Nanogels as pharmaceutical carriers: finite networks of infinite capabilities

Nanoparticles and modulation of immune responses

The exact role of engineered nanomaterials in immune system modulation remains unclear. The aim of this concise review is to give a comprehensive insight into recent published scientific data concerning the modulation of innate and adaptive immune responses by engineered nanoparticles, and to provide a basis for future experimental work related to designing safer, and more efficient biomaterials.

Hybrid micro-/nanogels for optical sensing and intracellular imaging

Hybrid micro-/nanogels are playing an increasing important part in a diverse range of applications, due to their tunable dimensions, large surface area, stable interior network structure, and a very short response time. We review recent advances and challenges in the developments of hybrid micro-/nanogels toward applications for optical sensing of pH, temperature, glucose, ions, and other species as well as for intracellular imaging. Due to their unique advantages, hybrid micro-/nanogels as optical probes are attracting substantial interests for continuous monitoring of chemical parameters in complex samples such as blood and bioreactor fluids, in chemical research and industry, and in food quality control. In particular, their intracellular probing ability enables the monitoring of the biochemistry and biophysics of live cells over time and space, thus contributing to the explanation of intricate biological processes and the development of novel diagnoses. Unlike most other probes, hybrid micro-/nanogels could also combine other multiple functions into a single probe. The rational design of hybrid micro-/nanogels will not only improve the probing applications as desirable, but also implement their applications in new arenas. With ongoing rapid advances in bionanotechnology, the well-designed hybrid micro-/nanogel probes will be able to provide simultaneous sensing, imaging diagnosis, and therapy toward clinical applications.

Hollow core-porous shell structure poly(acrylic acid) nanogels with a superhigh capacity of drug loading

Poly(acrylic acid) (PAA) nanogels with a hollow core-porous shell structure were prepared by the direct polymerization of an acrylic acid monomer in the presence of hydroxypropylcellulose (HPC) and a cross-linking agent, N,N-methylenebisacrylamide, followed by removal of HPC from the generated HPC-PAA nanoparticles in a basic environment. The properties of PAA nanogel were characterized by dynamic light scattering, FT-IR, transmission electron microscopy, and atomic force microscopy. It is found that the nanogels have a hollow core-porous shell structure. Protein, bovine serum albumin (BSA), and an antitumor agent, doxorubicin hydrochloride, were used as model drugs to investigate their loading abilities as versatile drug-delivery vehicles. The nanogel exhibits surprisingly high loading ability to both protein and small molecular drugs. For example, the maximum BSA loading capacity of PAA nanogel can reach as high as 800% (i.e., 1 mg of nanogel can load about 8.0 mg of BSA). This high loading capacity may be related with the hollow core-porous shell structure of PAA nanogels. PAA nanogels have also shown sustained drug release properties and can cross biological barriers to deliver loaded cargo inside cells. Considering the high stability of the materials, simple and mild preparation procedure, high loading capacity, sustained-release property, and ability to protect biological agents from denaturation, PAA nanogels should be promising drug-delivery carriers for drug-delivery systems.

The complex role of multivalency in nanoparticles targeting the transferrin receptor for cancer therapies

Transferrin receptor (TfR, CD71) has long been a therapeutic target due to its overexpression in many malignant tissues. In this study, PRINT() nanoparticles were conjugated with TfR ligands for targeted drug delivery. Cylindrical poly(ethylene glycol)-based PRINT nanoparticles (diameter (d) = 200 nm, height (h) = 200 nm) labeled with transferrin receptor antibody (NP-OKT9) or human transferrin (NP-hTf) showed highly specific TfR-mediated uptake by all human tumor cell lines tested, relative to negative controls (IgG1 for OKT9 or bovine transferrin (bTf) for hTf). The targeting efficiency was dependent on particle concentration, ligand density, dosing time, and cell surface receptor expression level. Interestingly, NP-OKT9 or NP-hTf showed little cytotoxicity on all solid tumor cell lines tested but were very toxic to Ramos B-cell lymphoma, whereas free OKT9 or hTf was not toxic. There was a strong correlation between TfR ligand density on the particle surface and cell viability and particle uptake. NP-OKT9 and NP-hTf were internalized into acidic intracellular compartments but were not localized in EEA1-enriched early endosomes or lysosomes. Elevated caspase 3/7 activity indicates activation of apoptosis pathways upon particle treatment. Supplementation of iron suppressed the toxicity of NP-OKT9 but not NP-hTf, suggesting different mechanisms by which NP-hTf and NP-OKT9 exerts cytotoxicity on Ramos cells. On the basis of such an observation, the complex role of multivalency in nanoparticles is discussed. In addition, our data clearly reveal that one must be careful in making claims of "lack of toxicity" when a targeting molecule is used on nanoparticles and also raise concerns for unanticipated off-target effects when one is designing targeted chemotherapy nanodelivery agents.

