(+)-4-phosphonophenylglycine (PPG) a new group III selective metabotropic glutamate receptor agonist

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.

Proton dissociation properties of arylphosphonates: Determination of accurate Hammett equation parameters

Determination of the proton dissociation constants of several arylphosphonic acid derivatives was carried out to investigate the accuracy of the Hammett equations available for this family of compounds. For the measurement of the pK_a values modern, accurate methods, such as the differential potentiometric titration and NMR-pH titration were used. We found our results significantly different from the pK_a values reported before (pK_a1: MAE = 0.16 pK_a2: MAE=0.59). Based on our recently measured pK_a values, refined Hammett equations were determined that might be used for predicting highly accurate ionization constants of newly synthesized compounds (pK_a1=1.70-0.894σ, pK_a2=6.92-0.934σ).

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

L-2-Amino-4-phosphonobutyric acid (l-AP4), l-2-amino-4-thiophosphonobutyric acid (l-thioAP4), and l-2-amino-4-(hydroxy)phosphinylbutyric acid (desmethylphosphinothricin, DMPT) were synthesized from protected vinylglycine. They were tested as agonists at group III metabotropic glutamate receptors (mGluR) along with phosphinothricin (PT). DMPT and PT display a much lower potency at mGlu4 receptor (EC50 = 4.0 and 1100 microM, respectively) in comparison to l-AP4 (EC50 = 0.08 microM), whereas l-thioAP4 has a 2-fold higher potency (EC50 = 0.039 microM). Similar rank orders of potency were observed at mGlu6,7 and mGlu8 receptors. The higher potency of l-thioAP4 is due to its stronger second acidity compared to l-AP4. These pKa values of 5.56 and 6.88, respectively, were determined using 31P NMR chemical shift variations. The second distal negative charge of l-AP4/l-thioAP4 probably provides stronger binding to specific basic residues of the binding sites of group III mGluRs, which stabilizes the active conformation of the receptor.

Synthesis and biological evaluation of (2S)- and (2R)-2-(3'-phosphonobicyclo[1.1.1]pentyl)glycines as novel group III selective metabotropic glutamate receptor ligands

The synthesis of (2S)- and (2R)-2-(3'-phosphonobicyclo[1.1.1]pentyl)glycine isomers (10 and 11), characterized by the bioisosteric replacement of the distal carboxylic group of 2-(3'-carboxybicyclo[1.1.1]pent-1-yl)glycine by the phosphonate moiety, was accomplished by a stereoselective Ugi condensation. The two isomers were tested for their activity against an array of metabotropic glutamate receptors, and the S-isomer (10) turned out to be a moderately potent and selective mGluR4 agonist.

Synthesis and preliminary biological evaluation of (2S,1′R,2′S)- and (2S,1′S,2′R)-2-(2′-phosphonocyclopropyl)glycines, two novel conformationally constrained l-AP4 analogues

Two novel constrained l-AP4 analogues, (2S,1'R,2'S)- and (2S,1'S,2'R)-2-(2'-phosphonocyclopropyl)glycines (7) and (8), were synthesized and evaluated as mGluR ligands. Compound 7 showed to be a group III mGluRs agonist with micromolar activity.

2,3-Ethylene- and 2,3-trimethylene-bridged analogues of the group III metabotropic glutamate receptor ligand 2-amino-4-phosphonobutanoic acid

The racemic trans- and cis-isomers of 1-amino-2-phosphonomethyl-cyclobutanecarboxylic acid (5 and 6) and 1-amino-2-phosphonomethyl-cyclopentanecarboxylic acid (7 and 8) were synthesized as extensions of the mGluR4 agonists trans- and cis-1-amino-2-phosphonomethyl-cyclopropanecarboxylic acid (3 and 4). Although the methylene bridge in 3 and 4 allows for retention of affinity toward the mGluR4 receptor, increasing the bridging unit to the ethylene group as in 5 and 6 or to the trimethylene group as in 7 and 8 introduces sufficient steric hindrance to eliminate affinity for the mGluR4 receptor.

Neuroprotective activity of metabotropic glutamate receptor ligands

Metabotropic glutamate receptors form a family of currently eight subtypes (mGluR1-8), subdivided into three groups (I-III). Activation of group-II (mGluR2 and -3) or group-III metabotropic glutamate receptors (mGluR4, -6, -7 and -8) has been established to be neuroprotective in vitro and in vivo. In contrast, group-I mGluRs (mGluR1 and -5) need to be antagonized in order to evoke protection. Initially, all neuroprotective mGluR ligands were analogues of L-glutamate. Those compounds were valuable to demonstrate protection in vitro, but showed limited applicability in animal models, particularly in chronic tests, due to low blood-brain-barrier penetration. Recently, systemically active and more potent and selective ligands became available, e.g., the group-II mGluR agonists LY354740 and LY379268 or group-I antagonists like MPEP (mGluR5-selective) and BAY36-7620 (mGluR1-selective). This new generation of pharmacological agents allows a more stringent assessment of the role of individual mGluR-subtypes or groups of receptors in various nervous system disorders, including ischaemia-induced brain damage, traumatic brain injury, Huntington's and Parkinson's-like pathology or epilepsy. Moreover, the use of genetically modified animals (e.g., knock-out mice) is starting to shed light on specific functions of mGluR-subtypes in experimental neuropathologies.

Common and selective molecular determinants involved in metabotopic glutamate receptor agonist activity

Several potent and group selective agonists of metabotropic glutamate receptors (mGluRs) have been docked at mGlu1,2,4R binding sites in the closed conformation of the bilobate extracellular domain. Quisqualic acid and (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) were selected for mGlu1R, dicarboxycyclopropylglycine (DCG-IV), LY354740, (S)-4-carboxyphenylglycine (4CPG) for mGlu2R, and (S)-2-amino-4-phosphonobutyric acid (AP4), 1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I), (S)-4-phosphonophenylglycine (PPG) for mGlu4R. The models show a conserved binding pattern for the glycine moiety (alpha-amino and alpha-acidic functions) and group specific bindings for the distal acidic function. The best agonists allow optimized interaction with both lobes of the binding domain. Interlobe connections around the ligand are also described and participate in stabilizing the closed form of the amino-terminal domain. Altogether, the docking models support the proposal that the stabilization of a closed state represents a key step in agonist activation of mGluRs.

Metabotropic glutamate receptors: targets for therapy of cerebral ischaemia

Several pathophysiological processes are involved in the progression of the cerebral ischaemic injury, among which are inflammation, peri-infarct depolarisation, apoptosis and excitotoxicity. Overactivation of glutamate receptors, both ionotropic and metabotropic, constitutes the central step in the excitotoxic cascade of events leading to neuronal cell death following acute brain ischaemia. Owing to their peculiar characteristics of modulatory receptors and due to their wide molecular and biological diversity, metabotropic glutamate receptors constitute an attractive target for the development of potential neuroprotective agents. Recent achievements in developing novel chemical entities and in the characterisation of the physiological and pathological role of individual metabotropic glutamate receptors in postischaemic degeneration will be reviewed.

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.