In Vitro Antiproliferative Effects of Benzothiazole-Based Aminosquaraine Dyes Against Cancer Cell Lines

The introduction of amine groups into the four-membered central ring of squaraine dyes is expected to enhance the intrinsic cytotoxicity of this scaffold. In this study, the biological effects of eight benzothiazole-based aminosquaraine dyes were investigated by varying the length of the N-alkyl chains and the nature of the introduced amine. Biological activity was evaluated through different assays in several cell lines, including apoptosis and cell cycle analysis, as well as confocal microscopy studies. Overall, the incorporation of amino, methylamino, and ethanolamino groups significantly increased cytotoxicity, with the corresponding dyes displaying low half-maximal inhibitory concentration values, in some cases even lower than that of the positive control, 5-fluorouracil. In contrast, derivatives bearing diethanolamino or ethylenediamino groups exhibited lower cytotoxicity, particularly the dye containing N-decyl chain. For the dyes selected for further investigation, bearing a methylamino group and N-butyl or -hexyl chains, the induction of apoptosis was evident, especially for methylaminosquaraine dye 3. These dyes also exhibited antiproliferative effects, as evidenced by their ability to induce cell cycle arrest. Confocal microscopy revealed a predominantly cytoplasmic distribution, likely associated with cytoplasmic organelles, with limited penetration into the nucleus. Overall, although squaraines are widely recognized as promising photodynamic agents, the present results suggest that they may also be explored in chemotherapeutic approaches, particularly when exhibiting high intrinsic cytotoxicity.