A tumor-acidity-activated charge-conversional nanogel as an intelligent vehicle for promoted tumoral-cell uptake and drug delivery

Using the Power of Organic Synthesis for Engineering the Interactions of Nanoparticles with Biological Systems

The surface properties of nanoparticles (NPs) dictate their interaction with the outside world. The use of precisely designed molecular ligands to control NP surface properties provides an important toolkit for modulating their interaction with biological systems, facilitating their use in biomedicine. In this review we will discuss the application of the atom-by-atom control provided by organic synthesis to the generation of engineered nanoparticles, with emphasis on how the functionalization of NPs with these "small" organic molecules (Mw < 1,000) can be used to engineer NPs for a wide range of applications.

Organic Nanoparticles in Foods: Fabrication, Characterization, and Utilization

In the context of food systems, organic nanoparticles (ONPs) are fabricated from proteins, carbohydrates, lipids, and other organic compounds to a characteristic dimension, such as a radius smaller than 100 nm. ONPs can be fabricated with bottom-up and top-down approaches, or a combination of both, on the basis of the physicochemical properties of the source materials and the fundamental principles of physical chemistry, colloidal and polymer sciences, and materials science and engineering. ONPs are characterized for dimension, morphology, surface properties, internal structures, and biological properties to understand structure-function correlations and to explore their applications. These potential applications include modifying physical properties, improving sensory attributes and food quality, protecting labile compounds, and delivering encapsulated bioactive compounds for improved bioactivity and bioavailability. Because ONPs can have digestion and absorption properties different from conventional materials, the eventual applications of ONPs require in vitro and in vivo studies to guide the development of safe food products that utilize the unique functionalities of ONPs.

A facile way to fabricate pH-sensitive charge-conversion polymeric nanoparticles with tunable pH conversion point

pH-Sensitive charge-conversion polymeric nanoparticles could significantly enhance drug bioavailability due to improved tumor cell internalization.

Synergetic enhancement of antitumor efficacy with charge-reversal and reduction-sensitive polymer micelles

An amphiphilic block copolymer PLA-SS-PAEMA/DMMA was used to encapsulate and deliver Doxorubicin for synergetic enhancement of antitumor efficacy by the combinational effect of charge-reversal on cellular uptake and reduction-sensitivity on intracellular DOX release.

Active Tumor Permeation and Uptake of Surface Charge-Switchable Theranostic Nanoparticles for Imaging-Guided Photothermal/Chemo Combinatorial Therapy

To significantly promote tumor uptake and penetration of therapeutics, a nanovehicle system comprising poly(lactic-co-glycolic acid) (PLGA) as the hydrophobic cores coated with pH-responsive N-acetyl histidine modified D-α-tocopheryl polyethylene glycol succinate (NAcHis-TPGS) is developed in this work. The nanocarriers with switchable surface charges in response to tumor extracellular acidity (pH_e) were capable of selectively co-delivering indocyanine green (ICG), a photothermal agent, and doxorubicin (DOX), a chemotherapy drug, to tumor sites. The in vitro cellular uptake of ICG/DOX-loaded nanoparticles by cancer cells and macrophages was significantly promoted in weak acidic environments due to the increased protonation of the NAcHis moieties. The results of in vivo and ex vivo biodistribution studies demonstrated that upon intravenous injection the theranostic nanoparticles were substantially accumulated in TRAMP-C1 solid tumor of tumor-bearing mice. Immunohistochemical examination of tumor sections confirmed the active permeation of the nanoparticles into deep tumor hypoxia due to their small size, pH_e-induced near neutral surface, and the additional hitchhiking transport via tumor-associated macrophages. The prominent imaging-guided photothermal therapy of ICG/DOX-loaded nanoparticles after tumor accumulation induced extensive tumor tissue/vessel ablation, which further promoted their extravasation and DOX tumor permeation, thus effectively suppressing tumor growth.

Fluorene–morpholine-based organic nanoparticles: lysosome-targeted pH-triggered two-photon photodynamic therapy with fluorescence switch on–off

We synthesized fluorene–morpholine NPs that showed reversible fluorescence switch ON–OFF properties, which rendered the real time monitoring of PDT activity.

