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

New peptide architectures through C-H activation stapling between tryptophan-phenylalanine/tyrosine residues

Natural peptides show high degrees of specificity in their biological action. However, their therapeutical profile is severely limited by their conformational freedom and metabolic instability. Stapled peptides constitute a solution to these problems and access to these structures lies on a limited number of reactions involving the use of non-natural amino acids. Here, we describe a synthetic strategy for the preparation of unique constrained peptides featuring a covalent bond between tryptophan and phenylalanine or tyrosine residues. The preparation of such peptides is achieved in solution and on solid phase directly from the corresponding sequences having an iodo-aryl amino acid through an intramolecular palladium-catalysed C–H activation process. Moreover, complex topologies arise from the internal stapling of cyclopeptides and double intramolecular arylations within a linear peptide. Finally, as a proof of principle, we report the application to this new stapling method to relevant biologically active compounds.

Macrocyclic, constrained peptides show promise in therapeutic applications due to the stable and defined conformations that can be produced. Here, the authors report a method to form macrocyclic peptides through C–H activation on tryptophan and coupling with iodo-substituted aryl amino acids

Pharmacological Inhibition of β3 Integrin Reduces the Inflammatory Toxicities Caused by Oncolytic Adenovirus without Compromising Anticancer Activity

Adenoviruses have been clinically tested as anticancer therapies but their utility has been severely limited by rapid, systemic cytokine release and consequent inflammatory toxicity. Here, we describe a new approach to tackling these dangerous side effects. Using human ovarian cancer cell lines as well as malignant epithelial cells harvested from the ascites of women with ovarian cancer, we show that tumor cells do not produce cytokines in the first 24 hours following in vitro infection with the oncolytic adenovirus dl922-947. In contrast, dl922-947 does induce inflammatory cytokines at early time points following intraperitoneal delivery in mice with human ovarian cancer intraperitoneal xenografts. In these animals, cytokines originate predominantly in murine tissues, especially in macrophage-rich organs such as the spleen. We use a nonreplicating adenovirus to confirm that early cytokine production is independent of adenoviral replication. Using β3 integrin knockout mice injected intraperitoneally with dl922-947 and β3 null murine peritoneal macrophages, we confirm a role for macrophage cell surface β3 integrin in this dl922-947-induced inflammation. We present new evidence that co-administration of a cyclic RGD-mimetic-specific inhibitor of β3 integrin significantly attenuates the cytokine release and inflammatory hepatic toxicity induced by dl922-947 in an intraperitoneal murine model of ovarian cancer. Importantly, we find no evidence that β3 inhibition compromises viral infectivity and oncolysis in vitro or anticancer efficacy in vivo. By enabling safe, systemic delivery of replicating adenoviruses, this novel approach could have a major impact on the future development of these effective anticancer agents.

Design, synthesis, conformational analysis and application of indolizidin-2-one dipeptide mimics

Growth in the field of peptide mimicry over the past few decades has resulted in the synthesis of many new compounds and the investigation of novel pharmacological agents. Azabicyclo[X.Y.0]alkanone amino acids are among the attractive classes of constrained mimics, because they can create rigid peptide structures for probing the conformation and roles of natural motifs in recognition events important for biological activity. Herein, we review the last ten years of the synthesis, conformational analysis and activity of analogs of the azabicyclo[4.3.0]alkan-2-one amino acid subclass, so-called indolizidin-2-one amino acids, with particular attention on their employment as inputs for biological applications.

PTEN expression in endothelial cells is down-regulated by uPAR to promote angiogenesis

The tumour suppressor phosphatase and tensin homologue (PTEN), mutated or lost in many human cancers, is a major regulator of angiogenesis. However, the cellular mechanism of PTEN regulation in endothelial cells so far remains elusive. Here, we characterise the urokinase receptor (uPAR, CD87) and its tumour-derived soluble form, suPAR, as a key molecule of regulating PTEN in endothelial cells. We observed uPAR-deficient endothelial cells to express enhanced PTEN mRNA- and protein levels. Consistently, uPAR expression in endogenous negative uPAR cells, down-regulated PTEN and activated the PI3K/Akt pathway. Additionally, we found that integrin adhesion receptors act as trans-membrane signaling partners for uPAR to repress PTEN transcription in a NF-κB-dependent manner. Functional in vitro assays with endothelial cells, derived from uPAR-deficient and PTEN heterozygous crossbred mice, demonstrated the impact of uPAR-dependent PTEN regulation on cell motility and survival. In an in vivo murine angiogenesis model uPAR-deficient PTEN heterozygous animals increased the impaired angiogenic phenotype of uPAR knockout mice and were able to reverse the high invasive potential of PTEN heterozygots. Our data provide first evidence that endogenous as well as exogenous soluble uPAR down-regulated PTEN in endothelial cells to support angiogenesis. The uPAR-induced PTEN regulation might represent a novel target for drug interference, and may lead to the development of new therapeutic strategies in anti-angiogenic treatment.

