Quinoline-2,4-dicarboxylic acids: synthesis and evaluation as inhibitors of the glutamate vesicular transport system

Six-Membered Aromatic Nitrogen Heterocyclic Anti-Tumor Agents: Synthesis and Applications

Cancer stands as a serious malady, posing substantial risks to human well-being and survival. This underscores the paramount necessity to explore and investigate novel antitumor medications. Nitrogen-containing compounds, especially those derived from natural sources, form a highly significant category of antitumor agents. Among these, antitumor agents with six-membered aromatic nitrogen heterocycles have consistently attracted the attention of chemists and pharmacologists. Accordingly, we present a comprehensive summary of synthetic strategies and clinical implications of these compounds in this review. This entails an in-depth analysis of synthesis pathways for pyridine, quinoline, pyrimidine, and quinazoline. Additionally, we explore the historical progression, targets, mechanisms of action, and clinical effectiveness of small molecule inhibitors possessing these structural features.

Leveraging VGLUT3 Functions to Untangle Brain Dysfunctions

Vesicular glutamate transporters (VGLUTs) were long thought to be specific markers of glutamatergic excitatory transmission. The discovery, two decades ago, of the atypical VGLUT3 has thoroughly modified this oversimplified view. VGLUT3 is strategically expressed in discrete populations of glutamatergic, cholinergic, serotonergic, and even GABAergic neurons. Recent reports show the subtle, but critical, implications of VGLUT3-dependent glutamate co-transmission and its roles in the regulation of diverse brain functions and dysfunctions. Progress in the neuropharmacology of VGLUT3 could lead to decisive breakthroughs in the treatment of Parkinson's disease (PD), addiction, eating disorders, anxiety, presbycusis, or pain. This review summarizes recent findings on VGLUT3 and its vesicular underpinnings as well as on possible ways to target this atypical transporter for future therapeutic strategies.

Inhibition of the Vesicular Glutamate Transporter (VGLUT) with Congo Red Analogs: New Binding Insights

The vesicular glutamate transporter (VGLUT) facilitates the uptake of glutamate (Glu) into neuronal vesicles. VGLUT has not yet been fully characterized pharmacologically but a body of work established that certain azo-dyes bearing two Glu isosteres via a linker were potent inhibitors. However, the distance between the isostere groups that convey potent inhibition has not been delineated. This report describes the synthesis and pharmacologic assessment of Congo Red analogs that contain one or two glutamate isostere or mimic groups; the latter varied in the interatomic distance and spacer properties to probe strategic binding interactions within VGLUT. The more potent inhibitors had two glutamate isosteres symmetrically linked to a central aromatic group and showed IC₅₀ values ~ 0.3-2.0 μM at VGLUT. These compounds contained phenyl, diphenyl ether (PhOPh) or 1,2-diphenylethane as the linker connecting 4-aminonaphthalene sulfonic acid groups. A homology model for VGLUT2 using D-galactonate transporter (DgoT) to dock and identify R88, H199 and F219 as key protein interactions with Trypan Blue, Congo Red and selected potent analogs prepared and tested in this report.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

VGLUT substrates and inhibitors: A computational viewpoint

The vesicular glutamate transporters (VGLUTs) bind and move glutamate (Glu) from the cytosol into the lumen of synaptic vesicles using a H⁺-electrochemical gradient (ΔpH and Δψ) generated by the vesicular H⁺-ATPase. VGLUTs show very low Glu binding and to date, no three-dimensional structure has been elucidated. Prior studies have attempted to identify the key residues involved in binding VGLUT substrates and inhibitors using homology models and docking experiments. Recently, the inward and outward oriented crystal structures of d-galactonate transporter (DgoT) emerged as possible structure templates for VGLUT. In this review, a new homology model for VGLUT2 based on DgoT has been developed and used to conduct docking experiments to identify and differentiate residues and binding orientations involved in ligand interactions. This review describes small molecule-ligand interactions including docking using a VGLUT2 homology model derived from DgoT.

A dual removable activating group enabled the Povarov reaction of N-arylalanine esters: synthesis of quinoline-4-carboxylate esters

A dual removable activating group enabled Povarov reaction of N-arylalanine esters was reported.

