Structure-activity relationship studies of allosteric inhibitors of EYA2 tyrosine phosphatase

Human eyes absent (EYA) proteins possess Tyr phosphatase activity, which is critical for numerous cancer and metastasis promoting activities, making it an attractive target for cancer therapy. In this work, we demonstrate that the inhibitor-bound form of EYA2 does not favour binding to Mg2+ , which is indispensable for the Tyr phosphatase activity. We further describe characterization and optimization of this class of allosteric inhibitors. A series of analogues were synthesized to improve potency of the inhibitors and to elucidate structure-activity relationships. Two co-crystal structures confirm the binding modes of this class of inhibitors. Our medicinal chemical, structural, biochemical, and biophysical studies provide insight into the molecular interactions of EYA2 with these allosteric inhibitors. The compounds derived from this study are useful for exploring the function of the Tyr phosphatase activity of EYA2 in normal and cancerous cells and serve as reference compounds for screening or developing allosteric phosphatase inhibitors. Finally, the co-crystal structures reported in this study will aid in structure-based drug discovery against EYA2.

Structural and Functional Analyses of an Allosteric EYA2 Phosphatase Inhibitor That Has On-Target Effects in Human Lung Cancer Cells

EYA proteins (EYA1-4) are critical developmental transcriptional cofactors that contain an EYA domain (ED) harboring Tyr phosphatase activity. EYA proteins are largely downregulated after embryogenesis but are reexpressed in cancers, and their Tyr phosphatase activity plays an important role in the DNA damage response and tumor progression. We previously identified a class of small-molecule allosteric inhibitors that specifically inhibit the Tyr phosphatase activity of EYA2. Herein, we determined the crystal structure of the EYA2 ED in complex with NCGC00249987 (a representative compound in this class), revealing that it binds to an induced pocket distant from the active site. NCGC00249987 binding leads to a conformational change of the active site that is unfavorable for Mg²⁺ binding, thereby inhibiting EYA2's Tyr phosphatase activity. We demonstrate, using genetic mutations, that migration, invadopodia formation, and invasion of lung adenocarcinoma cells are dependent on EYA2 Tyr phosphatase activity, whereas growth and survival are not. Further, we demonstrate that NCGC00249987 specifically targets migration, invadopodia formation, and invasion of lung cancer cells, but that it does not inhibit cell growth or survival. The compound has no effect on lung cancer cells carrying an EYA2 F290Y mutant that abolishes compound binding, indicating that NCGC00249987 is on target in lung cancer cells. These data suggest that the NCGC00249987 allosteric inhibitor can be used as a chemical probe to study the function of the EYA2 Tyr phosphatase activity in cells and may have the potential to be developed into an antimetastatic agent for cancers reliant on EYA2's Tyr phosphatase activity.

Fragment-based Discovery of a Small-Molecule Protein Kinase C-iota Inhibitor Binding Post-kinase Domain Residues

The atypical protein kinase C-iota (PKC-ι) enzyme is implicated in various cancers and has been put forward as an attractive target for developing anticancer therapy. A high concentration biochemical screen identified pyridine fragment weakly inhibiting PKC-ι with IC₅₀ = 424 μM. Driven by structure-activity relationships and guided by docking hypothesis, the weakly bound fragment was eventually optimized into a potent inhibitor of PKC-ι (IC₅₀= 270 nM). Through the course of the optimization, an intermediate compound was crystallized with the protein, and careful analysis of the X-ray crystal structure revealed a unique binding mode involving the post-kinase domain (C-terminal tail) of PKC-ι.

Fragment-Based Drug Discovery of Potent Protein Kinase C Iota Inhibitors

Protein kinase C iota (PKC-ι) is an atypical kinase implicated in the promotion of different cancer types. A biochemical screen of a fragment library has identified several hits from which an azaindole-based scaffold was chosen for optimization. Driven by a structure-activity relationship and supported by molecular modeling, a weakly bound fragment was systematically grown into a potent and selective inhibitor against PKC-ι.

Backbone resonance assignments for the SET domain of human methyltransferase NSD3 in complex with its cofactor

NSD3 is a histone H3 methyltransferase that plays an important role in chromatin biology. A construct containing the methyltransferase domain encompassing residues Q1049-K1299 of human NSD3 was obtained and biochemical activity was demonstrated using histone as a substrate. Here we report the backbone HN, N, Cα, C', and side chain Cβ assignments of the construct in complex with S-adenosyl-L-methionine (SAM). Based on these assignments, secondary structures of NSD3/SAM complex in solution were determined.

Crystal structure of unlinked NS2B-NS3 protease from Zika virus

Zika virus (ZIKV) has rapidly emerged as a global public health concern. Viral NS2B-NS3 protease processes viral polyprotein and is essential for the virus replication, making it an attractive antiviral drug target. We report crystal structures at 1.58-angstrom resolution of the unlinked NS2B-NS3 protease from ZIKV as free enzyme and bound to a peptide reversely oriented at the active site. The unlinked NS2B-NS3 protease adopts a closed conformation in which NS2B engages NS3 to form an empty substrate-binding site. A second protease in the same crystal binds to the residues K14K15G16E17 from the neighboring NS3 in reverse orientation, resisting proteolysis. These features of ZIKV NS2B-NS3 protease may accelerate the discovery of structure-based antiviral drugs against ZIKV and related pathogenic flaviviruses.

