Cyclodextrin

Cyclodextrins (CDs) are a family of cyclic polysaccharides used to form inclusion complexes with a wide variety of substances used in pharmaceuticals, drug delivery systems, cosmetics, and in the food and chemical industries [56]. Their molecular structure is composed of a cavity size, which is determined by the number of glucose units, where the space inside the cyclodextrin molecules allows the formation of inclusion complexes with poorly soluble compounds (Figure 3C) [56]. The inclusion of gues<sup>t</sup> molecules into CDs can change their physical and chemical properties, as well as increasing their water solubility and stability [57]. The CDs are an excellent alternative for the inclusion of a variety of natural compounds, such as oils [58,59] and other compounds [60,61]. The AST inclusion complexes with cyclodextrin are summarized in Table 4.

One of the first studies on the inclusion of AST in CDs to enhance its solubility for topical applications was developed by Lockwood et al. [62]. When used in proportions from 0 to 60% (*w/v*), a sulfobutyl ether β-cyclodextrin was shown to complex with crystalline AST. At 60%, AST water solubility has increased by more than 50 times, and the implementation of a pre-solubilization process could increase it by 71 times over the parent compound in water.

Kim et al. [63] prepared AST inclusion complexes with various types of CDs in different ratios, characterized each formulation by HPLC, SEM and FT-IR, and evaluated their 28-day stability and water solubility under different conditions of pH, light, temperature and oxidation. To minimize the costs, a β-cyclodextrin (β-CD) that is widely used in food and cosmetics applications was used for comparison. Inclusion complexes formed at the AST ratio of 1:200, and the host molecule showed a uniform shape and particle size. β-CD was proved to incorporate AST, with an inclusion yield higher than 90%, and the solubility of the resulting AST-loaded inclusion complex was 13-fold at 25 ◦C and about 100-fold that of free AST at a pH of 6.5. In the stability study, the yield of the inclusion complex

remained above 80% after 21 days of UV irradiation, while the free AST was completely degraded. In addition, it proved stable against oxidation, was favored by acidic conditions and exhibited greater temperature resistance for industrial processing.

**Table 4.** Summary of AST cyclodextrins (CDs) delivery systems, describing the preparation characterization, storage and stability data, and assays (in vitro/in vivo) that were performed for each system.


Chen et al. [57] prepared an AST β-cyclodextrin complex and measured its water solubility and stability to heat and light. Complex formation was checked by infrared spectroscopy and HPLC, showing an inclusion yield of 48.96%. The water solubility of AST was slightly increased, while its heat stability was greatly enhanced compared to the free bioactive compound.

Hydroxypropyl-β-cyclodextrin (HP-β-CD) is a hydroxyalkyl derivative alternative to parent CDs, which offers improved water solubility and is slightly more friendly from a toxicological standpoint [68]. Yuan et al. [64] prepared a new water-soluble formulation of AST with HP-β-CD, analyzed its thermal behavior, and investigated its stability in heat and light. The inclusion yield of the formulation was 46.5%, and no less than 200 mg/mL of it was dispersed in water. Water solubility was greatly enhanced (>1.0 mg/mL) in comparison with the previous study, due to the higher solubility of HP-β-CD compared with β-CD. On the other hand, the overall amount of AST entrapped in the inclusion complex was lower, likely because the hydroxypropyl substituent concentrated at the edge of the CD cavity made the entry of AST molecules more difficult. In 2012, the same research group studied this complex via UV-Vis, FT-IR, 1H nuclear magnetic resonance (NMR) spectroscopies and molecular modeling, to enhance knowledge about the molecule structure [65]. Storage stability at 4 and 25 ◦C was higher than that of free AST, while in vitro antioxidant tests showed greater antioxidant activity than ascorbic acid [66].

To prevent antioxidative stress on endothelial cells, Zuluaga et al. [67] developed a similar complex with AST and HP-β-CD. The differential of this study was the direct and indirect measurement of its antioxidant capacity by understanding the cells' molecular mechanisms involved in gene expression. Results showed that the inclusion complex formulation could protect cells by activating endogenous AST systems through the Nrf2/HO- 1/NQO1 pathway, in addition to the enhancement of solubility due to its incorporation into the cyclodextrin core.
