Cell-penetrating peptides and their analogues as novel nanocarriers for drug delivery

Current developments and prospects of the antibiotic delivery systems

Antibiotics have remained the cornerstone for the treatment of bacterial infections ever since their discovery in the twentieth century. The uproar over antibiotic resistance among bacteria arising from genome plasticity and biofilm development has rendered current antibiotic therapies ineffective, urging the development of innovative therapeutic approaches. The development of antibiotic resistance among bacteria has further heightened the clinical failure of antibiotic therapy, which is often linked to its low bioavailability, side effects, and poor penetration and accumulation at the site of infection. In this review, we highlight the potential use of siderophores, antibodies, cell-penetrating peptides, antimicrobial peptides, bacteriophages, and nanoparticles to smuggle antibiotics across impermeable biological membranes to achieve therapeutically relevant concentrations of antibiotics and combat antimicrobial resistance (AMR). We will discuss the general mechanisms via which each delivery system functions and how it can be tailored to deliver antibiotics against the paradigm of mechanisms underlying antibiotic resistance.

The Current and Promising Oral Delivery Methods for Protein- and Peptide-Based Drugs

Drugs based on peptides and proteins (PPs) have been widely used in medicine, beginning with insulin therapy in patients with diabetes mellitus over a century ago. Although the oral route of drug administration is the preferred one by the vast majority of patients and improves compliance, medications of this kind due to their specific chemical structure are typically delivered parenterally, which ensures optimal bioavailability. In order to overcome issues connected with oral absorption of PPs such as their instability depending on digestive enzymes and pH changes in the gastrointestinal (GI) system on the one hand, but also their limited permeability across physiological barriers (mucus and epithelium) on the other hand, scientists have been strenuously searching for novel delivery methods enabling peptide and protein drugs (PPDs) to be administered enterally. These include utilization of different nanoparticles, transport channels, substances enhancing permeation, chemical modifications, hydrogels, microneedles, microemulsion, proteolytic enzyme inhibitors, and cell-penetrating peptides, all of which are extensively discussed in this review. Furthermore, this article highlights oral PP therapeutics both previously used in therapy and currently available on the medical market.

The light chain of tetanus toxin bound to arginine-rich cell-penetrating peptide inhibits cortical reaction in mouse oocytes

Introduction: Cortical reaction is a secretory process that occurs after a spermatozoon fuses with the oocyte, avoiding the fusion of additional sperm. During this exocytic event, the cortical granule membrane fuses with the oocyte plasma membrane. We have identified several molecular components involved in this process and confirmed that SNARE proteins regulate membrane fusion during cortical reaction in mouse oocytes. In those studies, we microinjected different nonpermeable reagents to demonstrate the participation of a specific protein in the cortical reaction. However, the microinjection technique has several limitations. In this work, we aimed to assess the potential of cell-penetrating peptides (CPP) as biotechnological tools for delivering molecules into oocytes, and to evaluate the functionality of the permeable tetanus toxin (bound to CPP sequence) during cortical reaction. Methods: Arginine-rich cell-penetrating peptides have demonstrated the optimal internalization of small molecules in mammalian cells. Two arginine-rich CPP were used in the present study. One, labeled with 5-carboxyfluorescein, to characterize the factors that can modulate its internalization, and the other, the permeable light chain of tetanus toxin, that cleaves the SNAREs VAMP1 and VAMP3 expressed in mouse oocytes. Results: Results showed that fluorescent CPP was internalized into the oocyte cytoplasm and that internalization was dependent on the concentration, time, temperature, and maturation stage of the oocyte. Using our functional assay to study cortical reaction, the light chain of tetanus toxin bound to arginine-rich cell-penetrating peptide inhibited cortical granules exocytosis. Discussion: Results obtained from the use of permeable peptides demonstrate that this CPP is a promising biotechnological tool to study functional macromolecules in mouse oocytes.

Peptides as carriers of active ingredients: A review

Bioactive compounds are highly valuable in the fields of food and medicine, but their application is limited due to easy deterioration after oral or skin administration. In recent years, the use of peptides as delivery systems for bioactive compounds has been intensively researched because of their special physicochemical characteristics. Peptides can be assembled using various preparation methods and can form several composite materials such as hydrogels, micelles, emulsions and particles. The composite material properties are determined by peptides, bioactive compounds and the construction methods employed. Herein, this paper provides a comprehensive review of the peptides used for active ingredients delivery, fabrication methods for creating delivery systems, structures, targeting characteristics, functional activities and mechanism of delivery systems, as well as their absorption and metabolism, which provided theoretical basis and reference for further research and development of functional composites.

Cell penetrating peptides: Highlighting points in cancer therapy

Cell-penetrating peptides (CPPs), first identified in HIV a few decades ago, deserved great attention in the last two decades; especially to support the penetration of anticancer drug means. In the drug delivery discipline, they have been involved in various approaches from mixing with hydrophobic drugs to the use of genetically conjugated proteins. The early classification as cationic and amphipathic CPPs has been extended to a few more classes such as hydrophobic and cyclic CPPs so far. Developing potential sequences utilized almost all methods of modern science: choosing high-efficiency peptides from natural protein sequences, sequence-based comparison, amino acid substitution, obtaining chemical and/or genetic conjugations, in silico approaches, in vitro analysis, animal experiments, etc. The bottleneck effect in this discipline reveals the complications that modern science faces in drug delivery research. Most CPP-based drug delivery systems (DDSs) efficiently inhibited tumor volume and weight in mice, but only in rare cases reduced their levels and continued further processes. The integration of chemical synthesis into the development of CPPs made a significant contribution and even reached the clinical stage as a diagnostic tool. But constrained efforts still face serious problems in overcoming biobarriers to reach further achievements. In this work, we reviewed the roles of CPPs in anticancer drug delivery, focusing on their amino acid composition and sequences. As the most suitable point, we relied on significant changes in tumor volume in mice resulting from CPPs. We provide a review of individual CPPs and/or their derivatives in a separate subsection.

