Cyclic isoDGR and RGD peptidomimetics containing bifunctional diketopiperazine scaffolds are integrin antagonists

Small molecule- and peptide-drug conjugates addressing integrins: A story of targeted cancer treatment

Targeted cancer treatment should avoid side effects and damage to healthy cells commonly encountered during traditional chemotherapy. By combining small molecule or peptidic ligands as homing devices with cytotoxic drugs connected by a cleavable or non-cleavable linker in peptide-drug conjugates (PDCs) or small molecule-drug conjugates (SMDCs), cancer cells and tumours can be selectively targeted. The development of highly affine, selective peptides and small molecules in recent years has allowed PDCs and SMDCs to increasingly compete with antibody-drug conjugates (ADCs). Integrins represent an excellent target for conjugates because they are overexpressed by most cancer cells and because of the broad knowledge about native binding partners as well as the multitude of small-molecule and peptidic ligands that have been developed over the last 30 years. In particular, integrin αVβ3 has been addressed using a variety of different PDCs and SMDCs over the last two decades, following various strategies. This review summarises and describes integrin-addressing PDCs and SMDCs while highlighting points of great interest.

2,5-Diketopiperazines (DKPs): Promising Scaffolds for Anticancer Agents

2,5-Diketopiperazine (2,5-DKP) derivatives represent a family of secondary metabolites widely produced by bacteria, fungi, plants, animals, and marine organisms. Many natural products with DKP scaffolds exhibited various pharmacological activities such as antiviral, antifungal, antibacterial, and antitumor. 2,5-DKPs are recognized as privileged structures in medicinal chemistry, and compounds that incorporate the 2,5-DKP scaffold have been extensively investigated for their anticancer properties. This review is a thorough update on the anti-cancer activity of natural and synthesized 2,5-DKPs from 1997 to 2022. We have explored various aspects of 2,5-DKPs modifications and summarized their structure-activity relationships (SARs) to gain insight into their anticancer activities. We have also highlighted the novel approaches to enhance the specificity and pharmacokinetics of 2,5-DKP-based anticancer agents.

Optimizing the enzymatic release of MMAE from isoDGR-based small molecule drug conjugate by incorporation of a GPLG-PABC enzymatically cleavable linker

Antibody-Drug Conjugates (ADCs) and Small Molecule-Drug Conjugates (SMDCs) represent successful examples of targeted drug-delivery technologies for overcoming unwanted side effects of conventional chemotherapy in cancer treatment. In both strategies, a cytotoxic payload is connected to the tumor homing moiety through a linker that releases the drug inside or in proximity of the tumor cell, and that represents a key component for the final therapeutic effect of the conjugate. Here, we show that the replacement of the Val-Ala-p-aminobenzyloxycarbamate linker with the Gly-Pro-Leu-Gly-p-aminobenzyloxycarbamate (GPLG-PABC) sequence as enzymatically cleavable linker in the SMDC bearing the cyclo[DKP-isoDGR] α_Vβ₃ integrin ligand as tumor homing moiety and the monomethyl auristatin E (MMAE) as cytotoxic payload led to a 4-fold more potent anti-tumoral effect of the final conjugate on different cancer cell lines. In addition, the synthesized conjugate resulted to be significantly more potent than the free MMAE when tested following the "kiss-and-run" protocol, and the relative potency were clearly consistent with the expression of the α_Vβ₃ integrin receptor in the considered cancer cell lines. In vitro enzymatic cleavage tests showed that the GPLG-PABC linker is cleaved by lysosomal enzymes, and that the released drug is observable already after 15 min of incubation. Although additional data are needed to fully characterize the releasing capacity of GPLG-PABC linker, our findings are of therapeutic significance since we are introducing an alternative to other well-established enzymatically sensitive peptide sequences that might be used in the future for generating more efficient and less toxic drug delivery systems.

