Synthesis and pharmacological characterisation of 2,4-dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists

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.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Hydrazonoyl Chlorides as Precursors for Synthesis of Novel Bis-Pyrrole Derivatives

A convenient synthesis of some novel bis-pyrrole derivatives via hydrazonoyl halides is described. Antimicrobial evaluation of some selected examples of the synthesized products was carried out. The bis-pyrrole derivative having chloro substituents showed good activity against all of the used microbes. The molecular docking of the bis-pyrrole derivatives was performed by the Molecular Operating Environment (MOE) program.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

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.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Rapid hit to lead evaluation of pyrazolo[3,4-d]pyrimidin-4-one as selective and orally bioavailable mGluR1 antagonists

Our HTS effort yielded a preferential mGluR1 pyrimidinone antagonist 1 with lead-like characteristics. Rapid hit to lead (HTL) study identified compounds with improved functional activity and selectivity such as 1b with little improvements in ADME properties. Addition of an aminosulfonyl group on the N-1 aromatic ring led to 2f, a compound with similar in vitro biochemical profiles as those of 1b but drastically improved in vitro ADME properties. These improvements were paralleled by rat PK study characterized by low clearance and quantitative bioavailability. Compound 2f represented a true lead-like molecule that is amenable for further lead optimization (LO) evaluation.

Asymmetric Synthesis of Mono- and Dinuclear Bis(dipyrrinato) Complexes

The diastereoselective syntheses of Zn(II) bis(dipyrrinato) helicates is reported, involving ligands templated by the incorporation of homochiral binol within the linker joining the two dipyrrinato units. The most diastereoselective formation of dinuclear bis(dipyrrinato) helicates to date is reported. The formation of either mononuclear or dinuclear helicates can be tuned by varying the length of the linker between the dipyrrinato units and by varying the complexation procedure. The neutral dipyrrinato helicates were readily analyzed by HPLC to ascertain diastereoselectivity, and circular dichroism studies revealed the helical nature of the complexes. The molar ellipticities of the helicates produced by diastereoselective complexation are very large in the visible region and typically correspond to binol moieties in the UV region. Extensive X-ray crystallographic investigations further confirmed the helicity of the mononuclear Zn(II) helicates and identified significant interlayer displacement and bending within crystals.

From pyrroles to 1-oxo-2,3,4,9-tetrahydro-1H-β-carbolines: A new class of orally bioavailable mGluR1 antagonists

Exploiting the SAR of the known pyrrole derivatives, a new class of mGluR1 antagonists was designed by replacement of the pyrrole core with an indole scaffold and consequent cyclization of the C-2 position into a tricyclic beta-carboline template. The appropriate exploration of the position C-6 with a combination of H-bond acceptor groups coupled with bulky/lipophilic moieties led to the discovery of a new series of mGluR1 antagonists. These compounds exhibited a non-competitive behavior, excellent pharmacokinetic properties, and good in vivo activity in animal models of acute and chronic pain, after oral administration.

13C bis-labeled pyrroles: A tool for the identification of the rat metabolism of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester

Following the recent disclosure of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester as a potent and selective mGluR1 non-competitive antagonist, the use of a doubly (13)C-labeled analogue to identify, and consequently prevent, metabolically labile positions is reported.

Cyclothiazide selectively inhibits mGluR1 receptors interacting with a common allosteric site for non-competitive antagonists

Metabotropic glutamate receptors mGluR1 and mGluR5 stimulate phospholipase C, leading to an increased inositol trisphosphate level and to Ca(2+) release from intracellular stores. Cyclothiazide (CTZ), known as a blocker of AMPA receptor desensitization, produced a non-competitive inhibition of [Ca(2+)](i) increases induced by mGluR agonists in HEK 293 cells transfected with rat mGluR1a but had no effect on the [Ca(2+)](i) signals in cells expressing rat mGluR5a. In cells expressing mGluR1, CTZ also inhibited phosphoinositide hydrolysis, as well as cAMP accumulation and arachidonic acid release induced by mGluR1 agonists, indicating a direct inhibition of the receptor and not of a particular signal transduction system. However, CTZ failed to antagonize cAMP inhibition stimulated by rat mGluR2, -3, -4, -6, -7 and -8 receptors confirming its selectivity for mGluR1. The use of chimeric receptors with substituted N-terminal domains showed that CTZ did not interact with the N-terminal mGluR1a domain. Instead, mutation analysis revealed that CTZ interacts with the Thr-815 and Ala-818 residues, located at the 7th transmembrane domain, similarly as the mGluR1-selective antagonist CPCCOEt. In primary cultures of cerebellar granule neurons, expressing native metabotropic and ionotropic glutamate receptors, the final outcome of CTZ effects depended on its combined ability to potentiate AMPA receptors and inhibit mGluR1 receptors.

