Enhancing Endosomal Escape for Intracellular Delivery of Macromolecular Biologic Therapeutics

Current approaches to genetic modification of marine bacteria and considerations for improved transformation efficiency

Marine bacteria play vital roles in symbiosis, biogeochemical cycles and produce novel bioactive compounds and enzymes of interest for the pharmaceutical, biofuel and biotechnology industries. At present, investigations into marine bacterial functions and their products are primarily based on phenotypic observations, -omic type approaches and heterologous gene expression. To advance our understanding of marine bacteria and harness their full potential for industry application, it is critical that we have the appropriate tools and resources to genetically manipulate them in situ. However, current genetic tools that are largely designed for model organisms such as E. coli, produce low transformation efficiencies or have no transfer ability in marine bacteria. To improve genetic manipulation applications for marine bacteria, we need to improve transformation methods such as conjugation and electroporation in addition to identifying more marine broad host range plasmids. In this review, we aim to outline the reported methods of transformation for marine bacteria and discuss the considerations for each approach in the context of improving efficiency. In addition, we further discuss marine plasmids and future research areas including CRISPR tools and their potential applications for marine bacteria.

Towards targeted Cas9 (CRISPR-Cas) delivery: Preparation of IgG antibody-Cas9 conjugates using a split intein

The CRISPR-Cas9 system has revolutionized the field of genetic engineering, but targeted cellular delivery remains a central problem. The delivery of the preformed ribonuclease-protein (RNP) complex has the advantages of fewer side effects and avoidance of potential permanent effects. We reasoned that an internalizing IgG antibody as a targeting device could address the delivery of Cas9-RNP. We opted for protein trans-splicing mediated by a split intein to facilitate posttranslational conjugation of the two large protein entities. We recently described the cysteine-less CL split intein that efficiently performs under oxidizing conditions and does not interfere with disulfide bonds or thiol bioconjugation chemistries. Using the CL split intein, we report for the first time the ligation of monoclonal IgG antibody precursors, expressed in mammalian cells, and a Cas9 precursor, obtained from bacterial expression. A purified IgG-Cas9 conjugate was loaded with sgRNA to form the active RNP complex and introduced a double-strand break in its target DNA in vitro. Furthermore, a synthetic peptide variant of the short N-terminal split intein precursor proved useful for chemical modification of Cas9. The split intein ligation procedure reported here for IgG-Cas9 provides the first step towards a novel CRISPR-Cas9 targeting approach involving the preformed RNP complex.

Covalent conjugation and non-covalent complexation strategies for intracellular delivery of proteins using cell-penetrating peptides

Therapeutic proteins provided new opportunities for patients and high sales volumes. However, they are formulated for extracellular targets. The lipophilic barrier of the plasma membrane renders the vast array of intracellular targets out of reach. Peptide-based delivery systems, namely cell-penetrating peptides (CPPs), have few safety concerns, and low immunogenicity, with control over administered doses. This study investigates CPP-based protein delivery systems by classifying them into CPP-protein "covalent conjugation" and CPP: protein "non-covalent complexation" categories. Covalent conjugates ensure the proximity of the CPP to the cargo, which can improve cellular uptake and endosomal escape. We will discuss various aspects of covalent conjugates through non-cleavable (stable) or cleavable bonds. Non-cleavable CPP-protein conjugates are produced by recombinant DNA technology to express the complete fusion protein in a host cell or by chemical ligation of CPP and protein, which ensures stability during the delivery process. CPP-protein cleavable bonds are classified into pH-sensitive and redox-sensitive bonds, enzyme-cleavable bonds, and physical stimuli cleavable linkers (light radiation, ultrasonic waves, and thermo-responsive). We have highlighted the key characteristics of non-covalent complexes through electrostatic and hydrophobic interactions to preserve the conformational integrity of the CPP and cargo. CPP-mediated protein delivery by non-covalent complexation, such as zippers, CPP adaptor methods, and avidin-biotin technology, are featured. Conclusively, non-covalent complexation methods are appropriate when a high number of CPP or protein samples are to be screened. In contrast, when the high biological activity of the protein is critical in the intracellular compartment, conjugation protocols are preferred.

Microglial-derived C1q integrates into neuronal ribonucleoprotein complexes and impacts protein homeostasis in the aging brain

Neuroimmune interactions mediate intercellular communication and underlie critical brain functions. Microglia, CNS-resident macrophages, modulate the brain through direct physical interactions and the secretion of molecules. One such secreted factor, the complement protein C1q, contributes to complement-mediated synapse elimination in both developmental and disease models, yet brain C1q protein levels increase significantly throughout aging. Here, we report that C1q interacts with neuronal ribonucleoprotein (RNP) complexes in an age-dependent manner. Purified C1q protein undergoes RNA-dependent liquid-liquid phase separation (LLPS) in vitro, and the interaction of C1q with neuronal RNP complexes in vivo is dependent on RNA and endocytosis. Mice lacking C1q have age-specific alterations in neuronal protein synthesis in vivo and impaired fear memory extinction. Together, our findings reveal a biophysical property of C1q that underlies RNA- and age-dependent neuronal interactions and demonstrate a role of C1q in critical intracellular neuronal processes.

Comparing Cell Penetration of Biotherapeutics across Human Cell Lines

A major obstacle in biotherapeutics development is maximizing cell penetration. Ideally, assays would allow for optimization of cell penetration in the cell type of interest early in the drug development process. However, few assays exist to compare cell penetration across different cell types independent of drug function. In this work, we applied the chloroalkane penetration assay (CAPA) in seven mammalian cell lines as well as primary cells. Careful controls were used to ensure that data could be compared across cell lines. We compared the nuclear penetration of several peptides and drug-like oligonucleotides and saw significant differences among the cell lines. To help explain these differences, we quantified the relative activities of endocytosis pathways in these cell lines and correlated them with the penetration data. Based on these results, we knocked down clathrin in a cell line with an efficient permeability profile and observed reduced penetration of peptides but not oligonucleotides. Finally, we used small-molecule endosomal escape enhancers and observed enhancement of cell penetration of some oligonucleotides, but only in some of the cell lines tested. CAPA data provide valuable points of comparison among different cell lines, including primary cells, for evaluating the cell penetration of various classes of peptides and oligonucleotides.

Selective targeting of chemically modified miR-34a to prostate cancer using a small molecule ligand and an endosomal escape agent

Use of tumor-suppressive microRNAs (miRNAs) as anti-cancer agents is hindered by the lack of effective delivery vehicles, entrapment of the miRNA within endocytic compartments, and rapid degradation of miRNA by nucleases. To address these issues, we developed a miRNA delivery strategy that includes (1) a targeting ligand, (2) an endosomal escape agent, nigericin and (3) a chemically modified miRNA. The delivery ligand, DUPA (2-[3-(1,3-dicarboxy propyl) ureido] pentanedioic acid), was selected based on its specificity for prostate-specific membrane antigen (PSMA), a receptor routinely upregulated in prostate cancer-one of the leading causes of cancer death among men. DUPA was conjugated to the tumor suppressive miRNA, miR-34a (DUPA-miR-34a) based on the ability of miR-34a to inhibit prostate cancer cell proliferation. To mediate endosomal escape, nigericin was incorporated into the complex, resulting in DUPA-nigericin-miR-34a. Both DUPA-miR-34a and DUPA-nigericin-miR-34a specifically bound to, and were taken up by, PSMA-expressing cells in vitro and in vivo. And while both DUPA-miR-34a and DUPA-nigericin-miR-34a downregulated miR-34a target genes, only DUPA-nigericin-miR-34a decreased cell proliferation in vitro and delayed tumor growth in vivo. Tumor growth was further reduced using a fully modified version of miR-34a that has significantly increased stability.

