1-Pyrene Carboxylic Acid: An Internalization Enhancer for Short Oligoarginines

Getting through the cell membrane is challenging, and transporting a therapeutic agent while entering the cell is even more complicated. Cell-penetrating peptides (CPPs) are valuable tools for solving this problem, although they have drawbacks. In this work, the synthesis and investigation of efficient CPPs are described. We used an aromatic group, 1-pyrene carboxylic acid (PCA), to enhance internalization. We designed oligoarginines to investigate the effect of PCA in different positions at the N-terminus or in the side chain. Our novel peptide derivatives showed remarkable internalization on tumor cell lines, and more than one endocytic pathway plays a role in their internalization mechanism. With this modification, there is an opportunity to design short oligoarginines that can rival well-known CPPs like octaarginine in internalization.

KCNN4 as a genomic determinant of cytosolic delivery by the attenuated cationic lytic peptide L17E

The development of a cytosolic delivery strategy for biopharmaceuticals is one of the central issues in drug development. Knowledge of the mechanisms underlying these processes may also pave the way for the discovery of novel delivery systems. L17E is an attenuated cationic amphiphilic lytic (ACAL) peptide developed by our research group that shows promise for cytosolic antibody delivery. In this study, given the high efficacy of L17E in cytosolic delivery, we investigated the mechanism of action of L17E in detail. L17E was found to achieve cytosolic delivery predominantly by transient disruption of the plasma membrane without the need for endocytosis. Importantly, the cell-line selectivity studies of L17E revealed a strong correlation between the efficiency of L17E-mediated delivery and the expression level of KCNN4, the gene encoding the calcium-activated potassium channel KCa3.1. Genetic and pharmacological regulation of KCNN4 expression and KCa3.1 activity, respectively, correlate closely with the efficiency of L17E-mediated cytosolic delivery, suggesting the importance of membrane-potential regulation by extracellular Ca2+ influx. Therefore, the activity of the L17E is relevant to the calcium-activated potassium channel.

Polysaccharide-Based Coacervate Microgel Bearing Cationic Peptides That Achieve Dynamic Cell-Membrane Structure Alteration and Facile Cytosolic Infusion of IgGs

Conjugates of the biocompatible polysaccharide pullulan with a cell membrane permeabilizing peptide L17E (PL-L17Es) were prepared with the aim of producing complex coacervates with pronounced intracellular antibody (IgG) delivery activity and stable structures. Coacervates with diameters of a few μm were formed simply by mixing PL-L17Es with IgG labeled with negatively charged fluorescent moieties of Alexa Fluor 488 [IgG(AF488)]. The coacervate resulted in a pronounced cytosolic infusion of IgG(AF488) and IgG binding to the target proteins inside the cell. The droplet structures were maintained even under high salt conditions, and the fluorescence in the droplet was not recovered after photobleaching, suggesting the formation of complex coacervate microgels. Dynamic changes in cell membrane structure to entrap the coacervate microgels were captured by confocal and electron microscopy, resulting in cytosolic IgG infusion. The use of M-lycotoxin instead of L17E resulted in a coacervate microgel with marked IgG delivery activity even in the presence of serum. Successful IgG delivery to primary hepatocytes, undifferentiated induced pluripotent stem (iPS) cells, and iPS cell-derived intestinal epithelial cells was also achieved. The construction of complex coacervate microgels with design flexibility and the validity of intracellular IgG delivery with high salt stability were thus demonstrated.

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.

Influence of Aza-Glycine Substitution on the Internalization of Penetratin

The cell-penetrating peptide (CPP) penetratin has gained much attention over many years due to its potential role as a transporter for a broad range of cargo into cells. The modification of penetratin has been extensively investigated too. Aza-peptides are peptide analogs in which one or more of the amino residues are replaced by a semicarbazide. This substitution results in conformational restrictions and modifications in hydrogen bonding properties, which affect the structure and may lead to enhanced activity and selectivity of the modified peptide. In this work, the Trp residues of penetratin were substituted by aza-glycine or glycine residues to examine the effect of these modifications on the cellular uptake and the internalization mechanism. The substitution of Trp48 or Trp48,56 dramatically reduced the internalization, showing the importance of Trp48 in cellular uptake. Interestingly, while aza-glycine in the position of Trp56 increased the cellular uptake, Gly reduced it. The two Trp-modified derivatives showed altered internalization pathways, too. Based on our knowledge, this is the first study about the effect of aza-amino acid substitution on the cell entry of CPPs. Our results suggest that aza-amino acid insertion is a useful modification to change the internalization of a CPP.

