l-(+)-2-Amino-4-thiophosphonobutyric Acid (l-thioAP4), a New Potent Agonist of Group III Metabotropic Glutamate Receptors: Increased Distal Acidity Affords Enhanced Potency

Targeting mGluR group III for the treatment of neurodegenerative diseases

Glutamate, an excitatory neurotransmitter, is essential for neuronal function, and it acts on ionotropic or metabotropic glutamate receptors (mGluRs). A disturbance in glutamatergic signaling is a hallmark of many neurodegenerative diseases. Developing disease-modifying treatments for neurodegenerative diseases targeting glutamate receptors is a promising avenue. The understudied group III mGluR 4, 6-8 are commonly found in the presynaptic membrane, and their activation inhibits glutamate release. Thus, targeted mGluRs therapies could aid in treating neurodegenerative diseases. This review describes group III mGluRs and their pharmacological ligands in the context of amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. Attempts to evaluate the efficacy of these drugs in clinical trials are also discussed. Despite a growing list of group III mGluR-specific pharmacological ligands, research on the use of these drugs in neurodegenerative diseases is limited, except for Parkinson's disease. Future efforts should focus on delineating the contribution of group III mGluR to neurodegeneration and developing novel ligands with superior efficacy and a favorable side effect profile for the treatment of neurodegenerative diseases.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

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.

Evaluation and Development of Methodologies for the Synthesis of Thiophosphinic Acids

Thiophosphorus acids R¹R²P(S)OH constitute an important class of organophosphorus compounds, in which the phosphorus atom is intrinsically chiral if R¹ ≠ R². In connection with a project aimed at the preparation of chiral thiophosphorus acids, various available literature methods were considered, but few fit the requirement of odorless reagents. Herein, the results of our studies on the synthesis of thiophosphinic acids are reported. Ultimately, two major approaches were selected: (1) the Stec reaction of phosphorus amides with carbon disulfide; and (2) the one-pot synthesis of thiophosphorus acids from H-phosphinates, an organometallic nucleophile, and quenching with elemental sulfur. An application to the preparation of a potential chiral phosphorus organocatalyst is also reported.

Enzymatic kinetic resolution of desmethylphosphinothricin indicates that phosphinic group is a bioisostere of carboxyl group

Escherichia coli glutamate decarboxylase (EcGadB), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, is highly specific for L-glutamate and was demonstrated to be effectively immobilised for the production of γ-aminobutyric acid (GABA), its decarboxylation product. Herein we show that EcGadB quantitatively decarboxylates the L-isomer of D,L-2-amino-4-(hydroxyphosphinyl)butyric acid (D,L-Glu-γ-P_H), a phosphinic analogue of glutamate containing C-P-H bonds. This yields 3-aminopropylphosphinic acid (GABA-P_H), a known GABA_B receptor agonist and provides previously unknown D-Glu-γ-P_H, allowing us to demonstrate that L-Glu-γ-P_H, but not D-Glu-γ-P_H, is responsible for D,L-Glu-γ-P_H antibacterial activity. Furthermore, using GABase, a preparation of GABA-transaminase and succinic semialdehyde dehydrogenase, we show that GABA-P_H is converted to 3-(hydroxyphosphinyl)propionic acid (Succinate-P_H). Hence, PLP-dependent and NADP⁺-dependent enzymes are herein shown to recognise and metabolise phosphinic compounds, leaving unaffected the P-H bond. We therefore suggest that the phosphinic group is a bioisostere of the carboxyl group and the metabolic transformations of phosphinic compounds may offer a ground for prodrug design.

Computationally Guided Identification of Allosteric Agonists of the Metabotropic Glutamate 7 Receptor

The metabotropic glutamate 7 (mGlu₇) receptor belongs to the group III of mGlu receptors. Since the mGlu₇ receptor can control excitatory neurotransmission in the hippocampus and cortex, modulation of the receptor may have therapeutic benefit in several CNS diseases. However, mGlu₇ remains relatively unexplored among the eight known mGlu receptors partly because of the limited availability of tool compounds to interrogate its potential therapeutic utility. Here we report the discovery of a new class of mGlu₇ allosteric agonists. Hits originating from virtual screening were followed up with further analogue searching and screening, leading to a novel series of mGlu₇ allosteric agonists. Guided by docking into a structural model of the mGlu₇ receptor the initial hit 5 was successfully optimized to analogues with comparable potencies and more attractive drug-like attributes than AMN082.

