Photophysicochemical Properties and In Vitro Phototherapeutic Effects of Iodoquinoline- and Benzothiazole-Derived Unsymmetrical Squaraine Cyanine Dyes

Abstract

The search to replace conventional cancer treatment therapies, such as chemotherapy, radiotherapy and surgery has led over the last ten years, to a substantial effort in the development of several classes of photodynamic therapy photosensitizers with desired photophysicochemical and photobiological properties. Herein we report the synthesis of 6-iodoquinoline- and benzothiazole-based unsymmetrical squaraine cyanine dyes functionalized with amine groups located in the four-membered central ring. Their photodegradation and singlet oxygen production ability, as well as their in vitro photocytotoxicity against Caco-2 and HepG2 cell lines using a 630.8 ± 0.8 nm centered light-emitting diode system, were also investigated. All photosensitizer candidates displayed strong absorption within the tissue transparency spectral region (650–850 nm). The synthesized dyes were found to have moderate light stability. The potential of these compounds is evidenced by their cytotoxic activity against both tumor cell lines, highlighting the zwitterionic unsubstituted dye, which showed more intense photodynamic activity. Although the singlet oxygen quantum yields of these iodinated derivatives are considered low, it could be concluded that their introduction into the quinoline heterocycle was highly advantageous as it played a role in increasing selective cytotoxicity in the presence of light. Thus, the novel synthesized dyes present photophysicochemical and in vitro photobiological properties that make them excellent photosensitizer candidates for photodynamic therapy.