Advancements in Transdermal Drug Delivery Systems: Harnessing the Potential of Macromolecular Assisted Permeation Enhancement and Novel Techniques

Transdermal drug delivery (TDD) represents a transformative paradigm in drug administration, offering advantages such as controlled drug release, enhanced patient adherence, and circumvention of hepatic first-pass metabolism. Despite these benefits, the inherent barrier function of the skin, primarily attributed to the stratum corneum, remains a significant impediment to the efficient permeation of therapeutic agents. Recent advancements have focused on macromolecular-assisted permeation enhancers, including carbohydrates, lipids, amino acids, nucleic acids, and cell-penetrating peptides, which modulate skin permeability by transiently altering its structural integrity. Concurrently, innovative methodologies such as iontophoresis, electroporation, microneedles, ultrasound, and sonophoresis have emerged as potent tools to enhance drug transport by creating transient microchannels or altering the skin's microenvironment. Among the novel approaches, the development of nanocarriers such as Liposome, niosomes, and transethosomes etc. has garnered substantial attention. These elastic vesicular systems, comprising lipids and edge activators, exhibit superior skin penetration owing to their deformability and enhanced payload delivery capabilities. Furthermore, the integration of nanocarriers with physical enhancement techniques demonstrates a synergistic potential, effectively addressing the limitations of conventional TDD systems. This comprehensive convergence of macromolecular-assisted enhancers, advanced physical techniques, and next-generation nanocarriers underscores the evolution of TDD, paving the way for optimized therapeutic outcomes.

Enhancing Tumor Targeted Therapy: The Role of iRGD Peptide in Advanced Drug Delivery Systems

Chemotherapy remains the primary therapeutic approach in treating cancer. The tumor microenvironment (TME) is the complex network surrounding tumor cells, comprising various cell types, such as immune cells, fibroblasts, and endothelial cells, as well as ECM components, blood vessels, and signaling molecules. The often stiff and dense network of the TME interacts dynamically with tumor cells, influencing cancer growth, immune response, metastasis, and resistance to therapy. The effectiveness of the treatment of solid tumors is frequently reduced due to the poor penetration of the drug, which leads to attaining concentrations below the therapeutic levels at the site. Cell-penetrating peptides (CPPs) present a promising approach that improves the internalization of therapeutic agents. CPPs, which are short amino acid sequences, exhibit a high ability to pass cell membranes, enabling them to deliver drugs efficiently with minimal toxicity. Specifically, the iRGD peptide, a member of CPPs, is notable for its capacity to deeply penetrate tumor tissues by binding simultaneously integrins ανβ3/ανβ5 and neuropilin receptors. Indeed, ανβ3/ανβ5 integrins are characteristically expressed by tumor cells, which allows the iRGD peptide to home onto tumor cells. Notably, the respective dual-receptor targeting mechanism considerably increases the permeability of blood vessels in tumors, enabling an efficient delivery of co-administered drugs or nanoparticles into the tumor mass. Therefore, the iRGD peptide facilitates deeper drug penetration and improves the efficacy of co-administered therapies. Distinctively, we will focus on the iRGD mechanism of action, drug delivery systems and their application, and deliberate future perspectives in developing iRGD-conjugated therapeutics. In summary, this review discusses the potential of iRGD in overcoming barriers to drug delivery in cancer to maximize treatment efficiency while minimizing side effects.

Protein-based delivery systems for RNA delivery

RNA-based therapeutics have emerged as promising approaches to modulate gene expression and generate therapeutic proteins or antigens capable of inducing immune responses to treat a variety of diseases, such as infectious diseases, cancers, immunologic disorders, and genetic disorders. However, the efficient delivery of RNA molecules into cells poses significant challenges due to their large molecular weight, negative charge, and susceptibility to degradation by RNase enzymes. To overcome these obstacles, viral and non-viral vectors have been developed, including lipid nanoparticles, viral vectors, proteins, dendritic macromolecules, among others. Among these carriers, protein-based delivery systems have garnered considerable attention due to their potential to address specific issues associated with nanoparticle-based systems, such as liver accumulation and immunogenicity. This review provides an overview of currently marketed RNA drugs, underscores the significance of RNA delivery vector development, delineates the essential characteristics of an ideal RNA delivery vector, and introduces existing protein carriers for RNA delivery. By offering valuable insights, this review aims to serve as a reference for the future development of protein-based delivery vectors for RNA therapeutics.

Enhancing Methotrexate Delivery in the Brain by Mesoporous Silica Nanoparticles Functionalized with Cell-Penetrating Peptide using in Vivo and ex Vivo Monitoring

The blood-brain barrier (BBB) acts as a physical/biochemical barrier that protects brain parenchyma from potential hazards exerted by different xenobiotics found in the systemic circulation. This barrier is created by "a lipophilic gate" as well as a series of highly organized influx/efflux mechanisms. The BBB bottleneck adversely affects the efficacy of chemotherapeutic agents in treating different CNS malignancies such as glioblastoma, an aggressive type of cancer affecting the brain. In the present study, mesoporous silica nanoparticles (MSNs) were conjugated with the transactivator of transcription (TAT) peptide, a cell-penetrating peptide, to produce MSN-NH-TAT with the aim of improving methotrexate (MTX) penetration into the brain. The TAT-modified nanosystem was characterized by Fourier transform infrared spectrometry (FTIR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and N₂ adsorption-desorption analysis. In vitro hemolysis and cell viability studies confirmed the biocompatibility of the MSN-based nanocarriers. In addition, in vivo studies showed that the MTX-loaded MSN-NH-TAT improved brain-to-plasma concentration ratio, brain uptake clearance, and the drug's blood terminal half-life, compared with the use of free MTX. Taken together, the results of the present study indicate that MSN functionalization with TAT is crucial for delivery of MTX into the brain. The present nanosystem represents a promising alternative drug carrier to deliver MTX into the brain via overcoming the BBB.

