Block copolymer micelles for delivery of gene and related compounds

Polymer genomics: shifting the gene and drug delivery paradigms

Pluronic, the A-B-A amphiphilic block copolymers of poly(ethylene oxide) and poly(propylene oxide), can up-regulate the expression of selected genes in cells and alter genetic responses to antineoplastic agents in cancer. Two key new findings are discussed in relation to current drug and gene delivery strategies. First, these block copolymers alone and in combination with a polycation, polyethyleneimine, can up-regulate the expression of reporter genes in stably transfected cells. This underscores the ability of selected synthetic polymers to enhance transgene expression through a mechanism that augments improved DNA delivery into a cell. Second, although, when used alone, Pluronic is "genetically benign," when combined with an antineoplastic agent, doxorubicin, it drastically alters pharmacogenomic responses to this agent and prevents the development of multidrug resistance in breast cancer cells. Collectively, these studies propose the need for a thorough assessment of pharmacogenomic effects of polymer therapeutics to maximize the clinical outcomes and understand the pharmacological and toxicological effects of polymer-based drugs and delivery systems.

Smart polymeric micelles for gene and drug delivery

Polymeric micelles, supramolecular assemblies of block copolymers, are useful nanocarriers for the systemic delivery of drugs and genes. Recently, novel polymeric micelles with smart functions, such as targetability and stimuli-sensitivity, have emerged as promising carriers that enhance the efficacy of drugs and genes with minimal side effects. This review focuses on the construction and characteristic behaviors of intracellular environment-sensitive micelles that selectively exert drug activity and gene expression in live cells.:

Nontoxic block copolymer nanospheres: design and characterization

Biodegradable polymers capable of self-assembly into hollow nanospheres of less than 100 nm have significant potential for biotechnology applications such as drug delivery and gene therapy. Here we describe the synthesis of a novel ABA-type triblock copolymer made from a hydrophobic tyrosine-derived core and two hydrophilic poly(ethylene glycol) end groups (poly(ethylene glycol)-block-oligo(desaminotyrosyltyrosine octyl ester suberate)-block-poly(ethylene glycol)). We describe the self-assembly of this triblock copolymer and characterize its particles as 100 nm size vesicular nanospheres. The vesicular nature of these particles was determined by light scattering and electron microscopy. The nanospheres did not exhibit any short-term cytotoxicity toward UMR-106 cells at a concentration up to 2 mg/mL.

Supramolecular assemblies based on copolymers of PEG600 and functionalized aromatic diesters for drug delivery applications

A chemoenzymatic approach has been developed to synthesize poly(ethylene glycol)-based amphiphilic copolymers under mild reaction conditions that self-assemble in aqueous media to form polymeric nanomicelles in the range of 20-50 nm. The supramolecular organization of polymeric nanomicelles was studied by 1H NMR longitudinal relaxation time (T1) and light scattering techniques (static and dynamic). Interestingly, the enzyme novozyme-435 plays an important role in controlling the polymerization and distribution of polymer chains, which is critical for the formation of nanomicelles with unimodal distributions. The methodology developed is highly flexible as it allows the introduction of various functionalities in the polymeric nanomicelles. These self-organized nanomicelles are highly efficient drug delivery vehicles for hydrophobic and partially hydrophilic drugs, both transdermally and orally, as they have the ability to encapsulate guest molecules during self-organization. In vivo studies by encapsulating anti-inflammatory agents (aspirin and naproxen) in these polymeric nanomicelles and by applying topically resulted in significant reduction in inflammation. The % reduction in inflammation using polymeric nanomicelles containing aspirin and naproxen was 62 and 64%, respectively.

Do spherical polyelectrolyte brushes interdigitate?

The structure of spherical micelles of the diblock copolymer poly(styrene-block-acrylic acid) in water was investigated with small angle neutron scattering and contrast matching. We have monitored intermicelle correlation and the extension of the polyelectrolyte chains in the coronal layer through the overlap concentration. Irrespective of ionic strength, the corona shrinks with increasing packing fraction. Furthermore, at high charge and minimal screening conditions, the corona layers interpenetrate once the volume fraction exceeds the critical value 0.53+/-0.02.

