Nanoplatforms for Targeted Stimuli-Responsive Drug Delivery: A Review of Platform Materials and Stimuli-Responsive Release and Targeting Mechanisms

To achieve the promise of stimuli-responsive drug delivery systems for the treatment of cancer, they should (1) avoid premature clearance; (2) accumulate in tumors and undergo endocytosis by cancer cells; and (3) exhibit appropriate stimuli-responsive release of the payload. It is challenging to address all of these requirements simultaneously. However, the numerous proof-of-concept studies addressing one or more of these requirements reported every year have dramatically expanded the toolbox available for the design of drug delivery systems. This review highlights recent advances in the targeting and stimuli-responsiveness of drug delivery systems. It begins with a discussion of nanocarrier types and an overview of the factors influencing nanocarrier biodistribution. On-demand release strategies and their application to each type of nanocarrier are reviewed, including both endogenous and exogenous stimuli. Recent developments in stimuli-responsive targeting strategies are also discussed. The remaining challenges and prospective solutions in the field are discussed throughout the review, which is intended to assist researchers in overcoming interdisciplinary knowledge barriers and increase the speed of development. This review presents a nanocarrier-based drug delivery systems toolbox that enables the application of techniques across platforms and inspires researchers with interdisciplinary information to boost the development of multifunctional therapeutic nanoplatforms for cancer therapy.

Surfactant-free synthesis of monodispersed organosilica particles with pure sulfide-bridged silsesquioxane framework chemistry via extension of Stöber method

Sulfide bond incorporated organosilica particles have been broadly applied to versatile biomedical applications, wherein the uniformity of particles and the sulfur content significantly dictate the ultimate performance. Unfortunately, due to the difficulty in controlling the chemical behavior of organosilica precursors in a sol-gel process, challenges still exist in developing a facile and green synthetic approach to fabricate organosilica particles with good dispersity and high sulfur content. In the present work, by extending the classic Stöber method, a surfactant-free synthesis of monodispersed organosilica particles with pure sulfide-bridged silsesquioxane framework chemistry is reported for the first time. By simply tailoring the ethanol-to-water ratio and amount of catalyst, the size of disulfide-bridged organosilica particles can be tuned from ~0.50 to ~1.20 µm. Moreover, this approach can be employed to prepare tetra-sulfide bridged silica nanoparticles with an extremely high sulfur content of 30.7 wt% and negligible cytotoxicity. Notably, taking advantage of this extended Stöber method, both hydrophilic (methylene blue) and hydrophobic (curcumin) molecules can be in-situ encapsulated into tetra-sulfide bridged silica nanoparticles, whose glutathione-triggered biodegradability is also demonstrated. Collectively, the innovative synthetic approach and organosilica particles developed in this work are expected to open up new opportunities in hybrid materials fabrication and bio-applications.

Ultrasmall MoS2 Nanodots-Doped Biodegradable SiO2 Nanoparticles for Clearable FL/CT/MSOT Imaging-Guided PTT/PDT Combination Tumor Therapy

Recently, we developed ultrasmall molybdenum disulfide (MoS₂) quantum dots for computed tomography (CT) and multispectral optoacoustic tomography (MSOT) imaging-guided photothermal therapy (PTT). But, due to rapid body elimination and limited blood circulation time, the tumor uptake of the dots is low. In our study, this problem was solved via designing an amino-modified biodegradable nanomaterial based on MoS₂ quantum-dots-doped disulfide-based SiO₂ nanoparticles (denoted MoS₂@ss-SiO₂) for multimodal application. By integrating the MoS₂ quantum dots into clearable SiO₂ nanoparticles, this nanoplatform with an appropriate particle size can not only degrade and excrete in a reasonable period induced by redox responsiveness of glutathione but also exhibit a high tumor uptake due to the longer blood circulation time. Moreover, hyaluronic acid and chlorin e6 (Ce6) were adsorbed on the outer shell for tumor-targeting effect and photodynamic therapy, respectively. So, this biodegradable and clearable theranostic nanocomposite, which is applicable in integrated fluorescence/CT/MSOT imaging-guided combined photothermal therapy (PTT) and photodynamic therapy, is very promising in biomedical applications in the future.

Porous Inorganic Carriers Based on Silica, Calcium Carbonate and Calcium Phosphate for Controlled/Modulated Drug Delivery: Fresh Outlook and Future Perspectives

Porous inorganic nanostructured materials are widely used nowadays as drug delivery carriers due to their adventurous features: suitable architecture, large surface area and stability in the biological fluids. Among the different types of inorganic porous materials, silica, calcium carbonate, and calcium phosphate have received significant attention in the last decade. The use of porous inorganic materials as drug carriers for cancer therapy, gene delivery etc. has the potential to improve the life expectancy of the patients affected by the disease. The main goal of this review is to provide general information on the current state of the art of synthesis of the inorganic porous particles based on silica, calcium carbonate and calcium phosphate. Special focus is dedicated to the loading capacity, controllable release of drugs under internal biological stimuli (e.g., pH, redox, enzymes) and external noninvasive stimuli (e.g., light, magnetic field, and ultrasound). Moreover, the diverse compounds to deliver with silica, calcium carbonate and calcium phosphate particles, ranging from the commercial drugs to genetic materials are also discussed.

