Discovery of a potent and selective small molecule hGPR91 antagonist

Methionine secreted by tumor-associated pericytes supports cancer stem cells in clear cell renal carcinoma

Here, we identify a subset of vascular pericytes, defined by expression of platelet-derived growth factor receptor beta (PDGFR-β) and G-protein-coupled receptor 91 (GPR91), that promote tumorigenesis and tyrosine kinase inhibitors (TKIs) resistance by functioning as the primary methionine source for cancer stem cells (CSCs) in clear cell renal cell carcinoma (ccRCC). Tumor-cell-derived succinate binds to GPR91 on pericyte to activate autophagy for methionine production. CSCs use methionine to create stabilizing N6-methyladenosine in ATPase-family-AAA-domain-containing 2 (ATAD2) mRNA, and the resulting ATAD2 protein complexes with SRY-box transcription factor 9 to assemble super enhancers and thereby dictate its target genes that feature prominently in CSCs. Targeting PDGFR-β+GPR91+ pericytes with specific GRP91 antagonists reduce intratumoral methionine level, eliminate CSCs, and enhance TKIs sensitivity. These results unraveled the mechanisms by which PDGFR-β+GPR91+ pericytes provide supportive niche for CSCs and could be used to develop targets for treating ccRCC.

GPCRs involved in metabolic diseases: pharmacotherapeutic development updates

G protein-coupled receptors (GPCRs) are expressed in a variety of cell types and tissues, and activation of GPCRs is involved in enormous metabolic pathways, including nutrient synthesis, transportation, storage or insulin sensitivity, etc. This review intends to summarize the regulation of metabolic homeostasis and mechanisms by a series of GPCRs, such as GPR91, GPR55, GPR119, GPR109a, GPR142, GPR40, GPR41, GPR43 and GPR120. With deep understanding of GPCR's structure and signaling pathways, it is attempting to uncover the role of GPCRs in major metabolic diseases, including metabolic syndrome, diabetes, dyslipidemia and nonalcoholic steatohepatitis, for which the global prevalence has risen during last two decades. An extensive list of agonists and antagonists with their chemical structures in a nature of small molecular compounds for above-mentioned GPCRs is provided as pharmacologic candidates, and their preliminary data of preclinical studies are discussed. Moreover, their beneficial effects in correcting abnormalities of metabolic syndrome, diabetes and dyslipidemia are summarized when clinical trials have been undertaken. Thus, accumulating data suggest that these agonists or antagonists might become as new pharmacotherapeutic candidates for the treatment of metabolic diseases.

Succinate-Directed Approaches for Warburg Effect-Targeted Cancer Management, an Alternative to Current Treatments?

Approximately a century ago, Otto Warburg discovered that cancer cells use a fermentative rather than oxidative metabolism even though the former is more inefficient in terms of energy production per molecule of glucose. Cancer cells increase the use of this fermentative metabolism even in the presence of oxygen, and this process is called aerobic glycolysis or the Warburg effect. This alternative metabolism is mainly characterized by higher glycolytic rates, which allow cancer cells to obtain higher amounts of total ATP, and the production of lactate, but there are also an activation of protumoral signaling pathways and the generation of molecules that favor cancer progression. One of these molecules is succinate, a Krebs cycle intermediate whose concentration is increased in cancer and which is considered an oncometabolite. Several protumoral actions have been associated to succinate and its role in several cancer types has been already described. Despite playing a major role in metabolism and cancer, so far, the potential of succinate as a target in cancer prevention and treatment has remained mostly unexplored, as most previous Warburg-directed anticancer strategies have focused on other intermediates. In this review, we aim to summarize succinate's protumoral functions and discuss the use of succinate expression regulators as a potential cancer therapy strategy.

Succinate metabolism and its regulation of host-microbe interactions

Succinate is a circulating metabolite, and the relationship between abnormal changes in the physiological concentration of succinate and inflammatory diseases caused by the overreaction of certain immune cells has become a research focus. Recent investigations have shown that succinate produced by the gut microbiota has the potential to regulate host homeostasis and treat diseases such as inflammation. Gut microbes are important for maintaining intestinal homeostasis. Microbial metabolites serve as nutrients in energy metabolism, and act as signal molecules that stimulate host cell and organ function and affect the structural balance between symbiotic gut microorganisms. This review focuses on succinate as a metabolite of both host cells and gut microbes and its involvement in regulating the gut - immune tissue axis by activating intestinal mucosal cells, including macrophages, dendritic cells, and intestinal epithelial cells. We also examined its role as the mediator of microbiota - host crosstalk and its potential function in regulating intestinal microbiota homeostasis. This review explores feasible ways to moderate succinate levels and provides new insights into succinate as a potential target for microbial therapeutics for humans.

Succinate Activation of SUCNR1 Predisposes Severely Injured Patients to Neutrophil-mediated ARDS

OBJECTIVES

Identify the metabolites that are increased in the plasma of severely injured patients that developed ARDS versus severely injured patients that did not, and assay if these increased metabolites prime pulmonary sequestration of neutrophils (PMNs) and induce pulmonary sequestration in an animal model of ARDS. We hypothesize that metabolic derangement due to advanced shock in critically injured patients leads to the PMNs, which serves as the first event in the ARDS. Summary of Background Data: Intracellular metabolites accumulate in the plasma of severely injured patients.

METHODS

Untargeted metabolomics profiling of 67 critically injured patients was completed to establish a metabolic signature associated with ARDS development. Metabolites that significantly increased were assayed for PMN priming activity in vitro. The metabolites that primed PMNs were tested in a 2-event animal model of ARDS to identify a molecular link between circulating metabolites and clinical risk for ARDS.

RESULTS

After controlling for confounders, 4 metabolites significantly increased: creatine, dehydroascorbate, fumarate, and succinate in trauma patients who developed ARDS ( P < 0.05). Succinate alone primed the PMN oxidase in vitro at physiologically relevant levels. Intravenous succinate-induced PMN sequestration in the lung, a first event, and followed by intravenous lipopolysaccharide, a second event, resulted in ARDS in vivo requiring PMNs. SUCNR1 inhibition abrogated PMN priming, PMN sequestration, and ARDS. Conclusion: Significant increases in plasma succinate post-injury may serve as the first event in ARDS. Targeted inhibition of the SUCNR1 may decrease ARDS development from other disease states to prevent ARDS globally.

