Acid-cleavable polymeric core–shell particles for delivery of hydrophobic drugs

Overview of polyethylene glycol-based materials with a special focus on core-shell particles for drug delivery application

Polyethylene glycols (PEG) are water-soluble nonionic polymeric molecules. PEG and PEG-based materials are used for various important applications such as solvents, adhesives, adsorbents, drug delivery agents, tissue engineering scaffolds, etc. The coating of nanoparticles with PEG forms core-shell nanoparticles. The PEG-based core-shell nanoparticles are synthesized for the development of high-quality drug delivery systems. In the present review, we first explained the basics and various applications of PEGs and PEG-based composites materials and then concentrated on the PEG-based core-shell nanoparticles for biomedical applications specifically their use in drug delivery.

Transiently thermoresponsive polymers and their applications in biomedicine

The focus of this review is on the class of transiently thermoresponsive polymers. These polymers are thermoresponsive, but gradually lose this property upon chemical transformation - often a hydrolysis reaction - in the polymer side chain or backbone. An overview of the different approaches used for the design of these polymers along with their physicochemical properties is given. Their amphiphilic properties and degradability into fully soluble compounds make this class of responsive polymers attractive for drug delivery and tissue engineering applications. Examples of these are also provided in this review.

A biomimetic and pH-sensitive polymeric micelle as carrier for paclitaxel delivery

As nano-scale drug delivery systems, smart micelles that are sensitive to specific biological environment and allowed for target site-triggered drug release by reversible stabilization of micelle structure are attractive. In this work, a biocompatible and pH-sensitive copolymer is synthesized through bridging poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) block and poly (D, L-lactide) (PLA) block by a benzoyl imine linkage (Blink). Biomimetic micelles with excellent biocompatibility based on such PLA-Blink-PMPC copolymer are prepared as carriers for paclitaxel (PTX) delivery. Due to the rapid breakage of the benzoyl imine linkage under acidic condition, the micelle structure is disrupted with accelerated PTX release. Such pH-sensitive triggered drug release behavior in synchronization with acidic conditions at tumor site is helpful for improving the utilization of drug and facilitating antitumor efficacy. These micelles can be used as promising drug delivery systems due to their biocompatible and smart properties.

A hyperbranched amphiphilic acetal polymer for pH-sensitive drug delivery

In this study, a novel hyperbranched amphiphilic block copolymer was synthesized using deactivation-enhanced atom transfer radical polymerization (DE-ATRP) for smart drug delivery.

A dimethacrylate cross-linker cleavable under thermolysis or alkaline hydrolysis conditions: synthesis, polymerization, and degradation

2,6-Pyridinediethanol diesters can be incorporated in polymers conveying selective alkaline hydrolytic lability and acid stability, in addition to thermolyzability.

High Tumor Penetration of Paclitaxel Loaded pH Sensitive Cleavable Liposomes by Depletion of Tumor Collagen I in Breast Cancer

The network of collagen I in tumors could prevent the penetration of drugs loaded in nanoparticles, and this would lead to impaired antitumor efficacy. In this study, free losartan (an angiotensin inhibitor) was injected before treatment to reduce the level of collagen I, which could facilitate the penetration of nanoparticles. Then the pH-sensitive cleavable liposomes (Cl-Lip) were injected subsequently to exert the antitumor effect. The Cl-Lip was constituted by PEG(5K)-Hydrazone-PE and DSPE-PEG(2K)-R8. When the Cl-Lip reached to the tumor site by the enhanced permeability and retention (EPR) effect, PEG(5K)-Hydrazone-PE was hydrolyzed from the Cl-Lip under the low extra-cellular pH conditions of tumors, then the R8 peptide was exposed, and finally liposomes could be internalized into tumor cells by the mediation of R8 peptide. In vitro experiments showed both the cellular uptake of Cl-Lip by 4T1 cells and cytotoxicity of paclitaxel loaded Cl-Lip (PTX-Cl-Lip) were pH sensitive. In vivo experiments showed the Cl-Lip had a good tumor targeting ability. After depletion of collagen I, Cl-Lip could penetrate into the deep place of tumors, the tumor accumulation of Cl-Lip was further increased by 22.0%, and the oxygen distributed in tumor tissues was also enhanced. The antitumor study indicated free losartan in combination with PTX-Cl-Lip (59.8%) was more effective than injection with PTX-Cl-Lip only (37.8%) in 4T1 tumor bearing mice. All results suggested that depletion of collagen I by losartan dramatically increased the penetration of PTX-Cl-Lip and combination of free losartan and PTX-CL-Lip could lead to better antitumor efficacy of chemical drugs. Thus, the combination strategy might be a promising tactic for better treatment of solid tumors with a high level of collagen I.

