Unravelling cytosolic delivery of cell penetrating peptides with a quantitative endosomal escape assay

MeCP2 is a naturally supercharged protein with cell membrane transduction capabilities

The intrinsically disordered protein MeCP2 is a global transcriptional regulator encoded by the MECP2 gene. Although the structured domains of MeCP2 have been the subject of multiple studies, its unstructured regions have not been that extensively characterized. In this work, we show that MeCP2 possesses properties akin to those of supercharged proteins. By utilizing its unstructured portions, MeCP2 can successfully transduce across cell membranes and localize to heterochromatic foci in the nuclei, displaying uptake levels a third lower than a MeCP2 construct fused to the cell-penetrating peptide TAT. MeCP2 uptake can further be enhanced by the addition of compounds that promote endosomal escape following cellular trafficking by means of macropinocytosis. Using a combination of in silico prediction algorithms and live-cell imaging experiments, we mapped the sequence in MeCP2 responsible for its cellular incorporation, which bears a striking resemblance to TAT itself. Transduced MeCP2 was shown to interact with HDAC3. These findings provide valuable insight into the properties of MeCP2 and may be beneficial for devising future protein-based treatment strategies.

A novel multitargeted self-assembling peptide-siRNA complex for simultaneous inhibition of SARS-CoV-2-host cell interaction and replication

Effective therapeutics are necessary for managing severe COVID-19 disease despite the availability of vaccines. Small interfering RNA (siRNA) can silence viral genes and restrict SARS-CoV-2 replication. Cell-penetrating peptides is a robust method for siRNA delivery, enhancing siRNA stability and targeting specific receptors. We developed a peptide HE25 that blocks SARS-CoV-2 replication by various mechanisms, including the binding of multiple receptors involved in the virus's internalization, such as ACE2, integrins and NRP1. HE25 not only acts as a vehicle to deliver the SARS-CoV-2 RNA-dependent RNA polymerase siRNA into cells but also facilitates their internalization through endocytosis. Once inside endosomes, the siRNA is released into the cytoplasm through the Histidine-proton sponge effect and the selective cleavage of HE25 by cathepsin B. These mechanisms effectively inhibited the replication of the ancestral SARS-CoV-2 and the Omicron variant BA.5 in vitro. When HE25 was administered in vivo, either by intravenous injection or inhalation, it accumulated in lungs, veins and arteries, endothelium, or bronchial structure depending on the route. Furthermore, the siRNA/HE25 complex caused gene silencing in lung cells in vitro. The SARS-CoV-2 siRNA/HE25 complex is a promising therapeutic for COVID-19, and a similar strategy can be employed to combat future emerging viral diseases.

Plasma membrane damage limits cytoplasmic delivery by conventional cell penetrating peptides

Intracellular delivery of large molecule cargo via cell penetrating peptides (CPPs) is an inefficient process and despite intense efforts in past decades, improvements in efficiency have been marginal. Utilizing a standardized and comparative analysis of the delivery efficiency of previously described cationic, anionic, and amphiphilic CPPs, we demonstrate that the delivery ceiling is accompanied by irreparable plasma membrane damage that is part of the uptake mechanism. As a consequence, intracellular delivery correlates with cell toxicity and is more efficient for smaller peptides than for large molecule cargo. The delivery of pharmaceutically relevant cargo quantities with acceptable toxicity thus seems hard to achieve with the CPPs tested in our study. Our results suggest that any engineered intracellular delivery system based on conventional cationic or amphiphilic CPPs, or the design principles underlying them, needs to accept low delivery yields due to toxicity limiting efficient cytoplasmic uptake. Novel peptide designs based on detailed study of uptake mechanisms are required to overcome these limitations.

Unraveling mRNA delivery bottlenecks of ineffective delivery vectors by co-transfection with effective carriers

The messenger RNA (mRNA) SARS-CoV-2 vaccines have demonstrated the therapeutic potential of this novel drug modality. Protein expression is the consequence of a multistep delivery process that relies on proper packaging into nanoparticle carriers to protect the mRNA against degradation enabling effective cellular uptake and endosomal release, and liberating the mRNA in the cytosol. Bottlenecks along this route remain challenging to pinpoint. Although methods to assess endosomal escape of carriers have been developed, versatile strategies to identify bottlenecks along the delivery trajectory are missing. Here, it is shown that co-incubating an inefficient nanoparticle formulation with an efficient one solves this problem. Cells were co-incubated with mRNA nanoparticles formed with either the efficient cell-penetrating peptide (CPP) PepFect14 or the inefficient CPP nona-arginine (R9). Co-transfection enhanced cellular uptake and endosomal escape of R9-formulated mRNA, resulting in protein expression, demonstrating that both vectors enter cells along the same route. In addition, cells were transfected with a galectin-9-mCherry fusion protein to detect endosomal rupture. Remarkably, despite endosomal release, mRNA remained confined to punctate structures, identifying mRNA liberation as a further bottleneck. In summary, co-transfection offers a rapid means to identify bottlenecks in cytosolic mRNA delivery, supporting the rational design and optimization of intracellular mRNA delivery systems.

Intracellular delivery strategies using membrane-interacting peptides and proteins

While the cellular cytosol and organelles contain attractive targets for disease treatments, it remains a challenge to deliver therapeutic biomacromolecules to these sites. This is due to the selective permeability of the plasma and endosomal membranes, especially for large and hydrophilic therapeutic cargos such as proteins and nucleic acids. In response, many different delivery systems and molecules have been devised to help therapeutics cross these barriers to reach cytosolic targets. Among them are peptide and protein-based systems, which have several advantages over other natural and synthetic materials including their ability to interact with cell membranes. In this review, we will describe recent advances and current challenges of peptide and protein strategies that leverage cell membrane association and modulation to enable cytosolic delivery of biomacromolecule cargo. The approaches covered here include peptides and proteins derived from or inspired by natural sequences as well as those designed de novo for delivery function.

Therapeutic applications of cell engineering using mRNA technology

Engineering and reprogramming cells has significant therapeutic potential to treat a wide range of diseases, by replacing missing or defective proteins, to provide transcription factors (TFs) to reprogram cell phenotypes, or to provide enzymes such as RNA-guided Cas9 derivatives for CRISPR-based cell engineering. While viral vector-mediated gene transfer has played an important role in this field, the use of mRNA avoids safety concerns associated with the integration of DNA into the host cell genome, making mRNA particularly attractive for in vivo applications. Widespread application of mRNA for cell engineering is limited by its instability in the biological environment and challenges involved in the delivery of mRNA to its target site. In this review, we examine challenges that must be overcome to develop effective therapeutics.