An overview of polyallylamine applications in gene delivery

A chief objective of gene transportation studies is to manipulate clinically accepted carriers that can be utilized to combat incurable diseases. Despite various strategies, efficiency and application of these vectors have been hindered, owing to different obstacles. Polyallylamine (PAA) is a synthetic water-soluble, weak base cationic polymer with different properties that could be administrated as an ideal candidate for biomedical applications such as gene delivery, drug delivery, or even tissue engineering. However, some intrinsic properties of this polymer limit its application. The two associated problems with the use of PAA in gene delivery are low transfection efficiency (because of low buffering capacity) and cytotoxic effects attributed to intense cationic character. Most of the strategies for structural modification of the PAA structure have focused on introducing hydrophobic groups to the polymeric backbone that target both cytotoxicity and transfection. In this perspective, we concentrate on PAA as a gene delivery vehicle and the existing approaches for modification of this cationic polymer to give insight to researchers for exploitation of PAA as an efficient carrier in biomedical applications.

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Cell-penetrating Peptide-mediated Nanovaccine Delivery

Vaccination with small antigens, such as proteins, peptides, or nucleic acids, is used to activate the immune system and trigger the protective immune responses against a pathogen. Currently, nanovaccines are undergoing development instead of conventional vaccines. The size of nanovaccines is in the range of 10-500 nm, which enables them to be readily taken up by cells and exhibit improved safety profiles. However, low-level immune responses, as the removal of redundant pathogens, trigger counter-effective activation of the immune system invalidly and present a challenging obstacle to antigen recognition and its uptake via antigen-presenting cells (APCs). In addition, toxicity can be substantial. To overcome these problems, a variety of cell-penetrating peptide (CPP)-mediated vaccine delivery systems based on nanotechnology have been proposed, most of which are designed to improve the stability of antigens in vivo and their delivery into immune cells. CPPs are particularly attractive components of antigen delivery. Thus, the unique translocation property of CPPs ensures that they remain an attractive carrier with the capacity to deliver cargo in an efficient manner for the application of drugs, gene transfer, protein, and DNA/RNA vaccination delivery. CPP-mediated nanovaccines can enhance antigen uptake, processing, and presentation by APCs, which are the fundamental steps in initiating an immune response. This review describes the different types of CPP-based nanovaccines delivery strategies.

Tumor microenvironment targeting with dual stimuli-responsive nanoparticles based on small heat shock proteins for antitumor drug delivery

Tumour microenvironment (TME)-targeting nanoparticles (NPs) were developed based on Methanococcus jannaschii small heat shock proteins (Mj-sHSPs). Transactivator of transcription (TAT) were modified on the surface of Mj-sHSPs (T-HSPs) to enhance their cellular internalization ability (CIA), and a pH/enzyme dual sensitive PEG/N-(2-aminoethyl)piperidine-hyaluronic acid (PAHA) coat was combined with T-HSPs (PT-HSPs). PT-HSP NPs exhibited multi-layered morphologies and good stability against plasma protein adsorption. The release of paclitaxel (PTX) from PT-HSP NPs was negligible at physiological pH. Under conditions similar to the TME (acidic pH and overexpressed hyaluronidase (HAase)), the PAHA coat deshielded from PT-HSP NPs because of two factors: charge reversal and HAase degradation. Once the PAHA coat was shed, the size of the NPs decreased; its surface charge became positive; and remarkable drug release was triggered. Cellular experiments indicated that the CIA of PT-HSPs was shielded in the microenvironment of normal cells and recovered in that of tumour cells. In vivo imaging exhibited that the PT-HSP NPs had an impressive tumour targeting ability compared with the uncoated controls. The antitumor efficacy in vivo demonstrated that tumour-bearing mice treated with PTX-loaded PT-HSP NPs achieved better anti-tumour effects and safety than the Taxol formulation. In summary, this study provided Mj-sHSP NPs with coats that could be shed in response to the particular pH and enzymes in the TME, which improved the efficacy of tumour therapy. STATEMENT OF SIGNIFICANCE: This study reports on tumor microenvironment-targeting protein-based nanoparticles (PT-HSP NPs) for targeted tumor therapy. The NPs had a multilayered structure: a protein cage, a TAT cationic layer, and a dual-sensitive coat. PT-HSP NPs exhibited multilayered morphology, with good stability against plasma protein adsorption, and PTX release negligible at physiological pH. Under the tumor microenvironment (acidic pH and overexpressed HAase), PAHA coat deshielded from PT-HSP NPs due to two factors: the charge reversal induced by protonation of piperidines in PAHA and HAase degradation. The results of cellular uptake, cytotoxicity, in vivo imaging, and tumor inhibition experiments confirmed that PT-HSP NPs exhibited promising tumor targeting efficacy in vitro and in vivo.

Cu(II) coordination to His-containing linear peptides and related branched ones: Equalities and diversities

The two branched peptides (AAHAWG)₄-PWT2 and (HAWG)₄-PWT2 where synthesized by mounting linear peptides on a cyclam-based scaffold (PWT2), provided with four maleimide chains, through a thio-Michael reaction. The purpose of this study was primarily to verify if the two branched ligands had a Cu(II) coordination behavior reproducing that of the single-chain peptides, namely AAHAWG-NH₂, which bears an Amino Terminal Cu(II)- and Ni(II)-Binding (ATCUN) Motif, and HAWG-NH₂, which presents a His residue as the N-terminal amino acid, in a wide pH range. The study of Cu(II) binding was performed by potentiometric, spectroscopic (UV-vis absorption, CD, fluorescence) and ESI-MS techniques. ATCUN-type ligands ((AAHAWG)₄-PWT2 and AAHAWG-NH₂) were confirmed to bind one Cu(II) per peptide fragment at both pH 7.4 and pH 9.0, with a [NH₂, 2N^-, N_Im] coordination mode. On the other hand, the ligand HAWG-NH₂ forms a [CuL₂]²⁺ species at neutral pH, while, at pH 9, the formation of 1:2 Cu(II):ligand adducts is prevented by amidic nitrogen deprotonation and coordination, to give rise solely to 1:1 species. Conversely, Cu(II) binding to (HAWG)₄-PWT2 resulted in the formation of 1:2 copper:peptide chain also at pH 9: hence, through the latter branched peptide we obtained, at alkaline pH, the stabilization of a specific Cu(II) coordination mode which results unachievable using the corresponding single-chain peptide. This behavior could be explained in terms of high local peptide concentration on the basis of the speciation of the Cu(II)/single-chain peptide systems.