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.

Cell-Permeable Ubiquitin and Histone Tools for Studying Post-translational Modifications

Protein post-translational modifications (PTMs) regulate nearly all biological processes in eukaryotic cells, and synthetic PTM protein tools are widely used to detect the activity of the related enzymes and identify the interacting proteins in cell lysates. Recently, the study of these enzymes and the interacting proteome has been accomplished in live cells using cell-permeable PTM protein tools. In this concept, we will introduce cell penetrating techniques, the syntheses of cell-permeable PTM protein tools, and offer some future perspective.

Enhancing Cell Penetration Efficiency of Cyclic Oligoarginines Using Rigid Scaffolds

Delivering therapeutic agents into cells has always been a major challenge. In recent years, cyclization emerged as a tool for designing CPPs to increase their internalization and stability. Cyclic ring(s) can protect the peptide from enzymatic degradation, so cyclic peptides remain intact. Therefore they can be good carrier molecules. In this work, the preparation and investigation of efficient cyclic CPPs are described. Different oligoarginines were designed to conjugate with rigid aromatic scaffolds or form disulfide bonds. The reaction between the scaffolds and the peptides forms stable thioether bonds, constraining the peptide into a cyclic structure. The constructs presented very efficient internalization on cancerous cell lines. Our peptides use more than one endocytic pathway for cellular uptake. In this way, short peptides, which can compete with the penetration of well-known CPPs such as octaarginine (Arg₈), may be synthesized through cyclization.

Immobilized nanoparticles-mediated enzyme therapy; promising way into clinical development

Enzyme (Enz)-mediated therapy indicated a remarkable effect in the treatment of many human cancers and diseases with an insight into clinical phases. Because of insufficient immobilization (Imb) approach and ineffective carrier, Enz therapeutic exhibits low biological efficacy and bio-physicochemical stability. Although efforts have been made to remove the limitations mentioned in clinical trials, efficient Imb-destabilization and modification of nanoparticles (NPs) remain challenging. NP internalization through insufficient membrane permeability, precise endosomal escape, and endonuclease protection following release are the primary development approaches. In recent years, innovative manipulation of the material for Enz immobilization (EI) fabrication and NP preparation has enabled nanomaterial platforms to improve Enz therapeutic outcomes and provide low-diverse clinical applications. In this review article, we examine recent advances in EI approaches and emerging views and explore the impact of Enz-mediated NPs on clinical therapeutic outcomes with at least diverse effects.

Exploring Cell-Penetrating Peptides as Penetration Enhancers in Eye Drop Formulations Using a Reconstructed Human Corneal Epithelial Model

Ocular tissues function as biological barriers that hinder drug delivery, depending on the target tissue and route of administration, and must be overcome to achieve the desired therapeutic effect. Penetration enhancers have long been investigated to improve corneal drug penetration via eye drop instillation; however, further development is warranted owing to potential safety concerns. In the present study, we focused on cell-penetrating peptides (CPPs) as a penetration enhancer to address the requirements and explored CPP candidates suitable for corneal drug delivery. Using a reconstructed human corneal epithelial tissue model, LabCyte CORNEA-MODEL24 as an alternative to animal testing that is expected to have higher reproducibility than extracted eyeballs and octa-arginine (R8) as a representative model CPP with simple structure, we investigated the enhancement of 6-carboxyfluorescein (6-FAM) uptake by fluorescence imaging and the potential of eye irritation by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Also, surface plasmon resonance (SPR) evaluated the interaction between R8 and model compounds, suggesting that the stronger interaction could facilitate the corneal uptake of compounds. A comparative screening study of corneal uptake using various CPPs showed that the CPPs other than R8 also have the potential to enhance the corneal uptake of 6-FAM. In particular, penetratin (PNT) showed stronger fluorescence intensity. Through these findings, this manuscript provides beneficial information for the development of a novel corneal penetration enhancer with CPPs. In the future, it is expected that the basic findings with R8 will be verified to be applicable to other CPPs for development as penetration enhancers for eye drop formulation.

