Cell penetrating peptide conjugates of steric block oligonucleotides

Context Dependent Effects of Chimeric Peptide Morpholino Conjugates Contribute to Dystrophin Exon-skipping Efficiency

We have recently reported that cell-penetrating peptides (CPPs) and novel chimeric peptides containing CPP (referred as B peptide) and muscle-targeting peptide (referred as MSP) motifs significantly improve the systemic exon-skipping activity of morpholino phosphorodiamidate oligomers (PMOs) in dystrophin-deficient mdx mice. In the present study, the general mechanistic significance of the chimeric peptide configuration on the activity and tissue uptake of peptide conjugated PMOs in vivo was investigated. Four additional chimeric peptide-PMO conjugates including newly identified peptide 9 (B-9-PMO and 9-B-PMO) and control peptide 3 (B-3-PMO and 3-B-PMO) were tested in mdx mice. Immunohistochemical staining, RT-PCR and western blot results indicated that B-9-PMO induced significantly higher level of exon skipping and dystrophin restoration than its counterpart (9-B-PMO), further corroborating the notion that the activity of chimeric peptide-PMO conjugates is dependent on relative position of the tissue-targeting peptide motif within the chimeric peptide with respect to PMOs. Subsequent mechanistic studies showed that enhanced cellular uptake of B-MSP-PMO into muscle cells leads to increased exon-skipping activity in comparison with MSP-B-PMO. Surprisingly, further evidence showed that the uptake of chimeric peptide-PMO conjugates of both orientations (B-MSP-PMO and MSP-B-PMO) was ATP- and temperature-dependent and also partially mediated by heparan sulfate proteoglycans (HSPG), indicating that endocytosis is likely the main uptake pathway for both chimeric peptide-PMO conjugates. Collectively, our data demonstrate that peptide orientation in chimeric peptides is an important parameter that determines cellular uptake and activity when conjugated directly to oligonucleotides. These observations provide insight into the design of improved cell targeting compounds for future therapeutics studies.

Synthesis and effects of conjugated tocopherol analogues on peptide nucleic acid hybridisation

To the N-terminus of a nonamer peptide nucleic acid sequence, H-GCACGACTT-NH2, was attached a number of lipophilic conjugate molecules including three synthetic tocopherol (vitamin E) analogues. Studies were then undertaken with complementary PNA and DNA sequences to explore the effects of the conjugates using the techniques of UV monitored melting curves and isothermal calorimetry. Duplex formation was observed when the benzopyran group of vitamin E was conjugated. However, in the presence of the phytyl chain of vitamin E, binding was found to be temperature dependent.

Therapeutic potential of cell-penetrating peptides

The ability of cell-penetrating peptides to cross plasma membranes has been used for various applications, including the delivery of bioactive molecules to inhibit disease-producing cellular mechanisms. Selective drug delivery into target cells improves drug distribution and decreases dosing and toxicity. In this review, the authors outline the main challenges in the field, namely clarification of mechanisms of entry into cells, as well as current and future perspectives regarding cell-penetrating peptides application for human therapeutics. Here, the authors discuss some of the factors that influence efficacy of delivery and review the current status of preclinical studies and clinical trials involving the use of cell-penetrating peptide-mediated delivery of therapeutics.

The potential role of cell penetrating peptides in the intracellular delivery of proteins for therapy of erythroid related disorders

The erythroid related disorders (ERDs) represent a large group of hematological diseases, which in most cases are attributed either to the deficiency or malfunction of biosynthetic enzymes or oxygen transport proteins. Current treatments for these disorders include histo-compatible erythrocyte transfusions or allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy delivered via suitable viral vectors or genetically modified HSCs have been under way. Protein Transduction Domain (PTD) technology has allowed the production and intracellular delivery of recombinant therapeutic proteins, bearing Cell Penetrating Peptides (CPPs), into a variety of mammalian cells. Remarkable progress in the field of protein transduction leads to the development of novel protein therapeutics (CPP-mediated PTs) for the treatment of monogenetic and/or metabolic disorders. The “concept” developed in this paper is the intracellular protein delivery made possible via the PTD technology as a novel therapeutic intervention for treatment of ERDs. This can be achieved via four stages including: (i) the production of genetically engineered human CPP-mediated PT of interest, since the corresponding native protein either is missing or is mutated in the erythroid progenitor cell (ErPCs) or mature erythrocytes of patients; (ii) isolation of target cells from the peripheral blood of the selected patients; (iii) ex vivo transduction of cells with the CPP-mediated PT of interest; and (iv) re-administration of the successfully transduced cells back into the same patients.

Flavin conjugates for delivery of peptide nucleic acids

Oligonucleotides and their analogues, such as peptide nucleic acids (PNAs), can be used in chemical strategies to artificially control gene expression. Inefficient cellular uptake and inappropriate cellular localization still remain obstacles in biological applications, however, especially for PNAs. Here we demonstrate that conjugation of PNAs to flavin resulted in efficient internalization into cells through an endocytic pathway. The flavin-PNAs exhibited antisense activity in the sub-micromolar range, in the context of a treatment facilitating endosomal escape. Increased endosomal release of flavin conjugates into the cytoplasm and/or nucleus was shown by chloroquine treatment and also--when the flavin-PNA was conjugated to rhodamine, a mild photosensitizer--upon light irradiation. In conclusion, an isoalloxazine moiety can be used as a carrier and attached to a cargo biomolecule, here a PNA, for internalization and functional cytoplasmic/nuclear delivery. Our findings could be useful for further design of PNAs and other oligonucleotide analogues as potent antisense agents.