Stimuli-Sensitive Cell Penetrating Peptide-Modified Nanocarriers

The integration of drugs into nanocarriers favorably altered their pharmacodynamics and pharmacokinetics compared to free drugs, and increased their therapeutic index. However, selective cellular internalization in diseased tissues rather than normal tissues still presents a formidable challenge. In this chapter I will cover solutions involving environment-responsive cell-penetrating peptides (CPPs). I will discuss properties of CPPs as universal cellular uptake enhancers, and the modifications imparted to CPP-modified nanocarriers to confine CPP activation to diseased tissues.

Improved Protein Toxin Delivery Based on ATTEMPTS Systems

BACKGROUND

Ribosome-inactivating proteins (RIPs) are wildly found in multiple species of plants, bacteria and fungi. As a special family of protein toxins, RIPs can inhibit protein synthesis and induce cell death via inactivating ribosome in eukaryotic cells. Thus, RIPs have been applied for anti-tumor therapy in the past two decades. However, because of poor cell permeability, nonselective mode of action for tumor cells, poor pharmacokinetic profiles and immunogenicity, their clinical application has been severely constrained. As an effort to overcome these obstacles, tumor-specific monoclonal antibodies (mAb) have been conjugated to RIPs (forming so called "immunotoxins") specifically to increase their cytotoxicity and provide tumor targeting. Nevertheless, immunotoxins yet have not fully resolved all the issues and critical challenges still remain, such as immunogenicity and inability to penetrate into the deep site of tumor.

OBJECTIVE

To overcome the constrain of immunotoxins, the novel cell-penetrating peptide (CPP)- modified ATTEMPTS systems based on combination of CPP-mediated penetration and antibodymediated tumor targeting, with triggerable drug release function, were developed to achieve effective and safe delivery of protein toxin.

RESULTS

The CPP-modified ATTEMPTS systems showed effective protamine-triggered CPP-toxin release and thus enhanced CPP-mediated cellular uptake and cytotoxicity. It also showed antibodymediated in vivo tumor targeting and significantly increased in vivo tumor growth suppression with limited systematic toxicity.

CONCLUSION

The CPP-modified ATTEMPTS systems were developed and demonstrated as a proof-ofconcept for CPP-based protein toxin delivery with triggerable antibody targeting to improve the druggability of protein toxin drugs. The systems showed the potential application of protein toxin clinical translation in anticancer treatment.

Versatility of cell-penetrating peptides for intracellular delivery of siRNA

The plasma membrane is a large barrier to systemic drug delivery into cells, and it limits the efficacy of drug cargo. This issue has been overcome using cell-penetrating peptides (CPPs). CPPs are short peptides (6-30 amino acid residues) that are potentially capable of intracellular penetration to deliver drug molecules. CPPs broadened biomedical applications and provide a means to deliver a range of biologically active molecules, such as small molecules, proteins, imaging agents, and pharmaceutical nanocarriers, across the plasma membrane with high efficacy and low toxicity. This review is focused on the versatility of CPPs and advanced approaches for siRNA delivery.

Bioresponsive polyplexes - chemically programmed for nucleic acid delivery

INTRODUCTION

The whole delivery process of nucleic acids is very challenging. Appropriate carrier systems are needed, which show extracellular stability and intracellular disassembly. Viruses have developed various strategies to meet these requirements, as they are optimized by biological evolution to transfer genetic information into host cells. Taking viruses as models, smart synthetic carriers can be designed, mimicking the efficient delivery process of viral infection. These 'synthetic viruses' are pre-programmed and respond to little differences in their microenvironment, caused by either exogenous or endogenous stimuli.

AREAS COVERED

This review deals with polymer-based, bioresponsive nanosystems (polyplexes) for the delivery of nucleic acids. Strategies utilizing pH-responsiveness, redox-responsiveness as well as sensitivity towards enzymes will be described more in detail. Systems, which respond to other endogenous triggers (i.e. reactive oxygen species, adenosine triphosphate, hypoxia), will be briefly illustrated. Moreover, some examples for combined bioresponsiveness will be presented.

EXPERT OPINION

Bioresponsive polyplexes are a smart way to facilitate programmed, timely delivery of nucleic acids to desired, specific sites. Nevertheless, further optimization is necessary to improve the still moderate transfection efficiency and specificity - also in regard to medical translation. For this purpose, precise carrier structures are desirable and stability issues of bioresponsive systems must be considered.

Deepened cellular/subcellular interface penetration and enhanced antitumor efficacy of cyclic peptidic ligand-decorated accelerating active targeted nanomedicines

Introduction

Acceleration and improvement of penetration across cell-membrane interfaces of active targeted nanotherapeutics into tumor cells would improve tumor-therapy efficacy by overcoming the issue of poor drug penetration. Cell-penetrating peptides, especially synthetic polyarginine, have shown promise in facilitating cargo delivery. However, it is unknown whether polyarginine can work to overcome the membrane interface in an inserted pattern for cyclic peptide ligand-mediated active targeting drug delivery. Here, we conducted a study to test the hypothesis that tandem-insert nona-arginine (tiR₉) can act as an accelerating component for intracellular internalization, enhance cellular penetration, and promote antitumor efficacy of active targeted cyclic asparagine–glycine–arginine (cNGR)-decorated nanoliposomes.

Methods

Polyarginine was coupled with the polyethylene glycol (PEG) chain and the cNGR moiety, yielding a cNGR–tiR₉–PEG_2,000–distearoylphosphatidylethanolamine conjugate.