Cell-Penetrating Dabcyl-Containing Tetraarginines with Backbone Aromatics as Uptake Enhancers

Cell-penetrating peptides represent an emerging class of carriers capable of effective cellular delivery. This work demonstrates the preparation and investigation of efficient CPPs. We have already shown that the presence of 4-((4-(dimethylamino)phenyl)azo)benzoic acid (Dabcyl) and Trp greatly increase the uptake of oligoarginines. This work is a further step in that direction. We have explored the possibility of employing unnatural, aromatic amino acids, to mimic Trp properties and effects. The added residues allow the introduction of aromaticity, not as a side-chain group, but rather as a part of the sequence. The constructs presented exceptional internalization on various cell lines, with an evident structure-activity relationship. The CPPs were investigated for their entry mechanisms, and our peptides exploit favorable pathways, yet one of the peptides relies highly on direct penetration. Confocal microscopy studies have shown selectivity towards the cell lines, by showing diffuse uptake in FADU cells, while vesicular uptake takes place in SCC-25 cell line. These highly active CPPs have proved their applicability in cargo delivery by successfully delivering antitumor drugs into MCF-7 and MDA-MB-231 cells. The modifications in the sequences allow the preparation of short yet highly effective constructs able to rival the penetration of well-known CPPs such as octaarginine (Arg8).

Stearylated Macropinocytosis-Inducing Peptides Facilitating the Cellular Uptake of Small Extracellular Vesicles

Macropinocytosis is a form of endocytosis that allows massive uptake of extracellular materials and is a promising route for intracellular delivery of biofunctional macromolecules and nanoparticles. Our laboratory developed a potent macropinocytosis-inducing peptide named P4A. However, the ability of this peptide is not apparent in the presence of serum. This study aims to endow P4A and related peptides with the ability to induce macropinocytosis in the presence of serum by N-terminal acylation with long-chain fatty acids (i.e., decanoic, myristic, and stearic acids). Stearylated P4A (stearyl-P4A) had the highest effect on stimulating macropinocytotic uptake. Moreover, the intramolecularly disulfide-bridged analogue, stearyl-oxP4A, showed an even higher ability. The effect of stearyl-oxP4A to facilitate the intracellular delivery of small extracellular vesicles (sEVs) was evaluated in terms of (i) cellular uptake using sEVs labeled with an enhanced green fluorescent protein (EGFP) and (ii) cytosolic liberation and expression of sEV-encapsulated luciferase mRNA in recipient cells. The two- to threefold uptake of both sEVs in the presence of stearyl-oxP4A suggests the potential of the peptide for sEV delivery in the presence of serum.

L17ER4: A cell-permeable attenuated cationic amphiphilic lytic peptide

We have developed a series of attenuated cationic amphiphilic lytic (ACAL) peptides that can efficiently bring immunoglobulin G (IgG) and other functional proteins into cells. Delivery is generally achieved through the coadministration of ACAL peptides with cargo proteins. However, conjugation of ACAL peptides with cargos may be a promising approach for in vivo application to link in vivo outcomes of ACAL peptides and cargos. This study describes the creation of a new cell-permeable ACAL peptide, L17ER4. L17E is an optimized prototype of ACAL peptides previously developed in our laboratory for efficient delivery of IgGs into cells. Delivery was improved by functionalizing L17E with a tetra-arginine (R4) tag. Compared to the use of R8, a representative cell-penetrating peptide with high intracellular delivery efficacy, conjugation with L17ER4 afforded approximately four-fold higher cellular uptake of model small-molecule cargos (fluorescein isothiocyanate and HiBiT peptide). L17ER4 was also able to deliver proteins to cells. Fused with L17ER4, Cre recombinase was delivered into cells. Intracerebroventricular injection of Cre-L17ER4 into green red reporter mice, R26GRR, led to significant in vivo gene recombination in ependymal cells, suggesting that L17ER4 may be used as a cell-penetrating peptide for delivering protein therapeutics into cells in vivo.

Redesigning of Cell-Penetrating Peptides to Improve Their Efficacy as a Drug Delivery System

Cell-penetrating peptides (CPP) are promising tools for the transport of a broad range of compounds into cells. Since the discovery of the first members of this peptide family, many other peptides have been identified; nowadays, dozens of these peptides are known. These peptides sometimes have very different chemical–physical properties, but they have similar drawbacks; e.g., non-specific internalization, fast elimination from the body, intracellular/vesicular entrapment. Although our knowledge regarding the mechanism and structure–activity relationship of internalization is growing, the prediction and design of the cell-penetrating properties are challenging. In this review, we focus on the different modifications of well-known CPPs to avoid their drawbacks, as well as how these modifications may increase their internalization and/or change the mechanism of penetration.

