Discovery and Kinetic Profiling of 7-Aryl-1,2,4-triazolo[4,3-a]pyridines: Positive Allosteric Modulators of the Metabotropic Glutamate Receptor 2

The translational value of ligand-receptor binding kinetics in drug discovery

The translation of in vitro potency of a candidate drug, as determined by traditional pharmacology metrics (such as EC50/IC50 and KD/Ki values), to in vivo efficacy and safety is challenging. Residence time, which represents the duration of drug-target interaction, can be part of a more comprehensive understanding of the dynamic nature of drug-target interactions in vivo, thereby enabling better prediction of drug efficacy and safety. As a consequence, a prolonged residence time may help in achieving sustained pharmacological activity, while transient interactions with shorter residence times may be favourable for targets associated with side effects. Therefore, integration of residence time into the early stages of drug discovery and development has yielded a number of clinical candidates with promising in vivo efficacy and safety profiles. Insights from residence time research thus contribute to the translation of in vitro potency to in vivo efficacy and safety. Further research and advances in measuring and optimizing residence time will bring a much-needed addition to the drug discovery process and the development of safer and more effective drugs. In this review, we summarize recent research progress on residence time, highlighting its importance from a translational perspective.

Kinetic profiling of novel spirobenzo-oxazinepiperidinone derivatives as equilibrative nucleoside transporter 1 inhibitors

Evaluation of kinetic parameters of drug-target binding, kon, koff, and residence time (RT), in addition to the traditional in vitro parameter of affinity is receiving increasing attention in the early stages of drug discovery. Target binding kinetics emerges as a meaningful concept for the evaluation of a ligand's duration of action and more generally drug efficacy and safety. We report the biological evaluation of a novel series of spirobenzo-oxazinepiperidinone derivatives as inhibitors of the human equilibrative nucleoside transporter 1 (hENT1, SLC29A1). The compounds were evaluated in radioligand binding experiments, i.e., displacement, competition association, and washout assays, to evaluate their affinity and binding kinetic parameters. We also linked these pharmacological parameters to the compounds' chemical characteristics, and learned that separate moieties of the molecules governed target affinity and binding kinetics. Among the 29 compounds tested, 28 stood out with high affinity and a long residence time of 87 min. These findings reveal the importance of supplementing affinity data with binding kinetics at transport proteins such as hENT1.

A review of electrochemical impedance as a tool for examining cell biology and subcellular mechanisms: merits, limits, and future prospects

Herein the development of cellular impedance biosensors, electrochemical impedance spectroscopy, and the general principles and terms associated with the cell-electrode interface is reviewed. This family of techniques provides quantitative and sensitive information into cell responses to stimuli in real-time with high temporal resolution. The applications of cell-based impedance biosensors as a readout in cell biology is illustrated with a diverse range of examples. The current state of the field, its limitations, the possible available solutions, and the potential benefits of developing biosensors are discussed.

Rigorous Characterization of Allosteric Modulation of the Human Metabotropic Glutamate Receptor 1 Reveals Probe- and Assay-Dependent Pharmacology