Succinate at the Crossroad of Metabolism and Angiogenesis: Roles of SDH, HIF1α and SUCNR1

Angiogenesis is an essential process by which new blood vessels develop from existing ones. While adequate angiogenesis is a physiological process during, for example, tissue repair, insufficient and excessive angiogenesis stands on the pathological side. Fine balance between pro- and anti-angiogenic factors in the tissue environment regulates angiogenesis. Identification of these factors and how they function is a pressing topic to develop angiogenesis-targeted therapeutics. During the last decade, exciting data highlighted non-metabolic functions of intermediates of the mitochondrial Krebs cycle including succinate. Among these functions is the contribution of succinate to angiogenesis in various contexts and through different mechanisms. As the concept of targeting metabolism to treat a wide range of diseases is rising, in this review we summarize the mechanisms by which succinate regulates angiogenesis in normal and pathological settings. Gaining a comprehensive insight into how this metabolite functions as an angiogenic signal will provide a useful approach to understand diseases with aberrant or excessive angiogenic background, and may provide strategies to tackle them.

Cancer-derived extracellular succinate: a driver of cancer metastasis

Succinate is a tricarboxylic acid (TCA) cycle intermediate normally confined to the mitochondrial matrix. It is a substrate of succinate dehydrogenase (SDH). Mutation of SDH subunits (SDHD and SDHB) in hereditary tumors such as paraganglioma or reduction of SDHB expression in cancer results in matrix succinate accumulation which is transported to cytoplasma and secreted into the extracellular milieu. Excessive cytosolic succinate is known to stabilize hypoxia inducible factor-1α (HIF-1α) by inhibiting prolyl hydroxylase. Recent reports indicate that cancer-secreted succinate enhances cancer cell migration and promotes cancer metastasis by activating succinate receptor-1 (SUCNR-1)-mediated signaling and transcription pathways. Cancer-derived extracellular succinate enhances cancer cell and macrophage migration through SUCNR-1 → PI-3 K → HIF-1α pathway. Extracellular succinate induces tumor angiogenesis through SUCNR-1-mediated ERK1/2 and STAT3 activation resulting in upregulation of vascular endothelial growth factor (VEGF) expression. Succinate increases SUCNR-1 expression in cancer cells which is considered as a target for developing new anti-metastasis drugs. Furthermore, serum succinate which is elevated in cancer patients may be a theranostic biomarker for selecting patients for SUCNR-1 antagonist therapy.

Activation of succinate receptor 1 boosts human mast cell reactivity and allergic bronchoconstriction

BACKGROUND

SUCNR1 is a sensor of extracellular succinate, a Krebs cycle intermediate generated in excess during oxidative stress and has been linked to metabolic regulation and inflammation. While mast cells express SUCNR1, its role in mast cell reactivity and allergic conditions such as asthma remains to be elucidated.

METHODS

Cord blood-derived mast cells and human mast cell line LAD-2 challenged by SUCNR1 ligands were analyzed for the activation and mediator release. Effects on mast cell-dependent bronchoconstriction were assessed in guinea pig trachea and isolated human small bronchi challenged with antigen and anti-IgE, respectively.

RESULTS

SUCNR1 is abundantly expressed on human mast cells. Challenge with succinate, or the synthetic non-metabolite agonist cis-epoxysuccinate, renders mast cells hypersensitive to IgE-dependent activation, resulting in augmented degranulation and histamine release, de novo biosynthesis of eicosanoids and cytokine secretion. The succinate-potentiated mast cell reactivity was attenuated by SUCNR1 knockdown and selective SUCNR1 antagonists and could be tuned by pharmacologically targeting protein kinase C and extracellular signal-regulated kinase. Both succinate and cis-epoxysuccinate dose-dependently potentiated antigen-induced contraction in a mast cell-dependent guinea pig airway model, associated with increased generation of cysteinyl-leukotrienes and histamine in trachea. Similarly, cis-epoxysuccinate aggravated IgE-receptor-induced contraction of human bronchi, which was blocked by SUCNR1 antagonism.

CONCLUSION

SUCNR1 amplifies IgE-receptor-induced mast cell activation and allergic bronchoconstriction, suggesting a role for this pathway in aggravation of allergic asthma, thus linking metabolic perturbations to mast cell-dependent inflammation.

Genetic impairment of succinate metabolism disrupts bioenergetic sensing in adrenal neuroendocrine cancer

Metabolic dysfunction mutations can impair energy sensing and cause cancer. Loss of function of the mitochondrial tricarboxylic acid (TCA) cycle enzyme subunit succinate dehydrogenase B (SDHB) results in various forms of cancer typified by pheochromocytoma (PC). Here we delineate a signaling cascade where the loss of SDHB induces the Warburg effect, triggers dysregulation of [Ca²⁺]_i, and aberrantly activates calpain and protein kinase Cdk5, through conversion of its cofactor from p35 to p25. Consequently, aberrant Cdk5 initiates a phospho-signaling cascade where GSK3 inhibition inactivates energy sensing by AMP kinase through dephosphorylation of the AMP kinase γ subunit, PRKAG2. Overexpression of p25-GFP in mouse adrenal chromaffin cells also elicits this phosphorylation signaling and causes PC. A potent Cdk5 inhibitor, MRT3-007, reverses this phospho-cascade, invoking a senescence-like phenotype. This therapeutic approach halted tumor progression in vivo. Thus, we reveal an important mechanistic feature of metabolic sensing and demonstrate that its dysregulation underlies tumor progression in PC and likely other cancers.

Metabolite G-protein coupled receptor signaling: Potential regulation of eicosanoids

Eicosanoids are a family of bioactive compounds derived from arachidonic acid (AA) that play pivotal roles in physiology and disease, including inflammatory conditions of multiple organ systems. The biosynthesis of eicosanoids requires a series of catalytic steps that are controlled by designated enzymes, which can be regulated by inflammatory and stress signals via transcriptional and translational mechanisms. In the past decades, evidence have emerged indicating that G-protein coupled receptors (GPCRs) can sense extracellular metabolites, and regulate inflammatory responses including eicosanoid production. This review focuses on the recent advances of metabolite GPCRs research, their role in regulation of eicosanoid biosynthesis, and the link to pathophysiological conditions.