Synthesis and characterization of glycopolymer-polypeptide triblock copolymers

Glycopolymer-polypeptide triblock copolymers of the structure, poly(l-alanine)-b-poly(2-acryloyloxyethyl-lactoside)-b-poly(l-alanine) (AGA), have been synthesized by sequential atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP). Controlled free radical polymerization of 2-O-acryloyl-oxyethoxyl-(2,3,4,6-tetra-O-acetyl-beta-d-galactopyranosyl)-(1-4)-2,3,6-tri-O-acetyl-beta-d-glucopyranoside (AEL) by ATRP with a dibromoxylene (DBX)/CuBr/bipy complex system was used to generate a central glycopolymer block. Telechelic glycopolymers with diamino end groups were obtained by end group transformation and subsequently used as macroinitiators for ROP of l-alanine N-carboxyanhydride monomers (Ala-NCA). Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy analysis demonstrated that copolymer molecular weight and composition were controlled by both the molar ratios of the Ala-NCA monomer to macroinitiator and monomer conversion and exhibited a narrow distribution (Mw/Mn = 1.06-1.26). FT-IR spectroscopy of triblock copolymers revealed that the ratio of alpha-helix/beta-sheet increased with poly(l-alanine) block length. Of note, transmission electron microscopy (TEM) demonstrated that selected amphiphilic glycopolymer-polypeptide triblock copolymers self-assemble in aqueous solution to form nearly spherical aggregates of several hundreds nanometer in diameter. Significantly, the sequential application of ATRP and ROP techniques provides an effective method for producing triblock copolymers with a central glycopolymer block and flanking polypeptide blocks of defined architecture, controlled molecular weight, and low polydispersity.

PAMAM-PEG-PAMAM: Novel Triblock Copolymer as a Biocompatible and Efficient Gene Delivery Carrier

A novel triblock copolymer, PAMAM-block-PEG-block-PAMAM was synthesized and applied as a gene carrier. PAMAM dendrimer is proven to be an efficient gene carrier itself, but it is associated with certain problems such as low water solubility and considerable cytotoxicity. Therefore, we introduced PEG to engineer a nontoxic and highly transfection efficient polymeric gene carrier because PEG is known to convey water-solubility and biocompatibility to the conjugated copolymer. This copolymer could achieve self-assembly with plasmid DNA, forming compact nanosized particles with a narrow size distribution. Fulfilling our expectations, the copolymer was found to form highly water-soluble polyplexes with plasmid DNA, showed little cytotoxicity despite its poor degradability, and finally achieved high transfection efficiency comparable to PEI in 293 cells. Consequently, these data show that an approach involving the introduction of PEG to create a tree-like cationic copolymer possesses a great potential for use in gene delivery systems.

Micellar carriers based on block copolymers of poly(ε-caprolactone) and poly(ethylene glycol) for doxorubicin delivery

Diblock copolymers of poly(epsilon-caprolactone) (PCL) and monomethoxy poly(ethylene glycol) (MPEG) with various compositions were synthesized. The amphiphilic block copolymers self-assembled into nanoscopic micelles and their hydrophobic cores encapsulated doxorubicin (DOX) in aqueous solutions. The micelle diameter increased from 22.9 to 104.9 nm with the increasing PCL block length (2.5-24.7 kDa) in the copolymer composition. Hemolytic studies showed that free DOX caused 11% hemolysis at 200 microg ml(-1), while no hemolysis was detected with DOX-loaded micelles at the same drug concentration. An in vitro study at 37 degrees C demonstrated that DOX-release from micelles at pH 5.0 was much faster than that at pH 7.4. Confocal laser scanning microscopy (CLSM) demonstrated that DOX-loaded micelles accumulated mostly in cytoplasm instead of cell nuclei, in contrast to free DOX. Consistent with the in vitro release and CLSM results, a cytotoxicity study demonstrated that DOX-loaded micelles exhibited time-delayed cytotoxicity in human MCF-7 breast cancer cells.