Diversity-Oriented Synthesis as a Strategy for Fragment Evolution against GSK3β

Traditional fragment-based drug discovery (FBDD) relies heavily on structural analysis of the hits bound to their targets. Herein, we present a complementary approach based on diversity-oriented synthesis (DOS). A DOS-based fragment collection was able to produce initial hit compounds against the target GSK3β, allow the systematic synthesis of related fragment analogues to explore fragment-level structure-activity relationship, and finally lead to the synthesis of a more potent compound.

Escherichia coli Topoisomerase IV E Subunit and an Inhibitor Binding Mode Revealed by NMR Spectroscopy

Bacterial topoisomerases are attractive antibacterial drug targets because of their importance in bacterial growth and low homology with other human topoisomerases. Structure-based drug design has been a proven approach of efficiently developing new antibiotics against these targets. Past studies have focused on developing lead compounds against the ATP binding pockets of both DNA gyrase and topoisomerase IV. A detailed understanding of the interactions between ligand and target in a solution state will provide valuable information for further developing drugs against topoisomerase IV targets. Here we describe a detailed characterization of a known potent inhibitor containing a 9H-pyrimido[4,5-b]indole scaffold against the N-terminal domain of the topoisomerase IV E subunit from Escherichia coli (eParE). Using a series of biophysical and biochemical experiments, it has been demonstrated that this inhibitor forms a tight complex with eParE. NMR studies revealed the exact protein residues responsible for inhibitor binding. Through comparative studies of two inhibitors of markedly varied potencies, it is hypothesized that gaining molecular interactions with residues in the α4 and residues close to the loop of β1-α2 and residues in the loop of β3-β4 might improve the inhibitor potency.

Targeting the Central Pocket in Human Transcription Factor TEAD as a Potential Cancer Therapeutic Strategy

The human TEAD family of transcription factors (TEAD1-4) is required for YAP-mediated transcription in the Hippo pathway. Hyperactivation of TEAD's co-activator YAP contributes to tissue overgrowth and human cancers, suggesting that pharmacological interference of TEAD-YAP activity may be an effective strategy for anticancer therapy. Here we report the discovery of a central pocket in the YAP-binding domain (YBD) of TEAD that is targetable by small-molecule inhibitors. Our X-ray crystallography studies reveal that flufenamic acid, a non-steroidal anti-inflammatory drug (NSAID), binds to the central pocket of TEAD2 YBD. Our biochemical and functional analyses further demonstrate that binding of NSAIDs to TEAD inhibits TEAD-YAP-dependent transcription, cell migration, and proliferation, indicating that the central pocket is important for TEAD function. Therefore, our studies discover a novel way of targeting TEAD transcription factors and set the stage for therapeutic development of specific TEAD-YAP inhibitors against human cancers.

Optimization of Inhibitors of Mycobacterium tuberculosis Pantothenate Synthetase Based on Group Efficiency Analysis

Ligand efficiency has proven to be a valuable concept for optimization of leads in the early stages of drug design. Taking this one step further, group efficiency (GE) evaluates the binding efficiency of each appendage of a molecule, further fine‐tuning the drug design process. Here, GE analysis is used to systematically improve the potency of inhibitors of Mycobacterium tuberculosis pantothenate synthetase, an important target in tuberculosis therapy. Binding efficiencies were found to be distributed unevenly within a lead molecule derived using a fragment‐based approach. Substitution of the less efficient parts of the molecule allowed systematic development of more potent compounds. This method of dissecting and analyzing different groups within a molecule offers a rational and general way of carrying out lead optimization, with potential broad application within drug discovery.

Characterization of the interaction between Escherichia coli topoisomerase IV E subunit and an ATP competitive inhibitor

Bacterial topoisomerase IV (ParE) is essential for DNA replication and serves as an attractive target for antibacterial drug development. The X-ray structure of the N-terminal 24 kDa ParE, responsible for ATP binding has been solved. Due to the accessibility of structural information of ParE, many potent ParE inhibitors have been discovered. In this study, a pyridylurea lead molecule against ParE of Escherichia coli (eParE) was characterized with a series of biochemical and biophysical techniques. More importantly, solution NMR analysis of compound binding to eParE provides better understanding of the molecular interactions between the inhibitor and eParE.

NMR structural characterization of the N-terminal active domain of the gyrase B subunit from Pseudomonas aeruginosa and its complex with an inhibitor

The N-terminal ATP binding domain of the DNA gyrase B subunit is a validated drug target for antibacterial drug discovery. Structural information for this domain (pGyrB) from Pseudomonas aeruginosa is still missing. In this study, the interaction between pGyrB and a bis-pyridylurea inhibitor was characterized using several biophysical methods. We further carried out structural analysis of pGyrB using NMR spectroscopy. The secondary structures of free and inhibitor bound pGyrB were obtained based on backbone chemical shift assignment. Chemical shift perturbation and NOE experiments demonstrated that the inhibitor binds to the ATP binding pocket. The results of this study will be helpful for drug development targeting P. aeruginosa.