“Stealthy” chitosan/mesoporous silica nanoparticle based complex system for tumor-triggered intracellular drug release

A pH- and redox-sensitive “stealthy” chitosan/mesoporous silica nanoparticle-based complex system is prepared for tumor-triggered intracellular drug release.

Photomediated Reactive Oxygen Species-Generable Nanoparticles for Triggered Release and Endo/Lysosomal Escape of Drug upon Attenuated Single Light Irradiation

Nanoparticles with "smart" stimuli-responsive materials and multiple therapeutic strategies in a single delivery platform have emerged for highly efficient cancer therapy. Here, photomediated reactive oxygen species (ROS)-generable nanoparticles are designed that can trigger drug release and endo/lysosomal escape upon attenuated single light irradiation, simultaneously, for synergistic chemo-photodynamic ablation. In this study, the self-ROS-generable nanoparticles (SRNs) are prepared from the polymer based on polysaccharide, chlorin e6 as ROS generator and lipoic acid as ROS scavenger covalently conjugated pullulan with anticancer drug (doxorubicin, DOX) through self-assembly, and can disassemble via the ROS-mediated reduction of lipoyl group in response to low level exogenous single light switch. After cellular internalization in hepatic cancer through asialoglycoprotein receptor (ASGPR, as pullulan receptor)-mediated endocytosis, once irradiated, SRNs are able to produce ROS that can simultaneously induce drug release triggering and endo/lysosomal escape of DOX into cytoplasm as well as directly photodynamic therapy for highly efficient chemo-photodynamic cancer therapy. This promising delivery system, which has huge potential in biomedical applications, may be optimal for smart delivery platform.

A surface charge-switchable and folate modified system for co-delivery of proapoptosis peptide and p53 plasmid in cancer therapy

To improve the tumor therapeutic efficiency and reduce undesirable side effects, ternary FK/p53/PEG-PLL(DA) complexes with a detachable surface shielding layer were designed. The FK/p53/PEG-PLL(DA) complexes were fabricated by coating the folate incorporated positively charged FK/p53 complexes with charge-switchable PEG-shield (PEG-PLL(DA)) through electrostatic interaction. At the physiological pH 7.4 in the bloodstream, PEG-PLL(DA) could extend the circulating time by shielding the positively charged FK/p53 complexes. After the accumulation of the FK/p53/PEG-PLL(DA) complexes in tumor sites, tumor-acidity-triggered charge switch led to the detachment of PEG-PLL(DA) from the FK/p53 complexes, and resulted in efficient tumor cell entry by folate-mediated uptake and electrostatic attraction. Stimulated by the high content glutathione (GSH) in cytoplasm, the cleavage of disulfide bond resulted in the liberation of proapoptosis peptide C-KLA(TPP) and the p53 gene, which exerted the combined tumor therapy by regulating both intrinsic and extrinsic apoptotic pathways. Both in vitro and in vivo studies confirmed that the ternary detachable complexes FK/p53/PEG-PLL(DA) could enhance antitumor efficacy and reduce adverse effects to normal cells. These findings indicate that the tumor-triggered decomplexation of FK/p53/PEG-PLL(DA) supplies a useful strategy for targeting delivery of different therapeutic agents in synergetic anticancer therapy.

Multifunctional magnetic silica nanotubes for MR imaging and targeted drug delivery

A multifunctional drug delivery vehicle consisting of a tubular shaped silica host, a compact superparamagnetic iron oxide nanoparticle layer and a hyaluronic acid surface coating was developed as a theranostic platform, for in vivo MR imaging and magnetically guided/cancer targeted drug delivery.

