Asymmetrically Disubstituted Pyrenebutyrate Complexes of Pt(IV) as Cisplatin Prodrugs with Improved Anticancer Activity.

Among the non-classical platinum complexes, Pt(IV) prodrugs are most promising as versatile scaffolds for structural modification and fine tuning of their activation-by-reduction mechanism of action and the resulting anticancer activity. Herein, four new asymmetrically disubstituted pyrenebutyrate complexes of Pt(IV) (2-5) were synthesized and thoroughly studied. In this series, the second axial ligand was derived from dicarboxylic acids of different length-4 and 5 C-atoms, or replacement of the C-atom in the middle with either O- or S-atom. The structural effects on reduction kinetics, lipophilicity and cellular internalization of the complexes were monitored by NMR, HPLC, fluorescence and ICP-MS measurements. Their cytotoxicity was tested on a panel of cancer cell lines and mechanistic insights were obtained from proteome analysis and microscope imaging. The data indicate that all complexes, especially complex 3, represent a promising class of Pt(IV) prodrugs, exhibiting significantly higher cytotoxic activity than cisplatin in all tested models, including a cisplatin-resistant line. This was explained with a stronger and more integrated apoptotic response than cisplatin: pronounced Bax upregulation (3.6-fold), maximal cleaved caspase-3 (4-fold), activation of both intrinsic and extrinsic pathways, and effective p53 Ser15/Ser46 phosphorylation. The consistent rank order of potency (3 > 4 > 5 ≈ 2 ≫ cisplatin) suggests that subtle ligand modifications can substantially enhance efficacy, possibly by improving cellular uptake or altering DNA binding.