Is there a future for cell-penetrating peptides in oligonucleotide delivery?

Get out or die trying: Peptide- and protein-based endosomal escape of RNA therapeutics

RNA therapeutics offer great potential to transform the biomedical landscape, encompassing the treatment of hereditary conditions and the development of better vaccines. However, the delivery of RNAs into the cell is hampered, among others, by poor endosomal escape. This major hurdle is often tackled using special lipids, polymers, or protein-based delivery vectors. In this review, we will focus on the most prominent peptide- and protein-based endosomal escape strategies with focus on RNA drugs. We discuss cell penetrating peptides, which are still incorporated into novel transfection systems today to promote endosomal escape. However, direct evidence for enhanced endosomal escape by the action of such peptides is missing and their transfection efficiency, even in permissive cell culture conditions, is rather low. Endosomal escape by the help of pore forming proteins or phospholipases, on the other hand, allowed to generate more efficient transfection systems. These are, however, often hampered by considerable toxicity and immunogenicity. We conclude that the perfect enhancer of endosomal escape has yet to be devised. To increase the chances of success, any new transfection system should be tested under relevant conditions and guided by assays that allow direct quantification of endosomal escape.

Peptide-assembled nanoparticles targeting tumor cells and tumor microenvironment for cancer therapy

Tumor cells and corrupt stromal cells in the tumor microenvironment usually overexpress cancer-specific markers that are absent or barely detectable in normal cells, providing available targets for inhibiting the occurrence and development of cancers. It is noticeable that therapeutic peptides are emerging in cancer therapies and playing more and more important roles. Moreover, the peptides can be self-assembled and/or incorporated with polymeric molecules to form nanoparticles via non-covalent bond, which have presented appealing as well as enhanced capacities of recognizing targeted cells, responding to microenvironments, mediating internalization, and achieving therapeutic effects. In this review, we will introduce the peptide-based nanoparticles and their application advances in targeting tumor cells and stromal cells, including suppressive immune cells, fibrosis-related cells, and angiogenic vascular cells, for cancer therapy.

Cell-penetrating peptide-mediated delivery of therapeutic peptides/proteins to manage the diseases involving oxidative stress, inflammatory response and apoptosis

OBJECTIVES

Peptides and proteins represent great potential for modulating various cellular processes including oxidative stress, inflammatory response, apoptosis and consequently the treatment of related diseases. However, their therapeutic effects are limited by their inability to cross cellular barriers. Cell-penetrating peptides (CPPs), which can transport cargoes into the cell, could resolve this issue, as would be discussed in this review.

KEY FINDINGS

CPPs have been successfully exploited in vitro and in vivo for peptide/protein delivery to treat a wide range of diseases involving oxidative stress, inflammatory processes and apoptosis. Their in vivo applications are still limited due to some fundamental issues of CPPs, including nonspecificity, proteolytic instability, potential toxicity and immunogenicity.

SUMMARY

Totally, CPPs could potentially help to manage the diseases involving oxidative stress, inflammatory response and apoptosis by delivering peptides/proteins that could selectively reach proper intracellular targets. More studies to overcome related CPP limitations and confirm the efficacy and safety of this strategy are needed before their clinical usage.

Structural Modifications to the Internal Oligoguanidinium Transporter Uncover Two Potent Analogues that Effectively Deliver the Proapoptotic Peptide in Multiple Cancer Cell Lines

Efficient cytosolic delivery with serum-independent kinetics and low toxicity are the ultimate challenges towards the transformation of an antisense oligonucleotide or a therapeutic peptide to a suitable drug candidate for clinical trials. Most delivery vehicles falter on at least one of the above requirements, which hinders their potential in in vivo models as well. Our previous reports on internal guanidinium transporters (IGTs) have established the diversity of this particular class of molecule with the efficient delivery of antisense phosphorodiamidate morpholino oligonucleotides. In this paper, we report twenty IGTs with different types of evidence-backed structural modifications with different types of head-group linkage R, which significantly change the transfection, toxicity, and endosomal escape. Based on these three criteria, the analogues were sorted systematically to find the more promising IGTs, which were then further examined by LysoTracker studies. Finally, two analogues, with cholesteryl linkage (R = Chol) and pentafluorobenzyl linkage (R = PF Cbz), were selected for a proapoptotic peptide delivery as the final validation using a long-chain di-acid linker conjugation. Detailed mechanistic studies also revealed that the primary pathway of endocytosis is macropinocytosis, and that other pathways play different roles depending on the head group of the IGT. Since endocytosis pathways for entry depend on the nature of the cell line, we have shown the mechanistic variations in two cell lines for validation.

Potential of cell-penetrating peptides (CPPs) in delivery of antiviral therapeutics and vaccines

Viral infections are a great threat to human health. Currently, there are no effective vaccines and antiviral drugs against the majority of viral diseases, suggesting the need to develop novel and effective antiviral agents. Since the intracellular delivery of antiviral agents, particularly the impermeable molecules, such as peptides, proteins, and nucleic acids, are essential to exert their therapeutic effects, using a delivery system is highly required. Among various delivery systems, cell-penetrating peptides (CPPs), a group of short peptides with the unique ability of crossing cell membrane, offer great potential for the intracellular delivery of various biologically active cargoes. The results of numerous in vitro and in vivo studies with CPP conjugates demonstrate their promise as therapeutic agents in various medical fields including antiviral therapy. The CPP-mediated delivery of various antiviral agents including peptides, proteins, nucleic acids, and nanocarriers have been associated with therapeutic efficacy both in vitro and in vivo. This review describes various aspects of viruses including their biology, pathogenesis, and therapy and briefly discusses the concept of CPP and its potential in drug delivery. Particularly, it will highlight a variety of CPP applications in the management of viral infections.

Antisense oligonucleotides: absorption, distribution, metabolism, and excretion

INTRODUCTION

Antisense oligonucleotides (ASOs) have emerged as a promising novel drug modality that aims to address unmet medical needs. A record of six ASO drugs have been approved since 2016, and more candidates are in clinical development. ASOs are the most advanced class within the RNA-based therapeutics field.

AREAS COVERED

This review highlights the two major backbones that are currently used to build the most advanced ASO platforms - the phosphorodiamidate morpholino oligomers (PMOs) and the phosphorothioates (PSs). The absorption, distribution, metabolism, and excretion (ADME) properties of the PMO and PS platforms are discussed in detail.