A robust platform for functional microgels via thiol-ene achemistry with reactive polyether-based nanoparticles

We herein report the development of crosslinked polyether particles as a reactive platform for the preparation of functional microgels. Thiol-ene crosslinking of poly(allyl glycidyl ether) in miniemulsion droplets - stabilized by a surface active, bio-compatible polyethylene glycol block copolymer - resulted in colloidal gels with a PEG corona and an inner polymeric network containing reactive allyl units. The stability of the allyl groups allows the microgels to be purified and stored before a second, subsequent thiol-ene functionalization step allows a wide variety of pH- and chemically-responsive groups to be introduced into the nanoparticles. The facile nature of this synthetic platform enables the preparation of microgel libraries that are responsive to different triggers but are characterized by the same size distribution, surface functionality, and crosslinking density. In addition, the utilization of a crosslinker containing cleavable ester groups renders the resulting hydrogel particles degradable at elevated pH or in the presence of esterase under physiological conditions.

Synthesis of Acid-Labile PEG and PEG-Doxorubicin-Conjugate Nanoparticles via Brush-First ROMP

A panel of acid-labile bis-norbornene cross-linkers was synthesized and evaluated for the formation of acid-degradable brush-arm star polymers (BASPs) via the brush-first ring-opening metathesis polymerization (ROMP) method. An acetal-based cross-linker was identified that, when employed in conjunction with a poly(ethylene glycol) (PEG) macromonomer, provided highly controlled BASP formation reactions. A combination of this new cross-linker with a novel doxorubicin (DOX)-branch-PEG macromonomer provided BASPs that simultaneously degrade and release cytotoxic DOX in vitro.

Magnetic nanoparticles with a pH-sheddable layer for antitumor drug delivery

A dually responsive nanocarrier with a multilayer core-shell architecture was prepared based on Fe3O4@SiO2 nanoparticles successively coated with poly(benzyl L-aspartate) (PBLA) and poly(ethylene glycol) (PEG) for the purpose of tumor specific drug delivery applications. In this system, PEG chains are connected to the surface via pH-sensitive benzoic-imine bonds and serve as a pH-sheddable hydrophilic corona. Meanwhile, the PBLA segments serve as a hydrophobic middle layer used to load the drugs via hydrophobic interactions. The Fe3O4@SiO2 nanoparticle functions as a superparamagnetic core used to direct the drug loaded nanocarrier to the target pathological site. The obtained materials were characterized with FT-IR, (1)H NMR, dynamic light scattering, zeta-potential, TEM, TGA, and hysteresis loop analysis. An anticancer drug doxorubicin (DOX) was selected as the model drug loaded into the nanocarrier, which was relatively stable under physiological conditions due to its neutral hydrophilic shell, and could quickly release the drug in response to increased acidity via shedding of the PEG shells through cleavage of the intermediate benzoic-imine bonds. Meanwhile, the neutral shell shedding would reveal a positively charged nanoparticle surface that is readily taken up by tumor cells. These pH- and magnetic-responsive nanoparticles showed significant potential for use in the targeted intracellular delivery of hydrophobic chemotherapeutics in cancer therapy.

Magnetic and pH sensitive drug delivery system through NCA chemistry for tumor targeting

Magnetic- and pH- dually sensitive drug delivery system.

Microspheres as therapeutic delivery agents: synthesis and biological evaluation of pH responsiveness

A soft template method was used for the synthesis of pH-responsive microcontainers with an inner cavity. Poly(glycidyl methacrylate) (PGMA) microspheres of narrow size distribution were synthesized by soap-free radical emulsion polymerization and the coating of the microspheres was carried out by the same procedure. The procedure consists of two steps. In the first step the sacrificial template is synthesized and in the second step the shell is formed. Acrylic acid was used as a coating monomer, with the aim of introducing pH sensitivity in the synthesized microcontainers. A loading and release study of the anthracycline drug doxorubicin (DOX) was also carried out. The toxicity evaluation of the drug was carried out using the MTT assay, and the necrotic effect was studied using trypan blue.

