Structural basis for the interaction of isoDGR with the RGD-binding site of alphavbeta3 integrin

Mechanism of RGD-conjugated nanodevice binding to its target protein integrin αVβ3 by atomistic molecular dynamics and machine learning

Active targeting strategies have been proposed to enhance the selective uptake of nanoparticles (NPs) by diseased cells, and recent experimental findings have proven the effectiveness of this approach. However, no mechanistic studies have yet revealed the atomistic details of the interactions between ligand-activated NPs and integrins. As a case study, here we investigate, by means of advanced molecular dynamics simulations (MD) and machine learning methods (namely equilibrium MD, binding free energy calculations and training of self-organized maps), the interaction of a cyclic-RGD-conjugated PEGylated TiO2 NP (the nanodevice) with the extracellular segment of integrin αVβ3 (the target), the latter experimentally well-known to be over-expressed in several solid tumors. Firstly, we proved that the cyclic-RGD ligand binding to the integrin pocket is established and kept stable even in the presence of the cumbersome realistic model of the nanodevice. In this respect, the unsupervised machine learning analysis allowed a detailed comparison of the ligand/integrin binding in the presence and in the absence of the nanodevice, which unveiled differences in the chemical features. Then, we discovered that unbound cyclic RGDs conjugated to the NP largely contribute to the interactions between the nanodevice and the integrin. Finally, by increasing the density of cyclic RGDs on the PEGylated TiO2 NP, we observed a proportional enhancement of the nanodevice/target binding. All these findings can be exploited to achieve an improved targeting selectivity and cellular uptake, and thus a more successful clinical outcome.

Immunotherapy targeting isoDGR-protein damage extends lifespan in a mouse model of protein deamidation

Aging results from the accumulation of molecular damage that impairs normal biochemical processes. We previously reported that age-linked damage to amino acid sequence NGR (Asn-Gly-Arg) results in "gain-of-function" conformational switching to isoDGR (isoAsp-Gly-Arg). This integrin-binding motif activates leukocytes and promotes chronic inflammation, which are characteristic features of age-linked cardiovascular disorders. We now report that anti-isoDGR immunotherapy mitigates lifespan reduction of Pcmt1-/- mouse. We observed extensive accumulation of isoDGR and inflammatory cytokine expression in multiple tissues from Pcmt1-/- and naturally aged WT animals, which could also be induced via injection of isoDGR-modified plasma proteins or synthetic peptides into young WT animals. However, weekly injection of anti-isoDGR mAb (1 mg/kg) was sufficient to significantly reduce isoDGR-protein levels in body tissues, decreased pro-inflammatory cytokine concentrations in blood plasma, improved cognition/coordination metrics, and extended the average lifespan of Pcmt1-/- mice. Mechanistically, isoDGR-mAb mediated immune clearance of damaged isoDGR-proteins via antibody-dependent cellular phagocytosis (ADCP). These results indicate that immunotherapy targeting age-linked protein damage may represent an effective intervention strategy in a range of human degenerative disorders.

The challenges and opportunities of αvβ3-based therapeutics in cancer: From bench to clinical trials

Integrins are main cell adhesion receptors serving as linker attaching cells to extracellular matrix (ECM) and bidirectional hubs transmitting biochemical and mechanical signals between cells and their environment. Integrin αvβ3 is a critical family member of integrins and interacts with ECM proteins containing RGD tripeptide sequence. Accumulating evidence indicated that the abnormal expression of integrin αvβ3 was associated with various tumor progressions, including tumor initiation, sustained tumor growth, distant metastasis, drug resistance development, maintenance of stemness in cancer cells. Therefore, αvβ3 has been explored as a therapeutic target in various types of cancers, but there is no αvβ3 antagonist approved for human therapy. Targeting-integrin αvβ3 therapeutics has been a challenge, but lessons from the past are valuable to the development of innovative targeting approaches. This review systematically summarized the structure, signal transduction, regulatory role in cancer, and drug development history of integrin αvβ3, and also provided new insights into αvβ3-based therapeutics in cancer from bench to clinical trials, which would contribute to developing effective targeting αvβ3 agents for cancer treatment.

RGD Cyclopeptide Equipped with a Lysine-Engaging Salicylaldehyde Showing Enhanced Integrin Affinity and Cell Detachment Potency

Salicylaldehyde (SA) derivatives are emerging as useful fragments to obtain reversible-covalent inhibitors interacting with the lysine residues of the target protein. Here the SA installation at the C terminus of an integrin-binding cyclopeptide, leading to enhanced ligand affinity for the receptor as well as stronger biological activity in cultured glioblastoma cells is reported.

Targeting integrin pathways: mechanisms and advances in therapy

Integrins are considered the main cell-adhesion transmembrane receptors that play multifaceted roles as extracellular matrix (ECM)-cytoskeletal linkers and transducers in biochemical and mechanical signals between cells and their environment in a wide range of states in health and diseases. Integrin functions are dependable on a delicate balance between active and inactive status via multiple mechanisms, including protein-protein interactions, conformational changes, and trafficking. Due to their exposure on the cell surface and sensitivity to the molecular blockade, integrins have been investigated as pharmacological targets for nearly 40 years, but given the complexity of integrins and sometimes opposite characteristics, targeting integrin therapeutics has been a challenge. To date, only seven drugs targeting integrins have been successfully marketed, including abciximab, eptifibatide, tirofiban, natalizumab, vedolizumab, lifitegrast, and carotegrast. Currently, there are approximately 90 kinds of integrin-based therapeutic drugs or imaging agents in clinical studies, including small molecules, antibodies, synthetic mimic peptides, antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T-cell therapy, imaging agents, etc. A serious lesson from past integrin drug discovery and research efforts is that successes rely on both a deep understanding of integrin-regulatory mechanisms and unmet clinical needs. Herein, we provide a systematic and complete review of all integrin family members and integrin-mediated downstream signal transduction to highlight ongoing efforts to develop new therapies/diagnoses from bench to clinic. In addition, we further discuss the trend of drug development, how to improve the success rate of clinical trials targeting integrin therapies, and the key points for clinical research, basic research, and translational research.

Can the SARS-CoV-2 Spike Protein Bind Integrins Independent of the RGD Sequence?

