Synthesis and characterization of Pt(IV) fluorescein conjugates to investigate Pt(IV) intracellular transformations

Pt(IV) anticancer compounds typically operate as prodrugs that are reduced in the hypoxic environment of cancer cells, losing two axial ligands in the process to generate active Pt(II) species. Here we report the synthesis of two fluorescent Pt(IV) prodrugs of cisplatin in order to image and evaluate the Pt(IV) reduction process in simulated and real biological environments. Treatment of the complexes dissolved in PBS buffer with reducing agents typically encountered in cells, glutathione or ascorbate, afforded a 3- to 5-fold fluorescence turn-on owing to reduction and loss of their fluorescein-based axial ligands, which are quenched when bound to platinum. Both Pt(IV) conjugates displayed moderate cytotoxicity against human cancer cell lines, with IC50 values higher than that of cisplatin. Immunoblotting and DNA flow cytometry analyses of one of the complexes, Pt(IV)FL2, revealed that it damages DNA, causes cell cycle arrest in S or G2/M depending on exposure time, and ultimately triggers apoptotic cell death. Fluorescence microscopic studies prove that Pt(IV)FL2 enters cells intact and undergoes reduction intracellularly. The results are best interpreted in terms of a model in which the axial fluorescein ligands are expelled through lysosomes, with the platinum(II) moiety generated in the process binding to genomic DNA, which results in cell death.