Metabolism and disposition of the αv-integrin ß3/ß5 receptor antagonist cilengitide, a cyclic polypeptide, in humans

Cilengitide (EMD 121974, manufactured by Merck KGaA, Darmstadt, Germany) is an αv-integrin receptor antagonist showing high affinity for αvβ3 and αvβ5.This study determined the mass balance of cilengitide in healthy volunteers receiving a single intravenous infusion of 2.1 MBq (14) C-cilengitide spiked into 250 mL of 2000 mg of cilengitide. Blood, urine, and feces were collected up to day 15 or until excretion of radioactivity was below 1% of the administered dose. Total radioactivity derived from the administration of (14) C-cilengitide and unlabeled cilengitide levels were determined and used for calculation of pharmacokinetic parameters.(14) C-cilengitide-related radioactivity was completely recovered (94.5%; 87.4%-100.6%) and was mainly excreted into urine (mean, 79.0%; range, 70.3%-88.2%) and to a lesser extent into feces (mean, 15.5%; range, 9.3%-20.3%). Of the administered dose, 77.5% was recovered as unchanged cilengitide in urine. The concentration profiles of cilengitide and total radioactivity in plasma were comparable. No circulating metabolites were identified in plasma and urine. Two metabolites,M606-1 and M606-2, were identified in feces considered to be formed by intestinal peptidases or by peptidases from fecal bacteria. In conclusion, the data show that following intravenous administration, (14) C-cilengitide was completely recovered, was excreted mainly via renal elimination, and was not metabolized systemically.

Molecular Simulation of Conformational Pre-Organization in Cyclic RGD Peptides

To test the ability of molecular simulations to accurately predict the solution-state conformational properties of peptidomimetics, we examined a test set of 18 cyclic RGD peptides selected from the literature, including the anticancer drug candidate cilengitide, whose favorable binding affinity to integrin has been ascribed to its pre-organization in solution. For each design, we performed all-atom replica-exchange molecular dynamics simulations over several microseconds and compared the results to extensive published NMR data. We find excellent agreement with experimental NOE distance restraints, suggesting that molecular simulation can be a useful tool for the computational design of pre-organized solution-state structure. Moreover, our analysis of conformational populations estimates that, despite the potential for increased flexibility due to backbone amide isomerizaton, N-methylation provides about 0.5 kcal/mol of reduced conformational entropy to cyclic RGD peptides. The combination of pre-organization and binding-site compatibility explains the strong binding affinity of cilengitide to integrin.

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

The cyclo[DKP-isoDGR] peptidomimetics 2-5, containing bifunctional diketopiperazine (DKP) scaffolds that differ in the configuration of the two DKP stereocenters and in the substitution at the DKP nitrogen atoms, were prepared and examined in vitro in competitive binding assays with purified αv β3 and αv β5 integrin receptors. IC50 values ranged from low nanomolar (ligand 3) to submicromolar with αv β3 integrin. The biological activities of ligands cyclo[DKP3-RGD] 1 and cyclo[DKP3-isoDGR] 3, bearing the same bifunctional DKP scaffold and showing similar αV β3 integrin binding values, were compared in terms of their cellular effects in human U373 glioblastoma cells. Compounds 1 and 3 displayed overlapping inhibitory effects on the FAK/Akt integrin activated transduction pathway and on integrin-mediated cell infiltration processes, and qualify therefore, despite the different RGD and isoDGR sequences, as integrin antagonists. Both compounds induced apoptosis in glioma cells after 72 hour treatment.

Dual-action combination therapy enhances angiogenesis while reducing tumor growth and spread

Increasing chemotherapy delivery to tumors, while enhancing drug uptake and reducing side effects, is a primary goal of cancer research. In mouse and human cancer models in vivo, we show that coadministration of low-dose Cilengitide and Verapamil increases tumor angiogenesis, leakiness, blood flow, and Gemcitabine delivery. This approach reduces tumor growth, metastasis, and minimizes side effects while extending survival. At a molecular level, this strategy alters Gemcitabine transporter and metabolizing enzyme expression levels, enhancing the potency of Gemcitabine within tumor cells in vivo and in vitro. Thus, the dual action of low-dose Cilengitide, in vessels and tumor cells, improves chemotherapy efficacy. Overall, our data demonstrate that vascular promotion therapy is a means to improve cancer treatment.

Prediction and interpretation of the lipophilicity of small peptides

Peptide-based drug discovery has considerably expanded and solid in silico tools for the prediction of physico-chemical properties of peptides are urgently needed. In this work we tested some combinations of descriptors/algorithms to find the best model to predict [Formula: see text] of a series of peptides. To do that we evaluate the models statistical performances but also their skills in providing a reliable deconvolution of the balance of intermolecular forces governing the partitioning phenomenon. Results prove that a PLS model based on VolSurf+ descriptors is the best tool to predict [Formula: see text] of neutral and ionised peptides. The mechanistic interpretation also reveals that the inclusion in the chemical structure of a HBD group is more efficient in decreasing lipophilicity than the inclusion of a HBA group.