RGD and Scutellarin Conjugate (WK001) Targeting Platelet Glycoprotein IIb/IIIa Receptor Protects from Myocardial Ischemia/Reperfusion Injury: Synthesis, Characterization, and Bioactivity Evaluation

Myocardial ischemia/reperfusion (MI/R) injury is an unresolved clinical challenge. The blockade of binding fibrinogen by glycoprotein IIb/IIIa (GPIIb-IIIa) inhibitors has become a new therapeutic approach against MI/R injury. In this study, we modified the RGD structure to combine with scutellarin and synthesized a novel peptide, scutellarin-HomoArg-Gly-Asp-Trp-NH₂ (WK001). Herein, reported experimental and docking evidence indicates that WK001 provides immediate and potent platelet inhibition, with stronger inhibition of platelet aggregation than eptifibatide and scutellarin. In particular, it is administered intravenously to prevent thrombus formation and attenuate myocardial fibrosis progression in vivo. Therefore, WK001 could be developed as an antiplatelet drug to treat thrombosis-associated diseases, such as stroke and myocardial infarction.

RGD Peptidomimetic MMAE-Conjugate Addressing Integrin αVβ3-Expressing Cells with High Targeting Index

Small molecule-drug conjugates (SMDCs) mimicking the RGD sequence (-Arg-Gly-Asp-) with a non-peptide moiety require a pharmacophore-independent attachment site. A library of 36 sulfonamide-modified RGD mimetics with nM to pM affinity for integrin α_V β₃ was synthesized and analysed via DAD mapping. The best structure of the conjugable RGD mimetic was used and a linker was attached to an aromatic ring by Negishi cross-coupling. The product retained high affinity and selectivity for integrin α_V β₃ . The conjugable RGD mimetic was then attached to an enzymatically cleavable GKGEVA linker equipped with a self-immolative PABC and the antimitotic drug monomethyl auristatin E (MMAE). The resulting SMDC preferred binding to integrin α_V β₃ over α₅ β₁ in a ratio of 1 : 4519 (ELISA) and showed selectivity for α_V β₃ -positive WM115 cells over α_V β₃ -negative M21-L cells in the in vitro cell adhesion assay as well as in cell viability assays with a targeting index of 134 (M21-L/WM115).

Cyclic Dipeptide: A Privileged Molecular Scaffold to Derive Structural Diversity and Functional Utility

Cyclic dipeptides (CDPs) are the simplest form of cyclic peptides with a wide range of applications from therapeutics to biomaterials. CDP is a versatile molecular platform endowed with unique properties such as conformational rigidity, intermolecular interactions, structural diversification through chemical synthesis, bioavailability and biocompatibility. A variety of natural products with the CDP core exhibit anticancer, antifungal, antibacterial, and antiviral activities. The inherent bioactivities have inspired the development of synthetic analogues as drug candidates and drug delivery systems. CDP plays a crucial role as conformation and molecular assembly directing core in the design of molecular receptors, peptidomimetics and fabrication of functional material architectures. In recent years, CDP has rapidly become a privileged scaffold for the design of advanced drug candidates, drug delivery agents, bioimaging, and biomaterials to mitigate numerous disease conditions. This review describes the structural diversification and multifarious biomedical applications of the CDP scaffold, discusses challenges, and provides future directions for the emerging field.

Cyclic RGD and isoDGR Integrin Ligands Containing cis-2-amino-1-cyclopentanecarboxylic (cis-β-ACPC) Scaffolds

Integrin ligands containing the tripeptide sequences Arg-Gly-Asp (RGD) and iso-Asp-Gly- Arg (isoDGR) were actively investigated as inhibitors of tumor angiogenesis and directing unit in tumor-targeting drug conjugates. Reported herein is the synthesis, of two RGD and one isoDGR cyclic peptidomimetics containing (1S,2R) and (1R,2S) cis-2-amino-1-cyclopentanecarboxylic acid (cis-β-ACPC), using a mixed solid phase/solution phase synthetic protocol. The three ligands were examined in vitro in competitive binding assays to the purified α_vβ₃ and α₅β₁ receptors using biotinylated vitronectin (α_vβ₃) and fibronectin (α₅β₁) as natural displaced ligands. The IC50 values of the ligands ranged from nanomolar (the two RGD ligands) to micromolar (the isoDGR ligand) with a pronounced selectivity for α_vβ₃ over α₅β₁. In vitro cell adhesion assays were also performed using the human skin melanoma cell line WM115 (rich in integrin α_vβ₃). The two RGD ligands showed IC₅₀ values in the same micromolar range as the reference compound (cyclo[RGDfV]), while for the isoDGR derivative an IC₅₀ value could not be measured for the cell adhesion assay. A conformational analysis of the free RGD and isoDGR ligands by NMR (VT-NMR and NOESY experiments) and computational studies (MC/EM and MD), followed by docking simulations performed in the α_Vβ₃ integrin active site, provided a rationale for the behavior of these ligands toward the receptor.