Advancement and application of novel cell-penetrating peptide in cancer management

Cell-penetrating peptides (CPPs) are small amino acid sequences with the potential to enter cell membranes. Along with nucleic acids, large proteins, and other chemical compounds, they can deliver several bioactive cargos inside cells. Numerous CPPs have been extracted from natural or synthetic materials since the discovery of the first CPP. In the past few decades, a significant variety of studies have shown the potential of CPPs to cure different diseases. The low toxicity in peptide compared to other drug delivery carriers is a significant benefit of CPP-based therapy, in addition to the high efficacy brought about by swift and effective delivery. A significant tendency for intracellular DNA delivery may also be observed when nanoparticles and the cell penetration peptide are combined. CPPs are frequently used to increase intracellular absorption of nucleic acid, and other therapeutic agents inside the cell. Due to long-term side effects and possible toxicity, its implementation is restricted. The use of cell-permeating peptides is a commonly used technique to increase their intracellular absorption. Additionally, CPPs have lately been sought for application in vivo, following their success in cellular studies. This review will go through the numerous CPPs, the chemical modifications that improve their cellular uptake, the various means for getting them across cell membranes, and the biological activity they acquire after being conjugate with specific chemicals.

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.

Anticancer peptides mechanisms, simple and complex

Peptide therapy has started since 1920s with the advent of insulin application, and now it has emerged as a new approach in treatment of diseases including cancer. Using anti-cancer peptides (ACPs) is a promising way of cancer therapy as ACPs are continuing to be approved and arrived at major pharmaceutical markets. Traditional cancer treatments face different problems like intensive adverse effects to patient's body, cell resistance to conventional chemical drugs and in some worse cases the occurrence of cell multidrug resistance (MDR) of cancerous tissues against chemotherapy. On the other hand, there are some benefits conceived for peptides usage in treatment of diseases specifically cancer, as these compounds present favorable characteristics such as smaller size, high activity, low immunogenicity, good biocompatibility in vivo, convenient and rapid way of synthesis, amenable to sequence modification and revision and there is no limitation for the type of cargo they carry. It is possible to achieve an optimum molecular and functional structure of peptides based on previous experience and bank of peptide motif data which may result in novel peptide design. Bioactive peptides are able to form pores in cell membrane and induce necrosis or apoptosis of abnormal cells. Moreover, recent researches have focused on the tumor recognizing peptide motifs with the ability to permeate to cancerous cells with the aim of cancer treatment at earlier stages. In this strategy the most important factors for addressing cancer are choosing peptides with easy accessibility to tumor cell without cytotoxicity effect towards normal cells. The peptides must also meet acceptable pharmacokinetic requirements. In this review, the characteristics of peptides and cancer cells are discussed. The various mechanisms of peptides' action proposed against cancer cells make the next part of discussion. It will be followed by giving information on peptides application, various methods of peptide designing along with introducing various databases. Future aspects of peptides for employing in area of cancer treatment come as conclusion at the end.

Carbon nanostructures: a comprehensive review of potential applications and toxic effects

There is no doubt that nanotechnology has revolutionized our life since the 1970s when it was first introduced. Nanomaterials have helped us to improve the current products and services we use. Among the different types of nanomaterials, the application of carbon-based nanomaterials in every aspect of our lives has rapidly grown over recent decades. This review discusses recent advances of those applications in distinct categories, including medical, industrial, and environmental applications. The first main section introduces nanomaterials, especially carbon-based nanomaterials. In the first section, we discussed medical applications, including medical biosensors, drug and gene delivery, cell and tissue labeling and imaging, tissue engineering, and the fight against bacterial and fungal infections. The next section discusses industrial applications, including agriculture, plastic, electronic, energy, and food industries. In addition, the environmental applications, including detection of air and water pollutions and removal of environmental pollutants, were vastly reviewed in the last section. In the conclusion section, we discussed challenges and future perspectives.

Using nanotechnology to deliver biomolecules from nose to brain - peptides, proteins, monoclonal antibodies and RNA

There is a growing number of biomolecules, including peptides, proteins, monoclonal antibodies and RNA, that could be potentially used for the treatment of central nervous system (CNS) diseases. However, the realization of their potential is being hampered by the extraordinary difficulties these complex biomolecules have to reach the brain in therapeutically meaningful amounts. Nose-to-brain (N-to-B) delivery is now being investigated as a potential option for the direct transport of biomolecules from the nasal cavity to different brain areas. Here, we discuss how different technological approaches enhance this N-to-B transport, with emphasis on those that have shown a potential for clinical translation. We also analyse how the physicochemical properties of nanocarriers and their modification with cell-penetrating peptides (CPPs) and targeting ligands affect their efficacy as N-to-B carriers for biomolecules.

Cell-penetrating peptides improve pharmacokinetics and pharmacodynamics of anticancer drugs

This review concentrates on the research concerning conjugates of anticancer drugs with versatile cell-penetrating peptides (CPPs). For a better insight into the relationship between the components of the constructs, it starts with the characteristic of the peptides and considers its following aspects: mechanisms of cellular internalization, interaction with cancer-modified membranes, selectivity against tumor tissue. Also, CPPs with anticancer activity have been distinguished and summarized with their mechanisms of action. With respect to the conjugates, the preclinical studies (in vitro, in vivo) indicated that they possess several merits in comparison to the parent drugs. They concerned not only better cellular internalization but also other improvements in pharmacokinetics (e.g. access to the brain tissue) and pharmacodynamics (e.g. overcoming drug resistance). The anticancer activity of the conjugates was usually superior to that of the unconjugated drug. Certain anticancer CPPs and conjugates entered clinical trials.

TAT for Enzyme/Protein Delivery to Restore or Destroy Cell Activity in Human Diseases

Much effort has been dedicated in the recent decades to find novel protein/enzyme-based therapies for human diseases, the major challenge of such therapies being the intracellular delivery and reaching sub-cellular organelles. One promising approach is the use of cell-penetrating peptides (CPPs) for delivering enzymes/proteins into cells. In this review, we describe the potential therapeutic usages of CPPs (mainly trans-activator of transcription protein, TAT) in enabling the uptake of biologically active proteins/enzymes needed in cases of protein/enzyme deficiency, concentrating on mitochondrial diseases and on the import of enzymes or peptides in order to destroy pathogenic cells, focusing on cancer cells.

