A novel approach using functional peptides for efficient intestinal absorption of insulin

Evaluation of Function and Features of Human Induced Pluripotent Stem Cell-Derived Small Intestinal Epithelial Cells for Analyzing Peptide Drug Intestinal Absorption Profiles

Caco-2 cell monolayers are widely employed as an in vitro model of the intestinal barrier, capable of accurately predicting the absorption of conventional small-molecule drugs. However, this model may not be applicable to all drugs, and the accuracy of absorption prediction is typically poor for high molecular weight drugs. Recently, human induced pluripotent stem (iPS) cell-derived small intestinal epithelial cells (hiPSC-SIECs), exhibiting properties similar to those of the small intestine when compared with Caco-2 cells, have been developed and are considered a novel candidate model for in vitro evaluation of intestinal drug permeability. Therefore, we evaluated the utility of human hiPSC-SIECs as a new in vitro model to predict the intestinal absorption of middle-molecular weight drugs and peptide drugs. Firstly, we showed that the hiPSC-SIEC monolayer allowed faster transport of peptide drugs (insulin and glucagon-like peptide-1) than the Caco- 2 cell monolayer. Second, we revealed that hiPSC-SIECs require divalent cations (Mg2+ and Ca2+) to maintain barrier integrity. Third, we demonstrated that experimental conditions established for Caco-2 cells are not persistently applicable to hiPSC-SICEs when analyzing absorption enhancers. Comprehensively clarifying the features of hiPSC-SICEs is essential to establish a new in vitro evaluation model.

Orally-delivered insulin-peptide nanocomplexes enhance transcytosis from cellular depots and improve diabetic blood glucose control

Insulin regulates blood glucose levels, and is the mainstay for the treatment of type-1 diabetes and type-2 when other drugs provide inadequate control. Therefore, effective oral Insulin delivery would be a significant advance in drug delivery. Herein, we report the use of the modified cell penetrating peptide (CPP) platform, Glycosaminoglycan-(GAG)-binding-enhanced-transduction (GET), as an efficacious transepithelial delivery vector in vitro and to mediate oral Insulin activity in diabetic animals. Insulin can be conjugated with GET via electrostatic interaction to form nanocomplexes (Insulin GET-NCs). These NCs (size and charge; 140 nm, +27.10 mV) greatly enhanced Insulin transport in differentiated in vitro intestinal epithelium models (Caco2 assays; >22-fold increased translocation) with progressive and significant apical and basal release of up-taken Insulin. Delivery resulted in intracellular accumulation of NCs, enabling cells to act as depots for subsequent sustained release without affecting viability and barrier integrity. Importantly Insulin GET-NCs have enhanced proteolytic stability, and retained significant Insulin biological activity (exploiting Insulin-responsive reporter assays). Our study culminates in demonstrating oral delivery of Insulin GET-NCs which can control elevated blood-glucose levels in streptozotocin (STZ)-induced diabetic mice over several days with serial dosing. As GET promotes Insulin absorption, transcytosis and intracellular release, along with in vivo function, our simplistic complexation platform could allow effective bioavailability of other oral peptide therapeutics and help transform the treatment of diabetes.

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

Protein transduction domain of translationally controlled tumor protein: characterization and application in drug delivery

Our research group reported in 2011 the discovery of a novel cell-penetrating moiety in the N-terminus of the human translationally controlled tumor protein (TCTP). This moiety was responsible for the previously noted membrane translocating ability of purified full-length TCTP. The hydrophobic nature of TCTP-derived protein transduction domain (TCTP-PTD) endowed it with unique characteristics compared to other well-known cationic PTDs, such as TAT-PTD. TCTP-PTD internalizes partly through lipid-raft/caveolae-dependent endocytosis and partly by macropinocytosis. After cell entry, caveosome-laden TCTP-PTD appears to move to the cytoplasm and cytoskeleton except for the nucleus possibly through the movement to endoplasmic reticulum (ER). TCTP-PTD efficiently facilitates delivery of various types of cargos, such as peptides, proteins, and nucleic acids in vitro and in vivo. It is noteworthy that TCTP-PTD and its variants promote intranasal delivery of antidiabetics including, insulin and exendin-4 and of antigens for immunization in vivo, suggesting its potential for drug delivery. In this review, we attempted to describe recent advances in the understanding regarding the identification of TCTP-PTD, the characteristics of its cellular uptake, and the usefulness as a vehicle for delivery into cells of a variety of drugs and macromolecules. Our investigative efforts are continuing further to delineate the details of the functions and the regulatory mechanisms of TCTP-PTD-mediated cellular penetration and posttranslational modification of TCTP in physiologic and pathological processes. This is a review of what we currently know regarding TCTP-PTD and its use as a vehicle for the transduction of drugs and other molecules.