The RGD motif in the Severe Acute Syndrome Coronavirus 2 (SARS-CoV-2) spike protein has been predicted to bind RGD-recognizing integrins. Recent studies have shown that the spike protein does, indeed, interact with αVβ3 and α5β1 integrins, both of which bind to RGD-containing ligands. However, computational studies have suggested that binding between the spike RGD motif and integrins is not favourable, even when unfolding occurs after conformational changes induced by binding to the canonical host entry receptor, angiotensin-converting enzyme 2 (ACE2). Furthermore, non-RGD-binding integrins, such as αx, have been suggested to interact with the SARS-CoV-2 spike protein. Other viral pathogens, such as rotaviruses, have been recorded to bind integrins in an RGD-independent manner to initiate host cell entry. Thus, in order to consider the potential for the SARS-CoV-2 spike protein to bind integrins independent of the RGD sequence, we investigate several factors related to the involvement of integrins in SARS-CoV-2 infection. First, we review changes in integrin expression during SARS-CoV-2 infection to identify which integrins might be of interest. Then, all known non-RGD integrin-binding motifs are collected and mapped to the spike protein receptor-binding domain and analyzed for their 3D availability. Several integrin-binding motifs are shown to exhibit high sequence similarity with solvent accessible regions of the spike receptor-binding domain. Comparisons of these motifs with other betacoronavirus spike proteins, such as SARS-CoV and RaTG13, reveal that some have recently evolved while others are more conserved throughout phylogenetically similar betacoronaviruses. Interestingly, all of the potential integrin-binding motifs, including the RGD sequence, are conserved in one of the known pangolin coronavirus strains. Of note, the most recently recorded mutations in the spike protein receptor-binding domain were found outside of the putative integrin-binding sequences, although several mutations formed inside and close to one motif, in particular, may potentially enhance binding. These data suggest that the SARS-CoV-2 spike protein may interact with integrins independent of the RGD sequence and may help further explain how SARS-CoV-2 and other viruses can evolve to bind to integrins.

Discovery of dual targeting PEGylated BG-P1600-TAT to norepinephrine transporter (NET) and thyrointegrin αvβ3 in the treatment of neuroblastoma

Polymer-drug conjugates are growing in interest as novel anticancer agents for targeted cancer therapy. The aim of this study was to synthesize a poly(ethylene glycol) (PEG) conjugated anticancer drug for neuroblastoma, which is the most common extracranial solid tumor of childhood and the deadliest tumor of infancy. In our previous studies, we designed and synthesized a dual targeting agent using benzylguanidine (BG) conjugated with the high affinity thyrointegrin αvβ3 antagonist TriAzole Tetraiodothyroacetic acid (TAT) via non-cleavable bonding to PEG400 to make BG-P₄₀₀-TAT and its derivatives as agents against neuroblastoma. Here, we improved the pharmacodynamic properties and increased the solubility by changing the polymer length to 1600 molecular weight. The TAT group, which acts as an integrin αvβ3 antagonist, and the BG group, which can be taken up by neuroblastoma cells through the norepinephrine transporter (NET) system, are conjugated to PEG₁₆₀₀ to make BG-PEG₁₆₀₀-TAT. The binding affinity of BG-PEG₁₆₀₀-TAT was 40-fold higher to integrin αvβ3 versus BG-P₄₀₀-TAT and was associated with greater anticancer activities against neuroblastoma cells (SK-N-F1 and SKNAS) implanted in SCID mice along with broad spectrum anti-angiogenesis activities versus the FDA approved anti-Vascular Endothelial Growth Factor (VEGF) monoclonal antibody Avastin (bevacizumab). In conclusion, our novel dual targeting of NET and αvβ3 receptor antagonist, BG-P1600-TAT demonstrated broad spectrum anti-angiogenesis and anti-cancer activities in suppressing neuroblastoma tumor progression and metastasis. Thus, BG-PEG₁₆₀₀-TAT represents a potential clinical candidate for targeted therapy in neuroblastoma management.

Engineering Calreticulin-Targeting Monobodies to Detect Immunogenic Cell Death in Cancer Chemotherapy

Surface-exposed calreticulin (ecto-CRT) plays a crucial role in the phagocytic removal of apoptotic cells during immunotherapy. Ecto-CRT is an immunogenic signal induced in response to treatment with chemotherapeutic agents such as doxorubicin (DOX) and mitoxantrone (MTX), and two peptides (KLGFFKR (Integrin-α) and GQPMYGQPMY (CRT binding peptide 1, Hep-I)) are known to specifically bind CRT. To engineer CRT-specific monobodies as agents to detect immunogenic cell death (ICD), we fused these peptide sequences at the binding loops (BC and FG) of human fibronectin domain III (FN3). CRT-specific monobodies were purified from E. coli by affinity chromatography. Using these monobodies, ecto-CRT was evaluated in vitro, in cultured cancer cell lines (CT-26, MC-38, HeLa, and MDA-MB-231), or in mice after anticancer drug treatment. Monobodies with both peptide sequences (CRT3 and CRT4) showed higher binding to ecto-CRT than those with a single peptide sequence. The binding affinity of the Rluc8 fusion protein-engineered monobodies (CRT3-Rluc8 and CRT4-Rluc8) to CRT was about 8 nM, and the half-life in serum and tumor tissue was about 12 h. By flow cytometry and confocal immunofluorescence of cancer cell lines, and by in vivo optical bioluminescence imaging of tumor-bearing mice, CRT3-Rluc8 and CRT4-Rluc8 bound specifically to ecto-CRT and effectively detected pre-apoptotic cells after treatment with ICD-inducing agents (DOX and MTX) but not a non-ICD-inducing agent (gemcitabine). Using CRT-specific monobodies, it is possible to detect ecto-CRT induction in cancer cells in response to drug exposure. This technique may be used to predict the therapeutic efficiency of chemo- and immuno-therapeutics early during anticancer treatment.

Structural Engineering of Fluorescent Self-Threaded Peptide Probes for Targeted Cell Imaging†

Squaraine figure-eight (SF8) molecules are a new class of deep-red fluorescent probes that are well suited for fluorescence cell microscopy due to their very high fluorescence brightness and excellent stability. Three homologous SF8 probes, with peptidyl loops that differ by very minor changes in the peptide sequence, were synthesized and assessed for probe uptake by cancer cells. One of probes included the RGD motif that is recognized by many classes of integrin receptors that reside on the surface of the cancer cells, and it permeated the cells by receptor-mediated endocytosis. In contrast, cell microscopy showed that there was negligible cell uptake of the two homologous SF8 probes indicating differences in probe targeting capability. The synthetic method allows for easy alteration of the peptide sequence; thus, it is straightforward to develop new classes of peptidyl SF8 probes with loop sequences that target other cancer biomarkers.

