Preparation and Photodynamic Activities of Silicon(IV) Phthalocyanines Substituted with Permethylated β-Cyclodextrins

A series of silicon(IV) phthalocyanines linked to two permethylated β‐cyclodextrin moieties through different spacers at the axial positions have been synthesized and characterized. The effects of these spacers on the photophysical properties and in vitro photodynamic activities have also been examined. Having two bulky hydrophilic substituents, all of these compounds are soluble and essentially nonaggregated in DMF and even in aqueous media. The fluorescence and singlet oxygen quantum yields in DMF are lower for the analogue with the shortest separation between the amino group in the spacer and the phthalocyanine ring. The fluorescence quantum yield of this compound increases in water probably due to protonation of the amino group, which inhibits the reductive quenching process. This series of compounds also exhibit photocytotoxicity toward HT29 human colon adenocarcinoma and HepG2 human hepatocarcinoma cells with IC50 values in the range of 0.04–1.32 μM. The analogue with an α,ω‐aminohydroxypentyl linker shows the highest potency, which can be ascribed to its high cellular uptake and high efficiency in generating intracellular reactive oxygen species. This compound also shows preferential localization in the lysosome, induces cell death mainly through apoptosis, and inhibits the growth of tumor in vivo. The results suggest that it is a promising photosensitizer for photodynamic therapy. Not to be kept in the dark: Permethylated cyclodextrins serve as excellent substituents to increase the hydrophilicity of silicon(IV) phthalocyanine (SiPc), reduce its aggregation in aqueous and biological media, and enhance its photodynamic activities, both in vitro and in vivo, by facilitating the cellular uptake and promoting the intracellular reactive oxygen species production (see figure; ROS=reactive oxygen species).