Noncovalent encapsulation stabilities in supramolecular nanoassemblies

Exchange dynamics of lipophilic guest molecules, encapsulated in supramolecular nanoassemblies in aqueous solutions, have implications in evaluating the stability of drug delivery vehicles. This is because exchange dynamics is related to the propensity of a nanocarrier to be leaky. We describe a fluorescence resonance energy transfer (FRET) based method to evaluate guest exchange dynamics in the aqueous phase. We have utilized this method to analyze the stability of encapsulation in polymeric nanogels and other related amphiphilic nanoassemblies.

Development of a liposomal nanoparticle formulation of 5-fluorouracil for parenteral administration: formulation design, pharmacokinetics and efficacy

5-Fluorouracil (5-FU) is a small, very membrane permeable drug that is poorly retained within the aqueous compartment of liposomal nanoparticles (LNP). To address this problem a novel method relying on formation of a ternary complex comprising copper, low molecular weight polyethylenimine (PEI) and 5-FU has been developed. More specifically, in the presence of entrapped copper and PEI, externally added 5-FU can be efficiently encapsulated (>95%) in DSPC/Chol (1,2-Distearoyl-sn-Glycero-3-Phosphocholine/cholesterol; 55:45 mol%) liposomes (130-170 nm) to achieve drug-to-lipid ratios of 0.1 (mol:mol). Drug release studies completed using this LNP formulation of 5-FU demonstrated significant improvements in drug retention in vitro and in vivo. Plasma concentrations of 5-FU were 7- to 23-fold higher when the drug was administered intravenously to mice as the LNP 5-FU formulation compared to free 5-FU. Further, the therapeutic effects of the LNP 5-FU formulation, as determined in a HT-29 subcutaneous colorectal cancer model where treatment was given QDx5, was greater than that which could be achieved with free 5-FU when compared at equivalent doses. This is the first time an active loading method has been described for 5-FU. The use of ternary metal complexation strategy to encapsulate therapeutic agents may define a unique platform for preparation of LNP drug formulations.

Nano-NRTIs: efficient inhibitors of HIV type-1 in macrophages with a reduced mitochondrial toxicity

BACKGROUND

Macrophages serve as a depot for HIV type-1 (HIV-1) in the central nervous system. To efficiently target macrophages, we developed nanocarriers for potential brain delivery of activated nucleoside reverse transcriptase inhibitors (NRTIs) called nano-NRTIs.

METHODS

Nanogel carriers consisting of poly(ethylene glycol) (PEG)- or Pluronic-polyethylenimine (PEI) biodegradable networks, star PEG-PEI or poly(amidoamine) dendrimer-PEI-PEG dendritic networks, as well as nanogels decorated with brain-targeting peptide molecules, specifically binding to the apolipoprotein E receptor, were synthesized and evaluated. Nano-NRTIs were obtained by mixing aqueous solutions of zidovudine 5'-triphosphate or didanosine 5'-triphosphate and nanocarriers, followed by freeze-drying. Intracellular accumulation, cytotoxicity and antiviral activity of nano-NRTIs were monitored in monocyte-derived macrophages (MDMs). HIV-1 viral activity in infected MDMs was measured by a reverse transcriptase activity assay following treatment with nano-NRTIs. Mitochondrial DNA depletion in MDMs and human HepG2 cells was assessed by quantitative PCR.