Endogenous stimuli-sensitive multistage polymeric micelleplex anticancer drug delivery system for efficient tumor penetration and cellular internalization

To efficiently deliver anticancer drugs to the entire tumor tissue and cancer cells, an endogenous stimuli-sensitive multistage polymeric micelleplex drug delivery system is developed via electrostatic complexation between poly(ethylene glycol)-block-poly[(N'-dimethylmaleoyl-2-aminoethyl)aspartamide]-block-poly(ε-caprolactone) (PEG-b-PAsp(EDA-DM)-b-PCL) triblock copolymer micelles and cisplatin prodrug (Pt(IV))-conjugated cationic poly(amidoamine) dendrimers (PAMAM-Pt(IV)). The micelleplexes maintain structural stability at pH 7.4 ensuring long blood circulation and high tumor accumulation level, while they exhibit triggered release of secondary PAMAM-Pt(IV) dendrimer nanocarriers at tumoral acidity (≈pH 6.8) due to acid-labile charge-reversal properties of PAsp(EDA-DM) component under mildly acidic condition. The released PAMAM delivery nanocarriers with small size and slightly positive charges exhibit significantly deep tumor tissue penetration and efficient cellular internalization, followed by release of active cisplatin anticancer drug in intracellular reducing medium. In vivo investigation reveals that the Pt(IV)-loading micelleplexes significantly suppress tumor growth via intravenous injection due to synergistic effect of long circulation in bloodstream, high tumor accumulation, deep tumor tissue penetration, and efficient cellular internalization. Thus, the micelleplexes with stimuli-responsive multistage release feature show great potentials for better therapeutic efficacy of cancer especially through enhanced tumor penetration and cellular internalization.

Hybrid polymer micelles capable of cRGD targeting and pH-triggered surface charge conversion for tumor selective accumulation and promoted uptake

This study presents both tumor-targeting ligands (cRGD) and pH-activated surface charge-conversional moiety (imidazole) decorated micelles for Dox delivery. cRGD is expected to induce preferential tumor accumulation, while imidazole switches on positive charge in a tumor acid environment, which leads to enhanced micelle uptake by tumor cells.

Nanoparticle-based luminescent probes for intracellular sensing and imaging of pH

Fluorescence imaging microscopy is an essential tool in biomedical research. Meanwhile, various fluorescent probes are available for the staining of cells, cell membranes, and organelles. Though, to monitor intracellular processes and dysfunctions, probes that respond to ubiquitous chemical parameters determining the cellular function such as pH, pO2 , and Ca(2+) are required. This review is focused on the progress in the design, fabrication, and application of photoluminescent nanoprobes for sensing and imaging of pH in living cells. The advantages of using nanoprobes carrying fluorescent pH indicators compared to single molecule probes are discussed as well as their limitations due to the mostly lysosomal uptake by cells. Particular attention is paid to ratiometric dual wavelength nanosensors that enable intrinsic referenced measurements. Referencing and proper calibration procedures are basic prerequisites to carry out reliable quantitative pH determinations in complex samples such as living cells. A variety of examples will be presented that highlight the diverseness of nanocarrier materials (polymers, micelles, silica, quantum dots, carbon dots, gold, photon upconversion nanocrystals, or bacteriophages), fluorescent pH indicators for the weak acidic range, and referenced sensing mechanisms, that have been applied intracellularly up to now. WIREs Nanomed Nanobiotechnol 2016, 8:378-413. doi: 10.1002/wnan.1366 For further resources related to this article, please visit the WIREs website.

Surfactant-Free Emulsion-Based Preparation of Redox-Responsive Nanogels

A surfactant-free emulsion-based approach is developed for preparation of nanogels. A water-in-oil emulsion is generated feasibly from a mixture of water and a solution of disulfide-containing hyperbranched PEGylated poly(amido amine)s, poly(BAC2-AMPD1)-PEG, in chloroform. The water droplets in the emulsion are stabilized and filled with poly(BAC2-AMPD1)-PEG, and the crosslinked poly(amido amine)s nanogels are formed via the intermolecular disulfide exchange reaction. FITC-dextran is loaded within the nanogels by dissolving the compound in water before emulsification. Transmission electron microscopy and dynamic light scattering are applied to characterize the emulsion and the nanogels. The effects of the homogenization rate and the ratio of water/polymer are investigated. Redox-induced degradation and FITC-dextran release profile of the nanogels are monitored, and the results show efficient loading and redox-responsive release of FITC-dextran. This is a promising approach for the preparation of nanogels for drug delivery, especially for neutral charged carbohydrate-based drugs.

