Identification and characterization of a new family of cell-penetrating peptides: cyclic cell-penetrating peptides

Hydrophobicity: The door to drug delivery

The engineering of intracellular delivery systems with the goal of achieving personalized medicine has been encouraged by advances in nanomaterial science as well as a greater understanding of diseases and of the biochemical pathways implicated in many disorders. The development of vectors able to transport the drug to a target location and release it only on demand is undoubtedly the primary issue. From a molecular perspective, the topography of drug carrier surfaces is directly related to the design of an effective drug carrier because it provides a physical hint to modifying its interactions with biological systems. For instance, the initial ratio of hydrophilic to hydrophobic surfaces and the changes brought about by external factors enable the release or encapsulation of a therapeutic molecule and the ability of the nanosystem to cross biological barriers and reach its target without causing systemic toxicity. The first step in creating new materials with enhanced functionality is to comprehend and characterize the interplay between hydrophilic and hydrophobic molecules at the molecular level. Therefore, the focus of this review is on the function of hydrophobicity, which is essential for matching the complexity of biological environments with the intended functionality.

Delivery to, and Reactivation of, the p53 Pathway in Cancer Cells Using a Grafted Cyclotide Conjugated with a Cell-Penetrating Peptide

Peptides are promising drug modalities that can modulate protein-protein interactions, but their application is hampered by their limited ability to reach intracellular targets. Here, we improved the cytosolic delivery of a peptide blocking p53:MDM2/X interactions using a cyclotide as a stabilizing scaffold. We applied several design strategies to improve intracellular delivery and found that the conjugation of the lead cyclotide to the cyclic cell-penetrating peptide cR10 was the most effective. Conjugation allowed cell internalization at micromolar concentration and led to elevated intracellular p53 levels in A549, MCF7, and MCF10A cells, as well as inducing apoptosis in A549 cells without causing membrane disruption. The lead peptide had >35-fold improvement in inhibitory activity and increased cellular uptake compared to a previously reported cyclotide p53 activator. In summary, we demonstrated the delivery of a large polar cyclic peptide in the cytosol and confirmed its ability to modulate intracellular protein-protein interactions involved in cancer.

Resistance is futile: targeting multidrug-resistant bacteria with de novo Cys-rich cyclic polypeptides

The search for novel antimicrobial agents to combat microbial pathogens is intensifying in response to rapid drug resistance development to current antibiotic therapeutics. The use of disulfide-rich head-to-tail cyclized polypeptides as molecular frameworks for designing a new type of peptide antibiotics is gaining increasing attention among the scientific community and the pharmaceutical industry. The use of macrocyclic peptides, further constrained by the presence of several disulfide bonds, makes these peptide frameworks remarkably more stable to thermal, biological, and chemical degradation showing better activities when compared to their linear analogs. Many of these novel peptide scaffolds have been shown to have a high tolerance to sequence variability in those residues not involved in disulfide bonds, able to cross biological membranes, and efficiently target complex biomolecular interactions. Hence, these unique properties make the use of these scaffolds ideal for many biotechnological applications, including the design of novel peptide antibiotics. This article provides an overview of the new developments in the use of several disulfide-rich cyclic polypeptides, including cyclotides, θ-defensins, and sunflower trypsin inhibitor peptides, among others, in the development of novel antimicrobial peptides against multidrug-resistant bacteria.

Cell penetrating peptides: Highlighting points in cancer therapy

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

Differential Blood-Brain Barrier Transport and Cell Uptake of Cyclic Peptides In Vivo and In Vitro

The blood-brain barrier (BBB) poses major challenges to drug delivery to the CNS. SFTI-1 and kalata B1 are cyclic cell-penetrating peptides (cCPPs) with high potential to be used as scaffolds for drug delivery. We here studied their transport across the BBB and distribution within the brain to gauge the potential of these two cCPPs as scaffolds for CNS drugs. In a rat model, SFTI-1 exhibited, for a peptide, high extent of BBB transport with a partitioning of unbound SFTI-1 across the BBB, K_p,uu,brain, of 13%, while only 0.5% of kalata B1 equilibrated across the BBB. By contrast, kalata B1, but not SFTI-1, readily entered neural cells. SFTI-1, but not kalata B1, could be a potential CNS delivery scaffold for drugs directed to extracellular targets. These findings indicate that differences between the BBB transport and cellular uptake abilities of CPPs are crucial in the development of peptide scaffolds.

Insights into the synthesis strategies of plant-derived cyclotides

Cyclotides are plant peptides characterized with a head-to-tail cyclized backbone and three interlocking disulfide bonds, known as a cyclic cysteine knot. Despite the variations in cyclotides peptide sequences, this core structure is conserved, underlying their most useful feature: stability against thermal and chemical breakdown. Cyclotides are the only natural peptides known to date that are orally bioavailable and able to cross cell membranes. Cyclotides also display bioactivities that have been exploited and expanded to develop as potential therapeutic reagents for a wide range of conditions (e.g., HIV, inflammatory conditions, multiple sclerosis, etc.). As such, in vitro production of cyclotides is of the utmost importance since it could assist further research on this peptide class, specifically the structure-activity relationship and its mechanism of action. The information obtained could be utilized to assist drug development and optimization. Here, we discuss several strategies for the synthesis of cyclotides using both chemical and biological routes.

Development of a Potent Cyclic Peptide Inhibitor of the nNOS/PSD-95 Interaction

The complex between the N-methyl-d-aspartate receptor (NMDAR), neuronal nitric oxide synthase (nNOS), and the postsynaptic density protein-95 (PSD-95) is an attractive therapeutic target for the treatment of acute ischemic stroke. The complex is formed via the PDZ protein domains of PSD-95, and efforts to disrupt the complex have generally been based on C-terminal peptides derived from the NMDAR. However, nNOS binds PSD-95 through a β-hairpin motif, providing an alternative starting point for developing PSD-95 inhibitors. Here, we designed a cyclic nNOS β-hairpin mimetic peptide and generated cyclic nNOS β-hairpin peptide arrays with natural and unnatural amino acids (AAs), which provided molecular insights into this interaction. We then optimized cyclic peptides and identified a potent inhibitor of the nNOS/PSD-95 interaction, with the highest affinity reported thus far for a peptide macrocycle inhibitor of PDZ domains, which serves as a template for the development of treatment for acute ischemic stroke.

