Isoquinoline small molecule ligands are agonists and probe-dependent allosteric modulators of the glucagon subfamily of GPCRs.

Class B1 G protein-coupled receptors (GPCRs) are peptide hormone receptors and well validated therapeutic targets, however development of non-peptide drugs targeting this class of receptors is challenging. Recently, a series of isoquinoline-based derivates were reported in the patent literature as allosteric ligands for the glucagon receptor subfamily, and two compounds, LSN3451217 and LSN3556672, were used to facilitate structural studies with the glucagon-like peptide-1 receptor (GLP-1R) and glucose dependent insulinotropic peptide receptor (GIPR) bound to orthosteric agonists. Here we pharmacologically characterized stereoisomers of LSN3451217 and LSN3556672, across the class B1 GPCR family. This revealed LSN3556672 isomers are agonists for the glucagon receptor (GCGR), GLP-1R, GIPR and the calcitonin receptor (CTR), albeit the degree of agonism varied at each receptor. In contrast, LSN3451217 isomers were more selective agonists at the GLP-1R, with lower potency at the GCGR and CTR and no activity at the GIPR. All compounds also modulated peptide-mediated cyclic adenosine monophosphate (cAMP) signaling at the GIPR, and to a lesser extent the GLP-1R, in a probe-dependent manner, with modest positive allosteric modulation observed for some peptides, and negligible effects observed with other peptides. In contrast neutral or weak negative/positive allosteric modulation was observed with peptides assessed at the GCGR and CTR. This study expands our knowledge on class B1 GPCR allosteric modulation and may have implications for future structural and drug discovery efforts targeting the class B1 GPCR subfamily.

Protons taken hostage: Dynamic H-bond networks of the pH-sensing GPR68.

Proton-sensing G Protein Coupled Receptors (GPCRs) sense changes in the extracellular pH to effect cell signaling for cellular homeostasis. They tend to be overexpressed in solid tumors associated with acidic extracellular pH, and are of direct interest as drug targets. How proton-sensing GPCRs sense extracellular acidification and activate upon protonation change is important to understand, because it may guide the design of therapeutics. Lack of publicly available experimental structures make it challenging to discriminate between conflicting mechanisms proposed for proton-binding, as main roles have been assigned to either an extracellular histidine cluster or to an internal carboxylic triad. Here we present a protocol to derive and evaluate structural models of the proton-sensing GPR68. This approach integrates state-of-the-art homology modeling with microsecond-timescale atomistic simulations, and with a detailed assessment of the compatibility of the structural models with known structural features of class A GPCRs. To decipher structural elements of potential interest for protonation-coupled conformational changes of GPR68, we used the best-compatible model as a starting point for independent atomistic simulations of GPR68 with different protonation states, and graph computations to characterize the response of GPR68 to changes in protonation. We found that GPR68 hosts an extended hydrogen-bond network that inter-connects the extracellular histidine cluster to the internal carboxylic triad, and which can even reach groups at the cytoplasmic G-protein binding site. Taken together, results suggest that GPR68 relies on dynamic, hydrogen-bond networks to inter-connect extracellular and internal proton-binding sites, and to elicit conformational changes at the cytoplasmic G-protein binding site.

Crystal structures of human and mouse ketohexokinase provide a structural basis for species- and isoform-selective inhibitor design.

A molecular understanding of the proteins involved in fructose metabolism is essential for controlling the current spread of fructose-related obesity, diabetes and related adverse metabolic states in Western populations. Fructose catabolism starts with the phosphorylation of D-fructose to fructose 1-phosphate by ketohexokinase (KHK). KHK exists in two alternatively spliced isoforms: the hepatic and intestinal isoform KHK-C and the peripheral isoform KHK-A. Here, the structure of apo murine KHK (mKHK), which differs from structures of human KHK in overall conformation, is reported. An isoform-selective ligand, which offers a 50-fold higher potency on mKHK and human KHK-A compared with KHK-C, is further characterized. In mKHK, large-scale conformational changes are observed upon ligand binding. The structures suggest a combined strategy for the design of species- and isoform-selective KHK inhibitors.

Discovery of BI 7446: A Potent Cyclic Dinucleotide STING Agonist with Broad-Spectrum Variant Activity for the Treatment of Cancer.

