Positive Allosteric Modulators of the Metabotropic Glutamate Receptor Subtype 2 (mGluR2)

Fragment-Based Optimization of Dihydropyrazino-Benzimidazolones as Metabotropic Glutamate Receptor-2 Positive Allosteric Modulators against Migraine

Our previous scaffold-hopping attempts resulted in dihydropyrazino-benzimidazoles as metabotropic glutamate receptor-2 (mGluR2) positive allosteric modulators (PAMs) with suboptimal drug-like profiles. Here, we report an alternative fragment-based optimization strategy applied on the new dihydropyrazino-benzimidazolone scaffold. Analyzing published high-affinity mGluR2 PAMs, we used a pharmacophore-guided approach to identify suitable growing vectors and optimize the scaffold in these directions. This strategy resulted in a new fragment like lead (34) with improved druglike properties that were translated to sufficient pharmacokinetics and validated proof-of-concept studies in migraine. Gratifyingly, compound 34 showed reasonable activity in the partial infraorbital nerve ligation, a migraine disease model that might open this indication for mGluR2 PAMs.

3-(Cyclopropylmethyl)-7-((4-(4-[11C]methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine: Synthesis and preliminary evaluation for PET imaging of metabotropic glutamate receptor subtype 2

Selective metabotropic glutamate receptor 2 (mGluR2) inhibitors have been demonstrated to show therapeutic effects by improving alleviating symptoms of schizophrenic patients in clinical studies. Herein we report the synthesis and preliminary evaluation of a ¹¹C-labeled positron emission tomography (PET) tracer originating from a mGluR2 inhibitor, 3-(cyclopropylmethyl)-7-((4-(4-methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine (CMTP, 1a). [¹¹C]CMTP ([¹¹C]1a) was synthesized by O-[¹¹C]methylation of desmethyl precursor 1b with [¹¹C]methyl iodide in 19.7 ± 8.9% (n = 10) radiochemical yield (based on [¹¹C]CO₂) with >98% radiochemical purity and >74 GBq/μmol molar activity. Autoradiography study showed that [¹¹C]1a possessed moderate in vitro specific binding to mGluR2 in the rat brain, with a heterogeneous distribution of radioactive accumulation in the mGluR2-rich brain tissue sections, such as the cerebral cortex and striatum. PET study indicated that [¹¹C]1a was able to cross the blood-brain barrier and enter the brain, but had very low specific binding in the rat brain. Further optimization for the chemical structure of 1a is necessary to increase binding affinity to mGluR2 and then improve in vivo specific binding in brain.

mGluR2 positive allosteric modulators: an updated patent review (2013-2018)

INTRODUCTION

Positive allosteric modulation of mGlu2 has attracted much interest as an alternative approach to classical orthosteric receptor activation. Two mGlu2 PAMS have advanced into the clinic. The results obtained in schizophrenia and MDD phase 2 clinical trials have tempered the high expectations put on selective mGlu2 receptor activation for treating these conditions; nevertheless, the search for novel therapeutic indications and novel chemotypes continues to be an active field of research.

AREAS COVERED

2013-2018 patent literature on mGlu2 receptor PAMs.

EXPERT OPINION

After a decade of intensive research, the mGlu2 PAM field has seen a deceleration in the last five years. Negative phase 2 schizophrenia clinical trials with JNJ-40411813 and AZD8529 seem to have tempered the high expectations of the scientific community on the utility of mGlu2 PAMs for the treatment of schizophrenia. Nevertheless, novel therapeutic indications continue to be explored and AZD8529 is currently in a phase 2 study for smoking cessation. The advances in medicinal chemistry and in pharmacology, with novel indications such as epilepsy, have set the stage in the field of mGlu2 receptor PAMs. Ongoing preclinical and clinical studies will contribute to define their optimal therapeutic indication and potential to become novel therapeutic agents.

