Tumor targeting of a cell penetrating peptide by fusing with a pH-sensitive histidine-glutamate co-oligopeptide

Tumor Microenvironment pH-Sensitive Peptidomimetics for Targeted Anticancer Drug Delivery

Cell-penetrating peptides (CPPs) are known for their effective intracellular transport of bioactives such as therapeutic proteins, peptides, nucleic acid, and small molecule drugs. However, the excessive cationic charges that promote their membrane permeability result in nonselective delivery and cellular toxicity. In this study, we report a decamer cell-penetrating peptidomimetic, Hkd, designed to selectively deliver anticancer drugs into tumor cells in response to the acidic microenvironment. The pH-sensitive histidine (H) imidazole side chain undergoes protonation in acidic environments, facilitating membrane permeability. The rigid cyclic dipeptide (CDP) core (kd) of Hkd has multiple hydrogen bond donor and acceptor sites, enabling selective interaction-driven cellular uptake. Pharmacokinetic studies revealed the excellent serum stability of Hkd. Cellular uptake studies of Hkd showed improved uptake at a lower pH than physiological pH. Conjugation of Hkd to the anticancer drug camptothecin (Cpt) reduced nonselective drug transport to normal cells while effectively delivering the drug into cancerous cells at the tumor microenvironment pH and retaining the therapeutic potential of the drug. The systematic design of pH-sensitive peptidomimetics offers a viable method to overcome the challenges of stability and selectivity faced by traditional highly cationic CPPs, potentially expanding the application range of this delivery system.

A novel multitargeted self-assembling peptide-siRNA complex for simultaneous inhibition of SARS-CoV-2-host cell interaction and replication

Effective therapeutics are necessary for managing severe COVID-19 disease despite the availability of vaccines. Small interfering RNA (siRNA) can silence viral genes and restrict SARS-CoV-2 replication. Cell-penetrating peptides is a robust method for siRNA delivery, enhancing siRNA stability and targeting specific receptors. We developed a peptide HE25 that blocks SARS-CoV-2 replication by various mechanisms, including the binding of multiple receptors involved in the virus's internalization, such as ACE2, integrins and NRP1. HE25 not only acts as a vehicle to deliver the SARS-CoV-2 RNA-dependent RNA polymerase siRNA into cells but also facilitates their internalization through endocytosis. Once inside endosomes, the siRNA is released into the cytoplasm through the Histidine-proton sponge effect and the selective cleavage of HE25 by cathepsin B. These mechanisms effectively inhibited the replication of the ancestral SARS-CoV-2 and the Omicron variant BA.5 in vitro. When HE25 was administered in vivo, either by intravenous injection or inhalation, it accumulated in lungs, veins and arteries, endothelium, or bronchial structure depending on the route. Furthermore, the siRNA/HE25 complex caused gene silencing in lung cells in vitro. The SARS-CoV-2 siRNA/HE25 complex is a promising therapeutic for COVID-19, and a similar strategy can be employed to combat future emerging viral diseases.

Polyarginine Cell-Penetrating Peptides Bind and Inhibit SERCA2

Cell-penetrating peptides (CPPs) are short peptide sequences that have the ability to cross the cell membrane and deliver cargo. Although it is critical that CPPs accomplish this task with minimal off-target effects, such actions have in many cases not been robustly screened. We presently investigated whether the commonly used CPPs TAT and the polyarginines Arg9 and Arg11 exert off-target effects on cellular Ca2+ homeostasis. In experiments employing myocytes and homogenates from the cardiac left ventricle or soleus muscle, we observed marked inhibition of Ca2+ recycling into the sarcoplasmic reticulum (SR) following incubation with polyarginine CPPs. In both tissues, the rate of SR Ca2+ leak remained unchanged, indicating that protracted Ca2+ removal from the cytosol stemmed from inhibition of the SR Ca2+ ATPase 2 (SERCA2). No such inhibition occurred following treatment with TAT, or in preparations from the SERCA1-expressing extensor digitorum longus muscle. Experiments in HEK cells overexpressing individual SERCA isoforms confirmed that polyarginine incubation specifically inhibited the activity of SERCA2a and 2b, but not SERCA1 or 3. The attenuation of SERCA2 activity was not dependent on the presence of phospholamban, and ELISA-based analyses rather revealed direct interaction between the polyarginines and the actuator domain of the protein. Surface plasmon resonance experiments confirmed strong binding within this region of SERCA2, and slow dissociation between the two species. Based on these observations, we urge caution when employing polyarginine CPPs. Indeed, as SERCA2 is expressed in diverse cell types, the wide-ranging consequences of SERCA2 binding and inhibition should be anticipated in both experimental and therapeutic settings.