Nutritional and metabolic signalling through GPCRs

Deregulated metabolism is a well-known feature of several challenging diseases, including diabetes, obesity and cancer. Besides their important role as intracellular bioenergetic molecules, dietary nutrients and metabolic intermediates are released in the extracellular environment. As such, they may achieve unconventional roles as hormone-like molecules by activating cell-surface G-protein-coupled receptors (GPCRs) that regulate several pathophysiological processes. In this review, we provide an insight into the role of lactate, succinate, fatty acids, amino acids, ketogenesis-derived and β-oxidation-derived intermediates as extracellular signalling molecules. Moreover, the mechanisms by which their cognate metabolite-sensing GPCRs integrate nutritional and metabolic signals with specific intracellular pathways will be described. A better comprehension of these aspects is of fundamental importance to identify GPCRs as novel druggable targets.

Mitochondrial Dysfunction in Advanced Liver Disease: Emerging Concepts

Mitochondria are entrusted with the challenging task of providing energy through the generation of ATP, the universal cellular currency, thereby being highly flexible to different acute and chronic nutrient demands of the cell. The fact that mitochondrial diseases (genetic disorders caused by mutations in the nuclear or mitochondrial genome) manifest through a remarkable clinical variation of symptoms in affected individuals underlines the far-reaching implications of mitochondrial dysfunction. The study of mitochondrial function in genetic or non-genetic diseases therefore requires a multi-angled approach. Taking into account that the liver is among the organs richest in mitochondria, it stands to reason that in the process of unravelling the pathogenesis of liver-related diseases, researchers give special focus to characterizing mitochondrial function. However, mitochondrial dysfunction is not a uniformly defined term. It can refer to a decline in energy production, increase in reactive oxygen species and so forth. Therefore, any study on mitochondrial dysfunction first needs to define the dysfunction to be investigated. Here, we review the alterations of mitochondrial function in liver cirrhosis with emphasis on acutely decompensated liver cirrhosis and acute-on-chronic liver failure (ACLF), the latter being a form of acute decompensation characterized by a generalized state of systemic hyperinflammation/immunosuppression and high mortality rate. The studies that we discuss were either carried out in liver tissue itself of these patients, or in circulating leukocytes, whose mitochondrial alterations might reflect tissue and organ mitochondrial dysfunction. In addition, we present different methodological approaches that can be of utility to address the diverse aspects of hepatocyte and leukocyte mitochondrial function in liver disease. They include assays to measure metabolic fluxes using the comparatively novel Biolog’s MitoPlates in a 96-well format as well as assessment of mitochondrial respiration by high-resolution respirometry using Oroboros’ O2k-technology and Agilent Seahorse XF technology.

Foresight regarding drug candidates acting on the succinate-GPR91 signalling pathway for non-alcoholic steatohepatitis (NASH) treatment

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and it is a liver manifestation of metabolic syndrome, with a histological spectrum from simple steatosis to non-alcoholic steatohepatitis (NASH). NASH can evolve into progressive liver fibrosis and eventually lead to liver cirrhosis. The pathological mechanism of NASH is multifactorial, involving a series of metabolic disorders and changes that trigger low-level inflammation in the liver and other organs. In the pathogenesis of NASH, the signal transduction pathway involving succinate and the succinate receptor (G-protein-coupled receptor 91, GPR91) regulates inflammatory cell activation and liver fibrosis. This review describes the mechanism of the succinate-GPR91 signalling pathway in NASH and summarizes the drugs that act on this pathway, with the aim of providing a new approach to NASH treatment.

Bioisosteres of the Phenyl Ring: Recent Strategic Applications in Lead Optimization and Drug Design

The benzene moiety is the most prevalent ring system in marketed drugs, underscoring its historic popularity in drug design either as a pharmacophore or as a scaffold that projects pharmacophoric elements. However, introspective analyses of medicinal chemistry practices at the beginning of the 21st century highlighted the indiscriminate deployment of phenyl rings as an important contributor to the poor physicochemical properties of advanced molecules, which limited their prospects of being developed into effective drugs. This Perspective deliberates on the design and applications of bioisosteric replacements for a phenyl ring that have provided practical solutions to a range of developability problems frequently encountered in lead optimization campaigns. While the effect of phenyl ring replacements on compound properties is contextual in nature, bioisosteric substitution can lead to enhanced potency, solubility, and metabolic stability while reducing lipophilicity, plasma protein binding, phospholipidosis potential, and inhibition of cytochrome P450 enzymes and the hERG channel.

Succinate Mediates Tumorigenic Effects via Succinate Receptor 1: Potential for New Targeted Treatment Strategies in Succinate Dehydrogenase Deficient Paragangliomas

Paragangliomas and pheochromocytomas (PPGLs) are chromaffin tumors associated with severe catecholamine-induced morbidities. Surgical removal is often curative. However, complete resection may not be an option for patients with succinate dehydrogenase subunit A-D (SDHx) mutations. SDHx mutations are associated with a high risk for multiple recurrent, and metastatic PPGLs. Treatment options in these cases are limited and prognosis is dismal once metastases are present. Identification of new therapeutic targets and candidate drugs is thus urgently needed. Previously, we showed elevated expression of succinate receptor 1 (SUCNR1) in SDHB PPGLs and SDHD head and neck paragangliomas. Its ligand succinate has been reported to accumulate due to SDHx mutations. We thus hypothesize that autocrine stimulation of SUCNR1 plays a role in the pathogenesis of SDHx mutation-derived PPGLs. We confirmed elevated SUCNR1 expression in SDHx PPGLs and after SDHB knockout in progenitor cells derived from a human pheochromocytoma (hPheo1). Succinate significantly increased viability of SUCNR1-transfected PC12 and ERK pathway signaling compared to control cells. Candidate SUCNR1 inhibitors successfully reversed proliferative effects of succinate. Our data reveal an unrecognized oncometabolic function of succinate in SDHx PPGLs, providing a growth advantage via SUCNR1.

Succinate Receptor 1: An Emerging Regulator of Myeloid Cell Function in Inflammation

The rapidly evolving area of immunometabolism has shed new light on the fundamental properties of products and intermediates of cellular metabolism (metabolites), highlighting their key signaling roles in cell-to-cell communication. Recent evidence identifies the succinate-succinate receptor 1 (SUCNR1) axis as an essential regulator of tissue homeostasis. Succinate signaling via SUCNR1 guides divergent responses in immune cells, which are tissue and context dependent. Herein, we explore the main cellular pathways regulated by the succinate-SUCNR1 axis and focus on the biology of SUCNR1 and its roles influencing the function of myeloid cells. Hence, we identify new therapeutic targets and putative therapeutic approaches aimed at resolving detrimental myeloid cell responses in tissues, including those occurring in the persistently inflamed central nervous system (CNS).

