cells in suspension culture and in the rat in vivo. 1. 1.|Callulear UTP and CTP pools can be depleted most effectively by combining an inhibitor of de novo UMP synthesis with sugar analogs diverting UMP to UDP-sugar analogs. The following UMP-trapping sugar analogs were employed: d-galactosamine, d-galactosone, d-glucosome, and d-glucosamine. These d-galactose and d-glucose analogs intensified the depletion of UTP and CTP pools induced by the following inhibitors of de novo UMP synthesis: acivicin, PALA, palachol, pyrazofurin, and 6-azauridine. 2. 2.|The sugar analogs, in the absence of inhibitors of the de novo pathway, enhanced the rate of de novo UMP synthesis several-fold, as indicated by incorporation of 14CO 2 into intermediates and the products of the pathway and by the expansion of the acid-soluble uracil nucleotide pool. 3. 3.|Reduction of UTP and CTP contents to less than 5 and 15% of control, respectively, by d-galactosamine and PALA resulted in a decrease of the rate of RNA synthesis to 19% of control as calculated from the changes in specific activities of [ 14C]CTP and of [ 14C]cytidine in RNA after labeling with [ 14C]uridine. [cating that pyrimidine nucleoside excretion is regulated by pyrimidine nucleotide levels, possibly by UTP and CTP regulation of uridine kinase. 4. 5.|Determinaton of the rates of de novo pyrimidine synthesis, of the formation of RNA pyrimidines, and of pyrimidine nucleoside excreton indicates that de novo synthesis provides only about 67% of the pyrimidines required for the consuming processes. The difference, as well as the dilution of labeled pyrimidine nucleotide pools under conditions of a blocked de novo pathway, suggests a considerable salvage of pyrimidine nucleosides derived from RNA. This salvage of pyrimidines may be intracellular and/or by an excretion and re-uptake process. 5. 6.|Depletion of UTP and CTP pools, induced in hepatoma cells by d-galactosamine and 6-azauridine, leads to growth inhibition in suspension culture; this inhibition becomes irreversible in an increasing percentage of cells, killing all cells after 20 hr of UTP deficiency. 6. 7.|The enhanced uptake of 5-fluorouridine by UTP-deficient cells was associated with an increase of FUMP incorporation into RNA up to 4-fold and with stronger inhibition of cell growth. The amounts of FUDP-sugars formed from 5-fluorouridine in hepatoma cells were smaller than the amounts of UDP-sugars formed from uridine. All the FUDP-sugars served as substrates i vitro of the respective enzymes of UDP-sugar metablism. 7. 8.|Combination of UMP-trapping sugar analogs with an inhibitor of de novo UMP synthesis and followed by a pyrimidine ribonucleoside analog such as 5-fluorouridine may improve the tissue selectivity and the effectiveness of tumor chemotherapy with antipyrimidines.