Aging-induced isoDGR-modified fibronectin activates monocytic and endothelial cells to promote atherosclerosis

BACKGROUND AND AIMS

Aging is the primary risk factor for cardiovascular disease (CVD), but the mechanisms underlying age-linked atherosclerosis remain unclear. We previously observed that long-lived vascular matrix proteins can acquire 'gain-of-function' isoDGR motifs that might play a role in atherosclerotic pathology.

METHODS

IsoDGR-specific mAb were generated and used for ELISA-based measurement of motif levels in plasma samples from patients with coronary artery diseases (CAD) and non-CAD controls. Functional consequences of isoDGR accumulation in age-damaged fibronectin were determined by bioassay for capacity to activate monocytes, macrophages, and endothelial cells (signalling activity, pro-inflammatory cytokine expression, and recruitment/adhesion potential). Mice deficient in the isoDGR repair enzyme PCMT1 were used to assess motif distribution and macrophage localisation in vivo.

RESULTS

IsoDGR-modified fibronectin and fibrinogen levels in patient plasma were significantly enhanced in CAD and further associated with smoking status. Functional assays demonstrated that isoDGR-modified fibronectin activated both monocytes and macrophages via integrin receptor 'outside in' signalling, triggering an ERK:AP-1 cascade and expression of pro-inflammatory cytokines MCP-1 and TNFα to drive additional recruitment of circulating leukocytes. IsoDGR-modified fibronectin also induced endothelial cell expression of integrin β1 to further enhance cellular adhesion and matrix deposition. Analysis of murine aortic tissues confirmed accumulation of isoDGR-modified proteins co-localised with CD68+ macrophages in vivo.

CONCLUSIONS

Age-damaged fibronectin features isoDGR motifs that increase binding to integrins on the surface of monocytes, macrophages, and endothelial cells. Subsequent activation of 'outside-in' signalling elicits a range of potent cytokines and chemokines that drive additional leukocyte recruitment to the developing atherosclerotic matrix.

Elucidating Solution Structures of Cyclic Peptides Using Molecular Dynamics Simulations

Protein-protein interactions are vital to biological processes, but the shape and size of their interfaces make them hard to target using small molecules. Cyclic peptides have shown promise as protein-protein interaction modulators, as they can bind protein surfaces with high affinity and specificity. Dozens of cyclic peptides are already FDA approved, and many more are in various stages of development as immunosuppressants, antibiotics, antivirals, or anticancer drugs. However, most cyclic peptide drugs so far have been natural products or derivatives thereof, with de novo design having proven challenging. A key obstacle is structural characterization: cyclic peptides frequently adopt multiple conformations in solution, which are difficult to resolve using techniques like NMR spectroscopy. The lack of solution structural information prevents a thorough understanding of cyclic peptides' sequence-structure-function relationship. Here we review recent development and application of molecular dynamics simulations with enhanced sampling to studying the solution structures of cyclic peptides. We describe novel computational methods capable of sampling cyclic peptides' conformational space and provide examples of computational studies that relate peptides' sequence and structure to biological activity. We demonstrate that molecular dynamics simulations have grown from an explanatory technique to a full-fledged tool for systematic studies at the forefront of cyclic peptide therapeutic design.

Growth Factors VEGF-A165 and FGF-2 as Multifunctional Biomolecules Governing Cell Adhesion and Proliferation

Vascular endothelial growth factor-A165 (VEGF-A165) and fibroblast growth factor-2 (FGF-2) are currently used for the functionalization of biomaterials designed for tissue engineering. We have developed a new simple method for heterologous expression and purification of VEGF-A165 and FGF-2 in the yeast expression system of Pichia pastoris. The biological activity of the growth factors was assessed in cultures of human and porcine adipose tissue-derived stem cells (ADSCs) and human umbilical vein endothelial cells (HUVECs). When added into the culture medium, VEGF-A165 stimulated proliferation only in HUVECs, while FGF-2 stimulated the proliferation of both cell types. A similar effect was achieved when the growth factors were pre-adsorbed to polystyrene wells. The effect of our recombinant growth factors was slightly lower than that of commercially available factors, which was attributed to the presence of some impurities. The stimulatory effect of the VEGF-A165 on cell adhesion was rather weak, especially in ADSCs. FGF-2 was a potent stimulator of the adhesion of ADSCs but had no to negative effect on the adhesion of HUVECs. In sum, FGF-2 and VEGF-A165 have diverse effects on the behavior of different cell types, which maybe utilized in tissue engineering.

Ceruloplasmin Deamidation in Neurodegeneration: From Loss to Gain of Function

Neurodegenerative disorders can induce modifications of several proteins; one of which is ceruloplasmin (Cp), a ferroxidase enzyme found modified in the cerebrospinal fluid (CSF) of neurodegenerative diseases patients. Cp modifications are caused by the oxidation induced by the pathological environment and are usually associated with activity loss. Together with oxidation, deamidation of Cp was found in the CSF from Alzheimer’s and Parkinson’s disease patients. Protein deamidation is a process characterized by asparagine residues conversion in either aspartate or isoaspartate, depending on protein sequence/structure and cellular environment. Cp deamidation occurs at two Asparagine-Glycine-Arginine (NGR)-motifs which, once deamidated to isoAspartate-Glycine-Arginine (isoDGR), bind integrins, a family of receptors mediating cell adhesion. Therefore, on the one hand, Cp modifications lead to loss of enzymatic activity, while on the other hand, these alterations confer gain of function to Cp. In fact, deamidated Cp binds to integrins and triggers intracellular signaling on choroid plexus epithelial cells, changing cell functioning. Working in concert with the oxidative environment, Cp deamidation could reach different target cells in the brain, altering their physiology and causing detrimental effects, which might contribute to the pathological mechanism.

