In vivo proof of concept of oral insulin delivery based on a co-administration strategy with the cell-penetrating peptide penetratin

Coating of PLA-nanoparticles with cyclic, arginine-rich cell penetrating peptides enables oral delivery of liraglutide

Until today, the oral delivery of peptide drugs is hampered due to their instability in the gastrointestinal tract and low mucosal penetration. To overcome these hurdles, PLA (polylactide acid)-nanoparticles were coated with a cyclic, polyarginine-rich, cell penetrating peptide (cyclic R9-CPP). These surface-modified nanoparticles showed a size and polydispersity index comparable to standard PLA-nanoparticles. The zeta potential showed a significant increase indicating successful CPP-coupling to the surface of the nanoparticles. Cryo-EM micrographs confirmed the appropriate size and morphology of the modified nanoparticles. A high encapsulation efficiency of liraglutide could be achieved. In vitro tests using Caco-2 cells showed high viability indicating the tolerability of this novel formulation. A strongly enhanced mucosal binding and penetration was demonstrated by a Caco-2 binding and uptake assay. In Wistar rats, the novel nanoparticles showed a substantial, 4.5-fold increase in the oral bioavailability of liraglutide revealing great potential for the oral delivery of peptide drugs.

Impact Of Penetratin Stereochemistry On The Oral Bioavailability Of Insulin-Loaded Solid Lipid Nanoparticles

Purpose

This study evaluated the stereoisomeric effect of L- and D-penetratin—cell-penetrating peptides (CPPs)—incorporated insulin-loaded solid lipid nanoparticles (INS-SLNs) on the bioavailability (BA) of oral insulin (INS).

Methods

Insulin-loaded solid nanoparticles, L-penetratin-INS-SLNs (LP-INS-SLNs), and D-penetratin-INS-SLNs (DP-INS-SLNs) were formulated by double emulsification. The developed SLNs were evaluated for particle size, zeta potential (ZP), and drug encapsulation and subjected to differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and evaluated for stability against enzymatic degradation in rat intestinal fluid. Finally, the SLNs were administered to rats to evaluate the BA of INS-SLNs that contained L- and D-penetratin.

Results

The mean particle size, PDI, and ZP values of INS-SLNs, LP-INS-SLNs, and DP-INS-SLNs ranged from 618.5 to 973.0 nm, 0.227 to 0.734, and −17.0 to −23.7 mV, respectively. The encapsulation efficiency (%EE) and drug loading (%DL) of INS-SLNs, LP-INS-SLNs, and DP-INS-SLNs ranged from 59.03% to 67.42% and from 1.62% to 1.82%, respectively. Differential scanning calorimetry and FTIR analyses indicated that INS was successfully encapsulated in SLNs. Enzymatic degradation of DP-INS-SLNs was slower in intestinal fluid, and the half-life (t_1/2) was significantly prolonged, compared to all other SLNs. The pharmacological availability (PA) and BA of orally administered LP-INS-SLNs, which were the most effective SLNs, were 13.1% and 15.7% relative to s.c. administration, respectively.

Conclusion

Penetratin stereochemistry significantly impacted oral BA of INS-SLNs, which are promising carriers for oral INS administration.

Encapsulation in Polymeric Nanoparticles Enhances the Enzymatic Stability and the Permeability of the GLP-1 Analog, Liraglutide, Across a Culture Model of Intestinal Permeability

