Novel cell-penetrating peptide-adaptors effect intracellular delivery and endosomal escape of protein cargos

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.

Utilization of a cell-penetrating peptide-adaptor for delivery of human papillomavirus protein E2 into cervical cancer cells to arrest cell growth and promote cell death

BACKGROUND

Human papillomavirus (HPV) is the causative agent of nearly all forms of cervical cancer, which can arise upon viral integration into the host genome and concurrent loss of viral regulatory gene E2. Gene-based delivery approaches show that E2 reintroduction reduces proliferative capacity and promotes apoptosis in vitro.

AIMS

This work explored if our calcium-dependent protein-based delivery system, TAT-CaM, could deliver functional E2 protein directly into cervical cancer cells to limit proliferative capacity and induce cell death.

MATERIALS AND RESULTS

TAT-CaM and the HPV16 E2 protein containing a CaM-binding sequence (CBS-E2) were expressed and purified from Escherichia coli. Calcium-dependent binding kinetics were verified by biolayer interferometry. Equimolar TAT-CaM:CBS-E2 constructs were delivered into the HPV16+ SiHa cell line and uptake verified by confocal microscopy. Proliferative capacity was measured by MTS assay and cell death was measured by release of lactate dehydrogenase. As a control, human microvascular cells (HMECs) were used. As expected, TAT-CaM bound CBS-E2 with high affinity in the presence of calcium and rapidly disassociated upon its removal. After introduction by TAT-CaM, fluorescently labeled CBS-E2 was detected in cellular interiors by orthogonal projections taken at the depth of the nucleus. In dividing cells, E2 relocalized to regions associated with the mitotic spindle. Cells receiving a daily dose of CBS-E2 for 4 days showed a significant reduction in metabolic activity at low doses and increased cell death at high doses compared to controls. This phenotype was retained for 7 days with no further treatments. When subcultured on day 12, treated cells regained their proliferative capacity.

CONCLUSIONS

Using the TAT-CaM platform, bioactive E2 protein was delivered into living cervical cancer cells, inducing senescence and cell death in a time- and dose-dependent manner. These results suggest that this nucleic acid and virus-free delivery method could be harnessed to develop novel, effective protein therapeutics.

A reversible cell penetrating peptide-cargo linkage allows dissection of cell penetrating peptide- and cargo-dependent effects on internalization and identifies new functionalities of putative endolytic peptides

Cell penetrating peptides (CPPs) are a promising technology for therapeutic delivery of macromolecular cargos. CPPs have generally used covalent linkages to cargo, ensuring a common fate as one molecule. Conversely, our CPP-adaptor, TAT-CaM, noncovalently binds calmodulin binding sequence (CBS)-containing cargos in calcium rich media then dissociates in the calcium-poor endosomal environment following internalization, enhancing endosomal escape relative to standard CPPs. In this study, we report cell entry of positively charged protein cargos that were not increased by TAT-CaM while cargos based on the negatively charged maltose binding protein (MBP) displayed little intrinsic internalization but were internalized by TAT-CaM. In addition, association of positively charged proteins with negatively charged nucleic acids reduced internalization. This evidence points to the dominant role cargo charge plays in apparent CPP effectiveness. There has been little systematic investigation as to how interaction between CPPs and cargos impacts internalization efficiency. Our adaptors provide a tool that allows combinatorial assays to detect emergent properties. Toward this end we added 4 endolytic peptide (EP) sequences between cargo CBS and MBP moieties to create 4 new cargos and between TAT and CaM to create 4 new adaptors. The new cargos were assayed for internalization alone and with a panel of CPP-adaptors to identify combinations that displayed increased internalization efficiency or other properties. Among the most important results, addition of the EP LAH4 improved adaptor performance and provided some CPP capability to cargos. MBP-LAH4-CBS was internalized more effectively by most adaptors, suggesting this sequence has general stimulatory ability. Two other EPs, Aurein 1.2 and HA2, also provided some CPP capability to their MBP cargos but were unexpectedly antagonistic to internalization by most adaptors due to retention of adaptor/cargo complexes on the cell surface. We thus identified LAH4 as stimulator of internalization in both adaptors and cargos and uncovered new functionality for Aurein 1.2 and HA2, which may be related to their identification as EPs. Future experiments will test new endolytic capabilities made possible with combinatorial approaches.