Stimuli-responsive nanoparticles for targeting the tumor microenvironment

One of the most challenging and clinically important goals in nanomedicine is to deliver imaging and therapeutic agents to solid tumors. Here we discuss the recent design and development of stimuli-responsive smart nanoparticles for targeting the common attributes of solid tumors such as their acidic and hypoxic microenvironments. This class of stimuli-responsive nanoparticles is inactive during blood circulation and under normal physiological conditions, but is activated by acidic pH, enzymatic up-regulation, or hypoxia once they extravasate into the tumor microenvironment. The nanoparticles are often designed to first "navigate" the body's vascular system, "dock" at the tumor sites, and then "activate" for action inside the tumor interstitial space. They combine the favorable biodistribution and pharmacokinetic properties of nanodelivery vehicles and the rapid diffusion and penetration properties of smaller drug cargos. By targeting the broad tumor habitats rather than tumor-specific receptors, this strategy has the potential to overcome the tumor heterogeneity problem and could be used to design diagnostic and therapeutic nanoparticles for a broad range of solid tumors.

Self-Assembled Nanoparticles Based on Amphiphilic Anticancer Drug-Phospholipid Complex for Targeted Drug Delivery and Intracellular Dual-Controlled Release

Integrating advantages of mitomycin C (MMC)-phospholipid complex for increased drug encapsulation efficiency and reduced premature drug release, DSPE-PEG-folate (DSPE-PEG-FA) for specific tumor targeting, we reported a simple one-pot self-assembly route to prepare the MMC-phospholipid complex-loaded DSPE-PEG-based nanoparticles (MP-PEG-FA NPs). Both confocal imaging and flow cytometry demonstrated that MMC was distributed into nuclei after cellular uptake and intracellular drug delivery. More importantly, the systemically administered MP-PEG-FA NPs led to increased blood persistence and enhanced tumor accumulation in HeLa tumor-bearing nude mice. This study introduces a simple and effective strategy to design the anticancer drug-phospholipid complex-based targeted drug delivery system for sustained/controlled drug release.

Bioresponsive polymer-based nucleic acid carriers

Nucleic acid carriers need to possess multifunctionality for overcoming biological barriers, such as the stable encapsulation of nucleic acids in extracellular milieu, internalization by target cells, controlled intracellular distribution, and release of nucleic acids at the target site of action. To fulfill these stepwise functionalities, "bioresponsive" polymers that can alter their structure responding to site-specific biological signals are highly useful. Notably, pH, redox potential, and enzymatic activities vary along with microenvironments in the body, and thus, the responsiveness to these signals enables to construct nucleic acid carriers with programmed functionalities. This chapter describes the design of bioresponsive polymers that respond to various biological microenvironments for smart nucleic acids delivery.

Passive targeting of thermosensitive diblock copolymer micelles to the lungs: synthesis and characterization of poly(N-isopropylacrylamide)-block-poly(ε-caprolactone)

BACKGROUND

Amphiphilic poly(N-isopropylacrylamide)-block-poly(ε-caprolactone) (PNiPAAm-b-PCL) copolymers were synthesized by ring-opening polymerization to form thermosensitive micelles as nanocarriers for bioimaging and carboplatin delivery.

RESULTS

The critical micelle concentration increased from 1.8 to 3.5 mg/l following the decrease of the PNiPAAm chain length. The copolymers revealed a lower critical solution temperature (LCST) between 33 and 40°C. The copolymers self-assembled to form spherical particles of 146-199 nm in diameter. Carboplatin in micelles exhibited a slower release at 37°C relative to that at 25°C due to the gel layer formation on the micellar shell above the LCST. The micelles containing dye or carboplatin were intravenously injected into the rats for in vivo bioimaging and drug biodistribution. The bioimaging profiles showed a significant accumulation of micelles in the lungs. The micelles could minimize the reticuloendothelial system (RES) recognition of the dye. In vivo biodistribution demonstrated an improved pulmonary accumulation of carboplatin from 2.5 to 3.4 μg/mg by the micelles as compared to the control solution. Carboplatin accumulation in the heart and kidneys was reduced after encapsulation by the micelles.