Cell Penetrating Peptides: Classification, Mechanisms, Methods of Study, and Applications

Cell-penetrating peptides (CPPs) encompass a class of peptides that possess the remarkable ability to cross cell membranes and deliver various types of cargoes, including drugs, nucleic acids, and proteins, into cells. For this reason, CPPs are largely investigated in drug delivery applications in the context of many diseases, such as cancer, diabetes, and genetic disorders. While sharing this functionality and some common structural features, such as a high content of positively charged amino acids, CPPs represent an extremely diverse group of elements, which can differentiate under many aspects. In this review, we summarize the most common characteristics of CPPs, introduce their main distinctive features, mechanistic aspects that drive their function, and outline the most widely used techniques for their structural and functional studies. We highlight current gaps and future perspectives in this field, which have the potential to significantly impact the future field of drug delivery and therapeutics.

Rational Design and Self-Assembly of Histidine-Rich Peptides on a Graphite Surface

Histidine-rich peptides (HRPs) have been investigated to create functional biomolecules based on the nature of histidine, such as ion binding and catalytic activity. The organization of these HRPs on a solid surface can lead to surface functionalization with the well-known properties of HRPs. However, immobilization of HRPs on the surface has not been realized. Here, we design a series of octapeptides with histidine repeat units, aiming to establish their self-assembly on a graphite surface to produce a highly robust and active nanoscaffold. The new design has (XH)₄, and we incorporated various types of hydrophobic amino acids at X in the sequence to facilitate their interaction with the surface. The effect of the pair of amino acids on their self-assembly was investigated by atomic force microscopy. Contact angle measurement revealed that these assemblies functionalized graphite surfaces with different wetting chemistry. Moreover, the secondary structure of peptides was characterized by Fourier transform infrared spectroscopy (FTIR), which gives us further insights into the conformation of histidine repeat peptides on the surface. Our results showed a new approach to applying histidine-rich peptides on the surface and tuning the self-assembly behavior by introducing different counter amino acids that could be integrated with a wide range of biosensing and biotechnology applications.

One New Acid-Activated Hybrid Anticancer Peptide by Coupling with a Desirable pH-Sensitive Anionic Partner Peptide

Anticancer peptides (ACPs) are promising antitumor resources, and developing acid-activated ACPs as more effective and selective antitumor drugs would represent new progress in cancer therapy. In this study, we designed a new class of acid-activated hybrid peptides LK-LE by altering the charge shielding position of the anionic binding partner LE based on the cationic ACP LK and investigated their pH response, cytotoxic activity, and serum stability, in hoping to achieve a desirable acid-activatable ACP. As expected, the obtained hybrid peptides could be activated and exhibit a remarkable antitumor activity by rapid membrane disruption at acidic pH, whereas its killing activity could be alleviated at normal pH, showing a significant pH response compared with LK. Importantly, this study found that the peptide LK-LE3 with the charge shielding in the N-terminal of LK displayed notably low cytotoxicity and more stability, demonstrating that the position of charge masking is extremely important for the improvement of peptide toxicity and stability. In short, our work opens a new avenue to design promising acid-activated ACPs as potential targeting agents for cancer treatment.

Constructing New Acid-Activated Anticancer Peptide by Attaching a Desirable Anionic Binding Partner Peptide

Improving the cell selectivity of anticancer peptides (ACPs) is a major hurdle in their clinical utilization. In this study, a new acid-activated ACP was designed by conjugating a cationic ACP LK to its anionic binding partner peptide (LEH) via a disulfide linker to trigger antitumor activity at acidic pH while masking its killing activity at normal pH. Three anionic binding peptides containing different numbers of glutamic acid (Glu) and histidine were engineered to obtain an efficient acid-activated ACP. The conjugates LK-LEH2 and LK-LEH3 exhibited 6.1 and 8.0-fold higher killing activity at pH 6.0 relative to at pH 7.4, respectively, suggesting their excellent pH-dependent antitumor activity; and their cytotoxicity was 10-fold lower than that of LK. However, LK-LEH4 had no pH-responsive killing effect. Interestingly, increasing the number of Glu from 2 to 4 increased the pH-response of the physical mixture of LK and LEH; conversely, they weakly decreased the cytotoxicity of LK, suggesting that the conjugate connection was required to achieve excellent pH dependence while maintaining minimum toxicity. LK-LEH2 and LK-LEH3 were more enzymatically stable than LK, indicating their potential for in vivo application. Our work provided a basis for designing promising ACPs with good selectivity and low toxicity.

