Yukio Nagasaki

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.

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.

Long-circulating poly(ethylene glycol)-poly(D,L-lactide) block copolymer micelles with modulated surface charge.

Reactive polymeric micelles consisting of an alpha-acetal-poly(ethylene glycol)-poly(D,L-lactide) block copolymer (acetal-PEG-PDLLA) with a narrow size distribution were prepared in this study to conjugate small peptidyl ligands, tyrosine (Tyr) and tyrosyl-glutamic acid (Tyr-Glu), through reductive amination after converting the alpha-acetal group to an aldehyde group, allowing modulation of the surface charge of the micelles from neutral (Tyr-) to anionic (Tyr-Glu-). Both of these micelles showed a significantly long circulating time in the blood compartment with 25% of injected dose still circulating even at 24 h. Further, an appreciably lowered uptake into the liver and spleen was demonstrated for the anionic Tyr-Glu-conjugated PEG-PDLLA micelle compared with a neutral Tyr-conjugated micelle, suggesting a substantial role of the slight anionic charge on the micelle surface in avoiding non-specific organ uptake. Stability of the micelle form in the blood compartment was directly observed for the Tyr-PEG-PDLLA micelle by a gel filtration assay of a plasma sample collected from the micelle-injected mice at 24 h. These results demonstrated that a surface-modulated PEG-PDLLA micelle with a suitable size and a narrowly distributed nature has promising potential as a long-circulating carrier system with desirable biocompatibility and biofunctionality.

Development of a novel systemic gene delivery system for cancer therapy with a tumor-specific cleavable PEG-lipid.

For successful cancer gene therapy via intravenous (i.v.) administration, it is essential to optimize the stability of carriers in the systemic circulation and the cellular association after the accumulation of the carrier in tumor tissue. However, a dilemma exists regarding the use of poly(ethylene glycol) (PEG), which is useful for conferring stability in the systemic circulation, but is undesirable for the cellular uptake and the following processes. We report the development of a PEG-peptide-lipid ternary conjugate (PEG-Peptide-DOPE conjugate (PPD)). In this strategy, the PEG is removed from the carriers via cleavage by a matrix metalloproteinase (MMP), which is specifically expressed in tumor tissues. An in vitro study revealed that the PPD-modified gene carrier (Multifunctional Envelope-type Nano Device: MEND) exhibited pDNA expression activity that was dependent on the MMP expression level in the host cells. In vivo studies further revealed that the PPD was potent in stabilizing MEND in the systemic circulation and facilitating tumor accumulation. Moreover, the i.v. administration of PPD or PEG/PPD dually-modified MEND resulted in the stimulation of pDNA expression in tumor tissue, as compared with a conventional PEG-modified MEND. Thus, MEND modified with PPD is a promising device, which has the potential to make in vivo cancer gene therapy achievable.

A pH-sensitive fusogenic peptide facilitates endosomal escape and greatly enhances the gene silencing of siRNA-containing nanoparticles in vitro and in vivo

Previously, we developed a multifunctional envelope-type nano device (MEND) for efficient delivery of both pDNA and siRNA. Modification of a MEND with polyuethylene glycol, i.e., PEGylation, is a potential strategy for in vivo delivery of MENDs to tumor tissue. However, PEGylation also inhibits both uptake and endosomal escape of MENDs. To overcome these limitations, we developed a PEG-peptide-DOPE (PPD) that can be cleaved in a matrix metalloproteinase (MMP)-rich environment. In this study, to further improve the silencing activity of encapsulated siRNA, we modified the PPD-MEND with a pH-sensitive fusogenic GALA peptide (GALA/PPD-MEND). First, we determined the GALA and PPD content that would optimize the synergistic functions of GALA and PPD. The most efficient gene silencing activity was achieved when GALA and either conventional PEG-lipid or PPD were used to modify the MEND at a molar ratio of 1:1. In this case, the silencing activity was comparable to that achieved when using a MEND that had not been modified with PEG (unmodified MEND). Furthermore, in vivo topical administration revealed that optimized PPD/GALA-MENDa resulted in more efficient gene silencing compared with unmodified MENDs. Collectively, data demonstrate that introduction of both of a pH-sensitive fusogenic GALA peptide and PPD into the MEND facilitates nanoparticle endosomal escape, thereby enhancing the efficiency of siRNA delivery and gene silencing.

Preparation and characterization of polymer micelles from poly(ethylene glycol)-poly(D,L-lactide) block copolymers as potential drug carrier

Poly(ethylene glycol)-poly(D,L-lactide) block copolymers (PEG-PLA) with varying composition were prepared through successive ring-opening polymerization of ethylene oxide and D,L-lactide using an anionic initiator, and their property of multimolecular micellization in aqueous milieu was examined in detail from the standpoint of designing carriers for hydrophobic drugs. The heterogeneity of PEG-PLA was found to crucially affect the size and distribution of micelles, and narrowly-distributed micelles with sizes of approximately 30 nm in diameter were formed only from PEG-PLA with a substantially narrow molecular weight distribution and an appropriate balance in the length ratio of the PEG and PLA segments in PEG-PLA, indicating the importance of establishing a reliable synthetic route for the block copolymers. PEG-PLA micelles have a considerably low critical association concentration (approximately 1.0 mg/l) which is apparently an advantage in utilizing these micelles as drug carriers in an extremely diluted condition.

