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.     

Block copolymer micelles for drug delivery: design, characterization and biological significance

Recently, colloidal carrier systems have been receiving much attention in the field of drug targeting because of their high loading capacity for drugs as well as their unique disposition characteristics in the body. This paper highlights the utility of polymeric micelles formed through the multimolecular assembly of block copolymers as novel core-shell typed colloidal carriers for drug and gene targeting. The process of micellization in aqueous milieu is described in detail based on differences in the driving force of core segregation, including hydrophobic interaction, electrostatic interaction, metal complexation, and hydrogen bonding of constituent block copolymers. The segregated core embedded in the hydrophilic palisade is shown to function as a reservoir for genes, enzymes, and a variety of drugs with diverse characteristics. Functionalization of the outer surface of the polymeric micelle to modify its physicochemical and biological properties is reviewed from the standpoint of designing micellar carrier systems for receptor-mediated drug delivery. Further, the distribution of polymeric micelles is described to demonstrate their long-circulating characteristics and significant tumor accumulation, emphasizing their promising utility in tumor-targeting therapy. As an important perspective on carrier systems based on polymeric micelles, their feasibility as non-viral gene vectors is also summarized in this review article.     

Block copolymer micelles for drug delivery: Design,characterization and biological significance

Recently, colloidal carrier systems have been receiving much attention in the field of drug targeting because of their high loading capacity for drugs as well as their unique disposition characteristics in the body. This paper highlights the utility of polymeric micelles formed through the multimolecular assembly of block copolymers as novel core–shell typed colloidal carriers for drug and gene targeting. The process of micellization in aqueous milieu is described in detail based on differences in the driving force of core segregation, including hydrophobic interaction, electrostatic interaction, metal complexation, and hydrogen bonding of constituent block copolymers. The segregated core embedded in the hydrophilic palisade is shown to function as a reservoir for genes, enzymes, and a variety of drugs with diverse characteristics. Functionalization of the outer surface of the polymeric micelle to modify its physicochemical and biological properties is reviewed from the standpoint of designing micellar carrier systems for receptor-mediated drug delivery. Further, the distribution of polymeric micelles is described to demonstrate their long-circulating characteristics and significant tumor accumulation, emphasizing their promising utility in tumor-targeting therapy. As an important perspective on carrier systems based on polymeric micelles, their feasibility as non-viral gene vectors is also summarized in this review article.     

Block copolymer micelles as long-circulating drug vehicles

The development of block copolymer micelles as long-circulating drug vehicles is described. As well, a recent fundamental study of block copolymer micelles, where much insight into their structures and properties has been realized, is briefly summarized in order to shed light on their properties in vivo. There is emphasis on block copolymer micelles having poly(ethylene oxide) as the hydrophilic block and poly(l-amino acid) as the hydrophobic block, with some discussion on the properties of poly(ethylene oxide). Comparisons are drawn with other drug vehicles and with micelles formed from low molecular weight surfactants. Micelle-forming, block copolymer-drug conjugates are described. Hydrophobic drugs, such as doxorubicin, distribute into block copolymer micelles, and details of several examples are given. Finally, the paper presents studies that evidence the long circulation times of block copolymer micelles. Like long-circulating liposomes, block copolymers that form micelles accumulate passively at solid tumors and thus have great potential for anti-cancer drug delivery.     

Formulation of drugs in block copolymer micelles: drug loading and release

Block copolymer micelles have become accepted as a viable strategy for drug formulation and delivery. Block copolymer micelles may serve as solubilizers and/or true drug carriers depending on their drug retention properties in vivo. Indeed the formulation of hydrophobic drugs in these micelle systems has been shown to provide up to a 30,000 fold increase in the water solubility of some compounds. In addition, the administration of drugs in copolymer micelles has been shown to reduce their toxicity and improve their therapeutic efficacy. The present review is focused on the drug loading and release properties of block copolymer micelles. Specifically, the properties of the drug, properties of the micelle core and the presence of interactions between the drug and the core-forming block are discussed in terms of their influence on the drug loading and release properties of the micelles. The various methods employed to prepare drug-loaded micelles are reviewed and the in vitro release assays used to predict the in vivo release characteristics of the formulations are discussed. The balance between drug loading and micelle stability is highlighted as a critical factor in the optimization of micelle-based formulations. The in vivo performance of micelles as delivery systems is evaluated by comparing the pharmacokinetics of free drug and drug administered in micelle-based formulations. Overall, the composition-property and property-performance relationships outlined in this review may aid in guiding the rational design of block copolymer micelles for drug delivery. In addition, suggestions for future research in this area are provided as a means to assist in furthering block copolymer micelles as one of the leading advanced drug delivery technologies for the systemic administration of drugs.     