The Interactome of Cancer-Related Lysyl Oxidase and Lysyl Oxidase-Like Proteins

The members of the lysyl oxidase (LOX) family are amine oxidases, which initiate the covalent cross-linking of the extracellular matrix (ECM), regulate ECM stiffness, and contribute to cancer progression. The aim of this study was to build the first draft of the interactome of the five members of the LOX family in order to determine its molecular functions, the biological and signaling pathways mediating these functions, the biological processes it is involved in, and if and how it is rewired in cancer. In vitro binding assays, based on surface plasmon resonance and bio-layer interferometry, combined with queries of interaction databases and interaction datasets, were used to retrieve interaction data. The interactome was then analyzed using computational tools. We identified 31 new interactions and 14 new partners of LOXL2, including the α5β1 integrin, and built an interactome comprising 320 proteins, 5 glycosaminoglycans, and 399 interactions. This network participates in ECM organization, degradation and cross-linking, cell-ECM interactions mediated by non-integrin and integrin receptors, protein folding and chaperone activity, organ and blood vessel development, cellular response to stress, and signal transduction. We showed that this network is rewired in colorectal carcinoma, leading to a switch from ECM organization to protein folding and chaperone activity.

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.

αV integrins in Schwann cells promote attachment to axons, but are dispensable in vivo

In the developing peripheral nervous system, Schwann cells (SCs) extend their processes to contact, sort, and myelinate axons. The mechanisms that contribute to the interaction between SCs and axons are just beginning to be elucidated. Using a SC-neuron coculture system, we demonstrate that Arg-Gly-Asp (RGD) peptides that inhibit α_V -containing integrins delay the extension of SCs elongating on axons. α_V integrins in SC localize to sites of contact with axons and are expressed early in development during radial sorting and myelination. Short interfering RNA-mediated knockdown of the α_V integrin subunit also delays SC extension along axons in vitro, suggesting that α_V -containing integrins participate in axo-glial interactions. However, mice lacking the α_V subunit in SCs, alone or in combination with the potentially compensating α₅ subunit, or the α_V partners β₃ or β₈ , myelinate normally during development and remyelinate normally after nerve crush, indicating that overlapping or compensatory mechanisms may hide the in vivo role of RGD-binding integrins.

Triazole Modified Tetraiodothyroacetic Acid Conjugated to Polyethylene Glycol: High Affinity Thyrointegrin αvβ3 Antagonist with Potent Anticancer Activities in Glioblastoma Multiforme

Discovery of bioactive molecules that target integrins has implicated their role in tumor angiogenesis, tumor growth, metastasis, and other pathological angiogenesis processes. Integrins are members of a family of cell surface receptors that play a critical role in the angiogenesis process. Tetraiodothyroacetic acid (tetrac), a deaminated derivative of l-thyroxine (T4), is a "thyrointegrin" antagonist that blocks the actions of l-triiodothyronine (T3) and T4 with an interaction site that is located at or near the RGD recognition site identified on integrin α_vβ₃'s binding pocket (thyrointegrin α_vβ₃ receptors). We have enhanced the biological activity of a tetrac-based inhibitor via significantly improving its α_vβ₃ receptor binding affinity by introducing a triazole ring on the outer ring of tetrac and covalently conjugating to polymer to increase the product's hydrophilicity via PEGylation. The product, P-bi-TAT, was restricted from nuclear translocation and demonstrated high blood brain barrier permeability and retention in contrast to the non-PEG conjugated derivative. Results of biological activity indicated that this macromolecule new chemical entity P-bi-TAT has greater than 400-fold potent integrin α_vβ₃ affinity versus the parent compound tetrac and has potent anticancer/anti-angiogenesis efficacy against glioblastoma multiforme (GBM). P-bi-TAT administered subcutaneously once daily for 21 days at 1-10 mg/kg mouse body weight resulted in a dose-dependent suppression of GBM tumor growth and viability as monitored with IVIS imaging (P < 0.001). GBM tumors had >95% volume loss and maximal loss of GBM cell viability during the 21 days ON-treatment experiment as well as in the 21 days ON followed by 21 days OFF-treatment experiment (P < 0.001). In conclusion, P-bi-TAT is a promising lead clinical candidate effective in the treatment of human GBM.

Novel Pure αVβ3 Integrin Antagonists That Do Not Induce Receptor Extension, Prime the Receptor, or Enhance Angiogenesis at Low Concentrations

The integrin αVβ3 receptor has been implicated in several important diseases, but no antagonists are approved for human therapy. One possible limitation of current small-molecule antagonists is their ability to induce a major conformational change in the receptor that induces it to adopt a high-affinity ligand-binding state. In response, we used structural inferences from a pure peptide antagonist to design the small-molecule pure antagonists TDI-4161 and TDI-3761. Both compounds inhibit αVβ3-mediated cell adhesion to αVβ3 ligands, but do not induce the conformational change as judged by antibody binding, electron microscopy, X-ray crystallography, and receptor priming studies. Both compounds demonstrated the favorable property of inhibiting bone resorption in vitro, supporting potential value in treating osteoporosis. Neither, however, had the unfavorable property of the αVβ3 antagonist cilengitide of paradoxically enhancing aortic sprout angiogenesis at concentrations below its IC50, which correlates with cilengitide's enhancement of tumor growth in vivo.

In Vivo Tumor Growth Inhibition and Antiangiogenic Effect of Cyclic NGR Peptide-Daunorubicin Conjugates Developed for Targeted Drug Delivery

Among various homing devices, peptides containing the NGR tripeptide sequence represent a promising approach to selectively recognize CD13 receptor isoforms on the surface of tumor cells. They have been successfully used for the delivery of various chemotherapeutic drugs to tumor vessels. Here, we report on the murine plasma stability, in vitro and in vivo antitumor activity of our recently described bioconjugates containing daunorubicin as payload. Furthermore, CD13 expression of KS Kaposi’s Sarcoma cell line and HT-29 human colon carcinoma cell line was investigated. Flow cytometry studies confirm the fast cellular uptake resulting in the rapid delivery of the active metabolite Dau = Aoa-Gly-OH to tumor cells. The increased in vitro antitumor effect might be explained by the faster rearrangement from NGR to isoDGR in case of conjugate 2 (Dau = Aoa-GFLGK(c[NleNGRE]-GG)-NH₂) in comparison with conjugate 1 (Dau = Aoa-GFLGK(c[KNGRE]-GG)-NH₂). Nevertheless, results indicated that both conjugates showed significant effect on inhibition of proliferation in the primary tumor and also on blood vessel formation making them a potential candidate for targeting angiogenesis processes in tumors where CD13 and integrins are involved.