Activating the stimulator of interferon genes (STING) pathway with STING agonists is an attractive immune oncology concept to treat patients with tumors that are refractory to single-agent anti-PD-1 therapy. For best clinical translatability and broad application to cancer patients, STING agonists with potent cellular activation of all STING variants are desired. Novel cyclic dinucleotide (CDN)-based selective STING agonists were designed and synthesized comprising noncanonical nucleobase, ribose, and phosphorothioate moieties. This strategy led to the discovery of 2',3'-CDN 13 (BI 7446), which features unprecedented potency and activates all five STING variants in cellular assays. ADME profiling revealed that CDN 13 has attractive drug-like properties for development as an intratumoral agent. Injection of low doses of CDN 13 into tumors in mice induced long-lasting, tumor-specific immune-mediated tumor rejection. Based on its compelling preclinical profile, BI 7446 has been advanced to clinical trials (monotherapy and in combination with anti-PD-1 antibody).

Anti-GARP Antibodies Inhibit Release of TGF-ß by Regulatory T Cells via Different Modes of Action, but Do Not Influence Their Function In Vitro.

Regulatory T cells (Treg) play a critical role in controlling immune responses in diseases such as cancer or autoimmunity. Activated Treg express the membrane protein GARP (LRRC32) in complex with the latent form of the immunosuppressive cytokine TGF-ß (L-TGF-ß). In this study, we confirmed that active TGF-ß was generated from its latent form in an integrin-dependent manner and induced TGF-ß receptor signaling in activated human Treg. We studied a series of Abs targeting the L-TGF-ß/GARP complex with distinct binding modes. We found that TGF-ß receptor signaling could be inhibited by anti-TGF-ß and by some, but not all, Abs against the L-TGF-ß/GARP complex. Cryogenic electron microscopy structures of three L-TGF-ß/GARP complex-targeting Abs revealed their distinct epitopes and allowed us to elucidate how they achieve blockade of TGF-ß activation. Three different modes of action were identified, including a novel unusual mechanism of a GARP-binding Ab. However, blockade of GARP or TGF-ß by Abs did not influence the suppressive activity of human Treg in vitro. We were also not able to confirm a prominent role of GARP in other functions of human Treg, such as FOXP3 induction and Treg stability. These data show that the GARP/TGF-ß axis can be targeted pharmacologically in different ways, but further studies are necessary to understand its complexity and to unleash its therapeutic potential.

Structural basis of inhibition of the human SGLT2-MAP17 glucose transporter.

Human sodium-glucose cotransporter 2 (hSGLT2) mediates the reabsorption of the majority of filtrated glucose in the kidney1. Pharmacological inhibition of hSGLT2 by oral small-molecule inhibitors, such as empagliflozin, leads to enhanced excretion of glucose and is widely used in the clinic to manage blood glucose levels for the treatment of type 2 diabetes1. Here we determined the cryogenic electron microscopy structure of the hSGLT2-MAP17 complex in the empagliflozin-bound state to an overall resolution of 2.95 Å. Our structure shows eukaryotic SGLT-specific structural features. MAP17 interacts with transmembrane helix 13 of hSGLT2. Empagliflozin occupies both the sugar-substrate-binding site and the external vestibule to lock hSGLT2 in an outward-open conformation, thus inhibiting the transport cycle. Our work provides a framework for understanding the mechanism of SLC5A family glucose transporters and also develops a foundation for the future rational design and optimization of new inhibitors targeting these transporters.

Discovery and Structure-Based Optimization of Fragments Binding the Mixed Lineage Kinase Domain-like Protein Executioner Domain.

Necroptosis is a form of programmed cell death that in case of misregulation can lead to inflammatory diseases. Mixed lineage kinase domain-like protein (MLKL), the effector protein in the canonical necroptosis signaling pathway, becomes activated by phosphorylation. Here, we report the identification of novel reversible binders of the MLKL executioner domain by a protein NMR-detected fragment-based screen. Determination of protein fragment costructures using NMR spectroscopy revealed a small molecule binding site that is distinct from the previously identified binding site of covalent MLKL inhibitors. Affinity optimization of the initially prioritized hit with millimolar affinity was achieved by NMR-guided structure-based design and yielded fragment-like molecules with a KD of 50 µM. Furthermore, we demonstrate that the improved fragment competes for the same binding site as nonyl-maltoside, a detergent that in conjunction with phytic acid activates the MLKL executioner domain.

Biophysical and structural investigation of the regulation of human GTP cyclohydrolase I by its regulatory protein GFRP.