Micelle formation and drug release behavior of polypeptide graft copolymer and its mixture with polypeptide block copolymer

Self-association behavior of polypeptide graft copolymer and its mixture with polypeptide block copolymer and drug carrier capability of the formed micelles was examined. The results gained through fluorescence spectroscopy, transmission electron microscopy and nuclear magnetic resonance spectroscopy revealed that both polypeptide graft copolymer and its mixture with polypeptide block copolymer can self-assemble to form polymeric micelles in aqueous media. The molecular structure of the graft copolymer and blending the graft with block copolymer exert marked effects on the critical micelle concentration and the shape of formed micelles. It was found that the hydrophobic inner core of the micelles formed either by graft copolymer or mixture of graft and block copolymers can act as an incorporation site for the hydrophobic drugs. The drug loading content of the graft copolymer micelles tends to be larger when the content of the polypeptide segments in the copolymer increases. The results obtained from the drug-release studies showed that the drug-release rates are dependent on the chemical nature of the graft copolymer, the composition of the graft and block copolymer mixture, and also the pH value of the release media.     

Preparation,characterization, and drug release behaviors of drug nimodipine-loaded poly(epsilon-caprolactone)-poly(ethylene oxide)-poly(epsilon-caprolactone) amphiphilic triblock copolymer micelles

Amphiphilic triblock copolymers, poly(epsilon-caprolactone)-poly(ethylene oxide)-poly(epsilon-caprolactone) (PCL-PEO-PCL), were synthesized by ring opening polymerization of epsilon-caprolactone initiated with the hydroxyl functional groups of poly(ethylene glycol) at both ends of the chain. The micelles composed of this type of copolymer had such a structure that both ends of the PEO chain were anchored to the micelle. The critical micelle concentration of the block copolymer in distilled water was determined by a fluorescence probe technique using pyrene. As the hydrophobic components of the block copolymer increased, the critical micelle concentration value decreased. To estimate the feasibility as novel drug carriers, the block copolymer micelles were prepared by precipitation of polymer from acetone solution into water. From the observation of transmission electron microscopy, the micelles exhibited a spherical shape. Nimodipine was incorporated into the hydrophobic inner core of micelles as a lipophilic model drug to investigate the drug release behavior. The PEO/PCL ratio of copolymer was a main factor in controlling micelle size, drug-loading content, and drug release behavior. As PCL weight ratio increased, the micelle size and drug-loading content increased, and the drug release rate decreased.     

Block copolymers as drug carriers.

The main subject of this review is to describe the concept and strategy of utilizing block copolymers as drug carriers and to present a perspective for this utilization. The first section points out preferable properties of block copolymers as drug carriers with a brief survey on classifications and synthesis of block copolymers and specifies the merits of block copolymers for this purpose in the historical background of polymeric drug carriers. The second section introduces several studies using block copolymers as drug carriers. These studies contain surface modification of microspheres with block copolymers, polymeric micelles, and a conjugate of an antibody and block copolymer. In conclusion, it is determined that block copolymers possess a high potential for use as drug carriers irrespective of some difficulties in their synthesis.     

Chirality plays critical roles in enhancing the aqueous solubility of nocathiacin I by block copolymer micelles

Objectives Although drug solubilization by block copolymer micelles has been extensively studied, the rationale behind the choice of appropriate block copolymer micelles for various poorly water‐soluble drugs has been of relatively less concern. The objective of this study was to use methoxy‐poly(ethylene glycol)‐polylactate micelles (MPEG‐PLA) to solubilize glycosylated antibiotic nocathiacin I and to compare the effects of chirality on the enhancement of aqueous solubility. Methods Nocathiacin I‐loaded MPEG‐PLA micelles with opposite optical property in PLA were synthesized and characterized. The drug release profile, micelle stability and preliminary safety properties of MPEG‐PLA micelles were evaluated. Meanwhile, three other poorly water‐soluble chiral compound‐loaded micelles were also prepared and compared. Key findings The aqueous solubility of nocathiacin I was greatly enhanced by both l ‐ and d ‐copolymers, with the degree of enhancement appearing to depend on the chirality of the copolymers. Comparison of different chiral compounds confirmed the trend that aqueous solubility of chiral compounds can be more effectively enhanced by block copolymer micelles with specific stereochemical configuration. Conclusions The present study introduced chiral concept on the selection and preparation of block copolymer micelles for the enhancement of aqueous solubility of poorly water‐soluble drugs.     