The potential of poly (lactic-co-glycolic acid) nanoparticles (PLGA NPs) to overcome the intestinal barrier that limits oral liraglutide delivery was evaluated. Liraglutide-loaded PLGA NPs were prepared by the double emulsion solvent evaporation method. In vitro release kinetics and enzymatic degradation studies were conducted, mimicking the gastrointestinal environment. The permeability of liraglutide solution, liraglutide-loaded PLGA NPs, and liraglutide in the presence of the absorption enhancer PN159 peptide was tested on the Caco-2 cell model. Liraglutide release from PLGA NPs showed a biphasic release pattern with a burst effect of less than 15%. The PLGA nanosystem protected the encapsulated liraglutide from the conditions simulating the gastric environment. The permeability of liraglutide encapsulated in PLGA NPs was 1.5-fold higher (24 × 10⁻⁶ cm/s) across Caco-2 cells as compared to liraglutide solution. PLGA NPs were as effective at elevating liraglutide penetration as the tight junction-opening PN159 peptide. No morphological changes were seen in the intercellular junctions of Caco-2 cells after treatment with liraglutide-PLGA NPs, confirming the lack of a paracellular component in the transport mechanism. PLGA NPs, by protecting liraglutide from enzyme degradation and enhancing its permeability across intestinal epithelium, hold great potential as carriers for oral GLP-1 analog delivery.

CPProtectides: Rapid uptake of well-folded β-hairpin peptides with enhanced resistance to intracellular degradation

Cell penetrating peptides (CPPs) have emerged as powerful tools for delivering bioactive cargoes, such as biosensors or drugs to intact cells. One limitation of CPPs is their rapid degradation by intracellular proteases. β-hairpin "protectides" have previously been demonstrated to be long-lived under cytosolic conditions due to their secondary structure. The goal of this work was to demonstrate that arginine-rich β-hairpin peptides function as both protectides and as CPPs. Peptides exhibiting a β-hairpin motif were found to be rapidly internalized into cells with their uptake efficiency dependent on the number of arginine residues in the sequence. Cellular internalization of the β-hairpin peptides was compared to unstructured, scrambled sequences and to commercially available, arginine-rich CPPs. The unstructured peptides displayed greater uptake kinetics compared to the structured β-hairpin sequences; however, intracellular stability studies revealed that the β-hairpin peptides exhibited superior stability under cytosolic conditions with a 16-fold increase in peptide half-life. This study identifies a new class of long-lived CPPs that can overcome the stability limitations of peptide-based reporters or bioactive delivery mechanisms in intact cells.

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.

HAI Peptide and Backbone Analogs-Validation and Enhancement of Biostability and Bioactivity of BBB Shuttles

Low effectiveness and resistance to treatments are commonplace in disorders of the central nervous system (CNS). These issues concern mainly the blood-brain barrier (BBB), which preserves homeostasis in the brain and protects this organ from toxic molecules and biohazards by regulating transport through it. BBB shuttles—short peptides able to cross the BBB—are being developed to help therapeutics to cross this barrier. BBB shuttles can be discovered by massive exploration of chemical diversity (e.g. computational means, phage display) or rational design (e.g. derivatives from a known peptide/protein able to cross). Here we present the selection of a peptide shuttle (HAI) from several candidates and the subsequent in-depth in vitro and in vivo study of this molecule. In order to explore the chemical diversity of HAI and enhance its biostability, and thereby its bioactivity, we explored two new protease-resistant versions of HAI (i.e. the retro-D-version, and a version that was N-methylated at the most sensitive sites to enzymatic cleavage). Our results show that, while both versions of HAI are resistant to proteases, the retro-D-approach preserved better transport properties.

In vivo hepatocellular expression of interleukin-22 using penetratin-based hybrid nanoparticles as potential anti-hepatitis therapeutics

Hepatocellular injury is the pathological hallmark of hepatitis and a crucial driver for the progression of liver diseases, while the treatment options are commonly restricted. Interleukin-22 (IL-22) has attracted special attention as a potent survival factor for hepatocytes that both prevents and repairs the injury of hepatocytes through activation of STAT3 signaling pathway. We hypothesized that the ability to generate potent expression of IL-22 locally for the treatment of severe hepatocellular injury in hepatitis was a promising strategy to enhance efficacy and overcome off-target effects. Accordingly, we developed a polypeptide penetratin-based hybrid nanoparticle system (PDPIA) carrying IL-22 gene by a self-assembly process. This nanocomplex modified with penetratin featured direct translocation across the cellular or endosomal membrane but mild zeta-potential to facilitate the high cellular internalization and endosomal escape of the gene cargos as well as scarcely Kupffer cells uptake. More importantly, PDPIA afforded preferential liver accumulation and predominant hepatocytes internalization following systemic administration, which showed pharmacologically suitable organ and sub-organ-selective properties. Subsequent studies confirmed a considerable protective role of PDPIA in a model of severe hepatitis induced by concanavalin A, evidenced by reduced hepatocellular injury and evaded immune response. The locally expressed IL-22 by PDPIA activated STAT3/Erk signal transduction, and thus promoted hepatocyte regeneration, inhibited reactive oxygen species (ROS) accumulation as well as prevented the dysfunction of mitochondrial. In addition, this system did not manifest side effects or systemic toxicity in mice. Collectively, the high versatility of PDPIA rendered its promising applications might be an effective agent to treat various hepatic disorders.

