DNA polyplexes based on degradable oligoethylenimine-derivatives: Combination with EGF receptor targeting and endosomal release functions

Application of Peptides in Construction of Nonviral Vectors for Gene Delivery

Gene therapy, which aims to cure diseases by knocking out, editing, correcting or compensating abnormal genes, provides new strategies for the treatment of tumors, genetic diseases and other diseases that are closely related to human gene abnormalities. In order to deliver genes efficiently to abnormal sites in vivo to achieve therapeutic effects, a variety of gene vectors have been designed. Among them, peptide-based vectors show superior advantages because of their ease of design, perfect biocompatibility and safety. Rationally designed peptides can carry nucleic acids into cells to perform therapeutic effects by overcoming a series of biological barriers including cellular uptake, endosomal escape, nuclear entrance and so on. Moreover, peptides can also be incorporated into other delivery systems as functional segments. In this review, we referred to the biological barriers for gene delivery in vivo and discussed several kinds of peptide-based nonviral gene vectors developed for overcoming these barriers. These vectors can deliver different types of genetic materials into targeted cells/tissues individually or in combination by having specific structure-function relationships. Based on the general review of peptide-based gene delivery systems, the current challenges and future perspectives in development of peptidic nonviral vectors for clinical applications were also put forward, with the aim of providing guidance towards the rational design and development of such systems.

Photochemical Internalization for Intracellular Drug Delivery. From Basic Mechanisms to Clinical Research

Photochemical internalisation (PCI) is a unique intervention which involves the release of endocytosed macromolecules into the cytoplasmic matrix. PCI is based on the use of photosensitizers placed in endocytic vesicles that, following light activation, lead to rupture of the endocytic vesicles and the release of the macromolecules into the cytoplasmic matrix. This technology has been shown to improve the biological activity of a number of macromolecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), gene-encoding plasmids, adenovirus and oligonucleotides and certain chemotherapeutics, such as bleomycin. This new intervention has also been found appealing for intracellular delivery of drugs incorporated into nanocarriers and for cancer vaccination. PCI is currently being evaluated in clinical trials. Data from the first-in-human phase I clinical trial as well as an update on the development of the PCI technology towards clinical practice is presented here.

Coming in and Finding Out: Blending Receptor-Targeted Delivery and Efficient Endosomal Escape in a Novel Bio-Responsive siRNA Delivery System for Gene Knockdown in Pulmonary T Cells

RNA interference (RNAi) offers the potential to selectively silence disease-related genes in defined cell subsets. Translation into the clinical routine is, however, still hampered by the lack of efficient carrier systems for therapeutic siRNA, endosomal entrapment presenting a major hurdle. A promising siRNA delivery system has previously been developed on the base of polyethylenimine (PEI) and the targeting ligand transferrin (Tf) to specifically reach activated T cells in the lung. In the present work, the focus is on optimizing Tf-PEI polyplexes for gene knockdown in primary activated T cells by improving their endosomal escape properties. Blending of the conjugate with membrane lytic melittin significantly enhanced endosomal release and thereby cytoplasmic delivery, while maintaining selective T cell targeting abilities and overall cell tolerability. The gathered data furthermore demonstrate that melittin addition also distinctly improves several other essential particle characteristics, such as siRNA encapsulation efficiency and stability in lung lining fluids. In conclusion, this results in a novel upgraded siRNA delivery system that is not only able to specifically deliver its payload to the desired target cells via receptor-mediated endocytosis, but also shows enhanced release from endosomal vesicles in order to initiate RNAi in the cytoplasm.

