NMR spectroscopy can help accelerate antiviral drug discovery programs.

Small molecule drugs have an important role to play in combating viral infections, and biophysics support has been central for contributing to the discovery and design of direct acting antivirals. Perhaps one of the most successful biophysical tools for this purpose is NMR spectroscopy when utilized strategically and pragmatically within team workflows and timelines. This report describes some clear examples of how NMR applications contributed to the design of antivirals when combined with medicinal chemistry, biochemistry, X-ray crystallography and computational chemistry. Overall, these multidisciplinary approaches allowed teams to reveal and expose compound physical properties from which design ideas were spawned and tested to achieve the desired successes. Examples are discussed for the discovery of antivirals that target HCV, HIV and SARS-CoV-2.

Genuine selective caspase-2 inhibition with new irreversible small peptidomimetics.

Caspase-2 (Casp2) is a promising therapeutic target in several human diseases, including nonalcoholic steatohepatitis (NASH) and Alzheimer's disease (AD). However, the design of an active-site-directed inhibitor selective to individual caspase family members is challenging because caspases have extremely similar active sites. Here we present new peptidomimetics derived from the VDVAD pentapeptide structure, harboring non-natural modifications at the P2 position and an irreversible warhead. Enzyme kinetics show that these new compounds, such as LJ2 or its specific isomers LJ2a, and LJ3a, strongly and irreversibly inhibit Casp2 with genuine selectivity. In agreement with the established role of Casp2 in cellular stress responses, LJ2 inhibits cell death induced by microtubule destabilization or hydroxamic acid-based deacetylase inhibition. The most potent peptidomimetic, LJ2a, inhibits human Casp2 with a remarkably high inactivation rate (k3/Ki ~5,500,000 M-1 s-1), and the most selective inhibitor, LJ3a, has close to a 1000 times higher inactivation rate on Casp2 as compared to Casp3. Structural analysis of LJ3a shows that the spatial configuration of Cα at the P2 position determines inhibitor efficacy. In transfected human cell lines overexpressing site-1 protease (S1P), sterol regulatory element-binding protein 2 (SREBP2) and Casp2, LJ2a and LJ3a fully inhibit Casp2-mediated S1P cleavage and thus SREBP2 activation, suggesting a potential to prevent NASH development. Furthermore, in primary hippocampal neurons treated with ß-amyloid oligomers, submicromolar concentrations of LJ2a and of LJ3a prevent synapse loss, indicating a potential for further investigations in AD treatment.

Mixed Polymeric Micelles for Rapamycin Skin Delivery.

Facial angiofibromas (FA) are one of the most obvious cutaneous manifestations of tuberous sclerosis complex. Topical rapamycin for angiofibromas has been reported as a promising treatment. Several types of vehicles have been used hitherto, but polymeric micelles and especially those made of d-α-tocopherol polyethylene glycol 1000 succinate (TPGS) seem to have shown better skin bioavailability of rapamycin than the so far commonly used ointments. To better understand the influence of polymeric micelles on the behavior of rapamycin, we explored it through mixed polymeric micelles combining TPGS and poloxamer, evaluating stability and skin bioavailability to define an optimized formulation to effectively treat FA. Our studies have shown that TPGS improves the physicochemical behavior of rapamycin, i.e., its solubility and stability, due to a strong inclusion in micelles, while poloxamer P123 has a more significant influence on skin bioavailability. Accordingly, we formulated mixed-micelle hydrogels containing 0.1% rapamycin, and the optimized formulation was found to be stable for up to 3 months at 2-8 °C. In addition, compared to hydroalcoholic gel formulations, the studied system allows for better biodistribution on human skin.

1H, 13C and 15N backbone resonance assignment of HIV-1 Gag (276-432) encompassing the C-terminal domain of the capsid protein, the spacer peptide 1 and the nucleocapsid protein.