Quantitative analysis of electroporation-mediated intracellular delivery via bioorthogonal luminescent reaction

Efficient intracellular delivery is crucial for biotherapeutics, such as proteins, oligonucleotides, and CRISPR/Cas9 gene-editing systems, to achieve their efficacy. Despite the great efforts of developing new intracellular delivery carriers, the lack of straightforward methods for intracellular delivery quantification limits further development in this area. Herein, we designed a simple and versatile bioorthogonal luminescent reaction (BioLure assay) to analyze intracellular delivery. Our results suggest that BioLure can be used to estimate the amount of intracellularly delivered molecules after electroporation, and the estimation by BioLure is in good correlation with the results from complementary methods. Furthermore, we used BioLure assay to correlate the intracellularly-delivered RNase A amount with its tumoricidal activity. Overall, BioLure is a versatile tool for understanding the intracellular delivery process on live cells, and establishing the link between the cytosolic concentration of intracellularly-delivered biotherapeutics and their therapeutic efficacy.

Recent advances in methods for quantifying the cell penetration of macromolecules

As the landscape of macromolecule therapeutics advances, drug developers are continuing to aim at intracellular targets. To activate, inhibit, or degrade these targets, the macromolecule must be delivered efficiently to intracellular compartments. Quite often, there is a discrepancy between binding affinity in biochemical assays and activity in cell-based assays. Identifying the bottleneck for cell-based activity requires robust assays that quantify total cellular uptake and/or cytosolic delivery. Recognizing this need, chemical biologists have designed a plethora of assays to make this measurement, each with distinct advantages and disadvantages. In this review, we describe the latest and most promising developments in the last 3 to 4 years.

The intracellular visualization of exogenous DNA in fluorescence microscopy

In the development of non-viral gene delivery vectors, it is essential to reliably localize and quantify transfected DNA inside the cell. To track DNA, fluorescence microscopy methods are commonly applied. These mostly rely on fluorescently labeled DNA, DNA binding proteins fused to a fluorescent protein, or fluorescence in situ hybridization (FISH). In addition, co-stainings are often used to determine the colocalization of the DNA in specific cellular compartments, such as the endolysosomes or the nucleus. We provide an overview of these DNA tracking methods, advice on how they should be combined, and indicate which co-stainings or additional methods are required to draw precise conclusions from a DNA tracking experiment. Some emphasis is given to the localization of exogenous DNA inside the nucleus, which is the last step of DNA delivery. We argue that suitable tools which allow for the nuclear detection of faint signals are still missing, hampering the rational development of more efficient non-viral transfection systems.

Cell-Penetrating Peptide-Mediated Biomolecule Transportation in Artificial Lipid Vesicles and Living Cells

Signal transduction and homeostasis are regulated by complex protein interactions in the intracellular environment. Therefore, the transportation of impermeable macromolecules (nucleic acids, proteins, and drugs) that control protein interactions is essential for modulating cell functions and therapeutic applications. However, macromolecule transportation across the cell membrane is not easy because the cell membrane separates the intra/extracellular environments, and the types of molecular transportation are regulated by membrane proteins. Cell-penetrating peptides (CPPs) are expected to be carriers for molecular transport. CPPs can transport macromolecules into cells through endocytosis and direct translocation. The transport mechanism remains largely unclear owing to several possibilities. In this review, we describe the methods for investigating CPP conformation, translocation, and cargo transportation using artificial membranes. We also investigated biomolecular transport across living cell membranes via CPPs. Subsequently, we show not only the biochemical applications but also the synthetic biological applications of CPPs. Finally, recent progress in biomolecule and nanoparticle transportation via CPPs into specific tissues is described from the viewpoint of drug delivery. This review provides the opportunity to discuss the mechanism of biomolecule transportation through these two platforms.

A Quantitative Method to Distinguish Cytosolic from Endosome-Trapped Cell-Penetrating Peptides

Cell-penetrating peptides are known to penetrate cells through endocytosis and translocation. The two pathways are hardly distinguished in current cell assays. We developed a reliable, simple and robust method to distinguish and quantify independently the two routes. The assay requires (DABCYL) 4-(dimethylaminoazo)benzene-4-carboxylic acid- and (CF) carboxyfluorescein-labeled peptides. When the labeled peptide is intact, the fluorescence signal is weak thanks to the dark quenching property of DABCYL. A 10-fold higher fluorescence signal is measured when the labeled peptide is degraded. By referring to a standard fluorescent curve according to the concentration of the hydrolyzed peptide, we have access to the internalized peptide quantity. Therefore, cell lysis after internalization permits to determine the total quantity of intracellular peptide. The molecular state of the internalized peptide (intact or degraded), depends on its location in cells (cytosol vs endo-lysosomes), and can be blocked by boiling cells. This boiling step results indeed in denaturation and inhibition of the cellular enzymes. The advantage of this method is the possibility to quantify translocation at 37 °C and to compare it to the 4 °C condition, where all endocytosis processes are inhibited. We found that ranking of the translocation efficacy is DABCYL-R6-(ϵCF)K≫DABCYL-R4-(ϵCF)K≥CF-R9.

Realizing time-staggered expression of nucleic acid-encoded proteins by co-delivery of messenger RNA and plasmid DNA on a single nanocarrier

