Design and Chemical Synthesis of Integrin Ligands

Effects of SPARC and Possible Receptors on Colon Cancer Cell Line

Objective: The aim of this study was to observe the apoptotic/cytotoxic effects of exogenous SPARC on colon cancer cell line HT-29, then to investigate the function of stabilin-1 and integrin αvβ3, which are possible receptors for SPARC in colon cancer cells and to determine the quantitation of their receptor numbers. Methods: Appropriate doses of exogenous SPARC and it’s inhibitor, cilengitide added to HT-29 cell line were determined by xCELLigence Real-Time Cell Analysis system, SPARC-mediated caspase 3 expressions were measured. Using the RT-PCR system, gene expression levels of SPARC, stabilin-1 and integrin αvβ3 receptors (silenced/nonsilenced with cilengitide) were detected then the numbers of receptors per cell were quantitated by flow cytometry. Results: IC50 value of SPARC was determined as 4.57 μg/mL and IC50 value of cilengitide was determined as 50 nM. 5 μg/mL exogenous SPARC caused increased apoptosis in the HT-29 line. Significant increase in gene expression of integrin αvβ3 receptor was observed in the group incubated with 5 μg/mL SPARC, contrarily, the addition of cilengitide decreased gene expressions. The integrin αvβ3 receptor numbers increased approximately 2-fold with SPARC compared to the control. No significant changes were observed in the gene expression and receptor numbers of stabilin-1. Conclusion: Exogenous SPARC was shown to reduce proliferation and induce apoptosis in colon cancer cells. Integrin αvβ3 is thought to be the possible receptor mediating SPARC in colon cancer cells. Quantification of surface receptors per cell, which we think we have done first, can be considered as a marker in the follow-up of anticancer treatments.

Combination Effect of Cilengitide with Erlotinib on TGF-β1-Induced Epithelial-to-Mesenchymal Transition in Human Non-Small Cell Lung Cancer Cells

The epithelial-to-mesenchymal transition (EMT) is important for morphogenesis during development and is mainly induced by transforming growth factor (TGF)-β. In lung cancer, EMT is characterized by the transformation of cancer cells into a mobile, invasive form that can transit to other organs. Here, using a non–small cell lung cancer (NSCLC) cell line, we evaluated the EMT-related effects of the epidermal growth factor receptor inhibitor erlotinib alone and in combination with cilengitide, a cyclic RGD-based integrin antagonist. Erlotinib showed anti-proliferative and inhibitory effects against the TGF-β1–induced EMT phenotype in NSCLC cells. Compared with erlotinib alone, combination treatment with cilengitide led to an enhanced inhibitory effect on TGF-β1–induced expression of mesenchymal markers and invasion in non–small cell lung cancer A549 cells. These results suggest that cilengitide could improve anticancer drug efficacy and contribute to improved treatment strategies to inhibit and prevent EMT-based cancer progression.

AaTs-1: A Tetrapeptide from Androctonus australis Scorpion Venom, Inhibiting U87 Glioblastoma Cells Proliferation by p53 and FPRL-1 Up-Regulations

Glioblastoma is an aggressive cancer, against which medical professionals are still quite helpless, due to its resistance to current treatments. Scorpion toxins have been proposed as a promising alternative for the development of effective targeted glioblastoma therapy and diagnostic. However, the exploitation of the long peptides could present disadvantages. In this work, we identified and synthetized AaTs-1, the first tetrapeptide from Androctonus australis scorpion venom (Aa), which exhibited an antiproliferative effect specifically against human glioblastoma cells. Both the native and synthetic AaTs-1 were endowed with the same inhibiting effect on the proliferation of U87 cells with an IC₅₀ of 0.56 mM. Interestingly, AaTs-1 was about two times more active than the anti-glioblastoma conventional chemotherapeutic drug, temozolomide (TMZ), and enhanced its efficacy on U87 cells. AaTs-1 showed a significant similarity with the synthetic peptide WKYMVm, an agonist of a G-coupled formyl-peptide receptor, FPRL-1, known to be involved in the proliferation of glioma cells. Interestingly, the tetrapeptide triggered the dephosphorylation of ERK, p38, and JNK kinases. It also enhanced the expression of p53 and FPRL-1, likely leading to the inhibition of the store operated calcium entry. Overall, our work uncovered AaTs-1 as a first natural potential FPRL-1 antagonist, which could be proposed as a promising target to develop new generation of innovative molecules used alone or in combination with TMZ to improve glioblastoma treatment response. Its chemical synthesis in non-limiting quantity represents a valuable advantage to design and develop low-cost active analogues to treat glioblastoma cancer.