Non-viral Delivery of Nucleic Acids: Insight Into Mechanisms of Overcoming Intracellular Barriers

Delivery of genes, including plasmid DNAs, short interfering RNAs (siRNAs), and messenger RNAs (mRNAs), using artificial non-viral nanotherapeutics is a promising approach in cancer gene therapy. However, multiple physiological barriers upon systemic administration remain a key challenge in clinical translation of anti-cancer gene therapeutics. Besides extracellular barriers including sequestration of gene delivery nanoparticles from the bloodstream by resident organ-specific macrophages, and their poor extravasation and tissue penetration in tumors, overcoming intracellular barriers is also necessary for successful delivery of nucleic acids. Whereas for RNA delivery the endosomal barrier holds a key importance, transfer of DNA cargo additionally requires translocation into the nucleus. Better understanding of crossing membrane barriers by nucleic acid nanoformulations is essential to the improvement of current non-viral carriers. This review aims to summarize relevant literature on intracellular trafficking of non-viral nanoparticles and determine key factors toward surmounting intracellular barriers. Moreover, recent data allowed us to propose new interpretations of current hypotheses of endosomal escape mechanisms of nucleic acid nanoformulations.

Magnetofection Enhances Adenoviral Vector-based Gene Delivery in Skeletal Muscle Cells

The goal of magnetic field-assisted gene transfer is to enhance internalization of exogenous nucleic acids by association with magnetic nanoparticles (MNPs). This technique named magnetofection is particularly useful in difficult-to-transfect cells. It is well known that human, mouse, and rat skeletal muscle cells suffer a maturation-dependent loss of susceptibility to Recombinant Adenoviral vector (RAd) uptake. In postnatal, fully differentiated myofibers, the expression of the primary Coxsackie and Adenoviral membrane receptor (CAR) is severely downregulated representing a main hurdle for the use of these vectors in gene transfer/therapy. Here we demonstrate that assembling of Recombinant Adenoviral vectors with suitable iron oxide MNPs into magneto-adenovectors (RAd-MNP) and further exposure to a gradient magnetic field enables to efficiently overcome transduction resistance in skeletal muscle cells. Expression of Green Fluorescent Protein and Insulin-like Growth Factor 1 was significantly enhanced after magnetofection with RAd-MNPs complexes in C2C12 myotubes in vitro and mouse skeletal muscle in vivo when compared to transduction with naked virus. These results provide evidence that magnetofection, mainly due to its membrane-receptor independent mechanism, constitutes a simple and effective alternative to current methods for gene transfer into traditionally hard-to-transfect biological models.

Increased cell survival of cells exposed to superparamagnetic iron oxide nanoparticles through biomaterial substrate-induced autophagy

The enhanced autophagy response by culturing cells on chitosan substrate is linked to a high cell survival rate under excessive NP endocytosis conditions.

Enhancing endosomal escape for nanoparticle mediated siRNA delivery

Gene therapy with siRNA is a promising biotechnology to treat cancer and other diseases. To realize siRNA-based gene therapy, a safe and efficient delivery method is essential. Nanoparticle mediated siRNA delivery is of great importance to overcome biological barriers for systemic delivery in vivo. Based on recent discoveries, endosomal escape is a critical biological barrier to be overcome for siRNA delivery. This feature article focuses on endosomal escape strategies used for nanoparticle mediated siRNA delivery, including cationic polymers, pH sensitive polymers, calcium phosphate, and cell penetrating peptides. Work has been done to develop different endosomal escape strategies based on nanoparticle types, administration routes, and target organ/cell types. Also, enhancement of endosomal escape has been considered along with other aspects of siRNA delivery to ensure target specific accumulation, high cell uptake, and low toxicity. By enhancing endosomal escape and overcoming other biological barriers, great progress has been achieved in nanoparticle mediated siRNA delivery.