RESULTS

Nanogels were efficiently captured by MDMs and demonstrated low cytotoxicity, and no antiviral activity without drugs. All nano-NRTIs demonstrated high efficacy of HIV-1 inhibition at drug levels as low as 1 μmol/l, representing a 4.9- to 14-fold decrease in 90% effective drug concentrations as compared with NRTIs, whereas 50% cytotoxicity effects started at 200× higher concentrations. Nano-NRTIs with a core-shell structure and decorated with brain-targeting peptides displayed the highest antiviral efficacy. Mitochondrial DNA depletion, a major cause of NRTI neurotoxicity, was reduced threefold compared with NRTIs at application of selected nano-NRTIs.

CONCLUSIONS

Nano-NRTIs demonstrated a promising antiviral efficacy against HIV-1 in MDMs and showed strong potential as nanocarriers for delivery of antiviral drugs to macrophages harbouring in the brain.

Poly(N-isopropylacrylamide) microgels at the oil-water interface: interfacial properties as a function of temperature

Highly monodisperse poly(N-isopropylacrylamide), PNiPAM, microgels were prepared by the conventional radical polymerization of NiPAM in the presence of dimethylamino ethyl methacrylate (DMAEMA) monomers at various concentrations. The effect of DMAEMA on the polymerization of PNiPAM microgels was examined at constant initiator (V50) and cross-linker (MBA) concentrations. The presence of DMAEMA in the synthesis batch allows for the preparation of PNiPAM microgels with controlled size and a narrow size distribution. The oil(dodecane)/water interfacial properties of the model PNiPAM microgels were then investigated. The pendant drop technique was used to measure the interfacial tensions as a function of temperature. Over the whole range of temperature (20-45 degrees C), the interfacial tension remains low (on the order of 17 mN/m) and goes through a minimum (12 mN/m) at a temperature of about 34 degrees C, which well matches the volume phase transition temperature (VPTT) of PNiPAM microgels. Below the VPTT, the decrease in the interfacial tension with temperature is likely to be due to the adsorption of dense layers because of the decrease of the excluded volume interactions. Above the VPTT, we suggest that the increase in the interfacial tension with temperature comes from the adsorption of loosely packed PNiPAM microgels. We also studied the effect of temperature on the stability of emulsions. Dodecane in water emulsions, which form at ambient temperature, are destabilized as the temperature exceeds the VPTT. In light of the interfacial tension results, we suggest that emulsion destabilization arises from the adsorption of aggregates above the VPTT and not from an important desorption of microgels. Aggregate adsorption would bring a sufficiently high number of dodecane molecules into contact with water to induce coalescence without changing the interfacial tension very much.

Polymeric micelles with ionic cores containing biodegradable cross-links for delivery of chemotherapeutic agents

Novel functional polymeric nanocarriers with ionic cores containing biodegradable cross-links were developed for delivery of chemotherapeutic agents. Block ionomer complexes (BIC) of poly(ethylene oxide)-b-poly(methacylic acid) (PEO-b-PMA) and divalent metal cations (Ca(2+)) were utilized as templates. Disulfide bonds were introduced into the ionic cores by using cystamine as a biodegradable cross-linker. The resulting cross-linked micelles with disulfide bonds represented soft, hydrogel-like nanospheres and demonstrated a time-dependent degradation in the conditions mimicking the intracellular reducing environment. The ionic character of the cores allowed to achieve a very high level of doxorubicin (DOX) loading (50% w/w) into the cross-linked micelles. DOX-loaded degradable cross-linked micelles exhibited more potent cytotoxicity against human A2780 ovarian carcinoma cells as compared to micellar formulations without disulfide linkages. These novel biodegradable cross-linked micelles are expected to be attractive candidates for delivery of anticancer drugs.