Fragment Based Optimization of Metabotropic Glutamate Receptor 2 (mGluR2) Positive Allosteric Modulators in the Absence of Structural Information

Metabotropic glutamate receptor 2 (mGluR2) positive allosteric modulators (PAMs) have been implicated as potential pharmacotherapy for psychiatric conditions. Screening our corporate compound deck, we identified a benzotriazole fragment (4) that was rapidly optimized to a potent and metabolically stable early lead (16). The highly lipophilic character of 16, together with its limited solubility, permeability, and high protein binding, however, did not allow reaching of the proof of concept in vivo. Since further attempts on the optimization of druglike properties were unsuccessful, the original hit 4 has been revisited and was optimized following the principles of fragment based drug discovery (FBDD). Lacking structural information on the receptor-ligand complex, we implemented a group efficiency (GE) based strategy and identified a new fragment like lead (60) with more balanced profile. Significant improvement achieved on the druglike properties nominated the compound for in vivo proof of concept studies that revealed the chemotype being a promising PAM lead targeting mGluR2 receptors.

Structure-function studies with G protein-coupled receptors as a paradigm for improving drug discovery and development of therapeutics

There are a great variety of human membrane proteins, and these currently form the largest group of targets for marketed drugs. Despite the advances in drug design, however, promiscuity between drug molecules and targets often leads to undesired signaling effects, which result in unintended side effects. In this review, one family of membrane proteins - the G protein-coupled receptors (GPCRs) - is used as a model to review experimental techniques that may be used to examine the activity of membrane proteins. As these receptors are highly relevant to healthy human physiology and represent the largest family of drug targets, they represent an excellent model for membrane proteins in general. We also review experimental evidence that suggests there may be multiple ways to target a GPCR - and by extension, membrane proteins - to more effectively target unhealthy phenotypes while reducing the occurrence and severity of side effects.

mGluR2 positive allosteric modulators: a patent review (2009 - present)

INTRODUCTION

The mGlu2 receptor, which belongs to the group II subfamily of metabotropic glutamate receptors (mGlu) along with the mGlu3 receptor, has proven to be of particular importance in neuropharmacology. Preferentially expressed on presynaptic nerve terminals, the mGlu2 receptor negatively modulates glutamate and GABA release and is widely distributed in the brain. High levels of mGlu2 receptors are seen in brain areas such as prefrontal cortex, hippocampus and amygdala where glutamate hyperfunction may be implicated in disorders and diseases such as anxiety and schizophrenia. Given the promise offered by mGlu2/3 receptor activation, there is increased interest in identifying small molecules which activate the receptor. A preferred approach is via positive allosteric modulators (PAMs) which bind at an alternative site to agonists.

AREAS COVERED

This review covers the patent applications which were published between April 2009 and December 2012 on PAMs of the mGlu2, and it is a continuation of an earlier review published in this journal.

EXPERT OPINION

Advances in medicinal chemistry and pharmacology have set the stage in the field of mGlu2 receptor PAMs. Compounds currently advancing in clinical trials will soon establish the therapeutic potential of this allosteric approach.

Radiosynthesis of PET radiotracer as a prodrug for imaging group II metabotropic glutamate receptors in vivo

Group II metabotropic glutamate receptors (mGluRs) have been implicated in a variety of neurological and psychiatric disorders in recent studies. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers the possibility to visualize and study group II mGluRs in vivo under physiologic and pathologic conditions. We synthesized a PET tracer, (S,S,S)-2-(2-carboxycyclopropyl)-2-(3-[(11)C]methoxyphenethyl) glycine dimethyl ester ([(11)C]CMGDE), as a prodrug for group II mGluRs, and studied its preliminary biological properties in Sprague-Dawley rats to visualize group II mGluRs. The microPET studies demonstrated that [(11)C]CMGDE readily penetrated into the brain and the radiotracer generated from [(11)C]CMGDE had fast reversible binding in the group II mGluRs rich regions including striatum, hippocampus and different cortical areas. Blocking studies with LY341495 showed 20-30% decrease of binding of the radiotracer generated from [(11)C]CMGDE in all brain areas with the highest decrease in the striatum 31.5±3.2%. The results show [(11)C]CMGDE is the first PET tracer that is brain penetrating and can be used to image group II mGluRs in vivo.

