CYTOTOXICITY OF ANTIFOLATE INHIBITORS OF THYMIDYLATE AND PURINE SYNTHESIS TO WIDR COLONIC-CARCINOMA CELLS

We have studied the cytotoxicity of 5,10-dideazatetrahydrofolate (DDAT HF) and of D-1694 to human WiDr colonic carcinoma cells as a model sys tem for the effects of pure inhibitors of either the de novo purine sy nthesis pathway or thymidylate synthesis. The growth of this cell line was inhibited by very low concentrations of either agent and the leth ality of DDATHF and D-1694 was completely prevented by continuous expo sure to either hypoxanthine or thymidine, respectively, indicating tha t these compounds were very potent metabolic inhibitors, each specific for one of these pathways. D- 1694 was highly cytotoxic (>3 logs of k ill) after a 4-h exposure to 1 muM drug, or a 24-h exposure to very lo w concentrations (0.04 muM). On the other hand, the cytotoxicity of DD ATHF was substantially lower, with 2 logs of cell kill requiring >> 10 0 muM with 4 h of exposure or approximately 40 muM for 72 h of exposur e. Maximal cell kill induced by D-1694 was 5-6 logs, consistent with e limination of all viable cells except preexisting mutants. A maximum o f 2-3 logs of cell kill was observed with DDATHF. Exposure of WiDr cel ls to either D-1694 or DDATHF caused striking cellular changes, but th e morphologies of cells treated with the two drugs were remarkably dif ferent. D-1694-treated cells detached from the dish within 1-2 days af ter a megaloblastosis, whereas DDATHF-treated cells remained adherent to the dishes for at least 10 days after treatment. The addition of th ymidine to D-1694-treated cultures or hypoxanthine to DDATHF-treated c ells after up to 20 h of drug exposure completely prevented cytotoxici ty of either drug. With longer exposures, cytotoxicity of both drugs p rogressively increased in spite of such rescue. Our results indicate t hat substantial (99-99.9%) tumor cell kill can be induced by a pure in hibitor of purine synthesis, but that the rate of commitment to cell d eath and the extent of cell kill is greater with a pure inhibitor of t hymidylate synthesis.