Co-delivery of different protein-encoding polynucleotide species with varying expression kinetics of their therapeutic product will become a prominent requirement in the realm of combined nucleic acid(NA)-based therapies in the upcoming years. The current study explores the capacity for time-staggered expression of encoded proteins by simultaneous delivery of plasmid DNA (pDNA) in the core and mRNA on the shell of the same nanocarrier. The core is based on a Gelatin Type A-pDNA coacervate, thermally stabilized to form an irreversible nanogel stable enough for the deposition of cationic coats namely, protamine sulfate or LNP-related lipid mixtures. Only the protamine-coated nanocarriers remained colloidally stable following mRNA loading and could successfully co-transfect murine dendritic cell line DC2.4 with fluorescent reporter mRNA(mCherry) and pDNA (pAmCyan1). Further investigation of the protamine-coated nanosystem only, the transfection efficiency (percentage of transfected cells) and level of protein expression (mean fluorescence intensity, MFI) of mRNA and pDNA, simultaneously delivered by the same nanocarrier, were compared and kinetically assessed over 48 h in DC2.4 using flow cytometry. The onset of transfection for both nucleotides was initially delayed, with levels < 5% at 6 h. Thereafter, mRNA transfection reached 90% after 24 h and continued to slightly increase until 48 h. In contrast, pDNA transfection was clearly slower, reaching approximately 40% after 24 h, but continuing to increase to reach 94% at 48 h. The time course of protein expression (represented by MFI) for both NAs essentially followed that of transfection. Model-independent as well as model-dependent kinetic parameters applied to the data further confirmed such time-staggered expression of the two NA's where mRNA's rate of transfection and protein expression initially exceeded those of pDNA in the first 24 h of the experiment whereas the opposite was true during the second 24 h of the experiment where pDNA displayed the higher response rates. We expect that innovative nanocarriers capable of time-staggered co-delivery of different nucleotides could open new perspectives for multi-dosing, pulsatile or sustained expression of nucleic acid-based therapeutics in protein replacement, vaccination, and CRISPR-mediated gene editing scenarios.

Delivery Strategies of siRNA Therapeutics for Hair Loss Therapy

Therapeutic needs for hair loss are intended to find small interfering ribonucleic acid (siRNA) therapeutics for breakthrough. Since naked siRNA is restricted to meet a druggable target in clinic,, delivery systems are indispensable to overcome intrinsic and pathophysiological barriers, enhancing targetability and persistency to ensure safety, efficacy, and effectiveness. Diverse carriers repurposed from small molecules to siRNA can be systematically or locally employed in hair loss therapy, followed by the adoption of new compositions associated with structural and environmental modification. The siRNA delivery systems have been extensively studied via conjugation or nanoparticle formulation to improve their fate in vitro and in vivo. In this review, we introduce clinically tunable siRNA delivery systems for hair loss based on design principles, after analyzing clinical trials in hair loss and currently approved siRNA therapeutics. We further discuss a strategic research framework for optimized siRNA delivery in hair loss from the scientific perspective of clinical translation.

Lipid-mediated intracellular delivery of recombinant bioPROTACs for the rapid degradation of undruggable proteins

Recently, targeted degradation has emerged as a powerful therapeutic modality. Relying on "event-driven" pharmacology, proteolysis targeting chimeras (PROTACs) can degrade targets and are superior to conventional inhibitors against undruggable proteins. Unfortunately, PROTAC discovery is limited by warhead scarcity and laborious optimization campaigns. To address these shortcomings, analogous protein-based heterobifunctional degraders, known as bioPROTACs, have been developed. Compared to small-molecule PROTACs, bioPROTACs have higher success rates and are subject to fewer design constraints. However, the membrane impermeability of proteins severely restricts bioPROTAC deployment as a generalized therapeutic modality. Here, we present an engineered bioPROTAC template able to complex with cationic and ionizable lipids via electrostatic interactions for cytosolic delivery. When delivered by biocompatible lipid nanoparticles, these modified bioPROTACs can rapidly degrade intracellular proteins, exhibiting near-complete elimination (up to 95% clearance) of targets within hours of treatment. Our bioPROTAC format can degrade proteins localized to various subcellular compartments including the mitochondria, nucleus, cytosol, and membrane. Moreover, substrate specificity can be easily reprogrammed, allowing modular design and targeting of clinically-relevant proteins such as Ras, Jnk, and Erk. In summary, this work introduces an inexpensive, flexible, and scalable platform for efficient intracellular degradation of proteins that may elude chemical inhibition.

Chandipura viral glycoprotein (CNV-G) promotes Gectosome generation and enables delivery of intracellular therapeutics

Overexpression of vesicular stomatitis virus G protein (VSV-G) elevates the secretion of EVs known as gectosomes, which contain VSV-G. Such vesicles can be engineered to deliver therapeutic macromolecules. We investigated viral glycoproteins from several viruses for their potential in gectosome production and intracellular cargo delivery. Expression of the viral glycoprotein (viral glycoprotein from the Chandipura virus [CNV-G]) from the human neurotropic pathogen Chandipura virus in 293T cells significantly augments the production of CNV-G-containing gectosomes. In comparison with VSV-G gectosomes, CNV-G gectosomes exhibit heightened selectivity toward specific cell types, including primary cells and tumor cell lines. Consistent with the differential tropism between CNV-G and VSV-G gectosomes, cellular entry of CNV-G gectosome is independent of the Low-density lipoprotein receptor, which is essential for VSV-G entry, and shows varying sensitivity to pharmacological modulators. CNV-G gectosomes efficiently deliver diverse intracellular cargos for genomic modification or responses to stimuli in vitro and in the brain of mice in vivo utilizing a split GFP and chemical-induced dimerization system. Pharmacokinetics and biodistribution analyses support CNV-G gectosomes as a versatile platform for delivering macromolecular therapeutics intracellularly.

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.

Polymeric Nanoparticles for Drug Delivery

The recent emergence of nanomedicine has revolutionized the therapeutic landscape and necessitated the creation of more sophisticated drug delivery systems. Polymeric nanoparticles sit at the forefront of numerous promising drug delivery designs, due to their unmatched control over physiochemical properties such as size, shape, architecture, charge, and surface functionality. Furthermore, polymeric nanoparticles have the ability to navigate various biological barriers to precisely target specific sites within the body, encapsulate a diverse range of therapeutic cargo and efficiently release this cargo in response to internal and external stimuli. However, despite these remarkable advantages, the presence of polymeric nanoparticles in wider clinical application is minimal. This review will provide a comprehensive understanding of polymeric nanoparticles as drug delivery vehicles. The biological barriers affecting drug delivery will be outlined first, followed by a comprehensive description of the various nanoparticle designs and preparation methods, beginning with the polymers on which they are based. The review will meticulously explore the current performance of polymeric nanoparticles against a myriad of diseases including cancer, viral and bacterial infections, before finally evaluating the advantages and crucial challenges that will determine their wider clinical potential in the decades to come.