Development of a reduction-sensitive diselenide-conjugated oligoethylenimine nanoparticulate system as a gene carrier

BACKGROUND

The reduction-sensitive cationic polymer is a promising nonviral carrier for gene delivery. Until now, disulfide bonds have been the only golden standard for its design. The aim of this research was to develop a novel reduction-responsive cationic polymer as a gene carrier.

METHODS

Polycationic carriers were synthesized by addition of branched oligoethylenimine 800 Da (OEI(800)) via an active ester containing diselenide bonds. Disulfide bonds cross-linked with OEI(800)-SS(x) and monoselenide bonds linked with OEI(800)-Se(x) were synthesized and compared. Their molecular weights and degradation properties were determined using gel permeation chromatography. Changes in particle size, morphology, and DNA binding were investigated by dynamic light scattering, transmission electron microscopy, and electrophoresis assay in a reduction environment. Cytotoxicity and transfection in vitro were evaluated in a murine melanoma cell line (B16F10) and a human cervical epithelial carcinoma cell line (HeLa), while intracellular degradation and dissociation with DNA were studied by confocal laser scanning microscopy with FITC-labeled OEI(800) derivatives and Cy5-labeled DNA.

RESULTS

Diselenide-conjugated OEI(800) (OEI(800)-SeSe(x)) polymer carriers of high molecular weight were successfully synthesized. After compacting with DNA, the OEI(800)-SeSe(x) polymers formed nanoparticles with an average size of 140 nm at an adequate C/P ratio. OEI(800)-SeSe(x) showed reduction-responsive degradation properties similar to those of the OEI(800)-SS(x) via gel permeation chromatography, dynamic light scattering, and transmission electron microscopy. OEI(800)-SeSe(x) showed much lower cytotoxicity than PEI(25k), and significantly higher transfection efficiency than OEI(800) in both B16F10 and HeLa cells. Transfection of luciferase in the OEI(800)-SeSe(x) group was comparable with that of standard PEI(25k) and traditional reduction-sensitive polymer OEI(800)-SS(x) groups. Furthermore, intracellular degradation of OEI(800)-SeSe(x) and dissociation with DNA were also confirmed by confocal laser scanning microscopy.

CONCLUSION

The OEI(800)-SeSe(x) obtained was able to bind plasmid DNA efficiently to yield nanosized particles and had reduction sensitivity which is as efficient as that for OEI(800)-SS(x). In vitro experiments confirmed its low cytotoxicity and high transfection ability. Diselenide bonds can be used as effective and novel reduction-sensitive linkages for gene delivery.

Core-crosslinked pH-sensitive degradable micelles: A promising approach to resolve the extracellular stability versus intracellular drug release dilemma

The extracellular stability versus intracellular drug release dilemma has been a long challenge for micellar drug delivery systems. Here, core-crosslinked pH-sensitive degradable micelles were developed based on poly(ethylene glycol)-b-poly(mono-2,4,6-trimethoxy benzylidene-pentaerythritol carbonate-co-acryloyl carbonate) (PEG-b-P(TMBPEC-co-AC)) diblock copolymer that contains acid-labile acetal and photo-crossslinkable acryloyl groups in the hydrophobic polycarbonate block for intracellular paclitaxel (PTX) release. The micelles following photo-crosslinking while displaying high stability at pH 7.4 were prone to rapid hydrolysis at mildly acidic pHs of 4.0 and 5.0, with half lives of ca. 12.5 and 38.5h, respectively. Notably, these micelles showed high drug loading efficiencies of 76.0-93.2% at theoretical PTX loading contents of 5-15wt.%. Depending on drug loading contents, PTX-loaded micelles had average sizes varying from 132.2 to 171.6nm, which were decreased by 17-22nm upon photo-crosslinking. The in vitro release studies showed that PTX release at pH 7.4 was greatly inhibited by crosslinking of micelles. Notably, rapid drug release was obtained under mildly acidic conditions, in which 90.0% and 78.1% PTX was released in 23h at pH 4.0 and 5.0, respectively. MTT assays showed that PTX-loaded crosslinked micelles retained high anti-tumor activity with a cell viability of 9.2% observed for RAW 264.7 cells following 72h incubation, which was comparable to PTX-loaded non-crosslinked counterparts (cell viability 7.5%) under otherwise the same conditions, supporting efficient drug release from PTX-loaded crosslinked micelles inside the tumor cells. These core-crosslinked pH-responsive biodegradable micelles with superior extracellular stability and rapid intracellular drug release provide a novel platform for tumor-targeting drug delivery.

