Drug targeting by surface cationization

Cationization of drug products and carriers involves a direct modification or attachment of conveying or accompanying components, either of which cause a charge modification. Cationization of macromolecules such as proteins and nucleotides and particulate drug carriers generally enhances their cellular uptake by endocytosis. The most common use of cationization today is in gene delivery. This is undertaken by either employing cationic polymers or entraping nucleotides in cationic carriers such as cationic liposomes. Cationized delivery systems are also used to overcome biological barriers and are suggested for drug targeting, in a nonspecific manner, to a variety of body organs, including brain, eyes, nose, and inflamed intestinal epithelium. Protein cationization is also suggested both for tumor immunotherapy and as a diagnostic tool in cancer therapy. Cationization has proven itself to be a straightforward tool for targeting to cells, tissues, and selected organs. This article reviews the extensive range of applications of cationization for improving drug and gene delivery and summarizes major technologies employed for that purpose.     

Photochemical internalization: a new tool for drug delivery

The utilisation of macromolecules in the therapy of cancer and other diseases is becoming increasingly important. Recent advances in molecular biology and biotechnology have made it possible to improve targeting and design of cytotoxic agents, DNA complexes and other macromolecules for clinical applications. In many cases the targets of macromolecular therapeutics are intracellular. However, degradation of macromolecules in endocytic vesicles after uptake by endocytosis is a major intracellular barrier for the therapeutic application of macromolecules having intracellular targets of action. Photochemical internalisation (PCI) is a novel technology for the release of endocytosed macromolecules into the cytosol. The technology is based on the activation by light of photosensitizers located in endocytic vesicles to induce the release of macromolecules from the endocytic vesicles. Thereby, endocytosed molecules can be released to reach their target of action before being degraded in lysosomes. PCI has been shown to stimulate intracellular delivery of a large variety of macromolecules and other molecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), DNA delivered as gene-encoding plasmids or by means of adenovirus or adeno-associated virus, peptide nucleic acids (PNAs) and chemotherapeutic agents such as bleomycin and in some cases doxorubicin. PCI of PNA may be of particular importance due to the low therapeutic efficacy of PNA in the absence of an efficient delivery technology and the 10-100-fold increased efficacy in combination with PCI. The efficacy and specificity of PCI of macromolecular therapeutics has been improved by combining the macromolecules with targeting moieties, such as the epidermal growth factor. In general, PCI can induce efficient light-directed delivery of macromolecules into the cytosol, indicating that it may have a variety of useful applications for site-specific drug delivery as for example in gene therapy, vaccination and cancer treatment.     

Targeting intracellular targets

Many therapeutic agents have intracellular compartments as their site of action. Targeted delivery of these agents to their specific intracellular targets could result in enhanced therapeutic efficacy and reduced toxicity. Various carriers have been shown useful in targeted delivery of different classes of therapeutic agents. Among these carriers, biodegradable nanoparticles formulated from biocompatible polymers poly(D,L-lactide-co-glycolide) (PLGA) and polylactide (PLA) have shown the potential for sustained intracellular delivery of different therapeutic agents. In this review, we discuss different intracellular targets, barriers to intracellular delivery, mechanism and pathways of intracellular delivery, and various carriers and approaches that have been investigated for intracellular drug delivery.     

Recent trends in the use of polysaccharides for improved delivery of therapeutic agents: pharmacokinetic and pharmacodynamic perspectives

New and innovative methods of delivery of therapeutic agents using polysaccharides have been recently developed, which target site of action, increase the intensity and/or prolong pharmacologic action, and/or reduce toxicity of small molecule drugs, proteins, or enzymes. This review is focused on the role of dextran, pullulan, and mannan polysaccharides in such applications. While dextran and pullulan are glucose polymers with different glucosidic linkages, mannan is composed of mannose units. In terms of pharmacokinetics of the carriers themselves, molecular weight (MW), electric charge, various chemical modifications, and degree of polydispersity and/or branching would mostly determine their fate in vivo. Generally, large MW polysaccharides (MWs > or = 40 kD) have low clearance and relatively long plasma half life, resulting in accumulation in reticuloendothelial or tumor tissues. The tumor accumulation in most cases is a passive targeting due to "enhanced permeation and retention" of macromolecules by tumors. Additionally, drugs such as anticancer agents may be actively targeted to specific cells by polysaccharides to which appropriate ligands are attached. In terms of mode of use, polysaccharides have been utilized in a variety of innovative ways for improvement of drug delivery. Their most important application has been as carriers for preparation of macromolecular prodrugs that are normally inactive and need to release the active drug at the site(s) of interest. Also, they have been used for preparation of macromolecule-protein conjugates, which may retain the activity of the proteins, in order to increase the duration of effect and decrease the immunogenicity of proteins. Several other new applications, such as polysaccharide-anchored liposomal formulations, have also been gained attention recently and are briefly reviewed here. Finally, four recent examples of polysaccharide-based delivery systems involving specific drugs/imaging agents are reviewed in detail in terms of their development, pharmacokinetics, and pharmacodynamics. Collectively, these data suggest that macromolecular polysaccharides are promising agents for improving drug delivery.     