Transferrin Receptor Targeted Polyplexes Completely Comprised of Sequence-Defined Components

Human transferrin protein (Tf) modified polyplexes have already displayed encouraging potential for receptor-mediated nucleic acid delivery into tumors. The use of a blood-derived targeting protein and polydisperse macromolecular cationic subunits however presents a practical challenge for pharmaceutical grade production. Here, Tf receptor (TfR) targeted small interfering RNA (siRNA) polyplexes are designed that are completely composed of synthetic, monodisperse, and sequence-defined subunits generated by solid-phase supported synthesis. An optimized cationizable lipo-oligoaminoamide (lipo-OAA) is used for siRNA core polyplex formation, and a retro-enantio peptide (reTfR) attached via a monodisperse polyethylene glycol (PEG) spacer via click chemistry is applied for targeting. Improved gene silencing is demonstrated in TfR-expressing KB and DU145 cells. Analogous plasmid DNA (pDNA) polyplexes are successfully used for receptor-mediated gene delivery in TfR-rich K562 cells and Neuro2a cells. Six lipo-OAAs differing in their lipidic domain and redox-sensitive attachment of lipid residues are tested in order to evaluate the impact of core polyplex stability on receptor-dependent gene transfer.

Cyclic RGD Pentapeptide Cilengitide Enhances Efficacy of Gefitinib on TGF-β1-Induced Epithelial-to-Mesenchymal Transition and Invasion in Human Non-Small Cell Lung Cancer Cells

During non-small cell lung cancer (NSCLC) progression, transforming growth factor (TGF)-β mediated epithelial-to-mesenchymal transition (EMT) is an important process leading to high mortality and poor prognosis. The EMT is a fundamental process for morphogenesis characterized by the transformation of cancer cells into invasive forms that can be transferred to other organs during human lung cancer progression. Gefitinib, an epidermal growth factor receptor (EGFR) inhibitor, has shown anti-proliferative effects in EGFR-mutated NSCLC cells and an inhibitory effect on migration and invasion of NSCLC cells to other organs. In this study, we evaluated the combinatorial treatment effect of cilengitide, a cyclic RGD pentapeptide, on TGF-β1-induced EMT phenotype and invasion. Gefitinib suppressed the expression of TGF-β1-induced mesenchymal markers by inhibiting Smad and non-Smad signaling pathways. Cilengitide enhanced the inhibitory effect of gefitinib on TGF-β1-induced expression of mesenchymal markers, phosphorylation of Smad2/3, and invasion of NSCLC A549 cells. We suggested that the use of cilengitide can improve the efficacy of anti-cancer drugs in combination drug-based chemotherapy. These results provide an improved therapeutic strategy for treating and preventing EMT-related disorders, such as NSCLC, lung fibrosis, cancer metastasis, and relapse.

Targeted-nanoliposomal combretastatin A4 (CA-4) as an efficient antivascular candidate in the metastatic cancer treatment

A number of antiangiogenic drugs have been approved by the Food and Drug Administration which are used in cancer therapy, and variety of other agents in several stages of clinical development or in preclinical assessment. Among these, combretastatin A4 (CA-4) is an under-researched inhibitor of angiogenesis that shows potential activity in the treatment of advanced tumors with migration capacity. However, its clinical application has been limited due to poor water solubility, low bioavailability, rapid metabolism, and systemic elimination. During the last decade, numerous investigations have been done to overcome these problems by using different CA-4 delivery systems or developing produgs of CA-4 or its structural analogs. Nevertheless, these strategies could not be efficient out of the undesired side effects on normal tissues. Nanoliposomal CA-4 not only benefits from the advantage of using liposomal drugs as opposed to free drugs but also can accumulate in the tumor site via specific targeting ligands, which leads to efficient targeting and enhancement of bioavailability. To the best of our knowledge, we consider an important attempt to understand different factors that might influence the CA-4 loading and release pattern of liposomes and the consequent results in tumor therapy. In this review, we shed light on various studied liposomal CA-4 formulations showing application thereof in cancer treatment.

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.

Liposomal formulation of Galbanic acid improved therapeutic efficacy of pegylated liposomal Doxorubicin in mouse colon carcinoma

Galbanic acid (Gba), a sesquiterpene coumarin, with strong antiangiogenic activity could serve as an excellent anti-cancer agent. However, Gba is a poor water-solube which hampered its clinical application. In this study, a pegylated liposomal Gba (PLGba) with HSPC/Cholesterol/mPEG2000-DSPE (56.2, 38.3, 5.3% molar ratio) was developed by the thin film hydration plus extrusion and calcium acetate gradient remote loading method, to address the issue of poor Gba solubility. Moreover, an integrin-targeting ligand (RGD peptide, cyclo[Arg-Gly-Asp-D-Tyr-Cys]) was post-inserted into liposomes in order to increase Gba cell delivery. Using fluorescently-labeled model liposomes, it was found that the targeting could improve the integrin-mediated cellular uptake of the liposomes in vitro in human umbilical vein endothelial cells (HUVECs), and in vivo as evidenced by chicken chorioallantoic membrane angiogenesis (CAM) model. It also could enrich the liposome accumulation in C26 tumor. Interestingly, co-treatment with PLGba and pegylated liposomal doxorubicin (PLD, also known as Doxil®) had a synergistic and antagonistic antiproliferative effect on the C26 tumor cell line and the normal HUVEC, respectively. In C26 tumor bearing BALB/c mice, the PLGba and PLD combinatorial therapy improved the antitumor efficacy of the treatment as compared to those of single agents. This results have clear implications for cancer therapy.