EXPERT OPINION

Understanding the ADME properties of existing ASOs can foster further improvement of this cutting-edge therapy, thereby enabling researchers to safely develop ASO drugs and enhancing their ability to innovate.

ABBREVIATIONS

2'-MOE, 2'-O-methoxyethyl; 2'PS, 2 modified PS; ADME, absorption, distribution, metabolism, and excretion; ASO, antisense oligonucleotide; AUC, area under the curve; BNA, bridged nucleic acid; CPP, cell-penetrating peptide; CMV, cytomegalovirus; CNS, central nervous system; CYP, cytochrome P; DDI, drug-drug interaction; DMD, Duchenne muscular dystrophy; FDA, Food and Drug Administration; GalNAc3, triantennary N-acetyl galactosamine; IT, intrathecal; IV, intravenous; LNA, locked nucleic acid; mRNA, messenger RNA; NA, not applicable; PBPK, physiologically based pharmacokinetics; PD, pharmacodynamic; PK, pharmacokinetic; PMO, phosphorodiamidate morpholino oligomer; PMOplus, PMOs with positionally specific positive molecular charges; PPMO, peptide-conjugated PMO; PS, phosphorothioate; SC, subcutaneous; siRNA, small-interfering RNA; SMA, spinal muscular atrophy.

Progress in tumour-targeted drug delivery based on cell-penetrating peptides

Since the discovery of cell-penetrating peptides (CPP) in the 1980s, they have played a unique role in various fields owing to their excellent and unique cell membrane penetration function. In particular, in the treatment of tumours, CPPS have been used to deliver several types of 'cargos' to cancer cells. To address the insufficient targeting ability, non-selectivity, and blood instability, activatable cell-penetrating peptides, which can achieve targeted drug delivery in tumour treatment, enhance curative effects, and reduce toxicity have been developed. This study reviews the application of different cell-penetrating peptides in tumour-targeted delivery, overcoming multidrug resistance, organelle targeting, tumour imaging, and diagnosis, and summarises the different mechanisms of activatable cell-penetrating peptides in detail.

Non-covalent Encapsulation of siRNA with Cell-Penetrating Peptides

SiRNAs may act as selective and potent therapeutics, but poor deliverability in vivo is a limitation. Among the recently proposed vectors, cell-penetrating peptides (CPPs), also referred as protein transduction domains (PTDs), allow siRNA stabilization and increased cellular uptake. This chapter aims to guide scientists in the preparation and characterization of CPP-siRNA complexes, particularly the evaluation of novel CPPs variants for siRNA encapsulation and delivery. Herein, we present a collection of methods to determine CPP-siRNA interaction, encapsulation, stability, conformation, transfection, and silencing efficiency.

Internal Oligoguanidinium Transporter: Mercury-Free Scalable Synthesis, Improvement of Cellular Localization, Endosomal Escape, Mitochondrial Localization, and Conjugation with Antisense Morpholino for NANOG Inhibition to Induce Chemosensitization of Taxol in MCF-7 Cells

A nontoxic delivery vehicle is essential for the therapeutic applications of antisense phosphorodiamidate morpholino oligonucleotides (PMOs). Though guanidinium-rich or arginine-rich cellular transporter conjugated Vivo-PMO or PPMO has been developed for in vivo application, however, either their toxicity or stability has become an issue. Previously, we reported nonpeptidic internal guanidinium transporter (IGT) mediated delivery of PMO for gene silencing and got encouraging results. In this paper, we report the synthesis of IGT using a Hg-free method for scale up and N-terminal modification of IGT with a suitable hydrophobic or lipophilic group to improve the cell permeability, endosomal escape, and mitochondrial localization and to reduce toxicity in the MTT assay. For the delivery of PMO, IGT-PMO conjugate was synthesized to target NANOG in cells, a transcription factor required for cancer stem cell proliferation and embryonic development and is involved in many cancers. Our data shows IGT-PMO-facilitated NANOG inhibition, and thereby the prevention of EpCAM-N-Cadherin-Vimentin axis mediated epithelial to mesenchymal transition (EMT) in MCF-7 cells. Moreover, unlike taxol, NANOG inhibition influences the expression of stemness factor c-Myc, Hh-Gli signaling proteins, other cancer related factors, and their respective phenotypes in cancer cells. To the best of our knowledge, this is the first report to illustrate that the IGT-PMO-mediated NANOG inhibition increases the therapeutic potential of taxol and induces G0-G1 arrest in cancer cells to prevent cancer progression. However, it warrants further investigation in other cancer cells and preclinical platforms.

Polyarginine-Mediated siRNA Delivery: A Mechanistic Study of Intracellular Trafficking of PCL-R15/siRNA Nanoplexes

As a cell-penetrating peptide, polyarginine is widely used in drug delivery systems based on its membrane permeation ability. Previously, we developed the mPEG-PLA-b-polyarginine(R15) triblock copolymer, which exhibited a high siRNA delivery efficiency both in vitro and in vivo. As a continued effort, here the amphiphilic diblock polymer PCL-R15 was synthesized as a simplified model to further elucidate the structure-activity relationship of arginine-based amphiphilic polymers as siRNA delivery systems, and the cellular trafficking mechanisms of the PCL-R15/siRNA nanoplexes were investigated to understand the interaction patterns between the nanoplexes and cells. Compared to the R15/siRNA complexes, the introduction of PCL moiety was found to result in the stronger interactions with cells and the enhanced transfection efficiency after the formation of condensed nanoplexes. Caveolae-mediated endocytosis and clathrin-mediated endocytosis were major routes for the internalization of PCL-R15/siRNA nanoplexes. The intracellular release of siRNA from nanoplexes was confirmed by fluorescence resonance energy transfer assay. It was also noticed that the internalized PCL-R15/siRNA nanoplexes were transported through digestive routes and trapped in lysosomes, which may be the bottleneck for efficient siRNA delivery of PCL-R15/siRNA nanoplexes. This study investigated the relationship between the polymer structure of PCL-R15 and the cellular interaction patterns, which may render implications on the rational design of polyarginine-based siRNA delivery systems.