Biomimetic supramolecular designs for the controlled release of growth factors in bone regeneration

The extracellular matrix (ECM) of tissues is an assembly of insoluble macromolecules that specifically interact with soluble bioactive molecules and regulate their distribution and availability to cells. Recapitulating this ability has been an important target in controlled growth factor delivery strategies for tissue regeneration and requires the design of multifunctional carriers. This review describes the integration of supramolecular interactions on the design of delivery strategies that encompass self-assembling and engineered affinity components to construct advanced biomimetic carriers for growth factor delivery. Several glycan- and peptide-based self-assemblies reported in the literature are highlighted and commented upon. These examples demonstrate how molecular design and chemistry are successfully employed to create versatile multifunctional molecules which self-assemble/disassemble in a precisely predicted manner, thus controlling compartmentalization, transport and delivery. Finally, we discuss whether recent advances in the design and preparation of supramolecular delivery systems have been sufficient to drive real translation towards a clinical impact.     

Targeted intracellular delivery of therapeutics: an overview

During the last decade, intracellular drug delivery has become an emerging area of research in the medical and pharmaceutical field. Many therapeutic agents such as drugs and DNA/oligonucleotides can be delivered not just to the cell but also to a particular compartment of that cell to achieve better activity e.g. proapoptotic drugs to the mitochondria, antibiotics and enzymes to the lysosomes and various anticancer drugs and gene to the nucleus. The lipidic nature of biological membrans is the major obstacle to the intracellular delivery of macromolecular and ionic drugs. Additionally, after endocytosis, the lysosome, the major degradation compartment, needs to be avoided for better activity. To avoid these problems, various carriers have been investigated for efficient intracellular delivery, either by direct entry to cytoplasm or by escaping the endosomal compartment. These include cell penetrating peptides, and carrier systems such as liposomes, cationic lipids and polymers, polymeric nanoparticles, etc. Various properties of these carriers, including size, surface charge, composition and the presence of cell specific ligands, alter their efficacy and specificity towards particular cells. This review summarizes various aspects of targeted intracellular delivery of therapeutics including pathways, mechanisms and approaches. Various carrier constructs having potential for targeted intracellular delivery are also been discussed.     

Nuclear delivery of macromolecules: barriers and carriers

Recent evidence for efficient delivery of macromolecules, such as peptides and nucleic acids, from the cell exterior to the nucleus offers the interesting possibility of developing novel treatments directed at intranuclear targets. The findings should also stimulate the search for physiological ligands that utilize similar transport mechanisms to regulate pathobiological processes. Cytokines, growth factors and their receptors, as well as morphogens have all been shown to enter the nucleus to evoke biological responses in target cells. The rational design of intracellular drug delivery vehicles requires an increased understanding of the elaborate systems that mediate cellular communication and coordination with the extracellular environment without inflicting on the integrity of the cell. This review discusses some aspects of the carriers and barriers in macromolecular transport.     

