Conformationally Restricted GABA with Bicyclo[3.1.0]hexane Backbone as the First Highly Selective BGT-1 Inhibitor

Rational Search for Betaine/GABA Transporter 1 Inhibitors─In Vitro Evaluation of Selected Hit Compound

Inhibitory neurotransmission mediated by γ-aminobutyric acid (GABA) plays an important role in maintaining body homeostasis. Disturbances in GABA signaling are implicated in a multitude of neurologic and psychiatric conditions, including epilepsy, ischemia, anxiety, depression, insomnia, and mood disorders. Clinically relevant increases in GABA neurotransmitter level can be achieved by inhibition of its uptake into presynaptic neurons and surrounding glial cells, driven by GABA transporters (GAT1, BGT1, GAT2, and GAT3). Herein, we focused on the search for inhibitors of the BGT1 transporter which is understudied and for which the therapeutic potential of its inhibition is partly unknown. We applied multilevel virtual screening to identify compounds with inhibitory properties. Among selected hits, compound 9 was shown to be a preferential inhibitor of BGT1 (IC50 13.9 μM). The compound also revealed some inhibitory activity against GAT3 (4x lower) while showing no or low activity (IC50 > 100 μM) toward GAT1 and GAT2, respectively. The predicted binding mode of compound 9 was confirmed by mutagenesis studies on E52A, E52Y, Q299L, and E52A+Q299L human BGT1 mutants. Subsequent evaluation showed that the selected hit displayed no affinity toward major GABAA receptor subtypes. Moreover, it was nontoxic when tested on normal human astrocytes and even showed some neuroprotective activity in SH-SY5Y cells. Compound 9 is considered a promising candidate for further evaluation of the therapeutic potential of BGT1 transporter inhibition and the development of novel inhibitors.

Ways of modulating GABA transporters to treat neurological disease

INTRODUCTION

The main inhibitory neurotransmitter in the central nervous system (CNS), γ-aminobutyric acid (GABA), is involved in a multitude of neurological and psychiatric disorders characterized by an imbalance in excitatory and inhibitory signaling. Regulation of extracellular levels of GABA is maintained by the four GABA transporters (GATs; GAT1, GAT2, GAT3, and BGT1), Na+/Cl--coupled transporters of the solute carrier 6 (SLC6) family. Despite mounting evidence for the involvement of the non-GAT1 GABA transporters in diseases, only GAT1 has successfully been translated into clinical practice via the drug tiagabine.

AREAS COVERED

In this review, all four GATs will be described in terms of their involvement in disease, and the most recent data on structure, function, expression, and localization discussed in relation to their potential role as drug targets. This includes an overview of various ways to modulate the GATs in relation to treatment of diseases caused by imbalances in the GABAergic system.

EXPERT OPINION

The recent publication of various GAT1 structures is an important milestone for future development of compounds targeting the GATs. Such information can provide much needed insight into mechanistic aspects of all GAT subtypes and be utilized to design improved ligands for this highly interesting drug target class.

A comparative review on the well-studied GAT1 and the understudied BGT-1 in the brain

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Its homeostasis is maintained by neuronal and glial GABA transporters (GATs). The four GATs identified in humans are GAT1 (SLC6A1), GAT2 (SLC6A13), GAT3 (SLC6A11), and betaine/GABA transporter-1 BGT-1 (SLC6A12) which are all members of the solute carrier 6 (SLC6) family of sodium-dependent transporters. While GAT1 has been investigated extensively, the other GABA transporters are less studied and their role in CNS is not clearly defined. Altered GABAergic neurotransmission is involved in different diseases, but the importance of the different transporters remained understudied and limits drug targeting. In this review, the well-studied GABA transporter GAT1 is compared with the less-studied BGT-1 with the aim to leverage the knowledge on GAT1 to shed new light on the open questions concerning BGT-1. The most recent knowledge on transporter structure, functions, expression, and localization is discussed along with their specific role as drug targets for neurological and neurodegenerative disorders. We review and discuss data on the binding sites for Na⁺, Cl^-, substrates, and inhibitors by building on the recent cryo-EM structure of GAT1 to highlight specific molecular determinants of transporter functions. The role of the two proteins in GABA homeostasis is investigated by looking at the transport coupling mechanism, as well as structural and kinetic transport models. Furthermore, we review information on selective inhibitors together with the pharmacophore hypothesis of transporter substrates.