Boosting Interleukin-12 Antitumor Activity and Synergism with Immunotherapy by Targeted Delivery with isoDGR-Tagged Nanogold

The clinical use of interleukin-12 (IL12), a cytokine endowed with potent immunotherapeutic anticancer activity, is limited by systemic toxicity. The hypothesis is addressed that gold nanoparticles tagged with a tumor-homing peptide containing isoDGR, an αvβ3-integrin binding motif, can be exploited for delivering IL12 to tumors and improving its therapeutic index. To this aim, gold nanospheres are functionalized with the head-to-tail cyclized-peptide CGisoDGRG (Iso1) and murine IL12. The resulting nanodrug (Iso1/Au/IL12) is monodispersed, stable, and bifunctional in terms of αvβ3 and IL12-receptor recognition. Low-dose Iso1/Au/IL12, equivalent to 18-75 pg of IL12, induces antitumor effects in murine models of fibrosarcomas and mammary adenocarcinomas, with no evidence of toxicity. Equivalent doses of Au/IL12 (a nanodrug lacking Iso1) fail to delay tumor growth, whereas 15 000 pg of free IL12 is necessary to achieve similar effects. Iso1/Au/IL12 significantly increases tumor infiltration by innate immune cells, such as NK and iNKT cells, monocytes, and neutrophils. NK cell depletion completely inhibits its antitumor effects. Low-dose Iso1/Au/IL12 can also increase the therapeutic efficacy of adoptive T-cell therapy in mice with autochthonous prostate cancer. These findings indicate that coupling IL12 to isoDGR-tagged nanogold is a valid strategy for enhancing its therapeutic index and sustaining adoptive T-cell therapy.

Conjugates of Cryptophycin and RGD or isoDGR Peptidomimetics for Targeted Drug Delivery

RGD-cryptophycin and isoDGR-cryptophycin conjugates were synthetized by combining peptidomimetic integrin ligands and cryptophycin, a highly potent tubulin-binding antimitotic agent across lysosomally cleavable Val-Ala or uncleavable linkers. The conjugates were able to effectively inhibit binding of biotinylated vitronectin to integrin αvβ3, showing a binding affinity in the same range as that of the free ligands. The antiproliferative activity of the novel conjugates was evaluated on human melanoma cells M21 and M21-L with different expression levels of integrin αvβ3, showing nanomolar potency of all four compounds against both cell lines. Conjugates containing uncleavable linker show reduced activity compared to the corresponding cleavable conjugates, indicating efficient intracellular drug release in the case of cryptophycin-based SMDCs. However, no significant correlation between the in vitro biological activity of the conjugates and the integrin αvβ3 expression level was observed, which is presumably due to a non-integrin-mediated uptake. This reveals the complexity of effective and selective αvβ3 integrin-mediated drug delivery.

A dimeric bicyclic RGD ligand displays enhanced integrin binding affinity and strong biological effects on U-373 MG glioblastoma cells

A C₂-symmetric dimeric bicyclic integrin ligand, bearing two RGD motifs, displays enhanced biological effects compared to monovalent RGD analogues.

Phosphate-Catalyzed Succinimide Formation from an NGR-Containing Cyclic Peptide: A Novel Mechanism for Deammoniation of the Tetrahedral Intermediate

Spontaneous deamidation in the Asn-Gly-Arg (NGR) motif that yields an isoAsp-Gly-Arg (isoDGR) sequence has recently attracted considerable attention because of the possibility of application to dual tumor targeting. It is well known that Asn deamidation reactions in peptide chains occur via the five-membered ring succinimide intermediate. Recently, we computationally showed by the B3LYP density functional theory method, that inorganic phosphate and the Arg side chain can catalyze the NGR deamidation using a cyclic peptide, c[CH₂CO⁻NGRC]⁻NH₂. In this previous study, the tetrahedral intermediate of the succinimide formation was assumed to be readily protonated at the nitrogen originating from the Asn side chain by the solvent water before the release of an NH₃ molecule. In the present study, we found a new mechanism for the decomposition of the tetrahedral intermediate that does not require the protonation by an external proton source. The computational method is the same as in the previous study. In the new mechanism, the release of an NH₃ molecule occurs after a proton exchange between the peptide and the phosphate and conformational changes. The rate-determining step of the overall reaction course is the previously reported first step, i.e., the cyclization to form the tetrahedral intermediate.

Succinimide-Based Conjugates Improve IsoDGR Cyclopeptide Affinity to αvβ3 without Promoting Integrin Allosteric Activation

The isoDGR sequence is an integrin-binding motif that has been successfully employed as a tumor-vasculature-homing molecule or for the targeted delivery of drugs and diagnostic agents to tumors. In this context, we previously demonstrated that cyclopeptide 2, the product of the conjugation of c(CGisoDGRG) (1) to 4-( N-maleimidomethyl)cyclohexane-1-carboxamide, can be successfully used as a tumor-homing ligand for nanodrug delivery to neoplastic tissues. Here, combining NMR, computational, and biochemical methods, we show that the succinimide ring contained in 2 contributes to stabilizing interactions with α_vβ₃, an integrin overexpressed in the tumor vasculature. Furthermore, we demonstrate that various cyclopeptides containing the isoDGR sequence embedded in different molecular scaffolds do not induce α_vβ₃ allosteric activation and work as pure integrin antagonists. These results could be profitably exploited for the rational design of novel isoDGR-based ligands and tumor-targeting molecules with improved α_vβ₃-binding properties and devoid of adverse integrin-activating effects.

Molecular Imaging of Aminopeptidase N in Cancer and Angiogenesis

This review focuses on recent advances in the molecular imaging of aminopeptidase N (APN, also known as CD13), a zinc metalloenzyme that cleaves N-terminal neutral amino acids. It is overexpressed in multiple cancer types and also on the surface of vasculature undergoing angiogenesis, making it a promising target for molecular imaging and targeted therapy. Molecular imaging probes for APN are divided into two large subgroups: reactive and nonreactive. The structures of the reactive probes (substrates) contain a reporter group that is cleaved and released by the APN enzyme. The nonreactive probes are not cleaved by the enzyme and contain an antibody, peptide, or nonpeptide for targeting the enzyme exterior or active site. Multivalent homotopic probes utilize multiple copies of the same targeting unit, whereas multivalent heterotopic molecular probes are equipped with different targeting units for different receptors. Several recent preclinical cancer imaging studies have shown that multivalent APN probes exhibit enhanced tumor specificity and accumulation compared to monovalent analogues. The few studies that have evaluated APN-specific probes for imaging angiogenesis have focused on cardiac regeneration. These promising results suggest that APN imaging can be expanded to detect and monitor other diseases that are associated with angiogenesis.