Restoration of patterned vision with an engineered photoactivatable G protein-coupled receptor

Retinitis pigmentosa results in blindness due to degeneration of photoreceptors, but spares other retinal cells, leading to the hope that expression of light-activated signaling proteins in the surviving cells could restore vision. We used a retinal G protein-coupled receptor, mGluR2, which we chemically engineered to respond to light. In retinal ganglion cells (RGCs) of blind rd1 mice, photoswitch-charged mGluR2 (“SNAG-mGluR2”) evoked robust OFF responses to light, but not in wild-type retinas, revealing selectivity for RGCs that have lost photoreceptor input. SNAG-mGluR2 enabled animals to discriminate parallel from perpendicular lines and parallel lines at varying spacing. Simultaneous viral delivery of the inhibitory SNAG-mGluR2 and excitatory light-activated ionotropic glutamate receptor LiGluR yielded a distribution of expression ratios, restoration of ON, OFF and ON-OFF light responses and improved visual acuity. Thus, SNAG-mGluR2 restores patterned vision and combinatorial light response diversity provides a new logic for enhanced-acuity retinal prosthetics.

To restore sight after retinal degeneration, one approach is to express light-sensitive proteins in remaining cells. Here the authors combine a light-sensitive engineered G protein-coupled receptor and ion channels to restore ON and OFF responses as well as superior visual pattern discrimination.

Synthesis and evaluation of the inhibitory activity of the four stereoisomers of the potent and selective human γ-glutamyl transpeptidase inhibitor GGsTop

2-Amino-4-{[3-(carboxymethyl)phenoxy](methoxy)phosphoryl}butanoic acid (GGsTop) is a potent, highly selective, nontoxic, and irreversible inhibitor of γ-glutamyl transpeptidase (GGT). GGsTop has been widely used in academic and medicinal research, and also as an active ingredient (Nahlsgen) in commercial anti-aging cosmetics. GGsTop consists of four stereoisomers due to the presence of two stereogenic centers, i.e., the α-carbon atom of the glutamate mimic (l/d) and the phosphorus atom (R_P/S_P). In this study, each stereoisomer of GGsTop was synthesized stereoselectively and their inhibitory activity against human GGT was evaluated. The l- and d-configurations of each stereoisomer were determined by a combination of a chiral pool synthesis and chiral HPLC analysis. The synthesis of the four stereoisomers of GGsTop used chiral synthetic precursors that were separated by chiral HPLC on a preparative scale. With respect to the configuration of the α-carbon atom of the glutamate mimic, the l-isomer (k_on=174M^-1s^-1) was ca. 8-fold more potent than the d-isomer (k_on=21.5M^-1s^-1). In contrast, the configuration of the phosphorus atom is critical for GGT inhibitory activity. Based on a molecular modeling approach, the absolute configuration of the phosphorus atom of the active GGsTop isomers was postulated to be S_P. The S_P-isomers inhibited human GGT (k_on=21.5-174M^-1s^-1), while the R_P-isomers were inactive even at concentrations of 0.1mM.

Synthesis and studies on the mGluR agonist activity of FAP4 stereoisomers

The four stereoisomers of 1-amino-2-fluoro-2-(phosphonomethyl)cyclopropane-1-carboxylic acid (FAP4) were synthesized via diastereoselective Rh(II)-catalysed cyclopropanation of a phosphonylated fluoroalkene. Different isomers of FAP4 and the corresponding non-fluorinated analogs showed a similar pharmacological profile against the isoforms of metabotropic glutamate receptor (mGluR). Within the fluorinated series, (-)-(Z)-FAP4 and (-)-(E)-FAP4 demonstrated the highest agonist activity against mGlu4 (EC50 0.10 μM). Our results suggest that fluorocyclopropanes bearing an amino-acid function can be suitable for the development of potent conformationally restricted mGluR agonists.

