Letter: Modification of radiation effects on cells by membrane binding agents--procaine HC1

Chemical radiosensitizers in cancer therapy

The development of effective low-LET radiation therapy for cancer has been hindered by the lack of consistent differential responses to radiation between tumor and normal tissues. One major difference between many solid tumors and the surrounding normal stroma is the presence of hypoxic foci in solid tumors due to the inadequate supply of nutritional needs as a result of the breakdown of microvasculature. Consequently, failure of conventional radiotherapy and local recurrences are in part attributed to the radioresistant hypoxic cell populations, present in the tumor. Local cure/control rates of a tumor can be increased only by an effective increase in the radiation dose. At the same time, an increase in such a dose would damage the oxic normal stroma, more than the hypoxic tumor cells. Hence, specific modification of tumor radiosensitivity by the use of chemical radiosensitizers, in combination with conventional radiotherapy, is an attractive alternative. Many clinicians and radiotherapists are skeptical about the outcome of using radiosensitizers in patients. Nevertheless, a vast amount of information is currently available regarding the first- and second-generation radiosensitizers both in murine and in human tumors. As a result, it is hoped that eventually a radiosensitizing drug would be discovered/synthesized that will overcome the drawbacks so far encountered in their use in the clinic. In this article, the development of chemical radiosensitizers since the early sixties, the basis for their selection, their mechanism(s) of action, and the results obtained with the various groups of radiosensitizers are reviewed.

Non-nitro radiation sensitizers

A review of the literature on radiosensitization reveals that at least some of the non-nitro compounds, such as metabolic inhibitors and membrane-active drugs, could be considered as potentially valuable radiosensitizers for possible use in future cancer radiotherapy.

Modification of radiation response of E. coli B/r cells by phenothiazines

Promethazine and trimeprazine sensitized anoxic E. coli B/r cells to 60Co gamma-rays, but both drugs showed a radioprotective effect under euoxic conditions. Their radiosensitizing effect was found to be due to the reaction of radiolytically induced hydroxyl radicals with the sensitizers. The radioprotective effect of these drugs is attributed to changes in the membrane structure conducive with chemical repair of the damaged sites in the gel region of the cellular membrane by intracellular sulphydryl compounds. Pre-irradiation depletion of sulphydryls from E. coli B/r by treatment with N-ethyl maleimide abolished the radio-protective effect of these drugs under euoxic conditions.

Local anaesthetics as hypoxic radiosensitizers, oxic radioprotectors and potentiators of hyperthermic killing in mammalian cells

Local anaesthetics with well-known pharmacology are found to radiosensitize hypoxic and radioprotect oxic murine L5178Y cells. Under the specified experimental conditions, radiation-modifying effects exerted by local anaesthetics are dose-dependent. It appears that in order that the drugs should act as oxic radioprotectors or as hypoxic radio-sensitizers, the agent must be present during the period of irradiation. Local anaesthetics are also found to potentiate the hyperthermic killing of cells. The possibility of applying compounds with such dual radiation-modifying properties plus hyperthermia-potentiating capacity to enhance the therapeutic gains in tumour therapy is briefly discussed.

Modification of radiation effects in single-cell systems by membrane-binding agents

Chlorpromazine sensitized E. COLI B/r, thymocytes and Yoshida ascites-tumour cells to 60Co gamma-rays, preferentially, under hypoxic conditions, Residual sensitization was observed in E. coli B/r. The radiosensitization by chlorpromazine was found to be due to the reaction of radiolytically-induced hydroxyl radicals with the sensitizer. When both procaine HCI and chlorpromazine were present during anoxic irradiation of E. coli B/r, there was no additive effect. But the addition of procaine HCL after irradiation to cell sensitized by chlorpromazine had an effect that was significantly greater than that produced even by oxygen. A similar effect was observed when lignocaine or tetracaine was used in place of procaine. The possible mechanisms involved in radiosensitization by chlorpromazine and in the post-irradiation effect of local anaesthetics in cells sensitized to 60Co gamma-rays by chlorpromazine are discussed.