Recent Progress in the Endosomal Escape Mechanism and Chemical Structures of Polycations for Nucleic Acid Delivery

Nucleic acid-based therapies are seeing a spiralling surge. Stimuli-responsive polymers, especially pH-responsive ones, are gaining widespread attention because of their ability to efficiently deliver nucleic acids. These polymers can be synthesized and modified according to target requirements, such as delivery sites and the nature of nucleic acids. In this regard, the endosomal escape mechanism of polymer-nucleic acid complexes (polyplexes) remains a topic of considerable interest owing to various plausible escape mechanisms. This review describes current progress in the endosomal escape mechanism of polyplexes and state-of-the-art chemical designs for pH-responsive polymers. The importance is also discussed of the acid dissociation constant (i.e., pKa) in designing the new generation of pH-responsive polymers, along with assays to monitor and quantify the endosomal escape behavior. Further, the use of machine learning is addressed in pKa prediction and polymer design to find novel chemical structures for pH responsiveness. This review will facilitate the design of new pH-responsive polymers for advanced and efficient nucleic acid delivery.

Endosomal escape: A bottleneck for LNP-mediated therapeutics

Lipid nanoparticles (LNPs) have recently emerged as a powerful and versatile clinically approved platform for nucleic acid delivery, specifically for mRNA vaccines. A major bottleneck in the field is the release of mRNA-LNPs from the endosomal pathways into the cytosol of cells where they can execute their encoded functions. The data regarding the mechanism of these endosomal escape processes are limited and contradicting. Despite extensive research, there is no consensus regarding the compartment of escape, the cause of the inefficient escape and are currently lacking a robust method to detect the escape. Here, we review the currently known mechanisms of endosomal escape and the available methods to study this process. We critically discuss the limitations and challenges of these methods and the possibilities to overcome these challenges. We propose that the development of currently lacking robust, quantitative high-throughput techniques to study endosomal escape is timely and essential. A better understanding of this process will enable better RNA-LNP designs with improved efficiency to unlock new therapeutic modalities.

Filamentous-Actin-Mimicking Nanoplatform for Enhanced Cytosolic Protein Delivery

Despite the potential of protein therapeutics, the cytosolic delivery of proteins with high efficiency and bioactivity remains a significant challenge owing to exocytosis and lysosomal degradation after endocytosis. Therefore, it is important to develop a safe and efficient strategy to bypass endocytosis. Inspired by the extraordinary capability of filamentous-actin (F-actin) to promote cell membrane fusion, a cyanine dye assembly-containing nanoplatform mimicking the structure of natural F-actin is developed. The nanoplatform exhibits fast membrane fusion to cell membrane mimics and thus enters live cells through membrane fusion and bypasses endocytosis. Moreover, it is found to efficiently deliver protein cargos into live cells and quickly release them into the cytosol, leading to high protein cargo transfection efficiency and bioactivity. The nanoplatform also results in the superior inhibition of tumor cells when loaded with anti-tumor proteins. These results demonstrate that this fusogenic nanoplatform can be valuable for cytosolic protein delivery and tumor treatment.

Investigating the Cytosolic Delivery of Proteins by Lipid Nanoparticles Using the Chloroalkane Penetration Assay

Protein therapeutics are an expanding area for research and drug development, and lipid nanoparticles (LNPs) are the most prominent nonviral vehicles for protein delivery. The most common methods for assessing protein delivery by LNPs include assays that measure the total amount of protein taken up by cells and assays that measure the phenotypic changes associated with protein delivery. However, assays for total cellular uptake include large amounts of protein that are trapped in endosomes or are otherwise nonfunctional. Assays for functional delivery are important, but the readouts are indirect and amplified, limiting the quantitative interpretation. Here, we apply an assay for cytosolic delivery, the chloroalkane penetration assay (CAPA), to measure the cytosolic delivery of a (-30) green fluorescent protein (GFP) fused to Cre recombinase (Cre(-30)GFP) fusion protein by LNPs. We compare these data to the data from total cellular uptake and functional delivery assays to provide a richer analysis of uptake and endosomal escape for LNP-mediated protein delivery. We also use CAPA for a screen of a small library of lipidoids, identifying those with a promising ability to deliver Cre(-30)GFP to the cytosol of mammalian cells. With careful controls and optimized conditions, we expect that CAPA will be a useful tool for investigating the rate, efficiency, and mechanisms of LNP-mediated delivery of therapeutic proteins.

The lipid-binding D4 domain of perfringolysin O facilitates the active loading of exogenous cargo into extracellular vesicles

Whereas extracellular vesicles (EVs) have been engineered for cargo loading, innovative strategies for it can still be developed. Here, we describe domain 4 (D4), a cholesterol-binding domain derived from perfringolysin O, as a viable candidate for EV cargo loading. D4 and its mutants localized to the plasma membrane and the membranes of different vesicular structures in the cytoplasm, and facilitate the transport of proteins of interest (POIs) into EVs. D4-EVs were internalized by recipient cells analogous to EVs engineered with CD9. Intracellular cargo discharge from D4-EVs was successfully detected with the assistance of vesicular stomatitis virus glycoprotein. This study presents a novel strategy for recruiting POIs into EVs via a lipid-binding domain that ensures content release in recipient cells.

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.

Integrin Facilitates the Internalization of TAT Peptide Conjugated to RGD Motif in Model Lipid Membranes

In recent years, targeted drug delivery has attracted a great interest for enhanced therapeutic efficiency, with diminished side effects, especially in cancer therapy. Cell penetrating peptides (CPPs) like HIV1-TAT peptides, appear to be the perfect vectors for translocating drugs or other cargoes across the plasma membrane, but their application is limited mostly due to insufficient specificity for intended targets. Although these molecules were successfully used, the mechanism by which the peptides enter the cell interior still needs to be clarified. The tripeptide motif RGD (arginine-glycine-aspartate), found in extracellular matrix proteins has high affinity for integrin receptors overexpressed in cancer and it is involved in different phases of disease progression, including proliferation, invasion and migration. Discovery of new peptides with high binding affinity for disease receptors and permeability of plasma membranes is desirable for both, development of targeted drug delivery systems and early detection and diagnosis. To complement the TAT peptide with specific targeting ability, we conjugated it with an integrin-binding RGD motif. Although the idea of RGD-CPPs conjugates is not entirely new,[1] here we describe the permeability abilities and specificity of integrin receptors of RGD-TAT peptides in model membranes. Our findings reveal that this novel RGD sequence based on TAT peptide maintains its ability to permeate lipid membranes and exhibits specificity for integrin receptors embedded in giant unilamellar vesicles. This promising outcome suggests that the RGD-TAT peptide has significant potential for applications in the field of targeted drug delivery systems.