β-Glucuronidase triggers extracellular MMAE release from an integrin-targeted conjugate

A non-internalizing αvβ3 integrin ligand was conjugated to the anticancer drug MMAE through a β-glucuronidase-responsive linker. In the presence of β-glucuronidase, only the conjugate bearing a PEG4 spacer inhibited the proliferation of integrin-expressing cancer cells at low nanomolar concentrations, indicating important structural requirements for the efficacy of these therapeutics.

Stem-Like Cancer Cells in a Dynamic 3D Culture System: A Model to Study Metastatic Cell Adhesion and Anti-Cancer Drugs

Metastatic spread is mainly sustained by cancer stem cells (CSC), a subpopulation of cancer cells that displays stemness features. CSC are thought to be derived from cancer cells that undergo epithelial to mesenchymal transition (EMT), thus acquiring resistance to anoikis and anti-cancer drugs. After detachment from the primary tumor mass, CSC reach the blood and lymphatic flow, and disseminate to the target tissue. This process is by nature dynamic and in vitro models are quite far from the in vivo situation. In this study, we have tried to reproduce the adhesion process of CSC to a target tissue by using a 3D dynamic cell culture system. We isolated two populations of 3D tumor spheroids displaying CSC-like features from breast carcinoma (MCF-7) and lung carcinoma (A549) cell lines. Human fibroblasts were layered on a polystyrene scaffold placed in a dynamically perfused millifluidic system and then the adhesion of tumor cell derived from spheroids to fibroblasts was investigated under continuous perfusion. After 24 h of perfusion, we found that spheroid cells tightly adhered to fibroblasts layered on the scaffold, as assessed by a scanning electron microscope (SEM). To further investigate mechanisms involved in spheroid cell adhesion to fibroblasts, we tested the effect of three RGD integrin antagonists with different molecular structures on cell adhesion; when injected into the circuit, only cilengitide was able to inhibit cell adhesion to fibroblasts. Although our model needs further refinements and improvements, we do believe this study could represent a promising approach in improving current models to study metastatic infiltration in vitro and a new tool to screen new potential anti-metastatic molecules.

Kiss and Run: Promoting Effective and Targeted Cellular Uptake of a Drug Delivery Vehicle Composed of an Integrin-Targeting Diketopiperazine Peptidomimetic and a Cell-Penetrating Peptide

Cell-penetrating peptides (CPPs) have emerged as powerful tools in terms of drug delivery. Those short, often cationic peptides are characterized by their usually low toxicity and their ability to transport diverse cargos inside almost any kinds of cells. Still, one major drawback is their nonselective uptake making their application in targeted cancer therapies questionable. In this work, we aimed to combine the power of a CPP (sC18) with an integrin-targeting unit (c[DKP-f3-RGD]). The latter is composed of the Arg-Gly-Asp peptide sequence cyclized via a diketopiperazine scaffold and is characterized by its high selectivity toward integrin α_vβ₃. The two parts were linked via copper-catalyzed alkyne-azide click reaction (CuAAC), while the CPP was additionally functionalized with either a fluorescent dye or the anticancer drug daunorubicin. Both functionalities allowed a careful biological evaluation of these novel peptide-conjugates regarding their cellular uptake mechanism, as well as cytotoxicity in α_vβ₃ integrin receptor expressing cells versus cells that do not express α_vβ₃. Our results show that the uptake follows a "kiss-and-run"-like model, in which the conjugates first target and recognize the receptor, but translocate mainly by CPP mediation. Thereby, we observed significantly more pronounced toxic effects in α_vβ₃ expressing U87 cells compared to HT-29 and MCF-7 cells, when the cells were exposed to the substances with only very short contact times (15 min). All in all, we present new concepts for the design of cancer selective peptide-drug conjugates.

