Concentration and time dependence of the toxicity of fluorinated pyrimidines to HT 29 colorectal carcinoma cells

To determine the optimal concentration time factors for the fluoropyrimidines 5-fluorouracil (FU), 5-fluorouridine (FUR), and 5-fluoro-2'-deoxyuridine (FUdR) in regional chemotherapy, we tested these drugs against the colorectal carcinoma cell line HT 29 at various dosages and exposure times. The measure of cytotoxicity used was the degree of inhibition of colony formation in soft agar after drug treatment compared with untreated control cells. Colonies were visible after 6 days of growth in soft agar, so the initial evaluation of toxicity was done at this time. Additional colonies were found 10 and 16 days after the first evaluation, so the dishes containing the treated cells were also evaluated for this delayed growth phenomenon ("regrowth"), which we considered to be due to a cell growth inhibition effect of the drugs rather than a cytocidal effect. Exposure times of the cells to the drugs ranged from 5 min to 24 h and the doses, between 0.01 and 1000 micrograms/ml. The toxicity of FUdR was concentration-dependent, but its time dependence ceased after a relatively short exposure time. There was a cell population that was not susceptible to FUdR regardless of dose and exposure time; consequently, FUdR treatment was always accompanied by substantial regrowth of colonies. With FU and FUR, conditions could be achieved that resulted in complete cell death (no regrowth), but high concentrations and long exposure times were required with FU. With FUR, on the other hand, both cytostasis and cytotoxicity could be achieved with substantially lower doses and shorter exposure times than with FU. These results indicate that FUR has the potential to be an effective drug in chemotherapy protocols not involving systemic administration.