Using the Cyclotide Scaffold for Targeting Biomolecular Interactions in Drug Development

This review provides an overview of the properties of cyclotides and their potential for developing novel peptide-based therapeutics. The selective disruption of protein–protein interactions remains challenging, as the interacting surfaces are relatively large and flat. However, highly constrained polypeptide-based molecular frameworks with cell-permeability properties, such as the cyclotide scaffold, have shown great promise for targeting those biomolecular interactions. The use of molecular techniques, such as epitope grafting and molecular evolution employing the cyclotide scaffold, has shown to be highly effective for selecting bioactive cyclotides.

Delivery of Small Molecules by Syndiotactic Peptides for Breast Cancer Therapy

The utilization of peptide-based drug delivery systems has been suboptimal due to their poor proteolytic susceptibility, poor cell permeability, and limited tumor homing capabilities. Earlier attempts in using d-enantiomers in peptide sequences increased proteolytic stability but have compromised the overall penetration capability. We designed a series of peptides (STRAPs) with a syndiotactic polypeptide backbone that can potentially form a spatial array of cationic groups, an important feature that facilitates cellular uptake. The peptides penetrate cell membranes through a combination of active and passive modes. Furthermore, the cellular uptake of the peptides was unaffected by the presence of or treatment with bovine serum and human plasma. The designed peptides successfully delivered methotrexate, an anticancer drug, to the in vitro and in vivo models of breast cancer, with the best performing peptide STRAP-4-MTX conjugate having an EC50 value of 1.34 μM. Peptide drug delivery in mouse xenograft models showed a greater reduction of primary tumor and metastasis of breast cancer, in comparison to methotrexate of the same dose. The in vivo biodistribution assay of the STRAP-4 peptide suggests that the peptide accumulates at the tumor site after 2 h of treatment, and in the absence of tumors, the peptide gets metabolized and excreted from the system.

Influence of Open Chain and Cyclic Structure of Peptidomimetics on Antibacterial Activity in E. coli Strains

An efficient method for the synthesis of functionalized peptidomimetics via multicomponent Ugi reaction has been developed. The application of trifluoroethanol (TFE) as a reaction medium provided desired products with good yields. Further, using the developed cyclisation reaction, the obtained peptidomimetics were transformed into the cyclic analogues (diketopiperazines, DKPs). The goal of the performed studies was to revised and compare whether the structure of the obtained structurally flexible acyclic peptidomimetics and their rigid cycling analogue DKPs affect antimicrobial activity. We studied the potential of synthesized peptidomimetics, both cyclic and acyclic, as antimicrobial drugs on model E. coli bacteria strains (k12, R2–R4). The biological assays reveal that DKPs hold more potential as antimicrobial drugs compared to open chain Ugi peptidomimetics. We believe that it can be due to the rigid cyclic structure of DKPs which promotes the membrane penetration in the cell of studied pathogens. The obtained data clearly indicate the high antibiotic potential of synthesized diketopiperazine derivatives over tested antibiotics.

Structural Bases for the Involvement of Phosphatidylinositol-4,5-bisphosphate in the Internalization of the Cell-Penetrating Peptide Penetratin

Cell-penetrating peptides cross cell membranes through various parallel internalization pathways. Herein, we analyze the role of the negatively charged lipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) in the internalization of Penetratin. Contributions of both inner leaflet and outer leaflet pools of PI(4,5)P₂ were revealed by quantifying the internalization of Penetratin in cells treated with PI(4,5)P₂ binders. Studies on model systems showed that Penetratin has a strong affinity for PI(4,5)P₂ and interacts selectively with this lipid, even in the presence of other negatively charged lipids, as demonstrated by affinity photo-crosslinking experiments. Differential scanning calorimetry experiments showed that Penetratin induces lateral segregation in PI(4,5)P₂-containing liposomes, which was confirmed by coarse-grained molecular dynamics simulations. NMR experiments indicated that Penetratin adopts a stabilized helical conformation in the presence of PI(4,5)P₂-containing membranes, with an orientation parallel to the bilayer plane, which was also confirmed by all-atom simulations. NMR and photo-crosslinking experiments also suggest a rather shallow insertion of the peptide in the membrane. Put together, our findings suggest that PI(4,5)P₂ is a privileged interaction partner for Penetratin and that it plays an important role in Penetratin internalization.

An Ultrapotent and Selective Cyclic Peptide Inhibitor of Human β-Factor XIIa in a Cyclotide Scaffold

Cyclotides are plant-derived peptides with complex structures shaped by their head-to-tail cyclic backbone and cystine knot core. These structural features underpin the native bioactivities of cyclotides, as well as their beneficial properties as pharmaceutical leads, including high proteolytic stability and cell permeability. However, their inherent structural complexity presents a challenge for cyclotide engineering, particularly for accessing libraries of sufficient chemical diversity to design potent and selective cyclotide variants. Here, we report a strategy using mRNA display enabling us to select potent cyclotide-based FXIIa inhibitors from a library comprising more than 10¹² members based on the cyclotide scaffold of Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II). The most potent and selective inhibitor, cMCoFx1, has a pM inhibitory constant toward FXIIa with greater than three orders of magnitude selectivity over related serine proteases, realizing specific inhibition of the intrinsic coagulation pathway. The cocrystal structure of cMCoFx1 and FXIIa revealed interactions at several positions across the contact interface that conveyed high affinity binding, highlighting that such cyclotides are attractive cystine knot scaffolds for therapeutic development.

