Selective and Potent Proteomimetic Inhibitors of Intracellular Protein-Protein Interactions

Sulfono-γ-AApeptides as Protein Helical Domain Mimetics to Manipulate the Angiogenesis

Sulfono-γ-AApeptides recently developed in our group have been proven to be a new class of unnatural foldamer with well-defined helical structure and have been demonstrated to mimic protein helical domains and disrupt biomedically relevant protein-protein interactions (PPIs). Based on our design concept in a recent report, we discovered two similar sulfono-γ-AApeptides V2 and V3 which were designed to mimic the VEGF N-terminal helix α1 known to directly interact with VEGFRs. Interestingly, V2 was shown to possess the pro-angiogenic effect, whereas V3 was proved to be a potent inhibitor for angiogenesis. We speculate that the distinct angiogenesis signaling was due to the selective binding of the two molecules to VEGFR1 and VEGFR2, respectively. Together with their remarkable resistance to proteolytic degradation, relatively small sizes, and amenability to modification with diverse functional groups, V2 and V3 could serve as lead molecules for the development of potential therapeutic agents and molecular probes. These findings highlight sulfono-γ-AApeptides as an alternative paradigm to mimic the α-helical domain to modulate a wide variety of PPIs in the future.

Selective Disruption of Survivin's Protein‐Protein Interactions: A Supramolecular Approach Based on Guanidiniocarbonylpyrrole

Targeting specific protein binding sites to interfere with protein‐protein interactions (PPIs) is crucial for the rational modulation of biologically relevant processes. Survivin, which is highly overexpressed in most cancer cells and considered to be a key player of carcinogenesis, features two functionally relevant binding sites. Here, we demonstrate selective disruption of the Survivin/Histone H3 or the Survivin/Crm1 interaction using a supramolecular approach. By rational design we identified two structurally related ligands (L_NES and L_HIS), capable of selectively inhibiting these PPIs, leading to a reduction in cancer cell proliferation.

Natural and Synthetic Oligoarylamides: Privileged Structures for Medical Applications

The term "privileged structure" refers to a single molecular substructure or scaffold that can serve as a starting point for high-affinity ligands for more than one receptor type. In this report, a hitherto overlooked group of privileged substructures is addressed, namely aromatic oligoamides, for which there are natural models in the form of cystobactamids, albicidin, distamycin A, netropsin, and others. The aromatic and heteroaromatic core, together with a flexible selection of substituents, form conformationally well-defined scaffolds capable of specifically binding to conformationally well-defined regions of biomacromolecules such as helices in proteins or DNA often by acting as helices mimics themselves. As such, these aromatic oligoamides have already been employed to inhibit protein-protein and nucleic acid-protein interactions. This article is the first to bring together the scattered knowledge about aromatic oligoamides in connection with biomedical applications.

Selective Affimers Recognise the BCL-2 Family Proteins BCL-xL and MCL-1 through Noncanonical Structural Motifs*

The BCL-2 family is a challenging group of proteins to target selectively due to sequence and structural homologies across the family. Selective ligands for the BCL-2 family regulators of apoptosis are useful as probes to understand cell biology and apoptotic signalling pathways, and as starting points for inhibitor design. We have used phage display to isolate Affimer reagents (non-antibody-binding proteins based on a conserved scaffold) to identify ligands for MCL-1, BCL-xL , BCL-2, BAK and BAX, then used multiple biophysical characterisation methods to probe the interactions. We established that purified Affimers elicit selective recognition of their target BCL-2 protein. For anti-apoptotic targets BCL-xL and MCL-1, competitive inhibition of their canonical protein-protein interactions is demonstrated. Co-crystal structures reveal an unprecedented mode of molecular recognition; where a BH3 helix is normally bound, flexible loops from the Affimer dock into the BH3 binding cleft. Moreover, the Affimers induce a change in the target proteins towards a desirable drug-bound-like conformation. These proof-of-concept studies indicate that Affimers could be used as alternative templates to inspire the design of selective BCL-2 family modulators and more generally other protein-protein interaction inhibitors.

Evolution in non-peptide α-helix mimetics on the road to effective protein-protein interaction modulators

Modulation of interactome networks, essentially protein-protein interactions (PPIs), might represent valuable therapeutic approaches to different pathological conditions. Since a high percentage of PPIs are mediated by α-helical structures at the interacting surface, the development of compounds able to reproduce the amino acid side-chain organization of α-helices (e.g. stabilized α-helix peptides and β-derivatives, proteomimetics, and α-helix small-molecule mimetics) focuses the attention of different research groups. This appraisal describes the recent progress in the non-peptide α-helix mimetics field, which has evolved from single-face to multi-face reproducing compounds and from oligomeric to monomeric scaffolds able to bear different substituents in similar spatial dispositions as the side-chains in canonical helices. Grouped by chemical structures, the review contemplates terphenyl-like molecules, oligobenzamides and heterocyclic analogues, benzamide-amino acid conjugates and non-oligomeric small-molecules mimetics, among others, and their effectiveness to stabilize/disrupt therapeutically relevant PPIs. The X-ray structures of a couple of oligomeric peptidomimetics and of some small-molecules complexed with the MDM2 protein, as well as the state of the art on their development in clinical trials, are also remarked. The discovery of a continuously increasing number of new disease-relevant PPIs could offer future opportunities for these and other forthcoming α-helix mimetics.