Subcellular trafficking and transfection efficacy of polyethylenimine-polyethylene glycol polyplex nanoparticles with a ligand to melanocortin receptor-1

We have synthesized and investigated properties of new PEI-PEG-based polyplexes containing MC1SP-peptide, a ligand specific for melanocortin receptor-1 (targeted polyplexes), and control polyplexes without this ligand peptide (non-targeted polyplexes). The targeted polyplexes demonstrated receptor-mediated transfection of Cloudman S91 (clone M-3) murine melanoma cells that was more efficient than with the non-targeted ones. Transfection with the targeted polyplexes was inhibited by chlorpromazine, an inhibitor of the clathrin-mediated endocytosis pathway, and, to a lesser extent, by filipin III or nystatin, inhibitors of the lipid-raft endocytosis pathway, whereas transfection with the non-targeted polyplexes was inhibited mainly by nystatin or filipin III. The targeted polyplexes caused significantly higher in vivo transfection of melanoma tumor cells after intratumoral administration compared to the non-targeted control. The targeted polyplexes carrying the HSVtk gene, after ganciclovir administration, more efficiently inhibited melanoma tumor growth and prolonged the lifespan of DBA/2 tumor-bearing mice compared to the non-targeted ones. Packed targeted polyplexes appeared and accumulated in the melanoma cells 6h earlier than the non-targeted ones. The targeted polyplexes enter into the nuclei of the melanoma cells more rapidly than the non-targeted control, and this difference may also be attributed to processes of receptor-mediated endocytosis. We believe that these data may be useful for the optimization of polyplex systems.

Emerging links between surface nanotechnology and endocytosis: impact on nonviral gene delivery

Significant effort continues to be exerted toward the improvement of transfection mediated by nonviral vectors. These endeavors are often focused on the design of particulate carriers with properties that encourage efficient accumulation at the membrane surface, particle uptake, and endosomal escape. Despite its demonstrated importance in successful nonviral transfection, relatively little investigation has been done to understand the pressures driving internalized vectors into favorable nondegradative endocytic pathways. Improvements in transfection efficiency have been noted for complexes delivered with a substrate-mediated approach, but the reasons behind such enhancements remain unclear. The phenotypic changes exhibited by cells interacting with nano- and micro-featured substrates offer hints that may explain these effects. This review describes nanoscale particulate and substrate parameters that influence both the uptake of nonviral gene carriers and the endocytic phenotype of interacting cells, and explores the molecular links that may mediate these interactions. Substrate-mediated control of endocytosis represents an exciting new design parameter that will guide the creation of efficient transgene carriers.

Multifunctional peptide-PEG intercalating conjugates: programmatic of gene delivery to the blood-brain barrier

PURPOSE

To enhance transfection efficacy of pDNA through the application of multifunctional peptide-PEG-tris-acridine conjugates (pPAC) and the formation of biodegradable core-shell polyplexes for gene delivery to the blood-brain barrier (BBB).

METHODS

pPAC-mediated transfection was compositionally optimized in mouse BBB cells (bEnd.3). Cellular uptake and trafficking, and brain accumulation of pDNA was evaluated by fluorescent imaging and histochemistry. We constructed anti-MRP4 siRNA-producing vectors and evaluated the efficacy of MRP4 down-regulation of MRP4 by Western blot and qPCR, and its effect on the uptake of (3)H-AZT, an MRP4 substrate.

RESULTS

A core-shell gene delivery system (GDS) was assembled from pDNA and pPAC, carrying multifunctional peptides with NLS, TAT, and brain-specific BH, or ApoE sequences, and biodegradable pLPEI polyamine. This GDS demonstrated better cellular and nuclear accumulation, and a 25-fold higher transfection efficacy in slow-dividing bEnd.3 cells compared to ExGen500. Inclusion of brain-targeting pPAC enhanced in vivo accumulation of functional pDNA in brain capillaries. Treatment by encapsulated anti-MRP4 siRNA-producing pDNA caused transient down-regulation of MRP4, and, after intravenous injection in Balb/c mice, enhanced AZT uptake in the brain by 230-270%.

CONCLUSIONS

The pPAC represent novel efficient components of GDS that could find various gene therapy applications, including genetic modulation of the BBB.