Results

The accelerating active targeted liposome (Lip) nanocarrier (cNGR-tiR₉-Lip–doxorubicin [Dox]) constructed in this study held suitable physiochemical features, such as appropriate particle size of ~150 nm and sustained-release profiles. Subsequently, tiR₉ was shown to enhance cellular drug delivery of Dox-loaded active targeted systems (cNGR-Lip-Dox) significantly. Layer-by-layer confocal microscopy indicated that the tandem-insert polyarginine accelerated active targeted system entry into deeper intracellular regions based on observations at marginal and center locations. tiR₉ enhanced the penetration depth of cNGR-Lip–coumarin 6 through subcellular membrane barriers and caused its specific accumulation in mitochondria, endoplasmic reticulum, and Golgi apparatus. It was also obvious that cNGR-tiR₉-Lip-Dox induced enhanced apoptosis and activated caspase 3/7. Moreover, compared with cNGR-Lip-Dox, cNGR-tiR₉-Lip-Dox induced a significantly higher antiproliferative effect and markedly suppressed tumor growth in HT1080-bearing nude mice.

Conclusion

This active tumor-targeting nanocarrier incorporating a tandem-insert polyarginine (tiR₉) as an accelerating motif shows promise as an effective drug-delivery system to accelerate translocation of drugs across tumor-cell/subcellular membrane barriers to achieve improved specific tumor therapy.

Transferrin and octaarginine modified dual-functional liposomes with improved cancer cell targeting and enhanced intracellular delivery for the treatment of ovarian cancer

Off-target effects of drugs severely limit cancer therapy. Targeted nanocarriers are promising to enhance the delivery of therapeutics to tumors. Among many approaches for active tumor-targeting, arginine-rich cell penetrating peptides (AR-CPP) and ligands specific to target over-expressed receptors on cancer-cell surfaces, are popular. Earlier, we showed that the attachment of an AR-CPP octaarginine (R8) to the surface of DOXIL^® (Doxorubicin encapsulated PEGylated liposomes) improved cytoplasmic and nuclear DOX delivery that enhanced the cytotoxic effect in vitro and improved therapeutic efficacy in vivo. Here, we report on DOX-loaded liposomes, surface-modified with, R8 and transferrin (Tf) (Dual DOX-L), to improve targeting of A2780 ovarian carcinoma cells via the over-expressed transferrin receptors (TfRs) with R8-mediated intracellular DOX delivery. Flow cytometry analysis with fluorescently labeled DualL (without DOX) showed two-fold higher cancer-cell association than other treatments after 4 h treatment. Blocking entry pathways of R8 (macropinocytosis) and Tf (receptor-mediated endocytosis, RME) resulted in a decreased cancer-cell association of DualL. Confocal microscopy confirmed involvement of both entry pathways and cytoplasmic liposome accumulation with nuclear DOX delivery for Dual DOX-L. Dual DOX-L exhibited enhanced cytotoxicity in vitro and was most effective in controlling tumor growth in vivo in an A2780 ovarian xenograft model compared to other treatments. A pilot biodistribution study showed improved DOX accumulation in tumors after Dual DOX-L treatment. All results collectively presented a clear advantage of the R8 and Tf combination to elevate the therapeutic potential of DOX-L by exploiting TfR over-expression imparting specificity followed by endosomal escape and intracellular delivery via R8.

Block Copolymer Micelles in Nanomedicine Applications

Polymeric micelles are demonstrating high potential as nanomedicines capable of controlling the distribution and function of loaded bioactive agents in the body, effectively overcoming biological barriers, and various formulations are engaged in intensive preclinical and clinical testing. This Review focuses on polymeric micelles assembled through multimolecular interactions between block copolymers and the loaded drugs, proteins, or nucleic acids as translationable nanomedicines. The aspects involved in the design of successful micellar carriers are described in detail on the basis of the type of polymer/payload interaction, as well as the interplay of micelles with the biological interface, emphasizing on the chemistry and engineering of the block copolymers. By shaping these features, polymeric micelles have been propitious for delivering a wide range of therapeutics through effective sensing of targets in the body and adjustment of their properties in response to particular stimuli, modulating the activity of the loaded drugs at the targeted sites, even at the subcellular level. Finally, the future perspectives and imminent challenges for polymeric micelles as nanomedicines are discussed, anticipating to spur further innovations.

Cell Penetrating Peptides as Molecular Carriers for Anti-Cancer Agents

Cell membranes with their selective permeability play important functions in the tight control of molecular exchanges between the cytosol and the extracellular environment as the intracellular membranes do within the internal compartments. For this reason the plasma membranes often represent a challenging obstacle to the intracellular delivery of many anti-cancer molecules. The active transport of drugs through such barrier often requires specific carriers able to cross the lipid bilayer. Cell penetrating peptides (CPPs) are generally 5-30 amino acids long which, for their ability to cross cell membranes, are widely used to deliver proteins, plasmid DNA, RNA, oligonucleotides, liposomes and anti-cancer drugs inside the cells. In this review, we describe the several types of CPPs, the chemical modifications to improve their cellular uptake, the different mechanisms to cross cell membranes and their biological properties upon conjugation with specific molecules. Special emphasis has been given to those with promising application in cancer therapy.

Improved method for synthesis of low molecular weight protamine-siRNA conjugate

RNAi technology has aroused wide public interest due to its high efficiency and specificity to treat multiple types of diseases. However, the effective delivery of siRNA remains a challenge due to its large molecular weight and strong anionic charge. Considering their remarkable functions in vivo and features that are often desired in drug delivery carriers, biomimetic systems for siRNA delivery become an effective and promising strategy. Based on this, covalent attachment of synthetic cell penetrating peptides (CPP) to siRNA has become of great interest. We developed a monomeric covalent conjugate of low molecular weight protamine (LMWP, a well-established CPP) and siRNA via a cytosol-cleavable disulfide linkage using PEG as a crosslinker. Results showed that the conjugates didn't generate coagulation, and exhibited much better RNAi potency and intracellular delivery compared with the conventional charge-complexed CPP/siRNA aggregates. Three different synthetic and purification methods were compared in order to optimize synthesis efficiency and product yield. The methodology using hetero-bifunctional NHS–PEG–OPSS as a crosslinker to synthesize LMWP–siRNA simplified the synthesis and purification process and produced the highest yield. These results pave the way towards siRNA biomimetic delivery and future clinical translation.