Studies on the oral absorption of cyclic peptides from Pseudostellaria heterophylla and interacting with membrane receptors

Pseudostellaria heterophylla (Miq.) Pax. (Taizishen, TZS) contains a variety of natural active cyclic-peptide compounds (CP). In this article, four kinds of CP monomers were isolated by HPLC and the structures were identified by mass spectrometry. The in vivo absorption of CP was detected by UPLC-MS/MS. The interaction between CP and membrane receptor was analyzed by SPR. As a result, the relative absorption rate of CP was Pesudostellarin B > Heterophyllin B > Pesudostellarin C > Pesudostellarin E. The difference in absorption rate of CP in vivo was related to its interaction with membrane receptors. The absorption mechanism of CP might be different. This is the first report that in vivo absorption study of different CP from TZS and explore its absorption mechanism, laying a theoretical foundation for the research and development of its oral drugs, and providing new ideas for the study of the absorption mechanism of CP from traditional Chinese medicine.

Engineered Cyclotides with Potent Broad in Vitro and in Vivo Antimicrobial Activity

The search for novel antimicrobial agents to combat microbial pathogens is intensifying in response to the rapid development of drug resistance to current antibiotic therapeutics. Respiratory failure and septicemia are the leading causes of mortality among hospitalized patients. Here, the development of a novel engineered cyclotide with effective broad-spectrum antibacterial activity against several ESKAPE bacterial strains and clinical isolates is reported. The most active antibacterial cyclotide was extremely stable in serum, showed little hemolytic activity, and provided protection in vivo in a murine model of P. aeruginosa peritonitis. These results highlight the potential of the cyclotide scaffold for the development of novel antimicrobial therapeutic leads for the treatment of bacteremia.

Molecular hybridization combining tumor homing and penetrating peptide domains for cellular targeting

A complete peptide-based drug delivery unit has been designed with a tumor homing domain chemically linked to a syndiotactic cell-penetrating domain. The designed peptides were synthesized, characterized, and tested in vitro for cellular uptake and cytotoxicity evaluation. The differential uptake, cellular internalization, negligible hemotoxicity, selective toxicity to MDA-MB-231 breast cancer cells, and the superior penetration in three-dimensional MDA-MB-231 tumorospheres confirm their utility as a promising delivery vector.

An Integrated Molecular Grafting Approach for the Design of Keap1-Targeted Peptide Inhibitors

Inhibiting the Nrf2:Keap1 interaction to trigger cytoprotective gene expression is a promising treatment strategy for oxidative stress-related diseases. A short linear motif from Nrf2 has the potential to directly inhibit this protein-protein interaction, but poor stability and limited cellular uptake impede its therapeutic development. To address these limitations, we utilized an integrated molecular grafting strategy to re-engineer the Nrf2 motif. We combined the motif with an engineered non-native disulfide bond and a cell-penetrating peptide onto a single multifunctionalizable and ultrastable molecular scaffold, namely, the cyclotide MCoTI-II, resulting in the grafted peptide MCNr-2c. The engineered disulfide bond enhanced the conformational rigidity of the motif, resulting in a nanomolar affinity of MCNr-2c for Keap1. The cell-penetrating peptide led to an improved cellular uptake and increased ability to enhance the intracellular expression of two well-described Nrf2-target genes NQO1 and TALDO1. Furthermore, the stability of the scaffold was inherited by the grafted peptide, which became resistant to proteolysis in serum. Overall, we have provided proof-of-concept for a strategy that enables the encapsulation of multiple desired and complementary activities into a single molecular entity to design a Keap1-targeted inhibitor. We propose that this integrated approach could have broad utility for the design of peptide drug leads that require multiple functions and/or biopharmaceutical properties to elicit a therapeutic activity.

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

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

Tuning the Anti-Angiogenic Effect of the P15 Peptide Using Cyclic Trypsin Inhibitor Scaffolds

Angiogenesis is important for tumor growth, and accordingly, targeting angiogenesis has become an important pathway for antitumor therapy. A novel proapoptotic peptide, CIGB-300 (P15-Tat), has been shown to be involved in the casein kinase II phosphorylation pathway, conferring it with antiangiogenic activity. Cyclic peptides have been widely used as scaffolds in drug design studies due to their high stability and favorable biopharmaceutical properties. Here, we chose two very stable cyclic trypsin inhibitors, MCoTI-II and SFTI-1, as frameworks to incorporate the bioactive epitope P15 into various backbone loops. NMR studies revealed that all re-engineered analogs had similar secondary structures to their native cyclic frameworks. One key analog, MCoP15, displayed significant improvement for inhibiting human umbilical vein endothelial cell migration, was nontoxic, and had higher stability than the P15 epitope alone. Overall, the results show the value of P15 being engineered into cyclic trypsin inhibitor scaffolds for improving antiangiogenic activity and stability. More broadly, the study highlights the versatility of cyclic peptide frameworks in drug design for antiangiogenic therapies.

Repurposing the McoTI-II Rigid Molecular Scaffold in to Inhibitor of 'Papain Superfamily' Cysteine Proteases

Clan C1A or ‘papain superfamily’ cysteine proteases are key players in many important physiological processes and diseases in most living systems. Novel approaches towards the development of their inhibitors can open new avenues in translational medicine. Here, we report a novel design of a re-engineered chimera inhibitor Mco-cysteine protease inhibitor (CPI) to inhibit the activity of C1A cysteine proteases. This was accomplished by grafting the cystatin first hairpin loop conserved motif (QVVAG) onto loop 1 of the ultrastable cyclic peptide scaffold McoTI-II. The recombinantly expressed Mco-CPI protein was able to bind with micromolar affinity to papain and showed remarkable thermostability owing to the formation of multi-disulphide bonds. Using an in silico approach based on homology modelling, protein–protein docking, the calculation of the free-energy of binding, the mechanism of inhibition of Mco-CPI against representative C1A cysteine proteases (papain and cathepsin L) was validated. Furthermore, molecular dynamics simulation of the Mco-CPI–papain complex validated the interaction as stable. To conclude, in this McoTI-II analogue, the specificity had been successfully redirected towards C1A cysteine proteases while retaining the moderate affinity. The outcomes of this study pave the way for further modifications of the Mco-CPI design for realizing its full potential in therapeutics. This study also demonstrates the relevance of ultrastable peptide-based scaffolds for the development of novel inhibitors via grafting.

