RADIATION RENDERED MORE CYTOTOXIC BY FLUDARABINE MONOPHOSPHATE IN A HUMAN OROPHARYNX CARCINOMA CELL-LINE THAN IN FETAL LUNG FIBROBLASTS

Purpose: Fludarabine monophosphate (fludarabine-P) is a relatively new drug in the treatment of different haematological diseases. The mecha nism of action also implies a possible role of this drug as a radiosen sitizer. Up to now no in vitro investigations dealing with radiosensit izing effects of fludarabine-P in carcinoma cell lines and fibroblasts have been published. The aim of our studies was to analyse the cytoto xic and radiosensitizing effects of different dosages and application schedules of fludarabine-P in a human squamous carcinoma cell line of the oropharynx (ZMK-1) and of fetal lung fibroblasts (MRC-5) in vitro. Possible mechanisms of interaction of fludarabine-P and radiation wer e investigated. Methods: ZMK-1 and MRC-5 cells were cultured under sta ndard conditions with different concentrations of fludarabine-P in com bination with escalating doses of radiation. Cytotoxic effects were me asured by colony-forming assays. Induction and rejoining of radiation- induced DNA double-strand breaks after incubation with fludarabine-P w ere measured using constant-field gel electrophoresis. Incubation time s for rejoining varied from 0 h to 24 h. Results: Fludarabine-P showed a radiosensitizing activity in ZMK-1 tumour cells and MRC-5 fibroblas ts. The observed effects depended on the concentration and the incubat ion time. The largest effect was demonstrable for an incubation of 5 d ays, which started shortly before irradiation, whereas an incubation s olely before irradiation did not have a clear effect on the cellular s urvival. The sensitizer enhancement ratio, at the 10% survival level, in the ZMK-1 cells was 2.2 in comparison to 1.6 in MRC-5 cells. The an alysis of the interaction of fludarabine-P and ionising radiation by m eans of the isobologram approach, revealed an overadditive effect in t he tumour cell line and an additive effect in the lung fibroblasts. Fl udarabine-P did not modify the rejoining of radiation-induced DNA doub le-strand breaks in either cell line. Conclusions: We conclude that fl udarabine-P in clinically attainable doses is a strong radiosensitizer in ZMK-1 cells and has a lower activity in the MRC-5 fibroblasts in v itro. The radiosensitization of fludarabine-P seems to be over additiv e in the malignant cells and additive in normal fetal fibroblasts. Thi s would indicate that fludarabine-P might enhance the therapeutic rati o of radiation. Further investigations are warranted to identify the p otential of this drug as a radiosensitizer in vivo and to elucidate th e mechanism of interaction of the drug and radiation.