Low molecular mass peptide dendrimers that express antimicrobial properties

A series of low-generation dendrimeric peptides was synthesized in an attempt to evaluate their antimicrobial potency. All tested dendrimeric peptides in which lysine was a starting and branching element expressed moderate activity against Staphylococcus aureus NCTC 4163, and Escherichia coli NCTC 8196.

Formation of spindlelike aggregates and flowerlike arrays of polystyrene-b-poly(acrylic acid) micelles

In this letter we describe a simple physical method for the ordered aggregation of scattered single spherical polystyrene-b-poly(acrylic acid) (PS-b-PAA) micelles. First, narrow dispersed spindlelike aggregates, about 60 nm in diameter and 1.5 microm in length, are obtained from the aggregation of single spherical PS-b-PAA micelles at 0 degrees C on a glass slide. Then, the yielding spindlelike units can further aggregate into long-ranged, close-packed, flowerlike arrays after a given amount of freeze-thaw cycles. The formation of the interesting arrays is ascribed to the templated aggregation of micelles on the water polycrystal at the freezing point.

Preparation and characterization of water-soluble pH-sensitive nanocarriers for drug delivery

pH-sensitive drug delivery systems can be engineered to release their contents or change their physicochemical properties in response to variations in the acidity of the surroundings. The present work describes the preparation and characterization of novel polymeric micelles (PM) composed of amphiphilic pH-responsive poly(N-isopropylacrylamide) (PNIPAM) or poly(alkyl(meth)acrylate) derivatives. On one hand, acidification of the PNIPAM copolymers induces a coil-to-globule transition that can be exploited to destabilize the intracellular vesicle membranes. In this work, PNIPAM-based PM were loaded with either doxorubicin or aluminium chloride phthalocyanine and their cytotoxicity was assessed in murine tumoral models. On the other hand, poly(alkyl(meth)acrylate) copolymers can be designed to interact with either hydrophobic drugs or polyions and release their cargo upon an increase in pH.

Determination of the Bioavailability of Biotin Conjugated onto Shell Cross-Linked (SCK) Nanoparticles

Shell cross-linked nanoparticles (SCKs) presenting surface- and bioavailable biotin functional groups were synthesized via a mixed micelle methodology, whereby co-micellization of chain terminal biotinylated poly(acrylic acid)-b-poly(methyl acrylate) (PAA-b-PMA) and nonbiotinylated PAA-b-PMA were cross-linked in an intramicellar fashion within the shell layer of the mixed micelles, between the carboxylic acid groups of PAA and the amine functionalities of 2,2'-(ethylenedioxy)diethylamine. The hydrodynamic diameters (D(h)) of the micelles and the SCKs with different biotinylated block copolymer contents were determined by dynamic light scattering (DLS), and the dimensions of the SCKs were characterized with tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The amount of surface-available biotin was tuned by varying the stoichiometric ratio of the biotinylated PAA-b-PMA versus the nonbiotinylated PAA-b-PMA, as demonstrated with solution-state, binding interaction analyses, an avidin/HABA (avidin/4'-hydroxyazobenzene-2-carboxylic acid) competitive binding assay, and fluorescence correlation spectroscopy (FCS). The avidin/HABA assay found the amount of available biotin at the surface of the biotinylated SCK nanoparticles to increase with increasing biotin-terminated block copolymer incorporation, but to be less than 25% of the theoretical value. FCS measurements showed the same trend.