Hydrophobicity: The door to drug delivery

The engineering of intracellular delivery systems with the goal of achieving personalized medicine has been encouraged by advances in nanomaterial science as well as a greater understanding of diseases and of the biochemical pathways implicated in many disorders. The development of vectors able to transport the drug to a target location and release it only on demand is undoubtedly the primary issue. From a molecular perspective, the topography of drug carrier surfaces is directly related to the design of an effective drug carrier because it provides a physical hint to modifying its interactions with biological systems. For instance, the initial ratio of hydrophilic to hydrophobic surfaces and the changes brought about by external factors enable the release or encapsulation of a therapeutic molecule and the ability of the nanosystem to cross biological barriers and reach its target without causing systemic toxicity. The first step in creating new materials with enhanced functionality is to comprehend and characterize the interplay between hydrophilic and hydrophobic molecules at the molecular level. Therefore, the focus of this review is on the function of hydrophobicity, which is essential for matching the complexity of biological environments with the intended functionality.

Oral peptide therapeutics for diabetes treatment: State-of-the-art and future perspectives

Diabetes, characterized by hyperglycemia, is a major cause of death and disability worldwide. Peptides, such as insulin and glucagon-like peptide-1 (GLP-1) analogs, have shown promise as treatments for diabetes due to their ability to mimic or enhance insulin's actions in the body. Compared to subcutaneous injection, oral administration of anti-diabetic peptides is a preferred approach. However, biological barriers significantly reduce the efficacy of oral peptide therapeutics. Recent advancements in drug delivery systems and formulation techniques have greatly improved the oral delivery of peptide therapeutics and their efficacy in treating diabetes. This review will highlight (1) the benefits of oral anti-diabetic peptide therapeutics; (2) the biological barriers for oral peptide delivery, including pH and enzyme degradation, intestinal mucosa barrier, and biodistribution barrier; (3) the delivery platforms to overcome these biological barriers. Additionally, the review will discuss the prospects in this field. The information provided in this review will serve as a valuable guide for future developments in oral anti-diabetic peptide therapeutics.

Peptide-based non-viral gene delivery: A comprehensive review of the advances and challenges

Gene therapy is the most effective treatment option for diseases, but its effectiveness is affected by the choice and design of gene carriers. The genes themselves have to pass through multiple barriers in order to enter the cell and therefore require additional vectors to carry them inside the cell. In gene therapy, peptides have unique properties and potential as gene carriers, which can effectively deliver genes into specific cells or tissues, protect genes from degradation, improve gene transfection efficiency, and enhance gene targeting and biological responsiveness. This paper reviews the research progress of peptides and their derivatives in the field of gene delivery recently, describes the obstacles encountered by foreign materials to enter the interior of the cell, and introduces the following classes of functional peptides that can carry materials into the interior of the cell, and assist in transmembrane translocation of carriers, thus breaking through endosomal traps to enable successful entry of genetic materials into the nucleus of the cell. The paper also discusses the combined application of peptide vectors with other vectors to enhance its transfection ability, explores current challenges encountered by peptide vectors, and looks forward to future developments in the field.

BenzoHTag, a fluorogenic self-labeling protein developed using molecular evolution

Self-labeling proteins are powerful tools in chemical biology as they enable the precise cellular localization of a synthetic molecule, often a fluorescent dye, with the genetic specificity of a protein fusion. HaloTag7 is the most popular self-labeling protein due to its fast labeling kinetics and the simplicity of its chloroalkane ligand. Reaction rates of HaloTag7 with different chloroalkane-containing substrates is highly variable and rates are only very fast for rhodamine-based dyes. This is a major limitation for the HaloTag system because fast labeling rates are critical for live-cell assays. Here, we report a molecular evolution system for HaloTag using yeast surface display that enables the screening of libraries up to 108 variants to improve reaction rates with any substrate of interest. We applied this method to produce a HaloTag variant, BenzoHTag, which has improved performance with a fluorogenic benzothiadiazole dye. The resulting system has improved brightness and conjugation kinetics, allowing for robust, no-wash fluorescent labeling in live cells. The new BenzoHTag-benzothiadiazole system has improved performance in live-cell assays compared to the existing HaloTag7-silicon rhodamine system, including saturation of intracellular enzyme in under 100 seconds and robust labeling at dye concentrations as low as 7 nM. It was also found to be orthogonal to the silicon HaloTag7-rhodamine system, enabling multiplexed no-wash labeling in live cells. The BenzoHTag system, and the ability to optimize HaloTag for a broader collection of substrates using molecular evolution, will be very useful for the development of cell-based assays for chemical biology and drug development.

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

Cell-Penetrating Peptide-Bismuth Bicycles

Cell-penetrating peptides (CPPs) play a significant role in the delivery of cargos into human cells. We report the first CPPs based on peptide-bismuth bicycles, which can be readily obtained from commercially available peptide precursors, making them accessible for a wide range of applications. These CPPs enter human cells as demonstrated by live-cell confocal microscopy using fluorescently labelled peptides. We report efficient sequences that demonstrate increased cellular uptake compared to conventional CPPs like the TAT peptide (derived from the transactivating transcriptional activator of human immunodeficiency virus 1) or octaarginine (R8 ), despite requiring only three positive charges. Bicyclization triggered by the presence of bismuth(III) increases cellular uptake by more than one order of magnitude. Through the analysis of cell lysates using inductive coupled plasma mass spectrometry (ICP-MS), we have introduced an alternative approach to examine the cellular uptake of CPPs. This has allowed us to confirm the presence of bismuth in cells after exposure to our CPPs. Mechanistic studies indicated an energy-dependent endocytic cellular uptake sensitive to inhibition by rottlerin, most likely involving macropinocytosis.

Biomedical application of metal-organic frameworks (MOFs) in cancer therapy: Stimuli-responsive and biomimetic nanocomposites in targeted delivery, phototherapy and diagnosis

The nanotechnology is an interdisciplinary field that has become a hot topic in cancer therapy. Metal-organic frameworks (MOFs) are porous materials and hybrid composites consisted of organic linkers and metal cations. Despite the wide application of MOFs in other fields, the potential of MOFs for purpose of cancer therapy has been revealed by the recent studies. High surface area and porosity, significant drug loading and encapsulation efficiency are among the benefits of using MOFs in drug delivery. MOFs can deliver genes/drugs with selective targeting of tumor cells that can be achieved through functionalization with ligands. The photosensitizers and photo-responsive nanostructures including carbon dots and gold nanoparticles can be loaded in/on MOFs to cause phototherapy-mediated tumor ablation. The immunogenic cell death induction and increased infiltration of cytotoxic CD8+ and CD4+ T cells can be accelerated by MOF platforms in providing immunotherapy of tumor cells. The stimuli-responsive MOF platforms responsive to pH, redox, enzyme and ion can accelerate release of therapeutics in tumor site. Moreover, MOF nanocomposites can be modified ligands and green polymers to improve their selectivity and biocompatibility for cancer therapy. The application of MOFs for the detection of cancer-related biomarkers can participate in the early diagnosis of patients.

Engineered Silica Nanoparticles for Nucleic Acid Delivery

The development of nucleic acid-based drugs holds great promise for therapeutic applications, but their effective delivery into cells is hindered by poor cellular membrane permeability and inherent instability. To overcome these challenges, delivery vehicles are required to protect and deliver nucleic acids efficiently. Silica nanoparticles (SiNPs) have emerged as promising nanovectors and recently bioregulators for gene delivery due to their unique advantages. In this review, a summary of recent advancements in the design of SiNPs for nucleic acid delivery and their applications is provided, mainly according to the specific type of nucleic acids. First, the structural characteristics and working mechanisms of various types of nucleic acids are introduced and classified according to their functions. Subsequently, for each nucleic acid type, the use of SiNPs for enhancing delivery performance and their biomedical applications are summarized. The tailored design of SiNPs for selected type of nucleic acid delivery will be highlighted considering the characteristics of nucleic acids. Lastly, the limitations in current research and personal perspectives on future directions in this field are presented. It is expected this opportune review will provide insights into a burgeoning research area for the development of next-generation SiNP-based nucleic acid delivery systems.