Oncometabolites lactate and succinate drive pro-angiogenic macrophage response in tumors

Macrophages are innate phagocytic leukocytes that are highly present in solid tumors, where they are referred to as tumor-associated macrophages (TAMs). In solid tumors, the microenvironment is often immunosuppressive and hypoxic regions are prevalent. These hypoxic conditions impose tumor cells to reprogram their metabolism, shifting from oxidative phosphorylation to anaerobic glycolysis. This so-called glycolytic switch enables hypoxic tumor cells to survive, proliferate, and eventually to outcompete untransformed cells. The hypoxia-induced change in tumor cell metabolism leads to the production of oncometabolites, among which are the glycolytic end-metabolite lactate and the tricarboxylic acid cycle intermediate succinate. TAMs can react to these oncometabolites, resulting in an altered maturation and the adoption of pro-angiogenic features. These angiogenesis-promoting TAMs have been reported to cooperate with tumor cells in the formation of new vessels, and even have been considered an important cause of resistance against anti-angiogenic therapies. For a long time, the mechanisms by which lactate and succinate activated pro-angiogenic TAMs were not understood. Researchers now start to unravel and understand some of the underlying mechanisms. Here, the importance of microenvironmental cues in inducing different macrophage activation states is discussed, as well as the role of hypoxia in the recruitment and activation of pro-angiogenic macrophages. In addition, the latest findings on the oncometabolites lactate and succinate in the activation of angiogenesis supporting macrophages are reviewed. Finally, various oncometabolite-targeting therapeutic strategies are proposed that could improve the response to anti-angiogenic therapies. SIGNIFICANCE STATEMENT: Tumor-associated macrophages (TAMs) are known promotors of tumor neovascularization, and significantly contribute to the emergence of resistance to anti-angiogenic therapies. Recent evidence suggests that the angiogenesis promoting phenotype of TAMs can be activated by hypoxic tumor cell-derived oncometabolites, including lactate and succinate. Here, the latest findings into the lactate- and succinate-mediated mechanistic activation of pro-angiogenic TAMs are reviewed, and therapeutic strategies that interfere with this mechanism and may delay or even prevent acquired resistance to anti-angiogenic agents are discussed.

GPR91 antagonist and TGF-β inhibitor suppressed collagen production of high glucose and succinate induced HSC activation

Activated hepatic stellate cells (HSCs) play a central role in fibrillary collagen production, the primary cause of liver fibrosis. Although it is known that primary cultured HSCs are activated by plastic culture dish stiffness, HSC activation and quiescent-state-reversion mechanisms are still unclear. In this study, we used cultured normal rat HSCs on 3.2 kPa collagen normal liver stiffness equivalent gel, to determine whether high glucose or high succinate conditions induce HSC activation, and examined whether activated HSCs reverted to a quiescent state when suppressed by GPR91 antagonists or TGF-β1 receptor inhibitors. We measured the gene expression levels of α-SMA and type I collagen HSC activation markers using real-time PCR. Our data indicated that high glucose conditions induced HSC activation, and showed that under continuous high glucose exposure HSC activation progressed. A GPR91 antagonist, 2 d, and a TGF-β1 receptor inhibitor, SB525334, suppressed the Col1α mRNA expression level of these activated HSCs. Similarly, under extended high succinate exposure, 2 d and SB525334 reduced Col1α mRNA expression levels of activated HSCs. From the above, we determined that even though HSCs had already been activated by high glucose or succinate conditions which persisted after activation, the GPR91 antagonist and the TGF-β1 receptor inhibitor were able to reduce the production of type I collagen from activated HSCs.

Activation of metabolite receptor GPR91 promotes platelet aggregation and transcellular biosynthesis of leukotriene C4

BACKGROUND

Succinate is a Krebs cycle intermediate whose formation is enhanced under metabolic stress, and for which a selective sensor GPR91 has been identified on various cell types including platelets. Platelet-derived eicosanoids play pivotal roles in platelet activation/aggregation, which is key to thrombus formation and progression of atherothrombosis.

OBJECTIVES

This study aims to decipher the molecular mechanism(s) and potential involvement of eicosanoids in succinate enhanced platelet activation/aggregation.

METHODS

We used liquid chromatography-mass spectrometry (LC-MS)/MS-based lipid mediator profiling to identify eicosanoids regulated by succinate. We ran light transmittance aggregometry and flow cytometry to assess platelet aggregation, P-selectin expression, and platelet-polymorphonuclear leukocyte (PMN) adherence. Various pharmacological tools were used to assess the contributions of GPR91 signalling and eicosanoids in platelet aggregation.

RESULTS

Succinate and two types of synthetic non-metabolite GPR91 agonists-cis-epoxysuccinate (cES) and Cmpd131-potentiated platelet aggregation, which was partially blocked by a selective GPR91 antagonist XT1. GPR91 activation increased production of 12-hydroxy-eicosatetraenoic acid (12-HETE), thromboxane (TX) A2 , and 12-hydroxy-heptadecatrienoic acid (12-HHT) in human platelets, associated with phosphorylation of cytosolic phospholipase A2 (cPLA2 ), suggesting increased availability of free arachidonic acid. Blocking 12-HETE and TXA2 synthesis, or antagonism of the TXA2 receptor, significantly reduced platelet aggregation enhanced by GPR91 signalling. Moreover, platelet-PMN suspensions challenged with succinate exhibited enhanced transcellular biosynthesis of leukotriene C4 (LTC4 ), a powerful proinflammatory vascular spasmogen.

CONCLUSION

Succinate signals through GPR91 to promote biosynthesis of eicosanoids, which contribute to platelet aggregation/activation and potentially vascular inflammation. Hence, GPR91 may be a suitable target for pharmacological intervention in atherothrombotic conditions.

Structural basis for GPCR signaling by small polar versus large lipid metabolites-discovery of non-metabolite ligands

Key metabolites act through specific G protein-coupled receptors (GPCRs) as extracellular signals of fuel availability and metabolic stress. Here, we focus on the succinate receptor SUCNR1/GPR91 and the long chain fatty acid receptor FFAR1/GPR40, for which 3D structural information is available. Like other small polar acidic metabolites, succinate is excreted from the cell by transporter proteins to bind to an extracellular, solvent-exposed pocket in SUCNR1. Non-metabolite pharmacological tool compounds are currently being designed based on the structure of the SUCNR1 binding pocket. In FFAR1, differently signaling lipid mimetics bind in two distinct membrane-exposed sites corresponding to each of the lipid bilayer leaflets. Conceivably endogenous lipid ligands gain access to these sites by way of the membrane and probably occupy both sites under physiological circumstances. Design of polar agonists for a dynamic, solvent-exposed pocket in FFAR1 underlines the possibility of structure-based approaches for development of novel tool compounds even in lipid sensing metabolite GPCRs.