Potential vaccines and post-exposure treatments for filovirus infections

Viruses of the family Filoviridae represent significant health risks as emerging infectious diseases as well as potentially engineered biothreats. While many research efforts have been published offering possibilities toward the mitigation of filoviral infection, there remain no sanctioned therapeutic or vaccine strategies. Current progress in the development of filovirus therapeutics and vaccines is outlined herein with respect to their current level of testing, evaluation, and proximity toward human implementation, specifically with regard to human clinical trials, nonhuman primate studies, small animal studies, and in vitro development. Contemporary methods of supportive care and previous treatment approaches for human patients are also discussed.

Cell-penetrating properties of the transactivator of transcription and polyarginine (R9) peptides, their conjugative effect on nanoparticles and the prospect of conjugation with arsenic trioxide

Cell-penetrating peptides (CPPs) are short chains of amino acids with the distinct ability to cross cell plasma membranes. They are usually between seven and 30 residues in length. The mechanism of action is still a highly debated subject among researchers; it seems that a commonality between all CPPs is the presence of positively charged residues within the amino acid chain. Polyarginine and the transactivator of transcription peptide are two widely used CPPs. One distinct application of these CPPs is the ability to further enhance the therapeutic properties of a range of different agents. One group of agents of particular importance are nanoparticles (NPs). Most NPs have no mechanism for cellular uptake. Hence, by conjugating CPPs to NPs, the amount of NPs taken up by cells can be increased, and therefore, the therapeutic benefits can be maximized. Some examples of this will be explored further in this review. In addition to CPPs, the concept of conjugation with the anticancer drug arsenic trioxide is reviewed and the prospect of transactivator of transcription-conjugated arsenic trioxide albumin microspheres is also discussed. Recent locked nucleic acid technology to stabilize nucleotides (RNA or DNA) aptamer complexes able to target cancer cells more specifically and selectively to kill tumour cells and spare normal body cells. NPs tagged with modified locked nucleic acid-aptamers have the potential to kill cancer cells more specifically and effectively while sparing normal cells.

The chemistry and biology of oligonucleotide conjugates

Short DNA or RNA oligonucleotides have tremendous potential as therapeutic agents. Because of their ability to engage in Watson-Crick base pairing, they can interact with mRNA or pre-mRNA targets with high selectivity. As a result, they could precisely manipulate gene expression. This possibility has engendered extensive efforts to develop oligonucleotides as drugs, and many candidates are already in clinical trials. However, a major impediment to the maturation of this field of oligonucleotide-based therapeutics remains: these relatively large and often highly charged molecules don't easily cross cellular membranes, making it difficult for them to reach their sites of action in the cytosol or nucleus. In this Account, we summarize some basic features of the biology of antisense and siRNA oligonucleotides. We then discuss chemical conjugation as an approach to improving the intracellular delivery and therapeutic potential of these agents. Instead of focusing on the details of conjugation chemistry, we emphasize the pharmacological ramifications of oligonucleotide conjugates. In one important approach to improving delivery and efficacy, researchers have conjugated oligonucleotides with ligands designed to bind to particular receptors and thus provide specific interactions with cells. In another strategy, researchers have coupled antisense or siRNA with agents such as cell penetrating peptides that are designed to provoke escape of the conjugate from intracellular vesicular compartments. Although both of these strategies have had some success, further research is needed before oligonucleotide conjugates can find an important place in human therapeutics.

Enhanced cell uptake of superparamagnetic iron oxide nanoparticles through direct chemisorption of FITC-Tat-PEG₆₀₀-b-poly(glycerol monoacrylate)

Magnetic nanoparticles (MNPs) functionalized with specific ligands are emerging as a highly integrated platform for cancer targeting, drug delivery, and magnetic resonance imaging applications. In this study, we describe a multifunctional magnetic nanoparticle system (FITC-Tat MNPs) consisting of a fluorescently labeled cell penetrating peptide (FITC-Tat peptide), a biocompatible block copolymer PEG(600)-b-poly(glycerol monoacrylate) (PEG(600)-b-PGA), and a superparamagnetic iron oxide (SPIO) nanoparticle core. The particles were prepared by direct chemisorption of PEG(600)-b-PGA conjugated with FITC-Tat peptide on the SPIO nanoparticles. FITC-MNPs without Tat were prepared for comparison. Flow cytometry assays revealed significantly higher uptake of FITC-Tat MNPs compared to FITC-MNPs in Caco-2 cells. These results were confirmed using confocal laser scanning microscopy (LSCM), which further demonstrated that the FITC-Tat MNPs accumulated in the cytoplasm and nucleus while the FITC-MNPs were localized in the cell membrane compartments. The FITC-Tat MNPs did not exhibit observable cytotoxicity in MTS assays.

Assaying macrophage activity in a murine model of inflammatory bowel disease using fluorine-19 MRI

Macrophages have an important role in the pathogenesis of most chronic inflammatory diseases. A means of non-invasively quantifying macrophage migration would contribute significantly towards our understanding of chronic inflammatory processes and aid the evaluation of novel therapeutic strategies. We describe the use of a perfluorocarbon tracer reagent and in vivo (19)F magnetic resonance imaging (MRI) to quantify macrophage burden longitudinally. We apply these methods to evaluate the severity and three-dimensional distribution of macrophages in a murine model of inflammatory bowel disease (IBD). MRI results were validated by histological analysis, immunofluorescence and quantitative real-time polymerase chain reaction. Selective depletion of macrophages in vivo was also performed, further validating that macrophage accumulation of perfluorocarbon tracers was the basis of (19)F MRI signals observed in the bowel. We tested the effects of two common clinical drugs, dexamethasone and cyclosporine A, on IBD progression. Whereas cyclosporine A provided mild therapeutic effect, unexpectedly dexamethasone enhanced colon inflammation, especially in the descending colon. Overall, (19)F MRI can be used to evaluate early-stage inflammation in IBD and is suitable for evaluating putative therapeutics. Due to its high macrophage specificity and quantitative ability, we envisage (19)F MRI having an important role in evaluating a wide range of chronic inflammatory conditions mediated by macrophages.

