Fluorouracil has been used in combination with radiotherapy for the treatment of several malignant tumors with considerable success. In this excellent review, the authors point out that considerable gain could derive from increasing our understanding of the mechanisms of resistance of cytotoxicity or radiation enhancement.
Much is already known about the mechanism of x-ray cell kill that is induced mainly by DNA double-strand breaks, the repair of which is an important modulator of lethality. With fluorouracil, several mechanisms of cytotoxicity and radiosensitization have been proposed including: (1) incorporation into RNA, leading to disruption of RNA function1; (2) inhibition of thymidylate synthase, leading to an alteration of nuecleotide pools and consequently of DNA synthesis; (3) incorporation into DNA; and (4) a newly proposed disturbance of the cell cycle and accumulation of cells at the relatively radiosensitive G1/S boundary.
The contributions of these phenomena to cytotoxicity and radiation enhancement may depend on the drug used (fluorouracil or its metabolite 5-fluorodeoxyuridine [floxuridine; FUDR]), the cell line, and the drug concentration and duration of treatment [2,3]. For example, in our laboratory studies we showed that postirradiation treatment of human colon adenocarcinoma clone A cells with fluorouracil had no effect on the overall rate of ligation of x-ray-induced double-stranded breaks , whereas double-stranded break rejoining was inhibited in another human adenocarcinoma cell line following protracted preirradiation treatment with floxuridine .
New Mechanism of Radiation Enhancement
Pu et al have continued their studies focusing on floxuridine in order to concentrate on DNA-directed effects. In their most recent work, they propose another mechanism of radiation enhancement through the alteration of checkpoints in the cell cycle. This is a relatively new and exciting area of investigation in radiobiology since proliferative status may be important for both cytotoxicity and radiation enhancement.
In a related manner, we looked at the influence of cellular proliferation on the ability of postirradiation 5-fluorouracil to radiosensitize cultured human colon adenocarcinoma clone A cells by comparing the effects of 5-fluorouracil on log-phase or plateau-phase cells . Consistent with the observations of other workers, we found that radiosensitization was related to the degree of cytotoxicity, and that both were greater in log-phase than in plateau-phase cells . Furthermore, we wanted to know whether the effect of 5-fluorouracil on the radiosensitivity of log-phase cells is based on selective killing of S-phase cells, which are considered to be the most resistant subpopulation in an asynchronous culture . Our results showed no evidence of selective cytotoxicity or radiosensitization of S-phase cells [4,6], suggesting that direct killing of S-phase cells by 5-fluorouracil probably does not account for the apparent radiosensitization observed in our experimental conditions. The possibility that fluorouracil causes a checkpoint arrest was not tested in our experimental conditions.
The prospect that therapeutic agents may induce cell cycle blocks is very exciting because it relates one potential mechanism of treatment resistance to other knowledge of specific genetic alterations associated with cancer, eg, tumor-suppressor genes such as p53. One can speculate that in the near future, it may be possible to measure molecular abnormalities from human tumor biopsies that may predict treatment resistance.
One current possibility is the analysis of thymidylate synthase, which has recently been shown to be associated with prognosis in the adjuvant therapy of colorectal cancer . Other markers specific for drug-radiation interactions may some day be available, based on the type of studies performed by the radiobiologists at the University of Michigan laboratory.
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