Synthesis and Biological Evaluation of RGD and isoDGR-Monomethyl Auristatin Conjugates Targeting Integrin αV β3

This work reports the synthesis of a series of small-molecule-drug conjugates containing the αV β3 -integrin ligand cyclo[DKP-RGD] or cyclo[DKP-isoDGR], a lysosomally cleavable Val-Ala (VA) linker or an "uncleavable" version devoid of this sequence, and monomethyl auristatin E (MMAE) or F (MMAF) as the cytotoxic agent. The conjugates were obtained via a straightforward synthetic scheme taking advantage of a copper-catalyzed azide-alkyne cycloaddition as the key step. The conjugates were tested for their binding affinity for the isolated αv β3 receptor and were shown to retain nanomolar IC50 values, in the same range as those of the free ligands. The cytotoxic activity of the conjugates was evaluated in cell viability assays with αv β3 integrin overexpressing human glioblastoma (U87) and human melanoma (M21) cells. The conjugates possess markedly lower cytotoxic activity than the free drugs, which is consistent with inefficient integrin-mediated internalization. In almost all cases the conjugates featuring isoDGR as integrin ligand exhibited higher potency than their RGD counterparts. In particular, the cyclo[DKP-isoDGR]-VA-MMAE conjugate has low nanomolar IC50 values in cell viability assays with both cancer cell lines tested (U87: 11.50±0.13 nm; M21: 6.94±0.09 nm) and is therefore a promising candidate for in vivo experiments.

Neutrophil Elastase Promotes Linker Cleavage and Paclitaxel Release from an Integrin-Targeted Conjugate

This work takes advantage of one of the hallmarks of cancer, that is, the presence of tumor infiltrating cells of the immune system and leukocyte-secreted enzymes, to promote the activation of an anticancer drug at the tumor site. The peptidomimetic integrin ligand cyclo(DKP-RGD) was found to accumulate on the surface of αv β3 integrin-expressing human renal cell carcinoma 786-O cells. The ligand was conjugated to the anticancer drug paclitaxel through a Asn-Pro-Val (NPV) tripeptide linker, which is a substrate of neutrophil-secreted elastase. In vitro linker cleavage assays and cell antiproliferative experiments demonstrate the efficacy of this tumor-targeting conjugate, opening the way to potential therapeutic applications.

An RGD small-molecule integrin antagonist induces detachment-mediated anoikis in glioma cancer stem cells

The malignancy of glioblastoma (GB) is primarily due to the ability of glioma cancer stem cells (GSC) to disseminate into surrounding brain tissues, despite surgery and chemotherapy, and to form new tumoral masses. Members of the RGD-binding integrin family, which recognize the arginine-glycine-aspartic acid (RGD) sequence present in components of the extracellular matrix, and which serve a crucial function in the dissemination of GCS, are overexpressed in GB. Small-molecule integrin antagonists (SMIAs) designed to recognize RGD-integrins may therefore be an effective tool for decreasing GB infiltration and recurrence. In the present study, in vitro pro-apoptotic and infiltrative effects elicited by the SMIA 1a‑RGD in human GSC were investigated. Reverse transcription-quantitative polymerase chain reaction analysis revealed that, compared with normal human astrocytes, GSC grown on laminin-coated dishes overexpressed stemness markers as well as αvβ3 and αvβ5 integrins. In addition, dissociated GSC were identified to exhibit tumorigenic capacity when injected into immunodeficient mice. Using annexin/fluorescence-activated cell sorting analysis and ELISA nucleosome assays, it was identified that treatment of GSC with 25 µM 1a‑RGD for 48 h elicited detachment‑dependent anoikis not accompanied by necrosis-dependent cell death. A colorimetric proliferation assay indicated that 1a‑RGD did not affect cell viability, but that, instead, it markedly inhibited GSC migration as assessed using a Transwell assay. Western blot experiments revealed a decrease in focal adhesion kinase and protein kinase B phosphorylation with a concomitant increase in caspase-9 and -3/7 activity following 1a‑RGD treatment, suggesting that the pro-anoikis effects of 1a‑RGD may be mediated by these molecular mechanisms. Western blot analysis revealed no changes in specific markers of autophagy, suggesting further that 1a‑RGD-induced cell death is primarily sustained by anoikis-associated mechanisms. In conclusion, the results of the present study indicate that SMIA have potential as a therapeutic tool for decreasing GSC dissemination.