Cell penetrating peptides in the delivery of biopharmaceuticals

The cell membrane is a highly selective barrier. This limits the cellular uptake of molecules including DNA, oligonucleotides, peptides and proteins used as therapeutic agents. Different approaches have been employed to increase the membrane permeability and intracellular delivery of these therapeutic molecules. One such approach is the use of Cell Penetrating Peptides (CPPs). CPPs represent a new and innovative concept, which bypasses the problem of bioavailability of drugs. The success of CPPs lies in their ability to unlock intracellular and even intranuclear targets for the delivery of agents ranging from peptides to antibodies and drug-loaded nanoparticles. This review highlights the development of cell penetrating peptides for cell-specific delivery strategies involving biomolecules that can be triggered spatially and temporally within a cell transport pathway by change in physiological conditions. The review also discusses conjugations of therapeutic agents to CPPs for enhanced intracellular delivery and bioavailability that are at the clinical stage of development.

In vivo Noninvasive Small Animal Molecular Imaging

The remarkable efforts that are made on molecular imaging technologies demonstrate its potential importance and range of applications. The generation of disease-specific animal models, and the developments of target-specific probes and genetically encoded reporters are another important component. Continued improvements in the instrumentation, the identification of novel targets and genes, and the availability of improved imaging probes should be made. Multimodal imaging probes should provide easier transitions between laboratory studies, including small animal studies and clinical applications. Here, we reviewed basic strategies of noninvasive in vivo imaging methods in small animals to introducing the concept of molecular imaging.

Exploring the role of chirality in nucleic acid recognition

The study of the base-pairing properties of nucleic acids with sugar moieties in the backbone belonging to the L-series (β-L-DNA, β-L-RNA, and their analogs) are reviewed. The major structural factors underlying the formation of stable heterochiral complexes obtained by incorporation of modified nucleotides into natural duplexes, or by hybridization between homochiral strands of opposite sense of chirality are highlighted. In addition, the perspective use of L-nucleic acids as candidates for various therapeutic applications, or as tools for both synthetic biology and etiology-oriented investigations on the structure and stereochemistry of natural nucleic acids is discussed.

Overview of alternative oligonucleotide chemistries for exon skipping

The chemistry of the oligonucleotide backbone is crucial to obtaining high activity in vivo in exon skipping applications. Apart from the ability to bind strongly and sequence-specifically to pre-mRNA targets, the type of backbone also influences cell delivery, in vivo pharmacology, bio-distribution, toxicology, and ultimately the therapeutic use in humans. Reviewed here are classes of oligonucleotide commonly used for exon skipping applications, namely negatively charged backbones typified by RNA analogues having 2'-O-substitution and a phosphorothioate linkage and charge-neutral backbones such as PNA and PMO. Also discussed are peptide conjugates of PNA and PMO that enhance cellular and in vivo delivery and their potential for drug development. Finally, the prospects for development of other analogue types in exon skipping applications are outlined.

Synthetic antisense oligodeoxynucleotides to transiently suppress different nucleus- and chloroplast-encoded proteins of higher plant chloroplasts

Selective inhibition of gene expression by antisense oligodeoxynucleotides (ODNs) is widely applied in gene function analyses; however, experiments with ODNs in plants are scarce. In this work, we extend the use of ODNs in different plant species, optimizing the uptake, stability, and efficiency of ODNs with a combination of molecular biological and biophysical techniques to transiently inhibit the gene expression of different chloroplast proteins. We targeted the nucleus-encoded phytoene desaturase (pds) gene, encoding a key enzyme in carotenoid biosynthesis, the chlorophyll a/b-binding (cab) protein genes, and the chloroplast-encoded psbA gene, encoding the D1 protein. For pds and psbA, the in vivo stability of ODNs was increased by phosphorothioate modifications. After infiltration of ODNs into juvenile tobacco (Nicotiana benthamiana) leaves, we detected a 25% to 35% reduction in mRNA level and an approximately 5% decrease in both carotenoid content and the variable fluorescence of photosystem II. In detached etiolated wheat (Triticum aestivum) leaves, after 8 h of greening, the mRNA level, carotenoid content, and variable fluorescence were inhibited up to 75%, 25%, and 20%, respectively. Regarding cab, ODN treatments of etiolated wheat leaves resulted in an up to 59% decrease in the amount of chlorophyll b, a 41% decrease of the maximum chlorophyll fluorescence intensity, the cab mRNA level was reduced to 66%, and the protein level was suppressed up to 85% compared with the control. The psbA mRNA and protein levels in Arabidopsis (Arabidopsis thaliana) leaves were inhibited by up to 85% and 72%, respectively. To exploit the potential of ODNs for photosynthetic genes, we propose molecular design combined with fast, noninvasive techniques to test their functional effects.