Correlation between brain/plasma ratios and efficacy in neuropathic pain models of selective metabotropic glutamate receptor 1 antagonists

We have discovered a novel, potent, and selective triazafluorenone series of metabotropic glutamate receptor 1 (mGluR1) antagonists with efficacy in various rat pain models. Pharmacokinetic and pharmacodynamic profiles of these triazafluorenone analogs revealed that brain/plasma ratios of these mGluR1 antagonists were important to achieve efficacy in neuropathic pain models. This correlation could be used to guide our in vivo SAR (structure-activity relationship) modification. For example, compound 4a has a brain/plasma ratio of 0.34, demonstrating only moderate efficacy in neuropathic pain models. On the other hand, antagonist 4b with a brain/plasma ratio of 2.70 was fully efficacious in neuropathic pain models.

From pyrroles to pyrrolo[1,2-a]pyrazinones: A new class of mGluR1 antagonists

Exploiting the SAR of the known pyrrole derivatives, a new class of mGluR1 antagonists was developed through a cyclization of the C-2 position on the pyrrole N-1 nitrogen. The resulting pyrrolo[1,2-a]pyrazinones are potent and noncompetitive antagonists.

Structure-activity relationship of triazafluorenone derivatives as potent and selective mGluR1 antagonists

SAR (structure-activity relationship) studies of triazafluorenone derivatives as potent mGluR1 antagonists are described. The triazafluorenone derivatives are non-amino acid derivatives and noncompetitive mGluR1 antagonists that bind at a putative allosteric recognition site located within the seven-transmembrane domain of the receptor. These triazafluorenone derivatives are potent, selective, and systemically active mGluR1 antagonists. Compound 1n, for example, was a very potent mGluR1 antagonist (IC50 = 3 nM) and demonstrated full efficacy in various in vivo animal pain models.

Molecular pharmacology and therapeutic prospects of metabotropic glutamate receptor allosteric modulators

The metabotropic glutamate receptors (mGluR) consist of a family of eight G-protein-coupled receptors that differ in their function, distribution and physiological roles within the central nervous system. In recent years substantial efforts have been made towards developing selective agonists and antagonists which have proven useful for elucidating their potential as novel targets for the treatment of psychiatric and neurological diseases. In the present review we will provide an update of the recent developments of functional allosteric modulators of the mGluR family and explore their therapeutic potential for anxiety/depression, schizophrenia, epilepsy/stroke, pain and Alzheimer's, Parkinson's and Huntington's diseases.

In vitro and in vivo pharmacological characterization of the novel UT receptor ligand [Pen5,DTrp7,Dab8]urotensin II(4-11) (UFP-803)

The novel urotensin-II (U-II) receptor (UT) ligand, [Pen(5),DTrp(7),Dab(8)]U-II(4-11) (UFP-803), was pharmacologically evaluated and compared with urantide in in vitro and in vivo assays. In the rat isolated aorta, UFP-803 was inactive alone but, concentration dependently, displaced the contractile response to U-II to the right, revealing a competitive type of antagonism and a pA(2) value of 7.46. In the FLIPR [Ca(2+)](i) assay, performed at room temperature in HEK293(hUT) and HEK293(rUT) cells, U-II increased [Ca(2+)](i) with pEC(50) values of 8.11 and 8.48. Urantide and UFP-803 were inactive as agonists, but antagonized the actions of U-II by reducing, in a concentration-dependent manner, the agonist maximal effects with apparent pK(B) values in the range of 8.45-9.05. In a separate series of experiments performed at 37 degrees C using a cuvette-based [Ca(2+)](i) assay and CHO(hUT) cells, urantide mimicked the [Ca(2+)](i) stimulatory effect of U-II with an intrinsic activity (alpha) of 0.80, while UFP-803 displayed a small (alpha=0.21) but consistent residual agonist activity. When the same experiments were repeated at 22 degrees C (a temperature similar to that in FLIPR experiments), urantide displayed a very small intrinsic activity (alpha=0.11) and UFP-803 was completely inactive as an agonist. In vivo in mice, UFP-803 (10 nmol kg(-1)) antagonized U-II (1 nmol kg(-1))-induced increase in plasma extravasation in various vascular beds, while being inactive alone. In conclusion, UFP-803 is a potent UT receptor ligand which displays competitive/noncompetitive antagonist behavior depending on the assay. While UFP-803 is less potent than urantide, it displayed reduced residual agonist activity and as such may be a useful pharmacological tool.