Conformational studies of RGDechi peptide by natural-abundance NMR spectroscopy

Integrins are heterodimeric cell-surface proteins that play important roles during developmental and pathological processes. Diverse human pathologies involve integrin adhesion including thrombotic diseases, inflammation, tumour progression, fibrosis, and infectious diseases. Although in the past decade, novel integrin-inhibitor drugs have been developed for integrin-based medical applications, the structural determinants modulating integrin-ligands recognition mechanisms are still poorly understood, reducing the number of integrin subtype exclusive antagonists. In this scenario, we have very recently showed, by means of chemical and biological assays, that a chimeric peptide (named RGDechi), containing a cyclic RGD motif linked to an echistatin C-terminal fragment, is able to interact with the components of integrin family with variable affinities, the highest for α_v β3. Here, in order to understand the mechanistic details driving the molecular recognition mechanism of α_v β₃ by RGDechi, we have performed a detailed structural and dynamics characterization of the free peptide by natural abundance nuclear magnetic resonance (NMR) spectroscopy. Our data indicate that RGDechi presents in solution an heterogeneous conformational ensemble characterized by a more constrained and rigid pentacyclic ring and a largely unstructured acyclic region. Moreover, we propose that the molecular recognition of α_v β₃ integrin by RGDechi occurs by a combination of conformational selection and induced fit mechanisms. Finally, our study indicates that a detailed NMR characterization, by means of natural abundance ¹⁵ N and ¹³ C, of a mostly unstructured bioactive peptide may provide the molecular basis to get essential structural insights into the binding mechanism to the biological partner.

Tuning the properties of a cyclic RGD-containing tetrapeptide through backbone fluorination

Stereoselective fluorination is investigated as a method for modulating the properties of a cyclic RGD-containing tetrapeptide. Three key outcomes of fluorination are assessed: (i) the effect on peptide cyclisation efficiency; (ii) the ability to fine-tune the molecular conformation; and (iii) the effect on the cyclic peptides' biological activity. Fluorination is found to exert pronounced effects against all three criteria.

A Promising Biocompatible Platform: Lipid-Based and Bio-Inspired Smart Drug Delivery Systems for Cancer Therapy

Designing new drug delivery systems (DDSs) for safer cancer therapy during pre-clinical and clinical applications still constitutes a considerable challenge, despite advances made in related fields. Lipid-based drug delivery systems (LBDDSs) have emerged as biocompatible candidates that overcome many biological obstacles. In particular, a combination of the merits of lipid carriers and functional polymers has maximized drug delivery efficiency. Functionalization of LBDDSs enables the accumulation of anti-cancer drugs at target destinations, which means they are more effective at controlled drug release in tumor microenvironments (TMEs). This review highlights the various types of ligands used to achieve tumor-specific delivery and discusses the strategies used to achieve the effective release of drugs in TMEs and not into healthy tissues. Moreover, innovative recent designs of LBDDSs are also described. These smart systems offer great potential for more advanced cancer therapies that address the challenges posed in this research area.

RGD peptide-based non-viral gene delivery vectors targeting integrin αvβ3 for cancer therapy

Integrin α_vβ₃ is restrictedly expressed on angiogenic blood vessels and tumour cells. It plays a key role in angiogenesis for tumour growth and metastasis. RGD peptide can specifically recognise the integrin α_vβ₃, which serves as targeted molecular for anti-angiogenesis strategies. Therefore, the targeted delivery of therapeutics by RGD peptide-based non-viral vectors to tumour vasculature and tumour cells is recognised as a promising approach for treating cancer. In this review, we illustrate the interaction between RGD peptide and integrin α_vβ₃ from different perspectives. Meanwhile, four types of RGD peptide-based non-viral gene delivery vectors for cancer therapy, including RGD-based cationic polymers, lipids, peptides and hybrid systems, are summarised. The aim is to particularly highlight the enhanced therapeutic effects and specific targeting ability exhibited by these vectors for cancer gene therapy both in vitro and in vivo.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Exploring the Role of RGD-Recognizing Integrins in Cancer

Integrins are key regulators of communication between cells and with their microenvironment. Eight members of the integrin superfamily recognize the tripeptide motif Arg-Gly-Asp (RGD) within extracelluar matrix (ECM) proteins. These integrins constitute an important subfamily and play a major role in cancer progression and metastasis via their tumor biological functions. Such transmembrane adhesion and signaling receptors are thus recognized as promising and well accessible targets for novel diagnostic and therapeutic applications for directly attacking cancer cells and their fatal microenvironment. Recently, specific small peptidic and peptidomimetic ligands as well as antibodies binding to distinct integrin subtypes have been developed and synthesized as new drug candidates for cancer treatment. Understanding the distinct functions and interplay of integrin subtypes is a prerequisite for selective intervention in integrin-mediated diseases. Integrin subtype-specific ligands labelled with radioisotopes or fluorescent molecules allows the characterization of the integrin patterns in vivo and later the medical intervention via subtype specific drugs. The coating of nanoparticles, larger proteins, or encapsulating agents by integrin ligands are being explored to guide cytotoxic reagents directly to the cancer cell surface. These ligands are currently under investigation in clinical studies for their efficacy in interference with tumor cell adhesion, migration/invasion, proliferation, signaling, and survival, opening new treatment approaches in personalized medicine.

