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Materials 2020, 13, 2646 19 of 24 cells. The amines’ introduction can thus essentially lead to a significative rise in the cytotoxicity of this dye’s core; however, for the dyes presented in this study, it was found that this was intensified in a balanced way. Despite the evidence of a marked increase in cytotoxicity, which proved to be extremely advantageous in the present research, the amines’ introduction in this dye’s core is of great relevance, since, in addition to allowing an increase in its ability for producing singlet oxygen [77,78], as we verified on a small scale, it favorably increases its uptake by cells due to its cationic character and increases its polarity, thus favoring its interaction with biological systems [79]. Compared to other studies, namely two of our most recent studies involving breast cancer cell line MCF-7 and the potential application of indolenine-based aminosquaraine dyes [25,41], unsymmetrical squaraines present in this work showed similar levels of cytotoxicity in the presence of radiation, but more selective photocytotoxicity, since, for example, in non-irradiated conditions, N-hexyl indolenine-derived dyes only presented half inhibitory concentration values below 1 μM [25,41]. This dark cytotoxicity was subsequently improved by reducing the number of carbons in the N-alkyl chains of this dye’s core [25,41]. Nevertheless, it should be noted that, according to previous studies, dyes with longer carbon chains have longer wavelengths of maximum absorption and greater production of reactive oxygen species [80]. The improved cytotoxicity in the dark for MCF-7 metastatic cell line is also enhanced by the comparison with benzothiazole-derived symmetrical squaraine dyes, which showed half inhibitory concentrations in non-irradiated treatments below 0.6 μM [79]. Regarding the BT-474 cell line, to the best of our knowledge, until now there were no studies involving the photodynamic application of dyes analogous to those presented here. Thus, in general, the aminosquaraine dyes synthesized in this work, despite having low weighting singlet oxygen quantum yields, showed worthy phototherapeutic effects for specific in vitro conditions. This photocytotoxicity, according to our experience, must come from type I reactions; still, studies focused on their action mechanism in the biological environment are ongoing. The moderate to good photostability, as well as the high absorption in the tissue transparency spectral region and photoselective cytotoxic action, make these compounds potential photosensitizer compounds to be applied in photodynamic therapy for breast cancer. 5. Conclusions A series of quinoline- and benzoselenazole-derived squaraine and aminosquaraine cyanine dyes were designed, prepared, and characterized. All dyes showed intense bands in the region of the electromagnetic spectrum, where the living tissues are transparent to light. In organic solvents, the absorption bands, besides being intense, were narrow and had maximum absorption wavelengths between 636 and 733 nm and logarithms of the molar absorptivity coefficients around 5–6. In aqueous media, wider bands were observed due to the formation of aggregates. The constitution of aqueous media was found to influence the type of aggregates formed. Despite the forming of dimers in aqueous media, the absorptivity levels of these dyes remained high without compromising the potential photodynamic activity in a biological environment. The dyes proved to be light stable, highlighting the stability of the zwitterionic dye compared to the commercially available methylene blue photosensitizer. Nevertheless, this dye scaffold showed a low production of singlet oxygen. Despite this, their in vitro phototherapeutic activity was evident for specific conditions tested against the studied breast cancer cell lines. Of the evaluated dyes, the ones that stood out the most were the aminosquaraine dyes, which, in both cell lines, showed conditions in which the degree of cytotoxicity produced in the dark is much lower than when irradiated. The incubation periods with the dyes showed that these compounds have a prolonged activity over time, since there was a tendency to decrease cell viability, which was more accentuated in the period of 24 h compared to that of 1 h after being irradiated. Insignificant differences were observed between treatments irradiated for 7 and 14 min, proving that the shortest irradiation was sufficient to trigger their maximum biological activity. It is assumed that the dyes’ photobiological effects are mostly from type I reactions, as the reduced capacity of these dyes to produce singlet oxygen does not justify it by type II mechanism reactions.PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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