Cell penetrating peptides: the potent multi-cargo intracellular carriers

Introduction: Cell penetrating peptides (CPPs) known as protein translocation domains (PTD), membrane translocating sequences (MTS), or Trojan peptides (TP) are able to cross biological membranes without clear toxicity using different mechanisms, and facilitate the intracellular delivery of a variety of bioactive cargos. CPPs could overcome some limitations of drug delivery and combat resistant strains against a broad range of diseases. Despite delivery of different therapeutic molecules by CPPs, they lack cell specificity and have a short duration of action. These limitations led to design of combined cargo delivery systems and subsequently improvement of their clinical applications. Areas covered: This review covers all our studies and other researchers in different aspects of CPPs such as classification, uptake mechanisms, and biomedical applications. Expert opinion: Due to low cytotoxicity of CPPs as compared to other carriers and final degradation to amino acids, they are suitable for preclinical and clinical studies. Generally, the efficiency of CPPs was suitable to penetrate the cell membrane and deliver different cargos to specific intracellular sites. However, no CPP-based therapeutic approach has approved by FDA, yet; because there are some disadvantages for CPPs including short half-life in blood, and nonspecific CPP-mediated delivery to normal tissue. Thus, some methods were used to develop the functions of CPPs in vitro and in vivo including the augmentation of cell specificity by activatable CPPs, specific transport into cell organelles by insertion of corresponding localization sequences, incorporation of CPPs into multifunctional dendrimeric or liposomal nanocarriers to improve selectivity and efficiency especially in tumor cells.

Current Transport Systems and Clinical Applications for Small Interfering RNA (siRNA) Drugs

Small interfering RNAs (siRNAs) are an attractive new agent with potential as a therapeutic tool because of its ability to inhibit specific genes for many conditions, including viral infections and cancers. However, despite this potential, many challenges remain, including off-target effects, difficulties with delivery, immune responses, and toxicity. Traditional genetic vectors do not guarantee that siRNAs will silence genes in vivo. Rational design strategies, such as chemical modification, viral vectors, and non-viral vectors, including cationic liposomes, polymers, nanocarriers, and bioconjugated siRNAs, provide important opportunities to overcome these challenges. We summarize the results of research into vector delivery of siRNAs as a therapeutic agent from their design to clinical trials in ophthalmic diseases, cancers, respiratory diseases, and liver virus infections. Finally, we discuss the current state of siRNA delivery methods and the need for greater understanding of the requirements.

Super-resolution Imaging of Structure, Molecular Composition, and Stability of Single Oligonucleotide Polyplexes

The successful application of gene therapy relies on the development of safe and efficient delivery vectors. Cationic polymers such as cell-penetrating peptides (CPPs) can condense genetic material into nanoscale particles, called polyplexes, and induce cellular uptake. With respect to this point, several aspects of the nanoscale structure of polyplexes have remained elusive because of the difficulty in visualizing the molecular arrangement of the two components with nanometer resolution. This limitation has hampered the rational design of polyplexes based on direct structural information. Here, we used super-resolution imaging to study the structure and molecular composition of individual CPP-mRNA polyplexes with nanometer accuracy. We use two-color direct stochastic optical reconstruction microscopy (dSTORM) to unveil the impact of peptide stoichiometry on polyplex structure and composition and to assess their destabilization in blood serum. Our method provides information about the size and composition of individual polyplexes, allowing the study of such properties on a single polyplex basis. Furthermore, the differences in stoichiometry readily explain the differences in cellular uptake behavior. Thus, quantitative dSTORM of polyplexes is complementary to the currently used characterization techniques for understanding the determinants of polyplex activity in vitro and inside cells.

In vitro characterization of odorranalectin for peptide-based drug delivery across the blood-brain barrier

BACKGROUND

The use of siRNA-based gene silencing has been recently underscored as a potential therapeutic strategy for the treatment of neurological disorders. However, the stability of siRNA and other small molecule therapeutics is challenged by their intrinsic instability and limited passage across the blood-brain barrier (BBB). Based on these premises, our objective was to characterize/optimize odorranalectin (OL), a small non-immunogenic lectin-like peptide, as a carrier for targeted delivery across the BBB. For this purpose, 5(6)-carboxyfluorescein-conjugated OL and scramble peptide were synthesized, and then their BBB cellular internalization/trafficking and stability were characterized versus temperature, pH and serum content in the media in hCMEC/D3 cells as a model of BBB endothelium. Specifically, integrity of the internalized peptide in cell lysates was analyzed by LC/MS while cellular distribution and intracellular trafficking of OL was examined by fluorescence microscopy with early-late endosome (pHRodo Red^®) and lysosome (Lysotracker^®) markers.

RESULTS

Our data show that cellular uptake of OL increased linearly with the concentrations tested in this study at 37 °C and the uptake was two to threefolds higher when compared to scramble peptide. While there were no differences for scramble peptide, the uptake of OL decreased by 50% at 4 °C incubation (vs. 37 °C). No effects of pH were observed on endothelial uptake of OL. Immunofluorescence studies also indicated a significant cellular internalization of OL that remained intact (as evaluated by LC-MS/MS) and co-localized with endosomal, but not lysosome marker. Importantly, OL was found non-toxic to cells at all concentrations tested.

CONCLUSIONS

In summary, our data suggest the existence of a receptor-mediated transcytosis pathway for cellular uptake of OL at the BBB endothelium. However, in vivo studies will be needed to assess the siRNA loading capacity of OL and its trans-BBB transport efficiency for targeted delivery in the brain.

Current Development of siRNA Bioconjugates: From Research to the Clinic

Small interfering RNAs (siRNAs) acting via RNA interference mechanisms are able to recognize a homologous mRNA sequence in the cell and induce its degradation. The main problems in the development of siRNA-based drugs for therapeutic use are the low efficiency of siRNA delivery to target cells and the degradation of siRNAs by nucleases in biological fluids. Various approaches have been proposed to solve the problem of siRNA delivery in vivo (e.g., viruses, cationic lipids, polymers, nanoparticles), but all have limitations for therapeutic use. One of the most promising approaches to solve the problem of siRNA delivery to target cells is bioconjugation; i.e., the covalent connection of siRNAs with biogenic molecules (lipophilic molecules, antibodies, aptamers, ligands, peptides, or polymers). Bioconjugates are "ideal nanoparticles" since they do not need a positive charge to form complexes, are less toxic, and are less effectively recognized by components of the immune system because of their small size. This review is focused on strategies and principles for constructing siRNA bioconjugates for in vivo use.