Virus-Derived Peptides for Clinical Applications

Novel affinity agents with high specificity are needed to make progress in disease diagnosis and therapy. Over the last several years, peptides have been considered to have fundamental benefits over other affinity agents, such as antibodies, due to their fast blood clearance, low immunogenicity, rapid tissue penetration, and reproducible chemical synthesis. These features make peptides ideal affinity agents for applications in disease diagnostics and therapeutics for a wide variety of afflictions. Virus-derived peptide techniques provide a rapid, robust, and high-throughput way to identify organism-targeting peptides with high affinity and selectivity. Here, we will review viral peptide display techniques, how these techniques have been utilized to select new organism-targeting peptides, and their numerous biomedical applications with an emphasis on targeted imaging, diagnosis, and therapeutic techniques. In the future, these virus-derived peptides may be used as common diagnosis and therapeutics tools in local clinics.

High Affinity vs. Native Fibronectin in the Modulation of αvβ3 Integrin Conformational Dynamics: Insights from Computational Analyses and Implications for Molecular Design

Understanding how binding events modulate functional motions of multidomain proteins is a major issue in chemical biology. We address several aspects of this problem by analyzing the differential dynamics of αvβ3 integrin bound to wild type (wtFN10, agonist) or high affinity (hFN10, antagonist) mutants of fibronectin. We compare the dynamics of complexes from large-scale domain motions to inter-residue coordinated fluctuations to characterize the distinctive traits of conformational evolution and shed light on the determinants of differential αvβ3 activation induced by different FN sequences. We propose an allosteric model for ligand-based integrin modulation: the conserved integrin binding pocket anchors the ligand, while different residues on the two FN10's act as the drivers that reorganize relevant interaction networks, guiding the shift towards inactive (hFN10-bound) or active states (wtFN10-bound). We discuss the implications of results for the design of integrin inhibitors.

Conjugates of Cell Adhesion Peptides for Therapeutics and Diagnostics Against Cancer and Autoimmune Diseases

Overexpressed cell-surface receptors are hallmarks of many disease states and are often used as markers for targeting diseased cells over healthy counterparts. Cell adhesion peptides, which are often derived from interacting regions of these receptor-ligand proteins, mimic surfaces of intact proteins and, thus, have been studied as targeting agents for various payloads to certain cell targets for cancers and autoimmune diseases. Because many cytotoxic agents in the free form are often harmful to healthy cells, the use of cell adhesion peptides in targeting their delivery to diseased cells has been studied to potentially reduce required effective doses and associated harmful side-effects. In this review, multiple cell adhesion peptides from extracellular matrix and ICAM proteins were used to selectively direct drug payloads, signal-inhibitor peptides, and diagnostic molecules, to diseased cells over normal counterparts. RGD constructs have been used to improve the selectivity and efficacy of diagnostic and drug-peptide conjugates against cancer cells. From this precedent, novel conjugates of antigenic and cell adhesion peptides, called Bifunctional Peptide Inhibitors (BPIs), have been designed to selectively regulate immune cells and suppress harmful inflammatory responses in autoimmune diseases. Similar peptide conjugations with imaging agents have delivered promising diagnostic methods in animal models of rheumatoid arthritis. BPIs have also been shown to generate immune tolerance and suppress autoimmune diseases in animal models of type-1 diabetes, rheumatoid arthritis, and multiple sclerosis. Collectively, these studies show the potential of cell adhesion peptides in improving the delivery of drugs and diagnostic agents to diseased cells in clinical settings.

Rational Design of Cyclic Peptide Inhibitors of U2AF Homology Motif (UHM) Domains To Modulate Pre-mRNA Splicing

U2AF homology motifs (UHMs) are atypical RNA recognition motif domains that mediate critical protein-protein interactions during the regulation of alternative pre-mRNA splicing and other processes. The recognition of UHM domains by UHM ligand motif (ULM) peptide sequences plays important roles during early steps of spliceosome assembly. Splicing factor 45 kDa (SPF45) is an alternative splicing factor implicated in breast and lung cancers, and splicing regulation of apoptosis-linked pre-mRNAs by SPF45 was shown to depend on interactions between its UHM domain and ULM motifs in constitutive splicing factors. We have developed cyclic peptide inhibitors that target UHM domains. By screening a focused library of linear and cyclic peptides and performing structure-activity relationship analysis, we designed cyclic peptides with 4-fold improved binding affinity for the SPF45 UHM domain compared to native ULM ligands and 270-fold selectivity to discriminate UHM domains from alternative and constitutive splicing factors. These inhibitors are useful tools to modulate and dissect mechanisms of alternative splicing regulation.