An Endosomal Escape Trojan Horse Platform to Improve Cytosolic Delivery of Nucleic Acids

Endocytosis is a major bottleneck toward cytosolic delivery of nucleic acids, as the vast majority of nucleic acid drugs remain trapped within endosomes. Current trends to overcome endosomal entrapment and subsequent degradation provide varied success; however, active delivery agents such as cell-penetrating peptides have emerged as a prominent strategy to improve cytosolic delivery. Yet, these membrane-active agents have poor selectivity for endosomal membranes, leading to toxicity. A hallmark of endosomes is their acidic environment, which aids in degradation of foreign materials. Here, we develop a pH-triggered spherical nucleic acid that provides smart antisense oligonucleotide (ASO) release upon endosomal acidification and selective membrane disruption, termed DNA EndosomaL Escape Vehicle Response (DELVR). We anchor i-Motif DNA to a nanoparticle (AuNP), where the complement strand contains both an ASO sequence and a functionalized endosomal escape peptide (EEP). By orienting the EEP toward the AuNP core, the EEP is inactive until it is released through acidification-induced i-Motif folding. In this study, we characterize a small library of i-Motif duplexes to develop a structure-switching nucleic acid sequence triggered by endosomal acidification. We evaluate antisense efficacy using HIF1a, a hypoxic indicator upregulated in many cancers, and demonstrate dose-dependent activity through RT-qPCR. We show that DELVR significantly improves ASO efficacy in vitro. Finally, we use fluorescence lifetime imaging and activity measurement to show that DELVR benefits synergistically from nuclease- and pH-driven release strategies with increased ASO endosomal escape efficiency. Overall, this study develops a modular platform that improves the cytosolic delivery of nucleic acid therapeutics and offers key insights for overcoming intracellular barriers.

Tumor-specific polycistronic miRNA delivered by engineered exosomes for the treatment of glioblastoma

BACKGROUND

Glioblastoma (GBM) has poor prognosis due to ineffective agents and poor delivery methods. MicroRNAs (miRs) have been explored as novel therapeutics for GBM, but the optimal miRs and the ideal delivery strategy remain unresolved. In this study, we sought to identify the most effective pan-subtype anti-GBM miRs and to develop an improved delivery system for these miRs.

METHODS

We conducted an unbiased screen of over 600 miRs against 7 glioma stem cell (GSC) lines representing all GBM subtypes to identify a set of pan-subtype-specific anti-GBM miRs and then used available TCGA GBM patient outcomes and miR expression data to hone in on miRs that were most likely to be clinically effective. To enhance delivery and expression of the miRs, we generated a polycistronic plasmid encoding 3 miRs (pPolymiR) and used HEK293T cells as biofactories to package pPolymiR into engineered exosomes (eExos) that incorporate viral proteins (Gag/VSVg) in their structure (eExos+pPolymiR) to enhance function.

RESULTS

Our stepwise screen identified miR-124-2, miR-135a-2, and let-7i as the most effective miRs across all GBM subtypes with clinical relevance. Delivery of eExos+pPolymiR resulted in high expression of all 3 miRs in GSCs, and significantly decreased GSC proliferation in vitro. eExos+pPolymiR prolonged survival of GSC-bearing mice in vivo when compared with eExos carrying each of the miRs individually or as a cocktail.

CONCLUSION

eExos+pPolymiR, which includes a pan-subtype anti-glioma-specific miR combination encoded in a polycistronic plasmid and a novel exosome delivery platform, represents a new and potentially powerful anti-GBM therapeutic.

Engineering highly stable variants of Corynactis californica green fluorescent proteins

Fluorescent proteins (FPs) are versatile biomarkers that facilitate effective detection and tracking of macromolecules of interest in real time. Engineered FPs such as superfolder green fluorescent protein (sfGFP) and superfolder Cherry (sfCherry) have exceptional refolding capability capable of delivering fluorescent readout in harsh environments where most proteins lose their native functions. Our recent work on the development of a split FP from a species of strawberry anemone, Corynactis californica, delivered pairs of fragments with up to threefold faster complementation than split GFP. We present the biophysical, biochemical, and structural characteristics of five full-length variants derived from these split C. californica GFP (ccGFP). These ccGFP variants are more tolerant under chemical denaturation with up to 8 kcal/mol lower unfolding free energy than that of the sfGFP. It is likely that some of these ccGFP variants could be suitable as biomarkers under more adverse environments where sfGFP fails to survive. A structural analysis suggests explanations of the variations in stabilities among the ccGFP variants.

Intracellular Protein Delivery: Approaches, Challenges, and Clinical Applications

Protein biologics are powerful therapeutic agents with diverse inhibitory and enzymatic functions. However, their clinical use has been limited to extracellular applications due to their inability to cross plasma membranes. Overcoming this physiological barrier would unlock the potential of protein drugs for the treatment of many intractable diseases. In this review, we highlight progress made toward achieving cytosolic delivery of recombinant proteins. We start by first considering intracellular protein delivery as a drug modality compared to existing Food and Drug Administration-approved drug modalities. Then, we summarize strategies that have been reported to achieve protein internalization. These techniques can be broadly classified into 3 categories: physical methods, direct protein engineering, and nanocarrier-mediated delivery. Finally, we highlight existing challenges for cytosolic protein delivery and offer an outlook for future advances.

Topical Application of Cell-Penetrating Peptide Modified Anti-VEGF Drug Alleviated Choroidal Neovascularization in Mice

Background

Age-related macular degeneration (AMD) stands as the foremost cause of irreversible central vision impairment, marked by choroidal neovascularization (CNV). The prevailing clinical approach to AMD treatment relies on intravitreal injections of anti-vascular endothelial growth factor (VEGF) drugs. However, this method is encumbered by diverse complications, prompting exploration of non-invasive alternatives such as ocular administration via eye drops for anti-VEGF therapy.

Methods

Two complexes, 5-FITC-CPP-Ranibizumab (5-FCR) and 5-FITC-CPP-Conbercept (5-FCC), were synthesized by incorporating the anti-VEGF drugs Ranibizumab (RBZ) or Conbercept (CBC) with cell-penetrating peptide (CPP). Circular dichroism spectrum (CD) facilitated complexes characterization. Eye drops was utilized to address laser-induced CNV in mice. Fluorescein fundus angiography (FFA) observe the CNV lesion, while FITC-dextran and IB4 dual fluorescent staining, along with hematoxylin-eosin (HE) staining, assessed in lesion size. Tissue immunofluorescence examined CD31 and VEGF expression in choroidal/retinal pigment epithelial (RPE) tissues. Biocompatibility and biosafety of 5-FCR and 5-FCC was evaluated through histological examination of various organs or cell experiments.

Results

Both 5-FCR and 5-FCC exhibited favorable biocompatibility and safety profiles. VEGF-induced migration of Human umbilical vein endothelial cells (HUVECs) significantly decreased post-5-FCR/5-FCC treatment. Additionally, both complexes suppressed VEGF-induced tube formation in HUVECs. FFA results revealed a significant improvement in retinal exudation in mice. Histological examination unveiled the lesion areas in the 5-FCR and 5-FCC groups showed a significant reduction compared to the control group. Similar outcomes were observed in histological sections of the RPE-choroid-sclera flat mounts.

Conclusion

In this study, utilizing the properties of CPP and two anti-VEGF drugs, we successfully synthesized two complexes, 5-FCR and 5-FCC, through a straightforward approach. Effectively delivering the anti-VEGF drugs to the target area in a non-invasive manner, suppressing the progression of laser-induced CNV. This offers a novel approach for the treatment of wet AMD.