During the maturation of the HIV-1 particle, the Gag polyprotein is cleaved by the viral protease into several proteins: matrix (MA), capsid (CA), spacer peptide 1 (SP1), nucleocapsid (NC), spacer peptide 2 (SP2) and p6. After cleavage, these proteins rearrange to form infectious viral particles. The final cleavage by the protease occurs between CA and SP1 and is the limiting step for the maturation of the particle. The CA-SP1 junction is the target of HIV-1 maturation inhibitors. CA is responsible for the formation of the viral capsid which protects the viral RNA inside. The SP1 domain is essential for viral assembly and infectivity, it is flexible and in helix-coil equilibrium. The presence of NC allows the SP1 domain to be less dynamic. The perturbation of the natural coil-helix equilibrium to helix interferes with protease cleavage and leads to non-completion of viral maturation. In this work, two mutations, W316A and M317A, that abolish the oligomerization of CA were introduced into the protein. The HIV-1 CACTDW316A, M317A-SP1-NC which contains the C-terminal monomeric mutant of CA, SP1 and NC was produced to study the mechanism of action of HIV-1 maturation inhibitors. Here we report the backbone assignment of the protein CACTDW316A, M317A-SP1-NC. These results will be useful to study the interaction between HIV-1 Gag and HIV-1 maturation inhibitors.

The HIV-1 maturation inhibitor, EP39, interferes with the dynamic helix-coil equilibrium of the CA-SP1 junction of Gag.

During the maturation of HIV-1 particle, the Gag polyprotein is cleaved into several proteins by the HIV-1 protease. These proteins rearrange to form infectious virus particles. In this study, the solution structure and dynamics of a monomeric mutated domain encompassing the C-terminal of capsid, the spacer peptide SP1 and the nucleocapsid from Gag was characterized by Nuclear Magnetic Resonance in the presence of maturation inhibitor EP39, a more hydro-soluble derivative of BVM. We show that the binding of EP39 decreases the dynamics of CA-SP1 junction, especially the QVT motif in SP1, and perturbs the natural coil-helix equilibrium on both sides of the SP1 domain by stabilizing the transient alpha helical structure. Our results provide new insight into the structure and dynamics of the SP1 domain and how HIV-1 maturation inhibitors interfere with this domain. They offer additional clues for the development of new second generation inhibitors targeting HIV-1 maturation.

Structural studies of the binding of an antagonistic cyclic peptide to the VEGFR1 domain 2.

Physiological and pathological angiogenesis is mainly regulated by the binding of the vascular endothelial growth factor (VEGF) to its receptors (VEGFRs). Antagonists of VEGFR are very attractive for the treatment of diseases related to excessive angiogenesis. Our previously designed C-terminal alkylated cyclic peptides [YKDEGLEE]-NHR (R = alkyl, arylalkyl) disrupt the interaction between VEGF and VEGFRs in biological assays. In this paper, we described the structural studies of the binding of one of these cyclic peptides named Peptide 3 to the VEGFR1 domain 2 (VEGFR1-D2). The molecular docking and NMR mapping identified the binding site on VEGFR1-D2. The anti-angiogenic effect of our peptide was evaluated by an experiment of VEGF-induced tube formation in two cell lines, retinal cell type RF6/A and vascular endothelial cell type HUVEC. Some new peptides were also synthesized and compared by an ELISA-based assay, in order to verify their ability to disrupt the formation of the complex VEGF-A/VEGFR1. In conclusion, the structural studies of Peptide 3 with VEGFR1-D2 will help the design of more efficient VEGFR antagonists. Moreover, Peptide 3, with improved receptor binding affinity, could be more suitable for VEGFR targeting bioimaging studies once labeled.

Insight into the mechanism of action of EP-39, a bevirimat derivative that inhibits HIV-1 maturation.

Maturation of human immunodeficiency virus type 1 (HIV-1) particles is a key step for viral infectivity. This process can be blocked using maturation inhibitors (MIs) that affect the cleavage of the capsid-spacer peptide 1 (CA-SP1) junction. Here, we investigated the mechanisms underlying the activity of EP-39, a bevirimat (BVM) derivative with better hydrosolubility. To this aim, we selected in vitro EP-39- and BVM-resistant mutants. We found that EP-39-resistant viruses have four mutations within the CA domain (CA-A194T, CA-T200N, CA-V230I, and CA-V230A) and one in the first residue of SP1 (SP1-A1V). We also identified six mutations that confer BVM resistance (CA-A194T, CA-L231F, CA-L231M, SP1-A1V, SP1-S5N and SP1-V7A). To characterize the EP-39 and BVM-resistant mutants, we studied EP-39 effects on mutant virus replication and performed a biochemical analysis with both MIs. We observed common and distinct characteristics, suggesting that, although EP-39 and BVM share the same chemical skeleton, they could interact in a different way with the Gag polyprotein precursor (Pr55Gag). Using an in silico approach, we observed that EP-39 and BVM present different predicted positions on the hexameric crystal structure of the CACTD-SP1 Gag fragment. To clearly understand the relationship between assembly and maturation, we investigated the impact of all identified mutations on virus assembly by expressing Pr55Gag mutants. Finally, using NMR, we have shown that the interaction of EP-39 with a peptide carrying the SP1-A1V mutation (CA-SP1(A1V)-NC) is almost suppressed in comparison with the wild type peptide. These results suggest that EP-39 and BVM could interact differently with the Pr55Gag lattice and that the mutation of the first SP1 residue induces a loss of interaction between Pr55Gag and EP-39.