CONCLUSION

This study supports the potential of PNiPAAm-b-PCL micelles to passively target the lungs and attenuate RES uptake and possible side effects.

Acylsulfonamide-Functionalized Zwitterionic Gold Nanoparticles for Enhanced Cellular Uptake at Tumor pH

A nanoparticle design featuring pH-responsive alkoxyphenyl acylsulfonamide ligands is reported herein. As a result of ligand structure, this nanoparticle is neutral at pH 7.4, becoming positively charged at tumor pH (<6.5). The particle uptake and cytotoxicity increase over this pH range. This pH-controlled uptake and toxicity makes this particle a promising tool for tumor selective therapy.

Enhanced Therapeutic siRNA to Tumor Cells by a pH-Sensitive Agmatine-Chitosan Bioconjugate

Charge-conversional naturally occurring chitosan-agmatine bioconjugates are prepared by dimethylmaleic anhydride (DMA) modification and the nucleophilic reaction between tosyl of tosylated chitosan and primary amine of agmatine. These bioconjugates (CS-DM-Agm) are shown to condense siRNA into nanocomplexes, which are stable in the presence of serum at physical pH values. Furthermore, the surface charge of complexes can tune from negative to positive while pH is changed to weak acid tumor micromilieu, thus facilitating the target cancer cell internalization in resisting serum adsorption. More importantly, this smart biogenic system shows remarkable gene silencing efficiency and a high apoptotic rate of tumor cells both in vitro and in vivo, indicating its great potential for cancer therapy.

Inhibition of MDR1 gene expression and enhancing cellular uptake for effective colon cancer treatment using dual-surface-functionalized nanoparticles

Nanomedicine options for colon cancer therapy have been limited by the lack of suitable carriers capable of delivering sufficient drug into tumors to cause lethal toxicity. To circumvent this limitation, we fabricated a camptothecin (CPT)-loaded poly(lactic-co-glycolic acid) nanoparticle (NP) with dual-surface functionalization-Pluronic F127 and chitosan-for inhibiting multi-drug resistant gene 1 (MDR1) expression and enhancing tumor uptake. The resultant spherical NPs-P/C had a desirable particle size (∼268 nm), slightly positive zeta-potential, and the ability to efficiently down-regulate the expression of MDR1. In vitro cytotoxicity tests revealed that the 24 and 48 h IC50 values of NPs-P/C1 were 2.03 and 0.67 μm, respectively, which were much lower than those for free CPT and other NPs. Interestingly, NPs-P/C1 showed the highest cellular uptake efficiency (approximately 85.5%) among the different drug formulations. Most importantly, treatment of colon tumor-bearing mice with various drug formulations confirmed that the introduction of Pluronic F127 and chitosan to the NP surface significantly enhanced the therapeutic efficacy of CPT, induced tumor cell apoptosis, and reduced systemic toxicity. Collectively, these findings suggest that our one-step-fabricated, dual-surface-functionalized NPs may hold promise as a readily scalable and effective drug carrier with clinical potential in colon cancer therapy.

Mucoadhesive polyacrylamide nanogel as a potential hydrophobic drug carrier for intravesical bladder cancer therapy