Systemic and brain delivery of antidiabetic peptides through nasal administration using cell-penetrating peptides

The intranasal route has emerged as a promising strategy that can direct delivery of drugs into the systemic circulation because the high-vascularized nasal cavity, among other advantages, avoids the hepatic first-pass metabolism. The nose-to-brain pathway provides a non-invasive alternative to other routes for the delivery of macromolecular therapeutics. A great variety of methodologies has been developed to enhance the efficiency of transepithelial translocation of macromolecules. Among these, the use of cell-penetrating peptides (CPPs), short protein transduction domains (PTDs) that facilitate the intracellular transport of various bioactive molecules, has become an area of extensive research in the intranasal delivery of peptides and proteins either to systemic or to brain compartments. Some CPPs have been applied for the delivery of peptide antidiabetics, including insulin and exendin-4, for treating diabetes and Alzheimer's disease. This review highlights the current status of CPP-driven intranasal delivery of peptide drugs and its potential applicability as a universal vehicle in the nasal drug delivery.

Screening for effective cell-penetrating peptides with minimal impact on epithelial cells and gut commensals in vitro

One of the biggest challenges for oral drug absorption is the epithelial barrier of the gastrointestinal tract. The use of cell-penetrating peptides (CPPs) to modulate the epithelial barrier function is known to be an effective strategy to improve drug absorption and bioavailability. In this study we compare side-by-side, 9 most promising CPPs to study their cytotoxicity (Cytotox Red dye staining) and cell viability (AlamarBlue staining) on epithelial cells and their effects on paracellular permeability of the intestinal barrier in vitro in a differentiated Caco-2 epithelial monolayer model. The data revealed that 4 out of 9 well-studied CPPs significantly improved Caco-2 paracellular permeability without compromising on cellular health. To assess the impact of CPPs on the human microbiota we studied the antimicrobial effects of the 4 effective CPPs from our permeation studies against 10 representative strains of the gut microbiota in vitro using microbroth dilution. Our data revealed that these 4 CPPs affected the growth of almost all tested commensal strains. Interestingly, we found that two synthetic CPPs (Shuffle and Penetramax) outperformed all the other CPPs in their ability to increase intestinal paracellular permeability at 50 µM and had only a small to moderate effect on the tested gut commensal strains. Based on these data Shuffle and Penetramax represent relevant CPPs to be further characterized in vivo for safe delivery of poorly absorbed therapeutics while minimizing negative impacts on the gut microbiota.

An Effective Strategy to Develop Potent and Selective Antifungal Agents from Cell Penetrating Peptides in Tackling Drug-Resistant Invasive Fungal Infections

The high mortality rate of invasive fungal infections and quick emergence of drug-resistant fungal pathogens urgently call for potent antifungal agents. Inspired by the cell penetrating peptide (CPP) octaarginine (R8), we elongated to 28 residues poly(d,l-homoarginine) to obtain potent toxicity against both fungi and mammalian cells. Further incorporation of glutamic acid residues shields positive charge density and introduces partial zwitterions in the obtained optimal peptide polymer that displays potent antifungal activity against drug-resistant fungi superior to antifungal drugs, excellent stability upon heating and UV exposure, negligible in vitro and in vivo toxicity, and strong therapeutic effects in treating invasive fungal infections. Moreover, the peptide polymer is insusceptible to antifungal resistance owing to the unique CPP-related antifungal mechanism of fungal membrane penetration followed by disruption of organelles within fungal cells. All these merits imply the effectiveness of our strategy to develop promising antifungal agents.

Hydrogels and Hydrogel Nanocomposites: Enhancing Healthcare through Human and Environmental Treatment

Humans are constantly exposed to exogenous chemicals throughout their life, which can lead to a multitude of negative health impacts. Advanced materials can play a key role in preventing or mitigating these impacts through a wide variety of applications. The tunable properties of hydrogels and hydrogel nanocomposites (e.g., swelling behavior, biocompatibility, stimuli responsiveness, functionality, etc.) have deemed them ideal platforms for removal of environmental contaminants, detoxification, and reduction of body burden from exogenous chemical exposures for prevention of disease initiation, and advanced treatment of chronic diseases, including cancer, diabetes, and cardiovascular disease. In this review, three main junctures where the use of hydrogel and hydrogel nanocomposite materials can intervene to positively impact human health are highlighted: 1) preventing exposures to environmental contaminants, 2) prophylactic treatments to prevent chronic disease initiation, and 3) treating chronic diseases after they have developed.

Acid-Resistant and Physiological pH-Responsive DNA Hydrogel Composed of A-Motif and i-Motif toward Oral Insulin Delivery

An acid-resistant DNA hydrogel that is stable in an extremely acidic environment with pH as low as 1.2 has not been reported before, largely due to the instability of DNA-hybridized structures. To achieve this, adenine (A)-rich and cytosine (C)-rich oligonucleotides are rationally designed and integrated to form copolymers with acrylamide monomers via free-radical polymerization. In an acidic environment (pH 1.2-6.0), the generated copolymers form a hydrogel state, which is cross-linked by parallel A-motif duplex configurations (pH 1.2-3.0) and quadruplex i-motif structures (pH 4.0-6.0) due to the protonation of A and C bases, respectively. Specifically, the protonated A-rich sequences under pH 1.2-3.0 form a stable parallel A-motif duplex cross-linking unit through reverse Hoogsteen interaction and electrostatic attraction. Hemi-protonated C bases under mildly acidic pH (4.0-6.0) form quadruplex i-motif cross-linking configuration via Hoogsteen interaction. Under physiological pH, both A and C bases deprotonated, resulting in the separation of A-motif and i-motif to A-rich and C-rich single strands, respectively, and thereby the dissociation of the DNA hydrogel into the solution state. The acid-resistant and physiological pH-responsive DNA hydrogel was further developed for oral drug delivery to the hostile acidic environment in the stomach (pH 1.2), duodenum (pH 5.0), and small intestine (pH 7.2), where the drug would be released and absorbed. As a proof of concept, insulin was encapsulated in the DNA hydrogel and orally administered to diabetic rats. In vitro and in vivo studies demonstrated the potential usage of the DNA hydrogel for oral drug delivery.