Expression of novel proteins by polyomaviruses and recent advances in the structural and functional features of agnoprotein of JC virus, BK virus, and simian virus 40.

Polyomavirus family consists of a highly diverse group of small DNA viruses. The founding family member (MPyV) was first discovered in the newborn mouse in the late 1950s, which induces solid tumors in a wide variety of tissue types that are the epithelial and mesenchymal origin. Later, other family members were also isolated from a number of mammalian, avian and fish species. Some of these viruses significantly contributed to our current understanding of the fundamentals of modern biology such as transcription, replication, splicing, RNA editing, and cell transformation. After the discovery of first two human polyomaviruses (JC virus [JCV] and BK virus [BKV]) in the early 1970s, there has been a rapid expansion in the number of human polyomaviruses in recent years due to the availability of the new technologies and brought the present number to 14. Some of the human polyomaviruses cause considerably serious human diseases, including progressive multifocal leukoencephalopathy, polyomavirus-associated nephropathy, Merkel cell carcinoma, and trichodysplasia spinulosa. Emerging evidence suggests that the expression of the polyomavirus genome is more complex than previously thought. In addition to encoding universally expressed regulatory and structural proteins (LT-Ag, Sm t-Ag, VP1, VP2, and VP3), some polyomaviruses express additional virus-specific regulatory proteins and microRNAs. This review summarizes the recent advances in polyomavirus genome expression with respect to the new viral proteins and microRNAs other than the universally expressed ones. In addition, a special emphasis is devoted to the recent structural and functional discoveries in the field of polyomavirus agnoprotein which is expressed only by JCV, BKV, and simian virus 40 genomes.

Synthesis and characterization of water-soluble macrocyclic peptides stabilizing protein α-turn.

Short peptides composed of naturally occurring amino acids are usually unstructured in aqueous media. The installation of covalent constraints within their side chains or backbones, resulting in the formation of macrocyclic peptides, is an appealing approach to stabilize them in defined secondary structures. Therefore, with the objective to stabilize α-turn conformation, we designed, synthesized and characterized constrained 13-membered macrocyclic peptides. Their design was inspired by previous work using the replacement of a hydrogen bond by a covalent bond, for the stabilization of α-helical secondary structures. Their synthesis employed our recently published solid-phase method based on Fukuyama-Mitsunobu alkylation reactions. We report herein an optimized synthesis leading to three water-soluble 13-membered macrocyclic peptides 10a-c, including respectively two, one and zero glycine residues. They were characterized by CD and NMR, which indicated the presence of equilibrating conformers. The detailed conformational analysis was based on extensive NMR and molecular dynamics studies. We found that the peptide without glycine residues 10c was mostly present as slowly interconverting conformers whereas the peptide with two glycine residues 10a was mostly present as rapidly interconverting conformers. We did not find a good match between the conformers of 10a and α-turns occurring in proteins, due to the high flexibility of the glycine backbone. Interestingly, we found that the major conformer of 10c accurately matched the "non-classical" or "tight" α-turn of type II-αLS, with a RMSD value of 0.42 Å for heavy atoms constituting the macrocycle. This is, to the best of our knowledge, the first molecule reported to mimic this type of α-turn found in proteins.

Nuclear Magnetic Resonance Structure of the Human Polyoma JC Virus Agnoprotein.

Agnoprotein is an important regulatory protein of the human polyoma JC virus (JCV) and plays critical roles during the viral replication cycle. It forms highly stable dimers and oligomers through its Leu/Ile/Phe-rich domain, which is important for the stability and function of the protein. We recently resolved the partial 3D structure of this protein by NMR using a synthetic peptide encompassing amino acids Thr17 to Gln52, where the Leu/Ile/Phe- rich region was found to adopt a major alpha-helix conformation spanning amino acids 23-39. Here, we report the resolution of the 3D structure of full-length JCV agnoprotein by NMR, which not only confirmed the existence of the previously reported major α-helix domain at the same position but also revealed the presence of an additional minor α-helix region spanning amino acid residues Leu6 to lys13. The remaining regions of the protein adopt an intrinsically unstructured conformation. J. Cell. Biochem. 118: 3268-3280, 2017. © 2017 Wiley Periodicals, Inc.