Binding sites in membrane proteins--diversity, druggability and prospects

The identification of novel drug targets is one of the major challenges in proteomics. Computational methods developed over the last decade have enhanced the process of drug design in both terms of time and quality. The main task is the design of selective compounds, which bind targets more specifically, dependent on the desired mode of action of the particular drug. This makes it necessary to create compounds, which either exhibit their functions on one single protein to exclude undesired cross-reactivity or to use the advantageous effect of less selective drugs that target numerous proteins and therefore exhibit their functions on whole protein classes. Main aspects in the assignment of interactions between ligands and putative targets involve the amino acid composition of the binding site, evolutionary conservation and similarity in sequence and structure of known targets. Similarities or differences within classified protein families can be the key to their function and give first hints to functional drug design. Hereby, binding site-based classification outnumbers sequence-based classifications since similar binding sites can also be found in more distant proteins. Membrane proteins are 'difficult targets', because of their special physicochemical characteristics and the general lack of structural information. Here, we describe recent advances in modeling methods dedicated to membrane proteins. Different descriptors of similarity between compounds and the similarity between binding sites are under development and elucidate important aspects like dynamics or entropy. The importance of computational drug design is undisputable. Nevertheless, the process of design is complicated by increasing complexity, which underlines the importance of accurate knowledge about the addressed target class(es) and particularly their binding sites. One main objective by considering named topics is to predict putative side effects and errant functions (off-target effects) of novel drugs, which requires a holistic (systems biology) view on drug-target-pathway relations. In the following, we give a brief summary about the recent discussion on drug-target interactions with emphasis on membrane proteins.

Recent progress in the synthesis and characterization of group II metabotropic glutamate receptor allosteric modulators

Group II metabotropic glutamate (mGlu) receptors consist of the metabotropic glutamate 2 (mGlu(2)) and metabotropic glutamate 3 (mGlu(3)) receptor subtypes which modulate glutamate transmission by second messenger activation to negatively regulate the activity of adenylyl cyclase. Excessive accumulation of glutamate in the perisynaptic extracellular region triggers mGlu(2) and mGlu(3) receptors to inhibit further release of glutamate. There is growing evidence that the modulation of glutamatergic neurotransmission by small molecule modulators of Group II mGlu receptors has significant potential for the treatment of several neuropsychiatric and neurodegenerative diseases. This review provides an overview of recent progress on the synthesis and pharmacological characterization of positive and negative allosteric modulators of the Group II mGlu receptors.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Allosteric modulation of metabotropic glutamate receptors

The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson's disease, and Fragile X syndrome.

The identification of structurally novel, selective, orally bioavailable positive modulators of mGluR2

The optimisation of an HTS hit series (1) leading to the identification of structurally novel, selective, orally bioavailable mGluR2 positive modulators GSK1331258 and GSK1331268 is described. Structure-activity relationships, attenuation of dopaminergic activity, and potentiation of mGluR2 responses in rat hippocampal MPP-DG synapses are also reported.

3-Benzyl-1,3-oxazolidin-2-ones as mGluR2 positive allosteric modulators: Hit-to lead and lead optimization

The discovery, synthesis and SAR of a novel series of 3-benzyl-1,3-oxazolidin-2-ones as positive allosteric modulators (PAMs) of mGluR2 is described. Expedient hit-to-lead work on a single HTS hit led to the identification of a ligand-efficient and structurally attractive series of mGluR2 PAMs. Human microsomal clearance and suboptimal physicochemical properties of the initial lead were improved to give potent, metabolically stable and orally available mGluR2 PAMs.

Drugability of extracellular targets: discovery of small molecule drugs targeting allosteric, functional, and subunit-selective sites on GPCRs and ion channels

Beginning with the discovery of the structure of deoxyribose nucleic acid in 1953, by James Watson and Francis Crick, the sequencing of the entire human genome some 50 years later, has begun to quantify the classes and types of proteins that may have relevance to human disease with the promise of rapidly identifying compounds that can modulate these proteins so as to have a beneficial and therapeutic outcome. This so called 'drugable space' involves a variety of membrane-bound proteins including the superfamily of G-protein-coupled receptors (GPCRs), ion channels, and transporters among others. The recent number of novel therapeutics targeting membrane-bound extracellular proteins that have reached the market in the past 20 years however pales in magnitude when compared, during the same timeframe, to the advancements made in the technologies available to aid in the discovery of these novel therapeutics. This review will consider select examples of extracellular drugable targets and focus on the GPCRs and ion channels highlighting the corticotropin releasing factor (CRF) type 1 and gamma-aminobutyric acid receptors, and the Ca(V)2.2 voltage-gated ion channel. These examples will elaborate current technological advancements in drug discovery and provide a prospective framework for future drug development.