Bio-Membrane Internalization Mechanisms of Arginine-Rich Cell-Penetrating Peptides in Various Species

Recently, membrane-active peptides or proteins that include antimicrobial peptides (AMPs), cytolytic proteins, and cell-penetrating peptides (CPPs) have attracted attention due to their potential applications in the biomedical field. Among them, CPPs have been regarded as a potent drug/molecules delivery system. Various cargoes, such as DNAs, RNAs, bioactive proteins/peptides, nanoparticles and drugs, can be carried by CPPs and delivered into cells in either covalent or noncovalent manners. Here, we focused on four arginine-rich CPPs and reviewed the mechanisms that these CPPs used for intracellular uptake across cellular plasma membranes. The varying transduction efficiencies of them alone or with cargoes were discussed, and the membrane permeability was also expounded for CPP/cargoes delivery in various species. Direct membrane translocation (penetration) and endocytosis are two principal mechanisms for arginine-rich CPPs mediated cargo delivery. Furthermore, the amino acid sequence is the primary key factor that determines the cellular internalization mechanism. Importantly, the non-cytotoxic nature and the wide applicability make CPPs a trending tool for cellular delivery.

Cell-Penetrating Peptides as Carriers for Transepithelial Drug Delivery

This chapter describes the use of cell-penetrating peptides (CPPs) as carriers for transepithelial delivery of therapeutic peptides. Assessment of transepithelial peptide permeation and the mechanisms of action that permeability enhancing drug carriers exert on the epithelium requires subtle sample preparation and analysis by orthogonal methods. Here, the preparation and use of CPP-insulin physical mixture samples including the quantification of insulin by enzyme-linked immunosorbent assay (ELISA) is described. In addition, effects of CPPs on the epithelium and its barrier properties immediately upon exposure and after a recovery period are evaluated by epithelial cell viability, transepithelial electrical resistance, immunostaining of the tight junction associated zonula occludens (ZO-1) protein, and actin cytoskeleton staining.

Oral drug delivery for immunoengineering

The systemic pharmacotherapeutic efficacy of immunomodulatory drugs is heavily influenced by its route of administration. A few common routes for the systemic delivery of immunotherapeutics are intravenous, intraperitoneal, and intramuscular injections. However, the development of novel biomaterials, in adjunct to current progress in immunoengineering, is providing an exciting area of interest for oral drug delivery for systemic targeting. Oral immunotherapeutic delivery is a highly preferred route of administration due to its ease of administration, higher patient compliance, and increased ability to generate specialized immune responses. However, the harsh environment and slow systemic absorption, due to various biological barriers, reduces the immunotherapeutic bioavailability, and in turn prevents widespread use of oral delivery. Nonetheless, cutting edge biomaterials are being synthesized to combat these biological barriers within the gastrointestinal (GI) tract for the enhancement of drug bioavailability and targeting the immune system. For example, advancements in biomaterials and synthesized drug agents have provided distinctive methods to promote localized drug absorption for the modulation of local or systemic immune responses. Additionally, novel breakthroughs in the immunoengineering field show promise in the development of vaccine delivery systems for disease prevention as well as combating autoimmune diseases, inflammatory diseases, and cancer. This review will discuss current progress made within the field of biomaterials and drug delivery systems to enhance oral immunotherapeutic availability, and how these new delivery platforms can be utilized to deliver immunotherapeutics for resolution of immune-related diseases.

Nanotechnologies for the delivery of biologicals: Historical perspective and current landscape

Biological macromolecule-based therapeutics irrupted in the pharmaceutical scene generating a great hope due to their outstanding specificity and potency. However, given their susceptibility to degradation and limited capacity to overcome biological barriers new delivery technologies had to be developed for them to reach their targets. This review aims at analyzing the historical seminal advances that shaped the development of the protein/peptide delivery field, along with the emerging technologies on the lead of the current landscape. Particularly, focus is made on technologies with a potential for transmucosal systemic delivery of protein/peptide drugs, followed by approaches for the delivery of antigens as new vaccination strategies, and formulations of biological drugs in oncology, with special emphasis on mAbs. Finally, a discussion of the key challenges the field is facing, along with an overview of prospective advances are provided.

Innovations in Biomaterial Design toward Successful RNA Interference Therapy for Cancer Treatment

Gene regulation using RNA interference (RNAi) therapy has been developed as one of the frontiers in cancer treatment. The ability to tailor the expression of genes by delivering synthetic oligonucleotides to tumor cells has transformed the way scientists think about treating cancer. However, its clinical application has been limited due to the need to deliver synthetic RNAi oligonucleotides efficiently and effectively to target cells. Advances in nanotechnology and biomaterials have begun to address the limitations to RNAi therapeutic delivery, increasing the likelihood of RNAi therapeutics for cancer treatment in clinical settings. Herein, innovations in the design of nanocarriers for the delivery of oligonucleotides for successful RNAi therapy are discussed.

