Cytotoxicity modulation of ruthenium(II) tris-bathophenanthroline complexes with systematically varied charge

Abstract

A series of four ruthenium(II) complexes Ru(bp)n(bps)3-n (n = 0-3; bp = 4,7-diphenyl-1,10-phenanthroline or bathophenanthroline; and bps = disulfonated 4,7-diphenyl-1,10-phenanthroline or bathophenanthroline disulfonate) bearing different charges were synthesized and characterized. In aqueous media, all complexes displayed similar photophysical properties which makes this series ideal to study the effect of charge on the cytotoxicity of Ru(II) complexes. Ru(bp)32+ (1) and Ru(bps)34− (4) are known photosensitizers that penetrate cancer cells and possess a significant light-induced cytotoxicity. The newly conceived neutral Ru(bp)2(bps)0 (2) and dianionic Ru(bp)(bps)22− (3) were also found to have significant uptake as well as light-activation properties. Importantly, the cytotoxicity in the dark was successfully tuned upon systematic charge modulation of the complexes. The cationic complex 1 was potent against 5 out of 6 cell lines tested (MDA-MB-231, MCF-7, B16, SF and ML2) and displayed the highest cytotoxicity among the tested complexes with IC50 values of 4.0, 3.6, 1.7, 1.0 and 2.9 μM, respectively. The two anionic complexes, 3 and 4, with respective overall charges of −2 and −4, were not potent against any of the cell lines in the dark (IC50 > 200 μM), whereas the neutral molecule, complex 2, was potent against 3 out of 6 cell lines in the dark (B16, SF and ML2) and exhibited an intermediate activity with IC50 values of 12.8, 3.0 and 10.3 μM, respectively. All complexes presented significant phototoxicity when activated at 6 h post-incubation, which is consistent with their fast uptake and ability to produce singlet oxygen. The localization of complexes 1–4 within the cells was found to be dependent on the charge and photoexcitation conditions. The charge-activity relationship elucidated in this work facilitates the development of new sensitizers for photodynamic therapy.