Synthesis and biological evaluation of RGD and isoDGR peptidomimetic-α-amanitin conjugates for tumor-targeting

RGD-α-amanitin and isoDGR-α-amanitin conjugates were synthesized by joining integrin ligands to α-amanitin via various linkers and spacers. The conjugates were evaluated for their ability to inhibit biotinylated vitronectin binding to the purified α_Vβ₃ receptor, retaining good binding affinity, in the same nanomolar range as the free ligands. The antiproliferative activity of the conjugates was evaluated in three cell lines possessing different levels of α_Vβ₃ integrin expression: human glioblastoma U87 (α_Vβ₃+), human lung carcinoma A549 (α_Vβ₃−) and breast adenocarcinoma MDA-MB-468 (α_Vβ₃−). In the U87, in the MDA-MB-468, and partly in the A549 cancer cell lines, the cyclo[DKP-isoDGR]-α-amanitin conjugates bearing the lysosomally cleavable Val-Ala linker were found to be slightly more potent than α-amanitin. Apparently, for all these α-amanitin conjugates there is no correlation between the cytotoxicity and the expression of α_Vβ₃ integrin. To determine whether the increased cytotoxicity of the cyclo[DKP-isoDGR]-α-amanitin conjugates is governed by an integrin-mediated binding and internalization process, competition experiments were carried out in which the conjugates were tested with U87 (α_Vβ₃+, α_Vβ₅+, α_Vβ₆−, α₅β₁+) and MDA-MB-468 (α_Vβ₃−, α_Vβ₅+, α_Vβ₆+, α₅β₁−) cells in the presence of excess cilengitide, with the aim of blocking integrins on the cell surface. Using the MDA-MB-468 cell line, a fivefold increase of the IC₅₀ was observed for the conjugates in the presence of excess cilengitide, which is known to strongly bind not only α_Vβ₃, but also α_Vβ₅, α_Vβ₆, and α₅β₁. These data indicate that in this case the cyclo[DKP-isoDGR]-α-amanitin conjugates are possibly internalized by a process mediated by integrins different from α_Vβ₃ (e.g., α_Vβ₅).

Investigating the Interaction of Cyclic RGD Peptidomimetics with αVβ₆ Integrin by Biochemical and Molecular Docking Studies

The interaction of a small library of cyclic RGD (Arg-Gly-Asp) peptidomimetics with α_Vβ₆ integrin has been investigated by means of competitive solid phase binding assays to the isolated receptor and docking calculations in the crystal structure of the α_Vβ₆ binding site. To this aim, a rigid receptor-flexible ligand docking protocol has been set up and then applied to predict the binding mode of the cyclic RGD peptidomimetics to α_Vβ₆ integrin. Although the RGD interaction with α_Vβ₆ recapitulates the RGD binding mode observed in α_Vβ₃, differences between the integrin binding pockets can strongly affect the ligand binding ability. In general, the peptidomimetics exhibited IC₅₀ values for integrin α_Vβ₆ (i.e., the concentration of compound required for 50% inhibition of biotinylated fibronectin binding to isolated α_Vβ₆ integrin) in the nanomolar range (77–345 nM), about 10–100 times higher than those for the related α_Vβ₃ receptor, with a single notable ligand displaying a low nanomolar IC₅₀ value (2.3 nM). Insights from the properties of the binding pocket combined with the analysis of the docking poses provided a rationale for ligand recognition and selectivity.