In this paper, amine-functionalized polyacrylamide nanogels (PAm-NH2) loaded with docetaxel (DTX) were evaluated as a mucoadhesive and sustained intravesical drug delivery (IDD) system for potential bladder cancer therapy. Nanogels have not been applied for such therapy before. The mucoadhesiveness of the PAm-NH2 nanogels, which is a critical factor for IDD application, was investigated using the mucin-particle method and by analyzing the direct attachment of the PAm-NH2 nanogels onto the luminal surface of porcine urinary bladder. DTX, as a model hydrophobic drug, was successfully loaded into hydrophilic PAm-NH2 nanogels with high loading efficiency (>90%), and sustained release of DTX from the nanogels over 9 days in artificial urine was achieved. The nanogels were also taken in by bladder cancer cells in a concentration-dependent manner. The efficiency of the DTX-loaded nanogels in killing UMUC3 and T24 bladder cancer cells was determined to be equivalent to free DTX, and the morphology of the bladder urothelium was not adversely altered by the PAm-NH2 nanogels. These findings indicate that such mucoadhesive nanogels are potentially a promising candidate for intravesical delivery of hydrophobic drugs in bladder cancer therapy.

Inhibition of MDR1 gene expression and enhancing cellular uptake for effective colon cancer treatment using dual-surface-functionalized nanoparticles

Nanomedicine options for colon cancer therapy have been limited by the lack of suitable carriers capable of delivering sufficient drug into tumors to cause lethal toxicity. To circumvent this limitation, we fabricated a camptothecin (CPT)-loaded poly(lactic-co-glycolic acid) nanoparticle (NP) with dual-surface functionalization-Pluronic F127 and chitosan-for inhibiting multi-drug resistant gene 1 (MDR1) expression and enhancing tumor uptake. The resultant spherical NPs-P/C had a desirable particle size (∼268 nm), slightly positive zeta-potential, and the ability to efficiently down-regulate the expression of MDR1. In vitro cytotoxicity tests revealed that the 24 and 48 h IC50 values of NPs-P/C1 were 2.03 and 0.67 μm, respectively, which were much lower than those for free CPT and other NPs. Interestingly, NPs-P/C1 showed the highest cellular uptake efficiency (approximately 85.5%) among the different drug formulations. Most importantly, treatment of colon tumor-bearing mice with various drug formulations confirmed that the introduction of Pluronic F127 and chitosan to the NP surface significantly enhanced the therapeutic efficacy of CPT, induced tumor cell apoptosis, and reduced systemic toxicity. Collectively, these findings suggest that our one-step-fabricated, dual-surface-functionalized NPs may hold promise as a readily scalable and effective drug carrier with clinical potential in colon cancer therapy.

Noncanonical amino acids to improve the pH response of pHLIP insertion at tumor acidity

The pH low insertion peptide (pHLIP) offers the potential to deliver drugs selectively to the cytoplasm of cancer cells based on tumor acidosis. The WT pHLIP inserts into membranes with a pH50 of 6.1, while most solid tumors have extracellular pH (pH(e)) of 6.5-7.0. To close this gap, a SAR study was carried out to search for pHLIP variants with improved pH response. Replacing Asp25 with α-aminoadipic acid (Aad) adjusts the pH50 to 6.74, matching average tumor acidity, and replacing Asp14 with γ-carboxyglutamic acid (Gla) increases the sharpness of pH response (transition over 0.5 instead of 1 pH unit). These effects are additive: the Asp14Gla/Asp25Aad double variant shows a pH50 of 6.79, with sharper transition than Asp25Aad. Furthermore, the advantage of the double variant over WT pHLIP in terms of cargo delivery was demonstrated in turn-on fluorescence assays and anti-proliferation studies (using paclitaxel as cargo) in A549 lung cancer cells at pH 6.6.

Acid/redox dual-activated liposomes for tumor-targeted drug delivery and enhanced therapeutic efficacy

Acid/redox dual-activated liposomes support enhanced therapeutic efficacy by overcoming multiple barriers to the intravenous delivery of an anticancer drug.

Tumor extracellular acidity activated “off–on” release of bortezomib from a biocompatible dendrimer

A nanoparticle with a specific response to tumor extracellular acidity provides a new option in the design of tumor-targeted delivery systems.

pH-sensitive ternary nanoparticles for nonviral gene delivery

In this study, a ternary DNA delivery system with the charge conversional ability by deshielding the PEG layer at tumor acidity was designed.

