Opportunities and challenges in the discovery of allosteric modulators of GPCRs for treating CNS disorders

Binding sites and design strategies for small molecule GLP-1R agonists

Glucagon-like peptide-1 receptor (GLP-1R) is a pivotal receptor involved in blood glucose regulation and influencing feeding behavior. It has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. Peptide GLP-1 receptor agonists (GLP-1RAs) have achieved tremendous success in the market, driving the vigorous development of small molecule GLP-1RAs. Currently, several small molecules have entered the clinical research stage. Additionally, recent discoveries of GLP-1R positive allosteric modulators (PAMs) are also unveiling new regulatory patterns and treatment methods. This article reviews the structure and functional mechanisms of GLP-1R, recent reports on small molecule GLP-1RAs and PAMs, as well as the optimization process. Furthermore, it combines computer simulations to analyze structure-activity relationships (SAR) studies, providing a foundation for exploring new strategies for designing small molecule GLP-1RAs.

Monitoring GPCR conformation with GFP-inspired dyes

Solvatochromic compounds have emerged as valuable environment-sensitive probes for biological research. Here we used thiol-reactive solvatochromic analogs of the green fluorescent protein (GFP) chromophore to track conformational changes in two proteins, recoverin and the A2A adenosine receptor (A2AAR). Two dyes showed Ca2+-induced fluorescence changes when attached to recoverin. Our best-performing dye, DyeC, exhibited agonist-induced changes in both intensity and shape of its fluorescence spectrum when attached to A2AAR; none of these effects were observed with other common environment-sensitive dyes. Molecular dynamics simulations showed that activation of the A2AAR led to a more confined and hydrophilic environment for DyeC. Additionally, an allosteric modulator of A2AAR induced distinct fluorescence changes in the DyeC spectrum, indicating a unique receptor conformation. Our study demonstrated that GFP-inspired dyes are effective for detecting structural changes in G protein-coupled receptors (GPCRs), offering advantages such as intensity-based and ratiometric tracking, redshifted fluorescence spectra, and sensitivity to allosteric modulation.

Phenolic Constituents with Glucose Uptake and GLUT4 Translocation Bioactivities from the Fruits of Cordia dichotoma

From the fruits of Cordia dichotoma, 11 new phenolic compounds, dichotomins A-K, were isolated, together with 19 known compounds. Through the analysis of detailed NMR data and HRESIMS data, the planar structures of all compounds were confirmed. Using NMR calculations, the absolute configuration of dichotomins A-K was elucidated by comparing their observed and computed electronic circular dichroism (ECD) spectra. Dichotomin H (8) and dichotomin I (9) were determined as two pairs of enantiomers. The enantiomers of compounds 8 and 9 were separated using chiral-phase high-performance liquid chromatography (HPLC), and the stereostructure of each enantiomer was determined by similarly calculating the ECD. Compounds 3, 5, 7, 17, 18, 23-25, and 27-30 increased glucose uptake by 1.04- to 2.85-folds at concentrations of 30 μg/mL. Further studies revealed that compounds 3 and 5 had a moderate effect on glucose transporter 4 (GLUT4) translocation activity in L6 cells. At 30 μg/mL, compound 3 significantly enhanced AMPK phosphorylation and GLUT4 expression. As a whole, compound 3 has the potential to be a drug candidate for the treatment of type 2 diabetes mellitus (T2DM).

SCA44- and SCAR13-associated GRM1 mutations affect metabotropic glutamate receptor 1 function through distinct mechanisms

BACKGROUND AND PURPOSE

Metabotropic glutamate receptor 1 (mGlu1) is a promising therapeutic target for neurodegenerative CNS disorders including spinocerebellar ataxias (SCAs). Clinical reports have identified naturally-occurring mGlu1 mutations in rare SCA subtypes and linked symptoms to mGlu1 mutations. However, how mutations alter mGlu1 function remains unknown, as does amenability of receptor function to pharmacological rescue. Here, we explored SCA-associated mutation effects on mGlu1 cell surface expression, canonical signal transduction and allosteric ligand pharmacology.

EXPERIMENTAL APPROACH

Orthosteric agonists, positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) were assessed at two functional endpoints (iCa2+ mobilisation and inositol 1-phosphate [IP1] accumulation) in FlpIn Trex HEK293A cell lines expressing five mutant mGlu1 subtypes. Key pharmacological parameters including ligand potency, affinity and cooperativity were derived using operational models of agonism and allostery.