pH-Responsive Self-Assembling Peptide-Based Biomaterials: Designs and Applications

Stimuli-responsive peptide-based biomaterials are increasingly gaining interest for various specific and targeted treatments, including drug delivery and tissue engineering. Among all stimuli, pH can be especially useful because endogenous pH changes are often associated with abnormal microenvironments. pH-Responsive amino acids and organic linkers can be easily incorporated into peptides that self-assemble into various nanostructures. Thus, these largely biocompatible and easily tunable platforms are ideal candidates for drug release and as fibrous materials capable of mimicking the native extracellular matrix. In this review, we highlight common design motifs and mechanisms of pH-responsiveness in self-assembling peptide-based biomaterials, focusing on recent advances of these biomaterials applied in drug delivery and tissue engineering. Finally, we suggest future challenges and areas for potential development in pH-responsive self-assembling peptide-based biomaterials.

Recent applications of cell-penetrating peptide guidance of nanosystems in breast and prostate cancer (Review)

Cell-penetrating peptides (CPPs) are small peptides from natural sources or designed from other protein sequences that can penetrate cell membranes. This property has been used in biomedicine to add them to biomolecules to improve their capacity for cell internalization and as a guidance tool for specific cell types. CPPs have been shown to enhance cellular uptake in vitro and in vivo, improving the efficacy of anticancer drugs such as doxorubicin and paclitaxel, while also limiting their cytotoxic effects on healthy cells and tissues. The current study reviews the internalization and major therapeutic results achieved from the functionalization of nanosystems with CPPs for guidance into breast and prostate cancer cells in vitro and in vivo. In addition, the practical results obtained are specifically discussed for use as a starting point for scientists looking to begin research in this field.

Metal Binding Antimicrobial Peptides in Nanoparticle Bio-functionalization: New Heights in Drug Delivery and Therapy

Peptides are considered very important due to the diversity expressed through their amino acid sequence, structure variation, large spectrum, and their essential role in biological systems. Antimicrobial peptides (AMPs) emerged as a potent tool in therapy owing to their antimicrobial properties but also their ability to trespass the membranes, specificity, and low toxicity. They comprise a variety of peptides from which specific amino acid-rich peptides are of interest to the current review due to their features in metal interaction and cell penetration. Histidine-rich peptides such as Histatins belong to the metal binding salivary residing peptides with efficient antibacterial, antifungal, and wound-healing activities. Furthermore, their ability to activate in acidic environment attracted the attention to their potential in therapy. The current review covers the current knowledge about AMPs and critically assess the potential of associating with metal ions both structurally and functionally. This review provides interesting hints for the advantages provided by AMPs and metal ions in biomedicine, making use of their direct properties in brain diseases therapy or in the creation of new bio-functionalized nanoparticles for cancer diagnosis and treatment.

Challenge to overcome current limitations of cell-penetrating peptides

The penetration of biological membranes is a prime obstacle for the delivery of pharmaceutical drugs. Cell-penetrating peptide (CPP) is an efficient vehicle that can deliver various cargos across the biological membranes. Since the discovery, CPPs have been rigorously studied to unveil the underlying penetrating mechanism as well as to exploit CPPs for various biomedical applications. This review will focus on the various strategies to overcome current limitations regarding stability, selectivity, and efficacy of CPPs.

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

pH-Activatable cell penetrating peptide dimers for potent delivery of anticancer drug to triple-negative breast cancer

We developed a pH-activatable cell-penetrating peptide dimer LH₂ with histidine residues, which can penetrate cells, specifically in weak acidic conditions, even at few tens of nanomolar concentrations. LH₂ effectively delivered paclitaxel into triple-negative breast cancer cells, MDA-MB-231, via formation of non-covalent complexes (PTX-LH₂(M)) or covalent conjugates (PTX-LH₂(C)). Moreover, LH₂ showed prolonged circulation in the body and enhanced accumulation in tumors. Both PTX-LH₂(M) and PTX-LH₂(C) showed strong antitumor effects in a triple-negative breast cancer grafted mouse model at an extremely low dosage.

pH-dependent reversibly activatable cell-penetrating peptides improve the antitumor effect of artemisinin-loaded liposomes