Piezo1 activation using Yoda1 inhibits macropinocytosis in A431 human epidermoid carcinoma cells

Macropinocytosis is a type of endocytosis accompanied by actin rearrangement-driven membrane deformation, such as lamellipodia formation and membrane ruffling, followed by the formation of large vesicles, macropinosomes. Ras-transformed cancer cells efficiently acquire exogenous amino acids for their survival through macropinocytosis. Thus, inhibition of macropinocytosis is a promising strategy for cancer therapy. To date, few specific agents that inhibit macropinocytosis have been developed. Here, focusing on the mechanosensitive ion channel Piezo1, we found that Yoda1, a Piezo1 agonist, potently inhibits macropinocytosis induced by epidermal growth factor (EGF). The inhibition of ruffle formation by Yoda1 was dependent on the extracellular Ca²⁺ influx through Piezo1 and on the activation of the calcium-activated potassium channel KCa3.1. This suggests that Ca²⁺ ions can regulate EGF-stimulated macropinocytosis. We propose the potential for macropinocytosis inhibition through the regulation of a mechanosensitive channel activity using chemical tools.

CPP-Based Bioactive Drug Delivery to Penetrate the Blood-Brain Barrier: A Potential Therapy for Glioblastoma Multiforme

BACKGROUND

A large number of studies have been conducted on the treatment of glioblastoma multiforme (GBM). Chemotherapeutic drugs cannot penetrate deeply into the brain parenchyma due to the presence of the blood-brain barrier (BBB). Hence, crossing BBB is the significant obstacle in developing new therapeutic methods for GBM.

OBJECTIVE

Cell penetrating peptides (CPPs) have emerged as new tools that can efficiently deliver various substances across BBB. CPPs beneficial properties, such as BBB penetration capacity, low toxicity, and the ability to achieve active targeting and controllable drug release, have made them worthy candidates for GBM treatment. However, their application is limited by several drawbacks, including lack of selectivity, insufficient transport efficacy, and low stability. In order to overcome the selectivity issue, tumor targeting peptides and sequences that can be activated at the target site have been embedded into the structure of CPPs. To overcome their insufficient transport efficacy into the cells, which is mostly due to endosomal entrapment, various endosomolytic moieties have been incorporated into CPPs. Finally, their instability in blood circulation can be solved through different modifications to their structures. As this field is moving beyond preclinical studies, the discovery of new and more efficient CPPs for GBM treatment has become crucial. Thus, by using display techniques, such as phage display, this encouraging treatment strategy can be developed further.

CONCLUSION

Consequently, despite several challenges in CPPs application, recent progress in studies has shown their potential for the development of the next generation GBM therapeutics.

Artificial Nanocage Formed via Self-Assembly of β-Annulus Peptide for Delivering Biofunctional Proteins into Cell Interiors

Nanocarriers that deliver functional proteins to cell interiors are an attractive platform for the intracellular delivery of intact proteins without further modification, with in vivo compatibility. Development of efficient methods for cargo protein encapsulation and release in recipient cell cytosol is needed. Herein, we assess the feasibility of the abovementioned requirements using a protein nanocage (artificial nanocage) without compromising the structure and functions of the original protein and allowing for design flexibility of the surfaces and interiors. The protein nanocage formed via the self-assembly of the β-annulus peptide (24-amino acid peptide) in water was used as a model framework. The nitrilotriacetic acid moiety was displayed on the nanocage lumen for effective encapsulation of hexahistidine-tagged proteins in the presence of Ni²⁺, and the amphiphilic cationic lytic peptide HAad was displayed on a nanocage surface to attain cell permeability. Successful intracellular delivery of cargo proteins and targeting of cytosolic proteins by a nanobody were achieved, indicating the validity of the approach employed in this study.

Incorporation of Endosomolytic Peptides with Varying Disruption Mechanisms into EGFR-Targeted Protein Conjugates: The Effect on Intracellular Protein Delivery and EGFR Specificity in Breast Cancer Cells

Intracellular delivery of protein therapeutics remains a significant challenge limiting the majority of clinically available protein drugs to extracellular targets. Strategies to deliver proteins to subcellular compartments have traditionally relied on cell-penetrating peptides, which can drive enhanced internalization but exhibit unreliable activity and are rarely able to target specific cells, leading to off-target effects. Moreover, few design rules exist regarding the relative efficacy of various endosomal escape strategies in proteins. Accordingly, we developed a simple fusion modification approach to incorporate endosomolytic peptides onto epidermal growth factor receptor (EGFR)-targeted protein conjugates and performed a systematic comparison of the endosomal escape efficacy, mechanism of action, and capacity to maintain EGFR-targeting specificity of conjugates modified with four different endosomolytic sequences of varying modes of action (Aurein 1.2, GALA, HA2, and L17E). Use of the recently developed Gal8-YFP assay indicated that the fusion of each endosomolytic peptide led to enhanced endosomal disruption. Additionally, the incorporation of each endosomolytic peptide increased the half-life of the internalized protein and lowered lysosomal colocalization, further supporting the membrane-disruptive capacity. Despite this, only EGFR-targeted conjugates modified with Aurein 1.2 or GALA maintained EGFR specificity. These results thus demonstrated that the choice of endosomal escape moiety can substantially affect targeting capability, cytotoxicity, and bioactivity and provided important new insights into endosomolytic peptide selection for the design of targeted protein delivery systems.