Probing Membrane Receptor-Ligand Specificity with Surface- and Tip- Enhanced Raman Scattering

The specific interaction between a ligand and a protein is a key component in minimizing off-target effects in drug discovery. Investigating these interactions with membrane protein receptors can be quite challenging. In this report, we show how spectral variance observed in surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) can be correlated with ligand specificity in affinity-based assays. Variations in the enhanced Raman spectra of three peptide ligands (i.e., cyclic-RGDFC, cyclic-isoDGRFC, and CisoDGRC), which have different binding affinity to αvβ3 integrin, are reported from isolated proteins and from receptors in intact cancer cell membranes. The SERS signal from the purified proteins provides basis spectra to analyze the signals in cells. Differences in the spectral variance within the SERS and TERS data for each ligand indicate larger variance for nonspecific ligand-receptor interactions. The SERS and TERS results are correlated with single particle tracking experiments of the ligand-functionalized nanoparticles with purified receptors on glass surfaces and living cells. These results demonstrate the ability to elucidate protein-ligand recognition using the observed vibrational spectra and provide perspective on binding specificity for small-molecule ligands in intact cell membranes, demonstrating a new approach for investigating drug specificity.

Integrins and Exosomes, a Dangerous Liaison in Cancer Progression

Integrin activity and function is classically related to the bi-directional regulation of cell-extracellular matrix (ECM) contacts that regulate a number of cell pathways linked to cell adhesion, cell detachment from ECM, cell migration, and anoikis. Interestingly, emerging data continue to uncover new roles for integrins in cancer-relevant pathways, particularly concerning the regulation of immune cell activity in the tumor niche, like myeloid cell differentiation and function and, very recently, the regulation of metastatic processes by exosomes. Exosomes are deeply involved in cell-cell communication processes and several studies have shown that integrins found in tumor-associated exosomes can promote cancer progression by two novel cooperative mechanisms: horizontal transfer of integrin transcripts as vescicle cargo, and selection of target tissues to form new tumor niches during metastatic spread by integrins carried on the exosome’s surface. In this review we will discuss mounting evidence that contribute to the development of a new picture for integrins in cancer, highlighting the role of integrins in the processes that leads to tumor niche formation. In particular, the role of the periostin pathway in the recruitment of tumor-associated macrophages, and the proposed contribution of exosome-derived integrins in the metastatic spread will be discussed. Finally, in light of the above considerations, an evaluation of integrins as possible therapeutic targets will be conducted.

Tumor Targeting with an isoDGR-Drug Conjugate

Herein we report the first example of an isoDGR-drug conjugate (2), designed to release paclitaxel selectively within cancer cells expressing integrin αV β3 . Conjugate 2 was synthesized by connecting the isoDGR peptidomimetic 5 with paclitaxel via the lysosomally cleavable Val-Ala dipeptide linker. Conjugate 2 displayed a low nanomolar affinity for the purified integrin αV β3 receptor (IC50 =11.0 nm). The tumor targeting ability of conjugate 2 was assessed in vitro in anti-proliferative assays on two isogenic cancer cell lines characterized by different integrin αV β3 expression: human glioblastoma U87 (αV β3 +) and U87 β3 -KO (αV β3 -). The isoDGR-PTX conjugate 2 displayed a remarkable targeting index (TI=9.9), especially when compared to the strictly related RGD-PTX conjugate 4 (TI=2.4).

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.

Guidance of Signaling Activations by Cadherins and Integrins in Epithelial Ovarian Cancer Cells

Epithelial ovarian cancer (EOC) is the deadliest tumor among gynecological cancer in the industrialized countries. The EOC incidence and mortality have remained unchanged over the last 30 years, despite the progress in diagnosis and treatment. In order to develop novel and more effective therapeutic approaches, the molecular mechanisms involved in EOC progression have been thoroughly investigated in the last few decades. At the late stage, peritoneal metastases originate from the attachment of small clusters of cancer cells that shed from the primary site and carried by the ascites adhere to the abdominal peritoneum or omentum. This behavior suggests that cell–cell or cell–matrix adhesion mechanisms regulate EOC growth and dissemination. Complex downstream signalings, which might be influenced by functional cross-talk between adhesion molecules and co-expressed and activated signaling proteins, can affect the proliferation/survival and the migration/invasion of EOC cells. This review aimed to define the impact of the mechanisms of cell–cell, through cadherins, and cell–extracellular matrix adhesion, through integrins, on the signaling cascades induced by membrane receptors and cytoplasmic proteins known to have a role in the proliferation, migration and invasion of EOC cells. Finally, some novel approaches using peptidomimetic ligands to cadherin and integrins are summarized.

