Radiosensitization of human tumor cells by alpha‐difluoromethylornithine

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Abstract
The effect of polyamine depletion on the radiosensitivity of a human tumor cell line was investigated. CAL 18 A cells, derived from a breast carcinoma, were incubated with alpha‐difluoromethylornithine (DFMO)—a specific and irreversible inhibitor of ornithine decarboxylase (ODC)—at a 1 mM or 10‐mw concentration for either 1 hr or 24 hr and irradiated thereafter. Survival curves of exponentially growing cells revealed a moderate but significant enhancement of radiosensitivity as compared to untreated irradiated cells. Maximum radiosensitization was observed at a concentration of 10 mw after I hr incubation. Plateauphase cells were used to study the effect of polyamine inhibition on repair of radiation‐induced potentially lethal damage (PLD). DFMO enhanced the radiation response and significantly inhibited PLD repair in these cells. Measurement of ODC indicated that this enzyme was markedly inactivated upon brief incubation of CAL 18 A cells with DFMO, reflecting a depletion of polyamine synthesis. These results extend findings that have demonstrated enhancement of drug‐induced cytotoxicity, and raise the possibility of clinical use of this substance for potentiation of radiation response.