Palladium-catalyzed alkylation of unactivated C(sp3)–H bonds with primary alkyl iodides at room temperature: facile synthesis of β-alkyl α-amino acids

Practical synthesis of β-alkyl α-amino acidsviaC(sp³)–H alkylation at room temperature.

Group III metabotropic glutamate receptors and transient receptor potential vanilloid 1 co-localize and interact on nociceptors

Several lines of evidence indicate group III metabotropic glutamate receptors (mGluRs) have systemic anti-hyperalgesic effects. We hypothesized this could occur through modulation of transient receptor potential vanilloid 1 (TRPV1) receptors on nociceptors. To address this question we performed anatomical studies to determine if group III mGluRs were expressed on cutaneous axons and if they co-localized with TRPV1. Immunostaining at the electron microscopic level demonstrated that 22% of unmyelinated axons labeled for mGluR8. Immunostaining at the light microscopic level in lumbar dorsal root ganglia (DRG) demonstrated that 80% and 28% of neurons labeled for mGluR8 or TRPV1, respectively. Of those neurons labeled for mGluR8, 25% labeled for TRPV1; of those labeled for TRPV1, 71% labeled for mGluR8. In behavior studies intraplantar injection of the group III mGluR agonist, L-(+)-2-amino-4-phosphonobutyric acid (L-AP-4: 0.1, 1.0, and 10.0 μM) had no effect on paw withdrawal latency (PWL) to heat in naïve rats but administration of 10 μM L-AP-4 prior to 0.05% capsaicin (CAP), significantly attenuated CAP-induced lifting/licking and reduced flinching behavior. The L-AP-4 effect was specific since administration of a group III antagonist α-methyl-3-methyl-4-phosphonophenylglycine (UBP1112) (100μM) blocked the L-AP-4 effect on CAP, resulting in behaviors similar to CAP alone. Intraplantar injection of UBP1112 alone did not result in nociceptive behaviors, indicating group III mGluRs are not tonically active. Finally, the anti-hyperalgesic effect of group III in this paradigm was local and not systemic since intraplantar administration of L-AP-4 in one hind paw did not attenuate nociceptive behaviors following CAP injection in the contralateral hind paw. Adenyl cyclase/cyclic AMP/PKA may be the second messenger pathway linking these two receptor families because intraplantar injection of forskolin (FSK, 10 μM) reduced PWL to heat and L-AP-4 reversed this FSK effect. Taken together, these results suggest group III mGluRs can negatively modulate TRPV1 through inhibition of adenyl cyclase and downstream intracellular activity, blocking TRPV1-induced activation of nociceptors.

Orthosteric versus allosteric GPCR activation: the great challenge of group-III mGluRs

Group-III metabotropic glutamate receptors (mGluRs) comprise four structurally related brain and retinal G protein-coupled receptors (GPCRs), mGluR4, mGluR6, mGluR7 and mGluR8, which receive much attention as promising targets for nervous system drugs. In particular, activation of mGluR4 is a major focus for the development of new therapeutics in Parkinson's disease, while mGluR7 activation is considered a potential approach for future treatments of specific psychiatric conditions. The first generation group-III mGluR agonists, e.g.l-AP4 and l-SOP, are characterized by an essential phosphonate functional group, which became a major limitation for the development of systemically active, potent and receptor subtype-selective drugs. Recently however, two approaches emerged in parallel providing resolution to this constraint: in silico high-throughput screening of chemical libraries against a 3D-model of the mGluR4 extracellular domain identified a hit that was optimized into a series of potent and subtype-selective orthosteric agonists with drug-like properties and novel chemotype structures; secondly, high-throughput random screening of chemical libraries against recombinantly expressed group-III receptors identified diverse chemical sets of allosteric agonists and positive modulators, which are drug-like, display selectivity for mGluR4, mGluR7, or mGluR8 and act via novel pharmacological sites. Here, we illustrate new scientific insights obtained via the use of those strategies. Also, we compare advantages and disadvantages of both approaches to identify the desired group-III mGluR activators and we conclude with suggestions how to employ those discovery strategies with success for the identification, optimization, and development of clinical drug candidates; this may have important implications for the entire field of GPCR research.