Artemisinin (ART) is well known as an antimalarial drug, and it can also be used to treat inflammation as well as cancer. Although many researchers have reported the antitumor activity of ART, most of these studies were investigated in vitro. In addition, ART is sparingly soluble in water, limiting its clinical relevance in drug development. Based on the data from our preliminary study, ART is not cytotoxic at low micromolar concentrations. Thus, we hypothesized that smart nanocarriers are beneficial for not only increasing the solubility of ART but also elevating the concentration of the drug at the target, thereby inducing the ideal antitumor effect. In this article, a reversibly activatable cell-penetrating peptide ((HE)₁₀-G₅-R₆ or HE-R₆) was introduced to modify artemisinin (ART)-loaded liposomes (ART-Lip-HE-R₆) against tumors, and in vitro and in vivo performance were investigated. ART-Lip-HE-R₆ exhibited sustained release under different pH conditions. The internalization and cytotoxicity of liposomes were enhanced at low pH, i.e., 6.5, after modification with HE-R₆ versus nonmodified liposomes. Moreover, a longer retention time in tumors could be observed in the ART-Lip-HE-R₆ group, followed by higher efficiency of tumor suppression. In conclusion, Lip-HE-R₆ might be a promising delivery system for ART in cancer therapy.

Formulation, Characterization and Evaluation against SH-SY5Y Cells of New Tacrine and Tacrine-MAP Loaded with Lipid Nanoparticles

Tacrine (TAC) was the first FDA approved drug for the treatment of Alzheimer's disease, resulting in increased memory and enhanced cognitive symptoms in patients. However, long-term therapy presents poor patient compliance associated with undesired side effects such as nausea, vomiting and hepatoxicity. To improve its therapeutic efficacy and decrease toxicity, the use of nanoparticles could be applied as a possible solution to delivery TAC. In this context, a project has been designed to develop a new nanostructured lipid carrier (NLC) as a delivery system for TAC and conjugate TAC and model amphipathic peptide (MAP) to decrease TAC limitations. Different formulations loaded with TAC and TAC + MAP were prepared using a combination of Compritol 888 ATO as the solid lipid and Transcutol HP as the liquid lipid component. Physical characterization was evaluated in terms of particle size, surface charge, encapsulation efficiency and in vitro drug release studies. Particle size distributions within the nanometer range were obtained with encapsulation efficiencies of 72.4% for the TAC and 85.6% for the TAC + MAP conjugate. Furthermore, cytotoxicity of all NLC formulations was determined against neuroblastoma cell line SH-SY5Y. The optimized TAC delivery system revealed low toxicity suggesting this could be a potential carrier system to deliver TAC. However, TAC + MAP conjugated even encapsulated in the NLC system demonstrated toxicity against the SH-SY5Y cell line.

The world of cell penetrating: the future of medical applications

Cell-penetrating peptides present huge biomedical applications in a variety of pathologies, thanks to their ability to penetrate membranes and carry a variety of cargoes inside cells. Progress in peptide synthesis has produced a greater availability of virtually any synthetic peptide, increasing their attractiveness. Most molecules when associated to a cell-penetrating peptides can be delivered into a cell, however, understanding of the critical factors influencing the uptake mechanism is of paramount importance to construct nanoplatforms for effective delivery in vitro and in vivo in medical applications. Focus is now on the state-of-art of the mechanisms enabling therapeutics/diagnostics to reach the site target of their activities, and in support of scientists developing platforms for drug delivery and personalized therapies.

Cell Penetrating Peptides, Novel Vectors for Gene Therapy

Cell penetrating peptides (CPPs), also known as protein transduction domains (PTDs), first identified ~25 years ago, are small, 6-30 amino acid long, synthetic, or naturally occurring peptides, able to carry variety of cargoes across the cellular membranes in an intact, functional form. Since their initial description and characterization, the field of cell penetrating peptides as vectors has exploded. The cargoes they can deliver range from other small peptides, full-length proteins, nucleic acids including RNA and DNA, liposomes, nanoparticles, and viral particles as well as radioisotopes and other fluorescent probes for imaging purposes. In this review, we will focus briefly on their history, classification system, and mechanism of transduction followed by a summary of the existing literature on use of CPPs as gene delivery vectors either in the form of modified viruses, plasmid DNA, small interfering RNA, oligonucleotides, full-length genes, DNA origami or peptide nucleic acids.