2-Oxa-5-azabicyclo[2.2.1]heptane as a Platform for Functional Diversity: Synthesis of Backbone-Constrained γ-Amino Acid Analogues

We communicate a versatile synthetic approach to C-3 disubstituted 2-oxa-5-azabicyclo[2.2.1]heptanes as carbon-atom bridged morpholines, starting with 4R-hydroxy-l-proline as a chiron. Attaching an acetic acid moiety on the C-3 carbon of the 2-oxa-5-azabicyclo[2.2.1]heptane core reveals the framework of an embedded γ-amino butyric acid (GABA). Variations in the nature of the substituent on the tertiary C-3 atom with different alkyls or aryls led to backbone-constrained analogues of the U.S. Food and Drug Administration-approved drugs baclofen and pregabalin.

Synthesis of γ-Aminobutyric Acid (GABA) Analogues Conformationally Restricted by Bicyclo[3.1.0]hexane/hexene or [4.1.0]Heptane/heptene Backbones as Potent Betaine/GABA Transporter Inhibitors

Novel γ-aminobutyric acid (GABA) analogues 3-5, having a bicyclo[3.1.0]hexene, [4.1.0]heptane, or [4.1.0]heptene backbone, respectively, were designed from the bioactive form analysis of the previous inhibitor 2 with a bicyclo[3.1.0]hexane backbone. Compounds 3-5 and 2 were synthesized from a common 1,7-diene intermediate 6 using ring-closing metathesis (RCM) to construct the key bicyclo backbones. Compounds 3-5 strongly inhibit betaine/GABA transporter 1 (BGT1) uptake, but compound 4 stands out with its selective low micromolar potency.

A stereochemical journey around spirocyclic glutamic acid analogs

A practical divergent synthetic approach is reported for the library of regio- and stereoisomers of glutamic acid analogs built on the spiro[3.3]heptane scaffold. Formation of the spirocyclic scaffold was achieved starting from a common precursor - an O-silylated 2-(hydroxymethyl)cyclobutanone derivative. Its olefination required using the titanium-based Tebbe protocol since the standard Wittig reaction did not work with this particular substrate. The construction of the second cyclobutane ring of the spirocyclic system was achieved through either subsequent dichloroketene addition or Meinwald oxirane rearrangement as the key synthetic steps, depending on the substitution patterns in the target compounds (1,6- or 1,5-, respectively). Further modified Strecker reaction of the resulting racemic spirocyclic ketones with the Ellman's sulfinamide as a chiral auxiliary had low to moderate diastereoselectivity; nevertheless, all stereoisomers were isolated in pure form via chromatographic separation, and their absolute configuration was confirmed by X-ray crystallography. Members of the library were tested for the inhibitory activity against H. pylori glutamate racemase.

Molecular Determinants and Pharmacological Analysis for a Class of Competitive Non-transported Bicyclic Inhibitors of the Betaine/GABA Transporter BGT1

The betaine/GABA transporter 1 (BGT1) is a member of the GABA transporter (GAT) family with still elusive function, largely due to a lack of potent and selective tool compounds. Based on modeling, we here present the design, synthesis and pharmacological evaluation of five novel conformationally restricted cyclic GABA analogs related to the previously reported highly potent and selective BGT1 inhibitor (1S,2S,5R)-5-aminobicyclo[3.1.0]hexane-2-carboxylic acid (bicyclo-GABA). Using [³H]GABA radioligand uptake assays at the four human GATs recombinantly expressed in mammalian cell lines, we identified bicyclo-GABA and its N-methylated analog (2) as the most potent and selective BGT1 inhibitors. Additional pharmacological characterization in a fluorescence-based membrane potential assay showed that bicyclo-GABA and 2 are competitive inhibitors, not substrates, at BGT1, which was validated by a Schild analysis for bicyclo-GABA (pK_B value of 6.4). To further elaborate on the selectivity profile both compounds were tested at recombinant α₁β₂γ₂ GABA_A receptors. Whereas bicyclo-GABA showed low micromolar agonistic activity, the N-methylated 2 was completely devoid of activity at GABA_A receptors. To further reveal the binding mode of bicyclo-GABA and 2 binding hypotheses of the compounds were obtained from in silico-guided mutagenesis studies followed by pharmacological evaluation at selected BGT1 mutants. This identified the non-conserved BGT1 residues Q299 and E52 as the molecular determinants driving BGT1 activity and selectivity. The binding mode of bicyclo-GABA was further validated by the introduction of activity into the corresponding GAT3 mutant L314Q (38 times potency increase cf. wildtype). Altogether, our data reveal the molecular determinants for the activity of bicyclic GABA analogs, that despite their small size act as competitive inhibitors of BGT1. These compounds may serve as valuable tools to selectively and potently target BGT1 in order to decipher its elusive pharmacological role in the brain and periphery such as the liver and kidneys.