A fusion protein composed of the DSL domain of Dll1 and RGD motif protects cryptic stem cells in irradiation injury

Intestine is vulnerable to irradiation injury, which induces cell death and compromises regeneration of intestinal crypts. It is well accepted that cryptic stem cells, which are responsible for cryptic regeneration under physiological and pathological conditions, are controlled by multiple cell-intrinsic and environmental signals such as Notch signaling. Therefore, in the present study, we tested whether a soluble Notch ligand tethered to endothelial cells-mD1R-the Delta-Serrate-Lag2 (DSL) domain of mouse Notch ligand Delta-like1 fused with a RGD motif could protect cryptic cells from irradiation-induced intestinal injury. The result showed that administration of mD1R, which activated Notch signaling in intestinal cells, ameliorated loss of body weight and reduction of cryptic structures in intestine after total body irradiation (TBI) in mice. Histological staining showed that injection of mD1R after TBI promoted cryptic cell proliferation and reduced cell apoptosis in crypts. Immunofluorescence staining and reverse transcription (RT)-PCR showed that mD1R increased the level of Lgr5, Bmi1, Olfactomedin-4 (OLFM4), and IRIG1 in crypts, suggesting a protective effect on cryptic stem and progenitor cells after irradiation. Moreover, we found that administration of mD1R increased the number of Paneth cells and the mRNA level of Defa1, and the number Alcian Blue+ Goblet cells decreased first and then increased after irradiation, suggesting that mD1R promoted the maturation of the intestinal crypt after irradiation injury. Our data suggested that mD1R could serve as a therapeutic agent for the treatment of irradiation-induced intestinal injury.

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).

Glycine N-methylation in NGR-Tagged Nanocarriers Prevents Isoaspartate formation and Integrin Binding without Impairing CD13 Recognition and Tumor Homing

NGR (asparagine-glycine-arginine) is a tumor vasculature-homing peptide motif widely used for the functionalization of drugs, nanomaterials and imaging compounds for cancer treatment and diagnosis. Unfortunately, this motif has a strong propensity to undergo rapid deamidation. This reaction, which converts NGR into isoDGR, is associated with receptor switching from CD13 to integrins, with potentially important manufacturing, pharmacological and toxicological implications. It is found that glycine N-methylation of NGR-tagged nanocarriers completely prevents asparagine deamidation without impairing CD13 recognition. Studies in animal models have shown that the methylated NGR motif can be exploited for delivering radiolabeled compounds and nanocarriers, such as tumor necrosis factor-α (TNF)-bearing nanogold and liposomal doxorubicin, to tumors with improved selectivity. These findings suggest that this NGR derivative is a stable and efficient tumor-homing ligand that can be used for delivering functional nanomaterials to tumor vasculature.

Joint inflammation related citrullination of functional arginines in extracellular proteins

We report the extent, specific sites and structural requirements of joint inflammation related citrullination in extracellular proteins. A total of 40 synovial fluid samples derived from chronically inflamed human joints were analysed by heparin-agarose fractionation and LC-MS/MS. Citrullination of 55 arginines in extracellular proteins was detected. Importantly, 20% of the sites have a characterized function related to the hallmarks of destructive joint inflammation. E.g. four arginine residues, shown here to be citrullinated, are also affected by mutations in inherited diseases causing haemolysis or blood clotting dysfunction. Citrullination of integrin ligands was selected for further studies since fibronectin R234 in isoDGR was among the most frequently citrullinated arginines in synovial fluid. Assays with synovial fibroblasts and integrin αVβ3 indicated decreased affinity to the enzymatically citrullinated integrin binding sites. To conclude, our data indicate that in inflamed joints extensive citrullination affects the functional arginine residues in extracellular proteins.

Monocyte adhesion to atherosclerotic matrix proteins is enhanced by Asn-Gly-Arg deamidation

Atherosclerosis arises from leukocyte infiltration and thickening of the artery walls and constitutes a major component of vascular disease pathology, but the molecular events underpinning this process are not fully understood. Proteins containing an Asn-Gly-Arg (NGR) motif readily undergo deamidation of asparagine to generate isoDGR structures that bind to integrin α_vβ₃ on circulating leukocytes. Here we report the identification of isoDGR motifs in human atherosclerotic plaque components including extracellular matrix (ECM) proteins fibronectin and tenascin C, which have been strongly implicated in human atherosclerosis. We further demonstrate that deamidation of NGR motifs in fibronectin and tenascin C leads to increased adhesion of the monocytic cell line U937 and enhanced binding of primary human monocytes, except in the presence of a α_vβ₃-blocking antibody or the α_v-selective inhibitor cilengitide. In contrast, under the same deamidating conditions monocyte-macrophages displayed only weak binding to the alternative ECM component vitronectin which lacks NGR motifs. Together, these findings confirm a critical role for isoDGR motifs in mediating leukocyte adhesion to the ECM via integrin α_vβ₃ and suggest that protein deamidation may promote the pathological progression of human atherosclerosis by enhancing monocyte recruitment to developing plaques.

The CNGRCLLII(KLAKLAK)2 peptide shows cytotoxicity against HUVECs by inducing apoptosis: An in vitro and in vivo study

Fibrinogen Asn-Gly-Arg motif can specifically recognize and bind to Aminopeptidase N (CD13) on vascular endothelial cells in newly formed tumor vessels. Adipose-derived stem cells can serve as ideal vectors for gene therapy because of their ability of migrating to tumor tissues. First, this study was aimed to design a new peptide (CNGRCLLII(KLAKLAK)2) named CNAK which contains cyclic Asn-Gly-Arg motif and test its biological activity against human umbilical vein endothelial cells. Second, we aimed to construct stably transfected adipose-derived stem cells which express the CNAK peptide and investigate their anti-angiogenic activity in vivo. Adipose-derived stem cells were employed to localize CNAK on vascular endothelial cells in tumors based on their homing property. First of all, the new peptide was synthesized, which effectively entered into CD13+ human umbilical vein endothelial cells and showed cytotoxicity against human umbilical vein endothelial cells. The peptide induced apoptosis of human umbilical vein endothelial cells in a time- and dose-dependent manner, inhibited the expression of Bcl-2, and promoted the expression of Caspase-3 in human umbilical vein endothelial cells. Furthermore, the migration and tube formation of human umbilical vein endothelial cells were inhibited by CNAK. Primary adipose-derived stem cells were then isolated and identified. Stably transfected adipose-derived stem cells which express CNAK peptide (CNAK-ASCs) were successfully established, and the migration of CNAK-ASCs was assessed. In vivo, CNAK-ASCs were found to inhibit the growth and angiogenesis of breast cancer xenografts. This effect may be through inhibiting the secretion of matrix metalloproteinase-2 and membrane type 1-matrix metalloproteinase in vivo. It was also found that CNAK-ASCs reduced the quantity of breast cancer stem cells in tumor tissues. Our data suggested that the new peptide CNAK containing Asn-Gly-Arg motif had anti-angiogenic activity in vitro and in vivo.