Deciphering variations in the endocytic uptake of a cell-penetrating peptide: the crucial role of cell culture protocols

Delivery tools, including cell-penetrating peptides (CPPs), are often inefficient due to a combination of poor endocytosis and endosomal escape. Aspects that impact the delivery of CPPs are typically characterized using tissue culture models. One problem of using cell culture is that cell culture protocols have the potential to contribute to endosomal uptake and endosomal release of CPPs. Hence, a systematic study to identify which aspects of cell culturing techniques impact the endocytic uptake of a typical CPP, the TMR-TAT peptide (peptide sequence derived from HIV1-TAT with the N-terminus labeled with tetramethylrhodamine), was conducted. Aspects of cell culturing protocols previously found to generally modulate endocytosis, such as cell density, washing steps, and cell aging, did not affect TMR-TAT endocytosis. In contrast, cell dissociation methods, media, temperature, serum starvation, and media composition all contributed to changes in uptake. To establish a range of endocytosis achievable by different cell culture protocols, TMR-TAT uptake was compared among protocols. These protocols led to changes in uptake of more than 13-fold, indicating that differences in cell culturing techniques have a cumulative effect on CPP uptake. Taken together this study highlights how different protocols can influence the amount of endocytic uptake of TMR-TAT. Additionally, parameters that can be exploited to improve CPP accumulation in endosomes were identified. The protocols identified herein have the potential to be paired with other delivery enhancing strategies to improve overall delivery efficiency of CPPs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-023-00591-1.

Self-Immolative Polymer Nanoparticles with Precise and Controllable pH-Dependent Degradation

Polymer nanoparticles have generated significant interest as delivery systems for therapeutic cargo. Self-immolative polymers (SIPs) are an interesting category of materials for delivery applications, as the characteristic property of end-to-end depolymerization allows for the disintegration of the delivery system, facilitating a more effective release of the cargo and clearance from the body after use. In this work, nanoparticles based on a pH-responsive polymer poly(ethylene glycol)-b-(2-diisopropyl)amino ethyl methacrylate) and a self-immolative polymer poly[N,N-(diisopropylamino)ethyl glyoxylamide-r-N,N-(dibutylamino)ethyl glyoxylamide] (P(DPAEGAm-r-DBAEGAm)) were developed. Four particles were synthesized based on P(DPAEGAm-r-DBAEGAm) polymers with varied diisopropylamino to dibutylamino ratios of 4:1, 2:1, 2:3, and 0:1, termed 4:1, 2:1, 2:3, and 0:1 PGAm particles. The pH of particle disassembly was tuned from pH 7.0 to pH 5.0 by adjusting the ratio of diisopropylamino to dibutylamino substituents on the pendant tertiary amine. The P(DPAEGAm-r-DBAEGAm) polymers were observed to depolymerize (60-80%) below the particle disassembly pH after ∼2 h, compared to <10% at pH 7.4 and maintained reasonable stability at pH 7.4 (20-50% depolymerization) after 1 week. While all particles exhibited the ability to load a peptide cargo, only the 4:1 PGAm particles had higher endosomal escape efficiency (∼4%) compared to the 2:3 or 0:1 PGAm particles (<1%). The 4:1 PGAm particle is a promising candidate for further optimization as an intracellular drug delivery system with rapid and precisely controlled degradation.

Control of cell penetration enhancer shielding and endosomal escape-kinetics crucial for efficient and biocompatible siRNA delivery

Although cationic liposomes are efficient carriers for nucleic acid delivery, their toxicity often hampers the clinical translation. Polyethylene glycol (PEG) coating has been largely used to improve their stability and reduce toxicity. Nevertheless, it has been found to decrease the transfection process. In order to exploit the advantages of cationic liposomes and PEG decoration for nucleic acid delivery, liposomes decorated with tetraArg-[G-1]-distearoyl glycerol (Arg4-DAG) dendronic oligo-cationic lipid enhancer (OCE) and PEG-lipid have been investigated. Non decorated or OCE-decorated lipoplexes (OCEfree-LPX and OCE-LPX, respectively) were obtained by lipid film hydration using oligonucleotide (ON) solutions. PEG and OCE/PEG decorated lipoplexes (PEG-OCEfree-LPX and PEG-OCE-LPX, respectively) were obtained by post-insertion of 2 or 5 kDa PEG-DSPE on preformed lipoplexes. The OCE decoration yielded lipoplexes with size of about 240 nm, 84% loading efficiency at 10 N/P ratio, ten times higher than OCEfree-LPX, and prevented the ON release when incubated with physiological heparin concentration or with plasma. The PEG decoration reduced the zeta potential, enhanced the lipoplex stability in serum and decreased both hemolysis and cytotoxicity, while it did not affect the lipoplex size and ON loading. With respect to OCEfree-LPX, the OCE-LPX remarkably associated with cells and were taken up by different cancer cell lines (HeLa and MDA-MB-231). Interestingly, 2 or 5 kDa PEG decoration did not reduce either the cell interaction or the cell up-take of the cationic lipoplexes. With siRNA as a payload, OCE enabled efficient internalization, but endosomal release was hampered. Post-transfection treatment with the lysosomotropic drug chloroquine allowed to identify the optimal time point for endosomal escape. Chloroquine treatment after 12 to 20 h of LPX pre-incubation enabled siRNA mediated target knockdown indicating that this is the time window of endo-lysosomal processing. This indicates that OCE can protect siRNA from lysosomal degradation for up to 20 h, as shown by these rescue experiments.

Nanomechanical action opens endo-lysosomal compartments

Endo-lysosomal escape is a highly inefficient process, which is a bottleneck for intracellular delivery of biologics, including proteins and nucleic acids. Herein, we demonstrate the design of a lipid-based nanoscale molecular machine, which achieves efficient cytosolic transport of biologics by destabilizing endo-lysosomal compartments through nanomechanical action upon light irradiation. We fabricate lipid-based nanoscale molecular machines, which are designed to perform mechanical movement by consuming photons, by co-assembling azobenzene lipidoids with helper lipids. We show that lipid-based nanoscale molecular machines adhere onto the endo-lysosomal membrane after entering cells. We demonstrate that continuous rotation-inversion movement of Azo lipidoids triggered by ultraviolet/visible irradiation results in the destabilization of the membranes, thereby transporting cargoes, such as mRNAs and Cre proteins, to the cytoplasm. We find that the efficiency of cytosolic transport is improved about 2.1-fold, compared to conventional intracellular delivery systems. Finally, we show that lipid-based nanoscale molecular machines are competent for cytosolic transport of tumour antigens into dendritic cells, which induce robust antitumour activity in a melanoma mouse model.