Efficient in vivo direct conversion of fibroblasts into cardiomyocytes using a nanoparticle-based gene carrier

The reprogramming of induced cardiomyocytes (iCMs) has shown potential in regenerative medicine. However, in vivo reprogramming of iCMs is significantly inefficient, and novel gene delivery systems are required to more efficiently and safely induce in vivo reprogramming of iCMs for therapeutic applications in heart injury. In this study, we show that cationic gold nanoparticles (AuNPs) loaded with Gata4, Mef2c, and Tbx5 function as nanocarriers for cardiac reprogramming. The AuNP/GMT/PEI nanocomplexes show high reprogramming efficiency in human and mouse somatic cells with low cytotoxicity and direct conversion into iCMs without integrating factors into the genome. Importantly, AuNP/GMT/PEI nanocomplexes led to efficient in vivo conversion into cardiomyocytes after myocardial infarction (MI), resulting in the effective recovery of cardiac function and scar area. Taken together, these results show that the AuNP/GMT/PEI nanocarrier can be used to develop effective therapeutics for heart regeneration in cardiac disease patients.

Cell Penetration, Herbicidal Activity, and in-vivo-Toxicity of Oligo-Arginine Derivatives and of Novel Guanidinium-Rich Compounds Derived from the Biopolymer Cyanophycin

Oligo-arginines are thoroughly studied cell-penetrating peptides (CPPs, Figures 1 and 2). Previous in-vitro investigations with the octaarginine salt of the phosphonate fosmidomycin (herbicide and anti-malaria drug) have shown a 40-fold parasitaemia inhibition with P. falciparum, compared to fosmidomycin alone (Figure 3). We have now tested this salt, as well as the corresponding phosphinate salt of the herbicide glufosinate, for herbicidal activity with whole plants by spray application, hoping for increased activities, i.e. decreased doses. However, both salts showed low herbicidal activity, indicating poor foliar uptake (Table 1). Another pronounced difference between in-vitro and in-vivo activity was demonstrated with various cell-penetrating octaarginine salts of fosmidomycin: intravenous injection to mice caused exitus of the animals within minutes, even at doses as low as 1.4 μmol/kg (Table 2). The results show that use of CPPs for drug delivery, for instance to cancer cells and tissues, must be considered with due care. The biopolymer cyanophycin is a poly-aspartic acid containing argininylated side chains (Figure 4); its building block is the dipeptide H-βAsp-αArg-OH (H-Adp-OH). To test and compare the biological properties with those of octaarginines we synthesized Adp8-derivatives (Figure 5). Intravenouse injection of H-Adp8-NH2 into the tail vein of mice with doses as high as 45 μmol/kg causes no symptoms whatsoever (Table 3), but H-Adp8-NH2 is not cell penetrating (HEK293 and MCF-7 cells, Figure 6). On the other hand, the fluorescently labeled octamers FAM-(Adp(OMe))8-NH2 and FAM-(Adp(NMe2))8-NH2 with ester and amide groups in the side chains exhibit mediocre to high cell-wall permeability (Figure 6), and are toxic (Table 3). Possible reasons for this behavior are discussed (Figure 7) and corresponding NMR spectra are presented (Figure 8).

PEG-Peptide Inhibition of Scavenger Receptor Uptake of Nanoparticles by the Liver

PEGylated polylysine peptides represent a new class of scavenger receptor inhibitors that may find utility at inhibiting DNA nanoparticle uptake by Kupffer cells in the liver. PEG-peptides inhibit scavenger receptors in the liver by a novel mechanism involving in situ formation of albumin nanoparticles. The present study developed a new in vivo assay used to explore the structure-activity-relationships of PEG-peptides to find potent scavenger receptor inhibitors. Radio-iodinated PEG-peptides were dosed i.v. in mice and shown to saturate liver uptake in a dose-dependent fashion. The inhibition potency (IC₅₀) was dependent on both the length of a polylysine repeat and PEG molecular weight. PEG_30kda-Cys-Tyr-Lys₂₅ was confirmed to be a high molecular weight (33.5 kDa) scavenger receptor inhibitor with an IC₅₀ of 18 μM. Incorporation of multiple Leu residues improved potency, allowing a decrease in PEG MW and Lys repeat, resulting in PEG_5kda-Cys-Tyr-Lys-(Leu-Lys₄)₃-Leu-Lys that inhibited scavenger receptors with an IC₅₀ = 20 μM. A further decrease in PEG MW to 2 kDa increased potency further, resulting in a low molecular weight (4403 g/mol) PEG-peptide with an IC₅₀ of 3 μM. Optimized low molecular weight PEG-peptides also demonstrated potency when inhibiting the uptake of radio-iodinated DNA nanoparticles by the liver. This study demonstrates an approach to discover low molecular weight PEG-peptides that serve as potent scavenger receptor inhibitors to block nanoparticle uptake by the liver.

Covalent Strategies for Targeting Messenger and Non-Coding RNAs: An Updated Review on siRNA, miRNA and antimiR Conjugates

Oligonucleotide-based therapy has become an alternative to classical approaches in the search of novel therapeutics involving gene-related diseases. Several mechanisms have been described in which demonstrate the pivotal role of oligonucleotide for modulating gene expression. Antisense oligonucleotides (ASOs) and more recently siRNAs and miRNAs have made important contributions either in reducing aberrant protein levels by sequence-specific targeting messenger RNAs (mRNAs) or restoring the anomalous levels of non-coding RNAs (ncRNAs) that are involved in a good number of diseases including cancer. In addition to formulation approaches which have contributed to accelerate the presence of ASOs, siRNAs and miRNAs in clinical trials; the covalent linkage between non-viral vectors and nucleic acids has also added value and opened new perspectives to the development of promising nucleic acid-based therapeutics. This review article is mainly focused on the strategies carried out for covalently modifying siRNA and miRNA molecules. Examples involving cell-penetrating peptides (CPPs), carbohydrates, polymers, lipids and aptamers are discussed for the synthesis of siRNA conjugates whereas in the case of miRNA-based drugs, this review article makes special emphasis in using antagomiRs, locked nucleic acids (LNAs), peptide nucleic acids (PNAs) as well as nanoparticles. The biomedical applications of siRNA and miRNA conjugates are also discussed.