Tumor accumulation and antitumor efficacy of docetaxel-loaded core-shell-corona micelles with shell-specific redox-responsive cross-links

A robust core-shell-corona micelle bearing redox-responsive shell-specific cross-links was evaluated as a carrier of docetaxel (DTX) for cancer therapy. The polymer micelles of poly(ethylene glycol)-b-poly(L-lysine)-b-poly(L-phenylalanine) (PEG-PLys-PPhe) in the aqueous phase provided the three distinct functional domains: the PEG outer corona for prolonged circulation, the PLys middle shell for disulfide cross-linking, and the PPhe inner core for DTX loading. The shell cross-linking was performed by the reaction of disulfide-containing cross-linkers with Lys moieties in the middle shells. The shell cross-linking did not change the micelle size or the spherical morphology. The shell cross-linked micelles exhibited enhanced serum stability. The DTX release from the DTX-loaded disulfide cross-linked micelles (DTX-SSCLM) was facilitated by increasing the concentration of glutathione (GSH). At an intracellular GSH level, DTX release was facilitated due to the reductive cleavage of the disulfide cross-links in the shell domains. The in vivo tissue distribution and tumor accumulation of the DTX-SSCLM that were labeled with a near-infrared fluorescence (NIRF) dye, Cy5.5, were monitored in MDA-MB231 tumor-bearing mice. Non-invasive real-time optical imaging results indicated that the DTX-SSCLM exhibited enhanced tumor specificity due to the prolonged stable circulation in blood and the enhanced permeation and retention (EPR) effect compared with the DTX-loaded non-cross-linked micelles (DTX-NCLM). The DTX-SSCLM exhibited enhanced therapeutic efficacy in tumor-bearing mice compared with free DTX and DTX-NCLM. The domain-specific shell cross-linking that is described in this work may serve as a useful guidance for enhancing the antitumor therapeutic efficacy of various polymer micelles and nano-aggregates.

Multi-responsive nanogels containing motifs of ortho ester, oligo(ethylene glycol) and disulfide linkage as carriers of hydrophobic anti-cancer drugs

A family of multi-responsive nanogels with different compositions and crosslinking degrees have been prepared by the miniemulsion copolymerization of monomethyl oligo(ethylene glycol) acrylate (OEGA) and an ortho ester-containing acrylic monomer, 2-(5,5-dimethyl-1,3-dioxan-2-yloxy) ethyl acrylate (DMDEA), with bis(2-acryloyloxyethyl) disulfide (BADS) as a crosslinker. These nanogels are thermoresponsive and labile in the weakly acidic or reductive environments. The thermoresponsive behaviors, acid-triggered hydrolysis, and reduction-induced degradation of these nanogels were studied by means of dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results indicate that the volume phase transition temperature (VPTT), thermally induced deswelling ratio, and acid-triggered swelling ratio of the nanogels are closely relevant to their compositions and crosslinking degrees. Although these nanogels could be reductively disrupted by dithiothreitol (DTT), single polymer chains with sizes smaller than 20 nm were not detected by DLS. This is probably due to the existence of some unbreakable linkages formed by chain transfer to the disulfide bond during the radical polymerization. These nanogels are capable of encapsulating hydrophobic compounds. The loading capability of the nanogels for Nile Red (NR), paclitaxel (PTX), and doxorubicin (DOX), and the release behaviors of the drug-loaded nanogels were investigated by UV-vis spectrometry and HPLC. As expected, drug release can be greatly accelerated by a cooperative effect of both acid-triggered hydrolysis and DTT-induced degradation. Finally, the PTX-loaded nanogels exhibit a concentration-dependent toxicity to MCF-7 cells while the intact unloaded nanogels are non-toxic, thereby they may be used as potential carriers for hydrophobic anticancer drugs.