Study of the intracellular delivery mechanism of a pH-sensitive peptide modified with enhanced green fluorescent protein

The targeted delivery of therapeutic agents is a promising approach to enhance the efficacy and reduce the toxicity of cancer treatments. Understanding the intracellular endocytic mechanisms of a cell penetrating peptide (CPP) in an acidic environment is important for targeted delivery of macromolecules to tumours. In this study, we constructed a pH-sensitive CPP-based delivery system for the intracellular delivery of macromolecules. A pH-sensitive CPP, HBHAc, was fused with a model protein, enhanced green fluorescent protein (EGFP), through recombinant DNA technology. We found that is essential that negatively charged proteoglycans on the cell surface interact with HBHAc-EGFP prior to the cellular uptake of HBHAc-EGFP. The uptake was significantly restricted at 4 °C under pH conditions of both 6.5 and 7.5. The increased positive charge of HBHAc-EGFP under the acidic condition leads to a pH-dependent cellular uptake, and we observed that the internalisation of HBHAc-EGFP was significantly higher at pH 6.5 than at pH 7.5 (p < .05). Thus, with pH-sensitive activity, HBHAc is expected to improve tumour-targeted intracellular protein delivery. Moreover, our findings provide a new insight that the endocytic pathway may change under different pH conditions and suggest that this unique phenomenon benefits pH-sensitive drug delivery for tumour therapy.

Acid-Induced Activated Cell-Penetrating Peptide-Modified Cholesterol-Conjugated Polyoxyethylene Sorbitol Oleate Mixed Micelles for pH-Triggered Drug Release and Efficient Brain Tumor Targeting Based on a Charge Reversal Mechanism

Glioblastoma multiforme is the most devastating malignant brain tumor in adults. Even with the standard care of therapy, the prognosis remains dismal due to tumor heterogeneity, tumor infiltration, and, more importantly, the restrictive nature of the blood-brain barrier (BBB). To overcome the challenge of effectively delivering therapeutic cargo into the brain, herein a "smart", multifunctional polymeric micelle was developed using a cholesterol-conjugated polyoxyethylene sorbitol oleate. A cell-penetrating peptide, arginine-glycine repeats (RG)₅, was incorporated into the micelles to improve cellular uptake, while a pH-sensitive masking sequence, histidine-glutamic acid repeats (HE)₅, was introduced for charge shielding to minimize nonspecific binding and uptake at physiological pH. Results demonstrated that (RG)₅- and (HE)₅-modified mixed micelles were optimized using this strategy to effectively mask the cationic charges of the activated cell-penetrating peptide (RG)₅ at physiological pH, i.e., limiting internalization, and were selectively triggered in response to a mildly acidic microenvironment in vitro based on a charge reversal mechanism. In vivo results further confirmed that such micelles preferentially accumulated in both brain and tumor tissues in both xenograft and orthotropic glioma mouse models. Furthermore, micelles significantly inhibited tumor growth with limited toxicity to peripheral tissues. The combination of BBB penetration, tumor targeting, potent efficacy, and high tolerance of these micelles strongly suggests that they could be a promising candidate for safe and effective drug delivery to the brain.

pH-Responsive self-assembly of cationic surfactants with a star-shaped tetra-carboxylate acid and the solubilization of hydrophobic drugs

This work involved the construction of pH-responsive self-assembly systems from a pH-sensitive four-arm carboxylate acid (4EOCOOH) and either the cationic single chain surfactant dodecyl trimethyl ammonium bromide (DTAB) or the cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12). It was found that the constructed oligomeric-like structures from the mixtures of 4EOCOOH with DTAB or 12-6-12 greatly enhance the aggregation ability of the mixtures, thus improving the pH-responsivity. In particular, surfactant concentrations significantly affect the pH-responsivity at a fixed 4EOCOOH concentration. At higher surfactant concentrations, the pH-responsivity is suppressed, while at lower surfactant concentrations, the mixed aggregates gradually change from micelles to unstable large spherical aggregates or vesicles, and then to stable spherical aggregates, with decreasing pH. Moreover, the surfactant/4EOCOOH systems have different solubilization abilities for three hydrophobic drugs. For quercetin and baicalein, the systems support much better solubilization at lower pH values, while for indomethacin, the systems show better solubilization at higher pH values. In particular, compared with DTAB, 12-6-12 is more efficient in constructing pH-responsive systems, and the 12-6-12/4EOCOOH mixture shows better ability for solubilizing hydrophobic drugs. This work will be helpful in the design of high-efficiency, pH-responsive surfactant systems for solubilizing hydrophobic drugs by simply mixing pH-sensitive molecules with surfactants.

Designing Smart Materials with Recombinant Proteins

Recombinant protein design allows modular protein domains with different functionalities and responsive behaviors to be easily combined. Inclusion of these protein domains can enable recombinant proteins to have complex responses to their environment (e.g., temperature-triggered aggregation followed by enzyme-mediated cleavage for drug delivery or pH-triggered conformational change and self-assembly leading to structural stabilization by adjacent complementary residues). These "smart" behaviors can be tuned by amino acid identity and sequence, chemical modifications, and addition of other components. A wide variety of domains and peptides have smart behavior. This review focuses on protein designs for self-assembly or conformational changes due to stimuli such as shifts in temperature or pH.