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).

RGD-Binding Integrins in Head and Neck Cancers

Alterations in integrin expression and function promote tumour growth, invasion, metastasis and neoangiogenesis. Head and neck cancers are highly vascular tumours with a tendency to metastasise. They express a wide range of integrin receptors. Expression of the αv and β1 subunits has been explored relatively extensively and linked to tumour progression and metastasis. Individual receptors αvβ3 and αvβ5 have proved popular targets for diagnostic and therapeutic agents but lesser studied receptors, such as αvβ6, αvβ8, and β1 subfamily members, also show promise. This review presents the current knowledge of integrin expression and function in squamous cell carcinoma of the head and neck (HNSCC), with a particular focus on the arginine-glycine-aspartate (RGD)-binding integrins, in order to highlight the potential of integrins as targets for personalised tumour-specific identification and therapy.

Succinimide Formation from an NGR-Containing Cyclic Peptide: Computational Evidence for Catalytic Roles of Phosphate Buffer and the Arginine Side Chain

The Asn-Gly-Arg (NGR) motif and its deamidation product isoAsp-Gly-Arg (isoDGR) have recently attracted considerable attention as tumor-targeting ligands. Because an NGR-containing peptide and the corresponding isoDGR-containing peptide target different receptors, the spontaneous NGR deamidation can be used in dual targeting strategies. It is well known that the Asn deamidation proceeds via a succinimide derivative. In the present study, we computationally investigated the mechanism of succinimide formation from a cyclic peptide, c[CH₂CO-NGRC]-NH₂, which has recently been shown to undergo rapid deamidation in a phosphate buffer. An H₂PO₄⁻ ion was explicitly included in the calculations. We employed the density functional theory using the B3LYP functional. While geometry optimizations were performed in the gas phase, hydration Gibbs energies were calculated by the SM8 (solvation model 8) continuum model. We have found a pathway leading to the five-membered ring tetrahedral intermediate in which both the H₂PO₄⁻ ion and the Arg side chain act as catalyst. This intermediate, once protonated at the NH₂ group on the five-membered ring, was shown to easily undergo NH₃ elimination leading to the succinimide formation. This study is the first to propose a possible catalytic role for the Arg side chain in the NGR deamidation.

Systemic and tumor-targeted delivery of siRNA by cyclic NGR and isoDGR motif-containing peptides

The drug development of siRNA has been seriously hindered by the lack of an effective, safe and clinically applicable delivery system. The cyclic NGR motif and its isomerization product isoDGR recruit CD13 and integrin as their specific receptors, both of which are overexpressed by tumor and neovascular cells. In this study, a bi-functional peptide, named NGR-10R, was designed and tested for siRNA delivery in vitro and in vivo. Through the formation of peptide/siRNA nanoparticles, RNase resistance was greatly enhanced for the siRNAs. Both FACS and confocal assays revealed that the peptide/siRNA complexes were effectively internalized by MDA-MB-231 cells. Gene silencing assays indicated that anti-Lamin A/C siRNA delivered by NGR-10R robustly repressed gene expression in MDA-MB-231 and HUVEC (a CD13(+)/αvβ3(+) cell). Importantly, the siRNAs were efficiently delivered into tumor tissues and localized around the nuclei, as revealed by in vivo imaging and cryosection examination. In summary, NGR-10R not only efficiently delivered siRNAs into MDA-MB-231 cells in vitro but also delivered siRNAs into tumor cells in vivo, taking advantage of its specific binding to CD13 (neovascular) or αvβ3 (MDA-MB-231). Therefore, the NGR-10R peptide provides a promising siRNA delivery reagent that could be used for drug development, particularly for anti-tumor therapeutics.

NGR-tagged nano-gold: A new CD13-selective carrier for cytokine delivery to tumors

Colloidal gold (Au), a well-tolerated nanomaterial, is currently exploited for several applications in nanomedicine. We show that gold nanoparticles tagged with a novel tumor-homing peptide containing Asn-Gly-Arg (NGR), a ligand of CD13 expressed by the tumor neovasculature, can be exploited as carriers for cytokine delivery to tumors. Biochemical and functional studies showed that the NGR molecular scaffold/linker used for gold functionalization is critical for CD13 recognition. Using fibrosarcoma-bearing mice, NGR-tagged nanodrugs could deliver extremely low, yet pharmacologically active doses of tumor necrosis factor (TNF), an anticancer cytokine, to tumors with no evidence of toxicity. Mechanistic studies confirmed that CD13 targeting was a primary mechanism of drug delivery and excluded a major role of integrin targeting consequent to NGR deamidation, a degradation reaction that generates the isoAsp-Gly-Arg (isoDGR) integrin ligand. NGR-tagged gold nanoparticles can be used, in principle, as a novel platform for single- or multi-cytokine delivery to tumor endothelial cells for cancer therapy.

Distal M domain of cobra ADAM-like metalloproteinase mediates the binding of positively charged cysteine-rich domain to αvβ3 integrin in the suppression of cell migration

We have previously identified two new P-III type ADAM-like snake venom metalloproteinases (SVMPs), i.e., atragin and kaouthiagin-like, from Taiwan cobra venom and determined their 3D structures with a distinct C- and I-shaped metalloproteinase/disintegrin-like/cysteine-rich (MDC) modular architecture. Herein, we investigated their functional targets to elucidate the role of cobra SVMPs in perturbing wound healing in snakebite victims. We showed that the non-RGD (Arg-Gly-Asp) C-shaped SVMP atragin binds about ten-fold stronger than the RGD-containing I-shaped SVMP kaouthiagin-like to αvβ3 integrin in the surface-immobilized form. Atragin binds to αvβ3 integrin through a novel interaction mode involving distal M and C domains via the RRN sequence motif in the hyper variable loop. In a cell adhesion assay, the adhesion of fibroblasts to atragin was mediated by αvβ3 integrin. Furthermore, atragin inhibited wound healing and suppressed cell migration in a αvβ3 integrin-dependent manner. These results, together with our previous demonstration of non-cytotoxic cobra CTX A5 in targeting αvβ3 integrin, suggest that cobra venom consists of several non-RGD toxins with integrin-binding specificity that could perturb wound healing in snakebite victims.

