Recent Applications of the Gated Mesoporous Silica Nanoparticles as Drug Delivery System for Cancer Therapy: Ph Sensitive Controlled Release

Mesoporous silica nanoparticles (MSNs) have attracted extensive attention as efficient drug delivery carriers on account of its adjustable pore size, specific surface area, stability, and good biocompatibility. This review summarizes the recent advances on the application of gated MSNs for controlled drug delivery. These advanced technologies demonstrate current challenges and provide a bright future for precision of cancer treatment.


Introduction
Silica nanoparticles show desirable properties to be implemented as multifunctional nanoplatforms for drug delivery purposes, due to their unique characteristics, comprising extensive surface area, large pore size, lack of toxicity, biocompatibility, biodegradability, uniform distribution of target molecules on the porous space, controllable superficial charge and free dissemination in the whole body [1][2][3]. This review intends to comprehensively analyses published papers that have used pH cleavable linkers (hydrazone linker, acetal linker, imine linker and boronic-ester linker) in combination with MSNs to design gated systems for controlled drug delivery applications.  [4].
The hydrazone bond which is stable at physiological pH could undergo hydrolysis in an acidic environment, has been widely used to prepare smart drug delivery systems. Zhang and coworkers [4] developed multiple functional capping agents, which integrated pore-capping, drug-loading, and tumor-targeting abilities. As depicted in Figure 1, carbonyl was introduced on the surface of MSNs and a one-pot method was employed to load the chemotherapeutic agent in the pores and linked multifunctional capping agents via an acid-cleavable hydrazone bond at the same time. The drug-loading content of this delivery system can be as high as 10.6%, but there was only 7.8% of drug leakage in three days in the blood circulation. Cui et al. [5] introduced Fe 3 O 4 nanoparticles as a core coating with a mesoporous silica shell to obtain Fe 3 O 4 @mesoporous silica (mSiO 2 ) core-shell nanoparticles as the host and then pH-sensitive hydrazone linkage was connected to the mesoporous silica by means of chemical modification ( Figure   2). Doxorubicin (DOX) was adopted as a typical anticancer drug to evaluate the controlled release. After drug loading, the as-prepared Au nanoparticles were used as nanoscopic caps to block the pore and inhibit the drug release. Because of the hydrazone bond, the drug-loading nanocomposites display acid-enhanced release with little premature release under neutral basic conditions (pH 7.4). In addition, cell experiments (with HeLa cells) were carried out, thus revealing the negligible toxicity and good biocompatibility of the pure nanocomposites (Fe 3 O 4 @mSiO 2 @Au). Furthermore, DOX@ Fe 3 O 4 @mSiO 2 @Au reveals the pronounced cytotoxicity to HeLa cells, which is even similar to that of free DOX with concentrations up to 50 μgmL -1 .  Zhang et al. [6] applied a simple method to prepare the pH-sensitive dextran/MSNs based drug carriers in their work.
As shown in Figure 3, MSNs were reacted with 3 aminopropyl triethoxysilane (APTES) to form the MSN-NH 2 , which were further reacted with succinic anhydride to obtain MSN-COOH. Then tertbutyl carbazate was reacted with MSN-COOH to obtain MSN-NH-NH 2 . After loading of DOX, three kinds of dextran dialdehydes (PADs) were applied to cap the pores of MSNs with hydrazone bond formed through the reaction between hydrazine of MSN-NH-NH 2 and aldehyde of PADs. With the pH reduced from 7.4 to 5.5, the drug release rates were further accelerated. According to the results they suggested that drug carrier could entrap most of the DOX at pH 7.4 and burst release of the drugs in the weakly acidic environments, i.e. intracellular environment. Dai et al. [7] constructed a pH responsive drug delivery system for targeted tumor therapy based on hollow mesoporous silica nanoparticles (HMSNs). Hyaluronic acid (HA) molecules were employed as both blocking and targeting agents, which were anchored onto the surface of HMSNs through hydrazone bonds acting as pH-sensitive linkers. The fabricated targeted drug delivery systems could be effectively accumulated at tumor sites due to the targeting effect of HA molecules. And then, the linkers between HA blocking agents and HMSNs would be broken down triggered by the low pH value at tumor sites, leading to the removal of HA blocking agents and local anti-tumor drug release (Figure 4). Wong et al. [8] were synthesized a smart system in which DOX was conjugated to a zinc (II) phthalocyanine  The results showed that the DOX can be controlled release from porous silica nanoparticles via pH and redox dual triggers. HA on the nanoparticle surface could lead to specific cellular uptake and the nanocarriers could targeted deliver to HeLa cells. These results demonstrated that the dual delivery system resulted in high local concentration inside tumor cells and lead to an effective cancer cell apoptosis, which may be promising for cancer therapeutic application. The conjugation of dexamethasone (DEX) onto modified-porous silica materials via a pH-responsive hydrazone bond has been reported to be highly efficient method to specifically deliver the DEX to diseased sites by Numpilai et al. [10]. In their paper, the impact of pore sizes, particle sizes and silanol contents on surface functionalization, drug loading and release behavior of porous silica materials conjugated with dexamethasone via pH-responsive hydrazone bond was investigated. The release of DEX from porous silica-DEX conjugates exhibited pH-dependent, sustained release, with the faster DEX release rate at an acidic pH of 4.5 being due to the hydrolysis of hydrazone bonds.
In order to improve the effects of medical therapy for cancer, Jiang and coworkers [11], prepared magnetic nanocomposites

