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Materials 2020, 13, 2646 14 of 24 3.1.5. Singlet Oxygen Formation Ability Singlet oxygen is reported as the most important reactive oxygen species in photodynamic therapy, as it has high reactivity and produces significant cytotoxicity from the biological point of view [66–68], and several changes in the dye structure have been carried out to enhance the increase in its ability to produce type II reactions, a mechanism from which these species are produced [40,69]. Among the most common of the structural modifications in squaraine cyanine dyes, the introduction of heavy atoms is noteworthy, since, according to the literature, it leads to an increase in the dyes’ ability to generate type II reactions [45,70]. Despite the importance given to singlet oxygen about the potential application of novel molecules in photodynamic therapy, several studies showed that other mechanisms of action may be involved in phototherapeutic activity, namely type I reactions, in which other reactive oxygen species are produced that, despite being less responsive, are also highly toxic agents at high concentrations [37,48]. The dyes synthesized in this work showed low singlet oxygen quantum yields (Table 3), so their ability to trigger type II reactions is reduced. Even so, it is possible to verify that the introduction of amine groups in the four-membered central ring led to a slight increase in its production ability of these reactive oxygen species, since, compared to the zwitterionic compound 9, the insertion of methylamine group (12) caused their quantum yield doubled. O-methylated derivative 10, as it was not subject to photodynamic evaluation, for the reasons mentioned above, was not studied for this property either. Table 3. Singlet oxygen quantum yield (Φ∆) of quinoline- and benzoselenazole-derived unsymmetrical squaraine cyanine dyes 9, 11 and 12 measured in chloroform (CFM) using phenazine (Φ∆ = 0.84) as reference. Φ∆ Dye 9 Dye 11 Dye 12 CFM 0.03 0.04 0.06 3.2. In Vitro Photoantiproliferative Effects In vitro studies are undoubtedly extremely advantageous in a first approach to assess the effectiveness of novel photosensitizer candidates, given that, through the use of previously characterized cell lines, it is possible to obtain information about the intrinsic cytotoxicity of the compounds after irradiation. The phototherapeutic potential of the prepared squaraine dyes in cell cultures at first interpellations is aligned with the principle of the 3 R’s (“reduction”, “refinement” and “replacement”) [71]. As a preliminary study about the medicinal potential of dyes for the phototherapeutic treatment of breast cancer, in this research, the adherent BT-474 and MCF-7 cell lines were incubated with the synthesized compounds. Both cell lines come from the mammary gland; however, while the BT-474 cell line comes directly from its duct, MCF-7 cells are a line of epithelial cells isolated from a pleural effusion from a woman with metastatic breast cancer [29,72]. To evaluate the effect of the squaraine cyanine dyes 9, 11 and 12 on BT-474 and on MCF-7 breast cancer cell proliferation, cells were treated at several concentrations (0.1, 1.0, 5.0 and 10.0 μM). Twenty-four hours after treatment, photodynamic irradiation was applied for 7 or 14 min on the cells using a self-designed light-emitting diode system with a red light of λ = 630.2 ± 0.8 nm and radiant flux of P = 4.3 ± 0.5 mW (see methods for details). After 1 h and after 24 h of cell contact with the irradiated dyes, squaraines’ antiproliferative effects were evaluated using the Alamar Blue colorimetric method. Negative controls related to the presence of squaraine dyes and irradiation were also performed, that are non-treated and non-irradiated cells. Analyzing the activity of the squaraine dyes in the BT-474 cell line (Figure 4), it appears that aminosquaraine dyes 11 and 12 were the ones that showed the best phototherapeutic effects. Indeed, zwitterionic squaraine dye 9 exhibited higher cytotoxic activity under non-irradiated conditions compared to irradiated conditions, except for the concentration of 5.0 μM after 24 h irradiation.PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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