The development of benzo- and naphtho-fused quinoline-2,4-dicarboxylic acids as vesicular glutamate transporter (VGLUT) inhibitors reveals a possible role for neuroactive steroids

Substituted quinoline-2,4-dicarboxylates (QDCs) are conformationally-restricted mimics of glutamate that were previously reported to selectively block the glutamate vesicular transporters (VGLUTs). We find that expanding the QDC scaffold to benzoquinoline dicarboxylic acids (BQDC) and naphthoquinoline dicarboxylic acids (NQDCs) improves inhibitory activity with the NQDCs showing IC50∼70 μM. Modeling overlay studies showed that the polycyclic QDCs resembled steroid structures and led to the identification and testing of estrone sulfate, pregnenolone sulfate and pregnanolone sulfate that blocked the uptake of l-Glu by 50%, 70% and 85% of control, respectively. Pregnanolone sulfate was further characterized by kinetic pharmacological determinations that demonstrated competitive inhibition and a Ki of ≈20 μM.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

Use of the hydantoin isostere to produce inhibitors showing selectivity toward the vesicular glutamate transporter versus the obligate exchange transporter system x(c)(-)

Evidence was acquired prior to suggest that the vesicular glutamate transporter (VGLUT) but not other glutamate transporters were inhibited by structures containing a weakly basic α-amino group. To test this hypothesis, a series of analogs using a hydantoin (pK(a)∼9.1) isostere were synthesized and analyzed as inhibitors of VGLUT and the obligate cystine-glutamate transporter (system x(c)(-)). Of the hydantoin analogs tested, a thiophene-5-carboxaldehyde analog 2l and a bis-hydantoin 4b were relatively strong inhibitors of VGLUT reducing uptake to less than 6% of control at 5mM but few inhibited system x(c)(-) greater than 50% of control. The benzene-2,4-disulfonic acid analog 2b and p-diaminobenzene analog 2e were also good hydantoin-based inhibitors of VGLUT reducing uptake by 11% and 23% of control, respectively, but neither analog was effective as a system x(c)(-) inhibitor. In sum, a hydantoin isostere adds the requisite chemical properties needed to produce selective inhibitors of VGLUT.

Rose Bengal analogs and vesicular glutamate transporters (VGLUTs)

Vesicular glutamate transporters (VGLUTs) allow the loading of presynaptic glutamate vesicles and thus play a critical role in glutamatergic synaptic transmission. Rose Bengal (RB) is the most potent known VGLUT inhibitor (Ki 25 nM); therefore we designed, synthesized and tested in brain preparations, a series of analogs based on this scaffold. We showed that among the two tautomers of RB, the carboxylic and not the lactonic form is active against VGLUTs and generated a pharmacophore model to determine the minimal structure requirements. We also tested RB specificity in other neurotransmitter uptake systems. RB proved to potently inhibit VMAT (Ki 64 nM) but weakly VACHT (Ki>9.7 microM) and may be a useful tool in glutamate/acetylcholine co-transmission studies.

Conformationally-restricted amino acid analogues bearing a distal sulfonic acid show selective inhibition of system x(c)(-) over the vesicular glutamate transporter

A panel of amino acid analogs and conformationally-restricted amino acids bearing a sulfonic acid were synthesized and tested for their ability to preferentially inhibit the obligate cysteine-glutamate transporter system x(c)(-) versus the vesicular glutamate transporter (VGLUT). Several promising candidate molecules were identified: R/S-4-[4'-carboxyphenyl]-phenylglycine, a biphenyl substituted analog of 4-carboxyphenylglycine and 2-thiopheneglycine-5-sulfonic acid both of which reduced glutamate uptake at system x(c)(-) by 70-75% while having modest to no effect on glutamate uptake at VGLUT.

Pharmacology of neurotransmitter transport into secretory vesicles

Many neuropsychiatric disorders appear to involve a disturbance of chemical neurotransmission, and the mechanism of available therapeutic agents supports this impression. Postsynaptic receptors have received considerable attention as drug targets, but some of the most successful agents influence presynaptic processes, in particular neurotransmitter reuptake. The pharmacological potential of many other presynaptic elements, and in particular the machinery responsible for loading transmitter into vesicles, has received only limited attention. The similarity of vesicular transporters to bacterial drug resistance proteins and the increasing evidence for regulation of vesicle filling and recycling suggest that the pharmacological potential of vesicular transporters has been underestimated. In this review, we discuss the pharmacological effects of psychostimulants and therapeutic agents on transmitter release.