Gut microbiota-derived succinate: Friend or foe in human metabolic diseases?

There is now a wealth of evidence showing that communication between microbiota and the host is critical to sustain the vital functions of the healthy host, and disruptions of this homeostatic coexistence are known to be associated with a range of diseases including obesity and type 2 diabetes. Microbiota-derived metabolites act both as nutrients and as messenger molecules and can signal to distant organs in the body to shape host pathophysiology. In this review, we provide a new perspective on succinate as a gut microbiota-derived metabolite with a key role governing intestinal homeostasis and energy metabolism. Thus, succinate is not merely a major intermediary of the TCA traditionally considered as an extracellular danger signal in the host, but also a by-product of some bacteria and a primary cross-feeding metabolite between gut resident microbes. In addition to maintain a healthy microbiome, specific functions of microbiota-derived succinate in peripheral tissues regulating host nutrient metabolism should not be rule out. Indeed, recent research point to some probiotic interventions directed to modulate succinate levels in the intestinal lumen, as a new microbiota-based therapies to treat obesity and related co-morbidities. While further research is essential, a large body of evidence point to succinate as a new strategic mediator in the microbiota-host cross-talk, which might provide the basis for new therapeutically approaches in a near future.

Regulation of leukocyte function by citric acid cycle intermediates

Cellular metabolism is a means of generating ATP to provide energy for key cellular functions. However, recent research shows that citric acid cycle intermediates target vital cellular functions of the innate immune system. Succinate, itaconate, citrate, and fumarate have been shown to mediate or regulate important myeloid cell functions during infection and inflammation. This review covers the regulatory functions of citric acid cycle intermediates in myeloid cells and discusses potential translational applications, key mechanistic questions, and future research directions.

Coupling Krebs cycle metabolites to signalling in immunity and cancer

Metabolic reprogramming has become a key focus for both immunologists and cancer biologists, with exciting advances providing new insights into underlying mechanisms of disease. Metabolites traditionally associated with bioenergetics or biosynthesis have been implicated in immunity and malignancy in transformed cells, with a particular focus on intermediates of the mitochondrial pathway known as the Krebs cycle. Among these, the intermediates succinate, fumarate, itaconate, 2-hydroxyglutarate isomers (D-2-hydroxyglutarate and L-2-hydroxyglutarate) and acetyl-CoA now have extensive evidence for "non-metabolic" signalling functions in both physiological immune contexts and in disease contexts, such as the initiation of carcinogenesis. This review will describe how metabolic reprogramming, with emphasis placed on these metabolites, leads to altered immune cell and transformed cell function. The latest findings are informative for new therapeutic approaches which could be transformative for a range of diseases.

A Novel Naphthyridine Derivative, 3u, Induces Necroptosis at Low Concentrations and Apoptosis at High Concentrations in Human Melanoma A375 Cells

Naphthyridine derivatives are a widely-used class of heterocycles due to their pharmacological activities. A novel compound (10-Methoxy-1,2,3,4-tetrahydrobenzo(g)(1,3) diazepino(1,2-a)-(1,8)naphthyridin-6-yl)(phenyl) methanone (named 3u), showed good anticancer activity in the human malignant melanoma cell line A375 via Thiazolyl Blue Tetrazolium Bromide (MTT) assay. After Western blotting confirmed, we found that 3u induces necroptosis at low concentrations and apoptosis at high concentrations via the upregulation of death receptors and scaffold protein in A375 cells. Furthermore, by combining 3u with the caspase inhibitor zVAD-fmk or Receptor Interacting Serine/Threonine Kinase 1 (RIP1) kinase inhibitor Necrostatin-1 (Nec-1), we found that the activity of caspase-8 was the crucial factor that determined whether either apoptosis or necroptosis occurred. The results indicate that 3u should be considered as a potential chemical substance for melanoma treatment.

The Role of Metabolite-Sensing G Protein-Coupled Receptors in Inflammation and Metabolic Disease

SIGNIFICANCE

Great attention has been placed on the link between metabolism and immune function giving rise to the term "immunometabolism." It is widely accepted that inflammation and oxidative stress are key processes that underlie metabolic complications during obesity, diabetes, and atherosclerosis. Therefore, identifying the mechanisms and mediators that are involved in the regulation of both inflammation and metabolic homeostasis is of high scientific and therapeutic interest. Recent Advances: G protein-coupled receptors (GPCRs) that signal in response to metabolites have emerged as attractive therapeutic targets in inflammatory disease. Critical Issues and Future Directions: In this review, we discuss recent findings about the physiological role of the main metabolite-sensing GPCRs, their implication in immunometabolic disorders, their principal endogenous and synthetic ligands, and their potential as drug targets in inflammation and metabolic disease. Antioxid. Redox Signal. 29, 237-256.

Structure-Activity Investigations and Optimisations of Non-metabolite Agonists for the Succinate Receptor 1

The succinate receptor 1 (SUCNR1) is a receptor for the metabolite succinate, which functions as a metabolic stress signal in the liver, kidney, adipose tissue and the retina. However, potent non-metabolite tool compounds are needed to reveal the physiological role and pharmacological potential of SUCNR1. Recently, we published the discovery of a computationally receptor-structure derived non-metabolite SUCNR1 agonist series with high target selectivity. We here report our structure-activity exploration and optimisation that has resulted in the development of agonists with nanomolar potency and excellent solubility and stability properties in a number of in vitro assays. Ligand-guided receptor models with high discriminative power between binding of active and inactive compounds were developed for design of novel chemotypes.