Allosteric modulation of metabotropic glutamate receptor subtype 1 (mGlu1) represents a viable therapeutic target for treating numerous central nervous system disorders. Although multiple chemically distinct mGlu1 positive (PAMs) and negative (NAMs) allosteric modulators have been identified, drug discovery paradigms have not included rigorous pharmacological analysis. In the present study, we hypothesized that existing mGlu1 allosteric modulators possess unappreciated probe-dependent or biased pharmacology. Using human embryonic kidney 293 (HEK293A) cells stably expressing human mGlu1, we screened mGlu1 PAMs and NAMs from divergent chemical scaffolds for modulation of different mGlu1 orthosteric agonists in intracellular calcium (iCa2+) mobilization and inositol monophosphate (IP1) accumulation assays. Operational models of agonism and allosterism were used to derive estimates for important pharmacological parameters such as affinity, efficacy, and cooperativity. Modulation of glutamate and quisqualate-mediated iCa2+ mobilization revealed probe dependence at the level of affinity and cooperativity for both mGlu1 PAMs and NAMs. We also identified the previously described mGlu5 selective NAM PF-06462894 as an mGlu1 NAM with a different pharmacological profile from other NAMs. Differential profiles were also observed when comparing ligand pharmacology between iCa2+ mobilization and IP1 accumulation. The PAMs Ro67-4853 and CPPHA displayed apparent negative cooperativity for modulation of quisqualate affinity, and the NAMs CPCCOEt and PF-06462894 had a marked reduction in cooperativity with quisqualate in IP1 accumulation and upon extended incubation in iCa2+ mobilization assays. These data highlight the importance of rigorous assessment of mGlu1 modulator pharmacology to inform future drug discovery programs for mGlu1 allosteric modulators. SIGNIFICANCE STATEMENT: Metabotropic glutamate receptor subtype 1 (mGlu1) positive and negative allosteric modulators have therapeutic potential in multiple central nervous system disorders. We show that chemically distinct modulators display differential pharmacology with different orthosteric ligands and across divergent signaling pathways at human mGlu1. Such complexities in allosteric ligand pharmacology should be considered in future mGlu1 allosteric drug discovery programs.

Estimation of Drug-Target Residence Time by Targeted Molecular Dynamics Simulations

Drug-target residence time has emerged as a key selection factor in drug discovery since the binding duration of a drug molecule to its protein target can significantly impact its in vivo efficacy. The challenge in studying the residence time, in early drug discovery stages, lies in how to cost-effectively determine the residence time for the systematic assessment of compounds. Currently, there is still a lack of computational protocols to quickly estimate such a measure, particularly for large and flexible protein targets and drugs. Here, we report an efficient computational protocol, based on targeted molecular dynamics, to rank drug candidates by their residence time and to obtain insights into ligand-target dissociation mechanisms. The method was assessed on a dataset of 10 arylpyrazole inhibitors of CDK8, a large, flexible, and clinically important target, for which the experimental residence time of the inhibitors ranges from minutes to hours. The compounds were correctly ranked according to their estimated residence time scores compared to their experimental values. The analysis of protein-ligand interactions along the dissociation trajectories highlighted the favorable contribution of hydrophobic contacts to residence time and revealed key residues that strongly affect compound residence time.

Insight into Thermodynamic and Kinetic Profiles in Small-Molecule Optimization

Structure-activity relationships (SARs) and structure-property relationships (SPRs) have been considered the most important factors during the drug optimization process. For medicinal chemists, improvements in the potencies and druglike properties of small molecules are regarded as their major goals. Among them, the binding affinity and selectivity of small molecules on their targets are the most important indicators. In recent years, there has been growing interest in using thermodynamic and kinetic profiles to analyze ligand-receptor interactions, which could provide not only binding affinities but also detailed binding parameters for small-molecule optimization. In this perspective, we are trying to provide an insight into thermodynamic and kinetic profiles in small-molecule optimization. Through a highlight of strategies on the small-molecule optimization with specific cases, we aim to put forward the importance of structure-thermodynamic relationships (STRs) and structure-kinetic relationships (SKRs), which could provide more guidance to find safe and effective small-molecule drugs.

Public Data Set of Protein-Ligand Dissociation Kinetic Constants for Quantitative Structure-Kinetics Relationship Studies

Protein-ligand binding affinity reflects the equilibrium thermodynamics of the protein-ligand binding process. Binding/unbinding kinetics is the other side of the coin. Computational models for interpreting the quantitative structure-kinetics relationship (QSKR) aim at predicting protein-ligand binding/unbinding kinetics based on protein structure, ligand structure, or their complex structure, which in principle can provide a more rational basis for structure-based drug design. Thus far, most of the public data sets used for deriving such QSKR models are rather limited in sample size and structural diversity. To tackle this problem, we have compiled a set of 680 protein-ligand complexes with experimental dissociation rate constants (k off), which were mainly curated from the references accumulated for updating our PDBbind database. Three-dimensional structure of each protein-ligand complex in this data set was either retrieved from the Protein Data Bank or carefully modeled based on a proper template. The entire data set covers 155 types of protein, with their dissociation kinetic constants (k off) spanning nearly 10 orders of magnitude. To the best of our knowledge, this data set is the largest of its kind reported publicly. Utilizing this data set, we derived a random forest (RF) model based on protein-ligand atom pair descriptors for predicting k off values. We also demonstrated that utilizing modeled structures as additional training samples will benefit the model performance. The RF model with mixed structures can serve as a baseline for testifying other more sophisticated QSKR models. The whole data set, namely, PDBbind-koff-2020, is available for free download at our PDBbind-CN web site (http://www.pdbbind.org.cn/download.php).