Spontaneous Membrane Translocating Peptides: The Role of Leucine-Arginine Consensus Motifs

We previously used an orthogonal high-throughput screen to select peptides that spontaneously cross synthetic lipid bilayers without bilayer disruption. Many of the 12-residue spontaneous membrane translocating peptides (SMTPs) selected from the library contained a 5-residue consensus motif, LRLLR in positions 5-9. We hypothesized that the conserved motif could be a necessary and sufficient minimal motif for translocation. To test this and to explore the mechanism of spontaneous membrane translocation, we synthesized seven arginine placement variants of LRLLRWC and compared their membrane partitioning, translocation, and perturbation to one of the parent SMTPs, called "TP2". Several motif variant peptides translocate into synthetic vesicles with rates that are similar to TP2. However, the peptide containing the selected motif, LRLLRWC, was not the fastest; sequence context is also important for translocation efficiency. Although none of these peptides permeabilize bilayers, the motif peptides translocate faster at higher peptide to lipid ratios, suggesting that bilayer perturbation and/or cooperative interactions are important for their translocation. On the other hand, TP2 translocates slower as its concentration is increased, suggesting that TP2 translocates as a monomer and is inhibited by lateral interactions in the membrane. TP2 and the LRLLR motif peptide induce lipid translocation, suggesting that lipids chaperone them across the bilayer. The other motif peptides do not induce lipid flip-flop, suggesting an alternate mechanism. Concatenated motifs translocate slower than the motifs alone. Variants of TP2 with shorter and longer arginine side-chain analogs translocate slower than TP2. In summary, these results suggest that multiple patterns of leucine and arginine can support spontaneous membrane translocation, and that sequence context is important for the contribution of the motifs. Because motifs do not make simple, additive contributions to spontaneous translocation, rational engineering of novel SMTPs will remain difficult, providing even more reason to pursue SMTP discovery with synthetic molecular evolution.

Anti-microRNA targeting using peptide-based nanocomplexes to inhibit differentiation of human pancreatic stellate cells

Aim: To develop novel peptide-based nanocomplexes (NCs) for delivery of anti-miRNA oligonucleotides to human-derived pancreatic stellate cells (hPSCs), precursors of cancer-associated fibroblasts. Materials & methods: NCs of anti-miRNA oligonucleotides and cell-penetrating peptides (different variants) were formed and characterized. The effects of anti-miR-199a delivery on hPSC differentiation and 3D heterospheroid formation were investigated. Results: Dimeric cell-penetrating peptide based NCs (NC-2) showed 130-fold higher uptake by hPSCs compared with monomer-based NCs (NC-1) and tenfold higher uptake compared with general fibroblasts and different pancreatic tumor cells. Interestingly, delivery of anti-miR-199a inhibited hPSC differentiation into cancer-associated fibroblasts and inhibited the size of 3D heterospheroids comprised of hPSCs and tumor cells. Conclusion: Our NCs present a highly efficient anti-miRNA delivery system to hPSCs to inhibit their protumorigenic activity.

Tandem Peptide Based on Structural Modification of Poly-Arginine for Enhancing Tumor Targeting Efficiency and Therapeutic Effect

The nonselectivity of cell penetrating peptides had greatly limited the application in systemic administration. By conjugating a dGR motif to the C-terminal of octa-arginine, the formed tandem peptide R8-dGR had been proved to specifically recognize both integrin αvβ3 and neuropilin-1 receptors. However, the positive charge of poly-arginine would still inevitably lead to rapid clearance in the circulation system. Therefore, in this study, we tried to reduce the positive charge of poly-arginine by decreasing the number of arginine, to thus achieve improved tumor targeting efficiency. We had designed several different Rx-dGR peptides (x = 4, 6, and 8) modified liposomes and investigated their tumor targeting and penetrating properties both in vitro and in vivo. Among all the liposomes, R6-dGR modified liposomes exhibited a long circulation time similar to that of PEGylated liposomes while they retained strong penetrating ability into both tumor cells and tumor tissues, and thus had displayed the most superior tumor targeting efficiency. Then, paclitaxel and indocyanine green coloaded liposomes were prepared, and R6-dGR modified coloaded liposomes also exhibited enhanced antitumor effect on C6 xenograft tumor bearing mice. Therefore, we suggest R6-dGR as a potential tumor targeting ligand with both strong penetrating ability and improved pharmacokinetic behavior, which could be further used for efficient antitumor therapy.

Penetration of Oxidized Carbon Nanospheres through Lipid Bilayer Membrane: Comparison to Graphene Oxide and Oxidized Carbon Nanotubes, and Effects of pH and Membrane Composition

Here we show that the ability of oxidized carbon particles to penetrate phospholipid bilayer membrane varies with the particle shapes, chemical functionalities on the particle surface, lipid compositions of the membrane and pH conditions. Among the similar surface charged oxidized carbon particles of spherical (oxidized carbon nanosphere, OCS), tubular (oxidized carbon nanotube, OCT), and sheet (oxidized graphene sheet, OGSh) morphologies, OCS possesses the highest levels of adhesion to lipid bilayer membrane and penetration into the cell-sized liposome. OCS preferably binds better to the disordered lipid bilayer membrane (consisting of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) as compared to the ordered membrane (consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and cholesterol). The process of OCS-induced leak on the membrane is pH responsive and most pronounced under an acidic condition. Covalently decorating the OCS's surface with poly(ethylene oxide) or (2-aminoethyl)trimethylammonium moieties decreases its ability to interact with the membrane. When used as carriers, OCSs can deliver curcumin into nucleus of A549 human lung cancer and human embryonic kidney cells, in contrast, curcumin molecules delivered by OCTs remain in the cytoplasm. OGShs cannot significantly enter cells and cannot induce noticeable cellular uptake of curcumin.

Polymers for siRNA Delivery: A Critical Assessment of Current Technology Prospects for Clinical Application

The number of polymer-based vectors for siRNA delivery in clinical trials lags behind other delivery strategies; however, the molecular architectures and chemical compositions available to polymers make them attractive candidates for further exploration. Polymer vectors are extensively investigated in academic laboratories worldwide with fundamental progress having recently been made in the areas of high-throughput screening, synthetic methods, cellular internalization, endosomal escape and computational prediction and analysis. This review assesses recent advances within the field and highlights relevant developments from within the complementary fields of nanotechnology and protein chemistry with the intent to propose future work that addresses key gaps within the current body of knowledge, potentially advancing the development of the next generation of polymeric vectors.