Synthesis and preclinical evaluation of a novel, selective 111In-labelled aminoproline-RGD-peptide for non-invasive melanoma tumor imaging

An ¹¹¹In-labelled Amp-based RGD-DOTA conjugate was synthesized and evaluated in preclinical models of human melanoma as a novel integrin-targeted SPECT imaging tracer.

Blocking the heat shock response and depleting HSF-1 levels through heat shock protein 90 (hsp90) inhibition: a significant advance on current hsp90 chemotherapies

C-terminal inhibitors of heat shock protein 90 (hsp90) modulate the C-terminus and do not elicit a heat shock response.

Meaningful prevention of breast cancer metastasis: candidate therapeutics, preclinical validation, and clinical trial concerns

The development of drugs to treat breast and other cancers proceeds through phase I dose finding, phase II efficacy, and phase III comparative studies in the metastatic setting, only then asking if metastasis can be prevented in adjuvant trials. Compounds without overt cytotoxic activity, such as those developed to inhibit metastatic colonization, will likely fail to shrink established lesions in the metastatic setting and never be tested in a metastasis prevention scenario where they were preclinically validated. We and others have proposed phase II primary and secondary metastasis prevention studies to address this need. Herein, we have asked whether preclinical metastasis prevention data agrees with the positive adjuvant setting trials. The data are limited but complimentary. We also review fundamental pathways involved in metastasis, including Src, integrins, focal adhesion kinase (FAK), and fibrosis, for their clinical progress to date and potential for metastasis prevention. Issues of inadequate preclinical validation and clinical toxicity profiles are discussed.

A chemically cross-linked knottin dimer binds integrins with picomolar affinity and inhibits tumor cell migration and proliferation

Molecules that target and inhibit α_vβ₃, α_vβ₅, and α₅β₁ integrins have generated great interest because of the role of these receptors in mediating angiogenesis and metastasis. Attempts to increase the binding affinity and hence the efficacy of integrin inhibitors by dimerization have been marginally effective. In the present work, we achieved this goal by using oxime-based chemical conjugation to synthesize dimers of integrin-binding cystine knot (knottin) miniproteins with low-picomolar binding affinity to tumor cells. A non-natural amino acid containing an aminooxy side chain was introduced at different locations within a knottin monomer and reacted with dialdehyde-containing cross-linkers of different lengths to create knottin dimers with varying molecular topologies. Dimers cross-linked through an aminooxy functional group located near the middle of the protein exhibited higher apparent binding affinity to integrin-expressing tumor cells compared with dimers cross-linked through an aminooxy group near the C-terminus. In contrast, the cross-linker length had no effect on the integrin binding affinity. A chemical-based dimerization strategy was critical, as knottin dimers created through genetic fusion to a bivalent antibody domain exhibited only modest improvement (less than 5-fold) in tumor cell binding relative to the knottin monomer. The best oxime-conjugated knottin dimer achieved an unprecedented 150-fold increase in apparent binding affinity over the knottin monomer. Also, this dimer bound 3650-fold stronger and inhibited tumor cell migration and proliferation compared with cilengitide, an integrin-targeting peptidomimetic that performed poorly in recent clinical trials, suggesting promise for further therapeutic development.

Integrin αvβ3 acting as membrane receptor for thyroid hormones mediates angiogenesis in malignant T cells

The interaction of lymphoid tumor cells with components of the extracellular matrix via integrin αvβ3 allows tumor survival and growth. This integrin was demonstrated to be the membrane receptor for thyroid hormones (THs) in several tissues. We found that THs, acting as soluble integrin αvβ3 ligands, activated growth-related signaling pathways in T-cell lymphomas (TCLs). Specifically, TH-activated αvβ3 integrin signaling promoted TCL proliferation and angiogenesis, in part, via the upregulation of vascular endothelial growth factor (VEGF). Consequently, genetic or pharmacologic inhibition of integrin αvβ3 decreased VEGF production and induced TCL cell death in vitro and in human xenograft models. In sum, we show that integrin αvβ3 transduces prosurvival signals into TCL nuclei, suggesting a novel mechanism for the endocrine modulation of TCL pathophysiology. Targeting this mechanism could constitute an effective and potentially low-toxicity chemotherapy-free treatment of TCL patients.

How pathogens use linear motifs to perturb host cell networks

Molecular mimicry is one of the powerful stratagems that pathogens employ to colonise their hosts and take advantage of host cell functions to guarantee their replication and dissemination. In particular, several viruses have evolved the ability to interact with host cell components through protein short linear motifs (SLiMs) that mimic host SLiMs, thus facilitating their internalisation and the manipulation of a wide range of cellular networks. Here we present convincing evidence from the literature that motif mimicry also represents an effective, widespread hijacking strategy in prokaryotic and eukaryotic parasites. Further insights into host motif mimicry would be of great help in the elucidation of the molecular mechanisms behind host cell invasion and the development of anti-infective therapeutic strategies.