Multiple faces of succinate beyond metabolism in blood

Succinate is an essential intermediate of the tricarboxylic acid cycle that exerts pleiotropic roles beyond metabolism in both physiological and pathological conditions. Recent evidence obtained in mouse models shows its essential role regulating blood cell function through various mechanisms that include pseudohypoxia responses by hypoxia-inducible factor-1α activation, post-translational modifications like succinylation, and communication mediated by succinate receptor 1. Hence, succinate links metabolism to processes like gene expression and intercellular communication. Interestingly, succinate plays key dual roles during inflammatory responses, leading to net inflammation or anti-inflammation depending on factors like the cellular context. Here, we further discuss current suggestions of the possible contribution of succinate to blood stem cell function and blood formation. Further study will be required in the future to better understand succinate biology in blood cells. This promising field may open new avenues to modulate inflammatory responses and to preserve blood cell homeostasis in the clinical setting.

Succinate as a Regulator of Hepatic Stellate Cells in Liver Fibrosis

The rapid increase of obesity rates worldwide is associated with chronic liver injury due to non-alcoholic fatty liver disease and non-alcoholic steatohepatitis. Chronic liver inflammation drives hepatic fibrosis, which is a highly conserved and coordinated protective response to tissue injury, and is a reversible process. Hepatocytes, immune cells, and hepatic stellate cells (HSCs) have been identified as key players in the mechanisms of hepatic fibrosis and inflammation. During the last decade, succinate, an intermediate of the tricarboxylic acid cycle in mitochondrial ATP production, has emerged as an important signaling molecule in various diseases. Succinate acts as an extracellular ligand for G-protein coupled receptor 91, also known as succinate receptor 1, which is mainly expressed in the kidney, heart, liver, immune cells, and retinal cells, suggesting a widespread function in cellular metabolism. Furthermore, succinate stabilizes hypoxia-inducible factor-1α in immune cells and tumors as a signaling molecule, and has been shown to post-translationally modify proteins. This review summarizes the recent evidence pointing to an additional role of succinate in profibrotic signaling, along with its downstream signaling pathways, and updates the current state of knowledge on the role of succinate in liver fibrosis through its action on HSCs. Further focus on this link can help identify succinate, its receptor, and its downstream signaling molecules as new targets for the treatment of liver fibrosis.

Receptor structure-based discovery of non-metabolite agonists for the succinate receptor GPR91

Objective

Besides functioning as an intracellular metabolite, succinate acts as a stress-induced extracellular signal through activation of GPR91 (SUCNR1) for which we lack suitable pharmacological tools.

Methods and results

Here we first determined that the cis conformation of the succinate backbone is preferred and that certain backbone modifications are allowed for GPR91 activation. Through receptor modeling over the X-ray structure of the closely related P2Y1 receptor, we discovered that the binding pocket is partly occupied by a segment of an extracellular loop and that succinate therefore binds in a very different mode than generally believed. Importantly, an empty side-pocket is identified next to the succinate binding site. All this information formed the basis for a substructure-based search query, which, combined with molecular docking, was used in virtual screening of the ZINC database to pick two serial mini-libraries of a total of only 245 compounds from which sub-micromolar, selective GPR91 agonists of unique structures were identified. The best compounds were backbone-modified succinate analogs in which an amide-linked hydrophobic moiety docked into the side-pocket next to succinate as shown by both loss- and gain-of-function mutagenesis. These compounds displayed GPR91-dependent activity in altering cytokine expression in human M2 macrophages similar to succinate, and importantly were devoid of any effect on the major intracellular target, succinate dehydrogenase.

Conclusions

These novel, synthetic non-metabolite GPR91 agonists will be valuable both as pharmacological tools to delineate the GPR91-mediated functions of succinate and as leads for the development of GPR91-targeted drugs to potentially treat low grade metabolic inflammation and diabetic complications such as retinopathy and nephropathy.

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The GPR91 binding site for succinate is identified with an adjacent empty pocket.

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The binding pocket structure is used to identify novel synthetic GPR91 agonists.

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The non-metabolite GPR91 ligands can be used as pharmacological tools and drug leads.

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Novel compounds demonstrate GPR91 control of cytokine expression in M2 macrophages.

NHC-Cu(I)-Catalyzed Friedländer-Type Annulation of Fluorinated o-Aminophenones with Alkynes on Water: Competitive Base-Catalyzed Dibenzo[b,f][1,5]diazocine Formation

An efficient, easily scalable synthesis of 4-trifluoromethylquinolines and naphthydrines (as well as their difluoro- and perfluoro-analogues) as a result of tandem direct catalytic alkynylation/dehydrative condensation of o-aminofluoromethylketones (o-FMKs), for the first time catalyzed by NHC-copper(I) complexes on water, is reported. A wide range of terminal alkynes is tolerated under the reaction conditions, including β-lactam-, steroid-, and sugar-derived ones, leading to desired quinolines and naphthydrines with good yields. Further investigations proved that o-FMKs could be efficiently transformed into a rare class of heterocyclic compounds-dibenzo[b,f][1,5]diazocines-by a base-catalyzed condensation, also on water. The developed method was applied for gram-scale synthesis of a fluorinated analogue of G protein-coupled receptor antagonist (GPR91).

G protein-coupled receptor 91 signaling in diabetic retinopathy and hypoxic retinal diseases

G protein-coupled receptor 91 (GPR91) is a succinate-specific receptor and activation of GPR91 could initiate a complex signal transduction cascade and upregulate inflammatory and pro-angiogenic cytokines. In the retina, GPR91 is predominately expressed in ganglion cells, a major cellular entity involved in the pathogenesis of diabetic retinopathy (DR) and other hypoxic retinal diseases. During the development of DR and retinopathy of prematurity (ROP), chronic hypoxia causes an increase in the levels of local succinate. Succinate-mediated GPR91 activation upregulates vascular endothelial growth factor (VEGF) through ERK1/2-C/EBP β (c-Fos) and/or ERK1/2-COX-2/PGE2 signaling pathways, which in turn, leads to the breakdown of blood-retina barriers in these disorders. In this review, we will have a brief introduction of GPR91 and its biological functions and a more detailed discussion about the role and mechanisms of GPR91 in DR and ROP. A better understanding of GPR91 regulation may be of great significance in identifying new biomarkers and drug targets for the prediction and treatment of DR, ROP, and hypoxic retinal diseases.