Biodegradable Silica-Based Nanoparticles: Dissolution Kinetics and Selective Bond Cleavage

Silica-based nanomaterials are extensively used in industrial applications and academic biomedical research, thus properly assessing their toxicity and biodegradability is essential for their safe and effective formulation and use. Unfortunately, there is often a lot of confusion in the literature with respect to the toxicity and biodegradability of silica since various studies have yielded contradictory results. In this contribution, we first endeavor to underscore that the simplistic model of silica should be discarded in favor of a more realistic model recognizing that all silicas are not created equal and should thus be considered in the plural as silicas and silica hybrids, which indeed hold various biocompatibility and biodegradability profiles. We then demonstrated that all silicas are-as displayed in Nature-degradable in water by dissolution, as governed by the laws of kinetics. Lastly, we explore the vast potential of tuning the degradability of silica by materials design using various silica hybrids for redox-, pH-, enzymatic-, and biochelation-mediated lysis mechanisms.

Degradability and Clearance of Silicon, Organosilica, Silsesquioxane, Silica Mixed Oxide, and Mesoporous Silica Nanoparticles

The biorelated degradability and clearance of siliceous nanomaterials have been questioned worldwide, since they are crucial prerequisites for the successful translation in clinics. Typically, the degradability and biocompatibility of mesoporous silica nanoparticles (MSNs) have been an ongoing discussion in research circles. The reason for such a concern is that approved pharmaceutical products must not accumulate in the human body, to prevent severe and unpredictable side-effects. Here, the biorelated degradability and clearance of silicon and silica nanoparticles (NPs) are comprehensively summarized. The influence of the size, morphology, surface area, pore size, and surface functional groups, to name a few, on the degradability of silicon and silica NPs is described. The noncovalent organic doping of silica and the covalent incorporation of either hydrolytically stable or redox- and enzymatically cleavable silsesquioxanes is then described for organosilica, bridged silsesquioxane (BS), and periodic mesoporous organosilica (PMO) NPs. Inorganically doped silica particles such as calcium-, iron-, manganese-, and zirconium-doped NPs, also have radically different hydrolytic stabilities. To conclude, the degradability and clearance timelines of various siliceous nanomaterials are compared and it is highlighted that researchers can select a specific nanomaterial in this large family according to the targeted applications and the required clearance kinetics.

Stimuli-responsive delivery vehicles based on mesoporous silica nanoparticles: recent advances and challenges

In the past decade, stimuli-responsive drug delivery vehicles based on surface-functionalized mesoporous silica nanoparticles have attracted intense interest as a new type of drug carrier.

Breakable mesoporous silica nanoparticles for targeted drug delivery

"Pop goes the particle". Here we report on the preparation of redox responsive mesoporous organo-silica nanoparticles containing disulfide (S-S) bridges (ss-NPs) that, even upon the exohedral grafting of targeting ligands, retained their ability to undergo structural degradation, and increase their local release activity when exposed to a reducing agent. This degradation could be observed also inside glioma C6 cancer cells. Moreover, when anticancer drug-loaded pristine and derivatized ss-NPs were fed to glioma C6 cells, the responsive hybrids were more effective in their cytotoxic action compared to non-breakable particles. The possibility of tailoring the surface functionalization of this hybrid, yet preserving its self-destructive behavior and enhanced drug delivery properties, paves the way for the development of effective biodegradable materials for in vivo targeted drug delivery.

Biodegradable ethylene-bis(propyl)disulfide-based periodic mesoporous organosilica nanorods and nanospheres for efficient in-vitro drug delivery

Periodic mesoporous organosilica nanorods and nanospheres are synthesized from 1,4-bis(triethoxysilyl)ethylene and bis(3-ethoxysilylpropyl)disulfide. The nanosystems present the long-range order of the hexagonal nanostructure. They are degraded in simulated physiological conditions. The loading and release of doxorubicin with these nanosystems are both pH dependent. These nanoparticles are endocytosed by breast cancer cells and are very efficient for doxorubicin delivery in these cells.

Fabrication of single-hole glutathione-responsive degradable hollow silica nanoparticles for drug delivery

In the present study, a kind of single-hole glutathione (GSH)-responsive degradable hollow silica nanoparticles (G-DHSNs) was synthesized and used as carriers of doxorubicin (DOX) (DOX-G-DHSNs). The G-DHSNs were accurately designed and fabricated with a simple and convenient method, and without any extra pernicious component. The composition, morphology and properties of the G-DHSNs had been characterized by (1)HNMR spectra, Fourier transform infrared spectrograph, thermo gravimetric analysis, transmission electron microscope, and scanning electron microscope. The degradation study of G-DHSNs showed that the G-DHSNs would be broken into pieces after interacting with GSH. Besides, the negligible hemolytic activity and low cytotoxicity of the G-DHSNs demonstrated its excellent biocompatibility. pH- and GSH-triggered release of DOX followed by the decomposition of G-DHSNs within TCA8113 cancer cells was further confirmed by flow cytometry and confocal laser scanning microscopy studies. All of these results indicated that G-DHSNs can be used as safe and promising drug nanocarriers.