Figure 7 In vitro biocompatibility of beta-CD nanocage (a). Schematic illustration of drug release of beta-CD nanocage in the acidic and reductive microenvironment of tumor cell (b). In vitro antitumor effect of DOX-loaded beta-CD nanocage in acidic environment (c), reduction environment (d) and normal physiological environment (e) as well as the in vitro antitumor effect of free DOX (f).
3.4 Intracellular fluorescence imaging analysis of coumarin 6-loaded beta-CD nanocage
To determine whether the beta-CD nanocage could enhance DOX release in acidic or GSH-specific cellular microenvironments, intracellular fluorescence imaging was conducted to evaluate the two distinct responsive properties of the beta-CD nanocage. Tumor cell environments were induced to intensify acid and GSH expression. Specifically, in acid-treated tumor cells, green fluorescence was evident at 3 h and continued to increase at 6 h, indicating that H+triggered the breakage of imine bonds, causing the nanocage to gradually disintegrate and release coumarin 6. When tumor cells were treated with GSH, the green fluorescence signal was also evident and increased over time, indicating that disulfide bonds also underwent breakage in the enhanced reducing environment, allowing coumarin 6 to escape from the beta-CD nanocage. In untreated tumor cells, the fluorescence was lower compared to acid- or GSH-treated tumor cells, and the contrast between treated and untreated groups was noticeable. These results demonstrated that beta-CD is highly sensitive to acidic and GSH environments. Therefore, the enhanced intracellular drug release and uptake could be improved using this proposed beta-CD nanocage.