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

High-Yield Synthesis of Monomeric LMWP(CPP)-siRNA Covalent Conjugate for Effective Cytosolic Delivery of siRNA

Because of the unparalleled efficiency and universal utility in treating a variety of disease types, siRNA agents have evolved as the future drug-of-choice. Yet, the inability of the polyanionic siRNA macromolecules to cross the cell membrane remains as the bottleneck of possible clinical applications. With the cell penetrating peptides (CPP) being discovered lately, the most effective tactic to achieve the highest intracellular siRNA delivery deems to be by covalently conjugating the drug to a CPP; for instance, the arginine-rich Tat or low molecular weight protamine (LMWP) peptides. However, construction of such a chemical conjugate has been referred by scientists in this field as the “Holy Grail” challenge due to self-assembly of the cationic CPP and anionic siRNA into insoluble aggregates that are deprived of the biological functions of both compounds. Based on the dynamic motion of PEG, we present herein a concise coupling strategy that is capable of permitting a high-yield synthesis of the cell-permeable, cytosol-dissociable LMWP-siRNA covalent conjugates. Cell culture assessment demonstrates that this chemical conjugate yields by far the most effective intracellular siRNA delivery and its corresponded gene-silencing activities. This work may offer a breakthrough advance towards realizing the clinical potential of all siRNA therapeutics and, presumably, most anionic macromolecular drugs such as anti-sense oligonucleotides, gene compounds, DNA vectors and proteins where conjugation with the CPP encounters with problems of aggregation and precipitation. To this end, the impact of this coupling technique is significant, far-reaching and wide-spread.

Intracellular Delivery of Nanobodies for Imaging of Target Proteins in Live Cells

PURPOSE

Cytosolic delivery of nanobodies for molecular target binding and fluorescent labeling in living cells.

METHODS

Fluorescently labeled nanobodies were formulated with sixteen different sequence-defined oligoaminoamides. The delivery of formulated anti-GFP nanobodies into different target protein-containing HeLa cell lines was investigated by flow cytometry and fluorescence microscopy. Nanoparticle formation was analyzed by fluorescence correlation spectroscopy.

RESULTS

The initial oligomer screen identified two cationizable four-arm structured oligomers (734, 735) which mediate intracellular nanobody delivery in a receptor-independent (734) or folate receptor facilitated (735) process. The presence of disulfide-forming cysteines in the oligomers was found critical for the formation of stable protein nanoparticles of around 20 nm diameter. Delivery of labeled GFP nanobodies or lamin nanobodies to their cellular targets was demonstrated by fluorescence microscopy including time lapse studies.

CONCLUSION

Two sequence-defined oligoaminoamides with or without folate for receptor targeting were identified as effective carriers for intracellular nanobody delivery, as exemplified by GFP or lamin binding in living cells. Due to the conserved nanobody core structure, the methods should be applicable for a broad range of nanobodies directed to different intracellular targets.

Sequence-defined shuttles for targeted nucleic acid and protein delivery

Molecular medicine opens into a space of novel specific therapeutic agents: intracellularly active drugs such as peptides, proteins or nucleic acids, which are not able to cross cell membranes and enter the intracellular space on their own. Through the development of cell-targeted shuttles for specific delivery, this restriction in delivery has the potential to be converted into an advantage. On the one hand, due to the multiple extra- and intracellular barriers, such carrier systems need to be multifunctional. On the other hand, they must be precise and reproducibly manufactured due to pharmaceutical reasons. Here we review the design of precise sequence-defined delivery carriers, including solid-phase synthesized peptides and nonpeptidic oligomers, or nucleotide-based carriers such as aptamers and origami nanoboxes.

Expression, purification and production of antisera against recombinant truncated VP22 protein

Cell-penetrating peptides (CPPs) are non-invasive vectors that can efficiently transport bioactive cargo across the cell membrane. Naturally occurring CPPs, such as the tegument protein VP22 of the Herpes simplex virus type 1, can potentiate protein-drug delivery into living cells. The aim of the present study was to construct anti-VP22 antibodies that can be used to detect VP22-fusion drugs. Therefore, 60- and 45-amino acid peptides corresponding to the N-terminus and C-terminus of VP22, respectively, were cloned, expressed and purified. Subsequently, polyclonal antisera against them were generated. The DNA sequence, cloned into the pGEX-5X-1 vector, was transformed into E. coli BL21 (DE3). After inducing expression with 1 mM isopropyl-β-d-thiogalactopyranoside (IPTG) at 25°C for 4 h, the recombinant VP22 proteins were purified by electroelution. The high titers of polyclonal antisera obtained subsequent to immunization of mice with the purified recombinant truncated VP22 was confirmed by ELISA. Western blot and immunofluorescence analysis showed that the antisera detected both the truncated and full-length VP22 protein. Therefore, the polyclonal antisera against VP22 may be used in the detection of the intracellular location of VP22-fusion protein drugs.

Cell-penetrating peptides as noninvasive transmembrane vectors for the development of novel multifunctional drug-delivery systems

Unique characteristics, such as nontoxicity and rapid cellular internalization, allow the cell-penetrating peptides (CPPs) to transport hydrophilic macromolecules into cells, thus, enabling them to execute biological functions. However, some CPPs have limitations due to nonspecificity and easy proteolysis. To overcome such defects, the CPP amino acid sequence can be modified, replaced, and reconstructed for optimization. CPPs can also be used in combination with other drug vectors, fused with their preponderances to create novel multifunctional drug-delivery systems that increase the stability during blood circulation, and also develop novel preparations capable of targeted delivery, along with sustainable and controllable release. Further improvements in CPP structure can facilitate the penetration of macromolecules into diverse biomembrane structures, such as the blood brain barrier, gastroenteric mucosa, and skin dermis. The ability of CPP to act as transmembrane vectors improves the clinical application of some biomolecules to treat central nervous system diseases, increase oral bioavailability, and develop percutaneous-delivery dosage form.

Proteases in cancer drug delivery

Whereas protease inhibitors have been developed successfully against hypertension and viral infections, they have failed thus far as cancer drugs. With advances in cancer profiling we now better understand that the tumor "degradome" (i.e. the repertoire of proteases and their natural inhibitors and interaction partners) forms a complex network in which specific nodes determine the global outcome of manipulation of the protease web. However, knowing which proteases are active in the tumor micro-environment, we may tackle cancers with the use of Protease-Activated Prodrugs (PAPs). Here we exemplify this concept for metallo-, cysteine and serine proteases. PAPs not only exist as small molecular adducts, containing a cleavable substrate sequence and a latent prodrug, they are presently also manufactured as various types of nanoparticles. Although the emphasis of this review is on PAPs for treatment, it is clear that protease activatable probes and nanoparticles are also powerful tools for imaging purposes, including tumor diagnosis and staging, as well as visualization of tumor imaging during microsurgical resections.