Application of Fragment-Based Drug Discovery against DNA Gyrase B

Bacterial resistance to antibiotics remains a serious threat to global health. The gyrase B enzyme is a well-validated target for developing antibacterial drugs. Despite being an attractive target for antibiotic development, there are currently no gyrase B inhibitory drugs on the market. A fragment screen using 1,800 compounds identified 14 fragments that bind to Escherichia coli (E. coli) gyrase B. The detailed characterization of binding is described for all 14 fragments. With the aid of X-ray crystallography, modifications on a low-affinity fragment (K_D =253 μM, IC₅₀ =634 μM) has led to the development of a new class of potent phenyl aminopyrazole inhibitors against E. coli gyrase B (IC₅₀ =160 nM). The study presented here combines the use of a set of biophysical techniques including differential scanning fluorimetry, nuclear magnetic resonance, isothermal titration calorimetry, and X-ray crystallography to methodically identify, quantify, and optimize fragments into new chemical leads.

Pathway-selective sensitization of Mycobacterium tuberculosis for target-based whole-cell screening

Whole-cell screening of Mycobacterium tuberculosis (Mtb) remains a mainstay of drug discovery, but subsequent target elucidation often proves difficult. Conditional mutants that underexpress essential genes have been used to identify compounds with known mechanism of action by target-based whole-cell screening (TB-WCS). Here, the feasibility of TB-WCS in Mtb was assessed by generating mutants that conditionally express pantothenate synthetase (panC), diaminopimelate decarboxylase (lysA), and isocitrate lyase (icl1). The essentiality of panC and lysA, and conditional essentiality of icl1 for growth on fatty acids, was confirmed. Depletion of PanC and Icl1 rendered mutants hypersensitive to target-specific inhibitors. Stable reporter strains were generated for use in high-throughput screening, and their utility was demonstrated by identifying compounds that display greater potency against a PanC-depleted strain. These findings illustrate the power of TB-WCS as a tool for tuberculosis drug discovery.

NMR analysis of a novel enzymatically active unlinked dengue NS2B-NS3 protease complex

The dengue virus (DENV) is a mosquito-borne pathogen responsible for an estimated 100 million human infections annually. The viral genome encodes a two-component trypsin-like protease that contains the cofactor region from the nonstructural protein NS2B and the protease domain from NS3 (NS3pro). The NS2B-NS3pro complex plays a crucial role in viral maturation and has been identified as a potential drug target. Using a DENV protease construct containing NS2B covalently linked to NS3pro via a Gly4-Ser-Gly4 linker ("linked protease"), previous x-ray crystal structures show that the C-terminal fragment of NS2B is remote from NS3pro and exists in an open state in the absence of an inhibitor; however, in the presence of an inhibitor, NS2B complexes with NS3pro to form a closed state. This linked enzyme produced NMR spectra with severe signal overlap and line broadening. To obtain a protease construct with a resolved NMR spectrum, we expressed and purified an unlinked protease complex containing a 50-residue segment of the NS2B cofactor region and NS3pro without the glycine linker using a coexpression system. This unlinked protease complex was catalytically active at neutral pH in the absence of glycerol and produced dispersed cross-peaks in a (1)H-(15)N heteronuclear single quantum correlation spectrum that enabled us to conduct backbone assignments using conventional techniques. In addition, titration with an active-site peptide aldehyde inhibitor and paramagnetic relaxation enhancement studies demonstrated that the unlinked DENV protease exists predominantly in a closed conformation in solution. This protease complex can serve as a useful tool for drug discovery against DENV.

Optimization of the interligand Overhauser effect for fragment linking: application to inhibitor discovery against Mycobacterium tuberculosis pantothenate synthetase

Fragment-based methods are a new and emerging approach for the discovery of protein binders that are potential new therapeutic agents. Several ways of utilizing structural information to guide the inhibitor assembly have been explored to date. One of the approaches, application of interligand Overhauser effect (ILOE) observations, is of particular interest, as it does not require the availability of a three-dimensional protein structure and is an NMR-based method that can be applied to targets that cannot be observed directly because of their size. Fragments, as small and often hydrophobic molecules, suffer from problems including compound aggregation in an aqueous environment and nonspecific binding contributions, especially when screened at higher concentrations suitable for ILOE observations. Here we report how this problem can be overcome by applying a step-by-step iterative procedure that includes the application of optimized probe molecules with known binding modes to elucidate the unknown binding modes of fragments. An enzyme substrate with well-characterized binding was used as a starting point, and the relative binding modes of modified fragments derived from ILOE observations were used to guide the fragment linking, leading to a potent inhibitor of our model system, Mycobacterium tuberculosis pantothenate synthetase, a potential drug target. We have supported our NMR data with crystal structures, thus establishing the guidelines for optimizing the ILOE observations. This model study should expand the application of the technique in drug discovery.