GTP Cyclohydrolase I (GCH1) catalyses the conversion of guanosine triphosphate (GTP) to dihydroneopterin triphosphate (H2NTP), the initiating step in the biosynthesis of tetrahydrobiopterin (BH4). BH4 functions as co-factor in neurotransmitter biosynthesis. BH4 homeostasis is a promising target to treat pain disorders in patients. The function of mammalian GCH1s is regulated by a metabolic sensing mechanism involving a regulator protein, GCH1 feedback regulatory protein (GFRP). Dependent on the relative cellular concentrations of effector ligands, BH4 and phenylalanine, GFRP binds GCH1 to form inhibited or activated complexes, respectively. We determined high-resolution structures of the ligand-free and -bound human GFRP and GCH1-GFRP complexes by X-ray crystallography. Highly similar binding modes of the substrate analogue 7-deaza-GTP to active and inhibited GCH1-GFRP complexes confirm a novel, dissociation rate-controlled mechanism of non-competitive inhibition to be at work. Further, analysis of all structures shows that upon binding of the effector molecules, the conformations of GCH1 or GFRP are altered and form highly complementary surfaces triggering a picomolar interaction of GFRP and GCH1 with extremely slow koff values, while GCH1-GFRP complexes rapidly disintegrate in absence of BH4 or phenylalanine. Finally, comparing behavior of full-length and N-terminally truncated GCH1 we conclude that the disordered GCH1 N-terminus does not have impact on complex formation and enzymatic activity. In summary, this comprehensive and methodologically diverse study helps to provide a better understanding of the regulation of GCH1 by GFRP and could thus stimulate research on GCH1 modulating drugs.

Action of Dipeptidyl Peptidase-4 Inhibitors on SARS-CoV-2 Main Protease.

In a recent publication, Eleftheriou et al. proposed that inhibitors of dipeptidyl peptidase-4 (DPP-4) are functional inhibitors of the main protease (Mpro ) of SARS-CoV-2. Their predictions prompted the authors to suggest linagliptin, a DPP-4 inhibitor and approved anti-diabetes drug, as a repurposed drug candidate against the ongoing COVID-19 pandemic. We used an enzymatic assay measuring the inhibition of Mpro catalytic activity in the presence of four different commercially available gliptins (linagliptin, sitagliptin, alogliptin and saxagliptin) and several structural analogues of linagliptin to study the binding of DPP-4 inhibitors to Mpro and their functional activity. We show here that DPP-4 inhibitors like linagliptin, other gliptins and structural analogues are inactive against Mpro .

A Single Second Shell Amino Acid Determines Affinity and Kinetics of Linagliptin Binding to Type 4 Dipeptidyl Peptidase and Fibroblast Activation Protein.

Drugs targeting type 4 dipeptidyl peptidase (DPP-4) are beneficial for glycemic control, whereas fibroblast activation protein alpha (FAP-α) is a potential target for cancer therapies. Unlike other gliptins, linagliptin displays FAP inhibition. We compared biophysical and structural characteristics of linagliptin binding to DPP-4 and FAP to better understand what differentiates linagliptin from other gliptins. Linagliptin exhibited high binding affinity (KD ) and a slow off-rate (koff ) when dissociating from DPP-4 (KD 6.6 pM; koff 5.1×10-5  s-1 ), and weaker inhibitory potency to FAP (KD 301 nM; koff >1 s-1 ). Co-structures of linagliptin with DPP-4 or FAP were similar except for one second shell amino acid difference: Asp663 (DPP-4) and Ala657 (FAP). pH dependence of enzymatic activities and binding of linagliptin for DPP-4 and FAP are dependent on this single amino acid difference. While linagliptin may not display any anticancer activity at therapeutic doses, our findings may guide future studies for the development of optimized inhibitors.

Locking mixed-lineage kinase domain-like protein in its auto-inhibited state prevents necroptosis.

As an alternative pathway of controlled cell death, necroptosis can be triggered by tumor necrosis factor via the kinases RIPK1/RIPK3 and the effector protein mixed-lineage kinase domain-like protein (MLKL). Upon activation, MLKL oligomerizes and integrates into the plasma membrane via its executioner domain. Here, we present the X-ray and NMR costructures of the human MLKL executioner domain covalently bound via Cys86 to a xanthine class inhibitor. The structures reveal that the compound stabilizes the interaction between the auto-inhibitory brace helix α6 and the four-helix bundle by stacking to Phe148. An NMR-based functional assay observing the conformation of this helix showed that the F148A mutant is unresponsive to the compound, providing further evidence for the importance of this interaction. Real-time and diffusion NMR studies demonstrate that xanthine derivatives inhibit MLKL oligomerization. Finally, we show that the other well-known MLKL inhibitor Necrosulfonamide, which also covalently modifies Cys86, must employ a different mode of action.

A hybrid approach reveals the allosteric regulation of GTP cyclohydrolase I.