Biophysical and cellular-uptake properties of mixed-sequence pyrrolidine-amide oligonucleotide mimics

Previously we introduced the positively charged pyrrolidine-amide oligonucleotide mimics (POM), which possess a pyrrolidine ring and amide linkage in place of the sugar-phosphodiester backbone of natural nucleic acids. Short POM homo-oligomers have shown promising DNA and RNA recognition properties. However, to better understand the properties of POM and to assess their potential for use as modulators of gene expression and bioanalytical or diagnostic tools, more biologically relevant, longer, mixed-sequence oligomers need to be studied. In light of this, several mixed-sequence POM oligomers were synthesised, along with fluorescently labelled POM oligomers and a POM-peptide conjugate. UV thermal denaturation showed that mixed-sequence POMs hybridise to DNA and RNA with high affinity but slow rates of association and dissociation. The sequence specificity, influence of terminal amino acids, and the effect of pH and ionic strength on the DNA and RNA hybridisation properties of POM were extensively investigated. In addition, isothermal titration calorimetry (ITC) was used to investigate the thermodynamic parameters of the binding of a POM-peptide conjugate to DNA. Cellular uptake experiments have also shown that a fluorescently labelled POM oligomer is taken up into HeLa cells. These findings demonstrate that POM has the potential for use in a variety of applications, alongside other modified nucleic acids developed to date, such as peptide nucleic acids (PNA) and phosphoramidate morpholino oligomers (PMO).

An activated triple bond linker enables 'click' attachment of peptides to oligonucleotides on solid support

A general procedure, based on a new activated alkyne linker, for the preparation of peptide-oligonucleotide conjugates (POCs) on solid support has been developed. With this linker, conjugation is effective at room temperature (RT) in millimolar concentration and submicromolar amounts. This is made possible since the use of a readily attachable activated triple bond linker enhances the Cu(I) catalyzed 1,3-dipolar cycloaddition ('click' reaction). The preferred scheme for conjugate preparation involves sequential conjugation to oligonucleotides on solid support of (i) an H-phosphonate-based aminolinker; (ii) the triple bond donor p-(N-propynoylamino)toluic acid (PATA); and (iii) azido-functionalized peptides. The method gives conversion of oligonucleotide to the POC on solid support, and only involves a single purification step after complete assembly. The synthesis is flexible and can be carried out without the need for specific automated synthesizers since it has been designed to utilize commercially available oligonucleotide and peptide derivatives on solid support or in solution. Methodology for the ready conversion of peptides into 'clickable' azidopeptides with the possibility of selecting either N-terminus or C-terminus connection also adds to the flexibility and usability of the method. Examples of synthesis of POCs include conjugates of oligonucleotides with peptides known to be membrane penetrating and nuclear localization signals.

Cellular uptake and intracellular trafficking of antisense and siRNA oligonucleotides

Significant progress is being made concerning the development of oligonucleotides as therapeutic agents. Studies with antisense, siRNA, and other forms of oligonucleotides have shown promise in cellular and animal models and in some clinical studies. Nonetheless, our understanding of how oligonucleotides function in cells and tissues is really quite limited. One major issue concerns the modes of uptake and intracellular trafficking of oligonucleotides, whether as "free" molecules or linked to various delivery moieties such as nanoparticles or targeting ligands. In this review, we examine the recent literature on oligonucleotide internalization and subcellular trafficking in the context of current insights into the basic machinery for endocytosis and intracellular vesicular traffic.

Cell-penetrating peptides-based strategies for the delivery of splice redirecting antisense oligonucleotides

Progress in our understanding of the molecular pathogenesis of human malignancies has provided therapeutic targets amenable to oligonucleotide (ON)-based strategies. Antisense ON-mediated splicing regulation in particular offers promising prospects since the majority of human genes undergo alternative splicing and since splicing defects have been found in many diseases. However, their implementation has been hampered so far by the poor bioavailability of nucleic acids-based drugs. Cell-penetrating peptides (CPPs) now appear as promising non-viral delivery vector for non-permeant biomolecules. We describe here new CPPs allowing the delivery of splice redirecting steric-block ON using either chemical conjugation or non-covalent complexation. We also describe a convenient and robust splice redirecting assay which allows the quantitative assessment of ON nuclear delivery.

Influence of the metal center and linker on the intracellular distribution and biological activity of organometal-peptide conjugates

Organometallic complexes conjugated to cell-penetrating peptides (CPPs) are promising systems for diagnostic imaging and therapeutic applications in human medicine. Recently, we reported on the synthesis of cymantrene(CpMn(CO)(3))-CPP conjugates with biological activity on different cancer cell lines. However, the precise mechanism of cytotoxicity remained elusive in these studies. To investigate the role of the metal center and the linker between the CpM(CO)(3) moiety and the peptide, a number of derivatives with manganese replaced by rhenium and the keto linker originally used substituted by a methylene group were prepared and fully characterized by (1)H NMR spectroscopy, infrared spectroscopy, electrospray ionization mass spectrometry, and elemental analysis as well as X-ray structure determination. The organometal-peptide conjugates as well as carboxyfluorescein-labeled derivatives thereof were prepared by solid-phase peptide synthesis, purified by high-performance liquid chromatography, and analyzed by mass spectrometry. Fluorescence microscopy studies of MCF-7 human breast cancer cells revealed an efficient cellular uptake and pronounced nuclear localization of the bioconjugates with the methylene linker compared with systems with the keto group. In addition, the latter also showed a higher cytotoxicity. In contrast, the variation of the metal center from manganese to rhenium had a negligible effect. The structure-activity relationships determined in the present work will aid in the further tuning of the biological activity of organometal-peptide conjugates.