GABA and glutamate systems as therapeutic targets in depression and mood disorders

Advances made in diverse areas of neuroscience suggest that neurotransmitter systems, additional to the monoaminergic, contribute to the pathophysiology of mood disorders. This ever accruing body of preclinical and clinical research is providing increased recognition of the contribution made by amino acid neurotransmitters to the neurobiology of mood disorders. This review examines evidence supporting the role of GABA and glutamate in these processes and explores the potential to target these systems in the development of novel compounds; the viability of these agents for treatment-related co-morbidities will also be considered.

JNJ16259685, a highly potent, selective and systemically active mGlu1 receptor antagonist

We examined the pharmacological profile of (3,4-dihydro-2H-pyrano[2,3]b quinolin-7-yl) (cis-4-methoxycyclohexyl) methanone (JNJ16259685). At recombinant rat and human metabotropic glutamate (mGlu) 1a receptors, JNJ16259685 non-competitively inhibited glutamate-induced Ca2+ mobilization with IC50 values of 3.24+/-1.00 and 1.21+/-0.53 nM, respectively, while showing a much lower potency at the rat and human mGlu5a receptor. JNJ16259685 inhibited [3H]1-(3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-2-phenyl-1-ethanone ([3H]R214127) binding to membranes prepared from cells expressing rat mGlu1a receptors with a Ki of 0.34+/-0.20 nM. JNJ16259685 showed no agonist, antagonist or positive allosteric activity toward rat mGlu2, -3, -4 or -6 receptors at concentrations up to 10 microM and did not bind to AMPA or NMDA receptors, or to a battery of other neurotransmitter receptors, ion channels and transporters. In primary cerebellar cultures, JNJ16259685 inhibited glutamate-mediated inositol phosphate production with an IC50 of 1.73+/-0.40 nM. Subcutaneously administered JNJ16259685 exhibited high potencies in occupying central mGlu1 receptors in the rat cerebellum and thalamus ( ED50=0.040 and 0.014 mg/kg, respectively). These data show that JNJ16259685 is a selective mGlu1 receptor antagonist with excellent potencies in inhibiting mGlu1 receptor function and binding and in occupying the mGlu1 receptor after systemic administration.

Allosteric modulators of metabotropic glutamate receptors: lessons learnt from mGlu1, mGlu2 and mGlu5 potentiators and antagonists

Although relatively few G-protein-coupled receptors are Class C, in recent years, this small family of receptors has become a focal point for the discovery of new and exciting allosteric modulators. The mGlu (metabotropic glutamate) receptors are illustrative in the discovery of both positive and/or negative allosteric modulators with unique pharmacological properties. For instance, allosteric modulators of the mGlu2 receptor act as potentiators of glutamate responses in clonal expression systems and in native tissue assays. These potentiators act to increase the affinity of orthosteric agonists for the mGlu2 receptor and shift potency curves for the agonist to the left. In electrophysiological experiments, the potentiators show a unique activation-state-dependent presynaptic inhibition of glutamate release and significantly enhance the receptor-mediated increase in G-protein binding, as seen with autoradiography. Similarly, potentiators of mGlu5 have been described, as well as allosteric antagonists or inverse agonists of mGlu1 and mGlu5. Binding and activity of the modulators have recently indicated that positive and negative allosteric sites can be, but are not necessarily, overlapping. Compared with orthosteric ligands, these modulators display a unique degree of subtype selectivity within the highly conserved mGlu family of receptors and can have very distinct pharmacological properties, such as neuronal frequency-dependent activity. This short review describes some of the unique features of these mGlu1, mGlu2 and mGlu5 allosteric modulators.

