Cellular Toxicity Induced by the Photorelease of a Caged Bioactive Molecule: Design of a Potential Dual-Action Ru(II) Complex

The series Ru(tpy)(CH3CN)3(2+) (1), cis-Ru(tpy)(CH3CN)2Cl(+) (2), and Ru(tpy)(5CNU)3(2+) (3), where tpy = 2,2':6',2″-terpyridine and 5CNU = 5-cyanouracil, was synthesized, and their photochemical properties were investigated for use as potential photodynamic therapy (PDT) agents. When irradiated with visible light, 1-3 exhibit efficient exchange of the axial CH3CN or 5CNU ligand with H2O solvent molecules. Complexes 1-3 also exhibit photoinitiated binding to DNA when irradiated with λirr ≥ 395 nm light, and DNA binding can be accessed for 2 with λirr > 645 nm, well within the PDT window. Since 3 binds DNA and simultaneously releases biologically active 5CNU, it has the potential to be a dual-action therapeutic agent. Indeed, 3 is cytotoxic upon irradiation with visible light, whereas 1 is not under similar experimental conditions. The lack of toxicity imparted by 1 is explained by the exchange of only one CH3CN ligand in the complex under the irradiation conditions used for the cellular studies. Strategies are being sought to increase the quantum yields of ligand exchange and the cellular penetration of these compounds.