Recent Advances in Mesoporous Silica Nanoparticles for Targeted Drug Delivery applications

Nanotechnology provides the means to design and fabricate delivery vehicles capable of overcoming physiologically imposed obstacles and undesirable side effects of systemic drug delivery. This protocol allows maximal targeting effectiveness and therefore enhances therapeutic efficiency. In recent years, mesoporous silica nanoparticles (MSNPs) have sparked interest in the nanomedicine research community, particularly for their promising applications in cancer treatment. The intrinsic physio-chemical stability, facile functionalization, high surface area, low toxicity, and great loading capacity for a wide range of chemotherapeutic agents make MSNPs very appealing candidates for controllable drug delivery systems. Importantly, the peculiar nanostructures of MSNPs enabled them to serve as an effective drug, gene, protein, and antigen delivery vehicle for a variety of therapeutic regimens. For these reasons, in this review article, we underscore the recent progress in the design and synthesis of MSNPs and the parameters influencing their characteristic features and activities. In addition, the process of absorption, dissemination, and secretion by injection or oral management of MSNPs are also discussed, as they are key directions for the potential utilization of MSNPs. Factors influencing the in vivo fate of MSNPs will also be highlighted, with the main focus on particle size, morphology, porosity, surface functionality, and oxidation. Given that combining other functional materials with MSNPs may increase their biological compatibility, monitor drug discharge, or improve absorption by tumor cells coated MSNPs; these aspects are also covered and discussed herein.

Applications of amphipathic and cationic cyclic cell-penetrating peptides: Significant therapeutic delivery tool

A new class of peptides, cyclic cell-penetrating peptides (CPPs), has great potential for delivering a vast variety of therapeutics intracellularly for treating diverse ailments. CPPs have been used previously; however, their further use is limited due to instability, toxicity, endosomal degradation, and insufficient cellular penetration. Cyclic CPPs are being investigated in delivering therapeutics to treat various ailments, including multi-drug resistant microbial infections, HIV, and cancer. They can act as a carrier for a variety of cargos and target intracellularly. Approximately 40 cyclic peptides-based therapeutics are available in the market, and annually one cyclic peptide-based drug enters the market. Numerous research and review articles have been published in the last decade about linear and cyclic peptides separately. This review is the first to provide a comprehensive deliberation about cationic and amphipathic cyclic CPPs. Herein, we highlights their structures, significant advantages, translocation mechanisms, and delivery application in the area of biomedical sciences.

Complying with the physiological functions of Golgi apparatus for secretory exocytosis facilitated oral absorption of protein drugs

Intestinal epithelial cells are the primary biological barriers for orally administrated nano-formulations and the delivered protein drugs. Thereinto, besides the cellular uptake, intracellular trafficking pathway and the related exocytosis are of great importance to the trans-epithelial transport of drug-loaded NPs. Herein, inspired by the physiological functions of Golgi apparatus for secreting proteins out of cells, Golgi localization-related amino acid l-cysteine (Cys) was modified on the surface of NPs to see whether and how this modification could guide the Golgi pathway-related transport and facilitate the exocytosis of drug-loaded NPs. Meanwhile, cell-penetrating peptide octa-arginine (R8) was co-modified to increase the cellular uptake. The proportion of R8 and Cys modification was explored to get the best effect of endocytosis and exocytosis of NPs. As a result, 25%R8 + 75%Cys NPs with most Cys modification showed efficient transcytosis with the highest transcytosis/endocytosis ratio (0.87). Interestingly, exocytosis mechanism studies indicated that they trafficked through the Golgi secretory pathway and bypassed lysosomes due to Cys modification. The detailed Golgi position mechanism studies further suggested that the thiol group from Cys was important for mediating Golgi transport. In particular, competitive inhibition studies demonstrated that Cys-modified NPs were more conducive to their exocytosis after being transported through the Golgi secretory pathway. We proved that cargos transported via Golgi apparatus tended to be trafficked out of the cells and avoid degradation, which contributed to the transcytosis of 25%R8 + 75%Cys NPs in vitro. Inspiringly, compared with unmodified NPs, 25%R8 + 75%Cys NPs also exhibited promoted intestinal penetration and oral absorption in vivo. Oral delivery of insulin-loaded 25%R8 + 75%Cys NPs showed stronger hypoglycemic effects in diabetic rats. In summary, this work provides a strategy for complying with the physiological functions of Golgi apparatus for secreting to facilitate the exocytosis of NPs, thus further improving the oral absorption of loaded protein drugs.

In vivo delivery of a multiepitope peptide and Nef protein using novel cell-penetrating peptides for development of HIV-1 vaccine candidate

OBJECTIVES

A potent HIV vaccine should overcome some limitations such as polymorphism of human HLA, the diversity of HIV-1 virus, and the lack of an effective delivery system. In this study, a DNA construct encoding Nef_60-84, Nef_126-144, Vpr_34-47, Vpr_60-75, Gp160_30-53, Gp160_308-323, and P24_8-151 epitopes was designed using bioinformatics tools. The pcDNA3.1-nef-vpr-gp160-p24 and pcDNA3.1-nef constructs were prepared in large scale as endotoxin-free form. Moreover, the recombinant Nef-Vpr-Gp160-p24 polypeptide and Nef protein were generated inE. coli. These constructs were delivered using cell penetrating peptides (CPPs) in vivo, and immune responses were assessed for different modalities in BALB/c mice.

RESULTS

The recombinant DNA constructs were confirmed as the ~ 867 bp and ~ 648 bp bands related tonef-vpr-gp160-p24 andnef genes on agarose gel. Moreover, the purified Nef-Vpr-Gp160-p24 polypeptide and Nef protein showed the ~ 32 kDa and ~ 30 kDa bands on SDS-PAGE, respectively. The results of immune responses indicated that the heterologous prime/boost regimens using both Nef-Vpr-Gp160-P24 and Nef antigens induced significantly the secretion of IgG2a, IgG2b, IFN-γ and Granzyme B compared to other groups. The levels of Granzyme B in mice immunized with Nef antigen were higher than those immunized with Nef-Vpr-Gp160-P24 antigen. The CPPs showed the same potency with Montanide adjuvant for eliciting immune responses.

CONCLUSIONS

The heterologous prime/boost regimens for both antigens could significantly direct immune responses toward Th1 and CTL activity compared to other regimens. Comparing the efficiency of Nef-Vpr-Gp160-P24 and Nef constructs, the Nef-Vpr-Gp160-P24 constructs delivered by CPPs showed promising results as an HIV vaccine candidate.