Stapled Peptides as HIF-1α/p300 Inhibitors: Helicity Enhancement in the Bound State Increases Inhibitory Potency

Protein-protein interactions (PPIs) control virtually all cellular processes and have thus emerged as potential targets for development of molecular therapeutics. Peptide-based inhibitors of PPIs are attractive given that they offer recognition potency and selectivity features that are ideal for function, yet, they do not predominantly populate the bioactive conformation, frequently suffer from poor cellular uptake and are easily degraded, for example, by proteases. The constraint of peptides in a bioactive conformation has emerged as a promising strategy to mitigate against these liabilities. In this work, using peptides derived from hypoxia-inducible factor 1 (HIF-1α) together with dibromomaleimide stapling, we identify constrained peptide inhibitors of the HIF-1α/p300 interaction that are more potent than their unconstrained sequences. Contrary to expectation, the increased potency does not correlate with an increased population of an α-helical conformation in the unbound state as demonstrated by experimental circular dichroism analysis. Rather, the ability of the peptide to adopt a bioactive α-helical conformation in the p300 bound state is better supported in the constrained variant as demonstrated by molecular dynamics simulations and circular dichroism difference spectra.

Structure Elucidation of Helical Aromatic Foldamer-Protein Complexes with Large Contact Surface Areas

The development of large synthetic ligands could be useful to target the sizeable surface areas involved in protein-protein interactions. Herein, we present long helical aromatic oligoamide foldamers bearing proteinogenic side chains that cover up to 450 Ų of the human carbonic anhydrase II (HCA) surface. The foldamers are composed of aminoquinolinecarboxylic acids bearing proteinogenic side chains and of more flexible aminomethyl-pyridinecarboxylic acids that enhance helix handedness dynamics. Crystal structures of HCA-foldamer complexes were obtained with a 9- and a 14-mer both showing extensive protein-foldamer hydrophobic contacts. In addition, foldamer-foldamer interactions seem to be prevalent in the crystal packing, leading to the peculiar formation of an HCA superhelix wound around a rod of stacked foldamers. Solution studies confirm the positioning of the foldamer at the protein surface as well as a dimerization of the complexes.

Targeted Nanoswitchable Inhibitors of Protein-Protein Interactions Involved in Apoptosis

Progress in drug delivery is hampered by a lack of efficient strategies to target drugs with high specificity and precise spatiotemporal regulation. The remote control of nanoparticles and drugs with light allows regulation of their action site and dosage. Peptide-based drugs are highly specific, non-immunogenic, and can be designed to cross the plasma membrane. In order to combine target specificity and remote control of drug action, here we describe a versatile strategy based on a generalized template to design nanoswitchable peptides that modulate protein-protein interactions upon light activation. This approach is demonstrated to promote photomodulation of two important targets involved in apoptosis (the interactions Bcl-xL-Bak and MDM2-p53), but can be also applied to a large pool of therapeutically relevant protein-protein interactions mediated by α-helical motifs. The template can be adjusted using readily available information about hot spots (residues contributing most to the binding energy) at the protein-protein interface of interest.

Peptide-Directed Binding for the Discovery of Modulators of α-Helix-Mediated Protein-Protein Interactions: Proof-of-Concept Studies with the Apoptosis Regulator Mcl-1

Targeting PPIs with small molecules can be challenging owing to large, hydrophobic binding surfaces. Herein, we describe a strategy that exploits selective α-helical PPIs, transferring these characteristics to small molecules. The proof of concept is demonstrated with the apoptosis regulator Mcl-1, commonly exploited by cancers to avoid cell death. Peptide-directed binding uses few synthetic transformations, requires the production of a small number of compounds, and generates a high percentage of hits. In this example, about 50 % of the small molecules prepared showed an IC50 value of less than 100 μm, and approximately 25 % had IC50 values below 1 μm to Mcl-1. Compounds show selectivity for Mcl-1 over other anti-apoptotic proteins, possess cytotoxicity to cancer cell lines, and induce hallmarks of apoptosis. This approach represents a novel and economic process for the rapid discovery of new α-helical PPI modulators.

New Modalities for Challenging Targets in Drug Discovery

Our ever-increasing understanding of biological systems is providing a range of exciting novel biological targets, whose modulation may enable novel therapeutic options for many diseases. These targets include protein-protein and protein-nucleic acid interactions, which are, however, often refractory to classical small-molecule approaches. Other types of molecules, or modalities, are therefore required to address these targets, which has led several academic research groups and pharmaceutical companies to increasingly use the concept of so-called "new modalities". This Review defines for the first time the scope of this term, which includes novel peptidic scaffolds, oligonucleotides, hybrids, molecular conjugates, as well as new uses of classical small molecules. We provide the most representative examples of these modalities to target large binding surface areas such as those found in protein-protein interactions and for biological processes at the center of cell regulation.