Development of tricyclic N-benzyl-4-hydroxybutanamide derivatives as inhibitors of GABA transporters mGAT1-4 with anticonvulsant, antinociceptive, and antidepressant activity

γ-Aminobutyric acid (GABA) neurotransmission has a significant impact on the proper functioning of the central nervous system. Numerous studies have indicated that inhibitors of the GABA transporters mGAT1-4 offer a promising strategy for the treatment of several neurological disorders, including epilepsy, neuropathic pain, and depression. Following our previous results, herein, we report the synthesis, biological evaluation, and structure-activity relationship studies supported by molecular docking and molecular dynamics of a new series of N-benzyl-4-hydroxybutanamide derivatives regarding their inhibitory potency toward mGAT1-4. This study allowed us to identify compound 23a (N-benzyl-4-hydroxybutanamide bearing a dibenzocycloheptatriene moiety), a nonselective GAT inhibitor with a slight preference toward mGAT4 (pIC₅₀ = 5.02 ± 0.11), and compound 24e (4-hydroxy-N-[(4-methylphenyl)-methyl]butanamide bearing a dibenzocycloheptadiene moiety) with relatively high inhibitory activity toward mGAT2 (pIC₅₀ = 5.34 ± 0.09). In a set of in vivo experiments, compound 24e successively showed predominant anticonvulsant activity and antinociception in the formalin model of tonic pain. In contrast, compound 23a showed significant antidepressant-like properties in mice. These results were consistent with the available literature data, which indicates that, apart from seizure control, GABAergic neurotransmission is also involved in the pathophysiology of several psychiatric diseases, however alternative mechanisms underlying this action cannot be excluded. Finally, it is worth noting that the selected compounds showed unimpaired locomotor skills that have been indicated to give reliable results in behavioral assays.

[Medicinal Chemical Studies Based on the Theoretical Design of Bioactive Compounds]

I have engaged in medicinal chemical studies based on the theoretical design of bioactive compounds. First, I present a three-dimensional structural diversity-oriented conformational restriction strategy for developing bioactive compounds based on the characteristic steric and stereoelectronic features of cyclopropane. Using this strategy, various biologically active small molecule compounds, such as receptor agonists/antagonists and enzyme inhibitors, were effectively developed. The strategy was also applied to develop versatile peptidomimetics and membrane-permeable cyclic peptides. Next, studies on Ca²⁺-mobilizing second messengers, cyclic ADP-ribose (cADPR) and myo-inositol trisphosphates (IP₃), are described. In these studies, stable equivalents of cADPR were developed, since biological studies of cADPR have been limited due to its instability. Various potent IP₃ receptor ligands, which were designed using the d-glucose structure as a bioisostere of the myo-inositol moiety of IP₃, have been identified. Organic chemistry studies have also been extensively performed, because excellent organic chemistry is essential for promoting high-level medicinal chemical studies. For examples, new methods for the synthesis of chiral cyclopropanes, new radical reactions with silicon tethers, and kinetic anomeric effect-dependent stereoselective glycosidations have been developed.

Conformational Restriction of Histamine with a Rigid Bicyclo[3.1.0]hexane Scaffold Provided Selective H3 Receptor Ligands

We designed and synthesized conformationally rigid histamine analogues with a bicyclo[3.1.0]hexane scaffold. All the compounds were selectively bound to the H₃ receptor subtype over the H₄ receptor subtype. Notably, compound 7 showed potent binding affinity and over 100-fold selectivity for the H₃ receptors (K_i = 5.6 nM for H₃ and 602 nM for H₄). These results suggest that the conformationally rigid bicyclo[3.1.0]hexane structure can be a useful scaffold for developing potent ligands selective for the target biomolecules.

Exploring the molecular determinants for subtype-selectivity of 2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic acid analogs as betaine/GABA transporter 1 (BGT1) substrate-inhibitors

