Investigations using fluorescent ligands to monitor platinum(IV) reduction and platinum(II) reactions in cancer cells

Coordination of the aniline containing fluorophores, coumarin 120 (C120) and coumarin 151 (C151) at the non-leaving group positions of cisplatin analogues (giving cis-[PtCl2(C120)(NH3)] and cis-[PtCl2(C151)(NH3)]) resulted in partial and complete quenching of the fluorescence, respectively. Oxidation of the coumarin 120 complex to the Pt(IV) form (cis, trans, cis-[PtCl2(OH)(2)(C120)(NH3)]) resulted in further quenching compared to that seen for the Pt(II) complex. The fluorescence profiles of these coumarin complexes were collected to evaluate their suitability for studying the metabolism of cisplatin-based anticancer drugs. C151 has the more suitable profile with a lower energy excitation peak and a better separation between the excitation and emission spectra. The complete damping of fluorescence on coordination to Pt(II) makes it unsuitable for monitoring the reduction process, but does allow it to be used to monitor loss of the aniline type ligand. All of the coumarin complexes revealed moderate cyotoxcities in the range 10-22 mu M indicating that they are suitable models of anticancer agents. DNA dampens the fluorescence of both Pt(II) complexes and that of C120 has a much higher DNA binding affinity (10 000 M-1) than does the complex of C151 (300 M-1). Treatment of A2780 human ovarian carcinoma cells with the Pt-coumarin complexes resulted in fluorescence visible by confocal microscopy, and co-localisation studies with organelle specific dyes suggest they are concentrated in the late endosomes or lysosomes. Cells treated with the Pt(IV) complex of C120 revealed strong fluorescence and a somewhat different distribution to cells treated with the Pt(II) complex indicating reduction following uptake.