Emerging applications at the interface of DNA nanotechnology and cellular membranes: Perspectives from biology, engineering, and physics

DNA nanotechnology has proven exceptionally apt at probing and manipulating biological environments as it can create nanostructures of almost arbitrary shape that permit countless types of modifications, all while being inherently biocompatible. Emergent areas of particular interest are applications involving cellular membranes, but to fully explore the range of possibilities requires interdisciplinary knowledge of DNA nanotechnology, cell and membrane biology, and biophysics. In this review, we aim for a concise introduction to the intersection of these three fields. After briefly revisiting DNA nanotechnology, as well as the biological and mechanical properties of lipid bilayers and cellular membranes, we summarize strategies to mediate interactions between membranes and DNA nanostructures, with a focus on programmed delivery onto, into, and through lipid membranes. We also highlight emerging applications, including membrane sculpting, multicell self-assembly, spatial arrangement and organization of ligands and proteins, biomechanical sensing, synthetic DNA nanopores, biological imaging, and biomelecular sensing. Many critical but exciting challenges lie ahead, and we outline what strikes us as promising directions when translating DNA nanostructures for future in vitro and in vivo membrane applications.

A brain tumor-homing tetra-peptide delivers a nano-therapeutic for more effective treatment of a mouse model of glioblastoma

Organ-specific cell-penetrating peptides (CPPs) are a class of molecules that can be highly effective at delivering therapeutic cargoes, and they are currently of great interest in cancer treatment strategies. Herein, we describe a new CPP (amino acid sequence serine-isoleucine-tyrosine-valine, or SIWV) that homes to glioblastoma multiforme (GBM) brain tumor tissues with remarkable specificity in vitro and in vivo. The SIWV sequence was identified from an isoform of annexin-A3 (AA3H), a membrane-interacting human protein. The mechanism of intracellular permeation is proposed to follow a caveolin-mediated endocytotic pathway, based on in vitro and in vivo receptor inhibition and genetic knockdown studies. Feasibility as a targeting agent for therapeutics is demonstrated in a GBM xenograft mouse model, where porous silicon nanoparticles (pSiNPs) containing the clinically relevant anticancer drug SN-38 are grafted with SIWV via a poly-(ethylene glycol) (PEG) linker. The formulation shows enhanced in vivo targeting ability relative to a formulation employing a scrambled control peptide, and significant (P < 0.05) therapeutic efficacy relative to free SN-38 in the GBM xenograft animal model.

HR9: An Important Cell Penetrating Peptide for Delivery of HCV NS3 DNA into HEK-293T Cells

BACKGROUND

The delivery of exogenous genes into cells for functional expression is required for development of DNA vaccine and gene therapy in medicine and pharmacology. Cell Penetrating Peptides (CPPs) were considered to mediate gene and drug delivery into living cells. In this study, an attempt was made to evaluate the efficiency of an arginine-rich CPP, HR9, in HCV NS3 gene delivery compared to TurboFect cationic polymer and supercharged +36 GFP into HEK-293T cells.

METHODS

The recombinant pEGFP-NS3 was constructed and their accuracy was confirmed by digestion and sequencing. Then, the recombinant plasmid was transfected into HEK-293T cells by TurboFect, +36 GFP and HR9 gene delivery systems. The expression of NS3 protein was assessed by fluorescent microscopy, flow cytometry and western blotting.

RESULTS

Our data indicated that HR9 peptide was able to form stable complexes with plasmid DNA and increased its delivery into HEK-293T cells in a non-covalent manner. Furthermore, treatment of cells with HR9 and HR9/DNA complexes resulted in a viability of 90-95% indicating this CPP was not cytotoxic. The analysis of zeta potential and size showed the importance of interactions between positively-charged HR9/pEGFP-NS3 complexes and negatively-charged plasma membranes.

CONCLUSION

The non-toxic HR9 CPP can be considered an effective carrier for delivering plasmid DNA harboring Hepatitis C virus (HCV) gene in therapeutic vaccine design.

Cell penetrating peptides: the potent multi-cargo intracellular carriers

Introduction: Cell penetrating peptides (CPPs) known as protein translocation domains (PTD), membrane translocating sequences (MTS), or Trojan peptides (TP) are able to cross biological membranes without clear toxicity using different mechanisms, and facilitate the intracellular delivery of a variety of bioactive cargos. CPPs could overcome some limitations of drug delivery and combat resistant strains against a broad range of diseases. Despite delivery of different therapeutic molecules by CPPs, they lack cell specificity and have a short duration of action. These limitations led to design of combined cargo delivery systems and subsequently improvement of their clinical applications. Areas covered: This review covers all our studies and other researchers in different aspects of CPPs such as classification, uptake mechanisms, and biomedical applications. Expert opinion: Due to low cytotoxicity of CPPs as compared to other carriers and final degradation to amino acids, they are suitable for preclinical and clinical studies. Generally, the efficiency of CPPs was suitable to penetrate the cell membrane and deliver different cargos to specific intracellular sites. However, no CPP-based therapeutic approach has approved by FDA, yet; because there are some disadvantages for CPPs including short half-life in blood, and nonspecific CPP-mediated delivery to normal tissue. Thus, some methods were used to develop the functions of CPPs in vitro and in vivo including the augmentation of cell specificity by activatable CPPs, specific transport into cell organelles by insertion of corresponding localization sequences, incorporation of CPPs into multifunctional dendrimeric or liposomal nanocarriers to improve selectivity and efficiency especially in tumor cells.

Cyclic Cell-Penetrating Peptides as Efficient Intracellular Drug Delivery Tools

Cyclic cell-penetrating peptides are relatively a newer class of peptides that have a huge potential for the intracellular delivery of therapeutic agents aimed at treating challenging ailments like multidrug-resistant bacterial diseases, cancer, and HIV infection. Cell-penetrating peptides (CPPs) have been extensively explored as intracellular delivery vehicles; however, they have some inherent limitations like poor stability, endosomal entrapment, toxicity, and suboptimal cell penetration. Owing to their favorable properties that avoid these limitations, cyclic CPPs can provide a good alternative to linear CPPs. Several Reviews have been published in the past decade that cover CPPs and cyclic peptides independently. To the best of our knowledge, this is one of the first Reviews that covers cyclic CPPs comprehensively in the light of studies published so far. In this Review, we have detailed examples of cyclic CPPs, their structures, and cyclization strategies followed by a detailed account of their advantages over their linear counterparts. A hot area in cyclic CPPs is the exploration of cell-penetration mechanisms; this Review highlights this topic in detail. Finally, we will review the applications of cyclic CPPs, followed by conclusions and future prospects.

