Smart mesoporous silica nanoparticles for controlled-release drug delivery

3.1 Redox-responsive MSN

The difference between the reduction potential in the intracellular and extracellular environments provides an opportunity to design nanoparticles that are specific for intracellular delivery [46]. The high concentration of glutathione in the intracellular milieu is the main reason for this difference; this is only 2–20 μm in the body fluids, while it is as high as 0.5–10 mm inside the cells [47]. Cancer tissues contain low concentrations of oxygen compared to normal tissue, and the concentration of GSH in cancer cells is at least four times higher than GSH in normal physiologic microenvironments [48]. In the presence of high concentration of glutathione, the thiol group of the cysteine residue acts as an electron donor and is responsible for the antioxidant scavenging activity of GSH. In reducing conditions, the thiol group donates an electron to the unstable disulfide bond of the gatekeepers in MSN. The reduced disulfide bond is broken down, the gatekeeper is removed, and two molecules of glutathione become joined together by a new disulfide bond as a GS-SG molecule [49]. Figure 2 illustrates the basic mechanism for the redox responsivity of MSN.

Figure 2:

Redox-responsive MSN: disulfide bonds are broken at a high concentration of GSH, and the caps (here PEG molecules) are removed, and the loaded dye is released. Reused from Reference [41] with the permission of Elsevier publisher.

The surface of MSN, due to the intrinsic property of silica, possesses an overall negative charge and can be easily noncovalently linked to cationic polymers or molecules such as the amphiphilic peptide RGD, IL13, collagen, polyethyleneimine (PEI), and poly (N-isopropylacrylamide) that can all act as a “cap” for sealing the MSN pores [50]. Wang et al. used amphiphilic polymers to function as a gatekeeper in doxorubicin (DOX)-loaded MSN. In this MSN, first, a stearic acid derivative (Cys-C1s) was attached though a disulfide linkage, to the surface of MSN, then an RGD sequence to target Integrin αv (3 (C18-DSDSDSDSRGDS) was linked via a hydrophobic interaction with the octadecyl group, and finally, DOX was loaded into the modified MSN pores. RGD targeted the MSN to the tumor neovasculature, and then, the cap was removed due to cleavage of disulfide bound by intracellular redox conditions, and anticancer drug DOX was released inside the cells [51].

In a recent study, Li et al. [52] generated MSN-SH with a mesopore diameter of 2.64 nm by using a co-condensation method, then attached S-(2-aminoethylthio)-2-thiopyridine hydrochloride to produce MSN-S-S-NH₂. This structure was then reacted with propargyl bromide to obtain MSN-S-S-NH-alkyne. This particle was then attached to an azidopeptide via “click chemistry” to generate MSN-S-S-RGD. The MSN was then loaded with DOX that showed good responsivity to the presence of GSH in tumor cells. Furthermore, Sun et al. [53] used redox stimulus-responsive MSN to inhibit cancer by suppressing neovascularization and encouraging vascular normalization. For this purpose, they immobilized PEI through disulfide bond formation onto the surface of siRNA-loaded MSN to increase the RNA half-life and its penetration into targeted cancer cells. The MSN-siRNA/CrPEI showed an efficient intracellular gene delivery and therapeutic effect on the vascular endothelial growth factor (VEGF) pathway.

3.2 Enzyme-responsive MSN

Enzymes have unique properties such as their isoelectric pH, exquisite substrate specificity, greater expression in particular organs and in subcellular organelles and can have large changes in concentration in inflammation and disease states. Proteases have a critical role in intracellular delivery [54], and matrix metalloproteinases (MMP) can be specific for the cancer microenvironment [55]. The concentration of elastase is increased in inflammation [56], and phospholipases are overexpressed in pancreatic cancer and can be used for antibiotic delivery [57]. Oxidoreductases also are taken advantage of in oxidase-responsive DDS [58]. MSN can be tailored to respond to these different enzymes by changing the linkers and capping agents on their functionalized surface.