Intracellular delivery and anti-cancer effect of self-assembled heparin-Pluronic nanogels with RNase A

A novel self-assembled nanogel was prepared for the intracellular delivery of ribonuclease A (RNase A) and the anti-cancer efficacy of RNase A delivery was investigated. The physical properties of self-assembled heparin-Pluronic (HP) nanogels incorporating RNase A (HPR nanogels) were characterized by dynamic light scattering (DLS), ξ-potential, and transmission electron microscopy (TEM). RNase A showed a strong affinity for the HP nanogel, resulting in a high loading efficiency (>78%) and significantly decreased hydrodynamic size (from 89 to ~29). HPR nanogels were efficiently internalized into HeLa cells and localized in the cytosol as well as the nucleus. In the mechanism study of cellular uptake, treating with methoxy β-cyclodextrin (Mβ-CD) decreased the uptake efficiency of HP nanogel, indicating that internalization occurs via caveolae/lipid-raft mediated endocytosis. Localization in the nucleus most likely occurred because the conjugated heparin facilitated nucleus penetration. The cytotoxicity of HPR nanogels was significantly increased when the RNase A concentration was increased, which resulted from the degradation of single stranded RNAs in the cytosol and the nucleus due to the intracellular localization of the HPR nanogels. These results demonstrate that self-assembled HP nanogels are a remarkable vehicle for intracellular protein delivery and hold promise for use as cancer chemotherapeutics.

Endocytosis of nanomedicines

Novel nanomaterials are being developed to improve diagnosis and therapy of diseases through effective delivery of drugs, biopharmaceutical molecules and imaging agents to target cells in disease sites. Such diagnostic and therapeutic nanomaterials, also termed "nanomedicines", often require site-specific cellular entry to deliver their payload to sub-cellular locations hidden beneath cell membranes. Nanomedicines can employ multiple pathways for cellular entry, which are currently insufficiently understood. This review, first, classifies various mechanisms of endocytosis available to nanomedicines including phagocytosis and pinocytosis through clathrin-dependent and clathrin-independent pathways. Second, it describes the current experimental tools to study endocytosis of nanomedicines. Third, it provides specific examples from recent literature and our own work on endocytosis of nanomedicines. Finally, these examples are used to ascertain 1) the role of particle size, shape, material composition, surface chemistry and/or charge for utilization of a selected pathway(s); 2) the effect of cell type on the processing of nanomedicines; and 3) the effect of nanomaterial-cell interactions on the processes of endocytosis, the fate of the nanomedicines and the resulting cellular responses. This review will be useful to a diverse audience of students and scientists who are interested in understanding endocytosis of nanomedicines.

Combining host-guest systems with nonfouling material for the fabrication of a biosurface: toward nearly complete and reversible resistance of cytochrome c

In this letter, a pH-responsive reactivated biointerface is fabricated using an inclusion reaction between an azobenzene-containing self-assembled monolayer and pH-responsive poly(ethylene glycol)-block-poly(acrylic acid) grafted with cyclodextrins. The pH-responsive interface can be switched between an extended state and a relaxed state for the reversible resistance of cytochrome c adsorption completely in cooperation with protein-resistant poly(ethylene glycol).

Probing protease activity by single-fluorescent-protein nanocapsules

We describe a FRET-based protease detection strategy, using a single-fluorescent-protein nanogel as donor and a dark quencher as acceptor linked by a photolabile caged-peptide. This design enables probing of protease activity in a UV-responsive fashion.

Microfluidic directed self-assembly of liposome-hydrogel hybrid nanoparticles

We present a microfluidic method to direct the self-assembly of temperature-sensitive liposome-hydrogel hybrid nanoparticles. Our approach yields nanoparticles with structural properties and highly monodisperse size distributions precisely controlled across a broad range relevant to the targeted delivery and controlled release of encapsulated therapeutic agents. We used microfluidic hydrodynamic focusing to control the convective-diffusive mixing of two miscible nanoparticle precursor solutions (a DPPC:cholesterol:DCP phospholipid formulation in isopropanol and a photopolymerizable N-isopropylacrylamide mixture in aqueous buffer) to form nanoscale lipid vesicles with encapsulated hydrogel precursors. These precursor nanoparticles were collected off-chip and were irradiated with ultraviolet (UV) light in bulk to polymerize the nanoparticle interiors into hydrogel cores. Multiangle laser light scattering in conjunction with asymmetric flow field-flow fractionation was used to characterize nanoparticle size distributions, which spanned the approximately 150 to approximately 300 nm diameter range as controlled by microfluidic mixing conditions, with a polydispersity of approximately 3% to approximately 5% (relative standard deviation). Transmission electron microscopy was then used to confirm the spherical shape and core-shell composition of the hybrid nanoparticles. This method may be extended to the directed self-assembly of other similar cross-linked hybrid nanoparticle systems with engineered size/structure-function relationships for practical use in healthcare and life science applications.

