Tuning Polyamidoamine Design To Increase Uptake and Efficacy of Ruthenium Complexes for Photodynamic Therapy

In this work, we report the synthesis of [Ru(phen)₃²⁺]-based complexes and their use as photosensitizers for photodynamic therapy (PDT), a treatment of pathological conditions based on the photoactivation of bioactive compounds, which are not harmful in the absence of light irradiation. Of these complexes, Ru-PhenISA and Ru-PhenAN are polymer conjugates containing less than 5%, (on a molar basis), photoactive units. Their performance is compared with that of a small [Ru(phen)₃²⁺] compound, [Ru(phen)₂BAP](OTf)₂ (BAP = 4-(4'-aminobutyl)-1,10-phenanthroline, OTf = triflate anion), used as a model of the photoactive units. The polymer ligands, PhenISA and PhenAN, are polyamidoamines with different acid-base properties. At physiological pH, the former is zwitterionic, the latter moderately cationic, and both intrinsically cytocompatible. The photophysical characterizations show that the complexation to macromolecules does not hamper the Ru(phen)₃²⁺ ability to generate toxic singlet oxygen upon irradiation, and phosphorescence lifetimes and quantum yields are similar in all cases. All three compounds are internalized by HeLa cells and can induce cell death upon visible light irradiation. However, their relative PDT efficiency is different: the zwitterionic PhenISA endowed with the Ru-complex lowers the PDT efficiency of the free complex, while conversely, the cationic PhenAN boosts it. Flow cytometry demonstrates that the uptake efficiency of the three agents reflects the observed differences in PDT efficacy. Additionally, intracellular localization studies show that while [Ru(phen)₂BAP](OTf)₂ remains confined in vesicular structures, Ru-PhenISA localization is hard to determine due to the very low uptake efficiency. Very interestingly, instead, the cationic Ru-PhenAN accumulates inside the nucleus in all treated cells. Overall, the results indicate that the complexation of [Ru(phen)₂BAP](OTf)₂ with a cationic polyamidoamine to give the Ru-PhenAN complex is an excellent strategy to increase the Ru-complex cell uptake and, additionally, to achieve accumulation at the nuclear level. These unique features together make this compound an excellent photosensitizer with very high PDT efficiency.