Analysis of the drug synergism between thymidine and arabinosyl cytosine using mouse S49 T lymphoma mutants

The synergism between arabinosyl cytosine (araC) and thymidine is characterized using two mutant S49 T lymphoma cell populations with altered deoxyribonucleotide metabolism. AraC-1-6 cells are deficient in dCMP deaminase activity resulting in a secondary elevation of intracellular dCTP pools, whereas dGuo-200-l cells have a mutation in the M₁ subunit of ribonucleotide diphosphate reductase, which also results in elevation of dCTP levels. These two mutant cell populations are partially resistant to araC cytotoxicity as compared to the wild type cells. The resistance to araC is contributed to the elevation of dCTP levels in these mutants which prevent araC incorporation into the DNA due to feedback inhibition of deoxycytidine kinase. Addition of extracellular thymidine to dCMP deaminase deficient cells causes a decrease in dCTP levels and in parallel increase their sensitivity to araC. In contrast, extracellular thymidine does not reduce dCTP levels in the mutant cells with altered ribonucleotide reductase and no synergism between araC and thymidine is observed in these cells. The expansion of dTTP pools in the presence of thymidine is similar in the two mutants. These results suggest that the depletion of dCTP pools by thymidine is responsible for the synergistic action of thymidine on araC cytotoxicity and that dTTP does not directly enhance the incorporation of araC into the DNA of T lymphoma cells.