Constructing Nucleic Acid Delivering Lipoproteoplexes from Coiled-Coil Supercharged Protein and Cationic Liposomes

The safe and efficient delivery of nucleic acids is crucial for both clinical applications of gene therapy and pre-clinical laboratory research. Such delivery strategies rely on vectors to condense nucleic acid payloads and escort them into the cell without being degraded in the extracellular environment; however, the construction and utilization of these vectors can be difficult and time-consuming. Here, we detail the steps involved in the rapid, laboratory-scale production and assessment of a versatile, nucleic acid delivery vehicle, known as the lipoproteoplex. In this chapter, we outline: (1) the recombinant synthesis and subsequent purification of the supercharged coiled-coil protein component known as N8; (2) the synthesis of cationic liposomes from dioleoyl-3-trimethylammonium propane (DOTAP) and sodium cholate; (3) and finally a protocol for the delivery of a model siRNA cargo into a cultured cell line.

Design, Synthesis, and Biochemical Analysis of a Molecule Designed to Enhance Endosomal Escape

RNA therapeutics, including siRNAs, ASOs, and PMOs, have great potential to treat human disease. However, RNA therapeutics are too large, too charged, and/or too hydrophilic to cross the cellular membrane and are instead taken up into cells by endocytosis. Unfortunately, the vast majority of RNA therapeutics remain trapped inside endosomes (≥ 99%), which is the sole reason preventing their use to treat cancer, COVID, and other diseases. In contrast, enveloped viruses, such as influenza, also have an endosomal escape problem, but have evolved a highly efficient endosomal escape mechanism using trimeric hemagglutinin (HA) fusogenic protein. HA contains an outer hydrophilic domain (HA1) that masks an inner hydrophobic fusogenic/endosomal escape domain (HA2). Once inside endosomes, HA1 is shed to expose HA2 that, due to hydrophobicity, buries itself into the endosomal lipid bilayer, driving escape into the cytoplasm in a non-toxic fashion. To begin to address the RNA therapeutics rate-limiting endosomal escape problem, we report here a first step in the design and synthesis of a universal endosomal escape domain (uEED) that biomimics the enveloped virus escape mechanism. uEED contains an outer hydrophilic mask covalently attached to an inner hydrophobic escape domain. In plasma, uEED is inert and highly metabolically stable; however, when placed in endo/lysosomal conditions, uEED is activated by enzymatic removal of the hydrophilic mask, followed by self-immolation of the linker resulting in exposure of the hydrophobic indole ring domain in the absence of any hydrophilic tags. Thus, uEED is a synthetic biomimetic of the highly efficient viral endosomal escape mechanism.

PEGylation of a Peptide-Based Amphiphilic Delivery Agent and Influence on Protein Delivery to Cells

The cell membrane is a restrictive biological barrier, especially for large, charged molecules, such as proteins. The use of cell-penetrating peptides (CPPs) can facilitate the delivery of proteins, protein complexes, and peptides across the membrane by a variety of mechanisms that are all limited by endosomal sequestration. To improve CPP-mediated delivery, we previously reported the rapid and effective cytosolic delivery of proteins in vitro and in vivo by their coadministration with the peptide S10, which combines a CPP and an endosomal leakage domain. Amphiphilic peptides with hydrophobic properties, such as S10, can interact with lipids to destabilize the cell membrane, thus promoting cargo internalization or escape from endosomal entrapment. However, acute membrane destabilization can result in a dose-limiting cytotoxicity. In this context, the partial or transient deactivation of S10 by modification with methoxy poly(ethylene glycol) (mPEG; i.e., PEGylation) may provide the means to alter membrane destabilization kinetics, thereby attenuating the impact of acute permeabilization on cell viability. This study investigates the influence of PEGylation parameters (molecular weight, architecture, and conjugation chemistry) on the delivery efficiency of a green fluorescent protein tagged with a nuclear localization signal (GFP-NLS) and cytotoxicity on cells in vitro. Results suggest that PEGylation mostly interferes with adsorption and secondary structure formation of S10 at the cell membrane, and this effect is exacerbated by the mPEG molecular weight. This effect can be compensated for by increasing the concentration of conjugates prepared with lower molecular weight mPEG (5 to ∼20 kDa) but not for conjugates prepared with higher molecular weight mPEG (40 kDa). For conjugates prepared with moderate-to-high molecular weight mPEG (10 to 20 kDa), partial compensation of inactivation could be achieved by the inclusion of a reducible disulfide bond, which provides a mechanism to liberate the S10 from the polymer. Grafting multiple copies of S10 to a high-molecular-weight multiarmed PEG (40 kDa) improved GFP-NLS delivery efficiency. However, these constructs were more cytotoxic than the native peptide. Considering that PEGylation could be harnessed for altering the pharmacokinetics and biodistribution profiles of peptide-based delivery agents in vivo, the trends observed herein provide new perspectives on how to manipulate the membrane permeabilization process, which is an important variable for achieving delivery.

A synthetic delivery vector for mucosal vaccination

The success of mRNA-based vaccines during the Covid-19 pandemic has highlighted the value of this new platform for vaccine development against infectious disease. However, the CD8+ T cell response remains modest with mRNA vaccines, and these do not induce mucosal immunity, which would be needed to prevent viral spread in the healthy population. To address this drawback, we developed a dendritic cell targeting mucosal vaccination vector, the homopentameric STxB. Here, we describe the highly efficient chemical synthesis of the protein, and its in vitro folding. This straightforward preparation led to a synthetic delivery tool whose biophysical and intracellular trafficking characteristics were largely indistinguishable from recombinant STxB. The chemical approach allowed for the generation of new variants with bioorthogonal handles. Selected variants were chemically coupled to several types of antigens derived from the mucosal viruses SARS-CoV-2 and type 16 human papillomavirus. Upon intranasal administration in mice, mucosal immunity, including resident memory CD8+ T cells and IgA antibodies was induced against these antigens. Our study thereby identifies a novel synthetic antigen delivery tool for mucosal vaccination with an unmatched potential to respond to an urgent medical need.

Cell-Surface-Retained Peptide Additives for the Cytosolic Delivery of Functional Proteins

The delivery of functional proteins remains a major challenge in advancing biological and pharmaceutical sciences. Herein, we describe a powerful, simple, and highly effective strategy for the intracellular delivery of functional cargoes. Previously, we demonstrated that cell-penetrating peptide (CPP) additives equipped with electrophilic thiol-reactive moieties temporarily attach to the cellular membrane, thereby facilitating the cellular uptake of protein- and antibody-CPP cargoes through direct membrane transduction at low concentrations. Now, we hypothesize that CPP-additives with an increased retention on the cellular membrane will further enhance intracellular uptake. We discovered that adding a small hydrophobic peptide sequence to an arginine-rich electrophilic CPP-additive further improved the uptake of protein-CPP conjugates, whereas larger hydrophobic anchors showed increased cytotoxicity. Cell viability and membrane integrity measurements, structure-activity relationship studies, and quantitative evaluation of protein-CPP uptake revealed important design principles for cell-surface-retained CPP-additives. These investigations allowed us to identify a nontoxic, thiol-reactive CPP-additive containing the hydrophobic ILFF sequence, which can deliver fluorescent model proteins at low micromolar concentrations. This hydrophobic CPP-additive allowed the addition of protein cargoes for intracellular delivery after initial additive incubation. Time-lapse fluorescence microscopy and membrane tension analysis of cells treated with fluorescent ILFF-CPP-additives supported the claim of increased cell surface retention and suggested that the protein-CPP cargoes enter the cell through a mechanism involving lowered cell membrane tension. Finally, we demonstrated that our newly engineered hydrophobic CPP-additive enabled the uptake of a functional macrocyclic peptidic MDM2-inhibitor and a recombinant genome editing protein. This indicates that the developed hydrophobic CPP-additive holds promise as a tool to enhance the intracellular delivery of peptide and protein cargoes.