Guanosine triphosphate (GTP) cyclohydrolase I (GCH1) catalyzes the conversion of GTP to dihydroneopterin triphosphate (H2NTP), the initiating step in the biosynthesis of tetrahydrobiopterin (BH4). Besides other roles, BH4 functions as cofactor in neurotransmitter biosynthesis. The BH4 biosynthetic pathway and GCH1 have been identified as promising targets to treat pain disorders in patients. The function of mammalian GCH1s is regulated by a metabolic sensing mechanism involving a regulator protein, GCH1 feedback regulatory protein (GFRP). GFRP binds to GCH1 to form inhibited or activated complexes dependent on availability of cofactor ligands, BH4 and phenylalanine, respectively. We determined high-resolution structures of human GCH1-GFRP complexes by cryoelectron microscopy (cryo-EM). Cryo-EM revealed structural flexibility of specific and relevant surface lining loops, which previously was not detected by X-ray crystallography due to crystal packing effects. Further, we studied allosteric regulation of isolated GCH1 by X-ray crystallography. Using the combined structural information, we are able to obtain a comprehensive picture of the mechanism of allosteric regulation. Local rearrangements in the allosteric pocket upon BH4 binding result in drastic changes in the quaternary structure of the enzyme, leading to a more compact, tense form of the inhibited protein, and translocate to the active site, leading to an open, more flexible structure of its surroundings. Inhibition of the enzymatic activity is not a result of hindrance of substrate binding, but rather a consequence of accelerated substrate binding kinetics as shown by saturation transfer difference NMR (STD-NMR) and site-directed mutagenesis. We propose a dissociation rate controlled mechanism of allosteric, noncompetitive inhibition.

Hybrid Screening Approach for Very Small Fragments: X-ray and Computational Screening on FKBP51.

Fragment-based drug discovery (FBDD) permits efficient sampling of the vast chemical space for hit identification. Libraries are screened biophysically and fragment:protein co-structures are determined by X-ray crystallography. In parallel, computational methods can derive pharmacophore models or screen virtual libraries. We screened 15 very small fragments (VSFs) (HA ≤ 11) computationally, using site identification by ligand competitive saturation (SILCS), and experimentally, by X-ray crystallography, to map potential interaction sites on the FKBP51 FK1 domain. We identified three hot spots and obtained 6 X-ray co-structures, giving a hit rate of 40%. SILCS FragMaps overlapped with X-ray structures. The compounds had millimolar affinities as determined by 15N HSQC NMR. VSFs identified the same interactions as known FK1 binder and provide new chemical starting points. We propose a hybrid screening strategy starting with SILCS, followed by a pharmacophore-derived X-ray screen and 15N HSQC NMR based KD determination to rapidly identify hits and their binding poses.

A small-molecule inhibitor of lectin-like oxidized LDL receptor-1 acts by stabilizing an inactive receptor tetramer state.

The C-type lectin family member lectin-like oxidized LDL receptor-1 (LOX-1) has been object of intensive research. Its modulation may offer a broad spectrum of therapeutic interventions ranging from cardiovascular diseases to cancer. LOX-1 mediates uptake of oxLDL by vascular cells and plays an important role in the initiation of endothelial dysfunction and its progression to atherosclerosis. So far only a few compounds targeting oxLDL-LOX-1 interaction are reported with a limited level of characterization. Here we describe the identification and characterization of BI-0115, a selective small molecule inhibitor of LOX-1 that blocks cellular uptake of oxLDL. Identified by a high throughput screening campaign, biophysical analysis shows that BI-0115 binding triggers receptor inhibition by formation of dimers of the homodimeric ligand binding domain. The structure of LOX-1 bound to BI-0115 shows that inter-ligand interactions at the receptor interfaces are key to the formation of the receptor tetramer thereby blocking oxLDL binding.

In situ crystallography as an emerging method for structure solution of membrane proteins: the case of CCR2A.

The in meso in situ serial X-ray crystallization method (Huang et al., (2015) Acta Crystallogr D Biol Crystallogr 71, 1238) combines lipid cubic phase crystallization, direct freezing of the crystallization droplet without handling of the crystals, and data collection in situ. Recently, this method was used to overcome the mechanical fragility of crystals which enabled the X-ray structure determination of chemokine receptor 2A (Apel et al., (2019) Structure 27, 427) at 2.7 Å resolution. The CCR2 structure provides the structural basis for ligand selectivity of CCR2 against chemokine receptor 5 and provides insights into the residence time of MK-0812 analogs based on molecular dynamics simulations. These findings offer new opportunities for drug discovery targeting chemokine receptors.

Crystal structure and receptor-interacting residues of MYDGF - a protein mediating ischemic tissue repair.