Van Rijid’s group [54] coated the external the surface of MSN with a hepatopeptide that contained a biotin group linked to the amino acid sequence SWMGLP, which can be recognized by MMP9. The enzyme MMP9 is a matrix metalloproteinase that breaks down the extracellular matrix in physiological conditions, and many basic functions of cells depend on this enzyme, such as embryonic and bone development, cell migration, and wound healing responses. MMP9 is frequently overexpressed on malignant cancer cells, where it is responsible for tissue invasion and metastasis. After loading of the mesopores with the anticancer drug cisplatin, the protein avidin was attached to the biotin as a bulky gatekeeper to keep the loaded drug in the MSN pores. In the presence of MMP9 on the surface of lung cancer cells, the peptide sequence was cleaved by the enzyme, and the drug was released to the cancer cells. In a similar study, Xu et al. [55] coated MSN with a gelatin corona susceptible to degradation by MMP for delivery of DOX into HT-29 human colon carcinoma cells and suggested that the same MSN could be used for delivery of different drugs into different cells.

In an interesting study, Li et al. [59] attached an oligocationic TAT nucleus-penetrating peptide to the surface of drug-loaded MSN and neutralized it with a cleavable anionic peptide. In the cancer cells and in the presence of cathepsin B, the anionic peptide was cleaved, and the carrier was transferred into the nucleus by the penetrating peptide. Therefore, the anticancer drug was released directly into the nucleus. This strategy could be an efficient system for drug-resistant tumor therapy. Figure 3 depicts the basis of this strategy.

Figure 3:

Schematic mechanism of nucleus-targeted drug delivery; MSN was synthesized by quantum dot method and TAT sequence along with cathepsin B-responsive peptide attached to its surface. Inside the cancer cell, the enzyme cuts the responsive peptide, and the nucleus-penetrating agent guided the carrier to the nucleus. Reused from Reference [59] with the permission of John Wiley and Sons’ publisher.

Mondragon et al. [60] used a nontoxic lysine polymer with an ε-amino group linkage for capping the MSN, which was biodegradable by amidases. They showed this gatekeeper had a zero cargo release when the protease was absent, but after it was taken up by HeLa cells, the cargo was efficiently released into lysosomes under the action of proteases. Recently, Cheng et al. [42] designed a three-segment oligopeptide-conjugated rotaxane MSN. This oligopeptide consisted of a RGGS tumor-targeting peptide, a sequence of seven arginine residues as a cell-penetrating peptide, and a GFLG enzyme-cleavable peptide. This nanocarrier bound to the integrin αvβ3 that was overexpressed in cancerous cells and was internalized into the cells by the arginine sequence. Inside the cells, the GFLG peptide was cleaved by cathepsin B, and 80% of the loaded drug was rapidly released inside the cell.

3.3 pH-responsive MSN

The high rate of glycolysis in cancerous cells, leading to the production of lactic acid and carbon dioxide directly results in their having a relatively low pH that can be used as an endogenous stimulus for targeted drug release. This biochemical characteristic of abnormal cells has been utilized as a strategy to design an appropriate gatekeeper on the MSN surface and tunnels that could be triggered in response to an acidic environment.

Polyelectrolyte polymers formed from repeating monomer units bearing electrolyte groups can either be absorbed or covalently bonded to the surface of MSN. They act as stimulus-responsive release systems by virtue of their structural transformation in response to different pH values. The multilayered polyelectrolytes formed from PAH/PSS were used to modify MSN by Shi et al. [61]. The positive charge of PAH coated the negatively charged surface of the MSN. The negatively charged PSS, then, was added to PAH. Their results showed that gentamicin molecules could be stored within MSN pores and released in response to pH changes in the range of 2–8. The basic principle of pH-responsive drug release is shown in Figure 4.

Figure 4:

Basic concept of drug release under acidic conditions. Reused from Reference [43] with the permission of Elsevier.

Three different forms of MSN (carboxylated mesoporous silica (MSN-COOH), animated MSN (MSN-NH2), and hollow MSN (H-MSN)) were used by Gao et al. [62] to load anticancer drugs. Using cationic and anionic polyelectrolyte-coated MSN provided an opportunity for loading different types of bioactive molecules through the layer-by-layer method. Positive-charged drugs were loaded better into MSN-COOH, whereas MSN-NH₂ was more suitable for anionic drugs, while H-MSN exhibited a high capacity to load both types of drugs. Their results showed great potential as a pH-responsive DDS.