Kinetic profiling and functional characterization of 8-phenylxanthine derivatives as A2B adenosine receptor antagonists

A_2B adenosine receptor (A_2BAR) antagonists have therapeutic potential in inflammation-related diseases such as asthma, chronic obstructive pulmonary disease and cancer. However, no drug is currently clinically approved, creating a demand for research on novel antagonists. Over the last decade, the study of target binding kinetics, along with affinity and potency, has been proven valuable in early drug discovery stages, as it is associated with improved in vivo drug efficacy and safety. In this study, we report the synthesis and biological evaluation of a series of xanthine derivatives as A_2BAR antagonists, including an isothiocyanate derivative designed to bind covalently to the receptor. All 28 final compounds were assessed in radioligand binding experiments, to evaluate their affinity and for those qualifying, kinetic binding parameters. Both structure-affinity and structure-kinetic relationships were derived, providing a clear relationship between affinity and dissociation rate constants. Two structurally similar compounds, 17 and 18, were further evaluated in a label-free assay due to their divergent kinetic profiles. An extended cellular response was associated with long A_2BAR residence times. This link between a ligand's A_2BAR residence time and its functional effect highlights the importance of binding kinetics as a selection parameter in the early stages of drug discovery.

The Structural and Optical Properties of 1,2,4-Triazolo[4,3-a]pyridine-3-amine

The structural and spectroscopic properties of a new triazolopyridine derivative (1,2,4-triazolo[4,3-a]pyridin-3-amine) are described in this paper. Its FTIR spectrum was recorded in the 100–4000 cm⁻¹ range and its FT-Raman spectrum in the range 80–4000 cm⁻¹. The molecular structure and vibrational spectra were analyzed using the B3LYP/6-311G(2d,2p) approach and the GAUSSIAN 16W program. The assignment of the observed bands to the respective normal modes was proposed on the basis of PED calculations. XRD studies revealed that the studied compound crystallizes in the centrosymmetric monoclinic space group P2₁/n with eight molecules per unit cell. However, the asymmetric unit contains two 1,2,4-triazolo[4,3-a]pyridin-3-amine molecules linked via N–H⋯N hydrogen bonds with a R₂²(8) graph. The stability of the studied molecule was considered using NBO analysis. Electron absorption and the luminescence spectra were measured and discussed in terms of the calculated singlet, triplet, HOMO and LUMO electron energies. The Stokes shifts derived from the optical spectra were equal to 9410 cm⁻¹ for the triazole ring and 7625 cm⁻¹ for the pyridine ring.

Exploring the binding mechanism of positive allosteric modulators in human metabotropic glutamate receptor 2 using molecular dynamics simulations