M918: A Novel Cell Penetrating Peptide for Effective Delivery of HIV-1 Nef and Hsp20-Nef Proteins into Eukaryotic Cell Lines

BACKGROUND

HIV-1 Nef protein is a possible attractive target in the development of therapeutic HIV vaccines including protein-based vaccines. The most important disadvantage of protein-based vaccines is their low immunogenicity which can be improved by heat shock proteins (Hsps) as an immunomodulator, and cell-penetrating peptides (CPPs) as a carrier.

METHODS

In this study, the HIV-1 Nef and Hsp20-Nef proteins were generated in E.coli expression system for delivery into the HEK-293T mammalian cell line using a novel cell-penetrating peptide, M918, in a non-covalent fashion. The size, zeta potential and morphology of the peptide/protein complexes were studied by scanning electron microscopy (SEM) and Zeta sizer. The efficiency of Nef and Hsp20-Nef transfection using M918 was evaluated by western blotting in HEK-293T cell line.

RESULTS

The SEM data confirmed the formation of discrete nanoparticles with a diameter of approximately 200-250 nm and 50-80 nm for M918/Nef and M918/Hsp20-Nef, respectively. The dominant band of ~ 27 kDa and ~ 47 kDa was detected in the transfected cells with the Nef/ M918 and Hsp20-Nef/ M918 nanoparticles at a molar ratio of 1:20 using anti-HIV-1 Nef monoclonal antibody. These bands were not detected in the un-transfected and transfected cells with Nef or Hsp20- Nef protein alone indicating that M918 could increase the penetration of Nef and Hsp20-Nef proteins into the cells.

CONCLUSION

These data suggest that M918 CPP can be used to enter HIV-1 Nef and Hsp20-Nef proteins inside mammalian cells efficiently as a promising approach in HIV-1 vaccine development.

Simultaneous use of natural adjuvants and cell penetrating peptides improves HCV NS3 antigen-specific immune responses

To improve an effective hepatitis C virus (HCV) therapeutic vaccine, induction of a strong and long term HCV antigen-specific immune response is an important parameter. HCV non-structural protein 3 (NS3) has antigenic properties and plays a major role in viral clearance. In this study, DNA constructs encoding HCV NS3 and heat shock protein 27 (Hsp27)-NS3 genes, and the recombinant (r) NS3 and rHsp27-NS3 proteins complexed with HR9 and Cady-2 cell penetrating peptides (CPPs) were utilized to evaluate antibody, cytokine and Granzyme B secretion in mice. Herein, the formation of NS3 and Hsp27-NS3 DNA/ HR9 CPP complexes were revealed by gel retardation assay and protection against DNase and protease. Cady-2 peptide was used to form the nanoparticles with rNS3 and rHsp27-NS3 proteins. The size and charge of the nanoparticles were confirmed by SEM and Zetasizer instruments. Next, in vitro transfection of the nanoparticles was assessed by flow cytometry and western blotting. Finally, humoral and cellular immune responses were evaluated using different modalities in mice. Our data showed that HR9 and Cady-2 could form stable nanoparticles with DNA and proteins, respectively and enhance their delivery into HEK-293 T cells in a non-covalent approach. Furthermore, the heterologous Hsp27-NS3 DNA + HR9 prime/rHsp27-NS3+Cady-2 protein boost elicited a higher Th1 cellular immune response with a predominant IgG2a, IgG2b, IFN-γ profile and strong Granzyme B secretion than those induced by other groups. Briefly, the combination of a natural adjuvant (Hsp27) and CPPs (HR9 and Cady-2) could significantly stimulate effective immune responses as a promising approach for development of HCV therapeutic vaccines.

Recent Advances in Cell Penetrating Peptide-Based Anticancer Therapies

Cell-penetrating-peptides (CPPs) are small amino-acid sequences characterized by their ability to cross cellular membranes. They can transport various bioactive cargos inside cells including nucleic acids, large proteins, and other chemical compounds. Since 1988, natural and synthetic CPPs have been developed for applications ranging from fundamental to applied biology (cell imaging, gene editing, therapeutics delivery). In recent years, a great number of studies reported the potential of CPPs as carriers for the treatment of various diseases. Apart from a good efficacy due to a rapid and potent delivery, a crucial advantage of CPP-based therapies is the peptides low toxicity compared to most drug carriers. On the other hand, they are quite unstable and lack specificity. Higher specificity can be obtained using a cell-specific CPP to transport the therapeutic agent or using a non-specific CPP to transport a cargo with a targeted activity. CPP-cargo complexes can also be conjugated to another moiety that brings cell- or tissue-specificity. Studies based on all these approaches are showing promising results. Here, we focus on recent advances in the potential usage of CPPs in the context of cancer therapy, with a particular interest in CPP-mediated delivery of anti-tumoral proteins.

Expression, purification, and cytotoxic evaluation of IL24-BR2 fusion protein

Interleukin (IL) 24 is a pro-inflammatory and tumor suppressor cytokine capable of inducing selective apoptosis in various cancer cells. BR2, on the other hand, is an anti-microbial peptide with selective penetrability to the cancer cells. In this study, we aimed to produce and purify a fusion protein containing IL24 as the toxic moiety fused to BR2, as targeting moiety, and then to evaluate its cytotoxic activities. For this purpose, the coding sequence of IL24-BR2 fusion protein and IL24 were cloned into the pET28a vector and used to transform E. coli BL21 (DE3) cells. Following induction of expression, protein purification performed using Ni-NTA chromatography. SDS-PAGE and western blotting were performed to confirm the expression and purification. Finally, cytotoxic effects of the purified proteins were evaluated on MCF-7 and HUVEC cell lines. Analysis of crude lysate of induced recombinant E. coli BL21 (DE3) bacteria and also purified proteins showed a band of approximately 22 and 18 KDa on SDS-PAGE and western blotting for IL24-BR2 and IL24, respectively. Finally, statistical analysis showed significant cytotoxic effects of IL24-BR2 on MCF-7 cells at 10, 20, and 40 µg/mL concentrations compared to IL24 alone, which showed no significant cytotoxic effects on cancer cells except in the highest concentration. In conclusion, production and purification of IL24-BR2 fusion protein with potential specific toxicity toward cancer cells was successfully achieved. However, further investigation of the cytotoxic effects of this fusion protein on other cell lines and in vivo cancer models must be performed.