An elastic liposomal formulation for RNAi-based topical treatment of skin disorders: Proof-of-concept in the treatment of psoriasis

RNA interference (RNAi) is a rapidly emerging approach for targeted gene silencing to alleviate disease pathology. However, lack of efficient carriers for targeted delivery delays the clinical translation of RNAi. An interesting target for local RNAi therapeutics is the skin as it allows direct access to target cells. Still, applications are limited due to the effective skin barrier which hinders penetration. Herein, a description is given of a liposomal carrier, called 'DDC642', capable of delivering RNAi molecules to the epidermis of impaired and intact human skin, without targeting the dermis or circulatory system. In a psoriasis tissue model, down-regulation of the psoriasis marker human beta-defensin 2 by DDC642-delivered siRNA was confirmed, providing proof-of-concept. These liposomes thus hold great potential as topical delivery system for RNAi therapeutics in the treatment of numerous skin diseases.

Cell Penetrating Peptide Conjugated Chitosan for Enhanced Delivery of Nucleic Acid

Gene therapy is an emerging therapeutic strategy for the cure or treatment of a spectrum of genetic disorders. Nevertheless, advances in gene therapy are immensely reliant upon design of an efficient gene carrier that can deliver genetic cargoes into the desired cell populations. Among various nonviral gene delivery systems, chitosan-based carriers have gained increasing attention because of their high cationic charge density, excellent biocompatibility, nearly nonexistent cytotoxicity, negligible immune response, and ideal ability to undergo chemical conjugation. However, a major shortcoming of chitosan-based carriers is their poor cellular uptake, leading to inadequate transfection efficiency. The intrinsic feature of cell penetrating peptides (CPPs) for transporting diverse cargoes into multiple cell and tissue types in a safe manner suggests that they can be conjugated to chitosan for improving its transfection efficiency. In this review, we briefly discuss CPPs and their classification, and also the major mechanisms contributing to the cellular uptake of CPPs and cargo conjugates. We also discuss immense improvements for the delivery of nucleic acids using CPP-conjugated chitosan-based carriers with special emphasis on plasmid DNA and small interfering RNA.

Sorting nexin 9 differentiates ligand-activated Smad3 from Smad2 for nuclear import and transforming growth factor β signaling

Sorting nexin 9 (SNX9) is shown to differentiate Smad3 from Smad2 nuclear delivery by mediating the association of phosphorylated Smad3 with importin 8 and the nuclear membrane. While the absence of SNX9 had negligible effects on transforming growth factor β receptor activity or Smad2 signaling, Smad3-dependent targets and phenotypes were inhibited.

Transforming growth factor β (TGFβ) is a pleiotropic protein secreted from essentially all cell types and primary tissues. While TGFβ’s actions reflect the activity of a number of signaling networks, the primary mediator of TGFβ responses are the Smad proteins. Following receptor activation, these cytoplasmic proteins form hetero-oligomeric complexes that translocate to the nucleus and affect gene transcription. Here, through biological, biochemical, and immunofluorescence approaches, sorting nexin 9 (SNX9) is identified as being required for Smad3-dependent responses. SNX9 interacts with phosphorylated (p) Smad3 independent of Smad2 or Smad4 and promotes more rapid nuclear delivery than that observed independent of ligand. Although SNX9 does not bind nucleoporins Nup153 or Nup214 or some β importins (Imp7 or Impβ), it mediates the association of pSmad3 with Imp8 and the nuclear membrane. This facilitates nuclear translocation of pSmad3 but not SNX9.

Cell penetrating peptides can exert biological activity: a review

Cell penetrating peptides (CPPs) have been successful in delivering cargo into many different cell types and are an important alternative to other methods of permeation that might damage the integrity of the cell membrane. The traditional view of CPPs is that they are inert molecules that can be successfully used to deliver many cargos intracellularly. The goal of this review is to challenge this traditional understanding of CPPs. Recent literature has demonstrated that CPPs themselves can convey biological activity, including the alteration of gene expression and inhibition of protein kinases and proteolytic activity. Further characterization of CPPs is required to determine the extent of this activity. Research into the use of CPPs for intracellular delivery should continue with investigators being aware of these recent results.

Targeted LC-MS quantification of intact TAT-fusion therapeutics: a case study

Background: While HIV-1 TAT peptide-conjugation shows great promise on improving intracellular delivery of biotherapeutics in vitro and in vivo, quantification of TAT-fusion therapeutics in biological matrices represents a daunting challenge. Materials & methods: A sensitive MS approach for accurate quantification of intact TAT-fusion protein/polypeptide in plasma was developed. i) A semi-automated 96-well ion-exchange solid phase extraction was developed; ii) a rapid LC separation on C4 was devised; iii) a TAT-fusion analog was constructed as internal standard. Results: We reported that low percentage of supercharging reagents enabled a significant sensitivity improvement of MS for intact TAT-fusion protein/polypeptide analysis. We showed a proof of concept by successfully developing a sensitive LC/MRM-MS method for quantifying GAP161, a TAT-conjugating RasGAP mimics, in rat plasma. Conclusion: This work represents the first quantification of TAT-fusion therapeutics in biological samples by an LC-MS based method.

A versatile pH-responsive platform for intracellular protein delivery using calcium phosphate nanoparticles

A highly biocompatible nanoplatform for the intracellular delivery of different proteins, exhibiting pH-responsive release and efficient endosomal escape.

