Basic characterization of 64Cu-ATSM as a radiotherapy agent.

64Cu-diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) is a promising radiotherapy agent for the treatment of hypoxic tumors. In an attempt to elucidate the radiobiological basis of 64Cu-ATSM radiotherapy, we have investigated the cellular response patterns in vitro cell line models. Cells were incubated with 64Cu-ATSM, and the dose-response curves were obtained by performing a clonogenic survival assay. Radiation-induced damage in DNA was evaluated using the alkali comet assay and apoptotic cells were detected using Annexin V-FITC and propidium iodide staining methods. Washout rate and subcellular distribution of 64Cu in cells were investigated to further assess the effectiveness of 64Cu-ATSM therapy on a molecular basis. A direct comparison of subcellular localization of Cu-ATSM was made with the flow tracer analog Cu-pyruvladehyde-bis(N4-methylthiosemicarbazone). In this study, 64Cu-ATSM was shown to reduce the clonogenic survival rate of tumor cells in a dose-dependent manner. Under hypoxic conditions, cells took up 64Cu-ATSM and radioactive 64Cu was highly accumulated in the cells. In the 64Cu-ATSM-treated cells, DNA damage by the radiation emitted from 64Cu was detected, and inhibition of cell proliferation and induction of apoptosis was observed at 24 and 36 h after the treatment. The typical features of postmitotic apoptosis induced by radiation were observed following 64Cu-ATSM treatment. The majority of the 64Cu taken up into the cells remained in the postmitochondrial supernatant (the cellular residue after removal of the nuclei and mitochondria), which indicates that the beta- particle emitted from 64Cu may be as effective as the Auger electrons in 64Cu-ATSM therapy. These data allow us to postulate that 64Cu-ATSM will be able to attack the hypoxic tumor cells directly, as well as potentially affecting the peripheral nonhypoxic regions indirectly by the beta- particle decay of 64Cu.