Mesoporous silica nanoparticles for therapeutic/diagnostic applications

Based on unique intrinsic properties of mesoporous silica nanoparticles (MSNs) such as high surface area, large pore size, good biocompatibility and biodegradability, stable aqueous dispersion, they have received much attention in the recent decades for their applications as a promising platform in the biomedicine field. These porous structures possess a pore size ranging from 2 to 50 nm which make them excellent candidates for various biomedical applications. Herein, at first we described the common approaches of cargo loading and release processes from MSNs. Then, the intracellular uptake, safety and cytotoxicity aspects of MSNs are discussed as well. This review also highlights the most recent advances in the biomedical applications of MSNs, including 1) MSNs-based carriers, 2) MSNs as bioimaging agents, 3) MSNs-based biosensors, 4) MSNs as therapeutic agents (photodynamic therapy), 5) MSN based quantum dots, 6) MSNs as platforms for upconverting nanoparticles, and 6) MSNs in tissue engineering.

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.

Gene delivery by peptide-assisted transport

The diverse amino acid chemistries and secondary structures in peptides provide 'minimalist' mimics of motifs in proteins and offer many ideal properties for targeted delivery approaches. Several non-viral vectors (polymers and lipids) have been studied for their potential applications in gene delivery. However, non-specific uptake, lack of targeting, inability to escape endosomes, and inefficient nuclear delivery limit their application. Peptide-assisted trafficking of non-viral vectors can potentially overcome these biological barriers to improve gene delivery through targeted uptake using key cell-surface receptors (e.g., integrins, growth factor receptors, and G-protein coupled receptors); membrane disruption for endosomal escape; and nuclear importation. Furthermore, the capacity of peptides to regulate spatio-temporal control over gene delivery opens multi-faceted avenues for effective gene delivery in a variety of complex applications. Rigorous on-going in vitro and in vivo studies utilizing peptides for targeted and microenvironment-sensitive gene delivery could promote their widespread clinical usage.

Improved Antibacterial Activity of the Marine Peptide N6 against Intracellular Salmonella Typhimurium by Conjugating with the Cell-Penetrating Peptide Tat11 via a Cleavable Linker

The poor penetration ability of antimicrobial agents limits their use in the treatment of intracellular bacteria. In this study, the conjugate CNC (6) was generated by connecting the cell-penetrating peptide Tat₁₁ (1) and marine peptide N6 (2) via a cathepsin-cleavable linker, and the C-terminal aminated N6 (7) and CNC (8) were first designed and synthesized to eliminate intracellular Salmonellae Typhimurium. The cellular uptake of 6 and stability of 7 were higher than those of 2, and conjugates 6, 8, and 7 had almost no hemolysis and cytotoxicity. The antibacterial activities of 6, 8, and 7 against S. Typhimurium in RAW264.7 cells were increased by 67.2-76.2%, 98.6-98.9%, and 96.3-97.6%, respectively. After treatment with 1-2 μmol/kg of 6, 8, or 7, the survival of the S. Typhimurium-infected mice was 66.7-100%, higher than that of 2 (33.4-66.7%). This result suggested that 6, 8, and 7 may be excellent candidates for novel antimicrobial agents to treat intracellular pathogens.

pH-sensitive folic acid and dNP2 peptide dual-modified liposome for enhanced targeted chemotherapy of glioma

Effective chemotherapy for clinical glioma treatment is still lacking due to the poor penetration of blood-brain barrier (BBB) and the poor internalization into tumor cells. To facilitate the transmigration across the BBB as well as the glioma targeting of chemotherapeutics, we constructed cell penetrating peptide dNP2 and tumor microenvironment-cleavable folic acid (FA) dual modified, paclitaxel (PTX) loaded liposome for the targeted delivery of glioma. The modification of dNP2 significantly enhanced the transmigration across the BBB in an in vitro BBB model. The acid-cleavable cFd-Lip/PTX exhibited sensitive cleavage of FA at pH 6.8, which led to enhanced cellular uptake mediated by both cell penetrating peptide dNP2 and the interaction between FA and folate receptor (FR) on the glioma cells. After intravenous injection, compared with non-cleavable Fd-Lip and single modified liposomes, cFd-Lip enhanced the accumulation in orthotropic glioma and improved the anti-tumor effect of glioma-bearing mice. The dual modified liposomes also facilitated deep penetration into tumor cells and consequently enhanced the cytotoxicity of PTX-loaded liposomes. The acid-cleavable dual modified strategy retained the BBB penetrating and tumor targeting ability, meanwhile, the cleavage of FA further maximized the cell permeability of dNP2, exhibiting enhanced tumor targeting effect. The multi-targeting strategy provides a promising approach towards targeted chemotherapy for glioma.

Characterization of low-abundance species in the active pharmaceutical ingredient of CIGB-300: A clinical-grade anticancer synthetic peptide

CIGB-300 is a first-in-class synthetic peptide-based drug of 25 amino acids currently undergoing clinical trials in cancer patients. It contains an amidated disulfide cyclic undecapeptide fused to the TAT cell-penetrating peptide through a beta-alanine spacer. CIGB-300 inhibits the CK2-mediated phosphorylation leading to apoptosis of tumor cells in vitro, and in vivo in cancer patients. Despite the clinical development of CIGB-300, the characterization of peptide-related impurities present in the active pharmaceutical ingredient has not been reported earlier. In the decision tree of ICHQ3A(R2) guidelines, the daily doses intake, the abundance, and the identity of the peptide-related species are pivotal nodes that define actions to be taken (reporting, identification, and qualification). For this, purity was first assessed by reverse-phase chromatography (>97%) and low-abundance impurities (≤0.27%) were collected and identified by mass spectrometry. Most of the impurities were generated during peptide synthesis, the spontaneous air oxidation of the reduced peptide, and the lyophilization step. The most abundant impurity, with no biological activity, was the full-length peptide containing Met¹⁷ transformed into a sulfoxide residue. Interestingly, parallel and antiparallel dimers of CIGB-300 linked by 2 intermolecular disulfide bonds exhibited a higher antiproliferative activity than the CIGB-300 monomer. Likewise, very low abundance trimers and tetramers of CIGB-300 linked by disulfide bonds (≤0.01%) were also detected. Here we describe for the first time the presence of active dimeric species whose feasibility as novel CIGB-300 derived entities merits further investigation.