N-Boc-histidine-capped PLGA-PEG-PLGA as a smart polymer for drug delivery sensitive to tumor extracellular pH

A pH-sensitive polymer was synthesized by introducing the N-Boc-histidine to the ends of a PLGA-PEG-PLGA block copolymer. The synthesized polymer was confirmed to be biodegradable and biocompatible, well dissolved in water and forming micelles above the CMC. DOX was employed as a model anticancer drug. In vitro drug release from micelles of N-Boc-histidine-capped PLGA-PEG-PLGA exhibited significant difference between pH = 6.2 and pH = 7.4, whereas DOX release from micelles composed of un-capped virgin polymers was not significantly sensitive to medium pH. Uptake of DOX from micelles of the new polymer into MDA-MB-435 solid tumor cells was also observed, and pH sensitivity was confirmed. Hence, the N-Boc-histidine capped PLGA-PEG-PLGA might be a promising material for tumor targeting.

Novel reduction-sensitive micelles for triggered intracellular drug release

Novel reduction-sensitive micelles based on poly(ethylene oxide)-b-poly(N-methacryloyl-N'-(t-butyloxycarbonyl)cystamine) (PEO-b-PMABC) diblock copolymers were developed and applied for triggered intracellular drug release. PEO-b-PMABC block copolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of MABC with dithioester-capped PEO as macroRAFT agent. Gel permeation chromatography (GPC) and (1)H NMR analysis showed that the copolymers have controlled compositions and molecular weights, indicating the living nature of polymerization. These copolymers were self-assembled into micelles. The physicochemical characteristics and reduction-sensitivity of the resultant micelles were investigated by fluorescence measurement, transmission electron microscopy (TEM), and dynamic light scattering (DLS). The results showed that PEO-b-PMABC micelles are stable at normal physiologic condition but readily cleaved into free copolymers under reducing environment. In vitro release of doxorubicin (DOX) and cell experiments showed that the drug-loaded PEO-b-PMABC micelles accomplished much faster drug release under reducing condition and higher anticancer efficacy as compared to the control without reduction-sensitivity, indicating great potential of PEO-b-PMABC micelles for efficient intracellular drug delivery.

Nanotechnology solutions for mucosal immunization

The current prevalence of infectious diseases in many developing regions of the world is a serious burden, impacting both the general health as well as economic growth of these communities. Additionally, treatment with conventional medication becomes increasingly challenging due to emergence of new and drug resistant strains jeopardizing the progress made in recent years towards control and elimination of certain types of infectious diseases. Thus, from a public health perspective, prevention such as through immunization by vaccination, which has proven to be most effective, might be the best alternative to prevent and combat infectious diseases in these regions. To achieve this, development of wide-scale immunization programs become necessary including vaccines that can easily and widely be distributed, stored and administered. Mucosal vaccines offer great potential since they can be administered via oral or intranasal delivery route which does not require trained personnel, avoids the use of needles and improves overall patient compliance and acceptance. However, it necessitates the implementation of specific immunization strategies to improve their efficacy. Application of nanotechnology to design and create particle mediated delivery systems that can efficiently encapsulate vaccine components for protection of the sensitive payload, target the mucosal immune system and incorporate mucosal adjuvants maximizing immune response is key strategy to improve the effectiveness of mucosal vaccines.

Engineering of nanometer-sized cross-linked hydrogels for biomedical applications

Microgels/nanogels (micro/nanogels) are promising drug-delivery systems (DDS) because of their unique properties, including tunable chemical and physical structures, good mechanical properties, high water content, and biocompatibility. They also feature sizes tunable to tens of nanometers, large surface areas, and interior networks. These properties demonstrate the great potential of micro/nanogels for drug delivery, tissue engineering, and bionanotechnology. This mini-review describes the current approaches for the preparation and engineering of effective micro/nanogels for drug-delivery applications. It emphasizes issues of degradability and bioconjugation, as well as loading/encapsulation and release of therapeutics from customer-designed micro/nanogels.