Supramolecular species such as β-cyclodextrin (β-CD) and proteins can act as another type of gatekeeper in pH-responsive MSN. An interesting study was carried out by Meng et al. in which a series of aromatic amines were used as a stalk to attach to MSN, and then, β-CD was attached to the stalk to prevent cargo release at normal pH. In acidic environments (pH<6), however, the stalk lost its attachment due to protonation of the aromatic amines, and the cargo was released immediately upon opening of the nanovalves. The cell lines THP-1 and KB-31 were studied in their in vitro investigations. The THP-1 macrophages took up the ingested particles into the lysosomal compartment, and KB-31 was a cancer cell model to test the efficacy of the procedure [63].

pH-sensitive linkers such as acetal, ester, and hydrazine bonds have also been utilized in the design of smart MSN. These bonds can be used to attach bulky groups to cover the MSN pores, thus, preventing drug release at a pH about 7.4 as in the blood circulation. Recently, acetalated dextran (a polysaccharide with water-soluble characteristic achieved by hydrolysis of starch or glycogen) was used by Huang et al. to produce pH-sensitive MSN. The water solubility of the dextran could be modified by attaching cholesterol as a lipid soluble moiety. By modifying MSN with CaCO₃, another kind of pH-sensitive MSN was produced to produce acid-triggered release of the drugs, DOX and ibuprofen. Their results showed that the CaCO₃ coating dissociated in acidic conditions, and the drug was released rapidly. At physiological pH by contrast, the release of the loaded drug occurred very slowly [64].

3.4 Light-responsive MSN

Light responsiveness has been suggested to be a promising strategy for external stimulus-triggered drug release due to the noninvasive nature and the ability to remotely control it from outside. Near-infrared (NIR) light is a safe wavelength range and is able to deeply penetrate into tissue without any hazardous effect on the internal organs [65]. The main mechanisms of light-triggered drug release are (a) a photothermal increase of temperature at the site of irradiation with a corresponding change in the steric structure; and (b) photoisomerization of a light-sensitive molecule. Some studies have shown that the coating of the MSN with more hydrophobic materials can enhance its response to light [66].

Li’s group constructed an NIR light-responsive nanovalve on the surface of MSN-coated gold nanorods by using sulfonatocalix [4] arene (SC [4] A) attached to a quaternary ammonium stalk as a capping switch. In this nanocarrier, the energy of NIR light was converted to heat by the plasmonic absorption by the gold nanorod core and the binding between the quaternary ammonium salt and the SC [4] A was weakened, leading to removal of the capping agent from the surface of the MSN pore, and drug was released [65]. In a similar approach, Lui et al. [67] used single-walled carbon nanotubes (SWCNT) as a light-absorbing agent attached to MSN. They found that the light absorbing material could absorb energy converting it to heat and cause drug release. They coated MSN with SWCNT and functionalized them with polyethylene glycol (PEG) to enhance biocompatibility, solubility, and the stability of the nanocarrier in physiological conditions. The loaded drug was released after exposure of HeLa cells to NIR light. This strategy is depicted in Figure 5. Further studies indicated that noncovalent binding of the capping agents was able to prevent drug release and enhanced the light-mediated response. Reduced graphene oxide (RGO) noncovalently assembled onto the surface of alkyl chain-functionalized MSN could be disassembled by remote irradiation with NIR light [68].

Figure 5:

(A) Synthetic route to SWCNT@MS-MEG showing the preparation of mesoporous silica, its pegylation, and addition of CNT to construct the complete SWCNT@MS-MEG. (B) TEM image of the nanoparticles before pegylation. (C) Illustration of desorption and adsorption of cargo and its corresponding pore size. Reused from Reference [67] with permission of John Wiley and Sons’ publisher.

In an interesting work, Wang’s group designed a reversible nano valve that was responsive to near-UV irradiation. In this system, α-cyclodextrin was attached to the trans-form of azobenzene. Trans-azobenzene was isomerized under near-UV light to the cis-form, which forced the release of cyclodextrin, and the cargo was released during light irradiation. When the light irradiation ceased, the azobenzene returned to the trans-form, and as a result, the cyclodextrin again bound to the porous surface. This system represents the first reversible drug-release system triggered by light [69].