Transdermal drug delivery of insulin with ultradeformable carriers

For a long time, scientists believed that macromolecules can only be introduced through the skin with a hypodermic needle or some other harsh treatment that locally damages the skin barrier. It is now clear that macromolecules can be administered epicutaneously, so that insulin, for example, can exhibit therapeutic effects in patients with type 1 diabetes mellitus. When carriers are employed for the purpose, the drugs must be associated with specifically designed vehicles in the form of highly deformable aggregates and applied on the skin non-occlusively. Using such optimised carriers, so-called Transfersomes, ensures reproducible and efficient transcutaneous carrier and drug transport. Insulin-loaded Transfersomes, for example, can deliver the drug through the non-compromised skin barrier with a reproducible drug effect that resembles closely that of an ultralente insulin injected under the skin; the pharmacokinetic and pharmacodynamic properties of the injected and transdermal insulin are also comparable. The efficacy of transcutaneously delivered insulin in Transfersomes is not affected by the previous therapy, similar results having been measured in patients normally receiving intensified insulin therapy or a continuous subcutaneous infusion of insulin solution. Systemic normoglycaemia that lasts at least 16 hours has been achieved using a single non-invasive, epicutaneous administration of insulin in Transfersomes. Experience with other drugs suggests that the biodistribution of injected and transcutaneously delivered drugs can be very similar. This notwithstanding, Transfersomes can be designed and applied so as to mediate site-specific drug delivery into peripheral musculoskeletal tissues or into the skin, as may be desired.     

Liposomal drug delivery systems--clinical applications

Liposomes have been widely investigated since 1970 as drug carriers for improving the delivery of therapeutic agents to specific sites in the body. As a result, numerous improvements have been made, thus making this technology potentially useful for the treatment of certain diseases in the clinics. The success of liposomes as drug carriers has been reflected in a number of liposome-based formulations, which are commercially available or are currently undergoing clinical trials. The current pharmaceutical preparations of liposome-based therapeutic systems mainly result from our understanding of lipid-drug interactions and liposome disposition mechanisms. The insight gained from clinical use of liposome drug delivery systems can now be integrated to design liposomes that can be targeted on tissues, cells or intracellular compartments with or without expression of target recognition molecules on liposome membranes. This review is mainly focused on the diseases that have attracted most attention with respect to liposomal drug delivery and have therefore yielded most progress, namely cancer, antibacterial and antifungal disorders. In addition, increased gene transfer efficiencies could be obtained by appropriate selection of the gene transfer vector and mode of delivery.     

pH-sensitive polymers for cytoplasmic drug delivery

The pH-sensitive drug delivery systems could be triggered by a mild acidic environment, such as that occurring in solid tumors, inflammatory tissues and intracellular endosomal compartments. Moreover, the cytoplasmic delivery of internalized macromolecules (such as oligonucleic acid, siRNA, DNA, proteins and polymer–antibody complex) will be possible. Synthetic polymers – such as polyanions (acrylic acid derivatives) and polycations (poly ethylenimine and chitosan complexes) – are among the most popular compounds studied for intracellular trafficking of drugs. As research is progressing in the area of cytoplasmic delivery, many novel and innovative applications making use of the unique properties of pH-sensitive polymers are expected in the future.     

Cell-based drug-delivery platforms

Cell systems have recently emerged as biological drug carriers, as an interesting alternative to other systems such as micro- and nano-particles. Different cells, such as carrier erythrocytes, bacterial ghosts and genetically engineered stem and dendritic cells have been used. They provide sustained release and specific delivery of drugs, enzymatic systems and genetic material to certain organs and tissues. Cell systems have potential applications for the treatment of cancer, HIV, intracellular infections, cardiovascular diseases, Parkinson's disease or in gene therapy. Carrier erythrocytes containing enzymes such us L-asparaginase, or drugs such as corticosteroids have been successfully used in humans. Bacterial ghosts have been widely used in the field of vaccines and also with drugs such as doxorubicin. Genetically engineered stem cells have been tested for cancer treatment and dendritic cells for immunotherapeutic vaccines. Although further research and more clinical trials are necessary, cell-based platforms are a promising strategy for drug delivery.     

Internalization of Fractionated 3H-Heparin by the Scavenger-Like Receptor in Rat Liver Parenchymal Cells in Primary Culture