The effect of the processing and formulation parameters on the size of nanoparticles based on block copolymers of poly(ethylene glycol) and poly(N-isopropylacrylamide) with and without hydrolytically sensitive groups

Block copolymers of poly(ethylene glycol) (PEG) as a hydrophilic block and N-isopropylacrylamide (PNIPAAm) or poly (NIPAAm-co-N-(2-hydroxypropyl) methacrylamide-dilactate) (poly(NIPAAm-co-HPMAm-dilactate)) as a thermosensitive block, are able to self-assemble in water into nanoparticles above the cloud point (CP) of the thermosensitive block. The influence of processing and the formulation parameters on the size of the nanoparticles was studied using dynamic light scattering. PNIPAAm-b-PEG 2000 polymers were not suitable for the formation of small and stable particles. Block copolymers with PEG 5000 and 10000 formed relatively small and stable particles in aqueous solutions at temperatures above the CP of the thermosensitive block. Their size decreased with increasing molecular weight of the thermosensitive block, decreasing polymer concentration and using water instead of phosphate buffered saline as solvent. Extrusion and ultrasonication were inefficient methods to size down the polymeric nanoparticles. The heating rate of the polymer solutions was a dominant factor for the size of the nanoparticles. When an aqueous polymer solution was slowly heated through the CP, rather large particles (> or = 200 nm) were formed. Regardless the polymer composition, small nanoparticles (50-70 nm) with a narrow size distribution were formed, when a small volume of an aqueous polymer solution below the CP was added to a large volume of heated water. In this way the thermosensitive block copolymers rapidly pass their CP ('heat shock' procedure), resulting in small and stable nanoparticles.

Effective accumulation of polyion complex micelle to experimental choroidal neovascularization in rats

Exudative age-related macular degeneration, characterized by choroidal neovascularization (CNV), is a major cause of visual loss. In this study, we examined the distribution of the polyion complex (PIC) micelle encapsulating FITC-P(Lys) in blood and in experimental CNV in rats to investigate whether PIC micelle can be used for treatment of CNV. We demonstrate that PIC micelle has long-circulating characteristics, accumulating to the CNV lesions and is retained in the lesion for as long as 168 h after intravenous administration. These results raise the possibility that PIC micelles can be used for achieving effective drug targeting to CNV.

Complexes of poly(ethylene glycol)-based cationic random copolymer and calf thymus DNA: a complete biophysical characterization

Complete biophysical characterization of complexes (polyplexes) of cationic polymers and DNA is needed to understand the mechanism underlying nonviral therapeutic gene transfer. In this article, we propose a new series of synthesized random cationic polymers (RCPs) from methoxy poly(ethylene glycol) monomethacrylate (MePEGMA) and (3-(methacryloylamino)propyl)trimethylammonium chloride with different mole ratios (32:68, 11:89, and 6:94) which could be used as a model system to address and answer the basic questions relating to the mechanism of the interaction of calf thymus DNA (CT-DNA) and cationic polymers. The solubility of the complexes of CT-DNA and RCP was followed by turbidity measurements. It has been observed that complexes of RCP with 68 mol % MePEGMA precipitate near the charge neutralization point, whereas complexes of the other two polymers are water-soluble and stable at all compositions. Dnase 1 digestion experiments show that DNA is inaccessible when it forms complexes with RCP. Ethidium bromide exclusion and gel electrophoretic mobility show that both polymers are capable of binding with CT-DNA. Atomic force microscopy images in conjunction with light scattering experiments showed that the complexes are spherical in nature and 75-100 nm in diameter. Circular dichroism spectroscopy studies indicated that the secondary structure of DNA in the complexes is not perturbed due to the presence of poly(ethylene glycol) segments in the polymer. Furthermore, we used a combination of spectroscopic and calorimetric techniques to determine complete thermodynamic profiles accompanying the helix-coil transition of CT-DNA in the complexes. UV and differential scanning calorimetry melting experiments revealed that DNA in the complexes is more stable than in the free state and the extent of stability depends on the polymer composition. Isothermal titration calorimetry experiments showed that the binding of these RCPs to CT-DNA is associated with small exothermic enthalpy changes. A complete thermodynamic profile showed that the RCP/DNA complex formation is entropically favorable. Much broader opportunities to vary the architecture of the polymers studied here make these systems promising in addressing various basic and practical problems in gene delivery systems.