Cilengitide downmodulates invasiveness and vasculogenic mimicry of neuropilin 1 expressing melanoma cells through the inhibition of αvβ5 integrin

During melanoma progression, tumour cells show increased adhesiveness to the vascular wall, invade the extracellular matrix (ECM) and frequently form functional channels similar to vascular vessels (vasculogenic mimicry). These properties are mainly mediated by the interaction of integrins with ECM components. Since we had previously identified neuropilin 1 (NRP-1), a coreceptor of vascular endothelial growth factor A (VEGF-A), as an important determinant of melanoma aggressiveness, aims of this study were to identify the specific integrins involved in the highly invasive phenotype of NRP-1 expressing cells and to investigate their role as targets to counteract melanoma progression. Melanoma aggressiveness was evaluated in vitro as cell ability to migrate through an ECM layer and to form tubule-like structures using transfected cells. Integrins relevant to these processes were identified using specific blocking antibodies. The αvβ5 integrin was found to be responsible for about 80% of the capability of NRP-1 expressing cells to adhere on vitronectin. In these cells αvβ5 expression level was twice higher than in low-invasive control cells and contributed to the ability of melanoma cells to form tubule-like structures on matrigel. Cilengitide, a potent inhibitor of αν integrins activation, reduced ECM invasion, vasculogenic mimicry and secretion of VEGF-A and metalloproteinase 9 by melanoma cells. In conclusion, we demonstrated that ανβ5 integrin is involved in the highly aggressive phenotype of melanoma cells expressing NRP-1. Moreover, we identified a novel mechanism that contributes to the antimelanoma activity of the αv integrin inhibitor cilengitide based on the inhibition of vasculogenic mimicry.

Structural determinants of integrin β-subunit specificity for latent TGF-β

Eight integrin α-β heterodimers recognize ligands with an Arg-Gly-Asp (RGD) motif. However, the structural mechanism by which integrins differentiate among extracellular proteins with RGD motifs is not understood. Here, crystal structures, mutations and peptide-affinity measurements show that αVβ6 binds with high affinity to a RGDLXXL/I motif within the prodomains of TGF-β1 and TGF-β3. The LXXL/I motif forms an amphipathic α-helix that binds in a hydrophobic pocket in the β6 subunit. Elucidation of the basis for ligand binding specificity by the integrin β subunit reveals contributions by three different βI-domain loops, which we designate specificity-determining loops (SDLs) 1, 2 and 3. Variation in a pair of single key residues in SDL1 and SDL3 correlates with the variation of the entire β subunit in integrin evolution, thus suggesting a paradigmatic role in overall β-subunit function.

Cationic liposome-nucleic acid complexes for gene delivery and gene silencing

Cationic liposomes (CLs) are studied worldwide as carriers of DNA and short interfering RNA (siRNA) for gene delivery and gene silencing, and related clinical trials are ongoing. Optimization of transfection efficiency and silencing efficiency by cationic liposome carriers requires a comprehensive understanding of the structures of CL-nucleic acid complexes and the nature of their interactions with cell membranes as well as events leading to release of active nucleic acids within the cytoplasm. Synchrotron x-ray scattering has revealed that CL-nucleic acid complexes spontaneously assemble into distinct liquid crystalline phases including the lamellar, inverse hexagonal, hexagonal, and gyroid cubic phases, and fluorescence microscopy has revealed CL-DNA pathways and interactions with cells. The combining of custom synthesis with characterization techniques and gene expression and silencing assays has begun to unveil structure-function relations in vitro. As a recent example, this review will briefly describe experiments with surface-functionalized PEGylated CL-DNA nanoparticles. The functionalization, which is achieved through custom synthesis, is intended to address and overcome cell targeting and endosomal escape barriers to nucleic acid delivery faced by PEGylated nanoparticles designed for in vivo applications.

Macrocyclic Inhibitors of GPCR's, Integrins and Protein–Protein Interactions

This chapter summarizes some highlights of macrocyclic drug discovery in the area of GPCRs, integrins, and protein–protein interactions spanning roughly the last 30 years. Several examples demonstrate that incorporation of pharmacophores derived from natural peptide ligands into the context of a constrained macrocycle (“lock of the bioactive conformation”) has proven a powerful approach for the discovery of potent and selective macrocyclic drugs. In addition, it will be shown that macrocycles, due to their semi-rigid nature, can exhibit unique properties that can be beneficially exploited by medicinal chemists. Macrocycles can adapt their conformation during binding to a flexible protein target surface (“induced fit”), and due to their size, can interact with larger protein interfaces (“hot spots”). Also, macrocycles can display favorable ADME properties well beyond the rule of 5 in particular exhibiting favorable cell penetrating properties and oral bioavailability.