Tumor-targeting delivery of herb-based drugs with cell-penetrating/tumor-targeting peptide-modified nanocarriers

Cancer has become one of the leading causes of mortality globally. The major challenges of conventional cancer therapy are the failure of most chemotherapeutic agents to accumulate selectively in tumor cells and their severe systemic side effects. In the past three decades, a number of drug delivery approaches have been discovered to overwhelm the obstacles. Among these, nanocarriers have gained much attention for their excellent and efficient drug delivery systems to improve specific tissue/organ/cell targeting. In order to enhance targeting efficiency further and reduce limitations of nanocarriers, nanoparticle surfaces are functionalized with different ligands. Several kinds of ligand-modified nanomedicines have been reported. Cell-penetrating peptides (CPPs) are promising ligands, attracting the attention of researchers due to their efficiency to transport bioactive molecules intracellularly. However, their lack of specificity and in vivo degradation led to the development of newer types of CPP. Currently, activable CPP and tumor-targeting peptide (TTP)-modified nanocarriers have shown dramatically superior cellular specific uptake, cytotoxicity, and tumor growth inhibition. In this review, we discuss recent advances in tumor-targeting strategies using CPPs and their limitations in tumor delivery systems. Special emphasis is given to activable CPPs and TTPs. Finally, we address the application of CPPs and/or TTPs in the delivery of plant-derived chemotherapeutic agents.

Design and Application of a DNA-Encoded Macrocyclic Peptide Library

A DNA-encoded macrocyclic peptide library was designed and synthesized with 2.4 × 10¹² members composed of 4-20 natural and non-natural amino acids. Affinity-based selection was performed against two therapeutic targets, VHL and RSV N protein. On the basis of selection data, some peptides were selected for resynthesis without a DNA tag, and their activity was confirmed.

Improved antibacterial activity of a marine peptide-N2 against intracellular Salmonella typhimurium by conjugating with cell-penetrating peptides-bLFcin6/Tat11

Salmonellae, gram-negative bacteria, are facultative intracellular pathogens that cause a number of diseases in animals and humans. The poor penetration ability of antimicrobial agents limits their use in the treatment of intracellular bacterial infections. In this study, the cell-penetrating peptides (CPPs) bLFcin₆ and Tat₁₁ were separately conjugated to the antimicrobial peptide N2, and the antibacterial activity and pharmacodynamics of the CPPs-N2 conjugates were first evaluated against Salmonellae typhimurium in vitro and in macrophage cells. The cytotoxicity, cellular uptake and mechanism of cellular internalization of the CPPs-N2 conjugates were also examined in RAW264.7 cells. Similar to N2, CPPs-N2 have two reverse β-sheets and three loops. The minimal inhibitory concentration (MIC) of CPPs-N2 was approximately 2 μM, which was higher than that of N2 (0.8 μM). The dose-time curves and cytotoxicity assay showed that both peptide conjugates were more effective than N2 alone at concentrations ranging from 0.25 to 1 × MIC, and they exhibited low cytotoxicity (9.78%-13.54%) at 100 μM. After 0.5 h incubation, the cell internalization ratio of B6N2 and T11N2 exceeded 28.3% and 93.5%, respectively, which was higher than that of N2. The uptake of B6N2 and T11N2 was reduced by low temperature (82.1%-91.7%), chlorpromazine (35.7%-75.1%), and amiloride (26.0%-52.1%), indicating that macropinocytosis and clathrin-mediated endocytosis may be involved. Approximately 98.85% and 91.35% of bacteria were killed within 3 h by T11N2 and B6N2, respectively, which was higher than the percentage killed by N2 (69.74%). Compared with the bactericidal activity of N2 alone, the bactericidal activity of T11N2 and B6N2 was increased by 53.7%-99.6% and 85.3-85.8%, respectively. Both CPPs-N2 conjugates may be excellent candidates for novel antimicrobial agents to treat infectious diseases caused by intracellular pathogens.

Advance of DNA and CCPs-based nanocarriers in drug delivery systems

Development of smart and functional polymeric carriers, which enable controlled or timed release of a bioactive material, thereby providing a better dosing pattern and minimizing side effects, becomes a new requirement in the field of drug delivery. In the recent few decades, a great many advancements of polymer synthetic methods have led to a new generation of bioactive polymers' applications as drug controlled release carriers. In this review, we focus on the use of bioactive polymers for drug delivery system, with a particular in the utility of DNA-based nanocarriers and cell-penetrating peptides (CCPs)-based nanocarriers to provide precision control for drug targeting or stimuli responsive systems.

Synthesis and in vitro evaluation of fluorescent and magnetic nanoparticles functionalized with a cell penetrating peptide for cancer theranosis

We synthesized rationally designed multifunctional nanoparticles (NPs) composed of a superparamagnetic iron oxide nanoparticle (SPION) core, cyanine fluorescent dye emitting in far red, polyethylene glycol (PEG₅₀₀₀) coating, and the membranotropic peptide gH625, from the cell-penetrating peptides (CPP) family. The peptide sequence was enriched with an additional cysteine so it can be involved as a reactive moiety in a certain orientation- and sequence-specific coupling of the CPP to the PEG shell of the NPs. Our data indicate that the presence of approximately 23 peptide molecules per SPION coated with approximately 137 PEG chains minimally changes the overall NP characteristics. The final CPP-capped NP hydrodynamic diameter was 98nm, the polydispersity index was 0.192, and the zeta potential was 4.08mV. The in vitro evaluation, performed using an original technique fluorescence confocal spectral imaging, showed that after a short incubation duration (maximum 30min), SPIONs-PEG-CPP uptake was 3-fold higher than that for SPIONs-PEG. The CPP also drives the subcellular distribution of a higher NP fraction towards low polarity cytosolic locations. Therefore, the major cellular uptake mechanism for the peptide-conjugated NPs should be endocytosis. Enhancement/acceleration of this mechanism by gH625 appears promising because of potential applications of SPIONs-PEG-gH625 as a multifunctional nanoplatform for cancer theranosis involving magnetic resonance imaging, optical imaging in far red, drug delivery, and hyperthermia.

