Effects of the bidentate malonate ligand on the utilization and cytotoxicity of platinum compounds in the L1210 cell line.

Previous studies have investigated the cytotoxicity of platinum(II) compounds with bidentate leaving ligands, but little is known about their uptake or mechanism of action inside the cell. We have compared the uptake and intracellular effects of cis-1,2-diaminocyclohexanemalonatoplatinum(II) [Pt(mal)(cis-DACH)] and cis-1,2-diaminocyclohexanedichloroplatinum(II) [PtCl2(cis-DACH)] in the L1210 cell line. In the 3-h colony formation assay, PtCl2(cis-DACH) is almost 10 times more cytotoxic than Pt(mal)(cis-DACH). However, when the time of incubation with platinum drug is increased from 3 to 24 h the cytotoxicity of PtCl2(cis-DACH) changes relatively little while the cytotoxicity of Pt(mal)(cis-DACH) increases over 10-fold. As a consequence, Pt(mal)(cis-DACH) is only slightly less cytotoxic than the dichloro compound by 24 h. These data can readily be explained by differences in the time course of uptake. Pt(mal)(cis-DACH) is initially taken by the cell at a much slower rate than PtCl2(cis-DACH), but the uptake continues linearly for at least 24 h with the malonate compound, while uptake for the dichloro compound begins to plateau between 5 and 8 h. The earlier plateauing of PtCl2(cis-DACH) uptake is most likely to be due to a quicker approach to equilibrium between intracellular and extracellular concentrations of unchanged drug which is caused by a more rapid inactivation of PtCl2(cis-DACH) in the media coupled with the more rapid uptake of PtCl2(cis-DACH) by the cells. When compared at equal cytotoxicity, the uptake of platinum into the intracellular pool of low molecular weight platinum compounds is significantly greater for Pt(mal)(cis-DACH) even though the rate of platinum incorporation into DNA is essentially the same for both compounds at early times. These data suggest that a smaller percentage of the low molecular weight platinum is in a reactive form inside the cell for Pt(mal)(cis-DACH). While the inhibition of cell survival exactly parallels incorporation of platinum into DNA for both compounds, there is a significant lag before the onset of inhibition of DNA synthesis for Pt(mal)(cis-DACH). This observation suggests that the conversion of monoadducts to diadducts may be slower for platinum compounds with bidentate leaving ligands.