Block Catiomer Polyplexes with Regulated Densities of Charge and Disulfide Cross-Linking Directed To Enhance Gene Expression

A block catiomer polyplex, showing a high stability in the extracellular medium and an efficient release of plasmid DNA (pDNA) in the intracellular compartment, was developed by controlling both the cationic charge and disulfide cross-linking densities of the backbone polycations. Poly(ethylene glycol)-poly(L-lysine) block copolymer (PEG-PLL) was thiolated using either of two thiolation reagents, N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) or 2-iminothiolane (Traut's reagent), to investigate the effects of both the charge and disulfide cross-linking densities on the properties of the polyplexes. The introduction of thiol groups by SPDP proceeded through the formation of amide linkages to concomitantly decrease the cationic charge density of PLL segment, whereas Traut's reagent promoted the thiolation with the introduction of cationic imino groups to keep the charge density constant. These thiolated PEG-PLLs were complexed with pDNA to form the disulfide cross-linked block catiomer polyplexes, which had the size of approximately 100 nm. Both thiolation methods were similarly effective in introducing disulfide cross-links to prevent the polyplex from the dissociation through a counter polyanion exchange in the extracellular oxidative condition. On the other hand, the efficient release of pDNA responding to the reductive condition mimicking the intracellular environment was only achieved for the polyplex thiolated with SPDP, a system compensating for the decrease in the charge density with the disulfide cross-linking. This distinctive sensitivity toward oxidative and reductive environments was nicely correlated with the remarkable difference in the transfection efficiency between these two types of thiolated polyplexes (SPDP and Traut's reagent types): the former revealed approximately 50 times higher transfection efficiency toward 293T cells than the latter. Obviously, the balance between the densities of the cationic charge and disulfide cross-linking in the thiolated polyplex played a crucial role in the delivery and controlled release of entrapped pDNA into the microenvironment of intracellular compartment to achieve the high transfection efficiency.

Synthesis and Physical Characterization of Poly(ethylene glycol)-Gelatin Conjugates

Poly(ethylene glycol) (PEG) with the terminal group of active ester was coupled to the amino group of gelatin to prepare PEG-grafted gelatin (PEG-gelatin). The affinity chromatographic study revealed that the PEG-gelatin with high degrees of PEGylation did not adsorb onto the gelatin affinity column, in remarked contrast to gelatin alone and the PEG-gelatin with low PEGylation degrees. The former PEG-gelatin showed a critical micelle concentration while it had the apparent molecular size of about 100 nm and a surface charge of almost zero. These findings indicate that the PEG-gelatin formed a micelle structure of which the surface is covered with PEG molecules grafted. When the body distribution of 125I-labeled gelatin and PEG-gelatin after intravenous injection was evaluated, the radioactivity of micellar PEG-gelatin was retained in the blood circulation compared with that of gelatin and the PEG-gelatin of no micelle formation. At the same PEGylation degree, the blood concentration was significantly higher for the PEG-gelatin prepared from PEG with a molecular weight of 12 000 than that of molecular weights of 2000 and 5000. It is concluded that the PEG-gelatin is a drug carrier with a micelle structure which retains in the blood circulation.

Biocompatibility of implantable synthetic polymeric drug carriers: focus on brain biocompatibility

Numerous polymeric biomaterials are implanted each year in human bodies. Among them, drug delivery devices are potent novel powerful therapeutics for diseases which lack efficient treatments. Controlled release systems are in direct and sustained contact with the tissues, and some of them degrade in situ. Thus, both the material itself and its degradation products must be devoid of toxicity. The knowledge and understanding of the criteria and mechanisms determining the biocompatibility of biomaterials are therefore of great importance. The classical tissue response to a foreign material leads to the encapsulation of the implant, which may impair the drug diffusion in the surrounding tissue and/or cause implant failure. This tissue response depends on different factors, especially on the implantation site. Indeed, several organs possess a particular immunological status, which may reduce the inflammatory and immune reactions. Among them, the central nervous system is of particular interest, since many pathologies still need curative treatments. This review describes the classical foreign body reaction and exposes the particularities of the central nervous system response. The recent in vivo biocompatibility studies of implanted synthetic polymeric drug carriers are summarized in order to illustrate the behavior of different classes of polymers and the methodologies used to evaluate their tolerance.