Cell-penetrating peptides containing 2,5-diketopiperazine (DKP) scaffolds as shuttles for anti-cancer drugs: conformational studies and biological activity

A series of linear and cyclic peptidomimetics composed of a cell-penetrating peptide and a non-natural, bifunctional 2,5-diketopiperazine scaffold is reported. Conformational studies revealed well-defined helical structures in micellar medium for linear structures, while cyclic peptidomimetics were more flexible. Biological investigations showed higher membrane-activity of cyclic derivatives allowing their use as shuttles for anti-cancer drugs.

Chemical Protein Synthesis in Medicinal Chemistry

Although the majority of proteins used for biomedical research are produced using living systems such as bacteria, biological means for producing proteins can be advantageously complemented by protein semisynthesis or total chemical synthesis. The latter approach is particularly useful when the proteins to be produced are toxic for the expression system or show unusual features that cannot be easily programmed in living organisms. The aim of this review is to provide a wide overview of the use of chemical protein synthesis in medicinal chemistry with a special focus on the production of post-translationally modified proteins and backbone cyclized proteins.

Harnessing cyclotides to design and develop novel peptide GPCR ligands

Cyclotides are plant-derived cyclic, disulfide-rich peptides with a unique cyclic cystine knot topology that confers them with remarkable structural stability and resistance to proteolytic degradation. Recently, cyclotides have emerged as promising scaffold molecules for designing peptide-based therapeutics. Here, we provide examples of how engineering cyclotides using molecular grafting may lead to the development of novel peptide ligands of G protein-coupled receptors (GPCRs), today's most exploited drug targets. Integrating bioactive epitopes into stable cyclotide scaffolds can lead to improved pharmacokinetics and oral activity as well as selectivity and high enzymatic stability. We also discuss and highlight the importance of engineered cyclotides as novel tools to study GPCR signaling.

Engineering Cell-Permeable Proteins through Insertion of Cell-Penetrating Motifs into Surface Loops

Effective delivery of proteins into the cytosol of mammalian cells would open the door to a wide range of applications. However, despite great efforts from numerous investigators, effective protein delivery in a clinical setting is yet to be accomplished. Herein we report a potentially general approach to engineering cell-permeable proteins by genetically grafting a short cell-penetrating peptide (CPP) to an exposed loop of a protein of interest. The grafted peptide is conformationally constrained, exhibiting enhanced proteolytic stability and cellular entry efficiency. Applying this technique to enhanced green fluorescent protein (EGFP), protein-tyrosine phosphatase 1B (PTP1B), and purine nucleoside phosphorylase (PNP) rendered all three proteins cell-permeable and biologically active in cellular assays. When added into growth medium at 0.5-5 μM concentrations, the engineered PTP1B dose-dependently reduced the phosphotyrosine levels of intracellular proteins, while the modified PNP corrected the metabolic deficiency of PNP-deficient mouse T lymphocytes, providing a potential enzyme replacement therapy for a rare genetic disease.

Isolation of antigen-specific, disulphide-rich knob domain peptides from bovine antibodies

As a novel alternative to established surface display or combinatorial chemistry approaches for the discovery of therapeutic peptides, we present a method for the isolation of small, cysteine-rich domains from bovine antibody ultralong complementarity-determining regions (CDRs). We show for the first time that isolated bovine antibody knob domains can function as autonomous entities by binding antigen outside the confines of the antibody scaffold. This yields antibody fragments so small as to be considered peptides, each stabilised by an intricate, bespoke arrangement of disulphide bonds. For drug discovery, cow immunisations harness the immune system to generate knob domains with affinities in the picomolar to low nanomolar range, orders of magnitude higher than unoptimized peptides from naïve library screening. Using this approach, knob domain peptides that tightly bound Complement component C5 were obtained, at scale, using conventional antibody discovery and peptide purification techniques.

This study describes a method for the isolation of knob domains (a disulfide-rich domain found in the ultra-long CDRH3 of a subset of bovine antibodies) to create a uniquely small antibody fragment. With a molecular weight 3-6 KDa, the knob domain fragment is so small as to be considered a peptide. This approach uniquely harnesses the bovine immune system to affinity maturate peptides in vivo.

Cellular Uptake and Cytosolic Delivery of a Cyclic Cystine Knot Scaffold

Cyclotides are macrocyclic peptides with exceptionally stable structures and have been reported to penetrate cells, making them promising scaffolds for the delivery of inhibitory peptides to target intracellular proteins. However, their cellular uptake and cytosolic localization have been poorly understood until now, which has limited their therapeutic potential. In this study, the recently developed chloroalkane penetration assay was combined with established assays to characterize the cellular uptake and cytosolic delivery of the prototypic cyclotide, kalata B1. We show that kalata B1 enters the cytosol at low efficiency. A structure-activity study of residues in loop 6 showed that some modifications, such as increasing cationic residue content, did not affect delivery efficiency, whereas others, including introducing a single hydrophobic amino acid, did significantly improve cytosolic delivery. Our results provide a foundation for the further development of a structurally unique class of scaffolds for the delivery of therapeutic cargoes into cells.

Orbitides isolated from flaxseed induce apoptosis against SGC-7901 adenocarcinoma cells

Unique plant-derived cyclic peptides family exhibiting various key biological activities has great possibility for anticancer therapy. In this study, we investigated the effects of orbitides isolated from flax (Linum usitatissimum L.) on the growth of SGC-7901 cancer cells and the potential mechanism. Results showed that flaxseed orbitides killed off cancer cells by inducing apoptosis in a dose-dependent manner, which was confirmed by the appearance of nuclear shrinkage and DNA fragmentation, and the inhibitory effect was stronger than that of pure orbitide [1-9-NαC]-linusorb B2 or [1-9-NαC]-linusorb B3. Besides, the mitochondrial apoptosis pathway-related protein cytochrome C (Cyt C) was released from mitochondria to cytosol, associated with the activation of caspases 9 and 3, and the cleavage of PARP. Taken together, these results indicated that flaxseed orbitides induced apoptosis via the mitochondrial pathway, releasing Cyt C, increasing Bax/Bcl-2 ratio and elevating the expression of cleaved caspase 9 and 3 in SGC-7901 cells.