Evaluation of the effect of a cell penetrating peptide (TAT) towards tailoring the targeting efficacy and tumor uptake of porphyrin

Cell penetrating peptides (CPPs) are known to possess a unique capacity to penetrate biological membranes and translocate various molecules into the cells. Therefore, porphyrin-CPP conjugates could be envisaged to boost the intracellular delivery of porphyrins thereby providing an improved tool for the development of agents for multi-modal applications for cancer management. Working in this direction, an unsymmetrically substituted porphyrin derivative was conjugated with a transactivating transcriptional activator peptide (TAT) and various in vitro and in vivo studies were carried out in order to study the effect of adding a CPP to the porphyrin derivative. MTT assay revealed the preferential light dependent toxicity of the porphyrin derivative which was further enhanced upon peptide conjugation. Fluorescence and flow cytometry studies revealed the relatively higher cellular internalization of the porphyrin-TAT conjugate in comparison with the porphyrin derivative. The elevated light dependent cell toxicity of the porphyrin-TAT conjugate along with its capability of generating cytotoxic singlet oxygen indicated the advantages of using the porphyrin-TAT conjugate for PDT applications. Also, porphyrin and the porphyrin-peptide conjugate were radiolabelled with 68Ga to investigate their possible potential as PET agents. In vivo biodistribution studies revealed a higher tumor uptake for the 68Ga-porphyrin-TAT conjugate (6.32 ± 1.24% IA per g) than for 68Ga-porphyrin (2.45 ± 0.88% IA per g) at 60 min post-administration. However, the observation of a higher non-target retention of the radiolabelled agents during in vivo studies might pose a limitation on their possible application in PET imaging.

The role of cell-penetrating peptides in potential anti-cancer therapy

Due to the complex physiological structure, microenvironment and multiple physiological barriers, traditional anti-cancer drugs are severely restricted from reaching the tumour site. Cell-penetrating peptides (CPPs) are typically made up of 5-30 amino acids, and can be utilised as molecular transporters to facilitate the passage of therapeutic drugs across physiological barriers. Up to now, CPPs have widely been used in many anti-cancer treatment strategies, serving as an excellent potential choice for oncology treatment. However, their drawbacks, such as the lack of cell specificity, short duration of action, poor stability in vivo, compatibility problems (i.e. immunogenicity), poor therapeutic efficacy and formation of unwanted metabolites, have limited their further application in cancer treatment. The cellular uptake mechanisms of CPPs involve mainly endocytosis and direct penetration, but still remain highly controversial in academia. The CPPs-based drug delivery strategy could be improved by clever design or chemical modifications to develop the next-generation CPPs with enhanced cell penetration capability, stability and selectivity. In addition, some recent advances in targeted cell penetration that involve CPPs provide some new ideas to optimise CPPs.

Development of a Gene-Activated Scaffold Incorporating Multifunctional Cell-Penetrating Peptides for pSDF-1α Delivery for Enhanced Angiogenesis in Tissue Engineering Applications

Non-viral gene delivery has become a popular approach in tissue engineering, as it permits the transient delivery of a therapeutic gene, in order to stimulate tissue repair. However, the efficacy of non-viral delivery vectors remains an issue. Our lab has created gene-activated scaffolds by incorporating various non-viral delivery vectors, including the glycosaminoglycan-binding enhanced transduction (GET) peptide into collagen-based scaffolds with proven osteogenic potential. A modification to the GET peptide (FLR) by substitution of arginine residues with histidine (FLH) has been designed to enhance plasmid DNA (pDNA) delivery. In this study, we complexed pDNA with combinations of FLR and FLH peptides, termed GET* nanoparticles. We sought to enhance our gene-activated scaffold platform by incorporating GET* nanoparticles into collagen-nanohydroxyapatite scaffolds with proven osteogenic capacity. GET* N/P 8 was shown to be the most effective formulation for delivery to MSCs in 2D. Furthermore, GET* N/P 8 nanoparticles incorporated into collagen-nanohydroxyapatite (coll-nHA) scaffolds at a 1:1 ratio of collagen:nanohydroxyapatite was shown to be the optimal gene-activated scaffold. pDNA encoding stromal-derived factor 1α (pSDF-1α), an angiogenic chemokine which plays a role in BMP mediated differentiation of MSCs, was then delivered to MSCs using our optimised gene-activated scaffold platform, with the aim of significantly increasing angiogenesis as an important precursor to bone repair. The GET* N/P 8 coll-nHA scaffolds successfully delivered pSDF-1α to MSCs, resulting in a significant, sustained increase in SDF-1α protein production and an enhanced angiogenic effect, a key precursor in the early stages of bone repair.