Discovery of New Potent anti-MERS CoV Fusion Inhibitors

Middle East respiratory syndrome coronavirus (MERS-CoV), capable of zoonotic transmission, has been associated with emerging viral pneumonia in humans. In this study, a set of highly potent peptides were designed to prevent MERS-CoV fusion through competition with heptad repeat domain 2 (HR2) at its HR1 binding site. We designed eleven peptides with stronger estimated HR1 binding affinities than the wild-type peptide to prevent viral fusion with the cell membrane. Eight peptides showed strong inhibition of spike-mediated MERS-CoV cell-cell fusion with IC50 values in the nanomolar range (0.25–2.3 µM). Peptides #4–6 inhibited 95–98.3% of MERS-CoV plaque formation. Notably, peptide four showed strong inhibition of MERS-CoV plaques formation with EC50 = 0.302 µM. All peptides demonstrated safe profiles without cytotoxicity up to a concentration of 10 μM, and this cellular safety, combined with their anti-MERS-CoV antiviral activity, indicate all peptides can be regarded as potential promising antiviral agents.

Cyclodextrins-Peptides/Proteins Conjugates: Synthesis, Properties and Applications

Cyclodextrins (CDs) are a family of macrocyclic oligosaccharides mostly composed of six, seven, or eight α-D-glucopyranose units with α-1,4-glycosidic bonds to form toroidal structures. The CDs possess a hydrophilic exterior and hydrophobic interior with the ability to form an inclusion complex, especially with hydrophobic molecules. However, most existing studies are about conjugation CDs with peptide/protein focusing on the formation of new systems. The CD-peptide/protein can possess new abilities; particularly, the cavity can be applied in modulation properties of more complexed proteins. Most studies are focused on drug delivery, such as targeted delivery in cell-penetrating peptides or co-delivery. The co-delivery is based mostly on polylysine systems; on the other hand, the CD-peptide allows us to understand biomolecular mechanisms such as fibryllation or stem cell behaviour. Moreover, the CD-proteins are more complexed systems with a focus on targeted therapy; these conjugates might be controllable with various properties due to changes in their stability. Finally, the studies of CD-peptide/protein are promising in biomedical application and provide new possibilities for the conjugation of simple molecules to biomolecules.

Oral insulin delivery: utopia, currently possible or a near reality?

Diabetes is still one of the main diseases worldwide due to its high incidence, prevalence and, unfortunately, very high mortality. Type 1 diabetes (and in some other types) is generally controlled by exogenous insulin. Several attempts of oral insulin administration to humans have been done so far. Some of them achieved interesting results, but it seems to exist a barrier to transpose these studies into clinical trials. A broad perspective about the oral insulin and approaches will be addressed. Representative (not all) examples of innovation are herein described, and they should represent a step forward to achieve the main goal: to orally deliver insulin and improve the life quality of millions of patients.

Discovery of New Fusion Inhibitor Peptides against SARS-CoV-2 by Targeting the Spike S2 Subunit

A novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), caused a worldwide pandemic. Our aim in this study is to produce new fusion inhibitors against SARS-CoV-2, which can be the basis for developing new antiviral drugs. The fusion core comprising the heptad repeat domains (HR1 and HR2) of SARS-CoV-2 spike (S) were used to design the peptides. A total of twelve peptides were generated, comprising a short or truncated 24-mer (peptide #1), a long 36-mer peptide (peptide #2), and ten peptide #2 analogs. In contrast to SARS-CoV, SARS-CoV-2 S-mediated cell-cell fusion cannot be inhibited with a minimal length, 24-mer peptide. Peptide #2 demonstrated potent inhibition of SARS-CoV-2 S-mediated cell-cell fusion at 1 µM concentration. Three peptide #2 analogs showed IC50 values in the low micromolar range (4.7-9.8 µM). Peptide #2 inhibited the SARS-CoV-2 pseudovirus assay at IC50=1.49 µM. Given their potent inhibition of viral activity and safety and lack of cytotoxicity, these peptides provide an attractive avenue for the development of new prophylactic and therapeutic agents against SARS-CoV-2.

Cell-Penetrating Peptides as a Potential Drug Delivery System for Effective Treatment of Diabetes

BACKGROUND/PURPOSE

Type 2 diabetes (T2D) is characterized by hyperglycemia resulting from the body's inability to produce and/or use insulin. Patients with T2D often have hyperinsulinemia, dyslipidemia, inflammation, and oxidative stress, which then lead to hypertension, chronic kidney disease, cardiovascular disease, and increased risk of morbidity and mortality (9th leading cause globally). Insulin and related pharmacological therapies are widely used to manage T2D, despite their limitations. Efficient drug delivery systems (DDS) that control drug kinetics may decrease side effects, allow for efficient targeting, and increase the bioavailability of drugs to achieve maximum therapeutic benefits. Thus, the development of effective DDS is crucial to beat diabetes.