Adhesion molecules and the extracellular matrix as drug targets for glioma

The formation of tumor vasculature and cell invasion along white matter tracts have pivotal roles in the development and progression of glioma. A better understanding of the mechanisms of angiogenesis and invasion in glioma will aid the development of novel therapeutic strategies. The processes of angiogenesis and invasion cause the production of an array of adhesion molecules and extracellular matrix (ECM) components. This review focuses on the role of adhesion molecules and the ECM in malignant glioma. The results of clinical trials using drugs targeted against adhesion molecules and the ECM for glioma are also discussed.

Mimicking of Arginine by Functionalized N(ω)-Carbamoylated Arginine As a New Broadly Applicable Approach to Labeled Bioactive Peptides: High Affinity Angiotensin, Neuropeptide Y, Neuropeptide FF, and Neurotensin Receptor Ligands As Examples

Derivatization of biologically active peptides by conjugation with fluorophores or radionuclide-bearing moieties is an effective and commonly used approach to prepare molecular tools and diagnostic agents. Whereas lysine, cysteine, and N-terminal amino acids have been mostly used for peptide conjugation, we describe a new, widely applicable approach to peptide conjugation based on the nonclassical bioisosteric replacement of the guanidine group in arginine by a functionalized carbamoylguanidine moiety. Four arginine-containing peptide receptor ligands (angiotensin II, neurotensin(8-13), an analogue of the C-terminal pentapeptide of neuropeptide Y, and a neuropeptide FF analogue) were subject of this proof-of-concept study. The N(ω)-carbamoylated arginines, bearing spacers with a terminal amino group, were incorporated into the peptides by standard Fmoc solid phase peptide synthesis. The synthesized chemically stable peptide derivatives showed high receptor affinities with Ki values in the low nanomolar range, even when bulky fluorophores had been attached. Two new tritiated tracers for angiotensin and neurotensin receptors are described.

Integrin Ligands with α/β-Hybrid Peptide Structure: Design, Bioactivity, and Conformational Aspects

Integrins are cell surface receptors for proteins of the extracellular matrix and plasma-borne adhesive proteins. Their involvement in diverse pathologies prompted medicinal chemists to develop small-molecule antagonists, and very often such molecules are peptidomimetics designed on the basis of the short native ligand-integrin recognition motifs. This review deals with peptidomimetic integrin ligands composed of α- and β-amino acids. The roles exerted by the β-amino acid components are discussed in terms of biological activity, bioavailability, and selectivity. Special attention is paid to the synthetic accessibility and efficiency of conformationally constrained heterocyclic scaffolds incorporating α/β-amino acid span.

Development of a novel cyclic RGD peptide for multiple targeting approaches of liposomes to tumor region

Liposomes containing cytotoxic agents and targeted with Arg-Gly-Asp based peptides have frequently been used against αvβ3 integrin on tumor neovasculature. However, like many other ligand modified liposomes these preparations suffered from enhanced uptake by the reticulo endothelial system (RES) and off-targeted interaction with integrin receptors vastly expressed in normal organs causing poor biodistribution and toxic effects. Here we mainly focus on development of a RGD-modified liposomal delivery system to enhance both targeting selectivity and tumor uptake. First, sterically stabilized liposomal doxorubicin (SSLD) prepared and decorated with cRGDfK and RGDyC peptides differ in their physical properties. Stability assessments as well as in vitro and in vivo studies revealed that increasing the peptide hydrophobicity promotes the therapeutic efficacy of RGD-SSLD in a C-26 tumor model due to decreased recognition by RES and opsonization and limited off-targeted interactions. Then a novel N-methylated RGD peptide was designed and its capability in targeting integrin presenting cells was comprehensively assessed both in vitro and in vivo. RGDf[N-methyl]C promotes the liposome internalization by HUVEC via integrin mediated endocytosis. Intravital microscopy in window chamber bearing mice illustrated the capability of RGDf[N-methyl]C-liposomes in targeting both tumor vasculature and tumor cells in murine B16F0 and human BLM tumor models. Quantitative biodistribution in mice bearing B16F0 tumor revealed its high affinity to tumor with no considerable affinity to normal organs. Treatment by high dose of RGDf[N-methyl]C-SSLD was found more effective than non-targeted SSLD and no toxic side effect was observed. In conclusion, the RGDf[N-methyl]C-liposome was found promising in targeting tumor vasculature as well as other cells inside the tumor.