Conformationally constrained peptides for drug delivery

Peptides have shown great potential in acting as template for developing versatile carrier platforms in nanomedicine, aimed at selective delivery of drugs to only pathological tissues saving its normal neighbors. Cell-penetrating peptides (CPPs) are short oligomeric peptides capable of translocating across the cell membrane while simultaneously employing multiple mechanisms of entry. Most CPPs exist as disordered structures in solution and may adopt a helical conformation on interaction with cell membrane, vital to their penetrative capability. Herein, we report a series of cationic helical amphipathic peptides (CHAPs), which are topologically constrained to be helical. The peptides were tested against cervical and breast cancer cells for their cell penetration and drug delivery potential. The cellular uptake of CHAP peptides is independent of temperature and energy availability. The activity of the peptides is biocompatible in bovine serum. CHAPs delivered functional methotrexate (MTX) inside the cell as CHAP-MTX conjugates. CHAP-MTX conjugates were more toxic to cancer cells than MTX alone. However, the CHAP-MTX conjugates were less toxic to HEK-293 cells compared with the cancer cells suggesting higher affinity towards cancer cells.

Chemical diversity and mode of action of natural products targeting lipids in the eukaryotic cell membrane

Covering: up to 2019Nature furnishes bioactive compounds (natural products) with complex chemical structures, yet with simple, sophisticated molecular mechanisms. When natural products exhibit their activities in cells or bodies, they first have to bind or react with a target molecule in/on the cell. The cell membrane is a major target for bioactive compounds. Recently, our understanding of the molecular mechanism of interactions between natural products and membrane lipids progressed with the aid of newly-developed analytical methods. New technology reconnects old compounds with membrane lipids, while new membrane-targeting molecules are being discovered through the screening for antimicrobial potential of natural products. This review article focuses on natural products that bind to eukaryotic membrane lipids, and includes clinically important molecules and key research tools. The chemical diversity of membrane-targeting natural products and the molecular basis of lipid recognition are described. The history of how their mechanism was unveiled, and how these natural products are used in research are also mentioned.

Recombinant Expression of Cyclotides Using Expressed Protein Ligation

Cyclotides are naturally occurring microproteins (≈30 residues long) present in several families of plants. All cyclotides share a unique head-to-tail circular knotted topology containing three disulfide bridges forming a cystine knot topology. Cyclotides possess high stability to chemical, physical, and biological degradation and have been reported to cross cellular membranes. In addition, naturally occurring and engineered cyclotides have shown to possess various pharmacologically relevant activities. These unique features make the cyclotide scaffold an excellent tool for the design of novel peptide-based therapeutics by using molecular evolution and/or peptide epitope grafting techniques. In this chapter, we provide protocols to recombinantly produce a natively folded cyclotide making use of a standard bacterial expression system in combination with an intein-mediated backbone cyclization with concomitant oxidative folding.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

Roseltide rT7 is a disulfide-rich, anionic, and cell-penetrating peptide that inhibits proteasomal degradation

Disulfide-rich plant peptides with molecular masses of 2-6 kDa represent an expanding class of peptidyl-type natural products with diverse functions. They are structurally compact, hyperstable, and underexplored as cell-penetrating agents that inhibit intracellular functions. Here, we report the discovery of an anionic, 34-residue peptide, the disulfide-rich roseltide rT7 from Hibiscus sabdariffa (of the Malvaceae family) that penetrates cells and inhibits their proteasomal activities. Combined proteomics and NMR spectroscopy revealed that roseltide rT7 is a cystine-knotted, six-cysteine hevein-like cysteine-rich peptide. A pair-wise comparison indicated that roseltide rT7 is >100-fold more stable against protease degradation than its S-alkylated analog. Confocal microscopy studies and cell-based assays disclosed that after roseltide rT7 penetrates cells, it causes accumulation of ubiquitinated proteins, inhibits human 20S proteasomes, reduces tumor necrosis factor-induced IκBα degradation, and decreases expression levels of intercellular adhesion molecule-1. Structure-activity studies revealed that roseltide rT7 uses a canonical substrate-binding mechanism for proteasomal inhibition enabled by an IIML motif embedded in its proline-rich and exceptionally long intercysteine loop 4. Taken together, our results provide mechanistic insights into a novel disulfide-rich, anionic, and cell-penetrating peptide, representing a potential lead for further development as a proteasomal inhibitor in anti-cancer or anti-inflammatory therapies.

Evaluation of Cyclic Peptide Inhibitors of the Grb7 Breast Cancer Target: Small Change in Cargo Results in Large Change in Cellular Activity

Grb7 is an adapter protein, overexpressed in HER2+ve breast and other cancers, and identified as a therapeutic target. Grb7 promotes both proliferative and migratory cellular pathways through interaction of its SH2 domain with upstream binding partners including HER2, SHC, and FAK. Here we present the evaluation of a series of monocyclic and bicyclic peptide inhibitors that have been developed to specifically and potently target the Grb7 SH2-domain. All peptides tested were found to inhibit signaling in both ERK and AKT pathways in SKBR-3 and MDA-MB-231 cell lines. Proliferation, migration, and invasion assays revealed, however, that the second-generation bicyclic peptides were not more bioactive than the first generation G7-18NATE peptide, despite their higher in vitro affinity for the target. This was found not to be due to steric hindrance by the cell-permeability tag, as ascertained by ITC, but to differences in the ability of the bicyclic peptides to interact with and penetrate cellular membranes, as determined using SPR and mass spectrometry. These studies reveal that just small differences to amino acid composition can greatly impact the effectiveness of peptide inhibitors to their intracellular target and demonstrate that G7-18NATE remains the most effective peptide inhibitor of Grb7 developed to date.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Cell Membrane Composition Drives Selectivity and Toxicity of Designed Cyclic Helix-Loop-Helix Peptides with Cell Penetrating and Tumor Suppressor Properties