Positive allosteric modulators (PAMs) of human metabotropic glutamate receptor 2 (hmGlu₂) are well-known in the treatment of psychiatric disorders for their higher selectivity and lower tolerance risk. A variety of PAMs have been reported over the last decade and two compounds were in Phase II clinical trials for schizophrenia and anxiety. These trials were discontinued on account of the unsatisfactory therapeutic efficacy, but PAMs were explored as novel treatments for addiction and epilepsy. Thus, it is still important to explore novel hmGlu₂ PAMs in the near future. Nowadays, the challenges in optimizing drug potency and improving scaffold diversity for PAMs are the noncomprehensive character analyses of multiple scaffolds; the exploration of the binding modes of PAMs in the allosteric binding site have been proposed to reduce this difficulty. However, there has been no comprehensive research about the binding profiles of PAMs in the hmGlu₂ receptor. To address this issue, this work explores the binding characters of eight PAMs representing five chemical series by multiple computational methods. As a result, the shared binding modes of the eight studied PAMs interacting with 15 residues in the allosteric binding site were defined. In addition, the reduced hydrophobicity with low electronegativity of R₁, increased hydrophobicity with low negative electron density of R₂ and the electronegativity of the linker were identified as indicators that regulate the affinity of PAMs. This finding agrees well with the physicochemical properties of reported multiple series PAMs. This comprehensive work sheds additional light on the binding mechanism and physicochemical regularity underlining PAMs affinity and could be further utilized as a structural and energetic blueprint for discovering and assessing novel PAMs for hmGlu₂.

Scaffold Hopping to Imidazo[1,2-a]pyrazin-8-one Positive Allosteric Modulators of Metabotropic Glutamate 2 Receptor

Glutamate hyperfunction is implicated in multiple neurological and psychiatric diseases. Activation of the mGlu2 receptor results in reduced glutamate release and decreased excitability representing a promising novel therapeutic agent for the treatment of disorders such as epilepsy, schizophrenia, mood, anxiety, and other neuropsychiatric disorders. We have previously reported substantial efforts leading to potent and selective mGlu2 PAMs from different chemical series. Herein, the discovery and optimization of a novel series of imidazopyrazinone mGlu2 PAMs are reported. This new scaffold originated from computational searching of fragment databases and comparison with our previously explored scaffolds. Optimization guided by our robust understanding of SAR from former series led to potent, selective, and brain-penetrant compounds.

Evolution of the drug-target residence time model

IntroductionThe pharmacological action of a drug is linked to its affinity for a specific molecular target as quantified by in vitro equilibrium measurements. However, it is clear that for many highly effective drugs, interactions with their molecular targets do not conform to simple, equilibrium conditions in vivo and this results in a temporal discordance between pharmacokinetics and pharmacodynamics. The drug-target residence time model was developed to provide a theoretical framework with which to understand cases in which very slow dissociation of the drug-target complex in vivo results in durable PD effects even after systemic concentrations of drug have waned.Area coveredIn this article, the author provides a brief description of the drug-target residence time model and focuses on the refinements that have been made to the original model to incorporate the influences of compound rebinding in cells and pharmacokinetic properties of drug molecules.Expert opinionThere is now overwhelming evidence for the utility of the drug-target residence time model as a framework for understanding in vivo drug action. The in vitro measured residence time (τ_R) must be used in concert with equilibrium measures of drug-target affinity (e.g. IC₅₀) and with in vivo measures of pharmacokinetic half-life, to afford the researcher a powerful approach to compound optimization for clinical effect. Despite the significant use and refinement of this model, continued studies are required to better understand the dynamic interplay between residence time, target pathobiology, drug distribution and drug pharmacokinetics.

Positive Allosteric Modulators of Metabotropic Glutamate Receptor 5 as Tool Compounds to Study Signaling Bias