The present study was aimed at clarifying the uptake mechanisms of fractionated ³H-heparin (FH) in rat liver parenchymal cells in an effort to explore further the clinical applications of mucopolysaccharides, including their utilization in drug delivery. The internalization and surface binding of FH were determined by removing surface-bound FH by the NaCl wash method in uptake experiments in rat liver parenchymal cells in primary culture. Initial and transient peaks were observed in the time course of the surface binding of FH, suggesting the involvement of receptor-mediated endocy tosis (RME) and downregulation of the receptors. Consistent with this suggestion, internalization of FH was reduced by lowering the temperature from 37 to 4°C, while total association was unchanged. Although the internalization of FH was slow and concentration independent, both total association and internalization were inhibited by ligands of the scavenger receptor and some anions, but not by inhibitors of the RME of polypeptides. All these results collectively suggest the involvement of the scavenger receptor or similar substance in terms of substrate specificity in the uptake of FH in rat liver parenchymal cells. This is the first suggestion of the existence of the scavenger-like receptor in liver parenchymal cells. Macromolecular compounds such as heparin have recently been increasingly investigated for their clinical applications, including utilization as carriers for drug delivery (Nishikawa et al. 1995). Although there have been a number of studies concerning the disposition of polypeptides in the body and the utilization of macromolecules as drug carriers, more extensive studies are required to exploit a variety of macromolecules for clinical applications. Heparin, a macromolecular drug with molecular weights ranging from 3000 to 30,000 d., has been used as an anticoagulant drug. The disposition of heparin after intravenous administration depends on the molecular weight     

树状大分子作为新型药物载体的研究进展

树状大分子是一类高度枝化的单分散性大分子，其内部结构呈疏水性，外表面呈亲水性，可称为“单分子胶束”。本文在简介树状大分子的发展及结构特点的基础上，阐述了树状大分子作为药物载体的作用特点及其与药物的结合方式。目前，树状大分子在介导药物靶向传递及基因转染等方面的应用也备受关注，是一种颇有发展潜力的新型载体。     

Curb challenges of the “Trojan Horse” approach: Smart strategies in achieving effective yet safe cell-penetrating peptide-based drug delivery

Cell-penetrating peptide (CPP)-mediated intracellular drug delivery system, often specifically termed as “the Trojan horse approach”, has become the “holy grail” in achieving effective delivery of macromolecular compounds such as proteins, DNA, siRNAs, and drug carriers. It is characterized by the unique cell- (or receptor-), temperature-, and payload-independent mechanisms, therefore offering potent means to improve poor cellular uptake of a variety of macromolecular drugs. Nevertheless, this “Trojan horse” approach also acts like a double-edged sword, causing serious safety and toxicity concerns to normal tissues or organs for in vivo application, due to lack of target selectivity of the powerful cell penetrating activity. To overcome this problem of potent yet non-selective penetration vs. targeting delivery, a number of “smart” strategies have been developed in recent years, including controllable CPP-based drug delivery systems based on various stimuli-responsive mechanisms. This review article provides a fundamental understanding of these smart systems, as well as a discussion of their real-time in vivo applicability.     

Drug delivery systems. 1. site-specific drug delivery using liposomes as carriers

Drug delivery systems, offering controlled delivery of biologically active agents, are rapidly gaining importance in pharmaceutical research and development. To achieve controlled drug delivery, i.e., the administration of drugs so that optimal amount reaches the target site to cure or control the disease state, increasingly sophisticated systems containing different carriers have been developed. Macromolecules represent one of the carriers involved, and they have taken on a significantly prominent role in various modes of administration of therapeutic agents. Among macromolecules, for example, synthetic copolymers, polysaccharides, liposomes, polyanions and antibodies, as drug carriers, liposomes have proved most effective for diseases affecting the reticuloendothelial system and blood cells in particular. Liposomes, which are vesicles consisting of one or more concentrically ordered assemblies of phospholipids bilayers, range in size from a nanometer to several micrometers. Phospholipids such as egg phosphatidylcholine, phosphatidylserine, synthetic dipalmitoyl-DL-alpha-phosphatidylcholine or phosphatidylinositol, have been used in conjunction with cholesterol and positively or negatively charged amphiphiles such as stearylamine or phosphatidic acid. Alteration of surface charge has been shown to enhance drug incorporation and also influence drug release. Because of the multifold characteristics as drug carriers, liposomes have been investigated extensively as carriers of anticancer agents for the past several years. Liposomal entrapments include a variety of pharmacologically active compounds such as antimalarial, antiviral, anti-inflammatory and anti-fungal agents as well as antibiotics, prostaglandins, steroids and bronchodilators to name a few. The liposomal entrapment has been shown to have considerable effect on the pharmacokinetics and tissue distribution of administered drugs. Despite the potential value of liposomes as unique carriers, the major obstacles are the first order targeting of a systemically given liposomes, physical stability and manufacture of the liposomal products and these problems still remain to be overcome. Drug delivery systems evolving in the 1980s have become increasingly dependent on fundamental cell-biology and receptor-mediated endocytotic mechanisms. Drug delivery systems during the 1990s may take advantage of the specificity of receptor-mediated uptake mechanisms as well as polymer chemistry and cell-biology in order to introduce more precise and efficient target-specific delivery systems that are based especially on the liposome technology.     