An α-Helix-Mimicking 12,13-Helix: Designed α/β/γ-Foldamers as Selective Inhibitors of Protein-Protein Interactions

A major current challenge in bioorganic chemistry is the identification of effective mimics of protein secondary structures that act as inhibitors of protein-protein interactions (PPIs). In this work, trans-2-aminocyclobutanecarboxylic acid (tACBC) was used as the key β-amino acid component in the design of α/β/γ-peptides to structurally mimic a native α-helix. Suitably functionalized α/β/γ-peptides assume an α-helix-mimicking 12,13-helix conformation in solution, exhibit enhanced proteolytic stability in comparison to the wild-type α-peptide parent sequence from which they are derived, and act as selective inhibitors of the p53/hDM2 interaction.

Probing Protein Surfaces: QSAR Analysis with Helix Mimetics

α-Helix-mediated protein-protein interactions (PPIs) are important targets for small-molecule inhibition; however, generic approaches to inhibitor design are in their infancy and would benefit from QSAR analyses to rationalise the noncovalent basis of molecular recognition by designed ligands. Using a helix mimetic based on an oligoamide scaffold, we have exploited the power of a modular synthesis to access compounds that can readily be used to understand the noncovalent determinants of hDM2 recognition by this series of cell-active p53/hDM2 inhibitors.

Protein Camouflage: Supramolecular Anion Recognition by Ubiquitin

Progress in the field of bio-supramolecular chemistry, the bottom-up assembly of protein-ligand systems, relies on a detailed knowledge of molecular recognition. To address this issue, we have characterised complex formation between human ubiquitin (HUb) and four supramolecular anions. The ligands were: pyrenetetrasulfonic acid (4PSA), p-sulfonato-calix[4]arene (SCLX4), bisphosphate tweezers (CLR01) and meso-tetrakis (4-sulfonatophenyl)porphyrin (TPPS), which vary in net charge, size, shape and hydrophobicity. All four ligands induced significant changes in the HSQC spectrum of HUb. Chemical shift perturbations and line-broadening effects were used to identify binding sites and to quantify affinities. Supporting data were obtained from docking simulations. It was found that these weakly interacting ligands bind to extensive surface patches on HUb. A comparison of the data suggests some general indicators for the protein-binding specificity of supramolecular anions. Differences in binding were observed between the cavity-containing and planar ligands. The former had a preference for the arginine-rich, flexible C terminus of HUb.

Solution Observation of Dimerization and Helix Handedness Induction in a Human Carbonic Anhydrase-Helical Aromatic Amide Foldamer Complex

The design of synthetic foldamers to selectively bind proteins is currently hindered by the limited availability of molecular data to establish key features of recognition. Previous work has described dimerization of human carbonic anhydrase II (HCA) through self-association of a quinoline oligoamide helical foldamer attached to a tightly binding HCA ligand. A crystal structure of the complex provided atomic details to explain the observed induction of single foldamer helix handedness and revealed an unexpected foldamer-mediated dimerization. Here, we investigated the detailed behavior of the HCA-foldamer complex in solution by using NMR spectroscopy. We found that the ability to dimerize is buffer-dependent and uses partially distinct intermolecular contacts. The use of a foldamer variant incapable of self-association confirmed the ability to induce helix handedness separately from dimer formation and provided insight into the dynamics of enantiomeric selection.

Solid-Phase Synthesis of Water-Soluble Helically Folded Hybrid α-Amino Acid/Quinoline Oligoamides

We report here a solid phase synthesis methodology that allows the incorporation of α-amino acids (X) into quinoline (Q) oligoamide foldamer sequences. Water-soluble hybrid oligoamides based on the XQ2 trimer repeat motif were shown to adopt helical conformations presenting α-amino acid side chains in a predictable linear array on one face of the helix. In contrast, sequences based on the XQ dimer motif expressed less well-defined behavior, most likely due to local conformational variability precluding long-range order. Also presented is a full structural investigation by NMR of a dodecameric XQ2-type foldamer containing four different amino acid residues (Lys, Ala, Asp, and Ser).

Cyclic and Macrocyclic Peptides as Chemical Tools To Recognise Protein Surfaces and Probe Protein-Protein Interactions

Targeting protein surfaces and protein-protein interactions (PPIs) with small molecules is a frontier goal of chemical biology and provides attractive therapeutic opportunities in drug discovery. The molecular properties of protein surfaces, including their shallow features and lack of deep binding pockets, pose significant challenges, and as a result have proved difficult to target. Peptides are ideal candidates for this mission due to their ability to closely mimic many structural features of protein interfaces. However, their inherently low intracellular stability and permeability and high in vivo clearance have thus far limited their biological applications. One way to improve these properties is to constrain the secondary structure of linear peptides by cyclisation. Herein we review various classes of cyclic and macrocyclic peptides as chemical probes of protein surfaces and modulators of PPIs. The growing interest in this area and recent advances provide evidence of the potential of developing peptide-like molecules that specifically target these interactions.