Efficient internalization of TAT peptide in zwitterionic DOPC phospholipid membrane revealed by neutron diffraction

The aim of this study is to investigate the interactions between TAT peptides and a neutral DOPC bilayer by using neutron lamellar diffraction. The distribution of TAT peptides and the perturbation of water distribution across the DOPC bilayer were revealed. When compared to our previous study on an anionic DOPC/DOPS bilayer (X. Chen et al., Biochim Biophys Acta. 2013. 1828 (8), 1982-1988), a much deeper insertion of TAT peptides was found in the hydrophobic core of DOPC bilayer at a depth of 6.0Å from the center of the bilayer, a position close to the double bond of fatty acyl chain. We conclude that the electrostatic attractions between the positively charged TAT peptides and the negatively charged headgroups of phospholipid are not essential for the direct translocation. Furthermore, the interactions of TAT peptides with the DOPC bilayer were found to vary in a concentration-dependent manner. A limited number of peptides first associate with the phosphate moieties on the lipid headgroups by using the guanidinium ions pairing. Then the energetically favorable water defect structures are adopted to maintain the arginine residues hydrated by drawing water molecules and lipid headgroups into the bilayer core. Such bilayer deformations consequently lead to the deep intercalation of TAT peptides into the bilayer core. Once a threshold concentration of TAT peptide in the bilayer is reached, a significant rearrangement of bilayer will happen and steady-state water pores will form.

In vitro and in vivo delivery of therapeutic proteins using cell penetrating peptides

The failure of proteins to penetrate mammalian cells or target tumor cells restricts their value as therapeutic tools in a variety of diseases such as cancers. Recently, protein transduction domains (PTDs) or cell penetrating peptides (CPPs) have been shown to promote the delivery of therapeutic proteins or peptides into live cells. The successful delivery of proteins mainly depends on their physicochemical properties. Although, linear cell penetrating peptides are one of the most effective delivery vehicles; but currently, cyclic CPPs has been developed to potently transport bioactive full-length proteins into cells. Up to now, several small protein transduction domains from viral proteins including Tat or VP22 could be fused to other peptides or proteins to entry them in various cell types at a dose-dependent approach. A major disadvantage of PTD-fusion proteins is primary uptake into endosomal vesicles leading to inefficient release of the fusion proteins into the cytosol. Recently, non-covalent complex formation (Chariot) between proteins and CPPs has attracted a special interest to overcome some delivery limitations (e.g., toxicity). Many preclinical and clinical trials of CPP-based delivery are currently under evaluation. Generally, development of more efficient protein transduction domains would significantly increase the potency of protein therapeutics. Moreover, the synergistic or combined effects of CPPs with other delivery systems for protein/peptide drug delivery would promote their therapeutic effects in cancer and other diseases. In this review, we will describe the functions and implications of CPPs for delivering the therapeutic proteins or peptides in preclinical and clinical studies.

The function of activatable cell-penetrating peptides in human intrahepatic bile duct epithelial cells

This study aimed to investigate the function of Activatable Cell-Penetrating Peptides (ACPP) in detecting the changes of human intrahepatic bile duct epithelial cell(hIBDEC). ACPP, which target matrix metalloproteinases, were constructed. All were labeled with FITC and Gd-DTPA at the N-terminal. Fluorescence microscopy was used to observe the fluorescence intensity inside hIBDEC after stimulating with different concentrations of LPS and incubating with different concentrations of ACPP to determine the optimal concentration range for LPS stimulation and the optimal concentration for FITC-ACPP effect. Flow cytometry and magnetic resonance imaging were used to detect fluorescence signal intensity and nuclear magnetic resonance signal intensity, respectively, after stimulating with different concentrations of LPS. LPS stimulation time and ACPP incubation time were also evaluated, and variance analysis was conducted to analyze intracellular signal change characteristics for every group. Activatable Cell-Penetrating Peptides (ACPP), which were marked with FITC and Gd-DTPA had target-penetrating activity. The intracellular signal intensity gradually increased with the increase in LPS stimulation time and ACPP incubation time within a certain range; however, it did not increase with the increase of LPS concentration. ACPP can be used for imaging hIBDEC with epithelial-mesenchymal transition.

Nanoformulated Antiretrovirals for Penetration of the Central Nervous System: State of the Art

The central nervous system is a very challenging HIV-1 sanctuary. But, despite complete suppression of plasmatic viral replication with current antiretroviral therapy, signs of HIV-1 replication can still be found in the cerebrospinal fluid in some patients. The main limitation to achieving HIV-1 eradication from the brain is related to the suboptimal concentrations of antiretrovirals within this site, due to their low permeation across the blood-brain barrier. In recent years, a number of reliable nanotechnological strategies have been developed with the aim of enhancing antiretroviral drug penetration across the blood-brain barrier. The aim of this review is to provide an overview of the different nanoformulated antiretrovirals, used in both clinical and preclinical studies, that are designed to improve their delivery into the brain by active or passive permeation mechanisms through the barrier. Different nanotechnological approaches have proven successful for optimizing antiretrovirals delivery to the central nervous system, with a likely benefit for HIV-associated neurocognitive disorders and a more debated contribution to the complete eradication of the HIV-1 infection.

Cell-penetrating peptides: Possible transduction mechanisms and therapeutic applications

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse peptides with 5-30 amino acids. CPPs are divided into cationic, amphipathic and hydrophobic CPPs. They are able to carry small molecules, plasmid DNA, small interfering RNA, proteins, viruses, imaging agents and other various nanoparticles across the cellular membrane, resulting in internalization of the intact cargos. However, the mechanisms of CPP internalization remain to be elucidated. Recently, CPPs have received considerable attention due to their high transduction efficiency and low cytotoxicity. These peptides have a significant potential for diagnostic and therapeutic applications, such as delivery of fluorescent or radioactive compounds for imaging, delivery of peptides and proteins for therapeutic application, and delivery of molecules into induced pluripotent stem cells for directing differentiation. The present study reviews the classifications and transduction mechanisms of CPPs, as well as their potential applications.