Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility.

The study of ordered mesoporous silica materials has exploded since their discovery by Mobil researchers 20 years ago. The ability to make uniformly sized, porous, and dispersible nanoparticles using colloidal chemistry and evaporation-induced self-assembly has led to many applications of mesoporous silica nanoparticles (MSNPs) as "nanocarriers" for delivery of drugs and other cargos to cells. The exceptionally high surface area of MSNPs, often exceeding 1000 m²/g, and the ability to independently modify pore size and surface chemistry, enables the loading of diverse cargos and cargo combinations at levels exceeding those of other common drug delivery carriers such as liposomes or polymer conjugates. This is because noncovalent electrostatic, hydrogen-bonding, and van der Waals interactions of the cargo with the MSNP internal surface cause preferential adsorption of cargo to the MSNP, allowing loading capacities to surpass the solubility limit of a solution or that achievable by osmotic gradient loading. The ability to independently modify the MSNP surface and interior makes possible engineered biofunctionality and biocompatibility. In this Account, we detail our recent efforts to develop MSNPs as biocompatible nanocarriers (Figure 1 ) that simultaneously display multiple functions including (1) high visibility/contrast in multiple imaging modalities, (2) dispersibility, (3) binding specificity to a particular target tissue or cell type, (4) ability to load and deliver large concentrations of diverse cargos, and (5) triggered or controlled release of cargo. Toward function 1, we chemically conjugated fluorescent dyes or incorporated magnetic nanoparticles to enable in vivo optical or magnetic resonance imaging. For function 2, we have made MSNPs with polymer coatings, charged groups, or supported lipid bilayers, which decrease aggregation and improve stability in saline solutions. For functions 3 and 4, we have enhanced passive bioaccumulation via the enhanced permeability and retention effect by modifying the MSNP surfaces with positively charged polymers. We have also chemically attached ligands to MSNPs that selectively bind to receptors overexpressed in cancer cells. We have used encapsulation of MSNPs within reconfigurable supported lipid bilayers to develop new classes of responsive nanocarriers that actively interact with the target cell. Toward function 4, we exploit the high surface area and tailorable surface chemistry of MSNPs to retain hydrophobic drugs. Finally, for function 5, we have engineered dynamic behaviors by incorporating molecular machines within or at the entrances of MSNP pores and by using ligands, polymers, or lipid bilayers. These provide a means to seal-in and retain cargo and to direct MSNP interactions with and internalization by target cells. Application of MSNPs as nanocarriers requires biocompatibility and low toxicity. Here the intrinsic porosity of the MSNP surface reduces the extent of hydrogen bonding or electrostatic interactions with cell membranes as does surface coating with polymers or lipid bilayers. Furthermore, the high surface area and low extent of condensation of the MSNP siloxane framework promote a high rate of dissolution into soluble silicic acid species, which are found to be nontoxic. Potential toxicity is further mitigated by the high drug capacity of MSNPs, which greatly reduces needed dosages compared with other nanocarriers. We anticipate that future generations of MSNPs incorporating molecular machines and encapsulated by membrane-like lipid bilayers will achieve a new level of controlled cellular interactions.

Involvement of lysosomal exocytosis in the excretion of mesoporous silica nanoparticles and enhancement of the drug delivery effect by exocytosis inhibition.

The exocytosis of phosphonate modified mesoporous silica nanoparticles (P-MSNs) is demonstrated and lysosomal exocytosis is identified as the mechanism responsible for this event. Regulation of P-MSN exocytosis can be achieved by inhibiting or accelerating lysosomal exocytosis. Slowing down P-MSN exocytosis enhances the drug delivery effect of CPT-loaded P-MSNs by improving cell killing.

pH-responsive dual cargo delivery from mesoporous silica nanoparticles with a metal-latched nanogate.

A nanogate composed of two iminodiacetic acid (IDA) molecules and a metal ion latch was designed, synthesized, and assembled on mesoporous silica nanoparticles. This gating mechanism is capable of storing and releasing metal ions and molecules trapped in the pores. Pore openings derivatized with IDA can be latched shut by forming a bis-IDA chelate complex with a metal ion. This system was tested by loading with Hoechst 33342 as the probe cargo molecule, and latching with cobalt, nickel, or calcium metal ions. No cargo release was observed in a neutral aqueous environment, but both cargoes were delivered after acid stimulation and/or the addition of a competitively binding ligand.

Two-photon-triggered drug delivery in cancer cells using nanoimpellers.

A therapy of cancer cells: Two-photon-triggered camptothecin delivery with nanoimpellers was studied in MCF-7 breast cancer cells. A fluorophore with a high two-photon absorption cross-section was first incorporated in the nanoimpellers. Fluorescence resonance energy transfer (FRET) from the fluorophore to the azobenzene moiety was demonstrated.