Positive allosteric modulation of metabotropic glutamate subtype 5 (mGlu5) receptor has emerged as a potential new therapeutic strategy for the treatment of schizophrenia and cognitive impairments. However, positive allosteric modulator (PAM) agonist activity has been associated with adverse side effects, and neurotoxicity has also been observed for pure PAMs. The structural and pharmacological basis of therapeutic versus adverse mGlu5 PAM in vivo effects remains unknown. Thus, gaining insights into the signaling fingerprints, as well as the binding kinetics of structurally diverse mGlu5 PAMs, may help in the rational design of compounds with desired properties. We assessed the binding and signaling profiles of N-methyl-5-(phenylethynyl)pyrimidin-2-amine (MPPA), 3-cyano-N-(2,5-diphenylpyrazol-3-yl)benzamide (CDPPB), and 1-[4-(4-chloro-2-fluoro-phenyl)piperazin-1-yl]-2-(4-pyridylmethoxy)ethenone [compound 2c, a close analog of 1-(4-(2-chloro-4-fluorophenyl)piperazin-1-yl)-2-(pyridin-4-ylmethoxy)ethanone] in human embryonic kidney 293A cells stably expressing mGlu5 using Ca2+ mobilization, inositol monophosphate (IP1) accumulation, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, and receptor internalization assays. Of the three allosteric ligands, only CDPPB had intrinsic agonist efficacy, and it also had the longest receptor residence time and highest affinity. MPPA was a biased PAM, showing higher positive cooperativity with orthosteric agonists in ERK1/2 phosphorylation and Ca2+ mobilization over IP1 accumulation and receptor internalization. In primary cortical neurons, all three PAMs showed stronger positive cooperativity with (S)-3,5-dihydroxyphenylglycine (DHPG) in Ca2+ mobilization over IP1 accumulation. Our characterization of three structurally diverse mGlu5 PAMs provides further molecular pharmacological insights and presents the first assessment of PAM-mediated mGlu5 internalization. SIGNIFICANCE STATEMENT: Enhancing metabotropic glutamate receptor subtype 5 (mGlu5) activity is a promising strategy to treat cognitive and positive symptoms in schizophrenia. It is increasingly evident that positive allosteric modulators (PAMs) of mGlu5 are not all equal in preclinical models; there remains a need to better understand the molecular pharmacological properties of mGlu5 PAMs. This study reports detailed characterization of the binding and functional pharmacological properties of mGlu5 PAMs and is the first study of the effects of mGlu5 PAMs on receptor internalization.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Design, Synthesis, and Characterization of Benzimidazole Derivatives as Positron Emission Tomography Imaging Ligands for Metabotropic Glutamate Receptor 2

Three benzimidazole derivatives (13-15) have been synthetized as potential positron emission tomography (PET) imaging ligands for mGluR2 in the brain. Of these compounds, 13 exhibits potent binding affinity (IC₅₀ = 7.6 ± 0.9 nM), positive allosteric modulator (PAM) activity (EC₅₀ = 51.2 nM), and excellent selectivity against other mGluR subtypes (>100-fold). [¹¹C]13 was synthesized via O-[¹¹C]methylation of its phenol precursor 25 with [¹¹C]methyl iodide. The achieved radiochemical yield was 20 ± 2% (n = 10, decay-corrected) based on [¹¹C]CO₂ with a radiochemical purity of >98% and molar activity of 98 ± 30 GBq/μmol EOS. Ex vivo biodistribution studies revealed reversible accumulation of [¹¹C]13 and hepatobiliary and urinary excretions. PET imaging studies in rats demonstrated that [¹¹C]13 accumulated in the mGluR2-rich brain regions. Pre-administration of mGluR2-selective PAM, 17 reduced the brain uptake of [¹¹C]13, indicating a selective binding. Therefore, [¹¹C]13 is a potential PET imaging ligand for mGluR2 in different central nervous system-related conditions.

A structure-kinetic relationship study using matched molecular pair analysis

The lifetime of a binary drug–target complex is increasingly acknowledged as an important parameter for drug efficacy and safety. With a better understanding of binding kinetics and better knowledge about kinetic parameter optimization, intentionally induced prolongation of the drug–target residence time through structural changes of the ligand could become feasible. In this study we assembled datasets from 21 publications and the K4DD (Kinetic for Drug Discovery) database to conduct large scale data analysis. This resulted in 3812 small molecules annotated to 78 different targets from five protein classes (GPCRs: 273, kinases: 3238, other enzymes: 240, HSPs: 160, ion channels: 45). Performing matched molecular pair (MMP) analysis to further investigate the structure–kinetic relationship (SKR) in this data collection allowed us to identify a fundamental contribution of a ligand's polarity to its association rate, and in selected cases, also to its dissociation rate. However, we furthermore observed that the destabilization of the transition state introduced by increased polarity is often accompanied by simultaneous destabilization of the ground state resulting in an unaffected or even worsened residence time. Supported by a set of case studies, we provide concepts on how to alter ligands in ways to trigger on-rates, off-rates, or both.