Arginine-rich cell-penetrating peptide dramatically enhances AMO-mediated ATM aberrant splicing correction and enables delivery to brain and cerebellum

Antisense morpholino oligonucleotides (AMOs) can reprogram pre-mRNA splicing by complementary binding to a target site and regulating splice site selection, thereby offering a potential therapeutic tool for genetic disorders. However, the application of this technology into a clinical scenario has been limited by the low correction efficiency in vivo and inability of AMOs to efficiently cross the blood brain barrier and target brain cells when applied to neurogenetic disorders such as ataxia-telangiecatasia (A-T). We previously used AMOs to correct subtypes of ATM splicing mutations in A-T cells; AMOs restored up to 20% of the ATM protein and corrected the A-T cellular phenotype. In this study, we demonstrate that an arginine-rich cell-penetrating peptide, (RXRRBR)(2)XB, dramatically improved ATM splicing correction efficiency when conjugated with AMOs, and almost fully corrected aberrant splicing. The restored ATM protein was close to normal levels in cells with homozygous splicing mutations, and a gene dose effect was observed in cells with heterozygous mutations. A significant amount of the ATM protein was still detected 21 days after a single 5 µm treatment. Systemic administration of an fluorescein isothiocyanate-labeled (RXRRBR)(2)XB-AMO in mice showed efficient uptake in the brain. Fluorescence was evident in Purkinje cells after a single intravenous injection of 60 mg/kg. Furthermore, multiple injections significantly increased uptake in all areas of the brain, notably in cerebellum and Purkinje cells, and showed no apparent signs of toxicity. Taken together, these results highlight the therapeutic potential of (RXRRBR)(2)XB-AMOs in A-T and other neurogenetic disorders.

Insights into the cellular trafficking of splice redirecting oligonucleotides complexed with chemically modified cell-penetrating peptides

Conjugates of cell-penetrating peptides (CPP) and splice redirecting oligonucleotides (ON) display clinical potential as attested by in vivo experimentation in murine models of Duchenne muscular dystrophy. However, micromolar concentrations of these conjugates are required to obtain biologically relevant responses as a consequence of extensive endosomal sequestration following endocytosis. Recent work from our group has demonstrated that appending stearic acid to CPPs increases their efficiency and that the inclusion of pH titrable entities leads to further improvement. Moreover, these modified CPPs form non covalent complexes with charged ON analogs or siRNAs, which allows decreasing the concentrations of ONs by nearly one log. These modified CPPs and the parent peptides have been compared here in the same in vitro model in terms of cell uptake, trafficking and splicing redirection activity. The increased splicing redirection activity of our modified CPPs cannot be explained by differences in cell uptake but rather by their enhanced ability to escape from endocytic vesicles. Accordingly, a clear correlation between membrane destabilizing activity and splicing redirection was observed using a liposome leakage assay. Studies of cellular trafficking for the most active PF6:ON complexes indicate uptake by clathrin-mediated endocytosis using either FACS cell uptake or a splicing redirection functional assay. Acidification of intracellular vesicles and membrane potential were found important for splicing redirection but not for cell uptake. These results do confirm that the increased potency of PF6:ON complexes is not due to the use of a non endocytic route of cell internalization as proposed for some CPPs.

Integrin targeted delivery of gene therapeutics

Integrins have become key targets for molecular imaging and for selective delivery of anti-cancer agents. Here we review recent work concerning the targeted delivery of antisense and siRNA oligonucleotides via integrins. A variety of approaches have been used to link oligonucleotides to ligands capable of binding integrins with high specificity and affinity. This includes direct chemical conjugation, incorporating oligonucleotides into lipoplexes, and use of various polymeric nanocarriers including dendrimers. The ligand-oligonucleotide conjugate or complex associates selectively with the integrin, followed by internalization into endosomes and trafficking through subcellular compartments. Escape of antisense or siRNA from the endosome to the cytosol and nucleus may come about through endogenous trafficking mechanisms, or because of membrane disrupting capabilities built into the conjugate or complex. Thus a variety of useful strategies are available for using integrins to enhance the pharmacological efficacy of therapeutic oligonucleotides.

Design, synthesis and characterization of a new anionic cell-penetrating peptide: SAP(E)

Cell-penetrating peptides (CPPs) are powerful tools to transport cell-impermeable cargoes into the cytoplasm without damaging the cell membrane. The vast majority of these peptides described to date share several features, among others, they are positively charged at physiological pH. In several cases a clear correlation between an increasing number of positive charges and internalization properties has been reported. Here, we describe what, to the best of our knowledge, is the first anionic CPP. This new compound SAP(E) internalizes into a range of cell lines with good efficiency and it shows low toxicity. We also report on the internalization mechanism. The discovery of this new class of CPP opens the way to the intracellular delivery of new molecular cargoes.

Synthesis and splice-redirecting activity of branched, arginine-rich peptide dendrimer conjugates of peptide nucleic acid oligonucleotides

Arginine-rich cell-penetrating peptides have found excellent utility in cell and in vivo models for enhancement of delivery of attached charge-neutral PNA or PMO oligonucleotides. We report the synthesis of dendrimeric peptides containing 2- or 4-branched arms each having one or more R-Ahx-R motifs and their disulfide conjugation to a PNA705 splice-redirecting oligonucleotide. Conjugates were assayed in a HeLa pLuc705 cell assay for luciferase up-regulation and splicing redirection. Whereas 8-Arg branched peptide−PNA conjugates showed poor activity compared to a linear (R-Ahx-R)₄−PNA conjugate, 2-branched and some 4-branched 12 and 16 Arg peptide−PNA conjugates showed activity similar to that of the corresponding linear peptide−PNA conjugates. Many of the 12- and 16-Arg conjugates retained significant activity in the presence of serum. Evidence showed that biological activity in HeLa pLuc705 cells of the PNA conjugates of branched and linear (R-Ahx-R) peptides is associated with an energy-dependent uptake pathway, predominantly clathrin-dependent, but also with some caveolae dependence.