Evaluation of Therapeutic Oligonucleotides for Familial Amyloid Polyneuropathy in Patient-Derived Hepatocyte-Like Cells

Familial amyloid polyneuropathy (FAP) is caused by mutations of the transthyretin (TTR) gene, predominantly expressed in the liver. Two compounds that knockdown TTR, comprising a small interfering RNA (siRNA; ALN-TTR-02) and an antisense oligonucleotide (ASO; IONIS-TTRRx), are currently being evaluated in clinical trials. Since primary hepatocytes from FAP patients are rarely available for molecular analysis and commercial tissue culture cells or animal models lack the patient-specific genetic background, this study uses primary cells derived from urine of FAP patients. Urine-derived cells were reprogrammed to induced pluripotent stem cells (iPSCs) with high efficiency. Hepatocyte-like cells (HLCs) showing typical hepatic marker expression were obtained from iPSCs of the FAP patients. TTR mRNA expression of FAP HLCs almost reached levels measured in human hepatocytes. To assess TTR knockdown, siTTR1 and TTR-ASO were introduced to HLCs. A significant downregulation (>80%) of TTR mRNA was induced in the HLCs by both oligonucleotides. TTR protein present in the cell culture supernatant of HLCs was similarly downregulated. Gene expression of other hepatic markers was not affected by the therapeutic oligonucleotides. Our data indicate that urine cells (UCs) after reprogramming and hepatic differentiation represent excellent primary human target cells to assess the efficacy and specificity of novel compounds.

Current preclinical small interfering RNA (siRNA)-based conjugate systems for RNA therapeutics

Recent promising clinical results of RNA therapeutics have drawn big attention of academia and industries to RNA therapeutics and their carrier systems. To improve their feasibility in clinics, systemic evaluations of currently available carrier systems under clinical trials and preclinical studies are needed. In this review, we focus on recent noticeable preclinical studies and clinical results regarding siRNA-based conjugates for clinical translations. Advantages and drawbacks of siRNA-based conjugates are discussed, compared to particle-based delivery systems. Then, representative siRNA-based conjugates with aptamers, peptides, carbohydrates, lipids, polymers, and nanostructured materials are introduced. To improve feasibility of siRNA conjugates in preclinical studies, several considerations for the rational design of siRNA conjugates in terms of cleavability, immune responses, multivalent conjugations, and mechanism of action are also presented. Lastly, we discuss lessons from previous preclinical and clinical studies related to siRNA conjugates and perspectives of their clinical applications.

Effect of poly-glutamate on uptake efficiency and cytotoxicity of cell penetrating peptides

Cell penetrating peptides (CPPs) were developed as vehicles for efficient delivery of various molecules. An ideal CPP-peptide should not display any toxicity against cancer cells as well as healthy cells and efficiently enter into the cell. Because of the cationic nature and the intrinsic vector capabilities, these peptides can cause cytotoxicity. One of the possible reasons for toxicity of CPPs is direct translocation and consequently, pore formation on the plasma membrane. In this study it was demonstrated that interaction of poly-glutamate with CPP considerably reduced their cytotoxicity in A549 cell. This strategy could be useful for efficient drug delivery mediated by CPP.

Oral delivery of macromolecular drugs: Where we are after almost 100years of attempts

Since the first attempt to administer insulin orally in humans more than 90years ago, the oral delivery of macromolecular drugs (>1000g/mol) has been rather disappointing. Although several clinical pilot studies have demonstrated that the oral absorption of macromolecules is possible, the bioavailability remains generally low and variable. This article reviews the formulations and biopharmaceutical aspects of orally administered biomacromolecules on the market and in clinical development for local and systemic delivery. The most successful approaches for systemic delivery often involve a combination of enteric coating, protease inhibitors and permeation enhancers in relatively high amounts. However, some of these excipients have induced local or systemic adverse reactions in preclinical and clinical studies, and long-term studies are often missing. Therefore, strategies aimed at increasing the oral absorption of macromolecular drugs should carefully take into account the benefit-risk ratio. In the absence of specific uptake pathways, small and potent peptides that are resistant to degradation and that present a large therapeutic window certainly represent the best candidates for systemic absorption. While we acknowledge the need for systemically delivering biomacromolecules, it is our opinion that the oral delivery to local gastrointestinal targets is currently more promising because of their accessibility and the lacking requirement for intestinal permeability enhancement.

Cell Type Preference of a Novel Human Derived Cell-Permeable Peptide dNP2 and TAT in Murine Splenic Immune Cells

Cell-permeable peptides (CPPs) have been widely studied as an attractive drug delivery system to deliver therapeutic macromolecules such as DNA, RNA, and protein into cells. However, its clinical application is still limited and controversial due to the lack of a complete understanding of delivery efficiency in target cells. Previously we identified and characterized the novel and superior CPP, named dNP2, and here we comparatively analyzed intracellular delivery efficiency of dNP2 and TAT in various immune cells of mouse spleen to demonstrate their cell type preference. dNP2- or TAT-conjugated fluorescent proteins were most efficiently taken up by phagocytic cells such as dendritic cells and macrophages while little protein uptake was seen by lymphocytes including T cells, B cells, and NK cells. Interestingly CD8⁺ lymphoid dendritic cells and CD62L^loCD44^hi memory like T cell subsets showed significantly better uptake efficiency in vitro and in vivo relative to other dendritic cells or T cells, respectively. In addition, activated macrophages, T cells, and B cells took up the proteins more efficiently relative to when in the resting state. Importantly, only dNP2, not TAT, shows significant intracellular protein delivery efficiency in vivo. Collectively, this study provides important information regarding heterogeneous intracellular delivery efficiency of CPPs such as dNP2 and TAT with cell type preference in the spleen needed for its application in phagocytic cells or activated immune cells.