A large-scale study employing matched molecular pair (MMP) analysis to uncover the contribution of a compound's polarity to its association and dissociation rates.

Detailed In Vitro Pharmacological Characterization of Clinically Tested Negative Allosteric Modulators of the Metabotropic Glutamate Receptor 5

Negative allosteric modulation of the metabotropic glutamate 5 (mGlu5) receptor has emerged as a potential strategy for the treatment of neurologic disorders. Despite the success in preclinical studies, many mGlu5 negative allosteric modulators (NAMs) that have reached clinical trials failed due to lack of efficacy. In this study, we provide a detailed in vitro pharmacological characterization of nine clinically and preclinically tested NAMs. We evaluated inhibition of l-glutamate-induced signaling with Ca2+ mobilization, inositol monophosphate (IP1) accumulation, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, and real-time receptor internalization assays on rat mGlu5 expressed in HEK293A cells. Moreover, we determined association rates (kon) and dissociation rates (koff), as well as NAM affinities with [3H]methoxy-PEPy binding experiments. kon and koff values varied greatly between the nine NAMs (34- and 139-fold, respectively) resulting in long receptor residence times (>400 min) for basimglurant and mavoglurant, medium residence times (10-30 min) for AZD2066, remeglurant, and (RS)-remeglurant, and low residence times (<10 mins) for dipraglurant, F169521, F1699611, and STX107. We found that all NAMs inhibited l-glutamate-induced mGlu5 receptor internalization, generally with a similar potency to IP1 accumulation and ERK1/2 phosphorylation, whereas Ca2+ mobilization was less potently inhibited. Operational model of allosterism analyses revealed that dipraglurant and (RS)-remeglurant were biased toward (affinity) receptor internalization and away (cooperativity) from the ERK1/2 phosphorylation pathway, respectively. Our study is the first to measure mGlu5 NAM binding kinetics and negative allosteric modulation of mGlu5 receptor internalization and adds significant new knowledge about the molecular pharmacology of a diverse range of clinically relevant NAMs. SIGNIFICANCE STATEMENT: The metabotropic glutamate 5 (mGlu5) receptor is important in many brain functions and implicated in several neurological pathologies. Negative allosteric modulators (NAMs) have shown promising results in preclinical models but have so far failed in human clinical trials. Here we provide the most comprehensive and comparative molecular pharmacological study to date of nine preclinically/clinically tested NAMs at the mGlu5 receptor, which is also the first study to measure ligand binding kinetics and negative allosteric modulation of mGlu5 receptor internalization.

Perspective: Implications of Ligand-Receptor Binding Kinetics for Therapeutic Targeting of G Protein-Coupled Receptors

The concept of ligand-receptor binding kinetics has been broadly applied in drug development pipelines focusing on G protein-coupled receptors (GPCRs). The ligand residence time (RT) for a receptor describes how long a ligand-receptor complex exists, and is defined as the reciprocal of the dissociation rate constant (k _off). RT has turned out to be a valuable parameter for GPCR researchers focusing on drug development as a good predictor of in vivo efficacy. The positive correlation between RT and in vivo efficacy has been established for several drugs targeting class A GPCRs (e.g., the neurokinin-1 receptor (NK₁R), the β₂ adrenergic receptor (β₂AR), and the muscarinic 3 receptor (M₃R)) and for drugs targeting class B1 (e.g., the glucagon-like peptide 1 receptor (GLP-1R)). Recently, the association rate constant (k _on) has gained similar attention as another parameter affecting in vivo efficacy. In the current perspective, we address the importance of studying ligand-receptor binding kinetics for therapeutic targeting of GPCRs, with an emphasis on how binding kinetics can be altered by subtle molecular changes in the ligands and/or the receptors and how such changes affect treatment outcome. Moreover, we speculate on the impact of binding kinetic parameters for functional selectivity and sustained receptor signaling from endosomal compartments; phenomena that have gained increasing interest in attempts to improve therapeutic targeting of GPCRs.