Characterization of Polyelectrolyte Complex Formation Between Anionic and Cationic Poly(amino acids) and Their Potential Applications in pH-Dependent Drug Delivery

Polyelectrolyte complexes (PECs) are self-assembling nano-sized constructs that offer several advantages over traditional nanoparticle carriers including controllable size, biodegradability, biocompatibility, and lack of toxicity, making them particularly appealing as tools for drug delivery. Here, we discuss potential application of PECs for drug delivery to the slightly acidic tumor microenvironment, a pH in the range of 6.5–7.0. Poly(l-glutamic acid) (E_n), poly(l-lysine) (K_n), and a copolymer composed of histidine-glutamic acid repeats ((HE)_n) were studied for their ability to form PECs, which were analyzed for size, polydispersity, and pH sensitivity. PECs showed concentration dependent size variation at residue lengths of E₅₁/K₅₅ and E₁₃₅/K₁₂₇, however, no complexes were observed when E₂₂ or K₂₁ were used, even in combination with the longer chains. (HE)₂₀/K₅₅ PECs could encapsulate daunomycin, were stable from pH 7.4–6.5, and dissociated completely between pH 6.5–6.0. Conversely, the E_51-dauno/K₅₅ PEC dissociated between pH 4.0 and 3.0. These values for pH-dependent particle dissociation are consistent with the pK_a’s of the ionizable groups in each formulation and indicate that the specific pH-sensitivity of (HE)_20-dauno/K₅₅ PECs is mediated by incorporation of histidine. This response within a pH range that is physiologically relevant to the acidic tumors suggests a potential application of these PECs in pH-dependent drug delivery.

Design of new acid-activated cell-penetrating peptides for tumor drug delivery

TH(AGYLLGHINLHHLAHL(Aib)HHIL-NH₂), a histidine-rich, cell-penetrating peptide with acid-activated pH response, designed and synthesized by our group, can effectively target tumor tissues with an acidic extracellular environment. Since the protonating effect of histidine plays a critical role in the acid-activated, cell-penetrating ability of TH, we designed a series of new histidine substituents by introducing electron donating groups (Ethyl, Isopropyl, Butyl) to the C-2 position of histidine. This resulted in an enhanced pH-response and improved the application of TH in tumor-targeted delivery systems. The substituents were further utilized to form the corresponding TH analogs (Ethyl-TH, Isopropyl-TH and Butyl-TH), making them easier to protonate for positive charge in acidic tumor microenvironments. The pH-dependent cellular uptake efficiencies of new TH analogs were further evaluated using flow cytometry and confocal laser scanning microscopy, demonstrating that ethyl-TH and butyl-TH had an optimal pH-response in an acidic environment. Importantly, the new TH analogs exhibited relatively lower toxicity than TH. In addition, these new TH analogs were linked to the antitumor drug camptothecin (CPT), while butyl-TH modified conjugate presented a remarkably stronger pH-dependent cytotoxicity to cancer cells than TH and the other conjugates. In short, our work opens a new avenue for the development of improved acid-activated, cell-penetrating peptides as efficient anticancer drug delivery vectors.

The Current Role of Cell-Penetrating Peptides in Cancer Therapy

Cell-penetrating peptides (CPPs) are a heterogeneous class of peptides with the ability to translocate across the plasma membrane and to carry attached cargos inside the cell. Two main entry pathways are discussed, as direct translocation and endocytosis , whereas the latter is often favored when bulky cargos are added to the CPP. Attachment to the CPP can be achieved by means of covalent coupling or non-covalent complex formation, depending on the chemical nature of the cargo. Owing to their striking abilities the further development and application of CPP-based delivery strategies has steadily emerged during the past years. However, one main pitfall when using CPPs is their non-selective uptake in nearly all types of cells. Thus, one particular interest lies in the design of targeting strategies that help to circumvent this drawback but still benefit from the potent delivery abilities of CPPs. The following review aims to summarize some of these very recent concepts and to highlight the current role of CPPs in cancer therapy.

