Potentiation of etoposide cytotoxicity against a human ovarian cancer cell line by pretreatment with non-toxic concentrations of methotrexate or aphidicolin

Exposure of human ovarian cancer SW626 cell line to 0.08 μmol/l methotrexate or 25 μmol/l aphidicolin for 24 h caused no cytotoxiciy but enhanced etoposide cytotoxicity. Methotrexate or aphidicolin treatment induced a reversible blockade at the beginning of S phase which was reversed upon drug removal with a consequent wave of synchronisation. The enhancement of etoposide cytotoxicity was not due to higher etoposide intracellular uptake in the methotrexate or aphidicolin-pretreated cells. The topoisomerase II content in methotrexate or aphidicolin pretreated SW626 cells was higher than in control cells assessed by western blotting or flow cytometry. The higher etoposide cytotoxicity observed after synchronization with methotrexate or aphidicolin was apparently unrelated to the number of drug-induced DNA-topoisomerase II complexes evaluated as DNA double strand breaks or DNA-protein crosslinks. These data support the view that etoposide-induced DNA-topoisomerase II complexes are more cytotoxic in cells which are in S-phase.