Discovery of Novel Latency-Associated Nuclear Antigen Inhibitors as Antiviral Agents Against Kaposi's Sarcoma-Associated Herpesvirus

With the aim to develop novel antiviral agents against Kaposi's Sarcoma Herpesvirus (KSHV), we are targeting the latency-associated nuclear antigen (LANA). This protein plays an important role in viral genome maintenance during latent infection. LANA has the ability to tether the viral genome to the host nucleosomes and, thus, ensures latent persistence of the viral genome in the host cells. By inhibition of the LANA-DNA interaction, we seek to eliminate or reduce the load of the viral DNA in the host. To achieve this goal, we screened our in-house library using a dedicated fluorescence polarization (FP)-based competition assay, which allows for the quantification of LANA-DNA-interaction inhibition by small organic molecules. We successfully identified three different compound classes capable of disrupting this protein-nucleic acid interaction. We characterized these compounds by IC₅₀ dose-response evaluation and confirmed the compound-LANA interaction using surface plasmon resonance (SPR) spectroscopy. Furthermore, two of the three hit scaffolds showed only marginal cytotoxicity in two human cell lines. Finally, we conducted STD-NMR competition experiments with our new hit compounds and a previously described fragment-sized inhibitor. Based on these results, future compound linking approaches could serve as a promising strategy for further optimization studies in order to generate highly potent KSHV inhibitors.

Structure-Kinetic Profiling of Haloperidol Analogues at the Human Dopamine D2 Receptor

Haloperidol is a typical antipsychotic drug (APD) associated with an increased risk of extrapyramidal side effects (EPSs) and hyperprolactinemia relative to atypical APDs such as clozapine. Both drugs are dopamine D₂ receptor (D₂R) antagonists, with contrasting kinetic profiles. Haloperidol displays fast association/slow dissociation at the D₂R, whereas clozapine exhibits relatively slow association/fast dissociation. Recently, we have provided evidence that slow dissociation from the D₂R predicts hyperprolactinemia, whereas fast association predicts EPS. Unfortunately, clozapine can cause severe side effects independent of its D₂R action. Our results suggest an optimal kinetic profile for D₂R antagonist APDs that avoids EPS. To begin exploring this hypothesis, we conducted a structure-kinetic relationship study of haloperidol and revealed that subtle structural modifications dramatically change binding kinetic rate constants, affording compounds with a clozapine-like kinetic profile. Thus, optimization of these kinetic parameters may allow development of novel APDs based on the haloperidol scaffold with improved side-effect profiles.

Bioluminescence Resonance Energy Transfer Based G Protein-Activation Assay to Probe Duration of Antagonism at the Histamine H3 Receptor

Duration of receptor antagonism, measured as the recovery of agonist responsiveness, is gaining attention as a method to evaluate the 'effective' target-residence for antagonists. These functional assays might be a good alternative for kinetic binding assays in competition with radiolabeled or fluorescent ligands, as they are performed on intact cells and better reflect consequences of dynamic cellular processes on duration of receptor antagonism. Here, we used a bioluminescence resonance energy transfer (BRET)-based assay that monitors heterotrimeric G protein activation via scavenging of released Venus-Gβ₁γ₂ by NanoLuc (Nluc)-tagged membrane-associated-C-terminal fragment of G protein-coupled receptor kinase 3 (masGRK3ct-Nluc) as a tool to probe duration of G protein-coupled receptor (GPCR) antagonism. The Gα_i-coupled histamine H₃ receptor (H₃R) was used in this study as prolonged antagonism is associated with adverse events (e.g., insomnia) and consequently, short-residence time ligands might be preferred. Due to its fast and prolonged response, this assay can be used to determine the duration of functional antagonism by measuring the recovery of agonist responsiveness upon washout of pre-bound antagonist, and to assess antagonist re-equilibration time via Schild-plot analysis. Re-equilibration of pre-incubated antagonist with agonist and receptor could be followed in time to monitor the transition from insurmountable to surmountable antagonism. The BRET-based G protein activation assay can detect differences in the recovery of H₃R responsiveness and re-equilibration of pre-bound antagonists between the tested H₃R antagonists. Fast dissociation kinetics were observed for marketed drug pitolisant (Wakix^®) in this assay, which suggests that short residence time might be beneficial for therapeutic targeting of the H₃R.