Succinate and its G-protein-coupled receptor stimulates osteoclastogenesis

The mechanism underlying bone impairment in patients with diabetes mellitus, a metabolic disorder characterized by chronic hyperglycaemia and dysregulation in metabolism, is unclear. Here we show the difference in the metabolomics of bone marrow stromal cells (BMSCs) derived from hyperglycaemic (type 2 diabetes mellitus, T2D) and normoglycaemic mice. One hundred and forty-two metabolites are substantially regulated in BMSCs from T2D mice, with the tricarboxylic acid (TCA) cycle being one of the primary metabolic pathways impaired by hyperglycaemia. Importantly, succinate, an intermediate metabolite in the TCA cycle, is increased by 24-fold in BMSCs from T2D mice. Succinate functions as an extracellular ligand through binding to its specific receptor on osteoclastic lineage cells and stimulates osteoclastogenesis in vitro and in vivo. Strategies targeting the receptor activation inhibit osteoclastogenesis. This study reveals a metabolite-mediated mechanism of osteoclastogenesis modulation that contributes to bone dysregulation in metabolic disorders.

Bone loss is common in patients with diabetes, but the underlying molecular and cellular mechanisms are unclear. Here the authors show high succinate levels in mice with type 2 diabetes and that succinate can signal through succinate receptor 1 on osteoclasts to induce bone resorption.

GPCR-Mediated Signaling of Metabolites

In addition to their bioenergetic intracellular function, several classical metabolites act as extracellular signaling molecules activating cell-surface G-protein-coupled receptors (GPCRs), similar to hormones and neurotransmitters. "Signaling metabolites" generated from nutrients or by gut microbiota target primarily enteroendocrine, neuronal, and immune cells in the lamina propria of the gut mucosa and the liver and, through these tissues, the rest of the body. In contrast, metabolites from the intermediary metabolism act mainly as metabolic stress-induced autocrine and paracrine signals in adipose tissue, the liver, and the endocrine pancreas. Importantly, distinct metabolite GPCRs act as efficient pro- and anti-inflammatory regulators of key immune cells, and signaling metabolites may thus function as important drivers of the low-grade inflammation associated with insulin resistance and obesity. The concept of key metabolites as ligands for specific GPCRs has broadened our understanding of metabolic signaling significantly and provides a number of novel potential drug targets.

Identification and pharmacological characterization of succinate receptor agonists

BACKGROUND AND PURPOSE

The succinate receptor (formerly GPR91 or SUCNR1) is described as a metabolic sensor that may be involved in homeostasis. Notwithstanding its implication in important (patho)physiological processes, the function of succinate receptors has remained ill-defined because no pharmacological tools were available. We report on the discovery of the first family of potent synthetic agonists.

EXPERIMENTAL APPROACH

We screened a library of succinate analogues and analysed their activity on succinate receptors. Also, we modelled a pharmacophore and a binding site for this receptor. New agonists were identified based on the information provided by these two approaches. Their activity was studied in various bioassays, including measurement of cAMP levels, [Ca²⁺ ]_i mobilization, TGF-α shedding and recruitment of arrestin 3. The in vivo effects of activating succinate receptors with these new agonists was evaluated on rat BP.

KEY RESULTS

We identified cis-epoxysuccinic acid and cis-1,2-cyclopropanedicarboxylic acid as agonists with an efficacy similar to that of succinic acid. Interestingly, cis-epoxysuccinic acid was 10- to 20-fold more potent than succinic acid on succinate receptors. For example, cis-epoxysuccinic acid reduced cAMP levels with a pEC₅₀  = 5.57 ± 0.02 (EC₅₀  = 2.7 μM), compared with succinate pEC₅₀  = 4.54 ± 0.08 (EC₅₀  = 29 μM). The rank order of potency of the three agonists was the same in all in vitro assays. Both cis-epoxysuccinic and cis-1,2-cyclopropanedicarboxylic acid were as potent as succinate in increasing rat BP.

CONCLUSIONS AND IMPLICATIONS

We describe new agonists at succinate receptors that should facilitate further research on this understudied receptor.

GPR91 senses extracellular succinate released from inflammatory macrophages and exacerbates rheumatoid arthritis

When SUCNR1/GPR91-expressing macrophages are activated by inflammatory signals, they change their metabolism and accumulate succinate. In this study, we show that during this activation, macrophages release succinate into the extracellular milieu. They simultaneously up-regulate GPR91, which functions as an autocrine and paracrine sensor for extracellular succinate to enhance IL-1β production. GPR91-deficient mice lack this metabolic sensor and show reduced macrophage activation and production of IL-1β during antigen-induced arthritis. Succinate is abundant in synovial fluids from rheumatoid arthritis (RA) patients, and these fluids elicit IL-1β release from macrophages in a GPR91-dependent manner. Together, we reveal a GPR91/succinate-dependent feed-forward loop of macrophage activation and propose GPR91 antagonists as novel therapeutic principles to treat RA.

Insight into SUCNR1 (GPR91) structure and function

SUCNR1 (or GPR91) belongs to the family of G protein-coupled receptors (GPCR), which represents the largest group of membrane proteins in human genome. The majority of marketed drugs targets GPCRs, directly or indirectly. SUCNR1 has been classified as an orphan receptor until a landmark study paired it with succinate, a citric acid cycle intermediate. According to the current paradigm, succinate triggers SUCNR1 signaling pathways to indicate local stress that may affect cellular metabolism. SUCNR1 implication has been well documented in renin-induced hypertension, ischemia/reperfusion injury, inflammation and immune response, platelet aggregation and retinal angiogenesis. In addition, the SUCNR1-induced increase of blood pressure may contribute to diabetic nephropathy or cardiac hypertrophy. The understanding of SUCNR1 activation, signaling pathways and functions remains largely elusive, which calls for deeper investigations. SUCNR1 shows a high potential as an innovative drug target and is probably an important regulator of basic physiology. In order to achieve the full characterization of this receptor, more specific pharmacological tools such as small-molecules modulators will represent an important asset. In this review, we describe the structural features of SUCNR1, its current ligands and putative binding pocket. We give an exhaustive overview of the known and hypothetical signaling partners of the receptor in different in vitro and in vivo systems. The link between SUCNR1 intracellular pathways and its pathophysiological roles are also extensively discussed.

GPR91: expanding the frontiers of Krebs cycle intermediates

Since it was discovered, the citric acid cycle has been known to be central to cell metabolism and energy homeostasis. Mainly found in the mitochondrial matrix, some of the intermediates of the Krebs cycle are also present in the blood stream. Currently, there are several reports that indicate functional roles for Krebs intermediates out of its cycle. Succinate, for instance, acts as an extracellular ligand by binding to a G-protein coupled receptor, known as GPR91, expressed in kidney, liver, heart, retinal cells and possibly many other tissues, leading to a wide array of physiological and pathological effects. Through GPR91, succinate is involved in functions such as regulation of blood pressure, inhibition of lipolysis in white adipose tissue, development of retinal vascularization, cardiac hypertrophy and activation of stellate hepatic cells by ischemic hepatocytes. Along the current review, these new effects of succinate through GPR91 will be explored and discussed.