Small interfering RNA (siRNA)-based therapeutic applications against viruses: principles, potential, and challenges

RNA has emerged as a revolutionary and important tool in the battle against emerging infectious diseases, with roles extending beyond its applications in vaccines, in which it is used in the response to the COVID-19 pandemic. Since their development in the 1990s, RNA interference (RNAi) therapeutics have demonstrated potential in reducing the expression of disease-associated genes. Nucleic acid-based therapeutics, including RNAi therapies, that degrade viral genomes and rapidly adapt to viral mutations, have emerged as alternative treatments. RNAi is a robust technique frequently employed to selectively suppress gene expression in a sequence-specific manner. The swift adaptability of nucleic acid-based therapeutics such as RNAi therapies endows them with a significant advantage over other antiviral medications. For example, small interfering RNAs (siRNAs) are produced on the basis of sequence complementarity to target and degrade viral RNA, a novel approach to combat viral infections. The precision of siRNAs in targeting and degrading viral RNA has led to the development of siRNA-based treatments for diverse diseases. However, despite the promising therapeutic benefits of siRNAs, several problems, including impaired long-term protein expression, siRNA instability, off-target effects, immunological responses, and drug resistance, have been considerable obstacles to the use of siRNA-based antiviral therapies. This review provides an encompassing summary of the siRNA-based therapeutic approaches against viruses while also addressing the obstacles that need to be overcome for their effective application. Furthermore, we present potential solutions to mitigate major challenges.

A cell-active cyclic peptide targeting the Nrf2/Keap1 protein-protein interaction

The disruption of the protein-protein interaction (PPI) between Nrf2 and Keap1 is an attractive strategy to counteract the oxidative stress that characterises a variety of severe diseases. Peptides represent a complementary approach to small molecules for the inhibition of this therapeutically important PPI. However, due to their polar nature and the negative net charge required for binding to Keap1, the peptides reported to date exhibit either mid-micromolar activity or are inactive in cells. Herein, we present a two-component peptide stapling strategy to rapidly access a variety of constrained and functionalised peptides that target the Nrf2/Keap1 PPI. The most promising peptide, P8-H containing a fatty acid tag, binds to Keap1 with nanomolar affinity and is effective at inducing transcription of ARE genes in a human lung epithelial cell line at sub-micromolar concentration. Furthermore, crystallography of the peptide in complex with Keap1 yielded a high resolution X-ray structure, adding to the toolbox of structures available to develop cell-permeable peptidomimetic inhibitors.

A super-resolution and transmission electron microscopy correlative approach to study intracellular trafficking of nanoparticles

Nanoparticles (NPs) are used to encapsulate therapeutic cargos and deliver them specifically to the target site. The intracellular trafficking of NPs dictates the NP-cargo distribution within different cellular compartments, and thus governs their efficacy and safety. Knowledge in this field is crucial to understand their biological fate and improve their rational design. However, there is a lack of methods that allow precise localization and quantification of individual NPs within distinct cellular compartments simultaneously. Here, we address this issue by proposing a correlative light and electron microscopy (CLEM) method combining direct stochastic optical reconstruction microscopy (dSTORM) and transmission electron microscopy (TEM). We aim at combining the advantages of both techniques to precisely address NP localization in the context of the cell ultrastructure. Individual fluorescently-labelled poly(lactide-co-glycolide)-poly(ethylene glycol) (PLGA-PEG) NPs were directly visualized by dSTORM and assigned to cellular compartments by TEM. We first tracked NPs along the endo-lysosomal pathway at different time points, then demonstrated the effect of chloroquine on their intracellular distribution (i.e. endosomal escape). The proposed protocol can be applied to fluorescently labelled NPs and/or cargo, including those not detectable by TEM alone. Our studies are of great relevance to obtain important information on NP trafficking, and crucial for the design of more complex nanomaterials aimed at cytoplasmic/nucleic drug delivery.

Chloroquine and cytosolic galectins affect endosomal escape of antisense oligonucleotides after Stabilin-mediated endocytosis

Non-DNA-binding Stabilin-2/HARE receptors expressed on liver sinusoidal endothelial cells specifically bind to and internalize several classes of phosphorothioate antisense oligonucleotides (PS-ASOs). After Stabilin-mediated uptake, PS-ASOs are trafficked within endosomes (>97%-99%), ultimately resulting in destruction in the lysosome. The ASO entrapment in endosomes lowers therapeutic efficacy, thereby increasing the overall dose for patients. Here, we use confocal microscopy to characterize the intracellular route transverse by PS-ASOs after Stabilin receptor-mediated uptake in stable recombinant Stabilin-1 and -2 cell lines. We found that PS-ASOs as well as the Stabilin-2 receptor transverse the classic path: clathrin-coated vesicle-early endosome-late endosome-lysosome. Chloroquine exposure facilitated endosomal escape of PS-ASOs leading to target knockdown by more than 50% as compared to untreated cells, resulting in increased PS-ASO efficacy. We also characterize cytosolic galectins as novel contributor for PS-ASO escape. Galectins knockdown enhances ASO efficacy by more than 60% by modulating EEA1, Rab5C, and Rab7A mRNA expression, leading to a delay in the endosomal vesicle maturation process. Collectively, our results provide additional insight for increasing PS-ASO efficacy by enhancing endosomal escape, which can further be utilized for other nucleic acid-based modalities.

Therapeutic Oligonucleotides: An Outlook on Chemical Strategies to Improve Endosomal Trafficking

The potential of oligonucleotide therapeutics is undeniable as more than 15 drugs have been approved to treat various diseases in the liver, central nervous system (CNS), and muscles. However, achieving effective delivery of oligonucleotide therapeutics to specific tissues still remains a major challenge, limiting their widespread use. Chemical modifications play a crucial role to overcome biological barriers to enable efficient oligonucleotide delivery to the tissues/cells of interest. They provide oligonucleotide metabolic stability and confer favourable pharmacokinetic/pharmacodynamic properties. This review focuses on the various chemical approaches implicated in mitigating the delivery problem of oligonucleotides and their limitations. It highlights the importance of linkers in designing oligonucleotide conjugates and discusses their potential role in escaping the endosomal barrier, a bottleneck in the development of oligonucleotide therapeutics.

Cell-Penetrating and Targeted Peptides Delivery Systems as Potential Pharmaceutical Carriers for Enhanced Delivery across the Blood-Brain Barrier (BBB)

Among the challenges to the 21st-century health care industry, one that demands special mention is the transport of drugs/active pharmaceutical agents across the blood-brain barrier (BBB). The epithelial-like tight junctions within the brain capillary endothelium hinder the uptake of most pharmaceutical agents. With an aim to understand more deeply the intricacies of cell-penetrating and targeted peptides as a powerful tool for desirable biological activity, we provide a critical review of both CPP and homing/targeted peptides as intracellular drug delivery agents, especially across the blood-brain barrier (BBB). Two main peptides have been discussed to understand intracellular drug delivery; first is the cell-penetrating peptides (CPPs) for the targeted delivery of compounds of interest (primarily peptides and nucleic acids) and second is the family of homing peptides, which specifically targets cells/tissues based on their overexpression of tumour-specific markers and are thus at the heart of cancer research. These small, amphipathic molecules demonstrate specific physical and chemical modifications aimed at increased ease of cellular internalisation. Because only a limited number of drug molecules can bypass the blood-brain barrier by free diffusion, it is essential to explore all aspects of CPPs that can be exploited for crossing this barrier. Considering siRNAs that can be designed against any target RNA, marking such molecules with high therapeutic potential, we present a synopsis of the studies on synthetic siRNA-based therapeutics using CPPs and homing peptides drugs that can emerge as potential drug-delivery systems as an upcoming requirement in the world of pharma- and nutraceuticals.

