Influence of Equatorial and Axial Carboxylato Ligands on the Kinetic Inertness of Platinum(IV) Complexes in the Presence of Ascorbate and Cysteine and within DLD‑1 Cancer Cells

The rapid and premature reduction of platinum­(IV) complexes in vivo is a significant impediment to these complexes being successfully employed as anticancer prodrugs. This study investigates the influence of the platinum­(IV) coordination sphere on the ease of reduction of the platinum center in various biological contexts. In the presence of the biological reductants, ascorbate and cysteine, platinum­(IV) complexes with dicarboxylato equatorial ligands were observed to exhibit lower reduction potentials and slower reduction rates than analogous platinum­(IV) complexes with dichlorido equatorial ligands. Diaminetetracarboxylatoplatinum­(IV) complexes exhibited unusually long half-lives in the presence of excess reductants; however, the complexes exhibited moderate potency in vitro, indicative of rapid reduction within the intracellular environment. By use of XANES spectroscopy, <i>trans</i>-[Pt­(OAc)₂(ox)­(en)] and <i>trans</i>-[PtCl₂­(OAc)₂(en)] were observed to be reduced at a similar rate within DLD-1 cancer cells. This large variability in kinetic inertness of diamine­tetracarboxylato­platinum­(IV) complexes in different biological contexts has significant implications for the design of platinum­(IV) prodrugs