Tetrapeptide inhibitors of the glutamate vesicular transporter (VGLUT)

Quinoline-2,4-dicarboxylic acids (QDCs) bearing lipophilic substituents in the 6- or 7-position were shown to be inhibitors of the glutamate vesicular transporter (VGLUT). Using the arrangement of the QDC lipophilic substituents as a template, libraries of X(1)X(2)EF and X(1)X(2)EW tetrapeptides were synthesized and tested as VGLUT inhibitors. The peptides QIEW and WNEF were found to be the most potent. Further stereochemical deconvolution of these two peptides showed dQlIdElW to be the best inhibitor (K(i)=828+/-252 microM). Modeling and overlay of the tetrapeptide inhibitors with the existing pharmacophore showed that H-bonding and lipophilic residues are important for VGLUT binding.

Inhibition of Vesicular Glutamate Uptake by Rose Bengal-Related Compounds: Structure–Activity Relationship

Synaptic vesicular accumulation of glutamate is a vital initial step in glutamate transmission. We have previously shown that Rose Bengal, a polyhalogenated fluorescein analog, is a potent inhibitor of glutamate uptake into synaptic vesicles. Here, we report the structural features of Rose Bengal required for this inhibition. Various Rose Bengal-related compounds, with systematic structural variations, were tested. Results indicate that the four iodo groups and the phenyl group attached to the xanthene moiety are critical for potent inhibitory activity. Replacement of these groups with two iodo groups and an alkyl group, respectively, results in substantial reduction in potency. Of further interest in creating high potency is the critical nature of the oxygen atom which links the two benzene rings of xanthene. Thus, the phenyl group and multiple iodo groups, as well as the bridging oxygen of xanthene, are crucial elements of Rose Bengal required for its potent inhibitory action.

Molecular pharmacology of glutamate transporters, EAATs and VGLUTs

L-Glutamate serves as a major excitatory neurotransmitter in the mammalian central nervous system (CNS) and is stored in synaptic vesicles by an uptake system that is dependent on the proton electrochemical gradient (VGLUTs). Following its exocytotic release, glutamate activates fast-acting, excitatory ionotropic receptors and slower-acting metabotropic receptors to mediate neurotransmission. Na+-dependent glutamate transporters (EAATs) located on the plasma membrane of neurons and glial cells rapidly terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels. Thus far, five Na+-dependent glutamate transporters (EAATs 1-5) and three vesicular glutamate transporters (VGLUTs 1-3) have been identified. Examination of EAATs and VGLUTs in brain preparations and by heterologous expression of the various cloned subtypes shows these two transporter families differ in many of their functional properties including substrate specificity and ion requirements. Alterations in the function and/or expression of these carriers have been implicated in a range of psychiatric and neurological disorders. EAATs have been implicated in cerebral stroke, epilepsy, Alzheimer's disease, HIV-associated dementia, Huntington's disease, amyotrophic lateral sclerosis (ALS) and malignant glioma, while VGLUTs have been implicated in schizophrenia. To examine the physiological role of glutamate transporters in more detail, several classes of transportable and non-transportable inhibitors have been developed, many of which are derivatives of the natural amino acids, aspartate and glutamate. This review summarizes the development of these indispensable pharmacological tools, which have been critical to our understanding of normal and abnormal synaptic transmission.

[Pharmacology of excitatory amino acid transporters (EAATs and VGLUTs)]

L-Glutamate is a major excitatory neurotransmitter in the mammalian central nervous system (CNS). It contributes not only to fast synaptic neurotransmission but also to complex physiological processes like plasticity, learning, and memory. Glutamate is synthesized in the cytoplasm and stored in synaptic vesicles by a proton gradient-dependent uptake system (VGLUTs). Following its exocytotic release, glutamate activates different kinds of glutamate receptors and mediates excitatory neurotransmission. To terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels, glutamate is quickly removed by Na(+)-dependent glutamate transporters (EAATs). Recently, three vesicular glutamate transporters (VGLUT1-3) and five Na(+)-dependent glutamate transporters (EAAT1-5) were identified. VGLUTs and EAATs are thought to play important roles in neuronal disorders, such as amyotrophic lateral sclerosis, Alzheimer's disease, cerebral ischemia, and Huntington's disease. In this review, the development of new compounds to regulate the function of VGLUTs and EAATs will be described.