Allosteric modulation of kinases and GPCRs: design principles and structural diversity

Allosteric binding sites, as opposed to traditional orthosteric binding sites, offer unparalleled opportunities for drug discovery by providing high levels of selectivity, mimicking physiological conditions, affording fewer side effects because of desensitization/downregulation, and engendering ligands with chemotypes divergent from orthosteric ligands. For kinases, allosteric mechanisms described to date include alteration of protein kinase conformation blocking productive ATP binding which appear 'ATP competitive' or blocking kinase activation by conformational changes that are 'ATP non-competitive'. For GPCRs, allosteric mechanisms impart multiple modes of target modulation (positive allosteric modulation (PAM), negative allosteric modulation (NAM), neutral cooperativity, partial antagonism (PA), allosteric agonism and allosteric antagonism). Here, we review recent developments in the design principles and structural diversity of allosteric ligands for kinases and GPCRs.

Unifying electrostatic mechanism for receptor-ligand activity

A prior article in skeletal form proposed an electrostatic mechanism for receptor-ligand activity. The present review provides an elaboration, including supporting evidence. The fundamental aspect entails the presence of molecular electrostatic potential associated with ions and dipoles in the ligand. The ligand can be regarded as an electrical link that joins prevalent electrostatic fields present in the surrounding protein matrix. The exact role of these fields is speculative. One possibility is to function as conduits for electrons and radicals in cell signaling. There is increasing support for important participation of these species in signal transduction. There might also be a favorable influence on energetics involving the electron transfer process. A summary of receptor biology is also provided, including receptors for acetylcholine (nicotinic and muscarinic), GABA, adrenergic, and glutamate.

Distribution of metabotropic glutamate 2 and 3 receptors in the rat forebrain: Implication in emotional responses and central disinhibition

The receptor localization of metabotropic glutamate receptors (mGlu) 2 and 3 was examined by using in situ hybridization and a well-characterized mGlu2-selective antibody in the rat forebrain. mGlu2 was highly and discretely expressed in cell bodies in almost all of the key regions of the limbic system in the forebrain, including the midline and intralaminar structures of the thalamus, the association cortices, the dentate gyrus of the hippocampus, the medial mammillary nucleus, and the lateral and basolateral nuclei of the amygdala. Moreover, presynaptic mGlu2 terminals were found in most of the forebrain structures, especially in the lateral part of the central nucleus of the amygdala, and the CA1 region of the hippocampus. Although some overlaps exist, such as in the hippocampus and the amygdala, the expression of mGlu3 mRNA, however, appeared to be more disperse, compared with that of mGlu2 mRNA. These distribution results support previous behavioral studies that the mGlu2 and 3 receptors may play important roles in emotional responses. In addition to its expression in glia, mGlu3 was distinctively expressed in cells in the GABAergic reticular nucleus of the thalamus. Local infusion of a non-selective mGlu2/3 agonist, LY379268, in the reticular nucleus of the thalamus, significantly reduced GABA release, suggesting that mGlu3 may also play a role in central disinhibition.

Keynote review: Allosterism in membrane receptors

Allosteric modulation of membrane receptors has been intensively studied in the past three decades and is now considered to be an important indirect mechanism for the control of receptor function. The allosteric site on the GABA(A) receptor is the target for the most widely prescribed sleep medicines, the benzodiazepines. Cinacalcet, an allosteric enhancer of the calcium-sensing receptor, is used to treat secondary hyperparathyroidism. Allosteric ligands might be especially valuable to control receptors for which the design of selective orthosteric agonists or antagonists has been elusive, such as muscarinic acetylcholine receptors.