Non-viral nanocarriers for siRNA delivery in breast cancer

Breast cancer is the most frequently diagnosed malignancy in American women. While significant progress has been made in the development of modern diagnostic tools and surgical treatments, only marginal improvements have been achieved with relapsed metastatic breast cancer. Small interfering RNAs (siRNAs) mediate gene silencing of a target protein by disrupting messenger RNAs in an efficient and sequence-specific manner. One application of this technology is the knockdown of genes responsible for tumorigenesis, including those driving oncogenesis, survival, proliferation and death of cells, angiogenesis, invasion and metastasis, and resistance to treatment. Non-viral nanocarriers have attracted attention based on their potential for targeted delivery of siRNA and efficient gene silencing without toxicity. Here, we review promising, non-viral delivery strategies employing liposomes, nanoparticles and inorganic materials in breast cancer.

A survey on "Trojan Horse" peptides: opportunities, issues and controlled entry to "Troy"

Cell-penetrating peptides (CPPs), often vividly termed as the "Trojan Horse" peptides, have attracted considerable interest for the intracellular delivery of a wide range of cargoes, such as small molecules, peptides, proteins, nucleic acids, contrast agents, nanocarriers and so on. Some preclinical and clinical developments of CPP conjugates demonstrate their promise as therapeutic agents for drug discovery. There is increasing evidence to suggest that CPPs have the potential to cross several bio-barriers (e.g., blood-brain barriers, intestinal mucosa, nasal mucosa and skin barriers). Despite revolutionary process in many aspects, there are a lot of basic issues unclear for these entities, such as internalization mechanisms, translocation efficiency, translocation kinetics, metabolic degradation, toxicity, side effect, distribution and non-specificity. Among them, non-specificity remains a major drawback for the in vivo application of CPPs in the targeted delivery of cargoes. So far, diverse organelle-specific CPPs or controlled delivery strategies have emerged and improved their specificity. In this review, we will look at the opportunities of CPPs in clinical development, bio-barriers penetration and nanocarriers delivery. Then, a series of basic problems of CPPs will be discussed. Finally, this paper will highlight the use of various controlled strategies in the organelle-specific delivery and targeted delivery of CPPs. The purpose of this review will be to emphasize most influential advance in this field and present a fundamental understanding for challenges and utilizations of CPPs. This will accelerate their translation as efficient vectors from the in vitro setting into the clinic arena, and retrieve the entry art to "Troy".

Label-free probe of HIV-1 TAT peptide binding to mimetic membranes

The transacting activator of transduction (TAT) protein plays a key role in the progression of AIDS. Studies have shown that a +8 charged sequence of amino acids in the protein, called the TAT peptide, enables the TAT protein to penetrate cell membranes. To probe mechanisms of binding and translocation of the TAT peptide into the cell, investigators have used phospholipid liposomes as cell membrane mimics. We have used the method of surface potential sensitive second harmonic generation (SHG), which is a label-free and interface-selective method, to study the binding of TAT to anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol (POPG) and neutral 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes. It is the SHG sensitivity to the electrostatic field generated by a charged interface that enabled us to obtain the interfacial electrostatic potential. SHG together with the Poisson-Boltzmann equation yielded the dependence of the surface potential on the density of adsorbed TAT. We obtained the dissociation constants Kd for TAT binding to POPC and POPG liposomes and the maximum number of TATs that can bind to a given liposome surface. For POPC Kd was found to be 7.5 ± 2 μM, and for POPG Kd was 29.0 ± 4.0 μM. As TAT was added to the liposome solution the POPC surface potential changed from 0 mV to +37 mV, and for POPG it changed from -57 mV to -37 mV. A numerical calculation of Kd, which included all terms obtained from application of the Poisson-Boltzmann equation to the TAT liposome SHG data, was shown to be in good agreement with an approximated solution.

Bioreducible polycations as shuttles for therapeutic nucleic acid and protein transfection

SIGNIFICANCE

Nucleic acids such as gene-encoding DNAs, gene-silencing small interfering RNAs, or recombinant proteins addressing intracellular molecular targets present a major new therapeutic modality, provided efficient solutions for intracellular delivery can be found. The different physiological redox environments inside and outside the cell can be utilized for optimizing the involved transport processes.

RECENT ADVANCES

Intracellular delivery of nucleic acids or proteins requires dynamic carriers that discriminate between different cellular locations. Bioreducible cationic polymers can package their therapeutic cargo stably in the extracellular environment, but sense the reducing intracellular cytosolic environment. Based on disulfide cleavage, carriers are degraded into biocompatible fragments and release the cargo in functional form. Disulfide linkages between oligocations, between the carrier and the cargo, or spatial caging of complexed cargo by disulfides have been pursued, with polymers or precise sequence-defined peptides and oligomers.

CRITICAL ISSUES

A quantitative knowledge of the bioreductive capacities within different biological compartments and the involved cellular reduction processes would be greatly helpful for improved carriers with disulfides cleaved within the right compartment at the right time.

FUTURE DIRECTIONS

Novel designs of multifunctional nanocarriers will incorporate macromolecular disulfide entry mechanisms previously optimized by natural evolution of toxins and viruses. In addition to extracellular stabilization and intracellular disassembly, tuned disulfides will contribute to deshielding at the cell surface, or translocation from intracellular compartments to the cytosol.

The intracellular delivery of TAT-aequorin reveals calcium-mediated sensing of environmental and symbiotic signals by the arbuscular mycorrhizal fungus Gigaspora margarita

Arbuscular mycorrhiza (AM) is an ecologically relevant symbiosis between most land plants and Glomeromycota fungi. The peculiar traits of AM fungi have so far limited traditional approaches such as genetic transformation. The aim of this work was to investigate whether the protein transduction domain of the HIV-1 transactivator of transcription (TAT) protein, previously shown to act as a potent nanocarrier for macromolecule delivery in both animal and plant cells, may translocate protein cargoes into AM fungi. We evaluated the internalization into germinated spores of Gigaspora margarita of two recombinant TAT fusion proteins consisting of either a fluorescent (GFP) or a luminescent (aequorin) reporter linked to the TAT peptide. Both TAT-fused proteins were found to enter AM fungal mycelia after a short incubation period (5-10 min). Ca2+ measurements in G. margarita mycelia pre-incubated with TAT-aequorin demonstrated the occurrence of changes in the intracellular free Ca2+ concentration in response to relevant stimuli, such as touch, cold, salinity, and strigolactones, symbiosis-related plant signals. These data indicate that the cell-penetrating properties of the TAT peptide can be used as an effective strategy for intracellularly delivering proteins of interest and shed new light on Ca2+ homeostasis and signalling in AM fungi.