Self-assembled nanogel made of mannan: synthesis and characterization

Amphiphilic mannan (mannan-C(16)) was synthesized by the Michael addition of hydrophobic 1-hexadecanethiol (C(16)) to hydroxyethyl methacrylated mannan (mannan-HEMA). Mannan-C(16) formed nanosized aggregates in water by self-assembly via the hydrophobic interaction among C(16) molecules as confirmed by hydrogen nuclear magnetic resonance ((1)H NMR), fluorescence spectroscopy, cryo-field emission scanning electron microscopy (cryo-FESEM), and dynamic light scattering (DLS). The mannan-C(16) critical aggregation concentration (cac), calculated by fluorescence spectroscopy with Nile red and pyrene, ranged between 0.04 and 0.02 mg/mL depending on the polymer degree of substitution of C(16) relative to methacrylated groups. Cryo-FESEM micrographs revealed that mannan-C(16) formed irregular spherical macromolecular micelles, in this work designated as nanogels, with diameters ranging between 100 and 500 nm. The influence of the polymer degree of substitution, DS(HEMA) and DS(C(16)), on the nanogel size and zeta potential was studied by DLS at different pH values and ionic strength and as a function of mannan-C(16) and urea concentrations. Under all tested conditions, the nanogel was negatively charged with a zeta potential close to zero. Mannan-C(16) with higher DS(HEMA) and DS(C(16)) values formed larger nanogels and were also less stable over a 6 month storage period and at concentrations close to the cac. When exposed to solutions of different pH and aggressive conditions of ionic strength and urea concentration, the size of mannan-C(16) varied to some extent but was always in the nanoscale range.

Influence of architecture on the interaction of negatively charged multisensitive poly(N-isopropylacrylamide)-co-methacrylic acid microgels with oppositely charged polyelectrolyte: absorption vs adsorption

Two sets of core-shell microgels composed of temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) with different spatial distribution of pH-sensitive methacrylic acid (MAA) groups were prepared. The cores consist of either PNiPAM (neutral core; nc) or PNiPAM-co-MAA (charged core; cc). A charged shell existing of PNiPAM-co-MAA was added to the neutral core (yielding neutral core-charged shell; nccs), on the charged core, on the other hand, a neutral shell of PNiPAM was added (charged core-neutral shell; ccns). Complexes of these microgels with positively charged poly(diallyldimethylammonium chloride) (PDADMAC) of different molar masses were prepared. The amount of bound polyelectrolyte was quantified, and the microgel-polyelectrolyte complexes were characterized with respect to electrophoretic mobility and hydrodynamic radius. The penetration of polyelectrolyte into the microgel was also monitored by means of lifetime analysis of a fluorescent dye covalently bound to poly(L-lysine) providing information on the probe's local environment. The architecture of the microgel has a significant influence on the interaction with oppositely charged polyelectrolyte. Complexes with microgel with the charged shell tend to flocculate at charge ratios of 1 and are thus similar to polyelectrolyte complexes with rigid colloidal particles. Complexes with microgels that consist of a charged core and a neutral shell show very different properties: They are still temperature sensitive and reveal an influence of the polyelectrolyte's chain length. Low molecular weight PDADMAC can penetrate through the neutral shell into the charged core, and thus nearly no charge reversal occurs. The high-MW polyelectrolyte does not penetrate fully and leads to charge reversal. The results demonstrate that microgels are able to absorb or adsorb polyelectrolytes depending on the polyelectrolyte's chain length and the microgels architecture. Complexes with different surface properties and different colloidal stability can be prepared, and polyelectrolytes can be encapsulated in the microgel core. Thus, multisensitive core-shell microgels combine permeability and compartmentalization on a nanometer length scale and provide unique opportunities for applications in controlled uptake and release.