METHODS

Here, we introduced a highly bioavailable vector, cell-penetrating peptides (CPPs), as a powerful DDS to overcome limitations of free drug administration.

RESULTS

CPPs are short peptides that serve as a potent tool for delivering therapeutic agents across cell membranes. Various cargoes, including proteins, DNA, RNA, liposomes, therapeutic molecules, and nanomaterials, generally retain their bioactivity upon entering cells. The mechanisms of CPPs/cargoes intracellular entry are classified into two parts: endocytic pathways and direct membrane translocation. In this article, we focus on the applications of CPPs/therapeutic agents in the treatment of diabetes. Hypoglycemic drugs with CPPs intervention can enhance therapeutic effectiveness, and CPP-mediated drug delivery can facilitate the actions of insulin. Numerous studies indicate that CPPs can effectively deliver insulin, produce synergistic effects with immunosuppressants for successful pancreatic islet xenotransplantation, prolong pharmacokinetics, and retard diabetic nephropathy.

CONCLUSION

We suggest that CPPs can be a new generation of drug delivery systems for effective treatment and management of diabetes and diabetes-associated complications.

Oral Coadministration of Zn-Insulin with d-Form Small Intestine-Permeable Cyclic Peptide Enhances Its Blood Glucose-Lowering Effect in Mice

Oral delivery of insulin remains a challenge owing to its poor permeability across the small intestine and enzymatic digestion in the gastrointestinal tract. In a previous study, we identified a small intestine-permeable cyclic peptide, C-DNPGNET-C (C-C disulfide bond, cyclic DNP peptide), which facilitated the permeation of macromolecules. Here, we showed that intraintestinal and oral coadministration of insulin with the cyclic DNP derivative significantly reduced blood glucose levels by increasing the portal plasma insulin concentration following permeation across the small intestine of mice. We also found that protecting the cyclic DNP derivative from enzymatic digestion in the small intestine of mice using d-amino acids and by the cyclization of DNP peptide was essential to enhance cyclic DNP derivative-induced insulin absorption across the small intestine. Furthermore, intraintestinal and oral coadministration of insulin hexamer stabilized by zinc ions (Zn-insulin) with cyclic D-DNP derivative was more effective in facilitating insulin absorption and inducing hypoglycemic effects in mice than the coadministration of insulin with the cyclic D-DNP derivative. Moreover, Zn-insulin was more resistant to degradation in the small intestine of mice compared to insulin. Intraintestinal and oral coadministration of Zn-insulin with cyclic DNP derivative also reduced blood glucose levels in a streptozotocin-induced diabetes mellitus mouse model. A single intraintestinal administration of the cyclic D-DNP derivative did not induce any cytotoxicity, either locally in the small intestine or systemically. In summary, we demonstrated that coadministration of Zn-insulin with cyclic D-DNP derivative could enhance oral insulin absorption across the small intestine in mice.

Overcoming negatively charged tissue barriers: Drug delivery using cationic peptides and proteins

Negatively charged tissues are ubiquitous in the human body and are associated with a number of common diseases yet remain an outstanding challenge for targeted drug delivery. While the anionic proteoglycans are critical for tissue structure and function, they make tissue matrix dense, conferring a high negative fixed charge density (FCD) that makes drug penetration through the tissue deep zones and drug delivery to resident cells extremely challenging. The high negative FCD of these tissues is now being utilized by taking advantage of electrostatic interactions to create positively charged multi-stage delivery methods that can sequentially penetrate through the full thickness of tissues, create a drug depot and target cells. After decades of work on attempting delivery using strong binding interactions, significant advances have recently been made using weak and reversible electrostatic interactions, a characteristic now considered essential to drug penetration and retention in negatively charged tissues. Here we discuss these advances using examples of negatively charged tissues (cartilage, meniscus, tendons and ligaments, nucleus pulposus, vitreous of eye, mucin, skin), and delve into how each of their structures, tissue matrix compositions and high negative FCDs create barriers to drug entry and explore how charge interactions are being used to overcome these barriers. We review work on tissue targeting cationic peptide and protein-based drug delivery, compare and contrast drug delivery designs, and also present examples of technologies that are entering clinical trials. We also present strategies on further enhancing drug retention within diseased tissues of lower FCD by using synergistic effects of short-range binding interactions like hydrophobic and H-bonds that stabilize long-range charge interactions. As electrostatic interactions are incorporated into design of drug delivery materials and used as a strategy to create properties that are reversible, tunable and dynamic, bio-electroceuticals are becoming an exciting new direction of research and clinical work.

Intestinal permeation enhancers to improve oral bioavailability of macromolecules: reasons for low efficacy in humans

INTRODUCTION

Intestinal permeation enhancers (PEs) are substances that transiently alter the intestinal epithelial barrier to facilitate permeation of macromolecules with low oral bioavailability (BA). While a number of PEs have progressed to clinical testing in conventional formulations with macromolecules, there has been only low single digit increases in oral BA, irrespective of whether the drug met primary or secondary clinical endpoints.

AREAS COVERED

This article considers the causes of sub-optimal BA of macromolecules from PE dosage forms and suggests approaches that may improve performance in humans.