Poly(I:C)-mediated tumor growth suppression in EGF-receptor overexpressing tumors using EGF-polyethylene glycol-linear polyethylenimine as carrier

PURPOSE

To develop a novel polyethylenimine (PEI)-based polymeric carrier for tumor-targeted delivery of cytotoxic double-stranded RNA polyinosinic:polycytidylic acid, poly(I:C). The novel carrier should be chemically less complex but at least as effective as a previously developed tetra-conjugate containing epidermal growth factor (EGF) as targeting ligand, polyethylene glycol (PEG) as shielding spacer, 25 kDa branched PEI as RNA binding and endosomal buffering agent, and melittin as endosomal escape agent.

METHODS

Novel conjugates were designed employing a simplified synthetic strategy based on 22 kDa linear polyethylenimine (LPEI), PEG spacers, and recombinant EGF. The efficacy of various conjugates (different PEG spacers, with and without targeting EGF) in poly(I:C)-mediated cell killing was evaluated in vitro using two human U87MG glioma cell lines. The most effective polyplex was tested for in vivo activity in A431 tumor xenografts.

RESULTS

Targeting conjugate LPEI-PEG2 kDa-EGF was found as most effective in poly(I:C)-triggered killing of tumor cells in vitro. The efficacy correlated with glioma cell EGFR levels. Repeated intravenous administration of poly(I:C) polypexes strongly retarded growth of A431 human tumor xenograft in mice.

CONCLUSIONS

The optimized LPEI-PEG2 kDa-EGF conjugate displays reduced chemical complexity and efficient poly(I:C)-mediated killing of EGFR overexpressing tumors in vitro and in vivo.

Transfection efficiency of depolymerized chitosan and epidermal growth factor conjugated to chitosan-DNA polyplexes

An efficient non-viral gene delivery for varieties of cells has been considered essential for gene therapy and tissue engineering. This study evaluated transfection efficiency of chitosan (HW) with molecular weights (Mw) at 470 and degree of deacetylation (DDA) 80% and its depolymerization product (LW) with Mw at 16 kDa and DDA 54%, as well as epidermal growth factor (EGF) conjugated to chitosan-DNA microparticles of both HW and LW by using either disulfide linkage or NHS-PEO(4)-Maleimide as a cross linker. The results revealed that the depolymerized LW at chitosan/DNA charge ratio 56:1 and pH 6.9 gave high transfection efficiency in both KB, a cancer cell line, and fibroblast cells at about the same level of Lipofectamine, but the EGF-conjugated chitosan-DNA polyplexes from these methods did not improve transfection efficiency, which may come from the aggregation and fusing of the complexes as shown in scanning electron microscopy. However, this depolymerized LW chitosan showed the potential for further development as a safe and cost-effective non-viral gene delivery vehicle.

Chemically programmed polymers for targeted DNA and siRNA transfection

Plasmid DNA and siRNA have a large potential for use as therapeutic nucleic acids in medicine. The way to the target cell and its proper compartment is full of obstacles. Polymeric carriers help to overcome the encountered barriers. Cationic polymers can interact with the nucleic acid in a nondamaging way but still require optimization with regard to transfer efficiency and biocompatibility. Aiming at virus-like features, as viruses are the most efficient natural gene carriers, the design of bioresponsive polymers shows promising results regarding DNA and siRNA delivery. By specific chemical modifications dynamic structures are created, programmed to respond towards changing demands on the delivery pathway by cleavage of labile bonds or conformational changes, thus enhancing biocompatible gene delivery.

Targeting the EGF receptor for ovarian cancer therapy

Ovarian carcinoma is the leading cause of death from gynecologic malignancy in the US. Factors such as the molecular heterogeneity of ovarian tumors and frequent diagnosis at advanced stages hamper effective disease treatment. There is growing emphasis on the identification and development of targeted therapies to disrupt molecular pathways in cancer. The epidermal growth factor (EGF) receptor is one such protein target with potential utility in the management of ovarian cancer. This paper will discuss contributions of EGF receptor activation to ovarian cancer pathogenesis and the status of EGF receptor inhibitors and EGF receptor targeted therapies in ovarian cancer treatment.