Mechanisms and biomaterials in pH-responsive tumour targeted drug delivery: A review

As the mainstay in the treatment of various cancers, chemotherapy plays a vital role, but still faces many challenges, such as poor tumour selectivity and multidrug resistance (MDR). Targeted drug delivery using nanotechnology has provided a new strategy for addressing the limitations of the conventional chemotherapy. In the last decade, the volume of research published in this area has increased tremendously, especially with functional nano drug delivery systems (nanocarriers). Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for improving therapeutic outcomes. Among the various stimuli, pH triggered delivery is regarded as the most general strategy, targeting the acidic extracellular microenvironment and intracellular organelles of solid tumours. In this review, we discuss recent advances in the development of pH-sensitive nanocarriers for tumour-targeted drug delivery. The review focuses on the chemical design of pH-sensitive biomaterials, which are used to fabricate nanocarriers for extracellular and/or intracellular tumour site-specific drug release. The pH-responsive biomaterials bring forth conformational changes in these nanocarriers through various mechanisms such as protonation, charge reversal or cleavage of a chemical bond, facilitating tumour specific cell uptake or drug release. A greater understanding of these mechanisms will help to design more efficient drug delivery systems to address the challenges encountered in conventional chemotherapy.

Selective Intracellular Delivery of Recombinant Arginine Deiminase (ADI) Using pH-Sensitive Cell Penetrating Peptides To Overcome ADI Resistance in Hypoxic Breast Cancer Cells

Arginine depletion strategies, such as pegylated recombinant arginine deiminase (ADI-PEG20), offer a promising anticancer treatment. Many tumor cells have suppressed expression of a key enzyme, argininosuccinate synthetase 1 (ASS1), which converts citrulline to arginine. These tumor cells become arginine auxotrophic, as they can no longer synthesize endogenous arginine intracellularly from citrulline, and are therefore sensitive to arginine depletion therapy. However, since ADI-PEG20 only depletes extracellular arginine due to low internalization, ASS1-expressing cells are not susceptible to treatment since they can synthesize arginine intracellularly. Recent studies have found that several factors influence ASS1 expression. In this study, we evaluated the effect of hypoxia, frequently encountered in many solid tumors, on ASS1 expression and its relationship to ADI-resistance in human MDA-MB-231 breast cancer cells. It was found that MDA-MB-231 cells developed ADI resistance in hypoxic conditions with increased ASS1 expression. To restore ADI sensitivity as well as achieve tumor-selective delivery under hypoxia, we constructed a pH-sensitive cell penetrating peptide (CPP)-based delivery system to carry ADI inside cells to deplete both intra- and extracellular arginine. The delivery system was designed to activate the CPP-mediated internalization only at the mildly acidic pH (6.5-7) associated with the microenvironment of hypoxic tumors, thus achieving better selectivity toward tumor cells. The pH sensitivity of the CPP HBHAc was controlled by recombinant fusion to a histidine-glutamine (HE) oligopeptide, generating HBHAc-HE-ADI. The tumor distribution of HBHAc-HE-ADI was comparable to ADI-PEG20 in a mouse xenograft model of human breast cancer cells in vivo. In addition, HBHAc-HE-ADI showed increased in vitro cellular uptake in cells incubated in a mildly acidic pH (hypoxic conditions) compared to normal pH (normoxic conditions), which correlated with pH-sensitive in vitro cytotoxicity in hypoxic MDA-MB-231 and human prostate cancer PC3 cells. Together, we conclude that the HBHAc-HE-based peptide delivery offers a useful means to overcome hypoxia-induced resistance to ADI in breast cancer cells, and to target the mildly acidic tumor microenvironment.

Cell-penetrating peptides transport therapeutics into cells

Nearly 30years ago, certain small, relatively nontoxic peptides were discovered to be capable of traversing the cell membrane. These cell-penetrating peptides, as they are now called, have been shown to not only be capable of crossing the cell membrane themselves but can also carry many different therapeutic agents into cells, including small molecules, plasmid DNA, siRNA, therapeutic proteins, viruses, imaging agents, and other various nanoparticles. Many cell-penetrating peptides have been derived from natural proteins, but several other cell-penetrating peptides have been developed that are either chimeric or completely synthetic. How cell-penetrating peptides are internalized into cells has been a topic of debate, with some peptides seemingly entering cells through an endocytic mechanism and others by directly penetrating the cell membrane. Although the entry mechanism is still not entirely understood, it seems to be dependent on the peptide type, the peptide concentration, the cargo the peptide transports, and the cell type tested. With new intracellular disease targets being discovered, cell-penetrating peptides offer an exciting approach for delivering drugs to these intracellular targets. There are hundreds of cell-penetrating peptides being studied for drug delivery, and ongoing studies are demonstrating their success both in vitro and in vivo.