Forskolin-free cAMP assay for Gi-coupled receptors

G protein-coupled receptors (GPCRs) represent the most successful receptor family for treating human diseases. Many are poorly characterized with few ligands reported or remain completely orphans. Therefore, there is a growing need for screening-compatible and sensitive assays. Measurement of intracellular cyclic AMP (cAMP) levels is a validated strategy for measuring GPCRs activation. However, agonist ligands for Gi-coupled receptors are difficult to track because inducers such as forskolin (FSK) must be used and are sources of variations and errors. We developed a method based on the GloSensor system, a kinetic assay that consists in a luciferase fused with cAMP binding domain. As a proof of concept, we selected the succinate receptor 1 (SUCNR1 or GPR91) which could be an attractive drug target. It has never been validated as such because very few ligands have been described. Following analyses of SUCNR1 signaling pathways, we show that the GloSensor system allows real time, FSK-free detection of an agonist effect. This FSK-free agonist signal was confirmed on other Gi-coupled receptors such as CXCR4. In a test screening on SUCNR1, we compared the results obtained with a FSK vs FSK-free protocol and were able to identify agonists with both methods but with fewer false positives when measuring the basal levels. In this report, we validate a cAMP-inducer free method for the detection of Gi-coupled receptors agonists compatible with high-throughput screening. This method will facilitate the study and screening of Gi-coupled receptors for active ligands.

Identification of lead compounds for (99m)Tc and (18)F GPR91 radiotracers

To develop the first radiotracer targeting GPR91, a cell membrane-bound receptor that modulates the cellular response to hyperglycemia and hypoxia, we designed and prepared a small series of compounds based on a published series of 1,8-naphthyridines with high affinity to GPR91. Our approach provides a mechanism to incorporate radioactive atoms ((99m)Tc and (18)F) into the GPR91 pharmacophore as the final synthetic step. Pharmacological assays confirmed lead compounds for (99m)Tc and (18)F GPR91 radiotracers within the series.

Metabolic reprograming of mononuclear phagocytes in progressive multiple sclerosis

Multiple sclerosis (MS) is an inflammatory and demyelinating disease of the central nervous system (CNS). Accumulation of brain damage in progressive MS is partly the result of mononuclear phagocytes (MPs) attacking myelin sheaths in the CNS. Although there is no cure yet for MS, significant advances have been made in the development of disease modifying agents. Unfortunately, most of these drugs fail to reverse established neurological deficits and can have adverse effects. Recent evidence suggests that MPs polarization is accompanied by profound metabolic changes, whereby pro-inflammatory MPs (M1) switch toward glycolysis, whereas anti-inflammatory MPs (M2) become more oxidative. It is therefore possible that reprograming MPs metabolism could affect their function and repress immune cell activation. This mini review describes the metabolic changes underpinning macrophages polarization and anticipates how metabolic re-education of MPs could be used for the treatment of MS.

KEY POINTS

Inflammation in progressive MS is mediated primarily by MPs.Cell metabolism regulates the function of MPs.DMAs can re-educate the metabolism of MPs to promote healing.

Regioselective construction of 1,3-diazaheterocycle fused [1,2-a][1,8]naphthyridine derivatives via cascade reaction of quinolines with heterocyclic ketene aminals: a joint experimental-computational approach

A one-step, transition-metal-free protocol, involving facile post-treatment, for the regioselective synthesis of 1,3-diazaheterocycle fused [1,2-a][1,8]naphthyridine derivatives (3) from 2-chloroquinoline-3-carbaldehydes (ClQuAlds) (1) and heterocyclic ketene aminals (HKAs) (2) was developed via a joint experimental-computational approach. The computational prediction of the reactivity of two series of synthons was applied in the process of optimizing the reaction conditions, which relied on density functional theory (DFT) calculations together with concepts of frontier molecular orbital (FMO) theory and quantitative structure-reactivity relationship (QSRR) presumptions. The combined results enabled the proposal of a pre-synthetic prediction of global reactivity. The fully consistent results of the synthetic experiments with the in silico evaluation confirmed the rationality, effectiveness, and practicability of the new strategy. Notably, the joint method is not limited to the laboratory, but has applications ranging from routine to industry. This approach is likely to yield numerous insights to accelerate HKA-related synthetic chemistry that can be extended to numerous heterocycles. It thus opens up a novel entry towards rapidly investigating the reactivity of novel synthons with unique properties, a further step towards exploiting cascade reactions by avoiding the futile waste of time and resources.

Succinate: a metabolic signal in inflammation

Succinate is an intermediate of the tricarboxylic acid (TCA) cycle, and plays a crucial role in adenosine triphosphate (ATP) generation in mitochondria. Recently, new roles for succinate outside metabolism have emerged. Succinate stabilizes the transcription factor hypoxia-inducible factor-1α (HIF-1α) in specific tumors and in activated macrophages, and stimulates dendritic cells via its receptor succinate receptor 1. Furthermore, succinate has been shown to post-translationally modify proteins. This expanding repertoire of functions for succinate suggests a broader role in cellular activation. We review the new roles of succinate and draw parallels to other metabolites such as NAD(+) and citrate whose roles have expanded beyond metabolism and into signaling.

(Biphenyl-4-yl)methylammonium chlorides: potent anticonvulsants that modulate Na+ currents

We have reported that compounds containing a biaryl linked unit (Ar-X-Ar') modulated Na(+) currents by promoting slow inactivation and fast inactivation processes and by inducing frequency (use)-dependent inhibition of Na(+) currents. These electrophysiological properties have been associated with the mode of action of several antiepileptic drugs. In this study, we demonstrate that the readily accessible (biphenyl-4-yl)methylammonium chlorides (compound class B) exhibited a broad range of anticonvulsant activities in animal models, and in the maximal electroshock seizure test the activity of (3'-trifluoromethoxybiphenyl-4-yl)methylammonium chloride (8) exceeded that of phenobarbital and phenytoin upon oral administration to rats. Electrophysiological studies of 8 using mouse catecholamine A-differentiated cells and rat embryonic cortical neurons confirmed that 8 promoted slow and fast inactivation in both cell types but did not affect the frequency (use)-dependent block of Na(+) currents.