Targeted folate-conjugated pluronic P85/poly(lactide-co-glycolide) polymersome for the oral delivery of insulin

Aim: To explore the better efficacy of targeted folic acid (FA)-Pluronic 85-poly(lactide-co-glycolide) (FA–P85–PLGA) polymersome in oral insulin delivery. Materials & methods: The cytotoxicity of the polymers, in vitro qualitative and quantitative cellular uptake and the internalization mechanism of insulin-loaded FA–P85–PLGA and PLGA–P85–PLGA polymersomes were studied with the human colon adenocarcinoma cells (Caco-2 cells). Their pharmacodynamics and pharmacokinetics properties were also studied with diabetic rats. Results & conclusion: Polymersomes have shown good biocompatibility. Polymersomes are mainly localized within the cytoplasm of Caco-2 cells from fluorescence microscopy images. FA–P85–PLGA exhibited higher cellular uptake than PLGA–P85–PLGA polymersomes and free fluorescein isothiocyanate-labeled insulin (FITC–insulin) did. The uptake process of targeted polymersomes included clathrin- and caveolae-mediated endocytosis, macropinocytosis and the folate receptor-mediated endocytosis. Insulin-loaded FA–P85–PLGA showed better hypoglycemic effects than insulin-loaded PLGA–P85–PLGA.

Closing the Loop: Constraining TAT Peptide by γPNA Hairpin for Enhanced Cellular Delivery of Biomolecules

Based on the exceptionally high stability of γPNA duplexes, we designed a peptide/γPNA chimera in which a cell-penetrating TAT peptide is flanked by two short complementary γPNA segments. Intramolecular hybridization of the γPNA segments results in a stable hairpin conformation in which the TAT peptide is constrained to form the loop. The TAT/γPNA hairpin (self-cyclized TAT peptide) enters cells at least 10-fold more efficiently than its nonhairpin analog in which the two γPNA segments are noncomplementary. Extending one of the γPNA segments in the hairpin results in an overhang that can be used for binding and delivering a variety of nucleic acid-conjugated molecules into cells via hybridization to the overhang. We demonstrated efficient cellular delivery of a protein (as low as 10 nM) and a DNA tetrahedron by a TAT/γPNA hairpin.

Interaction between porous silica gel microcarriers and peptides for oral administration of functional peptides

Functional peptides, peptides that have biological activities, have attracted attention as active ingredients of functional foods and health foods. In particular, for food applications, because orally ingested peptides are degraded by digestive enzymes in the stomach, novel oral administration methods that can prevent peptide degradation and successfully deliver them intestinally are desired. In the present study, we focused on porous silica gel, which has many useful characteristics, such as large surface area, pH responsive functional groups, size controllable pores, and approval as food additives. We investigated the possibility of using porous silica gel as a peptide degradation protective microcarrier. As a result, we found that heat treatment of the silica gel at 600 °C for 2 h remarkably enhanced the adsorbed amount of many peptides under acidic conditions, and negatively charged and highly hydrophobic peptides had suitable characteristics for oral intestinal delivery with silica gel. Finally, we demonstrated the degree of protection from pepsin degradation and found that the protection of DFELEDD peptide was 57.1 ± 3.9% when DFELEDD was mixed with the heat-treated silica gel. These results indicated that the heat-treated silica gel is promising for efficient oral intestinal delivery of hydrophobic negatively charged peptides.