3.5 Magnetic-responsive MSN

This idea, that the use of an external magnetic field could be a stimulus for drug release, was first suggested by Freeman in 1960 [70]. The entrapment of magnetic materials within MSN can be carried out by three different mechanisms that control the diameter, morphology, and structure of magnetic MSN. These strategies are 1) magnetic nanocrystals are embedded in the core and surrounded by an MSN shell; 2) a large magnetic core is embedded in a sandwich-like structure with MSN surrounding it; 3) hollow MSN can contain magnetic nanocrystals like a baby’s rattle [28]. These three mechanisms are depicted in Figure 6.

Figure 6:

Mechanisms of magnetic core entrapment in MSN shells. (a) Schematic construction mechanism; (b) TEM image of magnetic core nanoparticles; (c) TEM image of nanoparticle after pore formation. Reused from Reference [28] with permission of John Wiley and Sons’ publisher.

External magnetic fields are nontoxic, have a high ability to penetrate living organisms without any physical interaction with internal tissues [31]. As magnetic fields do not interact with living systems, they can be used for external guidance of magnetic nanocarriers, as contrast agents in magnetic resonance imaging (MRI), for tissue heating via application of alternating magnetic fields, and for triggering drug release. Application of magnetic fields can increase the cellular uptake of magnetic NP in a process known as “magetofection” [44, 71]. Magnetic field-induced heating might increase the temperature of the surrounding environment and have a synergistic effect on drug release and cancer therapy. One strategy to use this hyperthermia property is the entrapment of magnetic materials in the core of MSN. Dong’s group stabilized Fe₃O₄ NP within a silica layer. Then, hexadecyltrimethyammonium bromide (CTAB) was used as a surfactant, combined with mesitylene to cause swelling of the pores to generate M-MSN-CTAB. Next, the CTAB was removed by treatment with 3-(trim-thoxysily) propyl methacrylate (MPS) to graft vinyl groups on the external surface of the MSN nanoparticles. The removal of CTAB in an ammonia-ethanol solution led to pore formation in MSN. Finally, a smart thermoresponsive polymer, poly (N-isopropyl acrylamide) (PNIPAAm), was added as a gatekeeper to the pores. The Fe₃O₄ under an alternating magnetic field absorbed energy and generated heat. After the temperature was increased, the outer chains of PNIPAAm absorbed more water and became soluble, the pores became unblocked, and the loaded cargo was released [72]. The strategy is depicted in Figure 7.

Figure 7:

Schematic illustration of M-MSN-PNIPAAm preparation, functionalization, and drug release mechanism. Reused from Reference [72] with permission of John Wiley and Sons’ publisher.

In a similar approach, Thomas et al. covered zinc-doped iron oxide nanocrystals (ZnNC) inside the MSN and modified its surface by attaching pseudorotaxanes as a thermal-sensitive gatekeeper. The presence of ZnNC significantly improved the thermal responsivity four times, and the MRI contrast was increased 10 times compared to free iron oxide nanocrystals [73]. The unique structural properties of the MSN provided the possibility to combine magnetic sensitivity with temperature sensitivity as discussed above. This strategy has led to a generation of different MSN-based nanocarriers with magnetic cores and stimulus-sensitive nanovalves. For example, Lee’s group [74] used three different reactions; sol-gel reaction, solvothermal reaction, and amide coupling reaction, to construct magnetic core MSN with nanovalves composed of crown-ether macrocycles as ultrasound responsive moiteties and with CTAB as pH-responsive gatekeepers. This Fe₃O₄@SiO₂@CTAB-SiO₂-NH₂ MSN showed efficient multiresponsive behavior (i.e. ultrasound, pH, and magnetic responsivity) when both internal and external stimuli could be used.

3.6 Temperature-responsive MSN

Utilizing thermoresponsive MSN is another approach for targeted release of the drug that can be triggered by both external and internal stimulation. This concept was reported in 2003 for the first time. One of the characteristic properties of tumor tissues (caused by high metabolism rates and inflammation) is their higher temperature compared to normal tissue. Therefore, using phase-change polymers with melting temperature (Tm) higher than normal conditions as gatekeepers would be an efficient strategy for smart temperature-responsive DDS. These polymers remain in a solid state at temperatures lower than their Tm but are converted to a liquid phase when the surrounding temperature is raised to Tm [75]. Poly (N-isopropylacrylamide-co-acrylamide) is a thermoresponsive polymer that is widely used in the design of these carriers. Russell et al. [76] used this polymer for capping the MSN pores with diameters of 2–5 nm through covalent binding. This study found that the density of the grafted polymers could directly influence the efficiency of the capping agent in loading and releasing the bioactive molecules. Linking thermosensitive polymers to the surface of magnetic core MSN, as discussed above, has been used for generation of externally thermoresponsive MSN. Another strategy for the design of thermoresponsive MSN is capping the pores with paraffins. Different types of paraffins such as heneicosane (C21) and tetracosane (C24) with different Tm have been used. Solid paraffins reaching the Tm convert to the liquid phase and allow the loaded cargo to be released [75].