Endothelial endocytic pathways: gates for vascular drug delivery

Vascular endothelium plays strategic roles in many drug delivery paradigms, both as an important therapeutic target itself and as a barrier for reaching tissues beyond the vascular wall. Diverse means are being developed to improve vascular drug delivery including stealth liposomes and polymer carriers. Affinity carriers including antibodies or peptides that specifically bind to endothelial surface determinants, either constitutive or pathological, enhance targeting of drugs to endothelial cells (EC) in diverse vascular areas. In many cases, binding to endothelial surface determinants facilitates internalization of the drug/carrier complex. There are several main endocytic pathways in EC, including clathrin- and caveoli-mediated endocytosis, phagocytosis and macropinocytosis (these two are less characteristic of generic EC) and the recently described Cell Adhesion Molecule (CAM)-mediated endocytosis. The latter may be of interest for intracellular drug delivery to EC involved in inflammation or thrombosis. The metabolism and effects of internalized drugs largely depend on the routes of intracellular trafficking, which may lead to degrading lysosomal compartments or other organelles, recycling to the plasma membrane or transcytosis to the basal surface of endothelium. The latter route, characteristic of caveoli-mediated endocytosis, may serve for trans-endothelial drug delivery. Paracellular trafficking, which can be enhanced under pathological conditions or by auxiliary agents, represents an alternative for transcytosis. Endothelial surface determinants involved in endocytosis, mechanisms of the latter and trafficking pathways, as well as specific characteristics of EC in different vascular areas, are discussed in detail in the context of modern paradigms of vascular drug delivery.     

The role of macromolecular architecture in passively targeted polymeric carriers for drug and gene delivery

The use of polymeric carriers for drug delivery has become increasingly popular because of the ability to easily tune the physical and biological properties of macromolecules. With the growing commercial accessibility of branched and dendritic polymers, their incorporation into polymeric carriers is being explored with increased frequency. However, while a handful of systematic studies have explored the use of branched macromolecules for drug delivery, the role of polymer architecture in optimizing the polymeric carriers is not yet fully understood. Herein, the authors summarize the effect that architecture has on the basic physical properties of polymers, and review our preliminary understanding of the architectural effects on polymer-assisted drug delivery.     

Factors affecting drug and gene delivery: effects of interaction with blood components

Targeted drug delivery systems have been used extensively to improve the pharmacological and therapeutic activities of a wide variety of drugs and genes. In this article, we summarize the factors determining the tissue disposition of delivery systems: the physicochemical and biological characteristics of the delivery system and the anatomic and physiological characteristics of the tissues. There are several modes of drug and gene targeting, ranging from passive to active targeting, and each of these can be achieved by optimizing the design of the delivery system to suit a specific aim. After entering the systemic circulation, either by an intravascular injection or through absorption from an administration site, however, a delivery system encounters a variety of blood components, including blood cells and a range of serum proteins.These components are by no means inert as far as interaction with the delivery system is concerned, and they can sometimes markedly effect its tissue disposition. The interaction with blood components is known to occur with particulate delivery systems, such as liposomes, or with cationic charge-mediated delivery systems for genes. In addition to these rather nonspecific ones, interactions via the targeting ligand of the delivery system can occur. We recently found that mannosylated carriers interact with serum mannan binding protein, greatly altering their tissue disposition in a number of ways that depend on the properties of the carriers involved.     

外泌体作为药物递送载体的研究进展

外泌体(exosomes)是一种由细胞分泌的纳米尺度(40~100 nm)的囊泡,在细胞间物质运输和信号交流中发挥重要作用。外泌体在大小和功能上与合成的纳米颗粒类似,但作为天然内源性转运载体,具有毒性低、无免疫原性、渗透性好等优势,故可能成为更有应用前景的药物递送载体。本文主要介绍了外泌体的基本性质和获得方法、载药方法及其作为纳米载体在小分子和生物大分子药物递送和靶向研究中的应用进展情况,并分析探讨了外泌体在载药和靶向递送方面的不足。