Advance in oral delivery systems for therapeutic protein

Oral delivery is the most common method of drug administration with high safety and good compliance for patients. However, delivering therapeutic proteins to the target site via oral route involves tremendous challenge due to unfavourable conditions like biochemical barrier, mucus barrier and epithelial barriers. According to the functional differences of various protein drug delivery systems, the recent advances in pH responsive polymer-based drug delivery system, mucoadhesive polymer-based drug delivery system, absorption enhancers-based drug delivery system and composite polymer-based delivery system all were briefly summarised in this review, which not only clarified the clinic potential of these novel drug delivery systems, but also described the way for increasing oral bioavailability of therapeutic protein.

Stereochemistry as a determining factor for the effect of a cell-penetrating peptide on cellular viability and epithelial integrity

Cell-penetrating peptides (CPPs) comprise efficient peptide-based delivery vectors. Owing to the inherent poor enzymatic stability of peptides, CPPs displaying partial or full replacement of l-amino acids with the corresponding d-amino acids might possess advantages as delivery vectors. Thus, the present study aims to elucidate the membrane- and metabolism-associated effects of l-Penetratin (l-PEN) and its corresponding all-d analog (d-PEN). These effects were investigated when exerted on hepatocellular (HepG2) or intestinal (Caco-2 and IEC-6) cell culture models. The head-to-head comparison of these enantiomeric CPPs included evaluation of their effects on cell viability and morphology, epithelial membrane integrity, and cellular ultrastructure. In all investigated cell models, a rapid decrease in cell viability, pronounced membrane perturbation and an altered ultrastructure were detected upon exposure to d-PEN. At equimolar concentrations, these observations were less pronounced or even absent for cells exposed to l-PEN. Both CPPs remained stable for at least 2 h during exposure to proliferating cells (cultured for 24 h), although d-PEN exhibited a longer half-life when compared with that of l-PEN when exposed to well-differentiated cell monolayers (cultured for 18-20 days). Thus, the stereochemistry of the CPP penetratin significantly influences its effects on cell viability and epithelial integrity when profiled against a panel of mammalian cells.

Scalable production of core-shell nanoparticles by flash nanocomplexation to enhance mucosal transport for oral delivery of insulin

Scalable manufacturing continues to present a major barrier for clinical translation of nanotherapeutics. Methods available for fabricating protein-encapsulating nanoparticles in a scalable fashion are scarce. Protein delivery often requires multiple functionalities to be incorporated into the same vehicle. Specifically for nanoparticle-mediated oral delivery of protein therapeutics, protection in GI tract, site-specific release, facilitating transmucosal permeation, and enhancing epithelial transport are a few desirable features to be engineered into a nanoparticle system. Here we devised a sequential flash nanocomplexation (FNC) technique for the scalable production of a core-shell structured nanoparticle system by combining materials choice and particle size and structure to fulfill these functions, therefore enhancing the delivery efficiency of insulin. This method is highly effective in controlling the size, generating core-shell structure with high encapsulation efficiency (97%) and payload capacity (67%) using insulin/l-penetratin complex nanoparticles as a core coated with hyaluronic acid (HA). Both the in vitro and in vivo models confirmed that the HA coating on these core-shell nanoparticles enhanced the permeation of nanoparticles through the intestinal mucus layer and improved trans-epithelial absorption of insulin nanoparticles; and the enhancement effect was most prominent using HA with the highest average molecular weight. The insulin-loaded nanoparticles were then encapsulated into enteric microcapsules (MCs) in an FNC process to provide additional protection against the acidic environment in the stomach while allowing rapid release of insulin nanoparticles when they reach small intestine. The optimized multifunctional MCs delivered an effective glucose reduction in a Type I diabetes rat model following a single oral administration, yielding a relative bioavailability of 11% in comparison with subcutaneous injection of free-form insulin. This FNC technique is highly effective in controlling particle size and structure to improve delivery properties and function. It can be easily extended to oral delivery for other protein therapeutics.