3-Methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester: an exploration of the C-2 position. Part I

Following the recent disclosure of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester, a potent and selective mGluR1 non-competitive antagonist, we report here a detailed exploration of the C-2 position of this scaffold with the preparation of differently substituted amides. Great improvement of the pharmacokinetic properties has been achieved through this exercise.

3-Methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester: an exploration of the C-2 position. Part II, A solid-phase approach

Following the recent disclosure (Part I of this paper) of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) amides and of their improved pharmacokinetic profile with respect to the originally reported esters, a further exploration of the C-2 position through a solid-phase approach is reported here.

Stereoselective synthesis and preliminary evaluation of (+)- and (–)-3-methyl-5-carboxy-thien-2-yl-glycine (3-MATIDA): identification of (+)-3-MATIDA as a novel mGluR1 competitive antagonist

The synthesis of the (+)- and (-)-isomers of 3-methyl-5-carboxy-thyen-2-yl-glycine (3-MATIDA), heterocyle isosters of carboxyphenylglycines (CPGs), a known class of competitive metabotropic glutamate receptors, was accomplished by a stereoselective Ugi condensation. The two isomers were tested as potential rat mGluR1 ligand and the (+) isomer was found to be a moderately potent antagonist, while the (-) one was inactive.

Succinate increases neuronal post-synaptic excitatory potentials in vitro and induces convulsive behavior through N-methyl-d-aspartate-mediated mechanisms

Succinate is a dicarboxylic acid that accumulates due to succinate dehydrogenase inhibition by malonate and methylmalonate exposure. These neurotoxins cause increased excitability and excitotoxic damage, which can be prevented by administering high amounts of succinate. In the present study we investigated whether succinate alters hippocampal field excitatory post-synaptic potentials. Bath application of succinate at intermediate concentrations (0.3-1 mM) increased the slope of field excitatory post-synaptic potentials in hippocampal slices, and at high concentrations (above 1 mM) did not alter or decrease field excitatory post-synaptic potentials slope. Succinate-induced enhancement of field excitatory post-synaptic potentials slope was abolished by the addition of d-2-amino-5-phosphonovaleric acid (50 microM) to the perfusate, supporting the involvement of N-methyl-d-aspartate receptors in the excitatory effect of this organic acid. Accordingly, succinate (0.8-7.5 micromol) i.c.v. administration caused dose-dependent convulsive behavior in mice. The i.c.v. co-administration of MK-801 (7 nmol) fully prevented succinate-induced convulsions, further suggesting the involvement of N-methyl-d-aspartate receptors in the convulsant action of succinate. Our data indicate that accumulation of moderate amounts of succinate may contribute to the excitotoxicity induced by succinate dehydrogenase inhibitors, through the activation of N-methyl-d-aspartate receptors.

2,4-Dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists: an exploration of the role of the pyrrolic scaffold

Following the disclosure of 3-(1,2,2-trimethylpropyl) 4-[3,5-dimethyl-2-propyloxycarbonyl]pyrrolecarboxylate as a potent and selective mGluR1 non-competitive antagonist, the role and the importance of the pyrrole template were investigated. Different aromatic moieties were investigated as possible bio-isosteric replacement of the original scaffold and some of them were shown to be partially able to mimic the properties of the original pyrrole ring.

The distinct role of mGlu1 receptors in post-ischemic neuronal death

Metabotropic glutamate receptors of the mGlu(1) and mGlu(5) subtypes exhibit a high degree of sequence homology and are both coupled to phospholipase C and intracellular Ca(2+) mobilization. However, functional differences have been detected for these receptor subtypes when they are coexpressed in the same neuronal populations. Experimental evidence indicates that mGlu(1) and mGlu(5) receptors play a differential role in models of cerebral ischemia and that only mGlu(1) receptors are implicated in the pathways leading to post-ischemic neuronal injury. The localization of mGlu(1) receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted studies that have provided a new viewpoint on the neuroprotective mechanism of mGlu(1) receptor antagonists. The hypothesis predicts that these pharmacological agents attenuate post-ischemic injury by enhancing GABA-mediated neurotransmission.

2,4-Dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists: further characterization of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester and structure-activity relationships

Following the disclosure of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester [3,5-dimethyl PPP] as a potent and selective mGluR1 non-competitive antagonist, we report here further in vivo characterization of this important tool and disclose the investigation of the C-5 position, which led to very potent compounds.