Biophysical Studies of the Induced Dimerization of Human VEGF Receptor 1 Binding Domain by Divalent Metals Competing with VEGF-A.

Angiogenesis is tightly regulated through the binding of vascular endothelial growth factors (VEGFs) to their receptors (VEGFRs). In this context, we showed that human VEGFR1 domain 2 crystallizes in the presence of Zn2+, Co2+ or Cu2+ as a dimer that forms via metal-ion interactions and interlocked hydrophobic surfaces. SAXS, NMR and size exclusion chromatography analyses confirm the formation of this dimer in solution in the presence of Co2+, Cd2+ or Cu2+. Since the metal-induced dimerization masks the VEGFs binding surface, we investigated the ability of metal ions to displace the VEGF-A binding to hVEGFR1: using a competition assay, we evidenced that the metals displaced the VEGF-A binding to hVEGFR1 extracellular domain binding at micromolar level.

Guttiferone A Aggregates Modulate Silent Information Regulator 1 (SIRT1) Activity.

Natural products guttiferone A, hyperforin, and aristoforin were able to inhibit or increase SIRT1 catalytic activity, depending on protein concentration and presence of detergent. On the basis of NMR data for guttiferone A, we demonstrated that the aggregation state of the natural product played a crucial role for its interaction with the enzyme. These results are useful to interpret future in vitro structure-activity relationship studies on these natural products in the quest of their biological target(s).

Emerging From the Unknown: Structural and Functional Features of Agnoprotein of Polyomaviruses.

Agnoprotein is an important regulatory protein of polyomaviruses, including JCV, BKV, and SV40. In the absence of its expression, these viruses are unable to sustain their productive life cycle. It is a highly basic phosphoprotein that localizes mostly to the perinuclear area of infected cells, although a small amount of the protein is also found in nucleus. Much has been learned about the structure and function of this important regulatory protein in recent years. It forms highly stable dimers/oligomers in vitro and in vivo through its Leu/Ile/Phe-rich domain. Structural NMR studies revealed that this domain adopts an alpha-helix conformation and plays a critical role in the stability of the protein. It associates with cellular proteins, including YB-1, p53, Ku70, FEZ1, HP1α, PP2A, AP-3, PCNA, and α-SNAP; and viral proteins, including small t antigen, large T antigen, HIV-1 Tat, and JCV VP1; and significantly contributes the viral transcription and replication. This review summarizes the recent advances in the structural and functional properties of this important regulatory protein. J. Cell. Physiol. 231: 2115-2127, 2016. © 2016 Wiley Periodicals, Inc.

Nuclear magnetic resonance structure revealed that the human polyomavirus JC virus agnoprotein contains an α-helix encompassing the Leu/Ile/Phe-rich domain.

UNLABELLED: Agnoprotein is a small multifunctional regulatory protein required for sustaining the productive replication of JC virus (JCV). It is a mostly cytoplasmic protein localizing in the perinuclear area and forms highly stable dimers/oligomers through a Leu/Ile/Phe-rich domain. There have been no three-dimensional structural data available for agnoprotein due to difficulties associated with the dynamic conversion from monomers to oligomers. Here, we report the first nuclear magnetic resonance (NMR) structure of a synthetic agnoprotein peptide spanning amino acids Thr17 to Glu55 where Lys23 to Phe39 encompassing the Leu/Ile/Phe-rich domain forms an amphipathic α-helix. On the basis of these structural data, a number of Ala substitution mutations were made to investigate the role of the α-helix in the structure and function of agnoprotein. Single L29A and L36A mutations exhibited a significant negative effect on both protein stability and viral replication, whereas the L32A mutation did not. In addition, the L29A mutant displayed a highly nuclear localization pattern, in contrast to the pattern for the wild type (WT). Interestingly, a triple mutant, the L29A+L32A+L36A mutant, yielded no detectable agnoprotein expression, and the replication of this JCV mutant was significantly reduced, suggesting that Leu29 and Leu36 are located at the dimer interface, contributing to the structure and stability of agnoprotein. Two other single mutations, L33A and E34A, did not perturb agnoprotein stability as drastically as that observed with the L29A and L36A mutations, but they negatively affected viral replication, suggesting that the role of these residues is functional rather than structural. Thus, the agnoprotein dimerization domain can be targeted for the development of novel drugs active against JCV infection. IMPORTANCE: Agnoprotein is a small regulatory protein of JC virus (JCV) and is required for the successful completion of the viral replication cycle. It forms highly stable dimers and oligomers through its hydrophobic (Leu/Ile/Phe-rich) domain, which has been shown to play essential roles in the stability and function of the protein. In this work, the Leu/Ile/Phe-rich domain has been further characterized by NMR studies using an agnoprotein peptide spanning amino acids T17 to Q54. Those studies revealed that the dimerization domain of the protein forms an amphipathic α-helix. Subsequent NMR structure-based mutational analysis of the region highlighted the critical importance of certain amino acids within the α-helix for the stability and function of agnoprotein. In conclusion, this study provides a solid foundation for developing effective therapeutic approaches against the dimerization domain of the protein to inhibit its critical roles in JCV infection.