Poly(ester amide)-based hybrid hydrogels for efficient transdermal insulin delivery

Transdermal drug delivery is an attractive, non-invasive treatment.

Peptides as drug delivery vehicles across biological barriers

Peptides are small biological molecules that are attractive in drug delivery and materials engineering for applications including therapeutics, molecular building blocks and cell-targeting ligands. Peptides are small but can possess complexity and functionality as larger proteins. Due to their intrinsic properties, peptides are able to overcome the physiological and transport barriers presented by diseases. In this review, we discuss the progress of identifying and using peptides to shuttle across biological barriers and facilitate transport of drugs and drug delivery systems for improved therapy. Here, the focus of this review is on rationally designed, phage display peptides, and even endogenous peptides as carriers to penetrate biological barriers, specifically the blood-brain barrier(BBB), the gastrointestinal tract (GI), and the solid tumor microenvironment (T). We will discuss recent advances of peptides as drug carriers in these biological environments. From these findings, challenges and potential opportunities to iterate and improve peptide-based approaches will be discussed to translate their promise towards the clinic to deliver drugs for therapeutic efficacy.

Antimicrobial peptides in frog poisons constitute a molecular toxin delivery system against predators

Animals using toxic peptides and proteins for predation or defense typically depend on specialized morphological structures, like fangs, spines, or a stinger, for effective intoxication. Here we show that amphibian poisons instead incorporate their own molecular system for toxin delivery to attacking predators. Skin-secreted peptides, generally considered part of the amphibian immune system, permeabilize oral epithelial tissue and enable fast access of cosecreted toxins to the predator's bloodstream and organs. This absorption-enhancing system exists in at least three distantly related frog lineages and is likely to be a widespread adaptation, determining the outcome of predator-prey encounters in hundreds of species.

Cell-penetrating peptides (CPPs): From delivery of nucleic acids and antigens to transduction of engineered nucleases for application in transgenesis

Cell-penetrating peptides (CPPs) have been studied for their capacity to translocate across the lipid membrane of several cell types. In membrane translocation, these peptides can remarkably transport biologically active hydrophilic molecules, such as pharmaceuticals, nucleic acids (DNA and RNA) and even high-molecular-weight proteins, Fig. 3 into the cell cytoplasm and organelles. The development of CPPs as transduction agents includes the modification of gene and protein expression, the reprogramming and differentiation of induced pluripotent stem cells and the preparation of cellular vaccines. A relatively recent field of CPP application is the transduction of plasmid DNA vectors and CPP-fusion proteins to modify genomes and introduce new traits in cells and organisms. CPP-mediated transduction of components for genome editing is an advantageous alternative to viral DNA vectors. Engineered site-specific nucleases, such as Cre recombinase, ZFN, TALENs and CRISPR associated protein (Cas), have been coupled to CPPs, and the fused proteins have been used to permeate targeted cells and tissues. The functionally active fusion CPP-nucleases subsequently home to the nucleus, incise genomic DNA at specific sites and induce repair and recombination. This review has the objective of discussing CPPs and elucidating the prospective use of CPP-mediated transduction technology, particularly in genome modification and transgenesis.

Potential of single cationic amino acid molecule "Arginine" for stimulating oral absorption of insulin

We have reported that cell-penetrating peptides, such as oligoarginine, act as powerful absorption enhancers for the development of oral insulin delivery systems. However, the minimal essential sequence of oligoarginine that stimulates intestinal insulin absorption remains unclear. Therefore, the present study was conducted to clarify this minimum sequence of oligoarginine and to examine the effect of single cationic amino acid arginine on the intestinal and oral absorption of insulin. The results demonstrated that a remarkable enhancement of intestinal insulin absorption was observed after coadministration of insulin with l-arginine. The efficacy of d-forms of oligoarginine/arginine tended to decrease with a decreasing number of amino acid residues, whereas the effect of l-arginine was the strongest of any of the l-forms of oligoarginine/arginine. Interestingly, the effect of l-arginine was stronger than that of d-arginine at various concentrations, and the effect of other cationic amino acids such as lysine and histidine was relatively lower than that of arginine. In addition, no leakage of lactate dehydrogenase from the intestinal epithelium and no change in the transepithelial electrical resistance of a Caco-2 cell monolayer were detected after administration of l-arginine as the single amino acid, which suggests that there were no undesirable effects of arginine on the integrity of cell membranes and paracellular tight junctions. Oral administration study in mice demonstrated that the stronger hypoglycemic effects were observed after coadministration of insulin with l-arginine. In this study, we found that arginine is a key cationic amino acid for delivering insulin across intestinal epithelial barriers and hopefully accelerating the clinical development of oral insulin delivery systems.