Vectors for pulmonary gene therapy

The success of gene transfer in preclinical animal models and proof of principle clinical studies has made gene therapy an attractive concept for disease treatment. A variety of diseases affecting the lung are candidates for gene therapy. Delivery of genes to the lungs seems to be straightforward, because of the easy accessibility of epithelial cells via the airways. However, efficient delivery and expression of the therapeutic transgene at levels sufficient to result in phenotypic correction of the diseased state have proven elusive. This review presents a brief summary about current status and future prospects in the development of viral and non-viral strategies for pulmonary gene therapy.

Bioreducible hyperbranched poly(amido amine)s for gene delivery

A series of reducible hyperbranched poly(amido amine)s (RHB) with high transfection efficiency were designed and synthesized as nonviral gene delivery vectors. The polycations were synthesized by Michael addition copolymerization of N,N-dimethylaminodipropylenetriamine (DMDPTA) and two bisacrylamide monomers N,N'-hexamethylene bisacrylamide (HMBA) and N,N'-cystamine bisacrylamide (CBA). The density of disulfide linkages in the synthesized hyperbranched polymers was tuned by varying the feed molar ratio of the bisacrylamide monomers. The results demonstrate that disulfide content in RHB controls the molecular weight of the polycation degradation products, ease of polyplex disassembly, polycation cytotoxicity, and polyplex transfection activity. Due to their lower cytotoxicity, polyplexes based on reducible polycations could be used safely in a wider range of DNA doses than nonreducible controls. As a result, significantly increased transfection activity is achieved with optimized formulations of reducible polyplexes compared with nonreducible controls.

Barriers to successful delivery of short interfering RNA after systemic administration

1. RNA interference in vivo has tremendous potential, both with respect to the elucidation of protein function in animals and as a therapeutic platform in humans. In vitro, short interfering RNA (siRNA) has been shown to completely silence gene expression in mammalian cells at low picomolar concentrations. 2. Although many good publications have shown specific silencing to occur in vivo, there are few that have transferred the combination of maximal efficacy and high potency to this setting. The present review considers the biological barriers that limit the movement of siRNA from vascular lumen to target cell cytoplasm and the strategies that have been used to overcome them. 3. Intravenous administration of siRNA results in rapid, extensive removal of siRNA from the blood via renal excretion, tissue distribution and nuclease degradation. Movement across vascular capillaries appears to be a limiting factor in some cases; few examples of silencing have been reported in organs with a conventional capillary endothelium. 4. Cellular uptake and endosomal trapping are significant barriers, but can be overcome using strategies such as antibody mediated cellular uptake or polyethyleneimine-mediated endosomal escape.

Dynamics of photoinduced endosomal release of polyplexes

Endosomal escape is a well-known bottleneck for successful delivery of macromolecular drugs and genes. Photochemical disruption of endosomal membranes is an approach to overcome this bottleneck. In this study, we used the photosensitizer disulphonated meso-tetraphenylporphine with sulfonate groups on adjacent phenyl rings (TPPS(2a)) to investigate photoinduced endosomal release in living cells with high resolution fluorescence wide-field microscopy in real time. We studied the release dynamics of 10 kDa dextran and polyplexes consisting of DNA condensed with the cationic polymers linear polyethyleneimine (LPEI), poly-(L)-lysine (PLL) or poly-(D)-lysine (PDL). By means of dual-color microscopy and the use of double-labeled polyplexes DNA and polymer were imaged simultaneously. We show that the characteristics of the cationic polymer significantly influence the release behavior of the polyplexes. The release of dextran occurred within 100 ms. For LPEI/DNA particles, LPEI quickly spread throughout the cytosol similar to dextran, whereas DNA was released slowly (within 4 s) and remained immobile thereafter. In case of PLL particles, both DNA and polymer showed quick release. PDL particles remained condensed upon photosensitizer activation. In addition, we demonstrate that TPPS(2a) has biological side effects. Besides stop of microtubule dynamics in the dark, the movement of endosomes ceased after photosensitizer activation.