Zinc-induced heterodimer formation between metal-binding domains of intact and naturally modified amyloid-beta species: implication to amyloid seeding in Alzheimer's disease?

Zinc ions and modified amyloid-beta peptides (Aβ) play a critical role in the pathological aggregation of endogenous Aβ in Alzheimer's disease (AD). Zinc-induced Aβ oligomerization is mediated by the metal-binding domain (MBD) which includes N-terminal residues 1-16 (Aβ1-16). Earlier, it has been shown that Aβ1-16 as well as some of its naturally occurring variants undergoes zinc-induced homodimerization via the interface in which zinc ion is coordinated by Glu11 and His14 of the interacting subunits. In this study using surface plasmon resonance technique, we have found that in the presence of zinc ions Aβ1-16 forms heterodimers with MBDs of two Aβ species linked to AD: Aβ containing isoAsp7 (isoAβ) and Aβ containing phosphorylated Ser8 (pS8-Aβ). The heterodimers appear to possess the same interface as the homodimers. Simulation of 200 ns molecular dynamic trajectories in two constructed models of dimers ([Aβ1-16/Zn/Aβ1-16] and [isoAβ1-16/Zn/Aβ1-16]), has shown that conformational flexibility of the N-terminal fragments of the dimer subunits is controlled by the structure of corresponding sites 6-8. The data suggest that isoAβ and pS8-Aβ can be involved in the AD pathogenesis by means of their zinc-dependent interactions with endogenous Aβ resulting in the formation of heterodimeric seeds for amyloid aggregation.

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.

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.

Development of cyclic NGR peptides with thioether linkage: structure and dynamics determining deamidation and bioactivity

NGR peptides that recognize CD13 receptors in tumor neovasculature are of high interest, in particular due to their potential applications in drug targeting. Here we report the synthesis and structural analysis of novel thioether bond-linked cyclic NGR peptides. Our results show that their chemostability (resistance against spontaneous decomposition forming isoAsp and Asp derivatives) strongly depends on both sample handling conditions and structural properties. A significant correlation was found between chemostability and structural measures, such as NH(Gly)-CO(Asn-sc) distances. The side-chain orientation of Asn is a key determining factor; if it is turned away from HN(Gly), the chemostability increases. Structure stabilizing factors (e.g., H-bonds) lower their internal dynamics, and thus biomolecules become even more resistant against spontaneous decomposition. The effect of cyclic NGR peptides on cell adhesion was examined in A2058 melanoma cell lines. It was found that some of the investigated peptides gradually increased cell adhesion with long-term characteristics, indicating time-dependent formation of integrin binding isoAsp derivatives that are responsible for the adhesion-inducing effect.

Evaluation of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney aminopeptidase N expression

Aminopeptidase N (APN; CD13; EC 3.4.11.2) is a zinc-dependent membrane-bound exopeptidase that catalyzes the removal of N-terminal amino acids from peptides. APN is known to be highly expressed on renal cortical proximal tubules. APN expression levels are markedly decreased under the influence of nephrotoxins and in the tumor regions of renal cancers. Thus, molecular imaging of kidney APN expression could provide pathophysiological information about kidneys noninvasively. Probestin is a potent APN inhibitor and binds to APN. Abdominal SPECT imaging was conducted at 1 h postinjection of (99m)Tc-probestin in a group of 12 UPII-SV40T transgenic and wild-type mice. UPII-SV40T mice spontaneously develop urothelial carcinoma in situ and invasive transitional cell carcinoma (TCC) that invade kidneys. Histopathology and immunohistochemistry analysis were used to confirm the presence of tumor and to evaluate APN expression in kidney. Radioactivity in normal tissue regions of renal cortex was clearly visible in SPECT images, whereas tumor regions of renal cortex displayed significantly lower or no radioactivity uptake. Histopathological analysis of kidney sections showed normal morphology for both renal pelvic and cortical regions in wild-type mice and abnormal morphology in some transgenic mice. Proliferating cell nuclear antigen staining confirmed the presence of tumor in those abnormal regions. Immunohistochemical analysis of kidney sections using anti-CD13 antibody showed significantly lower APN expression in tumor regions compared to normal regions. Results obtained in this study demonstrate the potential use of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney APN expression.

13F-1, a novel 5-fluorouracil prodrug containing an Asn-Gly-Arg (NO2) COOCH3 tripeptide, inhibits human colonic carcinoma growth by targeting Aminopeptidase N (APN/CD13)

13F-1 is a 5-fluorouracil prodrug containing an Asn-Gly-Arg (NO2) COOCH3 tripeptide. 13F-1 might possess the activity against cancer growth by targeting Aminopeptidase N (APN/CD13). Our goal in this study was to evaluate the inhibitory effect of 13F-1 on the growth of human colonic carcinoma by both in vitro and in vivo studies. Experiments were performed in colonic carcinoma Colo205 cells, which highly express APN/CD13 on cell surface. The inhibition of 13F-1 on cancer cell growth was estimated by the colorimetric and clonogenic assays. The assays of Annexin V-FITC/PI and JC-1 fluorescence probe were employed to determine the apoptotic cells. Further experiment was performed in mice bearing Colo205 xenografts. 13F-1 was injected for three consecutive weeks. The specimens of Colo205 xenografts were removed for TUNEL staining and western blotting analysis. The expressions of APN/CD13 were analyzed by immunofluorescent flow cytometry and western blotting assays. 13F-1 significantly inhibited Colo205 cell proliferation. 13F-1 by injection delayed the expansion of Colo205 xenografts without significant toxicity to mice. The inhibitory effect of 13F-1 might arise from its role in apoptotic induction. Further analysis indicated that 13F-1 strongly inhibited APN/CD13 expression on cancer cell surface. In contrast, 5-FU did not affect APN/CD13 expression. These results indicated the mechanism of 13F-1 action that 13F-1׳s effect was associated with its role in suppression of APN/CD13 expression. Conclusion, 13F-1 could be developed as a promising agent for treatment of cancers with high expression of APN/CD13.