Renewable poly(δ-decalactone) based block copolymer micelles as drug delivery vehicle: in vitro and in vivo evaluation

Polymers from natural resources are attracting much attention in various fields including drug delivery as green alternatives to fossil fuel based polymers. In this quest, novel block copolymers based on renewable poly(δ-decalactone) (PDL) were evaluated for their drug delivery capabilities and compared with a fossil fuel based polymer i.e. methoxy-poly(ethylene glycol)-b-poly(ε-caprolactone) (mPEG-b-PCL). Using curcumin as a hydrophobic drug model, micelles of PDL block copolymers with different orientation i.e. AB (mPEG-b-PDL), ABA (PDL-b-PEG-b-PDL), ABC (mPEG-b-PDL-b-poly(pentadecalactone) and (mPEG-b-PCL) were prepared by nanoprecipitation method. The size, drug loading and curcumin stability studies results indicated that mPEG-b-PDL micelles was comparable to its counterpart mPEG-b-PCL micelles towards improved delivery of curcumin. Therefore, mixed micelles using these two copolymers were also evaluated to see any change in size, loading and drug release. Drug release studies proposed that sustained release can be obtained using poly(pentadecalactone) as crystalline core whereas rapid release can be achieved using amorphous PDL core. Further, mPEG-b-PDL micelles were found to be non-haemolytic, up to the concentration of 40?mg/mL. In vivo toxicity studies on rats advised low-toxic behaviour of these micelles up to 400?mg/kg dose, as evident by histopathological and biochemical analysis. In summary, it is anticipated that mPEG-b-PDL block copolymer micelles could serve as a renewable alternative for mPEG-b-PCL copolymers in drug delivery applications.     

Brushed Block Copolymer Micelles with pH-Sensitive Pendant Groups for Controlled Drug Delivery

Purpose

To investigate the effects of small aliphatic pendent groups conjugated through an acid-sensitive linker to the core of brushed block copolymer micelles on particle properties.

Methods

The brushed block copolymers were synthesized by conjugating five types of 2-alkanone (2-butanone, 2-hexanone, 2-octanone, 2-decanone, and 2-dodecanone) through an acid-labile hydrazone linker to poly(ethylene glycol)-poly(aspartate hydrazide) block copolymers.

Results

Only block copolymers with 2-hexanone and 2-octanone (PEG-HEX and PEG-OCT) formed micelles with a clinically relevant size (< 50 nm in diameter), low critical micelle concentration (CMC, < 20 μM), and drug entrapment yields (approximately 5 wt.%). Both micelles degraded in aqueous solutions in a pH-dependent manner, while the degradation was accelerated in an acidic condition (pH 5.0) in comparison to pH 7.4. Despite these similar properties, PEG-OCT micelles controlled the entrapment and pH-dependent release of a hydrophobic drug most efficiently, without altering particle size, shape, and stability. The molecular weight of PEG (12 kDa vs 5 kDa) induced no change in pH-controlled drug release rates of PEG-OCT micelles.

Conclusion

Acid-labile small aliphatic pendant groups are useful to control the entrapment and release of a hydrophobic drug physically entrapped in the core of brushed block copolymer micelles.     

多西他赛pH敏感嵌段共聚物胶束的制备

本文在合成pH敏感两亲性嵌段共聚物聚(2-乙基-2-噁唑啉)-聚乳酸(PEOz-PDLLA)的基础上，采用薄膜分散法制备多西他赛pH敏感嵌段共聚物胶束，利用芘荧光探针技术测定胶束的临界胶束浓度(CMC)；通过高效液相色谱测定胶束的载药量及包封率；分别利用透射电镜、动态光散射法和zeta电位分析仪对胶束的形态、粒径和表面电位进行了表征；采用透析法考察了载药聚合物胶束的体外释放行为。结果表明，胶束的临界胶束浓度值为1.0×10^-3 g·L^-1；载药量可达15.0%，包封率为91.1%；胶束的粒度分布很窄，平均粒径为28.7nm；胶束粒子为圆球形且分散良好，其表面zeta电位值为(1.19±0.12)mV；在pH 7.4释放介质中，多西他赛胶束具有缓释作用；而在pH 5.0条件下，胶束释药明显加快，体现出PEOz-PDLLA胶束释药行为的pH敏感性。综合上述研究可见，PEOz-PDLLA嵌段共聚物胶束作为疏水性抗肿瘤药物的给药系统具有很好的应用前景。     

Fate of micelles and quantum dots in cells.