Engineering Proteins for Cell Entry

Proteins are essential for life, as they participate in all vital processes in the body. In the past decade, delivery of active proteins to specific cells and organs has attracted increasing interest. However, most proteins cannot enter the cytoplasm due to the cell membrane acting as a natural barrier. To overcome this challenge, various proteins have been engineered to acquire cell-penetrating capacity by mimicking or modifying natural shuttling proteins. In this review, we provide an overview of the different types of engineered cell-penetrating proteins such as cell-penetrating peptides, supercharged proteins, receptor-binding proteins, and bacterial toxins. We also discuss some strategies for improving endosomal escape such as pore formation, the proton sponge effect, and hijacking intracellular trafficking pathways. Finally, we introduce some novel methods and technologies for designing and detecting engineered cell-penetrating proteins.

Intracellular protein delivery: New insights into the therapeutic applications and emerging technologies

The inability to cross the plasma membranes traditionally limited the therapeutic use of recombinant proteins. However, in the last two decades, novel technologies made delivering proteins inside the cells possible. This allowed researchers to unlock intracellular targets, once considered 'undruggable', bringing a new research area to emerge. Protein transfection systems display a large potential in a plethora of applications. However, their modality of action is often unclear, and cytotoxic effects are elevated, whereas experimental conditions to increase transfection efficacy and cell viability still need to be identified. Furthermore, technical complexity often limits in vivo experimentation, while challenging industrial and clinical translation. This review highlights the applications of protein transfection technologies, and then critically discuss the current methodologies and their limitations. Physical membrane perforation systems are compared to systems exploiting cellular endocytosis. Research evidence of the existence of either extracellular vesicles (EVs) or cell-penetrating peptides (CPPs)- based systems, that circumvent the endosomal systems is critically analysed. Commercial systems, novel solid-phase reverse protein transfection systems, and engineered living intracellular bacteria-based mechanisms are finally described. This review ultimately aims at finding new methodologies and possible applications of protein transfection systems, while helping the development of an evidence-based research approach.

A Comparison of Cellular Uptake Mechanisms, Delivery Efficacy, and Intracellular Fate between Liposomes and Extracellular Vesicles

A key aspect for successful drug delivery via lipid-based nanoparticles is their internalization in target cells. Two prominent examples of such drug delivery systems are artificial phospholipid-based carriers, such as liposomes, and their biological counterparts, the extracellular vesicles (EVs). Despite a wealth of literature, it remains unclear which mechanisms precisely orchestrate nanoparticle-mediated cargo delivery to recipient cells and the subsequent intracellular fate of therapeutic cargo. In this review, internalization mechanisms involved in the uptake of liposomes and EVs by recipient cells are evaluated, also exploring their intracellular fate after intracellular trafficking. Opportunities are highlighted to tweak these internalization mechanisms and intracellular fates to enhance the therapeutic efficacy of these drug delivery systems. Overall, literature to date shows that both liposomes and EVs are predominantly internalized through classical endocytosis mechanisms, sharing a common fate: accumulation inside lysosomes. Studies tackling the differences between liposomes and EVs, with respect to cellular uptake, intracellular delivery and therapy efficacy, remain scarce, despite its importance for the selection of an appropriate drug delivery system. In addition, further exploration of functionalization strategies of both liposomes and EVs represents an important avenue to pursue in order to control internalization and fate, thereby improving therapeutic efficacy.

Engineered Histidine-Rich Peptides Enhance Endosomal Escape for Antibody-Targeted Intracellular Delivery of Functional Proteins

Currently, the clinical application of protein/peptide therapeutics is mainly limited to the modulation of diseases in extracellular spaces. Intracellular targets are hardly accessed, owing largely to the endosomal entrapment of internalized proteins/peptides. Here, we report a strategy to design and construct peptides that enable endosome-to-cytosol delivery based on an extension of the "histidine switch" principle. By substituting the Arg/Lys residues in cationic cell-penetrating peptides (CPPs) with histidine, we obtained peptides with pH-dependent membrane-perturbation activity. These peptides do not randomly penetrate cells like CPPs, but imitate the endosomal escape of CPPs following cellular uptake. Working with one such 16-residue peptide (hsLMWP) with high endosomal escape capacity, we engineered modular fusion proteins and achieved antibody-targeted delivery of diverse protein cargoes-including the pro-apoptotic protein BID (BH3-interacting domain death agonist) and Cre recombinase-into the cytosol of multiple cancer cell types. After extensive in vitro testing, an in vivo analysis with xenograft mice ultimately demonstrated that a trastuzumab-hsLMWP-BID fusion conferred strong anti-tumor efficacy without apparent side effects. Notably, our fusion protein features a modular design, allowing flexible applications for any antibody/cargo combination of choice. Therefore, the potential applications extend throughout life science and biomedicine, including gene editing, cancer treatment, and immunotherapy.

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.

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.

Delivered complementation in planta (DCIP) enables measurement of peptide-mediated protein delivery efficiency in plants

Using a fluorescence complementation assay, Delivered Complementation in Planta (DCIP), we demonstrate cell-penetrating peptide-mediated cytosolic delivery of peptides and recombinant proteins in Nicotiana benthamiana. We show that DCIP enables quantitative measurement of protein delivery efficiency and enables functional screening of cell-penetrating peptides for in-planta protein delivery. Finally, we demonstrate that DCIP detects cell-penetrating peptide-mediated delivery of recombinantly expressed proteins such as mCherry and Lifeact into intact leaves. We also demonstrate delivery of a recombinant plant transcription factor, WUSCHEL (AtWUS), into N. benthamiana. RT-qPCR analysis of AtWUS delivery in Arabidopsis seedlings also suggests delivered WUS can recapitulate transcriptional changes induced by overexpression of AtWUS. Taken together, our findings demonstrate that DCIP offers a new and powerful tool for interrogating cytosolic delivery of proteins in plants and highlights future avenues for engineering plant physiology.