Metabotropic glutamate receptor 4 (mGlu4)-positive allosteric modulators for the treatment of Parkinson's disease: historical perspective and review of the patent literature

INTRODUCTION

Metabotropic glutamate receptor 4 (mGlu(4)) is a group III GPCR and has been demonstrated to play a major role in a number of therapeutic areas within the CNS. As the orthosteric site of all glutamate receptors is highly conserved, modulating mGlu(4) via allosteric modulation has emerged as a very attractive mode-of-action and has been validated preclinically in a number of animal models for Parkinson's disease, anxiety, pain, and neuroinflammation.

AREAS COVERED

In this review, the patent literature for mGlu(4)-positive allosteric modulators over the past 4 years will be provided. Patents from all companies are discussed and an overview of the chemical matter and relevant biological properties will be given.

EXPERT OPINION

Although there has yet to be an mGlu(4)-positive allosteric modulator progressed into clinical trials, there is a wealth of preclinical data from the primary literature that shows the promise of this emerging target. A number of academic and industry laboratories have recently published exciting patent data covering a multitude of chemical matter. Positive allosteric modulation of mGlu(4) remains one of the more attractive non-dopaminergic therapies for Parkinson's disease, as well as emerging data for other indications such as pain, neuroinflammation, schizophrenia and diabetes, which could potentially make mGlu(4) a significant therapeutic target going forward.

Recent advances in the drug discovery of metabotropic glutamate receptor 4 (mGluR4) activators for the treatment of CNS and non-CNS disorders

INTRODUCTION

The metabotropic glutamate receptor type 4 (mGluR4) plays a pivotal role in a plethora of therapeutic areas, as recently demonstrated in preclinical validation studies with several chemical classes of compounds in rodent models of central nervous system (CNS) and peripheral disorders. Activation of mGluR4 with orthosteric agonists, allosteric agonists or pure positive allosteric modulators (PAM) has been postulated to be of broad therapeutic use.

AREAS COVERED

The authors address past and current drug discovery efforts, insights and achievements in the field toward the identification of therapeutically promising and emerging class of mGluR4 activators, over the 2005 - 2011 period. Chemical structures, properties and in vivo pharmacological results discussed in the present review were retrieved from public literature including PubMed searches, Thomson Pharma and SciFinder databases searches, conferences, proceedings and posters.

EXPERT OPINION

Developing a subtype-selective, orally bioavailable brain penetrant mGluR4 orthosteric agonist remains challenging. Lack of subtype selectivity and low brain penetration has been a common limitation of the first generation of mGluR4 agonist and potentiators. However, significant progress has recently been made with the identification of several double- to single-digit nanomolar mGluR4 PAM having reasonable pharmacokinetic properties, oral bioavailability and brain penetration. The use of such compounds in research has led to advancement in understanding the central role of mGluR4 in multiple neurodegenerative and neuroinflammatory disorders, such as Parkinson's disease and multiple sclerosis. Our understanding of the potential application of mGluR4 as therapeutic target is expected to grow as these compounds advance into preclinical and clinical development.

Tailoring of integrin ligands: probing the charge capability of the metal ion-dependent adhesion site

Intervention in integrin-mediated cell adhesion and integrin signaling pathways is an ongoing area of research in medicinal chemistry and drug development. One key element in integrin-ligand interaction is the coordination of the bivalent cation at the metal ion-dependent adhesion site (MIDAS) by a carboxylic acid function, a consistent feature of all integrin ligands. With the exception of the recently discovered hydroxamic acids, all bioisosteric attempts to replace the carboxylic acid of integrin ligands failed. We report that phosphinates as well as monomethyl phosphonates represent excellent isosters, when introduced into integrin antagonists for the platelet integrin αIIbβ3. The novel inhibitors exhibit in vitro and ex vivo activities in the low nanomolar range. Steric and charge requirements of the MIDAS region were unraveled, thus paving the way for an in silico prediction of ligand activity and in turn the rational design of the next generation of integrin antagonists.