Integrins in glioblastoma: Still an attractive target?

Integrin-mediated signaling pathways have been found to promote the invasiveness and survival of glioma cells by modifying the brain microenvironment to support the formation of the tumoral niche. A variety of cells in the niche express integrin receptors, including tumor-associated macrophages, fibroblasts, endothelial cells and pericytes. In particular, RGD-binding integrins have been demonstrated to have an important role in the epithelial-mesenchymal transition process, considered the first step in the infiltration of tissue by cancer cells and molecular markers of which have been found in glioma cells. In simultaneous research, Small Molecule Integrin Antagonists (SMIA) yielded initially promising results in in vitro and in vivo studies, leading to clinical trials to test their safety and efficacy in combination with other anticancer drugs in the treatment of several tumor types. The initially high expectations, especially because of their antiangiogenic activity, which appeared to be a winning strategy against GBM, were not confirmed and this cast serious doubts on the real benefits to be gained from the use of SMIA for the treatment of cancer in humans. In this review, we provide an overview of recent findings concerning the functional roles of integrins, especially RGD-binding integrins, in the processes related to glioma cells survival and brain tissue infiltration. These findings disclose a new scenario in which recently developed SMIA might become useful tools to hinder glioblastoma cell dissemination.

Advancement in integrin facilitated drug delivery

The research of integrin-targeted anticancer agents has recorded important advancements in ingenious design of delivery systems, based either on the prodrug approach, or on nanoparticle carriers, but for now, none of these has reached a clinical stage of development. Past work in this area has been extensively reviewed by us and others. Thus, the purpose and scope of the present review is to survey the advancement reported in the last 3years, with focus on innovative delivery systems that appear to afford openings for future developments. These systems exploit the labelling with conventional and novel integrin ligands for targeting the interface of cancer cells and of endothelial cells involved in cancer angiogenesis, with the proteins of the extracellular matrix, in the circulation, in tissues, and in tumour stroma, as the site of progression and metastatic evolution of the disease. Furthermore, these systems implement the expertise in the development of nanomedicines to the purpose of achieving preferential biodistribution and uptake in cancer tissues, internalisation in cancer cells, and release of the transported drugs at intracellular sites. The assessment of the value of controlling these factors, and their combination, for future developments requires support of biological testing in appropriate mechanistic models, but also imperatively demand confirmation in therapeutically relevant in vivo models for biodistribution, efficacy, and lack of off-target effects. Thus, among many studies, we have tried to point out the results supported by relevant in vivo studies, and we have emphasised in specific sections those addressing the medical needs of drug delivery to brain tumours, as well as the delivery of oligonucleotides modulating gene-dependent pathological mechanism. The latter could constitute the basis of a promising third branch in the therapeutic armamentarium against cancer, in addition to antibody-based agents and to cytotoxic agents.

Synthesis, Characterization, and Biological Evaluation of a Dual-Action Ligand Targeting αvβ3 Integrin and VEGF Receptors

A dual-action ligand targeting both integrin αVβ3 and vascular endothelial growth factor receptors (VEGFRs), was synthesized via conjugation of a cyclic peptidomimetic αVβ3 Arg-Gly-Asp (RGD) ligand with a decapentapeptide. The latter was obtained from a known VEGFR antagonist by acetylation at the Lys13 side chain. Functionalization of the precursor ligands was carried out in solution and in the solid phase, affording two fragments: an alkyne VEGFR ligand and the azide integrin αVβ3 ligand, which were conjugated by click chemistry. Circular dichroism studies confirmed that both the RGD and VEGFR ligand portions of the dual-action compound substantially adopt the biologically active conformation. In vitro binding assays on isolated integrin αVβ3 and VEGFR-1 showed that the dual-action conjugate retains a good level of affinity for both its target receptors, although with one order of magnitude (10/20 times) decrease in potency. The dual-action ligand strongly inhibited the VEGF-induced morphogenesis in Human Umbilical Vein Endothelial Cells (HUVECs). Remarkably, its efficiency in preventing the formation of new blood vessels was similar to that of the original individual ligands, despite the worse affinity towards integrin αVβ3 and VEGFR-1.