The tumor suppressor protein p53 is inactive in a large number of cancers, including some forms of sarcoma, breast cancer, and leukemia, due to overexpression of its intrinsic inhibitors MDM2 and MDMX. Reactivation of p53 tumor suppressor activity, via disruption of interactions between MDM2/X and p53 in the cytosol, is a promising strategy to treat cancer. Peptides able to bind MDM2 and/or MDMX were shown to prevent MDM2/X:p53 interactions, but most possess low cell penetrability, low stability, and/or high toxicity to healthy cells. Recently, the designed peptide cHLH-p53-R was reported to possess high affinity for MDM2, resistance toward proteases, cell-penetrating properties, and toxicity toward cancer cells. This peptide uses a stable cyclic helix-loop-helix (cHLH) scaffold, which includes two helices connected with a Gly loop and cyclized to improve stability. In the current study, we were interested in examining the cell selectivity of cHLH-p53-R, its cellular internalization, and ability to reactivate the p53 pathway. We designed analogues of cHLH-p53-R and employed biochemical and biophysical methodologies using in vitro model membranes and cell-based assays to compare their structure, activity, and mode-of-action. Our studies show that cHLH is an excellent scaffold to stabilize and constrain p53-mimetic peptides with helical conformation, and reveal that anticancer properties of cHLH-p53-R are mediated by its ability to selectively target, cross, and disrupt cancer cell membranes, and not by activation of the p53 pathway. These findings highlight the importance of examining the mode-of-action of designed peptides to fully exploit their potential to develop targeted therapies.

Understanding Cell Penetration of Cyclic Peptides

Approximately 75% of all disease-relevant human proteins, including those involved in intracellular protein-protein interactions (PPIs), are undruggable with the current drug modalities (i.e., small molecules and biologics). Macrocyclic peptides provide a potential solution to these undruggable targets because their larger sizes (relative to conventional small molecules) endow them the capability of binding to flat PPI interfaces with antibody-like affinity and specificity. Powerful combinatorial library technologies have been developed to routinely identify cyclic peptides as potent, specific inhibitors against proteins including PPI targets. However, with the exception of a very small set of sequences, the vast majority of cyclic peptides are impermeable to the cell membrane, preventing their application against intracellular targets. This Review examines common structural features that render most cyclic peptides membrane impermeable, as well as the unique features that allow the minority of sequences to enter the cell interior by passive diffusion, endocytosis/endosomal escape, or other mechanisms. We also present the current state of knowledge about the molecular mechanisms of cell penetration, the various strategies for designing cell-permeable, biologically active cyclic peptides against intracellular targets, and the assay methods available to quantify their cell-permeability.

Development of Organelle Replacement Therapy Using a Stearyl-Polyhistidine Peptide against Lysosomal Storage Disease Cells

We previously reported on a polyhistidine peptide, His16 peptide, as a new cell-penetrating peptide. This peptide is anticipated to be a new carrier for drug delivery systems (DDSs) for targeting intracellular lysosomes because it can transport macromolecules (e.g., liposomes) into these organelles. In the present study, we examined the application of His16 peptide as a DDS carrier against lysosomal storage disease (LSD) cells. LSDs are metabolic disorders caused by loss of specific lysosomal enzymes. For the treatment of LSD cells, we devised a system designated organelle replacement therapy (ORT). ORT is a strategy for transporting exogenous lysosomes containing all kinds of lysosomal enzymes from normal cells into endogenous lysosomes in LSD cells using His16 peptide. To develop the ORT system, we prepared His16 peptide-modified healthy lysosomes (His16-Lyso) by insertion of a stearyl-His16 peptide into a hydrophobic region in the lysosomal membrane. His16-Lyso showed cellular uptake and localization to endogenous lysosomes in LSD cells. His16-Lyso also restored the proliferation of LSD cells, which otherwise showed slower proliferation than normal cells. These results suggested that His16-Lyso replenished deficient lysosomal enzymes in LSD cells. The results further suggest that His16-Lyso are promising candidates as a treatment tool for LSD cells and to establish a foundation for ORT.

Buried treasure: biosynthesis, structures and applications of cyclic peptides hidden in seed storage albumins

Covering: 1999 up to the end of 2017The small cyclic peptide SunFlower Trypsin Inhibitor-1 (SFTI-1) from sunflower seeds is the prototypic member of a novel family of natural products. The biosynthesis of these peptides is intriguing as their gene-encoded peptide backbone emerges from a precursor protein that also contains a seed storage albumin. The peptide sequence is cleaved out from the precursor and cyclised by the albumin-maturing enzymatic machinery. Three-dimensional solution NMR structures of a number of these peptides, and of the intact precursor protein preproalbumin with SFTI-1, have now been elucidated. Furthermore, the evolution of the family has been described and a detailed understanding of the biosynthetic steps, which are necessary to produce cyclic SFTI-1, is emerging. Macrocyclisation provides peptide stability and thus represents a key strategy in peptide drug development. Consequently the constrained structure of SFTI-1 has been explored as a template for protein engineering, for tuning selectivity towards clinically relevant proteases and for grafting in sequences with completely novel functions. Here we review the discovery of the SFTI-1 peptide family, their evolution, biosynthetic origin, and structural features, as well as highlight the potential applications of this unique class of natural products.