EXPERT OPINION

Permeation enhancement is most effective when the PE is co-localized with the macromolecule at the epithelial surface. Conditions in the GI tract impede optimal co-localization. Novel delivery systems that limit dilution and spreading of the PE and macromolecule in the small intestine have attempted to replicate promising enhancement efficacy observed in static drug delivery models.

Cell-penetrating peptide enhanced insulin buccal absorption

Non-injectable delivery of peptides and proteins is not feasible due to the limitations of large molecular mass, high hydrophilic properties, and gastrointestinal degradation. Therefore, proposing a new method to solve this problem is a burning issue. The objective of this study was to propose a novel protein delivery strategy to overcome the poor efficacy and irritation of buccal insulin delivery. In this study, we applied a conjugate of cell-penetrating peptides (LMWP) and insulin (INS-PEG-LMWP) for buccal delivery. INS-PEG-LMWP was prepared using insulin solution and mixture as references. The transport behaviour, in vivo bioactivity, hypoglycaemic effect, and safety of INS-PEG-LMWP were systematically characterised. An in vitro study demonstrated that the uptake and transportation of INS-PEG-LMWP across buccal mucosal multilayers significantly increased. By comparing the effects of different endocytic inhibitors on INS-PEG-LMWP uptake, the conjugate might be delivered via an energy independent, electrostatically adsorbed pathway. INS-PEG-LMWP's relative pharmacological bioavailability was high and its relative bioavailability was up to 26.86%, demonstrating no visible mucosal irritation. Cell-penetrating peptides are likely to become a reliable and safe tool for overcoming insulin's low permeability through the epithelial multilayers, the major barrier to buccal delivery.

Cell-Penetrating Peptides in Diagnosis and Treatment of Human Diseases: From Preclinical Research to Clinical Application

Cell-penetrating peptides (CPPs) are short peptides (fewer than 30 amino acids) that have been predominantly used in basic and preclinical research during the last 30 years. Since they are not only capable of translocating themselves into cells but also facilitate drug or CPP/cargo complexes to translocate across the plasma membrane, they have potential applications in the disease diagnosis and therapy, including cancer, inflammation, central nervous system disorders, otologic and ocular disorders, and diabetes. However, no CPPs or CPP/cargo complexes have been approved by the US Food and Drug Administration (FDA). Many issues should be addressed before translating CPPs into clinics. In this review, we summarize recent developments and innovations in preclinical studies and clinical trials based on using CPP for improved delivery, which have revealed that CPPs or CPP-based delivery systems present outstanding diagnostic therapeutic delivery potential.

Oral Delivery of Biologics for Precision Medicine

The emerging field of precision medicine is rapidly growing, fostered by the advances in genome mapping and molecular diagnosis. In general, the translation of these advances into precision treatments relies on the use of biological macromolecules, whose structure offers a high specificity and potency. Unfortunately, due to their complex structure and limited ability to overcome biological barriers, these macromolecules need to be administered via injection. The scientific community has devoted significant effort to making the oral administration of macromolecules plausible thanks to the implementation of drug delivery technologies. Here, an overview of the current situation and future prospects in the field of oral delivery of biologics is provided. Technologies in clinical trials, as well as recent and disruptive delivery systems proposed in the literature for local and systemic delivery of biologics including peptides, antibodies, and nucleic acids, are described. Strategies for the specific targeting of gastrointestinal regions-stomach, small bowel, and colon-cell populations, and internalization pathways, are analyzed. Finally, challenges associated with the clinical translation, future prospects, and identified opportunities for advancement in this field are also discussed.

Advances in the use of cell penetrating peptides for respiratory drug delivery

Introduction: Respiratory diseases are leading causes of death in the world, still inhalation therapies are the largest fail in drug development. There is an evident need to develop new therapies. Biomolecules represent apotential therapeutic agent in this regard, however their translation to the clinic is hindered by the lack of tools to efficiently deliver molecules. Cell penetrating peptides (CPPs) have arisen as apotential strategy for intracellular delivery that could theoretically enable the translation of new therapies.Areas covered: In this review, the use of CPPs as astrategy to deliver different molecules (cargoes) to treat lung-relateddiseases will be the focus. Abrief description of these molecules and the innovative methods in designing new CPPs is presented. The delivery of different cargoes (proteins, peptides, poorly soluble drugs and nucleic acids) using CPPs is discussed, focusing on benefits to treat different respiratory diseases like inflammatory disorders, cystic fibrosis and lung cancer.Expert opinion: The advantages of using CPPs to deliver biomolecules and poorly soluble drugs to the lungs is evident. This field has advanced in the past few years toward targeted intracellular delivery, although further studies are needed to fully understand its potential and limitations in vitro and in vivo.