Binding kinetics of ligands acting at GPCRs

The influence of drug-receptor binding kinetics has often been overlooked during the development of new therapeutics that target G protein-coupled receptors (GPCRs). Over the last decade there has been a growing understanding that an in-depth knowledge of binding kinetics at GPCRs is required to successfully target this class of proteins. Ligand binding to a GPCR is often not a simple single step process with ligand freely diffusing in solution. This review will discuss the experiments and equations that are commonly used to measure binding kinetics and how factors such as allosteric regulation, rebinding and ligand interaction with the plasma membrane may influence these measurements. We will then consider the molecular characteristics of a ligand and if these can be linked to association and dissociation rates.

Visible-Light-Induced Nickel-Catalyzed Negishi Cross-Couplings by Exogenous-Photosensitizer-Free Photocatalysis

The merging of photoredox and transition-metal catalysis has become one of the most attractive approaches for carbon-carbon bond formation. Such reactions require the use of two organo-transition-metal species, one of which acts as a photosensitizer and the other one as a cross-coupling catalyst. We report herein an exogenous-photosensitizer-free photocatalytic process for the formation of carbon-carbon bonds by direct acceleration of the well-known nickel-catalyzed Negishi cross-coupling that is based on the use of two naturally abundant metals. This finding will open new avenues in cross-coupling chemistry that involve the direct visible-light absorption of organometallic catalytic complexes.

Covalent Allosteric Probe for the Metabotropic Glutamate Receptor 2: Design, Synthesis, and Pharmacological Characterization

Covalent labeling of G protein-coupled receptors (GPCRs) by small molecules is a powerful approach to understand binding modes, mechanism of action, pharmacology, and even facilitate structure elucidation. We report the first covalent positive allosteric modulator (PAM) for a class C GPCR, the mGlu₂ receptor. Three putatively covalent mGlu₂ PAMs were designed and synthesized. Pharmacological characterization identified 2 to bind the receptor covalently. Computational modeling combined with receptor mutagenesis revealed T791^7.29×30 as the likely position of covalent interaction. We show how this covalent ligand can be used to characterize the PAM binding mode and that it is a valuable tool compound in studying receptor function and binding kinetics. Our findings advance the understanding of the mGlu₂ PAM interaction and suggest that 2 is a valuable probe for further structural and chemical biology approaches.

A binding kinetics study of human adenosine A3 receptor agonists

The human adenosine A₃ (hA₃) receptor has been suggested as a viable drug target in inflammatory diseases and in cancer. So far, a number of selective hA₃ receptor agonists (e.g. IB-MECA and 2-Cl-IB-MECA) inducing anti-inflammatory or anticancer effects are under clinical investigation. Drug-target binding kinetics is increasingly recognized as another pharmacological parameter, next to affinity, for compound triage in the early phases of drug discovery. However, such a kinetics-driven analysis has not yet been performed for the hA₃ receptor. In this study, we first validated a competition association assay for adenosine A₃ receptor agonists to determine the target interaction kinetics. Affinities and Kinetic Rate Index (KRI) values of 11 ribofurano and 10 methanocarba nucleosides were determined in radioligand binding assays. Afterwards, 15 analogues were further selected (KRI <0.70 or KRI >1.35) for full kinetics characterization. The structure-kinetics relationships (SKR) were derived and longer residence times were associated with methanocarba and enlarged adenine N⁶ and C2 substitutions. In addition, from a k_on-k_off-K_D kinetic map we divided the agonists into three subgroups. A residence time "cliff" was observed, which might be relevant to (N)-methanocarba derivatives' rigid C2-arylalkynyl substitutions. Our findings provide substantial evidence that, next to affinity, additional knowledge of binding kinetics is useful for developing and selecting new hA₃R agonists in the early phase of the drug discovery process.