Cell-penetrating peptide induces leaky fusion of liposomes containing late endosome-specific anionic lipid

Cationic cell-penetrating peptides (CPPs) are a promising vehicle for the delivery of macromolecular drugs. Although many studies have indicated that CPPs enter cells by endocytosis, the mechanisms by which they cross endosomal membranes remain elusive. On the basis of experiments with liposomes, we propose that CPP escape into the cytosol is based on leaky fusion (i.e., fusion associated with the permeabilization of membranes) of the bis(monoacylglycero)phosphate (BMP)-enriched membranes of late endosomes. In our experiments, prototypic CPP HIV-1 TAT peptide did not interact with liposomes mimicking the outer leaflet of the plasma membrane, but it did induce lipid mixing and membrane leakage as it translocated into liposomes mimicking the lipid composition of late endosome. Both membrane leakage and lipid mixing depended on the BMP content and were promoted at acidic pH, which is characteristic of late endosomes. Substitution of BMP with its structural isomer, phosphatidylglycerol (PG), significantly reduced both leakage of the aqueous probe from liposomes and lipid mixing between liposomes. Although affinity of binding to TAT was similar for BMP and PG, BMP exhibited a higher tendency to support the inverted hexagonal phase than PG. Finally, membrane leakage and peptide translocation were both inhibited by inhibitors of lipid mixing, further substantiating the hypothesis that cationic peptides cross BMP-enriched membranes by inducing leaky fusion between them.

Cellular delivery and antisense effects of peptide nucleic acid conjugated to polyethyleneimine via disulfide linkers

Peptide nucleic acid (PNA) is potentially an attractive antisense and antigene agent for which more efficient cellular delivery systems are still warranted. The cationic polymer polyethylenimine (PEI) is commonly used for cellular transfection of DNA and RNA complexes, but is not readily applicable for PNA due to the (inherent) charge neutrality of PNA. However, PEI could function as an efficient scaffold for PNA via chemical conjugation. Accordingly, we modified PEI with the amine-reactive heterobifunctional linker agent N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) (with and without a PEG moiety) and further reacted this with a cysteine PNA. The level of modification was determined spectrophotometrically with high accuracy, and the PNA transfection efficiency of the conjugates was evaluated in an antisense luciferase splice-correction assay using HeLa pLuc705 cells. We find that PEI is an efficient vector for PNA delivery yielding significantly higher (up to 10-fold) antisense activity than an analogous PNA-octaarginine conjugate, even in the presence of chloroquine, which only slightly enhances the PEI-PNA activity. The PEI-PEG conjugates are preferred due to lower acute cellular toxicity. Finally, the method can be easily modified to allow for co-conjugation of other small molecules in a high-throughput screening assay that does not require a purification step.

Efficiency of cellular delivery of antisense peptide nucleic acid by electroporation depends on charge and electroporation geometry

Electroporation is potentially a very powerful technique for both in vitro cellular and in vivo drug delivery, particularly relating to oligonucleotides and their analogs for genetic therapy. Using a sensitive and quantitative HeLa cell luciferase RNA interference mRNA splice correction assay with a functional luciferase readout, we demonstrate that parameters such as peptide nucleic acid (PNA) charge and the method of electroporation have dramatic influence on the efficiency of productive delivery. In a suspended cell electroporation system (cuvettes), a positively charged PNA (+8) was most efficiently transferred, whereas charge neutral PNA was more effective in a microtiter plate electrotransfer system for monolayer cells. Surprisingly, a negatively charged (-23) PNA did not show appreciable activity in either system. Findings from the functional assay were corroborated by pulse parameter variations, polymerase chain reaction, and confocal microscopy. In conclusion, we have found that the charge of PNA and electroporation system combination greatly influences the transfer efficiency, thereby illustrating the complexity of the electroporation mechanism.

A non-covalent peptide-based strategy for ex vivo and in vivo oligonucleotide delivery

The dramatic acceleration in identification of new nucleic acid-based therapeutic molecules such as short interfering RNA (siRNA) and peptide-nucleic acid (PNA) analogues has provided new perspectives for therapeutic targeting of specific genes responsible for pathological disorders. However, the poor cellular uptake of nucleic acids together with the low permeability of the cell membrane to negatively charged molecules remain major obstacles to their clinical development. Several non-viral strategies have been proposed to improve the delivery of synthetic short oligonucleotides both in cultured cells and in vivo. Cell-penetrating peptides constitute very promising tools for non-invasive cellular import of oligonucleotides and analogs. We recently described a non-covalent strategy based on short amphiphatic peptides (MPG8/PEP3) that have been successfully applied ex vivo and in vivo for the delivery of therapeutic siRNA and PNA molecules. PEP3 and MPG8 form stable nanoparticles with PNA analogues and siRNA, respectively, and promote their efficient cellular uptake, independently of the endosomal pathway, into a wide variety of cell lines, including primary and suspension lines, without any associated cytotoxicity. This chapter describes easy-to-handle protocols for the use of MPG-8 or PEP-3-nanoparticle technologies for PNA and siRNA delivery into adherent and suspension cell lines as well as in vivo into cancer mouse models.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Classes and prediction of cell-penetrating peptides

The classical view on how peptides enter cells has been changed due to the development in the research field of cell-penetrating peptides (CPPs). During the last 15 years, more than 100 peptide sequences have been published to enter cells and also to bring different biological cargoes with them. Here, we present an overview of CPPs, mainly trying to analyze their common properties yielding the prediction of their cell-penetrating properties. Furthermore, examples of recent research, ideas on classification and uptake mechanisms, as well as a summary of the therapeutic potential of CPPs are presented.