A dimethylmaleic acid-melittin-polylysine conjugate with reduced toxicity, pH-triggered endosomolytic activity and enhanced gene transfer potential

BACKGROUND

Poor endosomal release is one major barrier of gene delivery. Endosomolytic polyethylenimine-melittin conjugates have shown to enhance gene transfer efficiency; however, cytotoxicity due to their general membrane-destabilizing properties limits their application. To overcome this drawback we grafted a polycation with a masked pH-responsive melittin derivate and investigated lytic activity, gene transfer efficiency and cytotoxicity of the resulting conjugate.

METHODS

Melittin (Mel) was modified with dimethylmaleic anhydride (DMMAn) and covalently coupled to poly-L-lysine (PLL). The membrane lytic activity was analyzed after incubation at neutral or endosomal pH. PLL-DMMAn-Mel polyplexes were generated in HEPES-buffered glucose and tested in transfection experiments using luciferase as reporter gene. Cellular cytotoxicity was analyzed by measurement of membrane integrity and metabolic activity.

RESULTS

Covalent attachment of DMMAn-modified melittin to PLL resulted in a pH-responsive conjugate. No lytic activity was observed at neutral pH; after acidic cleavage of the protecting groups at pH 5 lytic activity was regained. Acute toxicity was greatly reduced (as compared to PLL-Mel or even unmodified PLL) and high gene expression levels (up to 1800-fold higher than unmodified PLL) were obtained.

CONCLUSIONS

Modification of the polycationic carrier PLL with DMMAn-masked melittin not only enhances gene transfer efficiency, but also strongly reduces the acute toxicity of melittin and PLL. Hence this modification might be useful for optimizing polycationic gene carriers.

Nanoconjugate based on polymalic acid for tumor targeting

A new prototype of polymer-derived drug delivery system, the nanoconjugate Polycefin, was tested for its ability to accumulate in tumors based on enhanced permeability and retention (EPR) effect and receptor mediated endocytosis. Polycefin was synthesized for targeted delivery of Morpholino antisense oligonucleotides into certain tumors. It consists of units that are covalently conjugated with poly(beta-l-malic acid) (M(w) 50,000, M(w)/M(n) 1.3) highly purified from cultures of myxomycete Physarum polycephalum. The units are active in endosomal uptake, disruption of endosomal membranes, oligonucleotide release in the cytoplasm, and protection against enzymatic degradation in the vascular system. The polymer is biodegradable, non-immunogenic and non-toxic. Polycefin was also coupled with AlexaFluor 680 C2-maleimide dye for in vivo detection. Nude mice received subcutaneous injections of MDA-MB 468 human breast cancer cells into the left posterior mid-dorsum or intracranial injections of human glioma cell line U87MG. Polycefin at concentration of 2.5mg/kg was injected via the tail vein. In vivo fluorescence tumor imaging was performed at different time points, 0-180 min up to 24h after the drug injection. The custom-made macro-illumination imaging MISTI system was used to examine the in vivo drug accumulation in animals bearing human breast and brain tumors. In breast tumors the fluorescence signal in large blood vessels and in the tumor increased rapidly until 60 min and remained in the tumor at a level 6 times higher than in non-tumor tissue (180 min) (p<0.003). In brain tumors drug accumulated selectively in 24h without any detectable signal in non-tumor areas. The results of live imaging were corroborated histologically by fluorescence microscopic examination of various organs. In addition to tumors, only kidney and liver showed some fluorescent signal.