Cyclic Cell-Penetrating Peptides as Efficient Intracellular Drug Delivery Tools

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

The Potential of the Cyclotide Scaffold for Drug Development

Cyclotides are a novel class of micro-proteins (≈30-40 residues long) with a unique topology containing a head-to-tail cyclized backbone structure further stabilized by three disulfide bonds that form a cystine knot. This unique molecular framework makes them exceptionally stable to physical, chemical, and biological degradation compared to linear peptides of similar size. The cyclotides are also highly tolerant to sequence variability, aside from the conserved residues forming the cystine knot, and are orally bioavailable and able to cross cellular membranes to modulate intracellular protein-protein interactions (PPIs), both in vitro and in vivo. These unique properties make them ideal scaffolds for many biotechnological applications, including drug discovery. This review provides an overview of the properties of cyclotides and their potential for the development of novel peptide-based therapeutics. The selective disruption of PPIs still remains a very challenging task, as the interacting surfaces are relatively large and flat. The use of the cell-permeable highly constrained polypeptide molecular frameworks, such as the cyclotide scaffold, has shown great promise, as it provides unique pharmacological properties. The use of molecular techniques, such as epitope grafting, and molecular evolution have shown to be highly effective for the selection of bioactive cyclotides. However, despite successes in employing cyclotides to target PPIs, some of the challenges to move them into the clinic still remain.

Syndiotactic peptides for targeted delivery

Lack of cell-type specificity and proteolytic susceptibility have long been the major bottlenecks for the development of peptide-based biomaterials for targeted drug delivery. Though a poly-l backbone provides the adaptability to re-conform the peptide structure to bind to a receptor, it also makes the peptide more susceptible to proteolytic cleavage. We have attempted to address this issue by designing a set of syndiotactic peptides de novo, with alternating l- and d-amino acids in succession. The designed peptides have higher rates of cellular uptake than the Tat (48-60) peptide in breast and cervical cancer cells. The uptake is independent of concentration, temperature and endocytosis (clathrin mediated). Importantly, the peptides are stable in both human plasma and bovine serum. The peptide-drug conjugates are much less toxic to the non-cancerous cells than cancer cells. The designed peptides are a step forward towards the development of targeted drug delivery vectors on peptide templates. STATEMENT OF SIGNIFICANCE: Present options in chemotherapy have multiple side effects arising from the lack of cell-type specificity, which makes them synonymous with "a Pyrrhic victory". Proteolytic susceptibility and non-specificity towards cancer cells has stunted the development of peptide-based biomaterials for targeted drug delivery. We have designed a set of peptides, addressing the above-mentioned roadblocks at an in vitro level. The peptides were designed on the template of a naturally existing peptide antibiotic from Bacillus brevis. The designed peptides have higher rates of cellular transduction than the model peptide (Tat), and is majorly membrane based. The peptides are stable in serum and selective towards cancer cells. Observations presented in this work can potentially take the discipline of de novo design of biomaterial conjugates forward.

Discovery and Characterization of Cyclic and Acyclic Trypsin Inhibitors from Momordica dioica

Momordica trypsin inhibitors (TIs) such as those isolated from the seeds of the gấc fruit, Momordica cochinchinensis (MCoTI-I and MCoTI-II), are widely used as scaffolds for drug design studies. To more effectively exploit these molecules in the development of therapeutics, there is a need for wider discovery of the natural sequence diversity among TIs from other species in the Momordica subfamily. Here we report the discovery of the encoding gene and six TIs from the seeds of the spiny gourd, Momordica dioica, four of which possess novel sequences (Modi 1, 3, 5, and 6) and two (Modi 2 and 4) of which are known peptides (TI-14, TI-17) previously identified in Momordica subangulata. Modi 6 is an acyclic peptide featuring a pyrrolidone carboxylic acid modification, whereas the remaining five TIs are cyclic. All Modi peptides display similar overall structures and trypsin inhibitory activities. No toxicity was observed for these peptides when tested against cancer and insect cells. All Modi peptides were exceptionally stable over 24 h in human serum, indicating a dual strategy to stabilize the peptides in nature, either head-to-tail cyclization or N-pyrolation, which suggests these peptides might be excellent candidates as scaffolds for epitope stabilization in drug design studies.

Optimised Extraction of Trypsin Inhibitors from Defatted Gac (Momordica cochinchinensis Spreng) Seeds for Production of a Trypsin Inhibitor‐Enriched Freeze Dried Powder

The seeds of the Gac fruit, Momordica cochinchinensis Spreng, are rich in trypsin inhibitors (TIs) but their optimal extraction and the effects of freeze drying are not established. This study aims to (1) compare aqueous solvents (DI water, 0.1 M NaCl, 0.02 M NaOH and ACN)/water/FA, 25:24:1) for extracting TIs from defatted Gac seed kernel powder, (2) to optimise the extraction in terms of solvent, time and material to solvent ratio and (3) to produce a TI-enriched freeze-dried powder (FD-TIP) with good characteristics. Based on the specific TI activity (TIA), the optimal extraction was 1 h using a ratio of 2.0 g of defatted powder in 30 mL of 0.05 M NaCl. The optimisation improved the TIA and specific TIA by 8% and 13%, respectively. The FD-TIP had a high specific TIA (1.57 ± 0.17 mg trypsin/mg protein), although it also contained saponins (43.6 ± 2.3 mg AE/g) and phenolics (10.5 ± 0.3 mg GAE/g). The FD-TIP was likely stable during storage due to its very low moisture content (0.43 ± 0.08%) and water activity (0.18 ± 0.07) and its ability to be easily reconstituted in water due to its high solubility index (92.4 ± 1.5%). Therefore, the optimal conditions for the extraction of TIs from defatted Gac seed kernel powder followed by freeze drying gave a high quality powder in terms of its highly specific TIA and physical properties.

Using backbone-cyclized Cys-rich polypeptides as molecular scaffolds to target protein-protein interactions

The use of disulfide-rich backbone-cyclized polypeptides, as molecular scaffolds to design a new generation of bioimaging tools and drugs that are potent and specific, and thus might have fewer side effects than traditional small-molecule drugs, is gaining increasing interest among the scientific and in the pharmaceutical industries. Highly constrained macrocyclic polypeptides are exceptionally more stable to chemical, thermal and biological degradation and show better biological activity when compared with their linear counterparts. Many of these relatively new scaffolds have been also found to be highly tolerant to sequence variability, aside from the conserved residues forming the disulfide bonds, able to cross cellular membranes and modulate intracellular protein-protein interactions both in vitro and in vivo These properties make them ideal tools for many biotechnological applications. The present study provides an overview of the new developments on the use of several disulfide-rich backbone-cyclized polypeptides, including cyclotides, θ-defensins and sunflower trypsin inhibitor peptides, in the development of novel bioimaging reagents and therapeutic leads.