Acetal Linker
The acetal linkers are stable under neutral conditions (pH 7.4), while can be cleaved under acidic conditions. Chen et al. [13] constructed drug delivery systems by immobilizing acetals on the surface of mesoporous silica, and then coupling to ultra-small lanthanide doped upconverting nanoparticle, which act as a gate keeper. The anti-cancer drug DOX, is thus locked in the pores, and its burst release can be achieved under acidic environment on account of the hydrolyzation reactions of acetals. The resulted system was believed to be able to efficiently transport DOX into the cancer cells, released rapidly in lysosomes and endosomes due to the acidic situation. The nanogated drug release system was highly efficacious for cancer therapy both in vitro and in vivo. After intravenous injection into the murine model, such nanocomposite, which accumulated in several organs, was degraded into apparently nontoxic products within a few days. Despite the enhanced therapeutic through the so-called Enhanced Permeability and Retention (EPR) effect, which was visualized by Magnetic Resonance Imaging (MRI).
Yang and coworkers [14] grafted an acid-labile acetal linker 3,9-   [16] reported the controlled release of guest molecules from mesoporous silica particles by using acid-labile acetal group linked gold nanoparticles as pH-responsive capping agents. The guest molecules were blocked from the hybrid materials at neutral pH and released at lower pH. The release profile is strongly dependent on the cleavage of the acetal linker at different pH's. The results make the system reported a promising candidate in the formulation of a pH-sensitive vehicle for in vivo delivery of therapeutic agents to low pH tissues, such as tumors and inflammatory sites ( Figure 6).

Volume 2 -Issue 5
Copyrights @ Pervin Deveci. Drug Des Int Prop Int J 250 Zhu and coworkers [18] designed a pH and redox dualresponsive supramolecular nanovalve system based on the hostguest interaction of β-cyclodextrins/ferrocenyl moiety (β-CD/Fc) inclusion complexes and an acetal linker to control release of cargos