Strategies to improve drug delivery across the blood-brain barrier

The blood-brain barrier (BBB), together with the blood-cerebrospinal-fluid barrier, protects and regulates the homeostasis of the brain. However, these barriers also limit the transport of small-molecule and, particularly, biopharmaceutical drugs such as proteins, genes and interference RNA to the brain, thereby limiting the treatment of many brain diseases. As a result, various drug delivery and targeting strategies are currently being developed to enhance the transport and distribution of drugs into the brain. In this review, we discuss briefly the biology and physiology of the BBB as the most important barrier for drug transport to the brain and, in more detail, the possibilities for delivering large-molecule drugs, particularly genes, by receptor-mediated nonviral drug delivery to the (human) brain. In addition, the systemic and intracellular pharmacokinetics of nonviral gene delivery, together with targeted brain imaging, are reviewed briefly.

Electrophoretic purification of tumor-targeted polyethylenimine-based polyplexes reduces toxic side effects in vivo

Non-viral vectors based on polyethylenimine (PEI) are usually generated with an excess of PEI. However, the amount of unbound polymer correlates with toxicity limiting the in vivo use of these gene carriers. Purification based on size exclusion chromatography of PEI/DNA polyplexes smaller than 200 nm has been shown to efficiently remove unbound PEI polymer. A novel purification method based on electrophoresis can purify PEI polyplexes independent of their size resulting in polyplexes with final PEI nitrogen/DNA phosphate ratios between 2.6 and 3.1. Also unbound PEI conjugates like PEGylated PEI and transferrin-conjugated PEI can be separated from the polyplexes, providing formulations with clearly defined compositions. Purified polyplexes can mediate in vitro gene transfer with high transfection efficiencies while demonstrating lower cellular toxicity. Purified polyplexes were well-tolerated when systemically delivered into tumor-bearing mice at 100 microg/20 g body weight, with tumor gene expression levels up to 5-fold higher than the non-purified polyplexes. Mice receiving non-purified gene carriers exhibited severe toxicity leading to high mortality and unfavourable gene expression patterns.

Optimizing targeted gene delivery: chemical modification of viral vectors and synthesis of artificial virus vector systems

In comparison to classical medicines, gene therapy has the potential to mediate the highest possible level of therapeutic specificity. Every normal or diseased cell can switch on or off a gene expression cassette in a tissue-, disease-, and time-dependent fashion, by use of specific transcription factors that are active only in a given unique situation. In practice, we face the problem in realizing the concept: the delivery of nucleic acids into target cells is very ineffective and presents a formidable challenge. Key issues for future developments include improved targeting, enhanced intracellular uptake, and reduced toxicity of gene vectors. The currently used classes of vectors have complementary characteristics, such as high intracellular efficiency of viral vectors on the one hand and low immunogenicity and greater flexibility of nonviral vectors on the other hand. The merge of viral and nonviral vector technologies is highlighted as an encouraging strategy for the future; concepts include chemically modified viral vectors ("chemo-viruses") and synthesis of virus-like systems ("synthetic viruses"). Examples for the development of vectors toward artificial synthetic viruses are presented.

Cell and tissue targeting of nucleic acids for cancer gene therapy

Tumor targeting--per definition--includes any strategy to improve the specificity of the therapeutic nucleic acid towards the tumor site, while highest biological activity should be maintained. Targeting has been successfully achieved at the transcriptional, transductional or delivery level. For tumor-specific delivery, physical targeting methods like electroporation, hyperthermia, magnetofection, photochemical internalization or ultrasound, and biological targeting systems, including active and passive tumor targeting, have been developed. Therapeutic effects could be demonstrated with various targeted nucleic acid formulations, such as tumor-targeted DNA plasmids expressing p53 or tumor necrosis factor alpha, small interfering RNAs knocking down gene expression from tumor specific chromosomal translocations or gene expression of tumor neoangiogenic processes, as well as double stranded RNA poly inosine-cytosine which triggers apoptosis in targeted tumor cells.