The role of carrier size in the pharmacodynamics of antisense and siRNA oligonucleotides

Effective therapeutic utilization of antisense and siRNA oligonucleotides represents a major challenge to drug delivery science. Although many strategies and technologies have been applied to oligonucleotide delivery, a key issue remains the role of molecular or carrier size. In this brief review, we address some of the size-related parameters that regulate the biodistribution of oligonucleotides. We also reprise several recent studies that have examined the inter-relationship of size and shape in influencing delivery.

Inhibition of influenza virus infection in human airway cell cultures by an antisense peptide-conjugated morpholino oligomer targeting the hemagglutinin-activating protease TMPRSS2

Influenza A viruses constitute a major and ongoing global public health concern. Current antiviral strategies target viral gene products; however, the emergence of drug-resistant viruses highlights the need for novel antiviral approaches. Cleavage of the influenza virus hemagglutinin (HA) by host cell proteases is crucial for viral infectivity and therefore presents a potential drug target. Peptide-conjugated phosphorodiamidate morpholino oligomers (PPMO) are single-stranded-DNA-like antisense agents that readily enter cells and can act as antisense agents by sterically blocking cRNA. Here, we evaluated the effect of PPMO targeted to regions of the pre-mRNA or mRNA of the HA-cleaving protease TMPRSS2 on proteolytic activation and spread of influenza viruses in human Calu-3 airway epithelial cells. We found that treatment of cells with a PPMO (T-ex5) designed to interfere with TMPRSS2 pre-mRNA splicing resulted in TMPRSS2 mRNA lacking exon 5 and consequently the expression of a truncated and enzymatically inactive form of TMPRSS2. Altered splicing of TMPRSS2 mRNA by the T-ex5 PPMO prevented HA cleavage in different human seasonal and pandemic influenza A viruses and suppressed viral titers by 2 to 3 log(10) units, strongly suggesting that TMPRSS2 is responsible for HA cleavage in Calu-3 airway cells. The data indicate that PPMO provide a useful reagent for investigating HA-activating proteases and may represent a promising strategy for the development of novel therapeutics to address influenza infections.

Antiviral and neuroprotective role of octaguanidinium dendrimer-conjugated morpholino oligomers in Japanese encephalitis

Background

Japanese encephalitis (JE), caused by a mosquito-borne flavivirus, is endemic to the entire south-east Asian and adjoining regions. Currently no therapeutic interventions are available for JE, thereby making it one of the most dreaded encephalitides in the world. An effective way to counter the virus would be to inhibit viral replication by using anti-sense molecules directed against the viral genome. Octaguanidinium dendrimer-conjugated Morpholino (or Vivo-Morpholino) are uncharged anti-sense oligomers that can enter cells of living organisms by endocytosis and subsequently escape from endosomes into the cytosol/nuclear compartment of cells. We hypothesize that Vivo-Morpholinos generated against specific regions of 3′ or 5′ untranslated regions of JEV genome, when administered in an experimental model of JE, will have significant antiviral and neuroprotective effect.

Methodology/Principal Findings

Mice were infected with JEV (GP78 strain) followed by intraperitoneal administration of Morpholinos (5 mg/kg body weight) daily for up to five treatments. Survivability of the animals was monitored for 15 days (or until death) following which they were sacrificed and their brains were processed either for immunohistochemical staining or protein extraction. Plaque assay and immunoblot analysis performed from brain homogenates showed reduced viral load and viral protein expression, resulting in greater survival of infected animals. Neuroprotective effect was observed by thionin staining of brain sections. Cytokine bead array showed reduction in the levels of proinflammatory cytokines in brain following Morpholino treatment, which were elevated after infection. This corresponded to reduced microglial activation in brain. Oxidative stress was reduced and certain stress-related signaling molecules were found to be positively modulated following Morpholino treatment. In vitro studies also showed that there was decrease in infective viral particle production following Morpholino treatment.

Conclusions/Significance

Administration of Vivo-Morpholino effectively resulted in increased survival of animals and neuroprotection in a murine model of JE. Hence, these oligomers represent a potential antiviral agent that merits further evaluation.

Japanese encephalitis (JE) is caused by a flavivirus that is transmitted to humans by mosquitoes belonging to the Culex sp. The threat of JE looms over a vast geographical realm, encompassing approximately 10 billion people. The disease is feared because currently there are no specific antiviral drugs available. There have been reports where other investigators have shown that agents that block viral replication can be used as effective therapeutic countermeasures. Vivo-Morpholinos (MOs) are synthetically produced analogs of DNA or RNA that can be modified to bind with specific targeted regions in a genome. In this study the authors propose that in an animal model of JE, MOs specifically designed to bind with specific region of JE virus (JEV) genome, blocks virus production in cells of living organisms. This results in reduced mortality of infected animals. As the major target of JEV is the nerve cells, analysis of brain of experimental animals, post treatment with MOs, showed neuroprotection. Studies in cultured cells were also supportive of the antiviral role of the MOs. The potent anti-sense effect in animals and lack of obvious toxicity at the effective dosage make these MOs good research reagents with future therapeutic applications in JE.

In vivo biodistribution and efficacy of peptide mediated delivery

To transverse the plasma membrane and gain access to the cellular interior is one of the major obstacles for many novel pharmaceutical molecules. Since the late 1990s, cell-penetrating peptides (CPPs) have been utilized as transport vectors for a broad spectrum of 'biological cargoes', ranging from inert gold particles to multifaceted macromolecules such as proteins and plasmids. Numerous studies have shown that CPPs are efficient carriers for bioactive cargoes in vitro. However, even though CPPs are versatile transport vectors, this does not guarantee they can be developed into useful pharmaceutical molecules. Nevertheless, recent progress in the field has shown CPPs to be effective for in vivo delivery with retained biological activity of a wide variety of bioactive cargoes into virtually any mammalian tissue. This review will focus on recent developments and applications for CPP delivery and distribution in vivo.