Micelles and quantum dots have been used as experimental drug delivery systems and imaging tools both in vitro and in vivo. Investigations of their fate at the subcellular level require different surface-core modifications. Among the most common modifications are those with fluorescent probes, dense-core metals or radionucleids. Cellular fate of several fluorescent probes incorporated into poly(caprolactone)-b-copolymer micelles (PCL-b-PEO) was followed by confocal microscopy, and colloidal gold incorporated in poly 4-vinyl pyridine-PEO micelles were developed to explore micelle fate by electron microscopy. More recently, we have examined quantum dots (QDs) as the next-generation-labels for cells and nanoparticulate drug carriers amenable both to confocal and electron microscopic analyses. Effects of QDs at the cellular and subcellular levels and their integrity were studied. Results from different studies suggest that size, charge and surface manipulations of QDs may play a role in their subcellular distribution. Examples of pharmacological agents incorporated into block copolymer micelles, administered or attached to QD surfaces show how the final biological outcome (e.g. cell death, proliferation or differentiation) depends on physical properties of these nanoparticles.     

Poly(ethylene oxide)-block-poly(L-amino acid) micelles for drug delivery

Block copolymer micelles encapsulate water insoluble drugs by chemical and physical means, and they may target therapeutics to their site of action in a passive or active way. In this review, we focus on micelles self-assembled from poly(ethylene oxide)-block-poly(L-amino acid) (PEO-b-PLAA). A common theme in these studies is the chemical modification of the core-forming PLAA block used to adjust and optimize the properties of PEO-b-PLAA micelles for drug delivery. Micelle-forming block copolymer-drug conjugates, micellar nanocontainers and polyion complex micelles have been obtained that mimic functional aspects of biological carriers, namely, lipoproteins and viruses. PEO-b-PLAA micelles may be advantageous in terms of safety, stability, and scale-up.     

Thermo-responsive shell cross-linked PMMA-b-P(NIPAAm-co-NAS) micelles for drug delivery

Thermo-responsive amphiphilic poly(methyl methacrylate)-b-poly(N-isopropylacrylamide-co-N-acryloxysuccinimide) (PMMA-b-P(NIPAAm-co-NAS)) block copolymer was synthesized by successive RAFT polymerizations. The uncross-linked micelles were facilely prepared by directly dissolving the block copolymer in an aqueous medium, and the shell cross-linked (SCL) micelles were further fabricated by the addition of ethylenediamine as a di-functional cross-linker into the micellar solution. Optical absorption measurements showed that the LCST of uncross-linked and cross-linked micelles was 31.0 °C and 40.8 °C, respectively. Transmission electron microscopy (TEM) showed that both uncross-linked and cross-linked micelles exhibited well-defined spherical shape in aqueous phase at room temperature, while the SCL micelles were able to retain the spherical shape with relatively smaller dimension even at 40 °C due to the cross-linked structure. In vitro drug release study demonstrated a slower and more sustained drug release behavior from the SCL micelles at high temperature as compared with the release profile of uncross-linked micelles, indicating the great potential of SCL micelles developed herein as novel smart carriers for controlled drug release.     

Shrinkage of a rapidly growing tumor by drug-loaded polymersomes: pH-triggered release through copolymer degradation

Carrier-mediated delivery of drugs into the cytosol is often limited by either release from the carrier or release from an internalizing endolysosome. Here, loading, delivery, and cytosolic uptake of drug mixtures from degradable polymersomes are shown to exploit both the thick membrane of these block copolymer vesicles and their aqueous lumen as well as pH-triggered release within endolysosomes. Our initial in vivo studies demonstrate growth arrest and shrinkage of rapidly growing tumors after a single intravenous injection of polymersomes composed of poly(ethylene glycol)-polyester. Vesicles are shown to break down into membrane-lytic micelles within hours at 37 degrees C and low pH, although storage at 4 degrees C allows retention of drug for over a month. It is then shown that cell entry of the polymersomes into endolysosomes is followed by copolymer-induced endolysosomal rupture with release of cytotoxic drugs. Above a critical poration concentration (CCPC) that is easily achieved within endolysosomes and that scales with copolymer proportions and molecular weight, the copolymer micelles are seen to disrupt lipid membranes and thereby enhance drug activity. Neutral polymersomes and related macrosurfactant assemblies can thus create novel pathways within cells for controlled release and delivery.     