A real-time assay for cell-penetrating peptide-mediated delivery of molecular cargos

Cell-penetrating peptides (CPPs) are capable of transporting molecules to which they are tethered across cellular membranes. Unsurprisingly, CPPs have attracted attention for their potential drug delivery applications, but several technical hurdles remain to be overcome. Chief among them is the so-called 'endosomal escape problem,' i.e. the propensity of CPP-cargo molecules to be endocytosed but remain entrapped in endosomes rather than reaching the cytosol. Previously, a CPP fused to calmodulin that bound calmodulin binding site-containing cargos was shown to efficiently deliver cargos to the cytoplasm, effectively overcoming the endosomal escape problem. The CPP-adaptor, "TAT-CaM," evinces delivery at nM concentrations and more rapidly than we had previously been able to measure. To better understand the kinetics and mechanism of CPP-adaptor-mediated cargo delivery, a real-time cell penetrating assay was developed in which a flow chamber containing cultured cells was installed on the stage of a confocal microscope to allow for observation ab initio. Also examined in this study was an improved CPP-adaptor that utilizes naked mole rat (Heterocephalus glaber) calmodulin in place of human and results in superior internalization, likely due to its lesser net negative charge. Adaptor-cargo complexes were delivered into the flow chamber and fluorescence intensity in the midpoint of baby hamster kidney cells was measured as a function of time. Delivery of 400 nM cargo was observed within seven minutes and fluorescence continued to increase linearly as a function of time. Cargo-only control experiments showed that the minimal uptake which occurred independently of the CPP-adaptor resulted in punctate localization consistent with endosomal entrapment. A distance analysis was performed for cell-penetration experiments in which CPP-adaptor-delivered cargo showing wider dispersions throughout cells as compared to an analogous covalently-bound CPP-cargo. Small molecule endocytosis inhibitors did not have significant effects upon delivery. The real-time assay is an improvement upon static endpoint assays and should be informative in a broad array of applications.

Intracellular Delivery of Proteins into Living Cells by Low-Molecular-Weight Polyethyleneimine

Introduction

Intracellular protein delivery is emerging as a potential strategy to revolutionize therapeutics in the field of biomedicine, aiming at treating a wide range of diseases including cancer, inflammatory diseases and other oxidative stress-related disorders with high specificity. However, the current challenges and limitations are addressed to either synthetically or biologically through multipotency of engineering, such as protein modification, insufficient delivery of large-size proteins, deficiency or mutation of proteins, and high cytotoxicity.

Methods

We prepared the nanocomposites by mixing protein with PEI1200 at a certain molar ratio and demonstrated that it can deliver proteins into living cells in high efficiency and safety through the following experiments, such as dynamic light scattering, fluorescent detection, agarose gel electrophoresis, ß-Galactosidase activity detection, immunofluorescence staining, digital fluorescent detection, cell viability assay and flow cytometry.

Results

The self-assembly of PEI1200/protein nanocomposites with appropriate molar ratio (4:1 and 8:1) could provide efficiently delivery of active proteins to a variety of cell types in the presence of serum. The nanocomposites could continuously release protein up to 96 h in their desired intracellular locations. In addition, these nanocomposites were able to preserve protein activity while maintain low cytotoxicity (when final concentration <1 μg/mL).

Conclusion

Collectively, PEI1200-based delivery system provided an alternative strategy to direct protein delivery in high efficiency and safety, offering increased potential applications in clinical biomedicine.

Intracellular Delivery of Active Proteins by Polyphosphazene Polymers

Achieving intracellular delivery of protein therapeutics within cells remains a significant challenge. Although custom formulations are available for some protein therapeutics, the development of non-toxic delivery systems that can incorporate a variety of active protein cargo and maintain their stability, is a topic of great relevance. This study utilized ionic polyphosphazenes (PZ) that can assemble into supramolecular complexes through non-covalent interactions with different types of protein cargo. We tested a PEGylated graft copolymer (PZ-PEG) and a pyrrolidone containing linear derivative (PZ-PYR) for their ability to intracellularly deliver FITC-avidin, a model protein. In endothelial cells, PZ-PYR/protein exhibited both faster internalization and higher uptake levels than PZ-PEG/protein, while in cancer cells both polymers achieved similar uptake levels over time, although the internalization rate was slower for PZ-PYR/protein. Uptake was mediated by endocytosis through multiple mechanisms, PZ-PEG/avidin colocalized more profusely with endo-lysosomes, and PZ-PYR/avidin achieved greater cytosolic delivery. Consequently, a PZ-PYR-delivered anti-F-actin antibody was able to bind to cytosolic actin filaments without needing cell permeabilization. Similarly, a cell-impermeable Bax-BH3 peptide known to induce apoptosis, decreased cell viability when complexed with PZ-PYR, demonstrating endo-lysosomal escape. These biodegradable PZs were non-toxic to cells and represent a promising platform for drug delivery of protein therapeutics.