Myeloid-derived growth factor (MYDGF) is a paracrine-acting protein that is produced by bone marrow-derived monocytes and macrophages to protect and repair the heart after myocardial infarction (MI). This effect can be used for the development of protein-based therapies for ischemic tissue repair, also beyond the sole application in heart tissue. Here, we report the X-ray structure of MYDGF and identify its functionally relevant receptor binding epitope. MYDGF consists of a 10-stranded ß-sandwich with a folding topology showing no similarities to other cytokines or growth factors. By characterizing the epitope of a neutralizing antibody and utilizing functional assays to study the activity of surface patch-mutations, we were able to localize the receptor interaction interface to a region around two surface tyrosine residues 71 and 73 and an adjacent prominent loop structure of residues 97-101. These findings enable structure-guided protein engineering to develop modified MYDGF variants with potentially improved properties for clinical use.

Crystal Structure of CC Chemokine Receptor 2A in Complex with an Orthosteric Antagonist Provides Insights for the Design of Selective Antagonists.

We determined two crystal structures of the chemokine receptor CCR2A in complex with the orthosteric antagonist MK-0812. Full-length CCR2A, stabilized by rubredoxin and a series of five mutations were resolved at 3.3 Å. An N- and C-terminally truncated CCR2A construct was crystallized in an alternate crystal form, which yielded a 2.7 Å resolution structure using serial synchrotron crystallography. Our structures provide a clear structural explanation for the observed key role of residue E2917.39 in high-affinity binding of several orthosteric CCR2 antagonists. By combining all the structural information collected, we generated models of co-structures for the structurally diverse pyrimidine amide class of CCR2 antagonists. Even though the representative Ex15 overlays well with MK-0812, it also interacts with the non-conserved H1213.33, resulting in a significant selectivity over CCR5. Insights derived from this work will facilitate drug discovery efforts directed toward highly selective CCR2 antagonists with potentially superior efficacy.

Biophysics in drug discovery: impact, challenges and opportunities.

Over the past 25 years, biophysical technologies such as X-ray crystallography, nuclear magnetic resonance spectroscopy, surface plasmon resonance spectroscopy and isothermal titration calorimetry have become key components of drug discovery platforms in many pharmaceutical companies and academic laboratories. There have been great improvements in the speed, sensitivity and range of possible measurements, providing high-resolution mechanistic, kinetic, thermodynamic and structural information on compound-target interactions. This Review provides a framework to understand this evolution by describing the key biophysical methods, the information they can provide and the ways in which they can be applied at different stages of the drug discovery process. We also discuss the challenges for current technologies and future opportunities to use biophysical methods to solve drug discovery problems.

Comparative Analysis of Binding Kinetics and Thermodynamics of Dipeptidyl Peptidase-4 Inhibitors and Their Relationship to Structure.

The binding kinetics and thermodynamics of dipeptidyl peptidase (DPP)-4 inhibitors (gliptins) were investigated using surface plasmon resonance and isothermal titration calorimetry. Binding of gliptins to DPP-4 is a rapid electrostatically driven process. Off-rates were generally slow partly because of reversible covalent bond formation by some gliptins, and partly because of strong and extensive interactions. Binding of all gliptins is enthalpy-dominated due to strong ionic interactions and strong solvent-shielded hydrogen bonds. Using a congeneric series of molecules which represented the intermediates in the lead optimization program of linagliptin, the onset of slow binding kinetics and development of the thermodynamic repertoire were analyzed in the context of incremental changes of the chemical structures. All compounds rapidly associated, and therefore the optimization of affinity and residence time is highly correlated. The major contributor to the increasing free energy of binding was a strong increase of binding enthalpy, whereas entropic contributions remained low and constant despite significant addition of lipophilicity.

Structure-Based Design of an in Vivo Active Selective BRD9 Inhibitor.

Components of the chromatin remodelling switch/sucrose nonfermentable (SWI/SNF) complex are recurrently mutated in tumors, suggesting that altering the activity of the complex plays a role in oncogenesis. However, the role that the individual subunits play in this process is not clear. We set out to develop an inhibitor compound targeting the bromodomain of BRD9 in order to evaluate its function within the SWI/SNF complex. Here, we present the discovery and development of a potent and selective BRD9 bromodomain inhibitor series based on a new pyridinone-like scaffold. Crystallographic information on the inhibitors bound to BRD9 guided their development with respect to potency for BRD9 and selectivity against BRD4. These compounds modulate BRD9 bromodomain cellular function and display antitumor activity in an AML xenograft model. Two chemical probes, BI-7273 (1) and BI-9564 (2), were identified that should prove to be useful in further exploring BRD9 bromodomain biology in both in vitro and in vivo settings.