We have previously identified 2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic acid (ATPCA) as the most potent substrate-inhibitor of the betaine/GABA transporter 1 (BGT1) (IC₅₀ 2.5 µM) reported to date. Herein, we characterize the binding mode of 20 novel analogs and propose the molecular determinants driving BGT1-selectivity. A series of N₁-, exocyclic-N-, and C₄-substituted analogs was synthesized and pharmacologically characterized in radioligand-based uptake assays at the four human GABA transporters (hGATs) recombinantly expressed in mammalian cells. Overall, the analogs retained subtype-selectivity for hBGT1, though with lower inhibitory activities (mid to high micromolar IC₅₀ values) compared to ATPCA. Further characterization of five of these BGT1-active analogs in a fluorescence-based FMP assay revealed that the compounds are substrates for hBGT1, suggesting they interact with the orthosteric site of the transporter. In silico-guided mutagenesis experiments showed that the non-conserved residues Q299 and E52 in hBGT1 as well as the conformational flexibility of the compounds potentially contribute to the subtype-selectivity of ATPCA and its analogs. Overall, this study provides new insights into the molecular interactions governing the subtype-selectivity of BGT1 substrate-inhibitors. The findings may guide the rational design of BGT1-selective pharmacological tool compounds for future drug discovery.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

Structural and molecular aspects of betaine-GABA transporter 1 (BGT1) and its relation to brain function

ɣ-aminobutyric-acid (GABA) functions as the principal inhibitory neurotransmitter in the central nervous system. Imbalances in GABAergic neurotransmission are involved in the pathophysiology of various neurological diseases such as epilepsy, Alzheimer's disease and stroke. GABA transporters (GATs) facilitate the termination of GABAergic signaling by transporting GABA together with sodium and chloride from the synaptic cleft into presynaptic neurons and surrounding glial cells. Four different GATs have been identified that all belong to the solute carrier 6 (SLC6) transporter family: GAT1-3 (SLC6A1, SLC6A13, SLC6A11) and betaine/GABA transporter 1 (BGT1, SLC6A12). BGT1 has emerged as an interesting target for treating epilepsy due to animal studies that reported anticonvulsant effects for the GAT1/BGT1 selective inhibitor EF1502 and the BGT1 selective inhibitor RPC-425. However, the precise involvement of BGT1 in epilepsy remains elusive because of its controversial expression levels in the brain and the lack of highly selective and potent tool compounds. This review gathers the current structural and functional knowledge on BGT1 with emphasis on brain relevance, discusses all available compounds, and tries to shed light on the molecular determinants driving BGT1 selectivity. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Feedback adaptation of synaptic excitability via Glu:Na+ symport driven astrocytic GABA and Gln release

Glutamatergic transmission composed of the arriving of action potential at the axon terminal, fast vesicular Glu release, postsynaptic Glu receptor activation, astrocytic Glu clearance and Glu→Gln shuttle is an abundantly investigated phenomenon. Despite its essential role, however, much less is known about the consequences of the mechanistic connotations of Glu:Na⁺ symport. Due to the coupled Na⁺ transport, Glu uptake results in significantly elevated intracellular astrocytic [Na⁺] that markedly alters the driving force of other Na⁺-coupled astrocytic transporters. The resulting GABA and Gln release by reverse transport through the respective GAT-3 and SNAT3 transporters help to re-establish the physiological Na⁺ homeostasis without ATP dissipation and consequently leads to enhanced tonic inhibition and replenishment of axonal glutamate pool. Here, we place this emerging astrocytic adjustment of synaptic excitability into the centre of future perspectives. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Organocatalytic Transannular Approach to Stereodefined Bicyclo[3.1.0]hexanes

A diastereodivergent approach to highly substituted bicyclo[3.1.0]hexanes has been developed through a transannular alkylation reaction that builds up the bicyclic core employing asymmetric organocatalysis as the tool for the installation of all stereocenters. On one hand, a Michael/Michael cascade process between enals and 4-alkenyl sulfamidate imines under the iminium/enamine activation manifold provides an oxathiazole-2,2-dioxide-fused cyclohexane adduct that, after isolation, is subsequently engaged in a transannular alkylation/hydrolysis through enamine activation by the use of a primary amine. On the other hand, the corresponding C-2 epimers are directly obtained from the same starting materials in a single operation through a cascade Michael/Michael/transannular alkylation/hydrolysis sequence through sequential iminium/enamine/enamine combination of aminocatalytic activation manifolds.

From Peptides to Peptidomimetics: A Strategy Based on the Structural Features of Cyclopropane

Peptidomimetics, non-natural mimicries of bioactive peptides, comprise an important class of drug molecules. The essence of the peptidomimetic design is to mimic the key conformation assumed by the bioactive peptides upon binding to their targets. Regulation of the conformation of peptidomimetics is important not only to enhance target binding affinity and selectivity, but also to confer cell-membrane permeability for targeting protein-protein interactions in cells. The rational design of peptidomimetics with suitable three-dimensional structures is challenging, however, due to the inherent flexibility of peptides and their dynamic conformational changes upon binding to the target biomolecules. In this Minireview, a three-dimensional structural diversity-oriented strategy based on the characteristic structural features of cyclopropane to address this challenging issue in peptidomimetic chemistry is described.