The Utilization of Cell-Penetrating Peptides in the Intracellular Delivery of Viral Nanoparticles

Viral particles (VPs) have evolved so as to efficiently enter target cells and to deliver their genetic material. The current state of knowledge allows us to use VPs in the field of biomedicine as nanoparticles that are safe, easy to manipulate, inherently biocompatible, biodegradable, and capable of transporting various cargoes into specific cells. Despite the fact that these virus-based nanoparticles constitute the most common vectors used in clinical practice, the need remains for further improvement in this area. The aim of this review is to discuss the potential for enhancing the efficiency and versatility of VPs via their functionalization with cell-penetrating peptides (CPPs), short peptides that are able to translocate across cellular membranes and to transport various substances with them. The review provides and describes various examples of and means of exploitation of CPPs in order to enhance the delivery of VPs into permissive cells and/or to allow them to enter a broad range of cell types. Moreover, it is possible that CPPs are capable of changing the immunogenic properties of VPs, which could lead to an improvement in their clinical application. The review also discusses strategies aimed at the modification of VPs by CPPs so as to create a useful cargo delivery tool.

Nanoparticle-Based Nanomedicines to Promote Cancer Immunotherapy: Recent Advances and Future Directions

Cancer immunotherapy is a promising cancer terminator by directing the patient's own immune system in the fight against this challenging disorder. Despite the monumental therapeutic potential of several immunotherapy strategies in clinical applications, the efficacious responses of a wide range of immunotherapeutic agents are limited in virtue of their inadequate accumulation in the tumor tissue and fatal side effects. In the last decades, increasing evidences disclose that nanotechnology acts as an appealing solution to address these technical barriers via conferring rational physicochemical properties to nanomaterials. In this Review, an imperative emphasis will be drawn from the current understanding of the effect of a nanosystem's structure characteristics (e.g., size, shape, surface charge, elasticity) and its chemical modification on its transport and biodistribution behavior. Subsequently, rapid-moving advances of nanoparticle-based cancer immunotherapies are summarized from traditional vaccine strategies to recent novel approaches, including delivery of immunotherapeutics (such as whole cancer cell vaccines, immune checkpoint blockade, and immunogenic cell death) and engineered immune cells, to regulate tumor microenvironment and activate cellular immunity. The future prospects may involve in the rational combination of a few immunotherapies for more efficient cancer inhibition and elimination.

Strategies to target bioactive molecules to subcellular compartments. Focus on natural compounds

Many potential pharmacological targets are present in multiple subcellular compartments and have different pathophysiological roles depending on location. In these cases, selective targeting of a drug to the relevant subcellular domain(s) may help to sharpen its impact by providing topological specificity, thus limiting side effects, and to concentrate the compound where needed, thus increasing its effectiveness. We review here the state of the art in precision subcellular delivery. The major approaches confer "homing" properties to the active principle via permanent or reversible (in pro-drug fashion) modifications, or through the use of special-design nanoparticles or liposomes to ferry a drug(s) cargo to its desired destination. An assortment of peptides, substituents with delocalized positive charges, custom-blended lipid mixtures, pH- or enzyme-sensitive groups provide the main tools of the trade. Mitochondria, lysosomes and the cell membrane may be mentioned as the fronts on which the most significant advances have been made. Most of the examples presented here have to do with targeting natural compounds - in particular polyphenols, known as pleiotropic agents - to one or the other subcellular compartment.

Pharmacological modulation of cell functional activity with valproic acid and erythropoietin

Introduction: Valproic acid (VA) is carboxylic acid with a branched chain, which is used as an antiepileptic drug.

Valproic acid influence on cells in vivo: VA, which is an antiepileptic drug, is also a teratogen, which causes defects of a neural tube and an axial skeleton, although the mechanisms are not yet fully clear.

