# Silica Mesoporous Structures: Effective Nanocarriers in Drug Delivery and Nanocatalysts
## 5 Application of Silica Mesopores in Drug Delivery
### Enzyme-Responsive Drug Delivery  
Due to their biocompatibility and specific biological activities, enzymes are suitable for the design of controlled-release delivery systems [190]. [PERSON] et al. designed a biocompatible matrix metalloproteinase (MMP)-responsive drug delivery system based on MSN (Figure 18) [191]. In this study, they used BSA as an end-capping agent, a functional polypeptide as an intermediate linker, and lactic acid (LA) as a targeting motif for releasing doxorubicin (DOX) in vivo. The in vivo experiments have shown that the system can inhibit tumor growth efficiently with minimal side effects. Other studies have investigated on the multifunctional stimulus-responsive nano vehicles that were constructed via peptide mediated core/satellite/shell assembly for targeted protease imaging and controlled drug release [192; 193]. [PERSON] et al. used gold nanoclusters based on MSNs through the conjugation of peptide cathepsin B to form MSNs@GNC with a core/satellite structure. The peptide-mediated core/satellite/shell nanocomplexes could be utilized as multifunctional nanovehicles for precise and dynamic imaging of cathepsin B and of controlled drug release [193].  
[PERSON] et al. used hyaluronic acid (HA) to modify silica mesopores in a smart drug delivery response to the enzyme hyaluronse (HD) [194]. HA, in addition to blocking mesoporous channels, can also operate as a target ligand. This modified nano-carrier has interesting features, including a simple chemical structure, high colloidal stability, good biocompatibility, good cell targeting, and good drug release control. HA decomposes when it reaches the infected cell due to excessive congestion with the HD enzyme, and the drug is released. The rhodamine model was utilized in this study. They confirmed the release rates of the drug in the presence and absence of the HD enzyme to ensure the system's responsiveness to the enzyme. The amount of drug released in the proximity of the HD enzyme was much greater than the amount of the drug released without the enzyme in an acetate buffer  
Figure 17: CdS/MSN-mercapto acetic acid as a nanocarrier for drug delivery system (DDS).  
(pH = 4.5) and a PBS buffer (pH = 7.4). [PERSON] et al. used lactose to fill mesoporous cavities [195]. Galactose has a large structure and, through this structure, prevents the release of pyridine ruthenium ([Ru (bipy)\({}_{3}\)]\({}^{2+}\)) in a drug model. When lactose is in contact with the galactosidase enzyme, it is hydrolyzed into galactose monosaccharide and glucose, and the resulting small structure causes the drug to escape from the cavities. [PERSON] et al. choose an enzyme-sensitive system that was able to reduce the azo linkage [196]. Mesoporous was converted to mesoporous with an iodine functional group; at a later stage, sulphasalazine was coupled to it, and two sulphapyridine and 5-amino-salicylic acid drug precursors were loaded. Due to the placement of the mesoporous in the surrounding enzyme reductase, the azo bond was broken, and the drug was released. [PERSON] et al. studied drug delivery to liver tissue using mesopores [197; 198]. Silica mesopores were modified via an intracellular cyclodextrin complex. The addition of the cyclodextrin and the loading of rhodamine as a drug material were accomplished simultaneously. To prevent a click reaction from taking place, a bulking group was attached to cyclodextrin, which connected to the mesopores with two esters and amid linkers. These two bonds are desirable when evaluating the effect of liver enzymes. However, the key factor is which linkage is hydrolyzed under these conditions. The results indicate that the ester bond was rapidly hydrolyzed. As a result, the bulk group was exited, and the drug was released.