Constraining cyclic peptides to mimic protein structure motifs

Many proteins exert their biological activities through small exposed surface regions called epitopes that are folded peptides of well-defined three-dimensional structures. Short synthetic peptide sequences corresponding to these bioactive protein surfaces do not form thermodynamically stable protein-like structures in water. However, short peptides can be induced to fold into protein-like bioactive conformations (strands, helices, turns) by cyclization, in conjunction with the use of other molecular constraints, that helps to fine-tune three-dimensional structure. Such constrained cyclic peptides can have protein-like biological activities and potencies, enabling their uses as biological probes and leads to therapeutics, diagnostics and vaccines. This Review highlights examples of cyclic peptides that mimic three-dimensional structures of strand, turn or helical segments of peptides and proteins, and identifies some additional restraints incorporated into natural product cyclic peptides and synthetic macrocyclic peptidomimetics that refine peptide structure and confer biological properties.

Nanoscale integrin ligand patterns determine melanoma cell behavior

Cells use integrin receptors to adhere onto surfaces by binding to ligands such as the arginine-glycine-aspartic acid (RGD) motif. Cancer cells make use of this adhesion process, which has motivated the development of integrin-directed drugs. However, those drugs may exert paradoxical effects on tumor progression, which raises the question of how integrin function is governed in tumor cells on the nanoscale. We have utilized precisely defined and tunable RGD ligand site densities spanning 1 order of magnitude, i.e., 103 to 1145 ligand sites/μm(2), by using RGD-functionalized gold nanoparticle patterns immobilized on glass by block copolymer (micellar) nanolithography. In an αVβ3 integrin-dependent fashion, human melanoma cells spread, formed focal contacts, and reorganized cytoskeletal fibers on a physiologically relevant RGD density of 349 sites/μm(2). Intriguingly, low doses of solute RGD "shifted" the optimal densities of immobilized ligand along with corresponding melanoma cell integrin clusters and cytoskeletal changes toward those typical for "intermediate" ligand presentation. Consequently, melanoma cells were forced into a "permissive" state, optimizing interactions with suboptimal nanostructured biomimetic surfaces, thus providing an explanation for the seemingly paradoxical effects on tumor progression and a potential clue for individualized antitumoral therapies.

Receptor-bound conformation of cilengitide better represented by its solution-state structure than the solid-state structure

The X-ray crystal and NMR spectroscopic structures of the peptide drug candidate Cilengitide (cyclo(RGDf(NMe)Val)) in various solvents are obtained and compared in addition to the integrin receptor bound conformation. The NMR-based solution structures exhibit conformations closely resembling the X-ray structure of Cilengitide bound to the head group of integrin αvβ3. In contrast, the structure of pure Cilengitide recrystallized from methanol reveals a different conformation controlled by the lattice forces of the crystal packing. Molecular modeling studies of the various ligand structures docked to the αvβ3 integrin revealed that utilization of the solid-state conformation of Cilengitide leads-unlike the solution-based structures-to a mismatch of the ligand-receptor interactions compared with the experimentally determined structure of the protein-ligand complex. Such discrepancies between solution and crystal conformations of ligands can be misleading during the structure-based lead optimization process and should thus be taken carefully into account in ligand orientated drug design.

Synthetic molecules that protect cells from anoikis and their use in cell transplantation

One of the major problems encountered in cell transplantation is the low level of survival of transplanted cells due to detachment-induced apoptosis, called anoikis. The present study reports on the chemical synthesis and biological evaluation of water-soluble molecules that protect suspended cells from anoikis. The synthetic molecules bind to and induce clusters of integrins and heparan-sulfate-bound syndecans, two classes of receptors that are important for extracellular matrix-mediated cell survival. Molecular biological analysis indicates that such molecules prolong the survival of suspended NIH3T3 cells, at least in part, by promoting clustering of syndecan-4 and integrin β1 on the cell surface, leading to the activation of small GTPase Rac-1 and Akt. In vivo experiments using animal disease models demonstrated the ability of the molecules to improve cell engraftment. The cluster-inducing molecules may provide a starting point for the design of new synthetic tools for cell-based therapy.

Cytoplasmic salt bridge formation in integrin αvß3 stabilizes its inactive state affecting integrin-mediated cell biological effects

Heterodimeric integrin receptors are mediators of cell adhesion, motility, invasion, proliferation, and survival. By this, they are crucially involved in (tumor) cell biological behavior. Integrins trigger signals bidirectionally across cell membranes: by outside-in, following binding of protein ligands of the extracellular matrix, and by inside-out, where proteins are recruited to ß-integrin cytoplasmic tails resulting in conformational changes leading to increased integrin binding affinity and integrin activation. Computational modeling and experimental/mutational approaches imply that associations of integrin transmembrane domains stabilize the low-affinity integrin state. Moreover, a cytoplasmic interchain salt bridge is discussed to contribute to a tight clasp of the α/ß-membrane-proximal regions; however, its existence and physiological relevance for integrin activation are still a controversial issue. In order to further elucidate the functional role of salt bridge formation, we designed mutants of the tumor biologically relevant integrin αvß3 by mutually exchanging the salt bridge forming amino acid residues on each chain (αvR995D and ß3D723R). Following transfection of human ovarian cancer cells with different combinations of wild type and mutated integrin chains, we showed that loss of salt bridge formation strengthened αvß3-mediated adhesion to vitronectin, provoked recruitment of cytoskeletal proteins, such as talin, and induced integrin signaling, ultimately resulting in enhanced cell migration, proliferation, and activation of integrin-related signaling molecules. These data support the notion of a functional relevance of integrin cytoplasmic salt bridge disruption during integrin activation.