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

Crotamine/siRNA Nanocomplexes for Functional Downregulation of Syndecan-1 in Renal Proximal Tubular Epithelial Cells

Proteinuria drives progressive tubulointerstitial fibrosis in native and transplanted kidneys, mainly through the activation of proximal tubular epithelial cells (PTECs). During proteinuria, PTEC syndecan-1 functions as a docking platform for properdin-mediated alternative complement activation. Non-viral gene delivery vectors to target PTEC syndecan-1 could be useful to slow down alternative complement activation. In this work, we characterize a PTEC-specific non-viral delivery vector composed of the cell-penetrating peptide crotamine complexed with a syndecan-1 targeting siRNA. Cell biological characterization was performed in the human PTEC HK2 cell line, using confocal microscopy, qRT-PCR, and flow cytometry. PTEC targeting in vivo was carried out in healthy mice. Crotamine/siRNA nanocomplexes are positively charged, about 100 nm in size, resistant to nuclease degradation, and showed in vitro and in vivo specificity and internalization into PTECs. The efficient suppression of syndecan-1 expression in PTECs mediated by these nanocomplexes significantly reduced properdin binding (p < 0.001), as well as the subsequent complement activation by the alternative complement pathway (p < 0.001), as observed in either normal or activated tubular conditions. To conclude, crotamine/siRNA-mediated downregulation of PTEC syndecan-1 reduced the activation of the alternative complement pathway. Therefore, we suggest that the present strategy opens new venues for targeted proximal tubular gene therapy in renal diseases.

Stimuli-responsive silk fibroin for on-demand drug delivery

Stimuli-responsive "smart" hydrogel biomaterials have attracted great attention in the biomedical field, especially in designing novel on-demand drug delivery systems. As a handful natural biomaterial approved by US Food and Drug Administration, silk fibroin (SF) has unique high temperature resistance as well as tunable structural composition. These properties make it one of the most ideal candidates for on-demand drug delivery. Meanwhile, recent advances in polymer modification and nanomaterials have fostered the development of various stimuli-responsive delivery systems. Here, we first review the recent advance in designing responsive SF-based delivery systems in different stimulus sources. These systems are able to release mediators in a desired manner in response to specific stimuli in active or passive manners. We then describe applications of these specially designed responsive delivery systems in wound healing, tumor therapy, as well as immunomodulation. We also discuss the future challenges and prospects of stimuli-responsive SF-based delivery systems.

Engineering protein-based therapeutics through structural and chemical design

Protein-based therapeutics have led to new paradigms in disease treatment. Projected to be half of the top ten selling drugs in 2023, proteins have emerged as rivaling and, in some cases, superior alternatives to historically used small molecule-based medicines. This review chronicles both well-established and emerging design strategies that have enabled this paradigm shift by transforming protein-based structures that are often prone to denaturation, degradation, and aggregation in vitro and in vivo into highly effective therapeutics. In particular, we discuss strategies for creating structures with increased affinity and targetability, enhanced in vivo stability and pharmacokinetics, improved cell permeability, and reduced amounts of undesired immunogenicity.

Protein Delivery to Insect Epithelial Cells In Vivo: Potential Application to Functional Molecular Analysis of Proteins in Butterfly Wing Development

Protein delivery to cells in vivo has great potential for the functional analysis of proteins in nonmodel organisms. In this study, using the butterfly wing system, we investigated a method of protein delivery to insect epithelial cells that allows for easy access, treatment, and observation in real time in vivo. Topical and systemic applications (called the sandwich and injection methods, respectively) were tested. In both methods, green/orange fluorescent proteins (GFP/OFP) were naturally incorporated into intracellular vesicles and occasionally into the cytosol from the apical surface without any delivery reagent. However, the antibodies were not delivered by the sandwich method at all, and were delivered only into vesicles by the injection method. A membrane-lytic peptide, L17E, appeared to slightly improve the delivery of GFP/OFP and antibodies. A novel peptide reagent, ProteoCarry, successfully promoted the delivery of both GFP/OFP and antibodies into the cytosol via both the sandwich and injection methods. These protein delivery results will provide opportunities for the functional molecular analysis of proteins in butterfly wing development, and may offer a new way to deliver proteins into target cells in vivo in nonmodel organisms.

Polymeric Micellar Systems-A Special Emphasis on "Smart" Drug Delivery

Concurrent developments in anticancer nanotechnological treatments have been observed as the burden of cancer increases every year. The 21st century has seen a transformation in the study of medicine thanks to the advancement in the field of material science and nanomedicine. Improved drug delivery systems with proven efficacy and fewer side effects have been made possible. Nanoformulations with varied functions are being created using lipids, polymers, and inorganic and peptide-based nanomedicines. Therefore, thorough knowledge of these intelligent nanomedicines is crucial for developing very promising drug delivery systems. Polymeric micelles are often simple to make and have high solubilization characteristics; as a result, they seem to be a promising alternative to other nanosystems. Even though recent studies have provided an overview of polymeric micelles, here we included a discussion on the "intelligent" drug delivery from these systems. We also summarized the state-of-the-art and the most recent developments of polymeric micellar systems with respect to cancer treatments. Additionally, we gave significant attention to the clinical translation potential of polymeric micellar systems in the treatment of various cancers.

Lipid-Specific Direct Translocation of the Cell-Penetrating Peptide NAF-144-67 across Bilayer Membranes

The cell-penetrating peptide NAF-1 has recently emerged as a promising candidate for selective penetration and destruction of cancer cells. It displays numerous membrane-selective behaviors including cell-specific uptake and organelle-specific degradation. In this work, we explore membrane penetration and translocation of NAF-1 in model lipid bilayer vesicles as a function of lipid identity in zwitterionic phosphatidylcholine lipids mixed with anionic phosphatidylserine, phosphatidylglycerol, and phosphatidic acid lipids. By monitoring the digestion of NAF-1 using the protease trypsin located inside but not outside the vesicles, we determined that the translocation of NAF-1 was significantly enhanced by the presence of phosphatidic acid in the membrane compared to the other three anionic or zwitterionic lipids. These findings were correlated to fluorescence measurements of dansyl-labeled NAF-1, which revealed whether noncovalent interactions between NAF-1 and the bilayer were most stable either at the membrane/solution interface or within the membrane interior. Phosphatidic acid promoted interactions with fatty acid tails, while phosphatidylcholine, phosphatidylserine, and phosphatidylglycerol stabilized interactions with polar lipid headgroups. Interfacial vibrational sum frequency spectroscopy experiments revealed that the phosphate moiety on phosphatidic acid headgroups was better hydrated than on the other three lipids, which helped to shuttle NAF-1 into the hydrophobic region. Our findings demonstrate that permeation does not depend on the net charge on phospholipid lipid headgroups in these model vesicles and suggest a model wherein NAF-1 crosses membranes selectively due to lipid-specific interactions at bilayer surfaces.