Targeted polymer-drug conjugates: Current progress and future perspective

The combination of polymer technology and targeted drug delivery may pave the way for more effective yet safer therapeutic options for cancer therapy. Polymer-drug conjugates belonging to polymer therapeutics represent an emerging approach for drug delivery. The development of smart targeted polymer-drug conjugates that can specifically deliver drugs at a sustained rate to tumor cells may substantially improve the therapeutic index of anticancer agents. In this update, we provide an overview of the most important targeting molecules, and systemically summarize the recent advances in the development of tumor-targeted polymer-drug conjugates. Additionally, several promising approaches for the future will also be presented.

Novel peptide-based platform for the dual presentation of biologically active peptide motifs on biomaterials

Biofunctionalization of metallic materials with cell adhesive molecules derived from the extracellular matrix is a feasible approach to improve cell-material interactions and enhance the biointegration of implant materials (e.g., osseointegration of bone implants). However, classical biomimetic strategies may prove insufficient to elicit complex and multiple biological signals required in the processes of tissue regeneration. Thus, newer strategies are focusing on installing multifunctionality on biomaterials. In this work, we introduce a novel peptide-based divalent platform with the capacity to simultaneously present distinct bioactive peptide motifs in a chemically controlled fashion. As a proof of concept, the integrin-binding sequences RGD and PHSRN were selected and introduced in the platform. The biofunctionalization of titanium with this platform showed a positive trend towards increased numbers of cell attachment, and statistically higher values of spreading and proliferation of osteoblast-like cells compared to control noncoated samples. Moreover, it displayed statistically comparable or improved cell responses compared to samples coated with the single peptides or with an equimolar mixture of the two motifs. Osteoblast-like cells produced higher levels of alkaline phosphatase on surfaces functionalized with the platform than on control titanium; however, these values were not statistically significant. This study demonstrates that these peptidic structures are versatile tools to convey multiple biofunctionality to biomaterials in a chemically defined manner.

Cilengitide, a small molecule antagonist, targeted to integrin αν inhibits proliferation and induces apoptosis of laryngeal cancer cells in vitro

Cilengitide is a chemical synthesis cyclopeptide containing RGD sequence, which can be used as a small molecule antagonist targeted to integrin αν (ITGAV). The aim of present study was to investigate the effect on proliferation and cell apoptosis of the cilengitide in laryngeal cancer cells. In the study, we have treatmented the cultured cells of laryngeal cancer (Hep-2) with cilengitide. After the medication, the proliferation of the Hep-2 cells was detected by MTT assay, the expression of ITGAV was detected by RT-PCR and the activity of caspase-3 protein was detected by a specialized kit. RGD linear peptides (GRGDSP), non-RGD linear peptide (GRGESP), and 5-fluorouracil (5-Fu) were used as controls. Results showed that the proliferation of Hep-2 cells was signally inhibited by the cilengitide with a time and dose compliance. Its inhibition effect was significantly higher than that of 5-Fu and GRGDSP, but the GRGESP showed no obvious inhibitory effect. After intervene of cilengitide, the activity of caspase-3 protein of Hep-2 cells was significantly increased, and the expression of ITGAV was significantly decreased. 5-Fu significantly inhibited the proliferation of Hep-2 cells, but no significant changes of ITGAV expression were observed. In conclusion, cilengitide can significantly down-regulate ITGAV expression and inhibit cell proliferation in laryngeal cancer cells, it will also to induce cell apoptosis through caspase-3 pathway. Therefore, it could be as a kind of effective chemotherapy drugs that will be used in clinical treatment of the laryngeal cancer.

Peptide-mediated targeting of cytokines to tumor vasculature: the NGR-hTNF example

A growing body of evidence suggests that the efficacy of cytokines in cancer therapy can be increased by targeting strategies based on conjugation with ligands that recognize receptors expressed by tumor cells or elements of the tumor microenvironment, including the tumor vasculature. The targeting approach is generally conceived to permit administration of low, yet pharmacologically active, doses of drugs, thereby avoiding toxic reactions. However, it is becoming clear that, in the case of cytokines, this strategy has another inherent advantage, i.e. the possibility of administering extremely low doses that do not activate systemic counter-regulatory mechanisms, which may limit their potential therapeutic effects. This review is focused on the use of tumor vasculature-homing peptides as vehicles for targeted delivery of cytokines to tumor blood vessel. In particular, we provide an overview of peptide-cytokine conjugates made with peptides containing the NGR, RGD, isoDGR or RGR sequences and describe, in more details, the biological and pharmacological properties of NGR-hTNF, a peptide-tumor necrosis factor-α conjugate that is currently being tested in phase II and III clinical studies. The results of preclinical and clinical studies performed with these products suggest that peptide-mediated vascular-targeting is indeed a viable strategy for delivering bioactive amounts of cytokines to tumor endothelial cells without causing the activation of counter-regulatory mechanisms and toxic reactions.