Slit diaphragm protein Neph1 and its signaling: a novel therapeutic target for protection of podocytes against glomerular injury

Podocytes are specialized epithelial cells that are critical components of the glomerular filtration barrier, and their dysfunction leads to proteinuria and renal failure. Therefore, preserving podocyte function is therapeutically significant. In this study, we identified Neph1 signaling as a therapeutic target that upon inhibition prevented podocyte damage from a glomerular injury-inducing agent puromycin aminonucleoside (PAN). To specifically inhibit Neph1 signaling, we used a protein transduction approach, where the cytoplasmic domain of Neph1 (Neph1CD) tagged with a protein transduction domain trans-activator of transcription was transduced in cultured podocytes prior to treatment with PAN. The PAN-induced Neph1 phosphorylation was significantly reduced in Neph1CD-transduced cells; in addition, these cells were resistant to PAN-induced cytoskeletal damage. The biochemical analysis using subfractionation studies showed that unlike control cells Neph1 was retained in the lipid raft fractions in the transduced cells following treatment with PAN, indicating that transduction of Neph1CD in podocytes prevented PAN-induced mislocalization of Neph1. In accordance, the immunofluorescence analysis further suggested that Neph1CD-transduced cells had increased ability to retain endogenous Neph1 at the membrane in response to PAN-induced injury. Similar results were obtained when angiotensin was used as an injury-inducing agent. Consistent with these observations, maintaining high levels of Neph1 at the membrane using a podocyte cell line overexpressing chimeric Neph1 increased the ability of podocytes to resist PAN-induced injury and PAN-induced albumin leakage. Using a zebrafish in vivo PAN and adriamycin injury models, we further demonstrated the ability of transduced Neph1CD to preserve glomerular function. Collectively, these results support the conclusion that inhibiting Neph1 signaling is therapeutically significant in preventing podocyte damage from glomerular injury.

Topical delivery of siRNA into skin using SPACE-peptide carriers

Short-interfering RNAs (siRNAs) offer a potential tool for the treatment of skin disorders. However, applications of siRNA for dermatological conditions are limited by their poor permeation across the stratum corneum of the skin and low penetration into the skin's viable cells. In this study, we report the use of SPACE-peptide in combination with a DOTAP-based ethosomal carrier system to enhance skin delivery of siRNA. A DOTAP-based SPACE Ethosomal System significantly enhanced siRNA penetration into porcine skin in vitro by 6.3±1.7-fold (p<0.01) with an approximately 10-fold (p<0.01) increase in epidermis accumulation of siRNA compared to that from an aqueous solution. Penetration of siRNA was also enhanced at the cellular level. Internalization of SPACE-peptide occurred in a concentration dependent manner marked by a shift in intracellular distribution from punctate spots to diffused cytoplasmic staining at a peptide concentration of 10mg/mL. In vitro delivery of GAPDH siRNA by SPACE peptide led to 83.3±3.0% knockdown relative to the control. In vivo experiments performed using female BALB/C mice also confirmed the efficacy of DOTAP-SES in delivering GAPDH-siRNA into skin. Topical application of DOTAP-SES on mice skin resulted in 63.2%±7.7% of GAPDH knockdown, which was significantly higher than that from GAPDH-siRNA PBS (p<0.05). DOTAP-SES formulation reported here may open new opportunities for cutaneous siRNA delivery.

Intracellular delivery of universal proteins using a lysine headgroup containing cationic liposomes: deciphering the uptake mechanism

An amino acid-based cationic lipid having a TFA counterion (trifluoroacetic acid counterion) in the lysine headgroup was used to deliver functional proteins into human cervical cancer cells, HeLa, in the presence of serum. Proteins used in the study were fluorescein isothiocyanate (FITC) labeled bovine serum albumin, mouse anti-F actin antibody [NH3], and goat anti mouse IgG conjugated with FITC. The formation of liposome/protein complexes was confirmed using native polyacrylamide gel electrophoresis. Furthermore, the complexes were characterized in terms of their size and zeta potential at different pH values and found to be responsive to changes in pH. The highest delivery efficiency of the liposome/albumin complexes was 99% at 37 °C. The liposomes effectively delivered albumin and antibodies as confirmed by confocal laser scanning microscopy (CLSM). Inhibition studies showed that the cellular uptake mechanism of the complexes was via caveolae-mediated endocytosis, and the proteins were subsequently released from either the early endosomes or the caveosomes as suggested by CLSM. Thus, lysine-based cationic liposomes can be a useful tool for intracellular protein delivery.

Gene delivery techniques for adult stem cell-based regenerative therapy

Over the past decade, stem cells have been considered to be a promising resource to cure and regenerate damaged or diseased tissues with research extending from basic studies to clinical application. Furthermore, genetically modified stem cells have the potential to reduce tumorigenic risks and achieve safe tissue formation. Recent advances in genetic modification of stem cells have rendered these cells more accessible and stable. The successful genetic modification of stem cells relies heavily on designing vector systems, either viral or nonviral vectors, which can efficiently deliver therapeutic genes to the cells with minimum toxicity. Currently, viral vectors showing high transfection efficiencies still raise safety issues, whereas safer nonviral vectors exhibit extremely poor transfection in stem cells. Here, we attempt to review and discuss the main factors raising concern in previous reports, and devise strategies to solve the issues in gene delivery systems for successful stem cell-targeting regenerative therapy.

Overcoming nonviral gene delivery barriers: perspective and future

A key end goal of gene delivery research is to develop clinically relevant vectors that can be used to combat elusive diseases such as AIDS. Despite promising engineering strategies, efficiency and ultimately gene modulation efficacy of nonviral vectors have been hindered by numerous in vitro and in vivo barriers that have resulted in subviral performance. In this perspective, we concentrate on the gene delivery barriers associated with the two most common classes of nonviral vectors, cationic-based lipids and polymers. We present the existing delivery barriers and summarize current vector-specific strategies to overcome said barriers.