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.

Intelligent substance delivery into cells using cell-penetrating peptides

Cell-penetrating peptides (CPPs) are oligopeptides that can permeate the cell membrane. The use of a CPP-mediated transport system could be an excellent method for delivering cell-impermeable substances such as proteins, antibodies, antisense oligonucleotides, siRNAs, plasmids, drugs, fluorescent compounds, and nanoparticles as covalently or noncovalently conjugated cargo into cells. Nonetheless, the mechanisms through which CPPs are internalized remain unclear. Endocytosis and direct translocation through the membrane are the generally accepted routes. Internalization via both pathways can occur simultaneously, depending on cellular conditions. However, the peculiar property of CPPs has attracted many researchers, especially in drug discovery or development, who intend to deliver impermeable substances into cells through the cell membrane. The delivery of drugs using CPPs may non-invasively solve the problem of drug penetration into cells with the added benefit of low cytotoxicity. Moreover, macromolecules can also be delivered by this transport system. In this review, I discuss the possibilities and advantages of substance delivery into cells using CPPs.

How to design the surface of peptide-loaded nanoparticles for efficient oral bioavailability?

The oral administration of proteins is a current challenge to be faced in the field of therapeutics. There is currently much interest in nanocarriers since they can enhance oral bioavailability. For lack of a clear definition, the key characteristics of nanoparticles have been highlighted. Specific surface area is one of these characteristics and represents a huge source of energy that can be used to control the biological fate of the carrier. The review discusses nanocarrier stability, mucus interaction and absorption through the intestinal epithelium. The protein corona, which has raised interest over the last decade, is also discussed. The universal ideal surface is a myth and over-coated carriers are not a solution either. Besides, common excipients can be useful on several targets. The suitable design should rather take into account the composition, structure and behavior of unmodified nanomaterials.

Intestinal permeation enhancers for oral peptide delivery

Intestinal permeation enhancers (PEs) are one of the most widely tested strategies to improve oral delivery of therapeutic peptides. This article assesses the intestinal permeation enhancement action of over 250 PEs that have been tested in intestinal delivery models. In depth analysis of pre-clinical data is presented for PEs as components of proprietary delivery systems that have progressed to clinical trials. Given the importance of co-presentation of sufficiently high concentrations of PE and peptide at the small intestinal epithelium, there is an emphasis on studies where PEs have been formulated with poorly permeable molecules in solid dosage forms and lipoidal dispersions.

Using peptides to increase transport across the intestinal barrier

The oral route is the preferred for the administration of drugs; however, it has some serious limitations. One of the main disadvantages is poor permeability across the intestinal barrier. Various approaches are currently being adopted to overcome this issue. In this review, we describe the alternatives that use peptides to enhance intestinal absorption. First, we define the various sources of peptide enhancers followed by the analysis of the absorption mechanism used. We then comment on the possible toxic effects derived from their use as permeation enhancers, as well as potential formulation strategies. Finally, the advantages and drawbacks of peptides as intestinal enhancers are examined.