Classes of Cell-Penetrating Peptides

During the three decades of cell-penetrating peptides era the superfamily of CPPs has rapidly expanded, and the quest for new sequences continues. CPPs have been well recognized by scientific community and they have been used for transduction of a wide variety of molecules and particles into cultured cells and in vivo. In parallel with application of CPPs for delivering of active payloads, the mechanisms that such peptides take advantage of for gaining access to cells' insides have been in the focus of intense studies. Although the common denominator "cell penetration" unites all CPPs, the interaction partners on the cell surface, evoked cellular responses and even the uptake mechanisms might greatly vary between different peptide types. Here we present some possibilities for classification of CPPs based on their type of origin, physical-chemical properties, and the extent of modifications and design efforts. We also briefly analyze the internalization mechanisms with regard to their classification into groups based on physical-chemical characteristics.

PepFects and NickFects for the Intracellular Delivery of Nucleic Acids

Nucleic acids can be utilized in gene therapy to restore, alter, or silence gene functions. In order to reveal the biological activity nucleic acids have to reach their intracellular targets by passing through the plasma membrane, which is impermeable for these large and negatively charged molecules. Cell-penetrating peptides (CPPs) condense nucleic acids into nanoparticles using non-covalent complexation strategy and mediate their delivery into the cell, whereas the physicochemical parameters of the nanoparticles determine the interactions with the membranes, uptake mechanism, and subsequent intracellular fate. The nanoparticles are mostly internalized by endocytosis that leads to the entrapment of them in endosomal vesicles. Therefore design of new CPPs that are applicable for non-covalent complex formation strategy and harness endosomolytic properties is highly vital. Here we demonstrate that PepFects and NickFects are efficient vectors for the intracellular delivery of various nucleic acids.This chapter describes how to form CPP/pDNA nanoparticles, evaluate stable nanoparticles formation, and assess gene delivery efficacy.

Challenges and opportunities for siRNA-based cancer treatment

As one of the life-threatening diseases involving multi-step genetic and epigenetic disorders, cancer has long been a dynamic research area for siRNA-based therapy as half of the current siRNA-based clinical trials are involved in oncology. However, despite consistent enthusiasm in the academic world, siRNA-based cancer treatment still faces obstacles and difficulties in clinical development. In this article, we discuss key challenges facing siRNA-based cancer treatment revealed from recent clinical and preclinical studies, including chemical modification, tumour penetration, endosomal escape, target selection and off-target effects. In addition, opportunities and avenues for translating siRNA technology from bench to oncologic clinics are explored.

Nuclease resistant oligonucleotides with cell penetrating properties

2'-O-AECM modified oligonucleotides provide an unusual combination of remarkable properties. This includes the combination of high resistance towards enzymatic degradation and the spontaneous cellular uptake of AECM oligonucleotides.

"Evolving nanoparticle gene delivery vectors for the liver: What has been learned in 30 years"

Nonviral gene delivery to the liver has been under evolution for nearly 30years. Early demonstrations established relatively simple nonviral vectors could mediate gene expression in HepG2 cells which understandably led to speculation that these same vectors would be immediately successful at transfecting primary hepatocytes in vivo. However, it was soon recognized that the properties of a nonviral vector resulting in efficient transfection in vitro were uncorrelated with those needed to achieve efficient nonviral transfection in vivo. The discovery of major barriers to liver gene transfer has set the field on a course to design biocompatible vectors that demonstrate increased DNA stability in the circulation with correlating expression in liver. The improved understanding of what limits nonviral vector gene transfer efficiency in vivo has resulted in more sophisticated, low molecular weight vectors that allow systematic optimization of nanoparticle size, charge and ligand presentation. While the field has evolved DNA nanoparticles that are stable in the circulation, target hepatocytes, and deliver DNA to the cytosol, breaching the nucleus remains the last major barrier to a fully successful nonviral gene transfer system for the liver. The lessons learned along the way are fundamentally important to the design of all systemically delivered nanoparticle nonviral gene delivery systems.

Therapeutic Oligonucleotides Targeting Liver Disease: TTR Amyloidosis

The liver has become an increasingly interesting target for oligonucleotide therapy. Mutations of the gene encoding transthyretin (TTR), expressed in vast amounts by the liver, result in a complex degenerative disease, termed familial amyloid polyneuropathy (FAP). Misfolded variants of TTR are linked to the establishment of extracellular protein deposition in various tissues, including the heart and the peripheral nervous system. Recent progress in the chemistry and formulation of antisense (ASO) and small interfering RNA (siRNA) designed for a knockdown of TTR mRNA in the liver has allowed to address the issue of gene-specific molecular therapy in a clinical setting of FAP. The two therapeutic oligonucleotides bind to RNA in a sequence specific manner but exploit different mechanisms. Here we describe major developments that have led to the advent of therapeutic oligonucleotides for treatment of TTR-related disease.

Cell-penetrating peptides: possibilities and challenges for drug delivery in vitro and in vivo

In this review, we discuss how cell-penetrating peptides (CPPs) might get access to their intracellular targets. We specifically focus on the challenge of deciding whether the positively-charged CPPs are just bound to the negatively-charged cell surface and subsequently endocytosed or actually transported into the cytosol, either by direct plasma membrane penetration or after endocytosis. This discussion includes comments about pitfalls when using pharmacological inhibitors in such studies. The possibility of exploiting CPPs as carriers for the delivery of drugs of different sizes in vitro is discussed, as is the use of CPPs as carriers for therapeutic drugs or contrast agents in vivo. We conclude that in many cases, more studies are needed to demonstrate conclusively whether increased delivery of a substance attached to CPPs is due to a membrane-penetrating property or whether the increase is a consequence of just changing the charge of the substance to be delivered. Finally, the expected dose needed for the use of such conjugates in vivo is discussed, including aspects to consider in order to bring potential products into clinical use.

Targeting the HIV RNA genome: high-hanging fruit only needs a longer ladder

Small molecules targeting the enzymes responsible for human immunodeficiency virus (HIV) maturation, DNA synthesis and its subsequent chromosomal integration as ribonucleotide-free double-stranded DNA remain the mainstay of combination antiretroviral therapy. For infected individuals harboring drug-susceptible virus, this approach has afforded complete or near-complete viral suppression. However, in the absence of a curative strategy, the predictable emergence of drug-resistant variants requires continued development of improved antiviral strategies, inherent to which is the necessity of identifying novel targets. Regulatory elements that mediate transcription, translation, nucleocytoplasmic transport, dimerization, packaging and reverse transcription of the (+) strand RNA genome should now be considered viable targets for small molecule, peptide- and oligonucleotide-based therapeutics. Where target specificity and cellular penetration and toxicity have been the primary obstacle to successful “macromolecule therapeutics”, this chapter summarizes (a) novel approaches targeting RNA motifs whose three-dimensional structure is critical for biological function and consequently may be less prone to resistance-conferring mutations and (b) improved methods for delivery.