Rhodamine 6G (R6G) as model drug molecule was encapsulated in
HMSNs-S1, at pH 7.4, almost no leakage was observed, while the R6G was released at acidic environment (Figure 7). A pH-sensitive drug release system using acetalated-dextran as valves was designed to manipulate smart intracellular release of anticancer drugs by Zhe et al. [19]. Dextran was grafted onto the exterior of MSNs through a click reaction, and followed by acetylation to generate the final carriers of MSN-Dex-Ac. The hydrophobic Dex-Ac would act as valves on the MSNs surface to block the entrapped drugs inside the MSNs pores. While under acidic conditions mimicking the microenvironment of endosomal/lysosomal compartments, the valves could be opened by acetal hydrolysis to recover the acetalateddextran to its hydrophilic state, resulting in fast drug release. In vitro drug release profile clearly showed that DOX release was restricted at pH 7.4 by the valves, while it was accelerated under acidic conditions. this study is to employ the drug itself as a pH-sensitive "gatekeeper" to minimize the potential risks of auxiliary capping agents that have been generally used in previous systems. In their design, DOX was used as the drug cargo as well as the "gatekeeper." A benzaldehydefunctionalized MSNs was loaded with DOX and then gated by DOX on pore outlets via the formation of benzoic-imine covalent bond between DOX and benzaldehyde groups. In case of being at weak acidic tumor tissue/cells (pH < 6.8), the loaded DOX could be on-demand released due to the pH-induced hydrolyzation of benzoic-imine bonds (Figures 8a & 8b). This is the first drug-selfgated system that does not require complex capping agents and also possesses switchable drug release behavior triggered by an internal biological stimulus.

Imine Linker
For the design of the dual-controlled release system Zhao and coworkers [21] chosed imine group and the complex of β-CD with azobenzene derivative, for pH and light trigger respectively. The system was prepared by surface modification of MSNs (MCM-41) with imine group and β-CD and azobenzene complex which were used for pH and light controlled delivery, respectively. The surface modification efficiently blocked the cargo release in pH = 7.0 PBS without 365nm UV irradiation. Plenty cargos would be delivered if both factors of acidic environment (pH=5.0) and UV light (365nm) irradiation were satisfied, meanwhile only few cargos were released when one factor was satisfied. Chen et al. [22] prepared MSN based drug carrier to load anti-cancer drug DOX and covered by mono-6-deoxy-6-EDA-b-cyclodextrine (β-CD-NH 2 ) to block the pores through pH-sensitive boronate ester bond. And the carriers were then coated with methoxy poly(ethylene glycol) (mPEG) through another pH-sensitive benzoic imine bond. mPEG leaving studies, in vitro cellular uptake studies and the flow cytometry analysis, proved that carriers was "stealthy" at pH 7.4, but could be "activated" for cytophagy by cancer cells in weakly acidic tumor tissues (pH 6.5) due to the departure of mPEG. β-CD-NH 2 leaving studies, the in vitro drug release studies and the in vitro cytotoxicity studies proved that boronate ester bond linking MSN and β-CD-NH 2 was stable at both pH 7.4 and 6.5 but could be hydrolyzed intracellular to release DOX for cellular apoptosis due to the lower pH (5.0). Cheng and coworkers [23] fabricated a facile and concise codelivery system using the anticancer drug DOX as the drug cargo as well as the gatekeeper of their system. The pore outlets of the MSNs were gated by the drug itself through the formation of a benzoicimine covalent bond between the amino groups of DOX and the benzaldehyde groups of benzaldehyde-functionalized MSNs under weakly alkaline conditions. Moreover, another distinct advantage of this co-delivery system is that the anticancer drug itself serves as a "gatekeeper", which can minimize potential risks brought by other extra capping agents (Figure 9) Zhang et al. [24] reported a novel "stealthy" chitosan (CHI)/MSN complex drug delivery system with tumor-triggered intracellular drug release properties. In this system, MSN was used as the nano-container for the loading of the anticancer drug DOX and was covered by CHI to block the pores through redox-sensitive disulfide bonds. Then mPEG was grafted to the surface of the nanoparticles via a pH-sensitive benzoic imine linker. The system was "stealthy" in physiological environments at pH 7.4 but was "activated" in weakly acidic environments, such as that found in solid tumor tissues, due to hydrolysis of the benzoic imine and the departure of the mPEG shields ( Figure 10).

Boronic-Ester Linker
Liu and coworkers [25] report a novel multifunctional tumor