Peptide-mediated delivery: an overview of pathways for efficient internalization

Poor cellular delivery and low bioavailability of novel potent therapeutic molecules continue to remain the bottleneck of modern cancer and gene therapy. Cell-penetrating peptides have provided immense opportunities for the intracellular delivery of bioactive cargos and have led to the first exciting successes in experimental therapy of muscular dystrophies. This review focuses on the mechanisms by which cell-penetrating peptides gain access to the cell interior and deliver cargos. Recent advances in augmenting delivery efficacy and facilitation of endosomal escape of cargo are presented, and the cell-penetrating peptide-mediated delivery of two of the most popular classes of cargo molecules, oligonucleotides and proteins, is analyzed. The arsenal of tools for oligonucleotide delivery has dramatically expanded in the last decade enabling harnessing of cell-surface receptors for targeted delivery.

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

INTRODUCTION

The impermeability of biological membranes is a major obstacle in drug delivery; however, some peptides have transition capabilities of biomembranes. In recent decades, cell-penetrating peptides (CPPs) have been introduced as novel biocarriers that are able to translocate into the cells. CPPs are biologically potent tools for non-invasive cellular internalization of cargo molecules. Nevertheless, the non-specificity of these peptides presents a restriction for targeting drug delivery; therefore, a peptidic nanocarrier sensitive to matrix metalloproteinase (MMP) has been prepared, called activatable cell-penetrating peptide (ACPP). In addition to the cell-penetrating peptide dendrimer (DCPP), other analogues of CPPs have been synthesized.

METHODS

In this study, the most recent literature in the field of biomedical application of CPPs and their analogues, ACPP and DCCP, were reviewed.

RESULTS

This review focuses on CPP and its analogues, ACPP and DCPP, as novel nanocarriers for drug delivery. In addition, nanoconjugates and bioconjugates of these peptide sequences are discussed.

CONCLUSION

DCCP, branched CPPs, compared to linear peptides have advantages such as resistance to rapid biodegradation, high loading capacities and large-scale production capability.

Screening of a specific peptide binding to esophageal squamous carcinoma cells from phage displayed peptide library

To select a specifically binding peptide for imaging detection of human esophageal squamous cell carcinoma (ESCC), a phage-displayed 12-mer peptide library was used to screen the peptide that bind to ESCC cells specifically. After four rounds of bio-panning, the phage recovery rate gradually increased, and specific phage clones were effectively enriched. The 60 randomly selected phage clones were tested using cellular enzyme-linked immunosorbent assay (ELISA), and 41 phage clones were identified as positive clones with the over 2.10 ratio of absorbance higher than other clones, IRP and PBS controls. From the sequencing results of the positive clones, 14 peptide sequences were obtained and ESCP9 consensus sequence was identified as the peptide with best affinity to ESCC cells via competitive inhibition, fluorescence microscopy, and flow cytometry. The results indicate that the peptide ESCP9 can bind to ESCC cells specifically and sensitively, and it is a potential candidate to be developed as an useful molecule to the imaging detection and targeting therapy for ESCC.

Interaction between cell-penetrating peptides and acid-sensitive anionic oligopeptides as a model for the design of targeted drug carriers

Overcoming the nonspecific cellular uptake of cell-penetrating peptides (CPPs) is a major hurdle in their clinical application. Using pH as the activation switch, histidine-glutamic acid (HE) dipeptide repeats were fused to CPPs to trigger the membrane-penetrating activity at mildly acidic pH environments (i.e., pH 6.5 or below) while masking the internalization at neutral pH (i.e., pH 7.0 or above). In this study, a series of recombinant GST-fusion proteins containing an HE oligopeptide sequence (i.e., (HE)n with n = 8, 10, or 12) and a cationic CPP (i.e., YG(RG)6, YGR6G6, or Tat) were engineered for a pH-sensitive study comparing their cellular uptake and surface binding in cultured HeLa cells. Circular dichroism (CD) spectroscopy was performed to correlate differences between CPPs in secondary structure with the pH sensitivity. YGR6G6 with clustered arginine residues exhibited greater pH sensitivity in cellular uptake than YG(RG)6 with separated arginine residues. Increasing the stretch of HE repeats decreased cellular uptake and surface binding for both YG(RG)6 and YGR6G6. The ratio of cellular internalization at pH 7.5 vs 6.0 was not changed by the presence of serum. CD spectral data revealed that both (HE)10-Tat and (HE)10-YGR6G6 exhibited an unordered secondary structure, whereas (HE)10-YG(RG)6 adopted an antiparallel β-sheet conformation. This β-sheet conformation presumably stabilized the association of (HE)10 with YG(RG)6, leading to weakened pH sensitivity of (HE)10-YG(RG)6. On the other hand, the random-coiled structures, that is, (HE)10-YGR6G6 and (HE)10-Tat, both showed higher pH sensitivity as determined in cell experiments. The data presented in this study provide a basis for the future design of pH-sensitive HE-CPP carrier for targeted drug delivery.