Micellar nanocontainers distribute to defined cytoplasmic organelles

Block copolymer micelles are water-soluble biocompatible nanocontainers with great potential for delivering hydrophobic drugs. An understanding of their cellular distribution is essential to achieving selective delivery of drugs at the subcellular level. Triple-labeling confocal microscopy in live cells revealed the localization of micelles in several cytoplasmic organelles, including mitochondria, but not in the nucleus. Moreover, micelles change the cellular distribution of and increase the amount of the agent delivered to the cells. These micelles may thus be worth exploring for their potential to selectively deliver drugs to specified subcellular targets.

Low-pH-sensitive PEG-stabilized plasmid-lipid nanoparticles: preparation and characterization

The acid-labile poly(ethyleneglycol)-diorthoester-distearoylglycerol lipid (POD), was used with a cationic lipid-phosphatidylethanolamine mixture to prepare stabilized plasmid-lipid nanoparticles (POD SPLP) that could mediate gene transfer in vitro by a pH triggered escape from the endosome. Nanoparticles of 60 nm diameter were prepared at pH 8.5 using a detergent dialysis method. The DNA encapsulation efficiency in the nanoparticles was optimal between 10 and 13 mol % ratio of cationic lipid and at a POD content of 20 mol %. The apparent zeta potential of the nanoparticles at 1 mM salt and pH 7.5 was positive, indicating cationic lipid on the external surface. However, the external layer of the nanoparticles was depleted in the cationic component compared to the starting mole ratio. Low pH sensitivity of the POD SPLP was characterized by a lag phase followed by a rapid collapse; at pH 5.3 the nanoparticles collapsed in 100 min. Nanoparticles prepared from a pH-insensitive PEG-lipid, PEG-distearoylglycerol had similar physicochemical characteristics as the POD SPLP but did not collapse at low pH. The POD SPLP had up to 3 orders of magnitude greater gene transfer activity than did the pH-insensitive nanoparticles. Both the pH-sensitive and pH-insensitive nanoparticles were internalized to a qualitatively similar extent in a punctate pattern into cultured cells within 2 h of incubation with the cells; thus, increased gene transfer of the POD SPLP was due to a more rapid escape from the endosome rather than to greater cell association of these nanoparticles. These results suggest that the pH-sensitive stabilized plasmid-lipid nanoparticles may be a useful component of a synthetic vector for parenterally administered gene therapy.

PEGylated nanoparticles for biological and pharmaceutical applications

The utility of polymeric micelles formed through the multimolecular assembly of block copolymer was comprehensively described as novel core-shell typed colloidal carriers for drug and gene targeting. Particularly, novel approaches for the formation of functionalized poly(ethylene glycol) (PEG) layers as hydrophilic outer shell were focused to attain receptor-mediated drug and gene delivery through PEG-conjugated ligands with a minimal non-specific interaction with other proteins. Surface organization of block copolymer micelles with cross-linking core was also described from a standpoint of the preparation of a new functional surface-coating with a unique macromolecular architecture. The micelle-attached surface and the thin hydrogel layer made by layered micelles exhibited nonfouling properties and worked as the reservoir for hydrophobic reagents. Furthermore, the potential utility of multimolecular assembly derived from heterobifunctional PEGs and block copolymers were explored to systematically modify the properties of metal and semiconductor nanostructures by controlling their structure and their surface properties, making them extremely attractive for use in biological and biomedical applications.