Cilengitide treatment for malignant glioma: current status and future direction

Malignant glioma is the most common primary brain tumor and accounts for the majority of diagnoses. Treatment has involved a combination of surgery, radiation, and chemotherapy, yet these modalities rarely extend the life of the patient to more than one year from diagnosis. Integrins are expressed in tumor cells and tumor endothelial cells, and are important in angiogenesis and invasion in glioma. αvβ3 and αvβ5 integrins regulate cell adhesion, and inhibitors of these integrins suppress tumor growth in certain pre-clinical models. Several integrin-targeted drugs are in clinical trials as potential compounds for the treatment of cancer. Among them, cilengitide is a novel integrin antagonist for the treatment of glioblastoma. The multimodal anti-glioma effects are based on its cytotoxic, anti-angiogenic, anti-invasive, and synergetic effects. Preclinical studies showed a promising synergy between cilengitide and radiochemotherapy in order to normalize tumor vasculature and attenuate tumor invasion. Cilengitide is currently being assessed in phase III trials for patients with glioblastoma multiforme and in phase II trials for other types of cancers, demonstrating promising therapeutic outcomes to date. The results of these and other clinical studies are expected with great hope and interest. A more clear understanding of the benefits and pitfalls of each approach can then lead to the design of strategies to derive maximal benefit from these therapies.

Biselectivity of isoDGR peptides for fibronectin binding integrin subtypes α5β1 and αvβ6: conformational control through flanking amino acids

Integrins are the major class of cell adhesion proteins. Their interaction with different ligands of the extracellular matrix is diverse. To get more insight into these interactions, artificial ligands endowed with a well-defined activity/selectivity profile are necessary. Herein, we present a library of cyclic pentapeptides, based on our previously reported peptide motif c(-phg-isoDGR-X-), in which high activity toward fibronectin binding integrins α5β1 and αvβ6 and not on vitronectin binding integrins αvβ3 and αvβ5 has been achieved by changing the flanking amino acids. The structure of the most promising candidates has been determined using a combined approach of NMR, distance geometry, and molecular dynamics simulations, and docking studies have been further used to elucidate the peptide-integrin interactions at the molecular level. The peptides' binding affinity has been characterized by enzyme linked immunosorbent assay experiments, and the results have been verified by cell adhesion experiments on specifically functionalized surfaces.

Cyclization of RGD peptide sequences via the macrocyclic chelator DOTA for integrin imaging

Two bicyclic compounds containing Arg-Gly-Asp (RGD) motifs (RGDf and RGD) were synthesized by cyclizing the peptide sequence across the macrocyclic ring of DOTA via two non-adjacent carboxylate pendent arms. The Lu(3+) or Cu(2+) complexes of these compounds, c(DOTA-RGDf) and c(DOTA-RGD), showed a metal dependent affinity towards integrin α(v)β(3)in vitro and the (177)Lu(3+) or (64)Cu(2+) labelled derivatives showed specific tumour uptake in MCF7 and U87MG tumour bearing mice.

Design of cyclic peptides featuring proline predominantly in the cis conformation under physiological conditions

Turns are secondary-structure elements that are omnipresent in natively folded polypeptide chains. A large variety of four-residue β-turns exist, which differ mainly in the backbone dihedral angle values of the two central residues i+1 and i+2. The βVI-type turns are of particular biological interest because the i+2 residue is always a proline in the cis conformation and might thus serve as target of peptidyl prolyl cis/trans isomerases (PPIases). We have designed cyclic hexapeptides containing two proline residues that predominantly adopt the cis conformation in aqueous solution. NMR data and MD calculations indicated that the cyclic peptide sequences c-(-DXaa-Ser-Pro-DXaa-Lys-Pro-) result in highly symmetric backbone structures when both prolines are in the cis conformation and the D-amino acids are either alanine or phenylalanine residues. Replacement of the serine residue either by phosphoserine or by tyrosine compromises this symmetry, but further increases the cis conformation content of both prolines. As a result, we obtained a cyclic hexapeptide that exists almost exclusively as the cis-Pro/cis-Pro conformer but shows no cis/trans interconversion even in the presence of the PPIase Pin1, apparently due to an energetically quite favorable but highly restricted conformational space.

Computational design of targeted inhibitors of polo-like kinase 1 (plk1)

Computational design of small molecule putative inhibitors of Polo-like kinase 1 (Plk1) is presented. Plk1, which regulates the cell cycle, is often over expressed in cancers. Down regulation of Plk1 has been shown to inhibit tumor progression. Most kinase inhibitors interact with the ATP binding site on Plk1, which is highly conserved. This makes the development of Plk1-specific inhibitors challenging, since different kinases have similar ATP sites. However, Plk1 also contains a unique region called the polo-box domain (PBD), which is absent from other kinases. In this study, the PBD site was used as a target for designed Plk1 putative inhibitors. Common structural features of several experimentally known Plk1 ligands were first identified. The findings were used to design small molecules that specifically bonded Plk1. Drug likeness and possible toxicities of the molecules were investigated. Molecules with no implied toxicities and optimal drug likeness values were used for docking studies. Several molecules were identified that made stable complexes only with Plk1 and LYN kinases, but not with other kinases. One molecule was found to bind exclusively the PBD site of Plk1. Possible utilization of the designed molecules in drugs against cancers with over expressed Plk1 is discussed.