Targeted drugs in combination with radiotherapy for the treatment of solid tumors: current state and future developments

The continuously rising use of novel drugs, especially of molecules belonging to the group of targeted drugs is now shaping the therapeutic landscape. However, treatment combinations of targeted drugs with radiotherapy are still rare. Only the monoclonal antibody cetuximab (Erbitux®) has been approved for the treatment of locally advanced squamous cell cancer of the head and neck in combination with radiotherapy. Several targeted compounds are in advanced stages of clinical development for combination treatments with radiotherapy, of which substances with either anti-EGFR or anti-angiogenic mechanisms, such as trastuzumab, panitumumab, erlotinib, cilengitide and bevacizumab are the most promising. Aim of this article is to provide, mainly from a radio-oncological point of view, an overview about the current state as well as to give an outlook on the near future of the most advanced targeted combined treatment concepts for solid tumors.

Uptake and transfection efficiency of PEGylated cationic liposome-DNA complexes with and without RGD-tagging

Steric stabilization of cationic liposome-DNA (CL-DNA) complexes is required for in vivo applications such as gene therapy. PEGylation (PEG: poly(ethylene glycol)) of CL-DNA complexes by addition of PEG2000-lipids yields sterically stabilized nanoparticles but strongly reduces their gene delivery efficacy. PEGylation-induced weakening of the electrostatic binding of CL-DNA nanoparticles to cells (leading to reduced uptake) has been considered as a possible cause, but experimental results have been ambiguous. Using quantitative live-cell imaging in vitro, we have investigated cell attachment and uptake of PEGylated CL-DNA nanoparticles with and without a custom synthesized RGD-peptide grafted to the distal ends of PEG2000-lipids. The RGD-tagged nanoparticles exhibit strongly increased cellular attachment as well as uptake compared to nanoparticles without grafted peptide. Transfection efficiency of RGD-tagged PEGylated CL-DNA NPs increases by about an order of magnitude between NPs with low and high membrane charge density (σM; the average charge per unit area of the membrane; controlled by the molar ratio of cationic to neutral lipid), even though imaging data show that uptake of RGD-tagged particles is only slightly enhanced by high σM. This suggests that endosomal escape and, as a result, transfection efficiency of RGD-tagged NPs is facilitated by high σM. We present a model describing the interactions between PEGylated CL-DNA nanoparticles and the anionic cell membrane which shows how the PEG grafting density and membrane charge density affect adhesion of nanoparticles to the cell surface.

Effects of a novel cyclic RGD peptidomimetic on cell proliferation, migration and angiogenic activity in human endothelial cells

BACKGROUND

Cyclic RGD peptidomimetics containing a bifunctional diketopiperazine scaffold are a novel class of high-affinity ligands for the integrins αVβ3 and αVβ5. Since integrins are a promising target for the modulation of normal and pathological angiogenesis, the present study aimed at characterizing the ability of the RGD peptidomimetic cyclo[DKP-RGD] 1 proliferation, migration and network formation in human umbilical vein endothelial cells (HUVEC).

METHODS

Cell viability was assessed by flow cytometry and annexin V (ANX)/propidium iodide (PI) staining. Cell proliferation was evaluated by the ELISA measurement of bromodeoxyuridine (BrdU) incorporation. Network formation by HUVEC cultured in Matrigel-coated plates was evaluated by optical microscopy and image analysis. Integrin subunit mRNA expression was assessed by real time-PCR and Akt phosphorylation by western blot analysis.

RESULTS

Cyclo[DKP-RGD] 1 does not affect cell viability and proliferation either in resting conditions or in the presence of the pro-angiogenic growth factors VEGF, EGF, FGF, and IGF-I. Addition of cyclo[DKP-RGD] 1 however significantly decreased network formation induced by pro-angiogenic growth factors or by IL-8. Cyclo[DKP-RGD] 1 did not affect mRNA levels of αV, β3 or β5 integrin subunits, however it significantly reduced the phosphorylation of Akt.

CONCLUSIONS

Cyclo[DKP-RGD] 1 can be a potential modulator of angiogenesis induced by different growth factors, possibly devoid of the adverse effects of cytotoxic RGD peptidomimetic analogues.