On the other hand, bioactive molecules can be used as thermosensitive materials on the surface of MSN. These molecules respond to a change in the temperature, by losing their conventional structure, either by opening of α-helix configuration or by altering hydrogen-linked subunits. This mechanism is depicted in Figure 8. Zhang’s group [45] attached a single-stranded DNA at the MSN pore sites, and its complementary strand was attached to a copper sulfide (CuS) nanoparticles. CuS is a nontoxic semiconductor compound that shows the effective photothermal ablation behavior. By irradiation of the CuS NP with NIR light, the temperature was increased and the two strands of DNA became separated and CuS cap was removed leading to release of the cargo. Interestingly, after cessation of the irradiation and temperature decrease the cap returned to the pore site by annealing of the two complementary DNA strands thus providing reversible controlled release. Using a similar concept, Chang’s group [77] utilized Au nanorods as a gatekeeper and a pair of oligonucleotides to form a temperature-responsive element under NIR light irradiation.

Figure 8:

Schematic of thermally responsive MSN: the normal structure of a thermally responsive material is changed under temperature fluctuation. Reused from Reference [45] with the permission of the Royal Society of Chemistry.

Based on changing the configuration of an α-helix, Torre’s group [78] used a 17-mer peptide anchored to a polymer as a gatekeeper. The α-helical structure of this peptide was converted to a random coil conformation at a raised temperature due to disorder arising in its structure. This transformation led to a reduction in the molecular crowding at the pore surface and led to the release of the cargo, whereas by reducing the temperature, the self-assembly property of the peptide covered the pore entrance and blocked the cargo release.

3.7 Toxicity of MSN

Silica materials, in general, are considered as being noncytotoxic, but the silica nano-composite materials may possess different properties owing to alteration of the physicochemical characteristics at the nanoscale [79]. Previously, it was shown that MSNs have significantly lower cytotoxicity toward phagocytes and inflammatory cells in in vitro conditions compared to amorphous colloidal silica NPs [80]. Some literature has indicated that the properties of SiO₂ nanoparticles can influence their interaction with targeted cells. The surface modification of silica nanoparticles enhances their cellular uptake [81], the pore diameter increases the adsorption capacity [82], and the different geometry of the particles can disturb normal cell functions [83]. The fact that the properties of silica nanoparticles could influence their biodistribution and biocompatibility cautions us to consider different parameters in the design of these nanoparticles in order to reduce their possible toxicity.

Yu’s group [84] focused on the impact of the charged surface groups, porosity, and geometry of the silica NPs on erythrocyte hemolysis and macrophage toxicity. They found that the toxicity of the SiO₂ was dependent on cell type, which is due to different physiological functions of different cells. Moreover, they suggested that the major factor in the cellular interactions was the surface properties and porosity of the SNPs, and comparison of different structures on erythrocyte hemolysis indicated that amine modification of SiO₂ reduced the toxicity of these NPs.

Fu et al. [85] studied the effect of the administration route on MSN toxicity. They found that after systemic administration, nanosilica particles accumulated in the liver and their distribution was not the same in the body after other administration routes. According to their results, as the MSN are excreted via urine and feces, the most compatible routes for administration of MSN were oral and intravenous injection. They also indicated that MSN excretion by the kidney could not cause any microstructural kidney damage.

Further studies suggested that in vitro assays might yield different results from assays after in vivo administration. In vivo interaction of SNPs with the reticuloendothelial system, (the major physiological system for blood purification) and interactions with complex serum protein concentrations could affect immune responses through the body. The presence of pores on the surface of SNPs could change the proliferation of splenocytes and increase immunoglobulin levels. MSN could possibly damage the immune system by disrupting the lymphocyte population patterns and produce alteration in lymph nodes [86].