Radiosensitization of EMT6 Cells by Four Platinum Complexes

The greatest research effort in producing radiation sensitizers has been directed toward organic compounds. However, platinum complexes also have radiosensitizing capabilities, perhaps because they bind to DNA. The compound described here are dichloro complexes of bivalent platinum with one or two potentially radiosensitizing ligands. The radiosensitization of oxygenated and hypoxic exponentially growing EMT6 cells in vitro was measured. The dose modifying factors obtained with 200 microM and 400 microM trans-bis(2-nitroimidazole)dichloroplatinum II (NIPt) in hypoxic cells were 1.5 and 2.1, respectively. For trans-bis(2-amino-5-nitrothiazole)dichloroplatinum II (Plant) under the same conditions, the dose modifying factor was 1.5 at 200 microM and 1.8 at 400 microM. Neither compound sensitized oxygenated cells when tested similar protocols. Unlike the trans complexes, (1,2-diamino-4-nitrobenzene)dichloroplatinum II (Plato) was cytotoxic toward the hypoxic cells in the absence of X rays. The time course of cytotoxicity for 100 microM Plato in exponentially growing cells showed rapid killing of hypoxic cells, and much less toxicity toward oxygenated cells. In radiosensitization studies, dose modifying factors of 1.6 and 2.0 were found with 200 microM and 400 microM Plato in hypoxic cells. The compound did not sensitize aerobic cells. The well-known platinum complex cis-dipyridinedichloroplatinum II (PyPt) represents a cis-platinum heterocyclic aromatic complex that does not have a nitro-functionality. The dose modifying factor obtained with 400 microM PyPt in hypoxic cells was 1.7. On a molar basis, the nitro-functional platinum complexes appear to be more effective as hypoxic cell radiosensitizers than the corresponding free ligands.