68Ga-NODAGA-RGDyK for αvβ3 integrin PET imaging. Preclinical investigation and dosimetry

AIM

To visualize neovasculature and/or tumour integrin αvβ3 we selected the binding moiety Arg-Gly-Asp-D-Tyr-Lys (RGDyK) coupled to NODAGA for labeling with 68Ga.

METHODS

NODAGA-RGDyK (ABX) was labeled with the 68Ga eluate from the 68Ge generator IGG100 using the processor unit PharmTracer. Biodistribution was measured in female Hsd mice sacrificed 10, 30, 60 and 90 min after i.v. injection of 68Ga-NODAGA-RGDyK for OLINDA dosimetry extrapolated to humans. Tumour targeting was studied in SCID mice bearing A431 and other tumour transplants using microPET and biodistribution measurements.

RESULTS

Effective half-life of 68Ga-NODAGA-RGDyK was ~25 min for total body and most organs except liver and spleen that showed stable activity retention. With a bladder voiding interval of 0.5 h the calculated effective dose (ED) was 0.012 and 0.016 mSv/MBq for males and females, respectively. Rapid uptake within 10 min was observed in A431 tumours with dynamic PET followed by a slow release. Biodistribution measurements showed a 68Ga-NODAGA-RGDyK uptake in A431 tumours of 3.4±0.4 and 2.7±0.3%ID/g at 1 and 2 h, respectively. Similar uptakes were observed in a mouse and human breast and ovarian cancer xenografts. Co-injection of excess (5 mg/kg) unlabeled NODAGA-RGDyK with the radiotracer reduced tumour uptake at one hour to 0.23±0.01%ID/g, but similarly decreased uptake in normal organs as well. When unlabeled peptide was injected 15 min after 68Ga-NODAGA-RGDyK, uptake diminished particularly in tumour and adrenals, suggestive of a different binding mode compared with other normal tissues.

CONCLUSION

NODAGA-RGDyK was reliably labeled with 68Ga and revealed a predicted ED of 0.014 mSv/MBq. Tumour uptake was rapid and significant and was chased with unlabeled RGDyK in a similar manner as adrenal uptake.

Advantages of RGD peptides for directing cell association with biomaterials

Despite many years of in vitro research confirming the effectiveness of RGD in promoting cell attachment to a wide variety of biomaterials, animal studies evaluating tissue responses to implanted RGD-functionalized substrates have yielded more variable results. The goals of this report are to present some of the reasons why cell culture studies may not always reliably predict in vivo responses, and more importantly, to highlight potential applications that may benefit from the use of RGD peptides.

Cilengitide: the first anti-angiogenic small molecule drug candidate design, synthesis and clinical evaluation

Cilengitide, a cyclic RGD pentapeptide, is currently in clinical phase III for treatment of glioblastomas and in phase II for several other tumors. This drug is the first anti-angiogenic small molecule targeting the integrins αvβ3, αvβ5 and αvβ1. It was developed by us in the early 90s by a novel procedure, the spatial screening. This strategy resulted in c(RGDfV), the first superactive αvβ3 inhibitor (100 to 1000 times increased activity over the linear reference peptides), which in addition exhibited high selectivity against the platelet receptor αIIbβ3. This cyclic peptide was later modified by N-methylation of one peptide bond to yield an even greater antagonistic activity in c(RGDf(NMe)V). This peptide was then dubbed Cilengitide and is currently developed as drug by the company Merck-Serono (Germany). This article describes the chemical development of Cilengitide, the biochemical background of its activity and a short review about the present clinical trials. The positive anti-angiogenic effects in cancer treatment can be further increased by combination with "classical" anti-cancer therapies. Several clinical trials in this direction are under investigation.

Exploring new near-infrared fluorescent disulfide-based cyclic RGD peptide analogs for potential integrin-targeted optical imaging

We synthesized disulfide-based cyclic RGD pentapeptides bearing a near-infrared fluorescent dye (cypate), represented by cypate-c(CRGDC) (1) for integrin-targeted optical imaging. These compounds were compared with the traditional lactam-based cyclic RGD counterpart, cypate-c(RGDfK) (2). Molecular modeling suggests that the binding affinity of 2 to integrin α(v)β(3) is an order of magnitude higher than that of 1. This was confirmed experimentally, which further showed that substitution of Gly with Pro, Val and Tyr in 1 remarkably hampered the α(v)β(3) binding. Interestingly, cell microscopy with A549 cells showed that 1 exhibited higher cellular staining than 2. These results indicate that factors other than receptor binding affinity to α(v)β(3) dimeric proteins mediate cellular uptake. Consequently, 1 and its analogs may serve as valuable molecular probes for investigating the selectivity and specificity of integrin targeting by optical imaging.