Easy preparation of a liposome-mediated protein delivery system by freeze-thawing a liposome–protein complex

Homeostasis can be achieved by adding a protein supplement; however, an appropriate vector is required to deliver the protein into the cell because of the low stability of proteins in...

Cytosolic protein delivery using pH-responsive, charge-reversible lipid nanoparticles

Although proteins have attractive features as biopharmaceuticals, the difficulty in delivering them into the cell interior limits their applicability. Lipid nanoparticles (LNPs) are a promising class of delivery vehicles. When designing a protein delivery system based on LNPs, the major challenges include: (i) formulation of LNPs with defined particle sizes and dispersity, (ii) efficient encapsulation of cargo proteins into LNPs, and (iii) effective cellular uptake and endosomal release into the cytosol. Dioleoylglycerophosphate-diethylenediamine (DOP-DEDA) is a pH-responsive, charge-reversible lipid. The aim of this study was to evaluate the applicability of DOP-DEDA-based LNPs for intracellular protein delivery. Considering the importance of electrostatic interactions in protein encapsulation into LNPs, a negatively charged green fluorescent protein (GFP) analog was successfully encapsulated into DOP-DEDA-based LNPs to yield diameters and polydispersity index of < 200 nm and < 0.2, respectively. Moreover, ~ 80% of the cargo proteins was encapsulated into the LNPs. Cytosolic distribution of fluorescent signals of the protein was observed for up to ~ 90% cells treated with the LNPs, indicating the facilitated endocytic uptake and endosomal escape of the cargo attained using the LNP system.

Liquid Droplet Formation and Facile Cytosolic Translocation of IgG in the Presence of Attenuated Cationic Amphiphilic Lytic Peptides

Fc region binding peptide conjugated with attenuated cationic amphiphilic lytic peptide L17E trimer [FcB(L17E)₃ ] was designed for immunoglobulin G (IgG) delivery into cells. Particle-like liquid droplets were generated by mixing Alexa Fluor 488 labeled IgG (Alexa488-IgG) with FcB(L17E)₃ . Droplet contact with the cellular membrane led to spontaneous influx and distribution of Alexa488-IgG throughout cells in serum containing medium. Involvement of cellular machinery accompanied by actin polymerization and membrane ruffling was suggested for the translocation. Alexa488-IgG negative charges were crucial in liquid droplet formation with positively charged FcB(L17E)₃ . Binding of IgG to FcB(L17E)₃ may not be necessary. Successful intracellular delivery of Alexa Fluor 594-labeled anti-nuclear pore complex antibody and anti-mCherry-nanobody tagged with supernegatively charged green fluorescence protein allowed binding to cellular targets in the presence of FcB(L17E)₃ .

Use of homoarginine to obtain attenuated cationic membrane lytic peptides

Our research group has been studying the design of intracellular delivery peptides based on cationic lytic peptides. By placing negatively charged amino acids on potentially hydrophobic faces of the peptides, membrane lytic activity is attenuated on the cell surface, whereas it recovers in endosomes, enabling cytosolic delivery of proteins including antibodies. These lytic peptides generally contain multiple lysines, facilitating cell surface interaction and membrane perturbation. This study evaluated the effect of lysine-to-homoarginine substitution using HAad as a model delivery peptide. The resulting peptide had a comparable or better delivery efficacy for Cre recombinase, antibodies, and the Cas9/sgRNA complex with one-quarter of the concentration of HAad, implying that a subtle structural difference can affect delivery activity.

Advances in peptide-mediated cytosolic delivery of proteins

The number of protein-based drugs is exponentially increasing. However, development of protein therapeutics against intracellular targets is hampered by the lack of efficient cytosolic delivery strategies. In recent years, the use of cell-penetrating peptides has been proposed as a strategy to promote protein internalization. In this article, we provide the reader with a succinct update on the strategies exploited to enable peptide-mediated cytosolic delivery of proteins. First, we analyse the various methods available for delivery. We then describe the most popular and the in vitro assays designed to assess the intracellular distribution of protein cargo.