Light-activated cytotoxicity of dicarbonyl Ru(II) complexes with a benzimidazole coligand towards breast cancer

Reaction between [RuCl2(CO)(2)](n) and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine ligands (L-R) functionalized with various electron-donating and electron-withdrawing substituents on the phenyl ring (R = H, 4-CH3, 4-Cl, 4-COOCH3, and 3-COOCH3) afforded the dark-stable photoactivatable carbon monoxide prodrugs of the general formula [RuCl2(CO)(2)L-R]. Release of the CO molecules from the Ru(II) compounds was examined by monitoring the electronic and IR spectra upon illumination at 365 nm. A noticeable decrease in the intensities of the two characteristic nu(C O) modes for Ru(CO)(2)(II) species, and the growth of two new bands for the mono-carbonyl species and free CO, were the main features of the photolysis profiles. The cytotoxicity of the complexes towards breast cancer (MCF-7) cells was assessed with and without illumination at 365 nm. All the complexes except that with a 4-COOCH3 group (IC50 = 45.08 +/- 3.5 mu M) are nontoxic under dark conditions. Upon illumination, all the compounds acquired cytotoxicity in the following order: H > 4-COOCH3 > 4-CH3 > 4-Cl > 3-COOCH3. Investigation of the cytotoxicity of the CO-depleted fragments showed that the light-induced cytotoxicity can be attributed to the liberated CO and CO-depleted metal fragments, including the liberated benzimidazole ligands.