Block copolymer micelles as delivery vehicles of hydrophobic drugs: micelle-cell interactions

One-third of drugs in development are water insoluble and one-half fail in trials because of poor pharmacokinetics. Block copolymer micelles are nanosized particles that can solubilize hydrophobic drugs and alter their kinetics in vitro and in vivo. However, block copolymer micelles are not solely passive drug containers that simply solubilize hydrophobic drugs; cells internalize micelles. To facilitate the development of advanced, controlled, micellar drug delivery vehicles, we have to understand the fate of micelles and micelle-incorporated drugs in cells and in vivo. With micelle-based drug formulations recently reaching clinical trials, the impetus for answers is ever so strong and detailed studies of interactions of micelles and cells are starting to emerge. Most notably, the question arises: Is the internalization of block copolymer micelles carrying small molecular weight drugs an undesired side effect or a useful means of improving the effectiveness of the incorporated drugs?     

Investigation on design of stable etoposide-loaded PEG-PCL micelles: effect of molecular weight of PEG-PCL diblock copolymer on the in vitro and in vivo performance of micelles

In the present study, six different molecular weight diblock copolymer of methoxy poly (ethylene glycol)-b-poly (ε-caprolactone) (MPEG-PCL) were synthesized and characterized and was used for fabrication of etoposide-loaded micelles by nanoprecipitation technique. The particle size and percentage drug entrapment of prepared micelles were found to be dependent on the molecular weight of PCL block and drug to polymer ratio. The maximum drug loading of 5.32% was found in micellar formulation MPEG5000-PCL10000, while MPEG2000-PCL2000 exhibited 2.73% of maximum drug loading. A variation in the fixed aqueous layer thickness and PEG surface density of micellar formulations was attributed to difference in MPEG molecular weight and interaction of PEG and PCL block of copolymer. The MPEG2000-PCL2000 micelles demonstrated poor in vitro stability among other micellar formulations, due to its interaction with bovine serum albumin and immediate release of drug from micelles. Furthermore, plain etoposide and MPEG2000-PCL2000 micelles exhibited greater extent of hemolysis, due to presence of surfactants and faster release of drug from micelles, respectively. The biodistribution studies carried out on Ehrlich ascites tumor-bearing Balb/C mice confirmed higher accumulation of etoposide-loaded micellar formulation at tumor site compared to plain etoposide due to enhanced permeability and retention effect.     

The Effect of Polymerization Method in Stereo-active Block Copolymers on the Stability of Polymeric Micelles and their Drug Release Profile

Purpose

To investigate the effect of polymerization method on the stability and drug release properties of polymeric micelles formed using stereo-active block copolymers.

Methods

Diblock copolymers consisting of methoxy poly ethylene oxide (MePEO) and poly(lactide)s (PLA)s of different stereochemistry were synthesized by bulk or solution polymerization. Polymers and micelles were characterized for their chemical structure by ¹H NMR, optical rotation by polarimetry, critical micellar concentration by fluorescence spectroscopy, thermal properties by differential scanning calorimetry, morphology by transmission electron microscopy and size as well as kinetic stability by dynamic light scattering. Release of encapsulated nimodipine from polymeric micelles at different levels of loading was also investigated.

Results

Solution polymerization yielded a higher degree of crystallinity for stereo-regular PLA blocks. Consequently, the related polymeric micelles were kinetically more stable than those prepared by bulk polymerization. At high drug loading levels, the release of nimodipine was more rapid from polymeric micelles with crystalline cores. At lower levels of drug loading, drug release was slower and independent of the stereochemistry of the core.

Conclusions

The results underline the effect of polymerization method in defining core crystallinity in stereoregular block copolymer micelles. It also shows the impact of core crystallinity on enhancing micellar stability and drug release.