Figure 5 The hydrodynamic diameters of beta-CD (a), aldehyde group functionalized beta-CD (b), crosslinked beta-CD nanocage (c) and crosslinked beta-CD nanocage loaded with DOX (d).
3.2 Self-regulate drug release of beta-CD nanocage in mimic tumor microenvironment
In this beta-CD nanocage, cystamine was used as a crosslinker in a simple manner to assemble this nanostructure, benefiting from its disulfide bond and amino groups at the two ends. This property makes it convenient to react with the aldehyde group-functionalized beta-CD. The new covalent framework with imine and disulfide bonds has been established, but its acidity/reduction-sensitive performance remained unclear. Therefore, understanding the acidity/reduction-responsive release of drugs is crucial for determining their impact on enhancing anti-tumor efficiency. To investigate the acidity/reduction-responsive drug release, pH 4.5 PBS solution and 5 mM GSH solution were used to mimic the acidic and highly reductive tumor microenvironments, respectively. In the acidic-mimic environment, the beta-CD nanocage exhibited a rapid drug release rate (over 60% drug release in conjugation of imine contributed to bond fission in an acidic environment. This led to the disassembly of the beta-CD nanocage and the release of DOX from the enclosed nanocage. In the GSH-mimic environment, the DOX release rate from the beta-CD nanocage increased (over 60% drug release in ~25 h) but was weaker compared to the acidic-mimic environment, suggesting that the controllable breakage of disulfide bonds was mediated by GSH. More importantly, the beta-CD nanocage exhibited weak release even after 50 h in a normal physiological environment. This contrasted markedly with its controllable drug release under acidic/GSH stimuli. Additionally, the hydrodynamic diameters of the DOX-loaded beta-CD nanocage treated with acidic and reductive agents, as well as neutral solutions, were measured (Figure S3). The change in hydrodynamic diameters was more pronounced when the DOX-loaded beta-CD nanocage was treated with acidic and reductive agents, indicating easier disintegration of the beta-CD nanocage structure in acidic and reductive environments. Therefore, the stimuli-responsive properties of the beta-CD nanocage were ultimately attributed to the covalent imine and disulfide bonds.