Self-assembly of poly(ethylene glycol)-based block copolymers for biomedical applications

Nanostructure fabrication from block copolymers is discussed in this review paper. Particularly, novel approaches for the construction of functionalized poly(ethylene glycol) (PEG) layers on surfaces were focused to attain the specific adsorption of a target protein through PEG-conjugated ligands with a minimal non-specific adsorption of other proteins. Furthermore, surface organization of block copolymer micelles with cross-linking cores was described from the standpoint of 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 non-fouling properties and worked as a reservoir for hydrophobic reagents. These PEG-functionalized surface in brush form or in micelle form can be used in diverse fields of medicine and biology to construct high-performance medical devices including scaffolds for tissue engineering and matrices for drug delivery systems.

Systemic delivery of siRNA to tumors using a lipid nanoparticle containing a tumor-specific cleavable PEG-lipid

Previously, we developed a multifunctional envelope-type nano device (MEND) for efficient delivery of nucleic acids. For tumor delivery of a MEND, PEGylation is a useful method, which confers a longer systemic circulation and tumor accumulation via the enhanced permeability and retention (EPR) effect. However, PEGylation inhibits cellular uptake and subsequent endosomal escape. To overcome this, we developed a PEG-peptide-DOPE (PPD) that is cleaved in a matrix metalloproteinase (MMP)-rich environment. In this study, we report on the systemic delivery of siRNA to tumors by employing a MEND that is modified with PPD (PPD-MEND). An in vitro study revealed that PPD modification accelerated both cellular uptake and endosomal escape, compared to a conventional PEG modified MEND. To balance both systemic stability and efficient activity, PPD-MEND was further co-modified with PEG-DSPE. As a result, the systemic administration of the optimized PPD-MEND resulted in an approximately 70% silencing activity in tumors, compared to non-treatment. Finally, a safety evaluation showed that the PPD-MEND showed no hepatotoxicity and innate immune stimulation. Furthermore, in a DNA microarray analysis in liver and spleen tissue, less gene alternation was found for the PPD-MEND compared to that for the PEG-unmodified MEND due to less accumulation in liver and spleen.

Endosomal release and intracellular delivery of anticancer drugs using pH-sensitive PEGylated nanogels

A pH-sensitive PEGylated nanogel was prepared by emulsion copolymerization of 2-(N,N-diethylamino)ethyl methacrylate (EAMA) with heterobifunctional poly(ethylene glycol) bearing a 4-vinylbenzyl group at the α-end and a carboxylic acid group at the ω-end (CH2CH–Ph–PEG–COOH; Mn = 8000) in the presence of potassium persulfate and ethylene glycol dimethacrylate (1.0 mol%) as cross-linker. The loading of the anticancer drug doxorubicin (DOX) into the pH-sensitive PEGylated nanogel was carried out by means of a solvent evaporation method, and the amount of DOX loaded into the PEAMA core was found to be 26 wt%. Furthermore, the DOX-loaded, pH-sensitive PEGylated nanogel showed almost no initial burst release of the DOX under physiological pH, whereas significant release of DOX from the pH-sensitive PEGylated nanogel was observed at the endosomal pH. The antitumor activity of the DOX-loaded, pH-sensitive, PEGylated nanogel against the human breast cancer cell line MCF-7 was lower than that of free DOX. On the other hand, the antitumor activity of the DOX-loaded, pH-sensitive, PEGylated nanogel against the human hepatoma cell line HuH-7, which is a natural drug-resistant tumor line, was superior to that of both free DOX and the DOX-loaded, pH-insensitive, PEGylated nanogel. Using fluorescence microscopy, pH-sensitive PEGylated nanogel in HuH-7 cells was found to be initially localized within the endosome and/or lysosome, with subsequent release of DOX from the nanogel in response to the endosomal pH, and ultimately, diffusion via the cytoplasm into the cell nucleus. These findings suggest that the pH-sensitive PEGylated nanogel represents a promising nano-sized carrier for anticancer drug delivery systems in vivo.

Two-dimensional multiarray formation of hepatocyte spheroids on a microfabricated PEG-brush surface.

A two‐dimensional microarray of ten thousand (100×100) hepatocyte heterospheroids, underlaid with endothelial cells, was successfully constructed with 100 μm spacing in an active area of 20×20 mm on microfabricated glass substrates that were coated with poly(ethylene glycol) brushes. Cocultivation of hepatocytes with endothelial cells was essential to stabilize hepatocyte viability and liver‐specific functions, allowing us to obtain hepatocyte spheroids with a diameter of 100 μm, functioning as a miniaturized liver to secret albumin for at least one month. The most important feature of this study is that these substrates are defined to provide an unprecedented control of substrate properties for modulating cell behavior, employing both surface engineering and synthetic polymer chemistry. The spheroid array constructed here is highly useful as a platform of tissue and cell‐based biosensors and detects a wide variety of clinically, pharmacologically, and toxicologically active compounds through a cellular physiological response.