Therapeutic potential of splice-switching oligonucleotides

Alternative splicing enables a single pre-messenger RNA transcript to yield multiple protein isoforms, making it a major contributor to the diversity of the proteome. While this process is essential for normal development, aberrations in alternative splicing are the cause of a multitude of human diseases. Methods for manipulating alternative splicing would thus be of therapeutic value. Chemically modified antisense oligonucleotides that alter alternative splicing by directing splice site selection have been developed to achieve this end. These splice-switching oligonucleotides (SSOs) have been applied to correct aberrant splicing, induce expression of a therapeutic splice variant, or induce expression of a novel therapeutic splice variant in a number of disease-relevant genes. Recently, in vivo efficacy of SSOs has been reported using animal disease models, as well as in results from the first clinical trial.

Noncovalent peptide-mediated delivery of chemically modified steric block oligonucleotides promotes splice correction: quantitative analysis of uptake and biological effect

Despite numerous encouraging reports in the literature, the efficiency of cell penetrating peptides (CPPs) in promoting cellular delivery of bioactive cargos is still limited. To extend our current understanding of the underlying limitations of such approaches, we performed quantitative uptake studies of different chemically modified (2'-O-methyl, LNA and PNA) steric block oligonucleotides, targeted against a mutated splice site inserted in a firefly luciferase reporter gene construct, applying the peptide carrier MPGalpha as a model system. The peptide formed stable noncovalent complexes with phosphorothioate oligonucleotide (PTO) and locked nucleic acid (LNA) modified oligonucleotides, whereas the neutral peptide nucleic acid (PNA) had to be hybridized to an unmodified DNA to allow for complex formation. Detailed quantitative uptake studies revealed comparable numbers of intracellular PTO and LNA oligonucleotides after peptide-mediated delivery. Surprisingly, the PTO derivative showed the strongest upregulation of reporter gene activity of about 100-fold followed by the PNA (40-fold) and LNA (10-fold). Electroporation and microinjection studies proved that delivery itself was not the limiting factor for the low activity of the LNA derivative. Maximal achievable reporter gene activity could be observed only after addition of chloroquine (CQ), indicative of an endosomal pathway involved. This is in line with nuclear microinjection experiments, which show that the minimal number of steric block molecules needed to trigger the observed reporter upregulation is about two orders of magnitude lower than determined after peptide or cationic lipid delivery.

Efficient inhibition of miR-155 function in vivo by peptide nucleic acids

MicroRNAs (miRNAs) play an important role in diverse physiological processes and are potential therapeutic agents. Synthetic oligonucleotides (ONs) of different chemistries have proven successful for blocking miRNA expression. However, their specificity and efficiency have not been fully evaluated. Here, we show that peptide nucleic acids (PNAs) efficiently block a key inducible miRNA expressed in the haematopoietic system, miR-155, in cultured B cells as well as in mice. Remarkably, miR-155 inhibition by PNA in primary B cells was achieved in the absence of any transfection agent. In mice, the high efficiency of the treatment was demonstrated by a strong overlap in global gene expression between B cells isolated from anti-miR-155 PNA-treated and miR-155-deficient mice. Interestingly, PNA also induced additional changes in gene expression. Our analysis provides a useful platform to aid the design of efficient and specific anti-miRNA ONs for in vivo use.

Building Cell Selectivity into CPP-Mediated Strategies

There is a pressing need for more effective and selective therapies for cancer and other diseases. Consequently, much effort is being devoted to the development of alternative experimental approaches based on selective systems, which are designed to be specifically directed against target cells. In addition, a large number of highly potent therapeutic molecules are being discovered. However, they do not reach clinical trials because of their low delivery, poor specificity or their incapacity to bypass the plasma membrane. Cell-penetrating peptides (CPPs) are an open door for cell-impermeable compounds to reach intracellular targets. Putting all these together, research is sailing in the direction of the design of systems with the capacity to transport new drugs into a target cell. Some CPPs show cell type specificity while others require modifications or form part of more sophisticated drug delivery systems. In this review article we summarize several strategies for directed drug delivery involving CPPs that have been reported in the literature.

Cell-Penetrating Peptides-Mechanisms of Cellular Uptake and Generation of Delivery Systems

The successful clinical application of nucleic acid-based therapeutic strategies has been limited by the poor delivery efficiency achieved by existing vectors. The development of alternative delivery systems for improved biological activity is, therefore, mandatory. Since the seminal observations two decades ago that the Tat protein, and derived peptides, can translocate across biological membranes, cell-penetrating peptides (CPPs) have been considered one of the most promising tools to improve non-invasive cellular delivery of therapeutic molecules. Despite extensive research on the use of CPPs for this purpose, the exact mechanisms underlying their cellular uptake and that of peptide conjugates remain controversial. Over the last years, our research group has been focused on the S4₁₃-PV cell-penetrating peptide, a prototype of this class of peptides that results from the combination of 13-amino-acid cell penetrating sequence derived from the Dermaseptin S4 peptide with the SV40 large T antigen nuclear localization signal. By performing an extensive biophysical and biochemical characterization of this peptide and its analogs, we have gained important insights into the mechanisms governing the interaction of CPPs with cells and their translocation across biological membranes. More recently, we have started to explore this peptide for the intracellular delivery of nucleic acids (plasmid DNA, siRNA and oligonucleotides). In this review we discuss the current knowledge of the mechanisms responsible for the cellular uptake of cell-penetrating peptides, including the S4₁₃-PV peptide, and the potential of peptide-based formulations to mediate nucleic acid delivery.