Enhanced cytotoxicity of a Pseudomonas Exotoxin A based immunotoxin against prostate cancer by addition of the endosomal escape enhancer SO1861

Immunotoxins consist of an antibody or antibody fragment that binds to a specific cell surface structure and a cytotoxic domain that kills the cell after cytosolic uptake. Pseudomonas Exotoxin A (PE) based immunotoxins directed against a variety of tumor entities have successfully entered the clinic. PE possesses a KDEL-like motif (REDLK) that enables the toxin to travel from sorting endosomes via the KDEL-receptor pathway to the endoplasmic reticulum (ER), from where it is transported into the cytosol. There, it ADP-ribosylates the eukaryotic elongation factor 2, resulting in ribosome inhibition and finally apoptosis. One major problem of immunotoxins is their lysosomal degradation causing the need for much more immunotoxin molecules than finally required for induction of cell death. The resulting dose limitations and substantially increased side effects require new strategies to achieve improved cytosolic uptake. Here we generated an immunotoxin consisting of a humanized single chain variable fragment (scFv) targeting the prostate specific membrane antigen (PSMA) and the de-immunized PE variant PE24mut. This immunotoxin, hD7-1(VL-VH)-PE24mut, showed high and specific cytotoxicity in PSMA-expressing prostate cancer cells. We deleted the REDLK sequence to prevent transport to the ER and achieve endosomal entrapment. The cytotoxicity of this immunotoxin, hD7-1(VL-VH)-PE24mutΔREDLK, was greatly reduced. To restore activity, we added the endosomal escape enhancer SO1861 and observed an up to 190,000-fold enhanced cytotoxicity corresponding to a 57-fold enhancement compared to the initial immunotoxin with the REDLK sequence. A biodistribution study with different routes of administration clearly showed that the subcutaneous injection of hD7-1(VL-VH)-PE24mutΔREDLK in mice resulted in the highest tumor uptake. Treatment of mice bearing prostate tumors with a combination of hD7-1(VL-VH)-PE24mutΔREDLK plus SO1861 resulted in inhibition of tumor growth and enhanced overall survival compared to the monotherapies. The endosomal entrapment of non-toxic anti-PSMA immunotoxins followed by enhanced endosomal escape by SO1861 provides new therapeutic options in the future management of prostate cancer.

A technology evaluation of the atypical use of a CPP-containing peptide in the formulation and performance of a clinical botulinum toxin product

INTRODUCTION

Cell-penetrating peptides (CPPs), are small peptides that facilitate cytosolic access and, thus, transport of therapeutic macromolecules to intracellular sites when conjugated to cargo proteins. As with all new delivery platforms, clinical development of CPP-containing therapeutics has faced considerable challenges.

AREAS COVERED

RTP004 is a novel, 35-amino acid, bi-CPP-containing excipient that binds noncovalently with its cargo (botulinum toxin type A) rather than conjugated as a fusion protein. An RTP004-containing neurotoxin formulation, daxibotulinumtoxinA-lanm for injection (DAXI), has recently been approved by the FDA. The formulation and pharmacological characteristics of RTP004 and the efficacy and safety of the RTP004-neurotoxin formulation are discussed.

EXPERT OPINION

RTP004 is a highly positively charged lysine- and arginine-rich structure that provides formulation stability, preventing self-aggregation of the cargo protein and adsorption to container surfaces. The presence of RTP004 in the formulation also appears to increase presynaptic binding of the neurotoxin, reduces post-injection diffusion, and thus facilitates an increase in the cleavage of the intracellular substrate for the botulinum toxin, likely through enhanced cellular uptake. The RTP004-neurotoxin formulation is the first CPP-containing product approved for clinical use. The potential for RTP004 to facilitate other therapeutic cargo molecules requires further research.

Basic Principles of RNA Interference: Nucleic Acid Types and In Vitro Intracellular Delivery Methods

Since its discovery in 1989, RNA interference (RNAi) has become a widely used tool for the in vitro downregulation of specific gene expression in molecular biological research. This basically involves a complementary RNA that binds a target sequence to affect its transcription or translation process. Currently, various small RNAs, such as small interfering RNA (siRNA), micro RNA (miRNA), small hairpin RNA (shRNA), and PIWI interacting RNA (piRNA), are available for application on in vitro cell culture, to regulate the cells' gene expression by mimicking the endogenous RNAi-machinery. In addition, several biochemical, physical, and viral methods have been established to deliver these RNAs into the cell or nucleus. Since each RNA and each delivery method entail different off-target effects, limitations, and compatibilities, it is crucial to understand their basic mode of action. This review is intended to provide an overview of different nucleic acids and delivery methods for planning, interpreting, and troubleshooting of RNAi experiments.

Stereoisomer-Dependent Membrane Association and Capacity for Insulin Delivery Facilitated by Penetratin

Cell-penetrating peptides (CPPs), such as penetratin, are often investigated as drug delivery vectors and incorporating d-amino acids, rather than the natural l-forms, to enhance proteolytic stability could improve their delivery efficiency. The present study aimed to compare membrane association, cellular uptake, and delivery capacity for all-l and all-d enantiomers of penetratin (PEN) by using different cell models and cargos. The enantiomers displayed widely different distribution patterns in the examined cell models, and in Caco-2 cells, quenchable membrane binding was evident for d-PEN in addition to vesicular intracellular localization for both enantiomers. The uptake of insulin in Caco-2 cells was equally mediated by the two enantiomers, and while l-PEN did not increase the transepithelial permeation of any of the investigated cargo peptides, d-PEN increased the transepithelial delivery of vancomycin five-fold and approximately four-fold for insulin at an extracellular apical pH of 6.5. Overall, while d-PEN was associated with the plasma membrane to a larger extent and was superior in mediating the transepithelial delivery of hydrophilic peptide cargoes compared to l-PEN across Caco-2 epithelium, no enhanced delivery of the hydrophobic cyclosporin was observed, and intracellular insulin uptake was induced to a similar degree by the two enantiomers.

Advanced Strategies for Overcoming Endosomal/Lysosomal Barrier in Nanodrug Delivery

Nanocarriers have therapeutic potential to facilitate drug delivery, including biological agents, small-molecule drugs, and nucleic acids. However, their efficiency is limited by several factors; among which, endosomal/lysosomal degradation after endocytosis is the most important. This review summarizes advanced strategies for overcoming endosomal/lysosomal barriers to efficient nanodrug delivery based on the perspective of cellular uptake and intracellular transport mechanisms. These strategies include promoting endosomal/lysosomal escape, using non-endocytic methods of delivery to directly cross the cell membrane to evade endosomes/lysosomes and making a detour pathway to evade endosomes/lysosomes. On the basis of the findings of this review, we proposed several promising strategies for overcoming endosomal/lysosomal barriers through the smarter and more efficient design of nanodrug delivery systems for future clinical applications.

Nanovaccine that activates the NLRP3 inflammasome enhances tumor specific activation of anti-cancer immunity

Neoantigen cancer vaccines that target tumor specific mutations are emerging as a promising modality for cancer immunotherapy. To date, various approaches have been adopted to enhance efficacy of these therapies, but the low immunogenicity of neoantigens has hindered clinical application. To address this challenge, we developed a polymeric nanovaccine platform that activates the NLRP3 inflammasome, a key immunological signaling pathway in pathogen recognition and clearance. The nanovaccine is comprised of a poly (orthoester) scaffold engrafted with a small-molecule TLR7/8 agonist and an endosomal escape peptide that facilitates lysosomal rupture and NLRP3 inflammasome activation. Upon solvent transfer, the polymer self-assembles with neoantigens to form ∼50 nm nanoparticles that facilitate co-delivery to antigen-presenting cells. This polymeric activator of the inflammasome (PAI) was found to induce potent antigen-specific CD8+ T cell responses characterized by IFN-γ and GranzymeB secretion. Moreover, in combination with immune checkpoint blockade therapy, the nanovaccine stimulated robust anti-tumor immune responses against established tumors in EG.7-OVA, B16·F10, and CT-26 models. Results from our studies indicate that NLRP3 inflammasome activating nanovaccines demonstrate promise for development as a robust platform to enhance immunogenicity of neoantigen therapies.