Synthesis and preliminary evaluation of trans-3,4-conformationally-restricted glutamate and pyroglutamate analogues as novel EAAT2 inhibitors

Select trans-4,5-[bi]cyclohexenylglutamic and pyroglutamic acids (3,4-substituted glutamates) were synthesized in three steps and were screened as potential inhibitors of the sodium dependent excitatory amino acid transporters 2 (EAAT2) and 3 (EAAT3), the chloride dependent glial cystine/glutamate exchanger system x(c)(-), and the glutamate vesicular transport system (VGLUT). Two glutamate analogues and one pyroglutamate analogue were found to inhibit EAAT2 with activity comparable to dihydrokainate.

Synthesis and in vitro pharmacology of substituted quinoline-2,4-dicarboxylic acids as inhibitors of vesicular glutamate transport

The vesicular glutamate transport (VGLUT) system selectively mediates the uptake of L-glutamate into synaptic vesicles. Uptake is linked to an H+-ATPase that provides coupling among ATP hydrolysis, an electrochemical proton gradient, and glutamate transport. Substituted quinoline-2,4-dicarboxylic acids (QDCs), prepared by condensation of dimethyl ketoglutaconate (DKG) with substituted anilines and subsequent hydrolysis, were investigated as potential VGLUT inhibitors in synaptic vesicles. A brief panel of substituted QDCs was previously reported (Carrigan et al. Bioorg. Med. Chem. Lett. 1999, 9, 2607-2612)(1) and showed that certain substituents led to more potent competitive inhibitors of VGLUT. Using these compounds as leads, an expanded series of QDC analogues were prepared either by condensation of DKG with novel anilines or via aryl-coupling (Suzuki or Heck) to dimethyl 6-bromoquinolinedicarboxylate. From the panel of almost 50 substituted QDCs tested as inhibitors of the VGLUT system, the 6-PhCH=CH-QDC (K(i) = 167 microM), 6-PhCH2CH2-QDC (K(i) = 143 microM), 6-(4'-phenylstyryl)-QDC (K(i) = 64 microM), and 6-biphenyl-4-yl-QDC (K(i) = 41 microM) were found to be the most potent blockers. A preliminary assessment of the key elements needed for binding to the VGLUT protein based on the structure-activity relationships for the panel of substituted QDCs is discussed herein. The substituted QDCs represent the first synthetically derived VGLUT inhibitors and are promising templates for the development of selective transporter inhibitors.

Analysis of conformationally restricted alpha-ketoglutarate analogues as substrates of dehydrogenases and aminotransferases

Five synthetic, conformationally restricted alpha-ketoglutarate analogues were tested as substrates of a variety of dehydrogenases and aminotransferases. The compounds were found not to be detectable substrates of glutamate dehydrogenase, L-leucine dehydrogenase, L-phenylalanine dehydrogenase, lactate dehydrogenase, malate dehydrogenase, glutamine transaminase K, aspartate aminotransferase, alanine aminotransferase, and alpha-ketoglutarate dehydrogenase complex. However, two thermostable aminotransferases were identified that catalyze transamination between several L-amino acids (e.g., phenylalanine, glutamate) and the alpha-ketoglutarate analogues of interest. Transamination between L-glutamate (or L-phenylalanine) and the alpha-ketoglutarate analogues was found to be 0.13 to 1.08 micromol/h/mg at 45 degrees C. The products resulting from transamination between L-phenylalanine and the alpha-ketoglutarate analogues were separated by reverse-phase HPLC, and the newly formed amino acid analogues were analyzed by LC-MS in an ion selective mode. In each case, the ions obtained were consistent with the expected product and a representative example is provided. The possibility existed that although the alpha-ketoglutarate analogues are not substrates of the dehydrogenases and most of the aminotransferases investigated, they might be good inhibitors. Weak inhibition of aminotransferases and glutamate dehydrogenase was found with some of the alpha-ketoglutarate analogues. The newly available thermostable aminotransferases may have general utility in the synthesis of bulky L-amino acids from the corresponding alpha-keto acids.