# Has the Toxicity of Therapeutic Deep Eutectic Systems
## 2 Cell Lines  
### Cell Viability  
The analysis of DESs' cytotoxicity has been an objective since they were firstly described and potential applications were proposed [26]. In fact, each time that a new mixture is characterized, and the goodness of its properties is demonstrated, its toxicity should be \begin{table}
\begin{tabular}{c c} \hline \hline
**Potential Advantages** & **Applications** \\ \hline Higher API solubility & Enhancement of therapeutic action of API \\ Higher API permeability & Faster drug delivery \\ Improved API absorption & Controlled drug release \\ Improved API bioavailability & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Potential advantages and applications of THEDESs. Figure 1: Focus of the review about toxicity studies developed with THEDESs. also checked. Despite being a mixture of components that are not harmful, toxicological behavior of the final resulting DES can change the expected result, taking into account the components separately. For example, [PERSON] et al. demonstrated a low cytotoxicity compared to ionic liquids, and they pointed that due to this fact, they could be useful for therapeutic applications. They examined the cytotoxicity of six different concentrations of several ammonium-based DESs toward seven cell lines (PC3, A375, HepG2, HT29, MCF-7, OKF6, and H413), most of them being cancer cell types, using MTT cell viability assay. When comparing the cytotoxicity results of DESs with their individual components, a synergistic effect was supposed to play a role, since the results could not be explained by the individual behavior of each one alone. Additionally, some components of the DES can show a higher toxicity, and even the effect in different cell lines can be variable [42]. One of the components that was pointed out as a potential factor that increases the cytotoxicity of DESs is oxalic acid, when fish (CCO) and human (MCF-7) cells are exposed to a eutectic mixture with ChCl. Nevertheless, the sensitivity of the cell lines was also different to this DES [43]. The HBA was considered as the main responsible factor of the cytotoxicity found by [PERSON] et al. in their study involving several THEDESs toward two cell lines [44]. When compared to ionic liquids, NADESs have also been recognized with a low cytotoxicity profile, although some constituents, such as tartaric acid, could affect the metabolism of cells [45]. Similarly, one study about the cytotoxicity of NADESs showed that the cytotoxicity was lower than in the former version, DESs, at the same time that it highlighted the importance of several factors such as their composition (e.g., presence of organic acids) or physical properties (e.g., viscosity) influencing it [46]. Sometimes, the toxicity of DESs is searched to destroy bacteria. [PERSON] et al. found that some phosphonium-based DESs were effective in the growth inhibition of both kinds of bacteria, Gram-positive (_Bacillus subtilis_ and _Staphylococcus aureus_) and Gram-negative (_Escherichia coli_ and _Pseudomonas aeruginosa_) [47]. [PERSON] et al. characterized different ChCl-based DESs and suggested that those DESs that are acid-based could be more harmful to bacteria [48]. If the attempt of confirming the antibacterial activity of DESs is successful, DESs can even enhance the antibacterial activity of drugs used for this purpose. Occasionally, DESs are not used to solubilize any chemotherapy agent or to enhance their activity, but they show their own cytotoxic usefulness for destroying cancer cells. In such cases, selectivity is the main property that must be verified in order to avoid side-effects [42]. Moreover, DESs have been also used as reaction media for the preparation of an anticancer compound, such as in the case of a derivate of a quinazoline. The final product was also tested using MTT assay with three cell lines (MCF-7, A549, and MCF-10A) in order to find a higher cytotoxic effect on the cancer cell lines than in the non-malignant one (MCF-10A) [49].