Advanced Approaches of Bioactive Peptide Molecules and Protein Drug Delivery Systems

Despite the fact that protein and peptide therapeutics are widely employed in the treatment of various diseases, their delivery is posing an unembellished challenge to the scientists. It was discovered that delivery of these therapeutic systems through oral route is easy with high patient compliance. However, proteolytic degradation and absorption through the mucosal epithelium are the barriers in this route. These issues can be minimized by the use of enzyme inhibitors, absorption enhancers, different carrier systems or either by direct modification. In the process of investigation, it was found that transdermal route is not posing any challenges of enzymatic degradation, but, still absorption is the limitation as the outer layer of skin acts as a barrier. To suppress the effect of the barrier and increase the rate of the absorption, various advanced technologies were developed, namely, microneedle technology, iontophoresis, electroporation, sonophoresis and biochemical enhancement. Indeed, even these molecules are targeted to the cells with the use of cell-penetrating peptides. In this review, delivery of the peptide and protein therapeutics using oral, transdermal and other routes is discussed in detail.

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.

A MALDI-TOF-based Method for Studying the Transport of BBB Shuttles-Enhancing Sensitivity and Versatility of Cell-Based In Vitro Transport Models

In recent decades, peptide blood-brain barrier shuttles have emerged as a promising solution for brain drugs that are not able to enter this organ. The research and development of these compounds involve the use of in vitro cell-based models of the BBB. Nevertheless, peptide transport quantification implies the use of large amounts of peptide (upper micromolar range for RP-HPLC-PDA) or of derivatives (e.g. fluorophore or quantum-dot attachment, radiolabeling) in the donor compartment in order to enhance the detection of these molecules in the acceptor well, although their structure is highly modified. Therefore, these methodologies either hamper the use of low peptide concentrations, thus hindering mechanistic studies, or do not allow the use of the unmodified peptide. Here we successfully applied a MALDI-TOF MS methodology for transport quantification in an in vitro BBB cell-based model. A light version of the acetylated peptide was evaluated, and the transport was subsequently quantified using a heavy internal standard (isotopically acetylated). We propose that this MALDI-TOF MS approach could also be applied to study the transport across other biological barriers using the appropriate in vitro transport models (e.g. Caco-2, PAMPA).

Hydrophobic Amino Acid Tryptophan Shows Promise as a Potential Absorption Enhancer for Oral Delivery of Biopharmaceuticals

Cell-penetrating peptides (CPPs) have great potential to efficiently deliver drug cargos across cell membranes without cytotoxicity. Cationic arginine and hydrophobic tryptophan have been reported to be key component amino acids for cellular internalization of CPPs. We recently found that l-arginine could increase the oral delivery of insulin in its single amino acid form. Therefore, in the present study, we evaluated the ability of another key amino acid, tryptophan, to enhance the intestinal absorption of biopharmaceuticals. We demonstrated that co-administration with l-tryptophan significantly facilitated the oral and intestinal absorption of the peptide drug insulin administered to rats. Furthermore, l-tryptophan exhibited the ability to greatly enhance the intestinal absorption of other peptide drugs such as glucagon-like peptide-1 (GLP-1), its analog Exendin-4 and macromolecular hydrophilic dextrans with molecular weights ranging from 4000 to 70,000 g/mol. However, no intermolecular interaction between insulin and l-tryptophan was observed and no toxic alterations to epithelial cellular integrity-such as changes to cell membranes, cell viability, or paracellular tight junctions-were found. This suggests that yet to be discovered inherent biological mechanisms are involved in the stimulation of insulin absorption by co-administration with l-tryptophan. These results are the first to demonstrate the significant potential of using the single amino acid l-tryptophan as an effective and versatile bioavailability enhancer for the oral delivery of biopharmaceuticals.

Combination Strategy with Complexation Hydrogels and Cell-Penetrating Peptides for Oral Delivery of Insulin

In previous studies we showed that the complexation hydrogels based in poly(methacrylic acid-g-ethylene glycol) [P(MAA-g-EG)] rapidly release insulin in the intestine owing to their pH-dependent complexation properties; they also exhibit a high insulin-loading efficiency, enzyme-inhibiting properties, and mucoadhesive characteristics. Cell-penetrating peptides (CPPs), such as oligoarginines [hexa-arginine (R6), comprising six arginine residues], have been employed as useful tools for the oral delivery of therapeutic macromolecules. The aim of our study was to investigate the combination strategy of using P(MAA-g-EG) hydrogels with R6-based CPPs to improve the intestinal absorption of insulin. A high efficiency of loading into crosslinked P(MAA-g-EG) hydrogels was observed for insulin (96.1±1.4%) and R6 (46.6±3.8%). In addition, immediate release of the loaded insulin and R6 from these hydrogels was observed at pH 7.4 (80% was released in approximately 30 min). Consequently, a strong hypoglycemic response was observed (approximately 18% reduction in blood glucose levels) accompanied by an improvement in insulin absorption after the co-administration of insulin-loaded particles (ILP) and R6-loaded particles (ALP) into closed rat ileal segments compared with that after ILP administration alone. These results indicate that the combination of P(MAA-g-EG) hydrogels with CPPs may be a promising strategy for the oral delivery of various insulin preparations as an alternative to conventional parenteral routes.