Structure-Guided Design of Novel, Potent, and Selective Macrocyclic Plasma Kallikrein Inhibitors

A series of macrocyclic analogues were designed and synthesized based on the cocrystal structure of small molecule plasma kallikrein (pKal) inhibitor, 2, with the pKal protease domain. This led to the discovery of a potent macrocyclic pKal inhibitor 29, with an IC₅₀ of 2 nM for one olefinic isomer and 42.3 nM for the other olefinic isomer.

Glial GABA Transporters as Modulators of Inhibitory Signalling in Epilepsy and Stroke

Imbalances in GABA-mediated tonic inhibition are involved in several pathophysiological conditions. A classical way of controlling tonic inhibition is through pharmacological intervention with extrasynaptic GABA_A receptors that sense ambient GABA and mediate a persistent GABAergic conductance. An increase in tonic inhibition may, however, also be obtained indirectly by inhibiting glial GABA transporters (GATs). These are sodium-coupled membrane transport proteins that normally act to terminate GABA neurotransmitter action by taking up GABA into surrounding astrocytes. The aim of the review is to provide an overview of glial GATs in regulating tonic inhibition, especially in epilepsy and stroke. This entails a comprehensive summary of changes known to occur in GAT expression levels and signalling following epileptic and ischemic insults. Further, we discuss the accumulating pharmacological evidence for targeting GATs in these diseases.

Highly Conformationally Restricted Cyclopropane Tethers with Three-Dimensional Structural Diversity Drastically Enhance the Cell Permeability of Cyclic Peptides

The conformation of cyclic peptides is closely related to their physicochemical and biological properties, but their rational design to obtain a conformation with the desired properties is difficult. Herein, we present a new strategy by using conformationally restricted cyclopropane tethers (CPTs) to control the conformation and improve the cell permeability of cyclic peptides regardless of the amino acid sequence. Newly designed cis- or trans-CPTs with three-dimensional structural diversity were introduced into a model cyclic peptide, and the relationship between the conformation of the cyclic peptides and their cell permeability was analyzed. Peptides containing a CPT exhibited conformational diversity due to the characteristic steric feature of cyclopropane, among which peptides containing a CPT, cis-NfCf had remarkably higher cell permeability than peptides containing other CPTs-even superior to that of cyclosporine A, a known permeable cyclic peptide.

Straightforward and effective synthesis of γ-aminobutyric acid transporter subtype 2-selective acyl-substituted azaspiro[4.5]decanes

Supply of major metabolites such as γ-aminobutyric acid (GABA), β-alanine and taurine is an essential instrument that shapes signalling, proper cell functioning and survival in the brain and peripheral organs. This background motivates the synthesis of novel classes of compounds regulating their selective transport through various fluid-organ barriers via the low-affinity γ-aminobutyric acid (GABA) transporter subtype 2 (GAT2). Natural and synthetic spirocyclic compounds or therapeutics with a range of structures and biological activity are increasingly recognised in this regard. Based on pre-validated GABA transport activity, straightforward and efficient synthesis method was developed to provide an azaspiro[4.5]decane scaffold, holding a variety of charge, substituent and 3D constrain of spirocyclic amine. Investigation of the azaspiro[4.5]decane scaffold in cell lines expressing the four GABA transporter subtypes led to the discovery of a subclass of a GAT2-selective compounds with acyl-substituted azaspiro[4.5]decane core.

Framing Neuro-Glia Coupling in Antiepileptic Drug Design

We delineate perspectives for the design and discovery of antiepileptic drugs (AEDs) with fewer side effects by focusing on astroglial modulation of spatiotemporal seizure dynamics. It is now recognized that the major inhibitory neurotransmitter of the brain, γ-aminobutyric acid (GABA), can be released through the reversal of astroglial GABA transporters. Synaptic spillover and subsequent glutamate (Glu) uptake in neighboring astrocytes evoke replacement of extracellular Glu for GABA, driving neurons away from the seizure threshold. Attenuation of synaptic signaling by this negative feedback through the interplay of Glu and GABA transporters of adjacent astroglia can result in shortened seizures. By contrast, long-range activation of astroglia through gap junctions may promote recurrent seizures on the model of pharmacoresistant temporal lobe epilepsy. From their first detection to our current understanding, we identify various targets that shape both short- and long-range neuro-astroglia coupling, as these are manifest in epilepsy phenomena and in the associated research promotions of AED.