Engineered Synthetic STxB for Enhanced Cytosolic Delivery

Many molecular targets for cancer therapy are located in the cytosol. Therapeutic macromolecules are generally not able to spontaneously translocate across membranes to reach these cytosolic targets. Therefore a strong need exists for tools that enhance cytosolic delivery. Shiga toxin B-subunit (STxB) is used to deliver therapeutic principles to disease-relevant cells that express its receptor, the glycolipid Gb3. Based on its naturally existing membrane translocation capacity, STxB delivers antigens to the cytosol of Gb3-positive dendritic cells, leading to the induction of CD8⁺ T cells. Here, we have explored the possibility of further increasing the membrane translocation of STxB to enable other therapeutic applications. For this, our capacity to synthesize STxB chemically was exploited to introduce unnatural amino acids at different positions of the protein. These were then functionalized with hydrophobic entities to locally destabilize endosomal membranes. Intracellular trafficking of these functionalized STxB was measured by confocal microscopy and their cytosolic arrival with a recently developed highly robust, sensitive, and quantitative translocation assay. From different types of hydrophobic moieties that were linked to STxB, the most efficient configuration was determined. STxB translocation was increased by a factor of 2.5, paving the path for new biomedical opportunities.

Computational design of non-porous, pH-responsive antibody nanoparticles

Programming protein nanomaterials to respond to changes in environmental conditions is a current challenge for protein design and important for targeted delivery of biologics. We describe the design of octahedral non-porous nanoparticles with the three symmetry axes (four-fold, three-fold, and two-fold) occupied by three distinct protein homooligomers: a de novo designed tetramer, an antibody of interest, and a designed trimer programmed to disassemble below a tunable pH transition point. The nanoparticles assemble cooperatively from independently purified components, and a cryo-EM density map reveals that the structure is very close to the computational design model. The designed nanoparticles can package a variety of molecular payloads, are endocytosed following antibody-mediated targeting of cell surface receptors, and undergo tunable pH-dependent disassembly at pH values ranging between to 5.9-6.7. To our knowledge, these are the first designed nanoparticles with more than two structural components and with finely tunable environmental sensitivity, and they provide new routes to antibody-directed targeted delivery.

Endosomal escape of RNA therapeutics: How do we solve this rate-limiting problem?

With over 15 FDA approved drugs on the market and numerous ongoing clinical trials, RNA therapeutics, such as small interfering RNAs (siRNAs) and antisense oligonucleotides (ASOs), have shown great potential to treat human disease. Their mechanism of action is based entirely on the sequence of validated disease-causing genes without the prerequisite knowledge of protein structure, activity or cellular location. In contrast to small molecule therapeutics that passively diffuse across the cell membrane's lipid bilayer, RNA therapeutics are too large, too charged, and/or too hydrophilic to passively diffuse across the cellular membrane and instead are taken up into cells by endocytosis. However, endosomes are also composed of a lipid bilayer barrier that results in endosomal capture and retention of 99% of RNA therapeutics with 1% or less entering the cytoplasm. Although this very low level of endosomal escape has proven sufficient for liver and some CNS disorders, it is insufficient for the vast majority of extra-hepatic diseases. Unfortunately, there are currently no acceptable solutions to the endosomal escape problem. Consequently, before RNA therapeutics can be used to treat widespread human disease, the rate-limiting delivery problem of endosomal escape must be solved in a nontoxic manner.

Engineered EVs designed to target diseases of the CNS

Diseases of the central nervous system (CNS) are challenging to treat, mainly due to the blood-brain barrier (BBB), which restricts drugs in circulation from entering target regions in the brain. To address this issue extracellular vesicles (EVs) have gained increasing scientific interest as carriers able to cross the BBB with multiplex cargos. EVs are secreted by virtually every cell, and their escorted biomolecules are part of an intercellular information gateway between cells within the brain and with other organs. Scientists have undertaken efforts to safeguard the inherent features of EVs as therapeutic delivery vehicles, such as protecting and transferring functional cargo, as well as loading them with therapeutic small molecules, proteins, and oligonucleotides and targeting them to specific cell types for the treatment of CNS diseases. Here, we review current emerging approaches that engineer the EV surface and cargo to improve targeting and functional responses in the brain. We summarize existing applications of engineered EVs as a therapeutic delivery platform for brain diseases, some of which have been evaluated clinically.

Characterization of PGua4, a Guanidinium-Rich Peptoid that Delivers IgGs to the Cytosol via Macropinocytosis

To investigate the structure-cellular penetration relationship of guanidinium-rich transporters (GRTs), we previously designed PGua4, a five-amino acid peptoid containing a conformationally restricted pattern of eight guanidines, which showed high cell-penetrating abilities and low cell toxicity. Herein, we characterized the cellular uptake selectivity, internalization pathway, and intracellular distribution of PGua4, as well as its capacity to deliver cargo. PGua4 exhibits higher penetration efficiency in HeLa cells than in six other cell lines (A549, Caco-2, fibroblast, HEK293, Mia-PaCa2, and MCF7) and is mainly internalized by clathrin-mediated endocytosis and macropinocytosis. Confocal microscopy showed that it remained trapped in endosomes at low concentrations but induced pH-dependent endosomal membrane destabilization at concentrations ≥10 μM, allowing its diffusion into the cytoplasm. Importantly, PGua4 significantly enhanced macropinocytosis and the cellular uptake and cytosolic delivery of large IgGs following noncovalent complexation. Therefore, in addition to its peptoid nature conferring high resistance to proteolysis, PGua4 presents characteristics of a promising tool for IgG delivery and therapeutic applications.

Endosomal escape for cell-targeted proteins. Going out after going in

Protein-based nanocarriers are versatile and biocompatible drug delivery systems. They are of particular interest in nanomedicine as they can recruit multiple functions in a single modular polypeptide. Many cell-targeting peptides or protein domains can promote cell uptake when included in these nanoparticles through receptor-mediated endocytosis. In that way, targeting drugs to specific cell receptors allows a selective intracellular delivery process, avoiding potential side effects of the payload. However, once internalized, the endo-lysosomal route taken by the engulfed material usually results in full degradation, preventing their adequate subcellular localization, bioavailability and subsequent therapeutic effect. Thus, entrapment into endo-lysosomes is a main bottleneck in the efficacy of protein-drug nanomedicines. Promoting endosomal escape and preventing lysosomal degradation would make this therapeutic approach clinically plausible. In this review, we discuss the mechanisms intended to evade lysosomal degradation of proteins, with the most relevant examples and associated strategies, and the methods available to measure that effect. In addition, based on the increasing catalogue of peptide domains tailored to face this challenge as components of protein nanocarriers, we emphasize how their particular mechanisms of action can potentially alter the functionality of accompanying protein materials, especially in terms of targeting and specificity in the delivery process.

Cell penetrating peptides are versatile tools for enhancing multimodal uptake into cells from pest insects

Cell penetrating peptides (CPPs) are small peptides defined by their ability to deliver molecular cargo into cells. While the subject of frequent investigation for pharmaceutical drug delivery, little consideration has been given to the possibility of CPPs for use as insecticides or insecticide enhancers. Here, we characterize the entry of four fluorescently tagged CPPs into two insect cell lines and dissected midgut tissues in terms of both total quantity and mode of penetration. Fluorescent microscopy showed that substantial amounts of CPPs penetrate the plasma membrane via endosomal uptake in ovarian (Sf9) and midgut derived (AW1) lepidopteran cells and that this process was sensitive to selected endocytosis inhibitors. Differences in the quantity of uptake was observed between CPPs, and further differences were found in the ability CPP-1838 to efficiently penetrate membranes through passive diffusion. These findings were extended to primary midgut derived cells and dissected tissues suggesting that CPPs show a preference for goblet cells and that CPP-1838 shows far higher rates of penetration. CPP-1838 thus shows extraordinary abilities to penetrate cells efficiency in both a diffusional and endocytotic manner. From these results more sophisticated delivery methods based on the utilization of CPPs can be developed.

