Studies on the mechanism of action of antitumor bis(aminophenolate) ruthenium(III) complexes

Two recently published Ru(III) complexes bearing (N₂O₂) tetradentate bis(aminophenolate) ligands, formulated as [Ru(III)(salan)(PPh₃)Cl] (salan is the tetradentate ligand 6,6′-(1S,2S)-cyclohexane-1,2-diylbis(azanediyl)bis(methylene)bis(3-methoxyphenol) in complex 1, or 2,2′-(1S,2S)-cyclohexane-1,2-diylbis(azanediyl)bis(methylene)bis(4-methoxyphenol) in complex 2; PPh₃ is triphenylphosphane) and found very active against ovarian and breast adenocarcinoma human cells were studied to outline their antitumor mode of action. The human cisplatin-sensitive ovarian adenocarcinoma line A2780 was used herein as the cell model. At a 24 h challenge (similarly as found before for 72 h) both complexes are active, their cytotoxicity being comparable to that of cisplatin in the same conditions. As a possible target in the cell for their action, the interaction of 1 and 2 with DNA was assessed through displacement of well-established DNA fluorescent probes (ethidium bromide, EB, and 4′,6-diamidino-2-phenylindole, DAPI) through steady-state and time-resolved fluorescence spectroscopy. The whole emission spectra were analyzed globally for the binary DNA–probe and ternary DNA–probe–Ru(III) complex systems. Both Ru(III) complexes can displace EB and bind to DNA with similar and moderate strong affinity with conditional stability constants of logK’ = (5.05 ± 0.01) for 1 and logK’ = (4.79 ± 0.01) for 2. The analysis of time-domain fluorescence intensity decays confirmed both qualitatively and quantitatively the model used to describe the binding and competition processes. Cell studies indicated that apoptosis is the major mechanism of cell death for both complexes, with 2 (the more active complex) promoting that process more efficiently than 1. Transmission electron micrographs revealed clear alterations on intracellular organization consistent with the induction of programmed cell death processes.