It is widely accepted that platinum(IV) complexes act as prodrugs and have to be activated by reduction to the respective platinum(II) analogs. Recently it could be shown that introduction of lipophilic carboxylato ligands in the axial position leads to diaminedichloridoplatinum(IV) compounds with exceptionally high cytotoxicity. With the aim of improving the antiproliferative properties of carboplatin, a series of twenty-one novel Pt(IV) complexes, featuring the equatorial ligand sphere of carboplatin as well as lipophilic axial carboxylato ligands, was synthesized. In depth characterization is based on elemental analysis, ESI-MS, ATR-IR, and multinuclear (1H, 13C, 15N, and 195Pt) NMR spectroscopy. Their cytotoxic activity in four cell lines (CH1, SK-OV-3, SW480, and A549), lipophilicity, electrochemistry and additionally the rate of reduction in the presence of ascorbic acid were investigated. In contrast to analogous diaminedicarboxylatodichloridoplatinum(IV) compounds, the cytotoxicity of novel diaminetetracarboxylato counterparts could not substantially be increased by simply enhancing their lipophilic character. It seems that not only the reduction potential, but also the rate of reduction has a tremendous influence on the cytotoxic properties. This has to be taken into account for the development of novel anticancer platinum(IV) agents.
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