Photodynamic therapy in nanomedicine: synthesis and multidisciplinary characterization of squaraine sensitizers for theranostics

The present dissertation deals with the study of 1,3- and 1,2-substituted squaraine dyes and their functionalized derivatives as efficient combined fluorescent markers and photodynamic therapy (PDT) agents. This well-established medical treatment involves the insurgence of citotoxic species in the cellular environment following up the irradiation of a suitable photosensitizer with visible light in the presence of molecular oxygen. Squaraine dyes (1,3-dicondensation products of squaric acid and electron-rich molecules) have been successfully employed for their unique optical properties. These cyanine-like chromophores are indeed characterized by an intense absorption and emission band localized in the transparency window of biological tissue (600–900 nm). Furthermore, squaraines are largely stable and non-toxic in the dark, but after irradiation they promote a strong dose-dependent phototoxic effect due to the formation of radical species. By an accurate choice of the reactants and of the synthesis conditions it has been possible to control the regiochemistry of substitution on the squaric acid core in order to selectively obtain the most common 1,3-regioisomers or the seldom mentioned 1,2-regioisomers. The photophysical and electrochemical behavior of 1,2-squaraines and their higher responsiveness to the chemical environment associates these chromophores to merocyanines. A challenge that occurs when dealing with photosensitizers is the way they are solubilized and specifically delivered to the biological target. This issue can be addressed by taking advantage of the high flexibility of the squaraine structure, which provides a powerful tool aimed at the improvement of bioavailability. In particular, different functional groups can be incorporated in the squaraine structure without alteration of the photophysical properties, obtaining a library of post-functionalizable squaraines. As representative examples, it is herein reported the study on the conjugation with biologically relevant groups (i.e. choline and galactose) and with zeolite nanocrystals. A promising third generation PDT photosensitizer has been thus obtained, which conjugates diagnostic imaging and therapy into a single integrated nanovector. Moreover, a fullero-squaraine dyad has been designed in order to combine the photophysical properties of the two portions, so that the conjugate unites the characteristic absorption of squaraine dyes and the high singlet oxygen generation of fullerene via intra-molecular energy transfer processes.