Valproic acid influence on mesenchymal stem cells (MSC) in vitro: It is shown that valproic acid reduces the intracellular level of oxygen active forms.

Valproic acid effect on tumor cells: VA inhibits tumor growth through several mechanisms, including the cell cycle stop, differentiation induction and inhibition of growth of tumor vessels.

Valproic acid influence on enzymes: It affects mainly GSK-3.

Valproic acid influence on animals’ cells: It is shown that VA can significantly improve an ability to develop in vitro and improve nuclear reprogramming of embryos.

Erythropoietin (EPO): Is an hypoxia-induced hormone and a cytokine, which is necessary for normal erythropoiesis. EPO is widely used in in vitro experiments.

Conclusion: Thus, the influence of VA and EPO on cells can be used in cell technologies.

Fluorescence Correlation Spectroscopy Reveals Efficient Cytosolic Delivery of Protein Cargo by Cell-Permeant Miniature Proteins

New methods for delivering proteins into the cytosol of mammalian cells are being reported at a rapid pace. Differentiating between these methods in a quantitative manner is difficult, however, as most assays for evaluating cytosolic protein delivery are qualitative and indirect and thus often misleading. Here we make use of fluorescence correlation spectroscopy (FCS) to determine with precision and accuracy the relative efficiencies with which seven different previously reported "cell-penetrating peptides" (CPPs) transport a model protein cargo-the self-labeling enzyme SNAP-tag-beyond endosomal membranes and into the cytosol. Using FCS, we discovered that the miniature protein ZF5.3 is an exceptional vehicle for delivering SNAP-tag to the cytosol. When delivered by ZF5.3, SNAP-tag can achieve a cytosolic concentration as high as 250 nM, generally at least 2-fold and as much as 6-fold higher than any other CPP evaluated. Additionally, we show that ZF5.3 can be fused to a second enzyme cargo-the engineered peroxidase APEX2-and reliably delivers the active enzyme to the cell interior. As FCS allows one to realistically assess the relative merits of protein transduction domains, we anticipate that it will greatly accelerate the identification, evaluation, and optimization of strategies to deliver large, intact proteins to intracellular locales.

Strategies for the enhanced intracellular delivery of nanomaterials

The intracellular delivery of nanomaterials and drugs has been attracting increasing research interest, mainly because of their important effects and functions in several organelles. Targeting specific organelles can help treat or decrease the symptoms of diabetes, cancer, infectious, and autoimmune diseases. Tuning biological and chemical properties enables the creation of functionalized nanomaterials with enhanced intracellular uptake, ability to escape premature lysosome degradation, and to reach a specific target. Here, we provide an update of recent advances in the intracellular delivery mechanisms that could help drugs reach their target more efficiently.

A quantitative comparison of cytosolic delivery via different protein uptake systems

Over many years, a variety of delivery systems have been investigated that have the capacity to shuttle macromolecular cargoes, especially proteins, into the cytosol. Due to the lack of an objective way to quantify cytosolic delivery, relative delivery efficiencies of the various transport systems have remained unclear. Here, we demonstrate the use of the biotin ligase assay for a quantitative comparison of protein transport to the cytosol via cell-penetrating peptides, supercharged proteins and bacterial toxins in four different cell lines. The data illustrate large differences in both the total cellular internalization, which denotes any intracellular location including endosomes, and in the cytosolic uptake of the transport systems, with little correlation between the two. Also, we found significant differences between the cell lines. In general, protein transport systems based on cell-penetrating peptides show a modest total uptake, and mostly do not deliver cargo to the cytosol. Systems based on bacterial toxins show a modest receptor-mediated internalization but an efficient delivery to the cytosol. Supercharged proteins, on the contrary, are not receptor-specific and lead to massive total internalization into endosomes, but only low amounts end up in the cytosol.

Breaking in and busting out: cell-penetrating peptides and the endosomal escape problem

Cell-penetrating peptides (CPPs) have long held great promise for the manipulation of living cells for therapeutic and research purposes. They allow a wide array of biomolecules from large, oligomeric proteins to nucleic acids and small molecules to rapidly and efficiently traverse cytoplasmic membranes. With few exceptions, if a molecule can be associated with a CPP, it can be delivered into a cell. However, a growing realization in the field is that CPP-cargo fusions largely remain trapped in endosomes and are eventually targeted for degradation or recycling rather than released into the cytoplasm or trafficked to a desired subcellular destination. This 'endosomal escape problem' has confounded efforts to develop CPP-based delivery methods for drugs, enzymes, plasmids, etc. This review provides a brief history of CPP research and discusses current issues in the field with a primary focus on the endosomal escape problem, for which several promising potential solutions have been developed. Are we on the verge of developing technologies to deliver therapeutics such as siRNA, CRISPR/Cas complexes and others that are currently failing because of an inability to get into cells, or are we just chasing after another promising but unworkable technology? We make the case for optimism.