Stimuli-responsive smart nanogels for cancer diagnostics and therapy

This article discusses stimuli-responsive poly(ethylene glycol) (PEG)-coated (PEGylated) nanogels and their biomedical applications. Preparation and characterization of stimuli-responsive PEGylated nanogels composed of a crosslinked poly(2-[N,N-diethylamino]ethyl methacrylate) (PEAMA) core and PEG tethered chains are initially described. Stimuli-responsive PEGylated nanogels show unique properties and functions in synchronizing with the reversible volume phase transition of the PEAMA core in response to the extracellular pH (7-6.5) of a tumor environment as well as endosomal/lysosomal pH (6.5-5.0) and temperature. We list several biomedical applications of stimuli-responsive PEGylated nanogels, including (19)F magnetic resonance spectroscopic imaging (MRS/I) probe to visualize acidosis (tumor tissue), intracellular drug and siRNA delivery, antennas for cancer photothermal therapy and apoptosis probe for monitoring response to cancer therapy. Thus, stimuli-responsive PEGylated nanogels can be utilized as smart nanomedicines for cancer diagnostics and therapy.

Doubly amphiphilic poly(2-oxazoline)s as high-capacity delivery systems for hydrophobic drugs

Solubilization of highly hydrophobic drugs with carriers that are non-toxic, non-immunogenic and well-defined remains a major obstacle in pharmaceutical sciences. Well-defined amphiphilic di- and triblock copolymers based on poly(2-oxazolines) were prepared and used for the solubilization of Paclitaxel (PTX) and other water-insoluble drugs. Probing the polymer micelles in water with the fluorescence probe pyrene, an unusual high polar microenvironment of the probe was observed. This coincides with an extraordinary large loading capacity for PTX of 45 wt.% active drug in the formulation as well as high water solubility of the resulting formulation. Physicochemical properties of the formulations, ease of preparation and stability upon lyophilization, low toxicity and immunogenicity suggest that poly(2-oxazoline)s are promising candidates for the delivery of highly challenging drugs. Furthermore, we demonstrate that PTX is fully active and provides superior tumor inhibition as compared to the commercial micellar formulation.

Hydrophobic pharmaceuticals mediated self-assembly of beta-cyclodextrin containing hydrophilic copolymers: novel chemical responsive nano-vehicles for drug delivery

Double hydrophilic copolymers with one polyethylene glycol (PEG) block and one beta-cyclodextrin (beta-CD) flanking block (PEG-b-PCDs) were synthesized through the post-modification of macromolecules. The self-assembly of PEG-b-PCDs in aqueous solutions was initially studied by a fluorescence technique. This measurement together with AFM and TEM characterizations demonstrated the formation of nanoparticles in the presence of lipophilic small molecules. The host-guest interaction between the beta-CD unit of a host copolymer and the hydrophobic group of a guest molecule was found to be the driving force for the observed self-assembly. This spontaneous assembly upon loading of guest molecules was also observed for hydrophobic drugs with various chemical structures. Relatively high drug loading was achieved by this approach. Desirable encapsulation was also achieved for the hydrophobic drugs that cannot efficiently interact with free beta-CD. In vitro release studies suggested that the payload in nano-assemblies could be released in a sustained manner. In addition, both the fluorescence measurement and the in vitro drug release studies suggested that these nano-assemblies mediated by the inclusion complexation exhibited a chemical sensitivity. The release of payload can be accelerated upon the triggering by hydrophobic guest molecules or free beta-CD molecules. These results support the potential applications of the synthesized copolymers for the delivery of hydrophobic drugs.

Engineering nanoscopic hydrogels via photo-crosslinking salt-induced polymer assembly for targeted drug delivery

We report the preparation of biodegradable nanoscopic hydrogels and their application for targeted drug delivery. The nanogel is synthesized in a template-free method by photo-crosslinking salt-induced polymer assemblies. With convenient incorporation of targeted lactosyl moieties, the nanogels efficiently deliver doxorubicin to HepG2 cells through receptor-mediated internalization.

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.