mGluR4-positive allosteric modulation as potential treatment for Parkinson's disease

Although Parkinson's disease was first diagnosed nearly 200 years ago, its effective treatment still remains elusive for most of those diagnosed. The gold standard of treatment for most patients is 3,4-dihydroxy-L-phenylalanine. This drug works for most individuals early in the disease; however, resistant symptoms start to emerge after several years of treatment. There has been increased interest in finding novel therapies to help Parkinson's disease patients. Such strategies may have the benefit of not only treating the symptomatic issues of the disorder, but might also offer promise in protecting dopaminergic neurons from further degeneration. One such target that is now receiving much attention from the scientific community is the metabotropic glutamate receptor mGluR4. In this article, we briefly review Parkinson's disease and then recent work in the mGluR area, with a focus on the efforts being made toward finding and optimizing novel mGluR4 positive allosteric modulators (PAMs). Preclinically in rodent models, mGluR4 activation has offered much promise as a novel treatment of Parkinson's disease. Additionally, the specific use of PAMs, rather than direct-acting agonists at the orthosteric glutamate site, continues to be validated as a viable treatment option for this target. It is anticipated that continued progress in this area will further our understanding of the potential of mGluR4 modulation as a novel symptomatic and potentially disease-modifying treatment for Parkinson's disease.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

Synthesis and in vitro evaluation of aspartate transcarbamoylase inhibitors

The design, synthesis, and evaluation of a series of novel inhibitors of aspartate transcarbamoylase (ATCase) are reported. Several submicromolar phosphorus-containing inhibitors are described, but all-carboxylate compounds are inactive. Compounds were synthesized to probe the postulated cyclic transition-state of the enzyme-catalyzed reaction. In addition, the associated role of the protonation state at the phosphorus acid moiety was evaluated using phosphinic and carboxylic acids. Although none of the synthesized inhibitors is more potent than N-phosphonacetyl-l-aspartate (PALA), the compounds provide useful mechanistic information, as well as the basis for the design of future inhibitors and/or prodrugs.

In vitro characterization of a heterologously expressed nonribosomal Peptide synthetase involved in phosphinothricin tripeptide biosynthesis

The late stages of biosynthesis of phosphinothricin tripeptide (PTT) involve peptide formation and methylation on phosphorus. The exact timing of these transformations is not known. To provide insight into this question, we developed a heterologous expression system for PhsA, one of three NRPS proteins in PTT biosynthesis. The apparent k(cat)/K(m) value for ATP-pyrophosphate exchange activity for d,l-N-acetylphosphinothricin was 3.5 muM(-1) min(-1), whereas the k(cat)/K(m,app) for l-N-acetyldemethylphosphinothricin was 0.5 microM(-1) min(-1), suggesting the former might be the physiological substrate. Each substrate could be loaded onto the phosphopantetheine arm of the thiolation domain as observed by Fourier transform mass spectrometry (FTMS).

Electrophysiological and behavioral evidence that modulation of metabotropic glutamate receptor 4 with a new agonist reverses experimental parkinsonism

Developing nondopaminergic palliative treatments for Parkinson's disease represents a major challenge to avoid the debilitating side effects produced by L-DOPA therapy. Increasing interest is addressed to the selective targeting of group III metabotropic glutamate (mGlu) receptors that inhibit transmitter release at presumably overactive synapses in the basal ganglia. Here we characterize the functional action of a new orthosteric group III mGlu agonist, LSP1-2111, with a preferential affinity for mGlu4 receptor. In mouse brain slices, LSP1-2111 inhibits striatopallidal GABAergic transmission by selectively activating the mGlu4 receptor but has no effect at a synapse modulated solely by the mGlu7 and mGlu8 receptors. Intrapallidal LSP1-2111 infusion reverses the akinesia produced by nigrostriatal dopamine depletion in a reaction time task, whereas an mGlu8-receptor agonist has no effect. Finally, systemic administration of LSP1-2111 counteracts haloperidol-induced catalepsy, opening promising perspectives for the development of antiparkinsonian therapeutic strategies focused on orthosteric mGlu4-receptor agonists.