Synthesis and biological evaluation of a new derivative of bevirimat that targets the Gag CA-SP1 cleavage site.

Bevirimat (2), the first-in-class HIV-1 maturation inhibitor, shows a low efficacy due essentially to the natural polymorphism of its target, the CA-SP1 junction. Moreover, its low hydrosolubility makes it difficult to study its interaction with the CA-SP1 junction. We have synthesized new derivatives of bevirimat by adding different hydrophilic substituents at the C-28 position to improve their hydrosolubility and perform the structural study of a complex by NMR. Synthesis of the new derivatives, the effect of substituents at the C-28 position and their hydrosolubility are discussed. The ability of these molecules to inhibit viral infection and their cytotoxicity is assessed. Compared to the well-known bevirimat (2), one of our compounds (16) shows a higher hydrosolubility associated with a 2.5 fold increase in activity, a higher selectivity index and a better antiviral profile. Moreover, for the first time a direct interaction between a derivative of bevirimat (16) and the domain CA-SP1-NC is shown by NMR. Information from this study should allow us to decipher the mechanism by which bevirimat inhibits HIV-1 maturation and how the natural polymorphism of the spacer peptide SP1 triggers resistance to inhibitors.

Targeting the proangiogenic VEGF-VEGFR protein-protein interface with drug-like compounds by in silico and in vitro screening.

Protein-protein interactions play a central role in medicine, and their modulation with small organic compounds remains an enormous challenge. Because it has been noted that the macromolecular complexes modulated to date have a relatively pronounced binding cavity at the interface, we decided to perform screening experiments over the vascular endothelial growth factor receptor (VEGFR), a validated target for antiangiogenic treatments with a very flat interface. We focused the study on the VEGFR-1 D2 domain, and 20 active compounds were identified. These small compounds contained a (3-carboxy-2-ureido)thiophen unit and had IC(50) values in the low micromolar range. The most potent compound inhibited the VEGF-induced VEGFR-1 transduction pathways. Our findings suggest that our best hit may be a promising scaffold to probe this macromolecular complex and for the development of treatments of VEGFR-1-dependent diseases.

Characterization of a novel type of HIV-1 particle assembly inhibitor using a quantitative luciferase-Vpr packaging-based assay.