Computational screening and selection of cyclic peptide hairpin mimetics by molecular simulation and kinetic network models

Designing peptidomimetic compounds to have a preorganized structure in solution is highly nontrivial. To show how simulation-based approaches can help speed this process, we performed an extensive simulation study of designed cyclic peptide mimics of a β-hairpin from bacterial protein LapD involved in a protein-protein interaction (PPI) pertinent to bacterial biofilm formation. We used replica exchange molecular dynamics (REMD) simulation to screen 20 covalently cross-linked designs with varying stereochemistry and selected the most favorable of these for massively parallel simulation on Folding@home in explicit solvent. Markov state models (MSMs) built from the trajectory data reveal how subtle chemical modifications can have a significant effect on conformational populations, leading to the overall stabilization of the target structure. In particular, we identify a key steric interaction between a methyl substituent and a valine side chain that acts to allosterically shift population between native and near-native states, which could be exploited in future designs. Visualization of this mechanism is aided considerably by the tICA method, which identifies degrees of freedom most important in slow conformational transitions. The combination of quantitative detail and human comprehension provided by MSMs suggests such approaches will be increasingly useful for design.

Redox-relevant aspects of the extracellular matrix and its cellular contacts via integrins

SIGNIFICANCE

The extracellular matrix (ECM) fulfills essential functions in multicellular organisms. It provides the mechanical scaffold and environmental cues to cells. Upon cell attachment, the ECM signals into the cells. In this process, reactive oxygen species (ROS) are physiologically used as signalizing molecules.

RECENT ADVANCES

ECM attachment influences the ROS-production of cells. In turn, ROS affect the production, assembly and turnover of the ECM during wound healing and matrix remodeling. Pathological changes of ROS levels lead to excess ECM production and increased tissue contraction in fibrotic disorders and desmoplastic tumors. Integrins are cell adhesion molecules which mediate cell adhesion and force transmission between cells and the ECM. They have been identified as a target of redox-regulation by ROS. Cysteine-based redox-modifications, together with structural data, highlighted particular regions within integrin heterodimers that may be subject to redox-dependent conformational changes along with an alteration of integrin binding activity.

CRITICAL ISSUES

In a molecular model, a long-range disulfide-bridge within the integrin β-subunit and disulfide bridges within the genu and calf-2 domains of the integrin α-subunit may control the transition between the bent/inactive and upright/active conformation of the integrin ectodomain. These thiol-based intramolecular cross-linkages occur in the stalk domain of both integrin subunits, whereas the ligand-binding integrin headpiece is apparently unaffected by redox-regulation.

FUTURE DIRECTIONS

Redox-regulation of the integrin activation state may explain the effect of ROS in physiological processes. A deeper understanding of the underlying mechanism may open new prospects for the treatment of fibrotic disorders.

Integrin modulators: a patent review

INTRODUCTION

Integrins are heterodimeric cell surface receptors, which enable adhesion, proliferation, and migration of cells by recognizing binding motifs in extracellular matrix (ECM) proteins. As transmembrane linkers between the cytoskeleton and the ECM, they are able to recruit a huge variety of proteins and to influence signaling pathways bidirectionally, thereby regulating gene expression and cell survival. Hence, integrins play a key role in various physiological as well as pathological processes, which has turned them into an attractive target for pharmaceutical research.

AREAS COVERED

In this review, the latest therapeutic developments of drug candidates and recently patented integrin ligands are summarized.

EXPERT OPINION

Integrins have been proven to be valuable therapeutic targets in the treatment of several inflammatory and autoimmune diseases, where leukocyte adhesion processes are regulated by them. Furthermore, they play an important role in pathological angiogenesis and tumor metastasis, being a promising target for cancer therapy.

Antiangiogenic activity of hypoxylonol C

Hypoxylonol C (1), isolated from the inedible mushroom Hypoxylon truncatum, exhibited inhibitory activities against the migration and tube formation of HUVECs. A cDNA microarray analysis was performed to investigate the target of hypoxylonol C (1) in HUVECs, and it was found that the genes related to cell cycle and adhesion were down-regulated. The down-regulation of mRNA levels of cell cycle and adhesion genes was confirmed by real-time RT-PCR. Cell cycle arrest and suppression of adhesion molecule expression might be plausible mechanisms of actions for the antiangiogenic activity of hypoxylonol C (1).

Apoptosis therapy in cancer: the first single-molecule co-activating p53 and the translocator protein in glioblastoma

In the complex scenario of cancer, treatment with compounds targeting multiple cell pathways has been emerging. In Glioblastoma Multiforme (GBM), p53 and Translocator Protein (TSPO), both acting as apoptosis inducers, represent two attractive intracellular targets. On this basis, novel indolylglyoxylyldipeptides, rationally designed to activate TSPO and p53, were synthesized and biologically characterized. The new compounds were able to bind TSPO and to reactivate p53 functionality, through the dissociation from its physiological inhibitor, murine double minute 2 (MDM2). In GBM cells, the new molecules caused Δψm dissipation and inhibition of cell viability. These effects resulted significantly higher with respect to those elicited by the single target reference standards applied alone, and coherent with the synergism resulting from the simultaneous activation of TSPO and p53. Taken together, these results suggest that TSPO/MDM2 dual-target ligands could represent a new attractive multi-modal opportunity for anti-cancer strategy in GBM.