A versatile cell-penetrating peptide-adaptor system for efficient delivery of molecular cargos to subcellular destinations

Cell penetrating peptides have long held great potential for delivery of biomolecular cargos for research, therapeutic and diagnostic purposes. They allow rapid, relatively nontoxic passage of a wide variety of biomolecules through the plasma membranes of living cells. However, CPP-based research tools and therapeutics have been stymied by poor efficiency in release from endosomes and a great deal of effort has been made to solve this ‘endosomal escape problem.’ Previously, we showed that use of a reversible, noncovalent coupling between CPP and cargo using calmodulin and a calmodulin binding motif allowed efficient delivery of cargo proteins to the cytoplasm in baby hamster kidney and other mammalian cell lines. The present report demonstrates the efficacy of our CPP-adaptor scheme for efficient delivery of model cargos to the cytoplasm using a variety of CPPs and adaptors. Effective overcoming of the endosomal escape problem is further demonstrated by the delivery of cargo to the nucleus, endoplasmic reticulum and peroxisomes by addition of appropriate subcellular localization signals to the cargos. CPP-adaptors were also used to deliver cargo to myotubes, demonstrating the feasibility of the system as an alternative to transfection for the manipulation of hard-to-transfect cells.

Single domain antibodies for the knockdown of cytosolic and nuclear proteins

Single domain antibodies (sdAbs) from camels or sharks comprise only the variable heavy chain domain. Human sdAbs comprise the variable domain of the heavy chain (VH) or light chain (VL) and can be selected from human antibodies. SdAbs are stable, nonaggregating molecules in vitro and in vivo compared to complete antibodies and scFv fragments. They are excellent novel inhibitors of cytosolic/nuclear proteins because they are correctly folded inside the cytosol in contrast to scFv fragments. SdAbs are unique because of their excellent specificity and possibility to target posttranslational modifications such as phosphorylation sites, conformers or interaction regions of proteins that cannot be targeted with genetic knockout techniques and are impossible to knockdown with RNAi. The number of inhibiting cytosolic/nuclear sdAbs is increasing and usage of synthetic single pot single domain antibody libraries will boost the generation of these fascinating molecules without the need of immunization. The most frequently selected antigenic epitopes belong to viral and oncogenic proteins, followed by toxins, proteins of the nervous system as well as plant- and drosophila proteins. It is now possible to select functional sdAbs against virtually every cytosolic/nuclear protein and desired epitope. The development of new endosomal escape protein domains and cell-penetrating peptides for efficient transfection broaden the application of inhibiting sdAbs. Last but not least, the generation of relatively new cell-specific nanoparticles such as polymersomes and polyplexes carrying cytosolic/nuclear sdAb-DNA or -protein will pave the way to apply cytosolic/nuclear sdAbs for inhibition of viral infection and cancer in the clinic.

Augmenting the Efficacy of Immunotoxins and Other Targeted Protein Toxins by Endosomal Escape Enhancers

The toxic moiety of almost all protein-based targeted toxins must enter the cytosol of the target cell to mediate its fatal effect. Although more than 500 targeted toxins have been investigated in the past decades, no antibody-targeted protein toxin has been approved for tumor therapeutic applications by the authorities to date. Missing efficacy can be attributed in many cases to insufficient endosomal escape and therefore subsequent lysosomal degradation of the endocytosed toxins. To overcome this drawback, many strategies have been described to weaken the membrane integrity of endosomes. This comprises the use of lysosomotropic amines, carboxylic ionophores, calcium channel antagonists, various cell-penetrating peptides of viral, bacterial, plant, animal, human and synthetic origin, other organic molecules and light-induced techniques. Although the efficacy of the targeted toxins was typically augmented in cell culture hundred or thousand fold, in exceptional cases more than million fold, the combination of several substances harbors new problems including additional side effects, loss of target specificity, difficulties to determine the therapeutic window and cell type-dependent variations. This review critically scrutinizes the chances and challenges of endosomal escape enhancers and their potential role in future developments.