The HIV-1 auxiliary protein Vpr and Vpr-fusion proteins can be copackaged with Gag precursor (Pr55Gag) into virions or membrane-enveloped virus-like particles (VLP). Taking advantage of this property, we developed a simple and sensitive method to evaluate potential inhibitors of HIV-1 assembly in a living cell system. Two proteins were coexpressed in recombinant baculovirus-infected Sf9 cells, Pr55Gag, which formed the VLP backbone, and luciferase fused to the N-terminus of Vpr (LucVpr). VLP-encapsidated LucVpr retained the enzymatic activity of free luciferase. The levels of luciferase activity present in the pelletable fraction recovered from the culture medium correlated with the amounts of extracellular VLP released by Sf9 cells assayed by conventional immunological methods. Our luciferase-based assay was then applied to the characterization of betulinic acid (BA) derivatives that differed from the leader compound PA-457 (or DSB) by their substituant on carbon-28. The beta-alanine-conjugated and lysine-conjugated DSB could not be evaluated for their antiviral potentials due to their high cytotoxicity, whereas two other compounds with a lesser cytotoxicity, glycine-conjugated and ε-NH-Boc-lysine-conjugated DSB, exerted a dose-dependent negative effect on VLP assembly and budding. A fifth compound with a low cytotoxicity, EP-39 (ethylene diamine-conjugated DSB), showed a novel type of antiviral effect. EP-39 provoked an aberrant assembly of VLP, resulting in nonenveloped, morula-like particles of 100-nm in diameter. Each morula was composed of nanoparticle subunits of 20-nm in diameter, which possibly mimicked transient intermediates of the HIV-1 Gag assembly process. Chemical cross-linking in situ suggested that EP-39 favored the formation or/and persistence of Pr55Gag trimers over other oligomeric species. EP-39 showed a novel type of negative effect on HIV-1 assembly, targeting the Pr55Gag oligomerisation. The biological effect of EP-39 underlined the critical role of the nature of the side chain at position 28 of BA derivatives in their anti-HIV-1 activity.

The inhibition of assembly of HIV-1 virus-like particles by 3-O-(3',3'-dimethylsuccinyl) betulinic acid (DSB) is counteracted by Vif and requires its Zinc-binding domain.

BACKGROUND: DSB, the 3-O-(3',3'dimethylsuccinyl) derivative of betulinic acid, blocks the last step of protease-mediated processing of HIV-1 Gag precursor (Pr55Gag), which leads to immature, noninfectious virions. When administered to Pr55Gag-expressing insect cells (Sf9), DSB inhibits the assembly and budding of membrane-enveloped virus-like particles (VLP). In order to explore the possibility that viral factors could modulate the susceptibility to DSB of the VLP assembly process, several viral proteins were coexpressed individually with Pr55Gag in DSB-treated cells, and VLP yields assayed in the extracellular medium. RESULTS: Wild-type Vif (Vifwt) restored the VLP production in DSB-treated cells to levels observed in control, untreated cells. DSB-counteracting effect was also observed with Vif mutants defective in encapsidation into VLP, suggesting that packaging and anti-DSB effect were separate functions in Vif. The anti-DSB effect was abolished for VifC133S and VifS116V, two mutants which lacked the zinc binding domain (ZBD) formed by the four H(108)C(114)C(133)H(139) coordinates with a Zn atom. Electron microscopic analysis of cells coexpressing Pr55Gag and Vifwt showed that a large proportion of VLP budded into cytoplasmic vesicles and were released from Sf9 cells by exocytosis. However, in the presence of mutant VifC133S or VifS116V, most of the VLP assembled and budded at the plasma membrane, as in control cells expressing Pr55Gag alone. CONCLUSION: The function of HIV-1 Vif protein which negated the DSB inhibition of VLP assembly was independent of its packaging capability, but depended on the integrity of ZBD. In the presence of Vifwt, but not with ZBD mutants VifC133S and VifS116V, VLP were redirected to a vesicular compartment and egressed via the exocytic pathway.

Rational design, structure, and biological evaluation of cyclic peptides mimicking the vascular endothelial growth factor.

Angiogenesis is the development of a novel vascular network from a pre-existing structure. Blocking angiogenesis is an attractive strategy to inhibit tumor growth and metastasis formation. Based on structural and mutagenesis data, we have developed novel cyclic peptides that mimic, simultaneously, two regions of the VEGF crucial for the interaction with the VEGF receptors. The peptides, displaying the best affinity for VEGF receptor 1 on a competition assay, inhibited endothelial cell transduction pathway, migration, and capillary-like tubes formation. The specificity of these peptides for VEGF receptors was demonstrated by microscopy using a fluorescent peptide derivative. The resolution of the structure of some cyclic peptides by NMR and molecular modeling has allowed the identification of various factors accounting for their inhibitory activity. Taken together, these results validate the selection of these two regions as targets to develop molecules able to disturb the development of cancer and angiogenesis-associated diseases.

On-resin cyclization of peptide ligands of the Vascular Endothelial Growth Factor Receptor 1 by copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition.

Cyclic peptides were obtained, on-resin, by the copper (I) catalysed 1,3-dipolar cycloaddition of azides and alkynes. The reaction led exclusively to the formation of the expected cyclomonomeric products which acted as ligands of the Vascular Endothelial Growth Factor receptor 1.