Cellular mechanisms of tissue fibrosis. 8. Current and future drug targets in fibrosis: focus on Rho GTPase-regulated gene transcription

Tissue fibrosis occurs with excessive extracellular matrix deposition from myofibroblasts, resulting in tissue scarring and inflammation. It is driven by multiple mediators, such as the G protein-coupled receptor ligands lysophosphatidic acid and endothelin, as well as signaling by transforming growth factor-β, connective tissue growth factor, and integrins. Fibrosis contributes to 45% of deaths in the developed world. As current therapeutic options for tissue fibrosis are limited and organ transplantation is the only effective treatment for end-stage disease, there is an imminent need for efficacious antifibrotic therapies. This review discusses the various molecular pathways involved in fibrosis. It highlights the Rho GTPase signaling pathway and its downstream gene transcription output through myocardin-related transcription factor and serum response factor as a convergence point for targeting this complex set of diseases.

Cyclic peptides as therapeutic agents and biochemical tools

There are many cyclic peptides with diverse biological activities, such as antibacterial activity, immunosuppressive activity, and anti-tumor activity, and so on. Encouraged by natural cyclic peptides with biological activity, efforts have been made to develop cyclic peptides with both genetic and synthetic methods. The genetic methods include phage display, intein-based cyclic peptides, and mRNA display. The synthetic methods involve individual synthesis, parallel synthesis, as well as split-and-pool synthesis. Recent development of cyclic peptide library based on split-and-pool synthesis allows on-bead screening, in-solution screening, and microarray screening of cyclic peptides for biological activity. Cyclic peptides will be useful as receptor agonist/antagonist, RNA binding molecule, enzyme inhibitor and so on, and more cyclic peptides will emerge as therapeutic agents and biochemical tools.

Pharmacophoric modifications lead to superpotent αvβ3 integrin ligands with suppressed α5β1 activity

The selective targeting of the αvβ3 integrin subtype without affecting the structurally closely related receptor α5β1 is crucial for understanding the details of their biological and pathological functions and thus of great relevance for diagnostic and therapeutic approaches in cancer treatment. Here, we present the synthesis of highly active RGD peptidomimetics for the αvβ3 integrin with remarkable selectivity against α5β1. Incorporation of a methoxypyridine building block into a ligand scaffold and variation of different functional moieties led to αvβ3-antagonistic activities in the low nanomolar or even subnanomolar range. Furthermore, docking studies were performed to give insights into the binding modes of the novel compounds. The presented library comprises powerful ligands for specific addressing and blocking of the αvβ3 integrin subtype, thereby representing privileged tools for integrin-based personalized medicine.

Benefits of NOPO as chelator in gallium-68 peptides, exemplified by preclinical characterization of (68)Ga-NOPO-c(RGDfK)

The αvβ3-integrin addressing cyclic pentapeptide cyclo(RGDfK) was conjugated to NOPO, 1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid], a bifunctional chelator with exceptional gallium-68 labeling properties. NOPO-c(RGDfK) and its Ga(III) and Cu(II) complexes showed high affinity to αvβ3 integrin (IC50 = 0.94 ± 0.06, 1.02 ± 0.09, and 0.51 ± 0.06 nM, respectively). (68)Ga labeling of NOPO-c(RGDfK) in an automated GMP-compliant procedure was performed with near-quantitative radiochemical yield, using precursor amounts as low as 0.5 nmol (approximately 0.6 μg). (68)Ga-NOPO-c(RGDfK) was obtained with high purity (>99% by radio-HPLC/TLC) and, optionally, could be produced with specific activities up to 6 TBq/μmol. M21/M21L (human melanoma with high/low αvβ3 integrin expression) xenografted athymic CD-1 nude mice were used for biodistribution, in vivo stability studies, and PET imaging. (68)Ga-NOPO-c(RGDfK) showed rapid and specific uptake in M21 tumor xenografts (2.02 ± 0.34% ID/g at 60 min p.i.) and was found stable in vivo. Its high hydrophilicity is reflected by an octanol-water distribution coefficient (log D = -4.6) which is more than 1 order of magnitude lower compared to respective NOTA or DOTA analogues. As expected, (68)Ga-NOPO-c(RGDfK) thus showed fast renal clearance from nontargeted tissues. We conclude that NOPO might generally prove a useful means to improve renal clearance of corresponding radiopharmaceuticals by increasing the polarity of its bioconjugates. Favorable labeling properties render NOPO conjugates highly recommendable for reliable routine production of (68)Ga-radiopharmaceuticals in a clinical setting.