Penetratin Peptide-Functionalized Gold Nanostars: Enhanced BBB Permeability and NIR Photothermal Treatment of Alzheimer's Disease Using Ultralow Irradiance

The structural changes of amyloid-beta (Aβ) from nontoxic monomers into neurotoxic aggregates are implicated with pathogenesis of Alzheimer's disease (AD). Over the past decades, weak disaggregation ability and low permeability to the blood-brain barrier (BBB) may be the main obstacles for major Aβ aggregation blockers. Here, we synthesized penetratin (Pen) peptide loaded poly(ethylene glycol) (PEG)-stabilized gold nanostars (AuNS) modified with ruthenium complex (Ru@Pen@PEG-AuNS), and Ru(II) complex as luminescent probes for tracking drug delivery. We revealed that Ru@Pen@PEG-AuNS could obviously inhibit the formation of Aβ fibrils as well as dissociate preformed fibrous Aβ under the irradiation of near-infrared (NIR) due to the NIR absorption characteristic of AuNS. More importantly, this novel design could be applied in medicine as an appropriate nanovehicle, being highly biocompatible and hemocompatible. In addition, Ru@Pen@PEG-AuNS had excellent neuroprotective effect on the Aβ-induced cellular toxicity by applying NIR irradiation. Meanwhile, Pen peptide could effectively improve the delivery of nanoparticles to the brain in vitro and in vivo, which overcame the major limitation of Aβ aggregation blockers. These consequences illustrated that the enhanced BBB permeability and efficient photothermolysis of Ru@Pen@PEG-AuNS are promising agents in AD therapy.

Cell-Penetrating Peptide Penetratin as a Potential Tool for Developing Effective Nasal Vaccination Systems

Nasal vaccination is considered an attractive strategy to prevent the infection and spread of viruses. However, the vaccine formulations available on the market remain imperfect on account of their limited effectiveness. In the present study, we hypothesized that the nasal coadministration of antigens with cell-penetrating peptides promotes antigen delivery immune response in the nasal mucosa, thereby enhancing the production of mucosal IgA and systemic IgG. The levels of ovalbumin (OVA)-specific IgG and IgA in plasma and nasal perfusate, respectively, increased after 2 or 4 weeks on nasal coadministration of OVA with l- or d-penetratin, suggesting that OVA antigen was effectively delivered by penetratin to the nasal epithelium. An additional study demonstrated that the production of systemic IgG and nasal mucosal IgA against influenza A virus was specifically promoted by nasal coadministration of influenza A virus with d-penetratin. The results of this study suggested that cell-penetrating peptides are a promising tool for the delivery of vaccines to the nasal mucosa and for the subsequent dual stimulation of systemic and mucosal immune responses.

Cell-penetrating peptides as tools to enhance non-injectable delivery of biopharmaceuticals

Non-injectable delivery of peptide and protein drugs is hampered by their labile nature, hydrophilicity, and large molecular size; thus limiting their permeation across mucosae, which represent major biochemical and physical barriers to drugs administered via e.g. the oral, nasal, and pulmonary routes. However, in recent years cell-penetrating peptides (CPP) have emerged as promising tools to enhance mucosal delivery of co-administered or conjugated peptide and protein cargo and more advanced CPP-cargo formulations are emerging. CPPs act as transepithelial delivery vectors, but the mechanism(s) by which CPPs mediate cargo translocation across an epithelium is so far poorly understood; both due to the fact that multiple factors influence the resulting uptake and trafficking mechanisms as well as to the complicated nature of sensitive studies of this. In addition to a proper mechanistic understanding, documentation of CPP-mediated delivery in higher animal species than rodent as well as extensive toxicological studies are necessary for CPP-containing non-injectable DDSs to reach the clinic.

Intracellular Delivery of Proteins with Cell-Penetrating Peptides for Therapeutic Uses in Human Disease

Protein therapy exhibits several advantages over small molecule drugs and is increasingly being developed for the treatment of disorders ranging from single enzyme deficiencies to cancer. Cell-penetrating peptides (CPPs), a group of small peptides capable of promoting transport of molecular cargo across the plasma membrane, have become important tools in promoting the cellular uptake of exogenously delivered proteins. Although the molecular mechanisms of uptake are not firmly established, CPPs have been empirically shown to promote uptake of various molecules, including large proteins over 100 kiloDaltons (kDa). Recombinant proteins that include a CPP tag to promote intracellular delivery show promise as therapeutic agents with encouraging success rates in both animal and human trials. This review highlights recent advances in protein-CPP therapy and discusses optimization strategies and potential detrimental effects.