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.

Comprehensive Analyses of the Intracellular and in Vivo Disposition of Fab- Small Interfering RNA Conjugate to Identify Key Issues to Improve Its in Vivo Activity

Comprehensive analyses of intracellular disposition and in vivo pharmacokinetics were performed for small interfering RNA (siRNA) conjugated with the Fab fragment of panitumumab, a fully humanized monoclonal antibody against epidermal growth factor receptor (EGFR). The Fab-siRNA conjugate was internalized into EGFR-expressing cancer cells in an antigen-dependent manner. Intracellular disposition was quantitatively evaluated using fluorescent-labeled panitumumab and confocal microscopy. The majority of internalized panitumumab was suggested to be transferred into lysosomes. In vivo pharmacokinetics were evaluated in EGFR-expressing tumor-bearing mice. Intact Fab-siRNA was measured by immunoprecipitation using anti-Fab antibody followed by quantitative polymerase chain reaction. The Fab portion was measured by a ligand binding assay. Intact Fab-siRNA concentrations rapidly decreased in the plasma and tumor, although the Fab portion concentration remained high, suggesting extensive degradation in the linker-siRNA portion. After incubation of Fab-siRNA in mouse plasma, samples were digested with proteinase K, and extracted siRNA tagged with Fab-derived peptide was subjected to an ion-pair reversed-phase liquid chromatography with mass spectrometry analysis. Results suggested that hydrolysis from the 3' end of the antisense strand of siRNA is the major metabolizing pathway. Based on these findings, endosomal escape and stability in lysosomes, blood, and tumor are key factors to improve to achieve efficient target gene knockdown in tumors, and stabilizing the 3' end of the antisense strand was suggested to be most efficient. Our approaches clearly identified the key issues of Fab-siRNA from a pharmacokinetics aspect, which will be useful for improving the in vivo activity of siRNA conjugated with not only Fab but also other immunoproteins. SIGNIFICANCE STATEMENT: The intracellular and in vivo disposition of Fab-small interfering RNA (siRNA) conjugate was comprehensively investigated using various approaches, including newly developed analytical methods. This study clearly shows that improvements in siRNA stability in lysosomes, blood, and tumor are needed for target gene knockdown in tumors. The major metabolic pathway of Fab-siRNA is 3' exonuclease degradation, suggesting that optimization of the conjugation site to Fab might help improve stability.

Plasma membrane depolarization reveals endosomal escape incapacity of cell-penetrating peptides

Cell-penetrating peptides (CPPs) are short (<30 amino acids), generally cationic, peptides that deliver diverse cargos into cells. CPPs access the cytosol either by direct translocation through the plasma membrane or via endocytosis followed by endosomal escape. Both direct translocation and endosomal escape can occur simultaneously, making it non-trivial to specifically study endosomal escape alone. Here we depolarize the plasma membrane and showed that it inhibits the direct translocation of several CPPs but does not affect their uptake into endosomes. Despite good endocytic uptake many CPPs previously considered to access the cytosol via endosomal escape, failed to access the cytosol once direct translocation was abrogated. Even CPPs designed for enhanced endosomal escape actually showed negligible endosomal escape into the cytosol. Our data reveal that cytosolic localization of CPPs occurs mainly by direct translocation across the plasma membrane. Cell depolarization represents a simple manipulation to stringently test the endosomal escape capacity of CPPs.

Single-cell quantification and dose-response of cytosolic siRNA delivery

Endosomal escape and subsequent cytosolic delivery of small interfering RNA (siRNA) therapeutics is believed to be highly inefficient. Since it has not been possible to quantify cytosolic amounts of delivered siRNA at therapeutic doses, determining delivery bottlenecks and total efficiency has been difficult. Here, we present a confocal microscopy-based method to quantify cytosolic delivery of fluorescently labeled siRNA during lipid-mediated delivery. This method enables detection and quantification of sub-nanomolar cytosolic siRNA release amounts from individual release events with measures of quantitation confidence for each event. Single-cell kinetics of siRNA-mediated knockdown in cells expressing destabilized eGFP unveiled a dose-response relationship with respect to knockdown induction, depth and duration in the range from several hundred to thousands of cytosolic siRNA molecules. Accurate quantification of cytosolic siRNA, and the establishment of the intracellular dose-response relationships, will aid the development and characterization of novel delivery strategies for nucleic acid therapeutics.

Nanomaterial-mediated photoporation for intracellular delivery

Translocation of extrinsic molecules into living cells is becoming increasingly crucial in biological studies ranging from cell engineering to biomedical applications. The concerns regarding biosafety and immunogenicity for conventional vectors and physical methods yet challenge effective intracellular delivery. Here, we begin with an overview of approaches for trans-membrane delivery up to now. These methods are featured with a relatively mature application but usually encounter low cell survival. Our review then proposes an advanced application for nanomaterial-sensitized photoporation triggered with a laser. We cover the mechanisms, procedures, and outcomes of photoporation-induced intracellular delivery with a highlight on its versatility to different living cells. We hope the review discussed here encourages researchers to further improvement and applications for photoporation-induced intracellular delivery. STATEMENT OF SIGNIFICANCE.

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.

Direct Cytosolic Delivery of Citraconylated Proteins

Current intracellular protein delivery strategies face the challenge of endosomal entrapment and consequent degradation of protein cargo. Methods to efficiently deliver proteins directly to the cytosol have the potential to overcome this hurdle. Here, we report the use of a straightforward approach of protein modification using citraconic anhydride to impart an overall negative charge on the proteins, enabling them to assemble with positively charged nano vectors. This strategy uses anhydride-modified proteins to electrostatically form polymer-protein nanocomposites with a cationic guanidinium-functionalized polymer. These supramolecular self-assemblies demonstrated the efficient cytosolic delivery of modified proteins through a membrane fusion-like mechanism. This approach was validated on five cell lines and seven proteins as cargo. Retention of protein function was confirmed through efficient cell killing via the intracellular enzymatic activity of RNase A. This platform provides a versatile, straightforward, and single-step method of protein modification and efficient direct cytosolic protein delivery.