Cytotoxicity of antifolate inhibitors of thymidylate and purine synthesis to WiDr colonic carcinoma cells.

We have studied the cytotoxicity of 5,10-dideazatetrahydrofolate (DDATHF) and of D-1694 to human WiDr colonic carcinoma cells as a model system for the effects of pure inhibitors of either the de novo purine synthesis pathway or thymidylate synthesis. The growth of this cell line was inhibited by very low concentrations of either agent and the lethality of DDATHF and D-1694 was completely prevented by continuous exposure to either hypoxanthine or thymidine, respectively, indicating that these compounds were very potent metabolic inhibitors, each specific for one of these pathways. D-1694 was highly cytotoxic (> 3 logs of kill) after a 4-h exposure to 1 microM drug, or a 24-h exposure to very low concentrations (0.04 microM). On the other hand, the cytotoxicity of DDATHF was substantially lower, with 2 logs of cell kill requiring >> 100 microM with 4 h of exposure or approximately 40 microM for 72 h of exposure. Maximal cell kill induced by D-1694 was 5-6 logs, consistent with elimination of all viable cells except preexisting mutants. A maximum of 2-3 logs of cell kill was observed with DDATHF. Exposure of WiDr cells to either D-1694 or DDATHF caused striking cellular changes, but the morphologies of cells treated with the two drugs were remarkably different. D-1694-treated cells detached from the dish within 1-2 days after a megaloblastosis, whereas DDATHF-treated cells remained adherent to the dishes for at least 10 days after treatment. The addition of thymidine to D-1694-treated cultures or hypoxanthine to DDATHF-treated cells after up to 20 h of drug exposure completely prevented cytotoxicity of either drug. With longer exposures, cytotoxicity of both drugs progressively increased in spite of such rescue. Our results indicate that substantial (99-99.9%) tumor cell kill can be induced by a pure inhibitor of purine synthesis, but that the rate of commitment to cell death and the extent of cell kill is greater with a pure inhibitor of thymidylate synthesis.