Applications and Challenges for Use of Cell-Penetrating Peptides as Delivery Vectors for Peptide and Protein Cargos

The hydrophilic nature of peptides and proteins renders them impermeable to cell membranes. Thus, in order to successfully deliver peptide and protein-based therapeutics across the plasma membrane or epithelial and endothelial barriers, a permeation enhancing strategy must be employed. Cell-penetrating peptides (CPPs) constitute a promising tool and have shown applications for peptide and protein delivery into cells as well as across various epithelia and the blood-brain barrier (BBB). CPP-mediated delivery of peptides and proteins may be pursued via covalent conjugation of the CPP to the cargo peptide or protein or via physical complexation obtained by simple bulk-mixing of the CPP with its cargo. Both approaches have their pros and cons, and which is the better choice likely relates to the physicochemical properties of the CPP and its cargo as well as the route of administration, the specific barrier and the target cell. Besides the physical barrier, a metabolic barrier must be taken into consideration when applying peptide-based delivery vectors, such as the CPPs, and stability-enhancing strategies are commonly employed to prolong the CPP half-life. The mechanisms by which CPPs translocate cell membranes are believed to involve both endocytosis and direct translocation, but are still widely investigated and discussed. The fact that multiple factors influence the mechanisms responsible for cellular CPP internalization and the lack of sensitive methods for detection of the CPP, and in some cases the cargo, further complicates the design and conduction of conclusive mechanistic studies.

Safety of the cell-penetrating peptide penetratin as an oral absorption enhancer

Novel delivery technology using cell-penetrating peptides (CPPs) have recently shown their potential and are emerging as promising candidates for an oral protein and peptide delivery systems. As with for the development of any absorption enhancer that is meant to function across an epithelial layer covering a surface highly exposed to pathogens such as the intestines, concern arises about the safety of such enhancers. The purpose of this study was to investigate the effect of 7 d of consecutive oral administrations of CPPs and a typical enterotoxin, lipopolysaccharide (LPS) to mice to determine the degree, if any, of damage caused to the hepatic tissue. Following the 7-d dosing regimen, we could not detect significantly increased levels of the liver enzymes alanine aminotransferase and aspartate aminotransferase in plasma of mice treated with CPP and LPS compared to the controls, whereas heightened levels were observed in animals receiving the bile salt. In conclusion, the repeated use of CPPs as an oral absorption enhancer for macromolecules was found to be a safe strategy.

Cell-Penetrating Peptides as Carriers for Oral Delivery of Biopharmaceuticals

Oral delivery of biopharmaceuticals, for example peptides and proteins, constitutes a great challenge in drug delivery due to their low chemical stability and poor permeation across the intestinal mucosa, to a large extent limiting the mode of administration to injections, which is not favouring patient compliance. Nevertheless, cell-penetrating peptides (CPPs) have shown promising potential as carriers to overcome the epithelium, and this minireview highlights recent knowledge gained within the field of CPP-mediated transepithelial delivery of therapeutic peptides and proteins from the intestine. Two approaches may be pursued: co-administration of the carrier and therapeutic peptide in the form of complexes obtained by simple bulk mixing, or administration of covalent conjugates demanding more advanced production methodologies. These formulation approaches have their pros and cons, and which is to be preferred depends on the physicochemical properties of both the specific CPP and the specific cargo. In addition to the physical epithelial barrier, a metabolic barrier must be overcome in order to obtain CPP-mediated delivery of a cargo drug from the intestine, and a number of strategies have been employed to delay enzymatic degradation of the CPP. The mechanisms by which CPPs translocate across membranes are not fully understood, but possibly involve endocytosis as well as direct translocation, and the CPP-mediated transepithelial delivery of cargo drugs thus likely involves similar mechanisms for the initial membrane interaction and translocation. However, the mechanisms responsible for transcytosis of the cargo drug, if taken up by an endocytic mechanism, or direct translocation across the epithelium are so far not known.