Current advances in lanthanide ion (Ln3+)-based upconversion nanomaterials for drug delivery

This review mainly focuses on the recent advances in various chemical syntheses of Ln³⁺-based upconversion nanomaterials, with special emphasis on their application in stimuli-response controlled drug release and subsequent therapy.

Surface charge-reversible polyelectrolyte complex nanoparticles for hepatoma-targeting delivery of doxorubicin

Polymeric nanoparticles are greatly advancing the field of nanomedicine due to their ability for targeted and controlled drug release.

Thermo and pH dual-controlled charge reversal amphiphilic graft copolymer micelles for overcoming drug resistance in cancer cells

A novel thermo and pH dual-controlled charge reversal PSMA₈₉-g-P(DMA₁₆-co-SD₅₆) graft copolymer micelle was developed with effectively enhanced cellular uptake for overcoming multi-drug resistance in cancer cells.

Reduction-triggered release of paclitaxel from in situ formed biodegradable core-cross-linked micelles

We provide a facile strategy to prepare redox-responsive core-crosslinked micelles for the controlled release of paclitaxel.

Integration of nanoassembly functions for an effective delivery cascade for cancer drugs

A "cluster-bomb"-like lipid-dendrimer nanoassembly synergizes the functions of its components and thereby efficiently accomplishes the drug delivery cascade for high efficacy in treating cancer. The nanoassembly successfully circulates in the blood and accumulates in the tumor. Once in the tumor, it releases small dendrimers that act like "bomblets", enabling tumor penetration, cell internalization, and drug release.

pH-sensitive polymeric nanoparticles for tumor-targeting doxorubicin delivery: concept and recent advances

Doxorubicin is a potent chemotherapeutic drug applied in the clinics for the treatment of various human cancers. It is typically administered as the hydrochloride salt or in liposomal forms, which are plagued with severe side effects. In recent years, pH-sensitive polymeric nanoparticles that are capable of retaining drug during circulation while actively releasing it at the tumor site and/or inside the target tumor cells have received an overwhelming interest for tumor-targeting cancer chemotherapy. This smart delivery approach has shown to elegantly resolve the in vivo stability versus intracellular drug release dilemma, as well as stealth versus tumor cell uptake dilemma. In this review, the concept and exciting new advances in pH-sensitive polymeric nanoparticles for doxorubicin delivery are presented and discussed.

Self-assembly of random copolymers

Self-assembly of random copolymers has attracted considerable attention recently. In this feature article, we highlight the use of random copolymers to prepare nanostructures with different morphologies and to prepare nanomaterials that are responsive to single or multiple stimuli. The synthesis of single-chain nanoparticles from random copolymers and their potential applications are also discussed in some detail. We aim to draw more attention to these easily accessible copolymers, which are likely to play an important role in translational polymer research.

Utilization of H-bond interaction of nucleobase Uralic with antitumor methotrexate to design drug carrier with ultrahigh loading efficiency and pH-responsive drug release

A novel Uralic (U)-rich linear-hyperbranched mono-methoxy poly (ethylene glycol)-hyperbranched polyglycerol-graft-Uralic (mPEG-HPG-g-U) nanoparticle (NP) was prepared as drug carrier for antitumor methotrexate (MTX). Due to the H-bond interaction of U with MTX and hydrophobic interaction, this NP exhibited high drug loading efficiency of up to 40%, which was significantly higher than that of traditional NPs based on U-absent copolymers (<15%). In addition, MTX-loaded mPEG-HPG-g-U NPs also demonstrated an acidity-accelerated drug release behavior.