Processing pathway dependence of amorphous silica nanoparticle toxicity: colloidal vs pyrolytic.

We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synthesized through low-temperature colloidal (e.g., Stöber silica) or high-temperature pyrolysis (e.g., fumed silica) routes. Through combined spectroscopic and physical analyses, we have determined the state of aggregation, hydroxyl concentration, relative proportion of strained and unstrained siloxane rings, and potential to generate hydroxyl radicals for Stöber and fumed silica NPs with comparable primary particle sizes (16 nm in diameter). On the basis of erythrocyte hemolytic assays and assessment of the viability and ATP levels in epithelial and macrophage cells, we discovered for fumed silica an important toxicity relationship to postsynthesis thermal annealing or environmental exposure, whereas colloidal silicas were essentially nontoxic under identical treatment conditions. Specifically, we find for fumed silica a positive correlation of toxicity with hydroxyl concentration and its potential to generate reactive oxygen species (ROS) and cause red blood cell hemolysis. We propose fumed silica toxicity stems from its intrinsic population of strained three-membered rings (3MRs) along with its chainlike aggregation and hydroxyl content. Hydrogen-bonding and electrostatic interactions of the silanol surfaces of fumed silica aggregates with the extracellular plasma membrane cause membrane perturbations sensed by the Nalp3 inflammasome, whose subsequent activation leads to secretion of the cytokine IL-1ß. Hydroxyl radicals generated by the strained 3MRs in fumed silica, but largely absent in colloidal silicas, may contribute to the inflammasome activation. Formation of colloidal silica into aggregates mimicking those of fumed silica had no effect on cell viability or hemolysis. This study emphasizes that not all amorphous silicas are created equal and that the unusual toxicity of fumed silica compared to that of colloidal silica derives from its framework and surface chemistry along with its fused chainlike morphology established by high-temperature synthesis (>1300 °C) and rapid thermal quenching.

A reversible light-operated nanovalve on mesoporous silica nanoparticles.

Two azobenzene α-cyclodextrin based nanovalves are designed, synthesized and assembled on mesoporous silica nanoparticles. Under aqueous conditions, the cyclodextrin cap is tightly bound to the azobenzene moiety and capable of holding back loaded cargo molecules. Upon irradiation with a near-UV light laser, trans to cis-photoisomerization of azobenzene initiates a dethreading process, which causes the cyclodextrin cap to unbind followed by the release of cargo. The addition of a bulky stopper to the end of the stalk allows this design to be reversible; complete dethreading of cyclodextrin as a result of unbinding with azobenzene is prevented as a consequence of steric interference. As a result, thermal relaxation of cis- to trans-azobenzene allows for the rebinding of cyclodextrin and resealing of the nanopores, a process which entraps the remaining cargo. Two stalks were designed with different lengths and tested with alizarin red S and propidium iodide. No cargo release was observed prior to light irradiation, and the system was capable of multiuse. On/off control was also demonstrated by monitoring the release of cargo when the light stimulus was applied and removed, respectively.

Use of size and a copolymer design feature to improve the biodistribution and the enhanced permeability and retention effect of doxorubicin-loaded mesoporous silica nanoparticles in a murine xenograft tumor model.

A key challenge for improving the efficacy of passive drug delivery to tumor sites by a nanocarrier is to limit reticuloendothelial system uptake and to maximize the enhanced permeability and retention effect. We demonstrate that size reduction and surface functionalization of mesoporous silica nanoparticles (MSNP) with a polyethyleneimine-polyethylene glycol copolymer reduces particle opsonization while enhancing the passive delivery of monodispersed, 50 nm doxorubicin-laden MSNP to a human squamous carcinoma xenograft in nude mice after intravenous injection. Using near-infrared fluorescence imaging and elemental Si analysis, we demonstrate passive accumulation of ∼12% of the tail vein-injected particle load at the tumor site, where there is effective cellular uptake and the delivery of doxorubicin to KB-31 cells. This was accompanied by the induction of apoptosis and an enhanced rate of tumor shrinking compared to free doxorubicin. The improved drug delivery was accompanied by a significant reduction in systemic side effects such as animal weight loss as well as reduced liver and renal injury. These results demonstrate that it is possible to achieve effective passive tumor targeting by MSNP size reduction as well as by introducing steric hindrance and electrostatic repulsion through coating with a copolymer. Further endowment of this multifunctional drug delivery platform with targeting ligands and nanovalves may further enhance cell-specific targeting and on-demand release.