Iron Oxide Nanoparticles Decorated with Functional Peptides for a Targeted siRNA Delivery to Glioma Cells

Glioma is a deadly form of brain cancer, and the difficulty of treating glioma is exacerbated by the chemotherapeutic resistance developed in the tumor cells over the time of treatment. siRNA can be used to silence the gene responsible for the increased resistance, and sensitize the glioma cells to drugs. Here, iron oxide nanoparticles functionalized with peptides (NP-CTX-R₁₀) were used to deliver siRNA to silence O6-methylguanine-DNA methyltransferase (MGMT) to sensitize tumor cells to alkylating drug, Temozolomide (TMZ). The NP-CTX-R₁₀ could complex with siRNA through electrostatic interactions and was able to deliver the siRNA to different glioma cells. The targeting ligand chlorotoxin and cell penetrating peptide polyarginine (R₁₀) enhanced the transfection capability of siRNA to a level comparable to commercially available Lipofectamine. The NP-siRNA was able to achieve up to 90% gene silencing. Glioma cells transfected with NP-siRNA targeting MGMT showed significantly elevated sensitivity to TMZ treatment. This nanoparticle formulation demonstrates the ability to protect siRNA from degradation and to efficiently deliver the siRNA to induce therapeutic gene knockdown.

RNAi-Based Therapeutics and Novel RNA Bioengineering Technologies

RNA interference (RNAi) provides researchers with a versatile means to modulate target gene expression. The major forms of RNAi molecules, genome-derived microRNAs (miRNAs) and exogenous small interfering RNAs (siRNAs), converge into RNA-induced silencing complexes to achieve posttranscriptional gene regulation. RNAi has proven to be an adaptable and powerful therapeutic strategy where advancements in chemistry and pharmaceutics continue to bring RNAi-based drugs into the clinic. With four siRNA medications already approved by the US Food and Drug Administration (FDA), several RNAi-based therapeutics continue to advance to clinical trials with functions that closely resemble their endogenous counterparts. Although intended to enhance stability and improve efficacy, chemical modifications may increase risk of off-target effects by altering RNA structure, folding, and biologic activity away from their natural equivalents. Novel technologies in development today seek to use intact cells to yield true biologic RNAi agents that better represent the structures, stabilities, activities, and safety profiles of natural RNA molecules. In this review, we provide an examination of the mechanisms of action of endogenous miRNAs and exogenous siRNAs, the physiologic and pharmacokinetic barriers to therapeutic RNA delivery, and a summary of the chemical modifications and delivery platforms in use. We overview the pharmacology of the four FDA-approved siRNA medications (patisiran, givosiran, lumasiran, and inclisiran) as well as five siRNAs and several miRNA-based therapeutics currently in clinical trials. Furthermore, we discuss the direct expression and stable carrier-based, in vivo production of novel biologic RNAi agents for research and development. SIGNIFICANCE STATEMENT: In our review, we summarize the major concepts of RNA interference (RNAi), molecular mechanisms, and current state and challenges of RNAi drug development. We focus our discussion on the pharmacology of US Food and Drug Administration-approved RNAi medications and those siRNAs and miRNA-based therapeutics that entered the clinical investigations. Novel approaches to producing new true biological RNAi molecules for research and development are highlighted.

Direct quantification of cytosolic delivery of drug nanocarriers using FlAsH-EDT2

The delivery of therapeutics across the cell membrane and into the cytoplasm is a major challenge that limits the development of new therapies. This challenge is compounded by the lack of a general assay for cytosolic delivery. Here we develop this assay based on the pro-fluorophore CrAsH-EDT2, and provide cytosolic penetration results for a variety of drug delivery agents (polyethyleneimine, poly-arginine, Ferritin, poly [maleic anhydride-alt-isobutene] grafted with dodecylamine, and cationic liposomes) into HeLa and T98G cells. Our results show that this method can be widely applicable to different cells and drug delivery agents, and yield statistically robust results. We later use this method to optimize and improve a model drug delivery agent's (Ferritin) cytosolic penetration.

Strategies and challenges for non-viral delivery of non-coding RNAs to the heart

Non-coding RNAs (ncRNAs), such as miRNAs and long non-coding RNAs (lncRNAs) have been reported as regulators of cardiovascular pathophysiology. Their transient effect and diversified mechanisms of action offer a plethora of therapeutic opportunities for cardiovascular diseases (CVDs). However, physicochemical RNA features such as charge, stability, and structural organization hinder efficient on-target cellular delivery. Here, we highlight recent preclinical advances in ncRNA delivery for the cardiovascular system using non-viral approaches. We identify the unmet needs and advance possible solutions towards clinical translation. Finding the optimal delivery vehicle and administration route is vital to improve therapeutic efficacy and safety; however, given the different types of ncRNAs, this may ultimately not be frameable within a one-size-fits-all approach.

Advances in nanoparticle-based mRNA delivery for liver cancer and liver-associated infectious diseases

The liver is a vital organ that functions to detoxify the body. Liver cancer and infectious diseases such as influenza and malaria can fatally compromise liver function. mRNA delivery is a relatively new means of therapeutic treatment which enables expression of tumor or pathogenic antigens, and elicits immune responses for therapeutic or prophylactic effect. Novel nanoparticles with unique biological properties serving as mRNA carriers have allowed mRNA-based therapeutics to become more clinically viable and relevant. In this review, we highlight recent progress in development of nanoparticle-based mRNA delivery systems for treatment of various liver diseases. First, we present developments in nanoparticle systems used to deliver mRNAs, with specific focus on enhanced cellular uptake and endosomal escape achieved through the use of these nanoparticles. To provide context for diseases that target the liver, we provide an overview of the function and structure of the liver, as well as the role of the immune system in the liver. Then, mRNA-based therapeutic approaches for addressing HCC are highlighted. We also discuss nanoparticle-based mRNA vaccines for treating hepatotropic infectious diseases. Finally, we present current challenges in the clinical translation of nanoparticle-based mRNA delivery systems and provide outlooks for their utilization in treating liver-related diseases.

The therapeutic prospects of N-acetylgalactosamine-siRNA conjugates

RNA interference has become increasingly used for genetic therapy following the rapid development of oligonucleotide drugs. Significant progress has been made in its delivery system and implementation in the treatment of target organs. After a brief introduction of RNA interference technology and siRNA, the efficiency and stability of GalNAc-siRNA conjugates are highlighted since several oligonucleotide drugs of GalNAc have been approved for clinical use in recent years. The structure and features of GalNAc-siRNA conjugates are studied and the clinical efficiency and limitations of oligonucleotide-based drugs are summarized and investigated. Furthermore, another delivery system, lipid nanoparticles, that confer many advantages, is concluded, includ-ing stability and mass production, compared with GalNAc-siRNA conjugates. Importantly, developing new approaches for the use of oligonucleotide drugs brings hope to genetic therapy.

Fusogenic peptide delivery of bioactive siRNAs targeting CSNK2A1 for treatment of ovarian cancer

Ovarian cancer has shown little improvement in survival among advanced-stage patients over the past decade. Current treatment strategies have been largely unsuccessful in treating advanced disease, with many patients experiencing systemic toxicity and drug-resistant metastatic cancer. This study evaluates novel fusogenic peptide carriers delivering short interfering RNA (siRNA) targeting casein kinase II, CSNK2A1, for reducing the aggressiveness of ovarian cancer. The peptides were designed to address two significant barriers to siRNA delivery: insufficient cellular uptake and endosomal entrapment. The three peptide variants developed, DIVA3, DIV3H, and DIV3W, were able to form monodisperse nanoparticle complexes with siRNA and protect siRNAs from serum and RNase degradation. Furthermore, DIV3W demonstrated optimal delivery of bioactive siRNAs into ovarian cancer cells with high cellular uptake efficiency and mediated up to 94% knockdown of CSNK2A1 mRNA compared with non-targeting siRNAs, resulting in decreased cell migration and recolonization in vitro. Intratumoral delivery of DIV3W-siCSNK2A1 complexes to subcutaneous ovarian tumors resulted in reduced CSNK2A1 mRNA and CK2α protein expression after 48 h and reduced tumor growth and migration in a 2-week multi-dosing regimen. These results demonstrate the potential of the DIV3W peptide to deliver bioactive siRNAs and confirms the role of CSNK2A1 in cell-cell communication and proliferation in ovarian cancer.