Cytochrome c end-capped mesoporous silica nanoparticles as redox-responsive drug delivery vehicles for liver tumor-targeted triplex therapy in vitro and in vivo

To develop carriers for efficient anti-cancer drug delivery with reduced side effects, a biocompatible and redox-responsive nanocontainer based on mesoporous silica nanoparticles (MSNs) for tumor-targeted triplex therapy was reported in this study. The nanocontainer was fabricated by immobilizing cytochrome c (CytC) onto the MSNs as sealing agent via intermediate linkers of disulfide bonds for redox-responsive intracellular drug delivery. AS1411 aptamer was further tailored onto MSNs for cell/tumor targeting. The successful construction of redox- responsive MSNs was confirmed by BET/BJH analysis, transmission electron microscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy and thermogravimetric analysis (TGA), respectively. Detailed investigations demonstrated that anticancer drug of doxorubicin (DOX) loaded nanocontainer could be triggered by reductant (e.g. glutathione) within cellular microenvironment and release DOX to induce tumor cell apoptosis in vitro. More importantly, the nanocontainer displayed great potential for tumor targeting and achieved triplex therapy effects on the tumor inhibition in vivo through the loading DOX, gatekeeper of CytC and AS1411 aptamer, which were reflected by the change of tumor size, TUNEL staining and HE staining assays.

Shieldable tumor targeting based on pH responsive self-assembly/disassembly of gold nanoparticles

A new approach to shield/deshield ligands for controllable tumor targeting was reported, which was based on amphiphilic self-assembly and disassembly of gold nanoparticles (Au NPs). Thanks to the excellent pH response of the system, glycyrrhetinic acid (GA) ligands can be buried inside the Au NPs' assembly at normal tissue pH (pH 7.4), while exposed when the nanostructure is disassembled at tumor extracellular pH (pHe 6.8). Hydrophobic GA molecules not only acted as ligands targeting tumor cells but also provided the major interparticle attractive force for Au NPs' assembling. An ordered assembly of Au NPs with regular shape, proper size and ultrasharp pH sensitivity (ΔpH ∼ 0.2) was achieved by fine-tuning of materials modified on Au NPs. Mechanism studies for assembly and disassembly of Au NPs indicated the possibility of a GA shield when the assembly formed, which was further demonstrated by bovine serum albumin absorption and cellular uptake. The assembly/disassembly process was reversible within extrinsic pH changes, which provides a perspective for reversible tumor targeting.

Environment-dependent guest exchange in supramolecular hosts

Dynamic exchange of guest molecules, encapsulated in host assemblies, is a phenomenon in supramolecular chemistry that has important implications in several applications. While the mechanism of exchange in micellar assemblies has been previously investigated, the effect of host and guest environment upon the guest-exchange dynamics has received little attention, if any. In this paper, we study the guest-exchange mechanism in pH-sensitive nanogels along with pH-insensitive nanogels as a control. By systematically comparing the behavior of these nanogels, we show that size, concentration, and hydrophobicity can all play a critical role in guest-exchange dynamics. More importantly, these studies reveal that the dominant mechanism of guest exchange can intimately depend on environmental factors.

Engineered nanoparticles for drug delivery in cancer therapy

In medicine, nanotechnology has sparked a rapidly growing interest as it promises to solve a number of issues associated with conventional therapeutic agents, including their poor water solubility (at least, for most anticancer drugs), lack of targeting capability, nonspecific distribution, systemic toxicity, and low therapeutic index. Over the past several decades, remarkable progress has been made in the development and application of engineered nanoparticles to treat cancer more effectively. For example, therapeutic agents have been integrated with nanoparticles engineered with optimal sizes, shapes, and surface properties to increase their solubility, prolong their circulation half-life, improve their biodistribution, and reduce their immunogenicity. Nanoparticles and their payloads have also been favorably delivered into tumors by taking advantage of the pathophysiological conditions, such as the enhanced permeability and retention effect, and the spatial variations in the pH value. Additionally, targeting ligands (e.g., small organic molecules, peptides, antibodies, and nucleic acids) have been added to the surface of nanoparticles to specifically target cancerous cells through selective binding to the receptors overexpressed on their surface. Furthermore, it has been demonstrated that multiple types of therapeutic drugs and/or diagnostic agents (e.g., contrast agents) could be delivered through the same carrier to enable combination therapy with a potential to overcome multidrug resistance, and real-time readout on the treatment efficacy. It is anticipated that precisely engineered nanoparticles will emerge as the next-generation platform for cancer therapy and many other biomedical applications.