PEG-PGA enveloped octaarginine-peptide nanocomplexes: An oral peptide delivery strategy

The objective of this work was the development of a new drug nanocarrier intended to overcome the barriers associated to the oral modality of administration and to assess its value for the systemic or local delivery of peptides. The nanocarrier was rationally designed taking into account the nature of the intestinal barriers and was loaded with insulin, which was selected as a model peptide. The nanocarrier consisted of a complex between insulin and a hydrophobically-modified cell penetrating peptide (CPP), enveloped by a protecting polymer. The selected CPP was octaarginine (r8), chemically conjugated with cholesterol (Chol) or lauric acid (C12), whereas the protecting polymer was poly (glutamic acid)-poly (ethylene glycol) (PGA-PEG). This enveloping material was intended to preserve the stability of the nanocomplex in the intestinal medium and facilitate its diffusion across the intestinal mucus. The enveloped nanocomplexes (ENCPs) exhibited a number of key features, namely (i) a unimodal size distribution with a mean size of 200 nm and a neutral zeta potential, (ii) the capacity to associate insulin (~100% association efficiency) and protect it from degradation in simulated intestinal fluids, (iii) the ability to diffuse through intestinal mucus and, most importantly, (iv) the capacity to interact with the Caco-2 model epithelium, resulting in a massive insulin cell uptake (47.59 ± 5.79%). This enhanced accumulation of insulin at the epithelial level was not translated into an enhanced insulin transport. In fact, only 2% of insulin was transported across the monolayer, and this was correlated with a moderate response of insulin following oral administration to healthy rats. Despite of this, the accumulation of the insulin-loaded nanocarriers in the intestinal mucosa could be verified in vivo upon their labeling with ^99mTc. Overall, these data underline the capacity of the nanocarriers to overcome substantial barriers associated to the oral modality of administration and to facilitate the accumulation of the associated peptide at the intestinal level.

Oral delivery of anti-diabetes therapeutics using cell penetrating and transcytosing peptide strategies

Oral delivery of insulin and other anti-diabetic peptides is inhibited by low intestinal absorption caused by the poor permeability across cellular membranes and the susceptibility to enzymatic degradation in the gastrointestinal tract. Cell-penetrating peptides (CPPs) have been investigated for a number of years as oral absorption enhancers for hydrophilic macromolecules by electrostatic or covalent conjugation on in conjunction with nanotechnology. Endogenous cellular uptake mechanisms present in the intestine can be exploited by engineering peptide conjugates that transcytose; entering cells by endocytosis and leaving by exocytosis. Efficiently delivering hydrophilic and sensitive peptide drugs to safely transverse the digestive barrier with no effect on gut physiology using remains a key driver for formulation research. Here we review the use of CPP and transcytosis peptide approaches, their modification and use in delivering anti-diabetic peptides (with the primary example of Insulin and engineered homologues) by direct oral administration to treat diabetes and associated metabolic disorders.

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.

Polyarginine nanocapsules: A versatile nanocarrier with potential in transmucosal drug delivery

The objective of this work was to investigate the potential utility of nanocapsules composed of an oily core decorated with a single polyarginine (PARG), or double PARG/polyacrylic acid (PAA) layer as oral peptide delivery carrier. A step-by-step formulation optimization process was designed, which involved the study of the influence of the surfactants, oils and polymer shells (PARG of different molecular weight and PAA) on the nanocapsules physicochemical properties, peptide loading efficiency, stability in simulated intestinal fluids (SIF) and capacity to enhance the permeability of the intestinal epithelium. Despite the lipophilic nature of the nanocapsules, it was possible to achieve a moderate loading of the hydrophilic model peptide salmon calcitonin and control its release in SIF, by adjusting the formulation conditions. Finally, studies in the Caco-2 epithelial cell line showed the capacity of the nanocapsules to reduce the transepithelial electric resistance of the monolayer, without compromising their viability. Overall, these properties suggest the capacity of polyarginine nanocapsules for enhancing the transport of peptides across epithelia.

Eudragit S100-Coated Chitosan Nanoparticles Co-loading Tat for Enhanced Oral Colon Absorption of Insulin

In order to improve oral absorption of insulin, especially the absorption at the colon, Eudragit S100® (ES)-coated chitosan nanoparticles loading insulin and a trans-activating transcriptional peptide (Tat) were employed as the vehicle. In vitro releases of insulin and Tat from ES-coated chitosan nanoparticles had a pH-dependant characteristic. A small amount of the contents was released from the coated nanoparticles at pH 1.2 simulated gastric fluid, while a fairly fast and complete release was observed in pH 7.4 medium. Caco-2 cell was used as the model of cellular transport and uptake studies. The results showed that the cellular transport and uptake of insulin for ES-coated chitosan nanoparticles co-loading insulin and Tat (ES-Tat-cNPs) were about 3-fold and 4-fold higher than those for the nanoparticles loading only insulin (ES-cNPs), respectively. The evaluations in vivo of ES-Tat-cNPs were conducted on diabetic rats and normal minipigs, respectively. The experimental results on rats revealed that the pharmacodynamical bioavailability of ES-Tat-cNPs had 2.16-fold increase compared with ES-cNPs. After oral administration of nanoparticle suspensions to the minipigs, insulin bioavailability of ES-Tat-cNPs was 1.73-fold higher than that of ES-cNPs, and the main absorption site of insulin was probably located in the colon for the two nanoparticles. In summary, this report provided an exploratory means for the improvement of oral absorption of insulin.

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.

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.