Lysine to arginine mutagenesis of chlorotoxin enhances its cellular uptake

Chlorotoxin (CTX), a disulfide-rich peptide from the scorpion Leiurus quinquestriatus, has several promising biopharmaceutical properties, including preferential affinity for certain cancer cells, high serum stability, and cell penetration. These properties underpin its potential for use as a drug design scaffold, especially for the treatment of cancer; indeed, several analogs of CTX have reached clinical trials. Here, we focus on its ability to internalize into cells-a trait associated with a privileged subclass of peptides called cell-penetrating peptides-and whether it can be improved through conservative substitutions. Mutants of CTX were made using solid-phase peptide synthesis and internalization into human cervical carcinoma (HeLa) cells was monitored by fluorescence and confocal microscopy. CTX_M1 (ie, [K15R/K23R]CTX) and CTX_M2 (ie, [K15R/K23R/Y29W]CTX) mutants showed at least a twofold improvement in uptake compared to CTX. We further showed that these mutants internalize into HeLa cells largely via an energy-dependent mechanism. Importantly, the mutants have high stability, remaining intact in serum for over 24 h; thus, retaining the characteristic stability of their parent peptide. Overall, we have shown that simple conservative substitutions can enhance the cellular uptake of CTX, suggesting that such type of mutations might be useful for improving uptake of other peptide toxins.

Potent, Selective, and Cell-Penetrating Inhibitors of Kallikrein-Related Peptidase 4 Based on the Cyclic Peptide MCoTI-II

Kallikrein-related peptidase 4 (KLK4) is a serine protease that has putative intracellular and extracellular functions in prostate cancer progression. Here we show that MCoTI-II, a 34-amino acid cyclic peptide found in the seeds of red gac (Momordica cochinchinensis), is an inhibitor of KLK4. By grafting a preferred KLK4 cleavage sequence into MCoTI-II, we produced a highly potent KLK4 inhibitor (K _i = 0.1 nM) that displayed 100,000-fold selectivity over related KLKs and the ability to penetrate cells. Additionally, by substituting positively charged noncontact residues in this compound, we produced a potent and selective KLK4 inhibitor that does not penetrate cells. The inhibitors were shown to be nontoxic to human cells and stable in human serum. These KLK4 inhibitors provide useful chemical tools to further define the role(s) of both intracellular and extracellular KLK4 in prostate cancer cell lines and disease models.

Bioactive Composition, Antioxidant Activity, and Anticancer Potential of Freeze-Dried Extracts from Defatted Gac (Momordica cochinchinensis Spreng) Seeds

Background: Gac (Momordica cochinchinensis Spreng) seeds have long been used in traditional medicine as a remedy for numerous conditions due to a range of bioactive compounds. This study investigated the solvent extraction of compounds that could be responsible for antioxidant activity and anticancer potential. Methods: Defatted Gac seed kernel powder was extracted with different solvents: 100% water, 50% methanol:water, 70% ethanol:water, water saturated butanol, 100% methanol, and 100% ethanol. Trypsin inhibitors, saponins, phenolics, and antioxidant activity using the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the ferric reducing antioxidant power (FRAP) assays; and anticancer potential against two melanoma cancer cell lines (MM418C1 and D24) were analysed to determine the best extraction solvents. Results: Water was best for extracting trypsin inhibitors (581.4 ± 18.5 mg trypsin/mg) and reducing the viability of MM418C1 and D24 melanoma cells (75.5 ± 1.3 and 66.9 ± 2.2%, respectively); the anticancer potential against the MM418C1 cells was highly correlated with trypsin inhibitors (r = 0.92, p < 0.05), but there was no correlation between anticancer potential and antioxidant activity. The water saturated butanol had the highest saponins (71.8 ± 4.31 mg aescin equivalents/g), phenolic compounds (20.4 ± 0.86 mg gallic acid equivalents/g), and antioxidant activity, but these measures were not related to anticancer potential. Conclusions: Water yielded a Gac seed extract, rich in trypsin inhibitors, which had high anticancer potential against two melanoma cell lines.

Membrane Active Peptides and Their Biophysical Characterization

In the last 20 years, an increasing number of studies have been reported on membrane active peptides. These peptides exert their biological activity by interacting with the cell membrane, either to disrupt it and lead to cell lysis or to translocate through it to deliver cargos into the cell and reach their target. Membrane active peptides are attractive alternatives to currently used pharmaceuticals and the number of antimicrobial peptides (AMPs) and peptides designed for drug and gene delivery in the drug pipeline is increasing. Here, we focus on two most prominent classes of membrane active peptides; AMPs and cell-penetrating peptides (CPPs). Antimicrobial peptides are a group of membrane active peptides that disrupt the membrane integrity or inhibit the cellular functions of bacteria, virus, and fungi. Cell penetrating peptides are another group of membrane active peptides that mainly function as cargo-carriers even though they may also show antimicrobial activity. Biophysical techniques shed light on peptide–membrane interactions at higher resolution due to the advances in optics, image processing, and computational resources. Structural investigation of membrane active peptides in the presence of the membrane provides important clues on the effect of the membrane environment on peptide conformations. Live imaging techniques allow examination of peptide action at a single cell or single molecule level. In addition to these experimental biophysical techniques, molecular dynamics simulations provide clues on the peptide–lipid interactions and dynamics of the cell entry process at atomic detail. In this review, we summarize the recent advances in experimental and computational investigation of membrane active peptides with particular emphasis on two amphipathic membrane active peptides, the AMP melittin and the CPP pVEC.