Peptide-mediated delivery: an overview of pathways for efficient internalization

Poor cellular delivery and low bioavailability of novel potent therapeutic molecules continue to remain the bottleneck of modern cancer and gene therapy. Cell-penetrating peptides have provided immense opportunities for the intracellular delivery of bioactive cargos and have led to the first exciting successes in experimental therapy of muscular dystrophies. This review focuses on the mechanisms by which cell-penetrating peptides gain access to the cell interior and deliver cargos. Recent advances in augmenting delivery efficacy and facilitation of endosomal escape of cargo are presented, and the cell-penetrating peptide-mediated delivery of two of the most popular classes of cargo molecules, oligonucleotides and proteins, is analyzed. The arsenal of tools for oligonucleotide delivery has dramatically expanded in the last decade enabling harnessing of cell-surface receptors for targeted delivery.

Cell-penetrating peptides and their analogues as novel nanocarriers for drug delivery

INTRODUCTION

The impermeability of biological membranes is a major obstacle in drug delivery; however, some peptides have transition capabilities of biomembranes. In recent decades, cell-penetrating peptides (CPPs) have been introduced as novel biocarriers that are able to translocate into the cells. CPPs are biologically potent tools for non-invasive cellular internalization of cargo molecules. Nevertheless, the non-specificity of these peptides presents a restriction for targeting drug delivery; therefore, a peptidic nanocarrier sensitive to matrix metalloproteinase (MMP) has been prepared, called activatable cell-penetrating peptide (ACPP). In addition to the cell-penetrating peptide dendrimer (DCPP), other analogues of CPPs have been synthesized.

METHODS

In this study, the most recent literature in the field of biomedical application of CPPs and their analogues, ACPP and DCCP, were reviewed.

RESULTS

This review focuses on CPP and its analogues, ACPP and DCPP, as novel nanocarriers for drug delivery. In addition, nanoconjugates and bioconjugates of these peptide sequences are discussed.

CONCLUSION

DCCP, branched CPPs, compared to linear peptides have advantages such as resistance to rapid biodegradation, high loading capacities and large-scale production capability.

Antisense oligonucleotides, microRNAs, and antibodies

The specificity of Watson-Crick base pairing and the development of several chemical modifications to oligonucleotides have enabled the development of novel drug classes for the treatment of different human diseases. This review focuses on promising results of recent preclinical or clinical studies on targeting HDL metabolism and function by antisense oligonucleotides and miRNA-based therapies. Although many hurdles regarding basic mechanism of action, delivery, specificity, and toxicity need to be overcome, promising results from recent clinical trials and recent approval of these types of therapy to treat dyslipidemia suggest that the treatment of HDL dysfunction will benefit from these unique clinical opportunities. Moreover, an overview of monoclonal antibodies (mAbs) developed for the treatment of dyslipidemia and cardiovascular disease and currently being tested in clinical studies is provided. Initial studies have shown that these compounds are generally safe and well tolerated, but ongoing large clinical studies will assess their long-term safety and efficacy.

Oligonucleotide delivery with cell surface binding and cell penetrating Peptide amphiphile nanospheres

A drug delivery system designed specifically for oligonucleotide therapeutics can ameliorate the problems associated with the in vivo delivery of these molecules. The internalization of free oligonucleotides is challenging, and cytotoxicity is the main obstacle for current transfection vehicles. To develop nontoxic delivery vehicles for efficient transfection of oligonucleotides, we designed a self-assembling peptide amphiphile (PA) nanosphere delivery system decorated with cell penetrating peptides (CPPs) containing multiple arginine residues (R4 and R8), and a cell surface binding peptide (KRSR), and report the efficiency of this system in delivering G-3129, a Bcl-2 antisense oligonucleotide (AON). PA/AON (peptide amphiphile/antisense oligonucleotide) complexes were characterized with regards to their size and secondary structure, and their cellular internalization efficiencies were evaluated. The effect of the number of arginine residues on the cellular internalization was investigated by both flow cytometry and confocal imaging, and the results revealed that uptake efficiency improved as the number of arginines in the sequence increased. The combined effect of cell penetration and surface binding property on the cellular internalization and its uptake mechanism was also evaluated by mixing R8-PA and KRSR-PA. R8 and R8/KRSR decorated PAs were found to drastically increase the internalization of AONs compared to nonbioactive PA control. Overall, the KRSR-decorated self-assembled PA nanospheres were demonstrated to be noncytotoxic delivery vectors with high transfection rates and may serve as a promising delivery system for AONs.

Peptide nanofiber complexes with siRNA for deep brain gene silencing by stereotactic neurosurgery

Peptide nanofibers (PNFs) are one-dimensional assemblies of amphiphilic peptides in a cylindrical geometry. We postulated that peptide nanofibers (PNFs) can provide the tools for genetic intervention and be used for delivery of siRNA, as they can be engineered with positively charged amino acids that can electrostatically bind siRNA. The aim of this work was to investigate the use of PNFs as vectors for siRNA delivery providing effective gene knockdown. We designed a surfactant-like peptide (palmitoyl-GGGAAAKRK) able to self-assemble into PNFs and demonstrated that complexes of PNF:siRNA are uptaken intracellularly and increase the residence time of siRNA in the brain after intracranial administration. The biological activity of the complexes was investigated in vitro by analyzing the down-regulation of the expression of a targeted protein (BCL2), as well as induction of apoptosis, as well as in vivo by analyzing the relative gene expression upon stereotactic administration into a deep rat brain structure (the subthalamic nucleus). Gene expression levels of BCL2 mRNA showed that PNF:siBCL2 constructs were able to silence the target BCL2 in specific loci of the brain. Silencing of the BCL2 gene resulted in ablation of neuronal cell populations, indicating that genetic interventions by PNF:siRNA complexes may lead to novel treatment strategies of CNS pathologies.