KEY RESULTS

mGlu1 mutants exhibited differential impacts on mGlu1 expression, with a C-terminus truncation significantly reducing surface expression. Mutations differentially influenced orthosteric ligand affinity, efficacy and functional cooperativity between allosteric and orthosteric ligands. Loss-of-function mutations L454F and N885del reduced orthosteric affinity and efficacy, respectively. A gain-of-function Y792C mutant mGlu1 displayed enhanced constitutive activity in IP1 assays, which manifested as reduced orthosteric agonist activity. The mGlu1 PAMs restored glutamate potency in iCa2+ mobilisation for loss-of-function mutations and mGlu1 NAMs displayed enhanced inverse agonist activity at Y792C relative to wild-type mGlu1.

CONCLUSION AND IMPLICATIONS

Collectively, these data highlight distinct mechanisms by which mGlu1 mutations affect receptor function and show allosteric modulators may present a therapeutic strategy to restore aberrant mGlu1 function in rare SCA subtypes.

Advances and Insights in Positron Emission Tomography Tracers for Metabotropic Glutamate Receptor 4 Imaging

Positron emission tomography (PET) imaging employs positron-emitting radioisotopes to visualize biological processes in living subjects with high sensitivity and quantitative accuracy. As the most translational molecular imaging modality, PET can detect and image a wide range of radiotracers with minimal or no modification to parent drugs or targeting molecules. This Perspective provides a comprehensive analysis of developing PET radioligands using allosteric modulators for the metabotropic glutamate receptor subtype 4 (mGluR4) as a therapeutic target for neurological disorders. We focus on the selection of lead compounds from various chemotypes of mGluR4 positive allosteric modulators (PAMs) and discuss the challenges and systematic characterization required in developing brain-penetrant PET tracers specific for mGluR4. Through this analysis, we offer insights into the development and evaluation of PET ligands. Our review concludes that further research and development in this field hold great promise for discovering effective treatments for neurological disorders.

ERNEST COST action overview on the (patho)physiology of GPCRs and orphan GPCRs in the nervous system

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a critical role in nervous system function by transmitting signals between cells and their environment. They are involved in many, if not all, nervous system processes, and their dysfunction has been linked to various neurological disorders representing important drug targets. This overview emphasises the GPCRs of the nervous system, which are the research focus of the members of ERNEST COST action (CA18133) working group 'Biological roles of signal transduction'. First, the (patho)physiological role of the nervous system GPCRs in the modulation of synapse function is discussed. We then debate the (patho)physiology and pharmacology of opioid, acetylcholine, chemokine, melatonin and adhesion GPCRs in the nervous system. Finally, we address the orphan GPCRs, their implication in the nervous system function and disease, and the challenges that need to be addressed to deorphanize them.

Novel-Type GABAB PAMs: Structure-Activity Relationship in Light of the Protein Structure

Selecting a known HTS hit with the pyrazolo[1,5-a]pyrimidine core, our project was started from CMPPE, and its optimization was driven by a ligand-based pharmacophore model developed on the basis of published GABAB positive allosteric modulators (PAMs). Our primary goal was to improve the potency by finding new enthalpic interactions. Therefore, we included the lipophilic ligand efficiency (LLE or LipE) as an objective function in the optimization that led to a carboxylic acid derivative (34). This lead candidate offers the possibility to improve potency without drastically inflating the physicochemical properties. Although the discovery of the novel carboxyl feature was surprising, it turned out to be an important element of the GABAB PAM pharmacophore that can be perfectly explained based on the new protein structures. Rationalizing the binding mode of 34, we analyzed the intersubunit PAM binding site of GABAB receptor using the publicly available experimental structures.

In vitro evaluation of PET radiotracers for imaging synaptic density, the acetylcholine transporter, AMPA-tarp-γ8 and muscarinic M4 receptors in Alzheimer's disease

Several therapeutics and biomarkers that target Alzheimer's disease (AD) are under development. Our clinical positron emission tomography (PET) research programs are interested in six radiopharmaceuticals to image patients with AD and related dementias, specifically [11C]UCB-J and [18F]SynVesT-1 for synaptic vesicle glycoprotein 2A as a marker of synaptic density, two vesicular acetylcholine transporter PET radiotracers: [18F]FEOBV and [18F]VAT, as well as the transmembrane AMPA receptor regulatory protein (TARP)-γ8 tracer, [18F]JNJ-64511070, and the muscarinic acetylcholine receptor (mAChR) M4 tracer [11C]MK-6884. The goal of this study was to compare all six radiotracers (labeled with tritium or 18F) by measuring their density variability in pathologically diagnosed cases of AD, mild cognitive impairment (MCI) and normal healthy volunteer (NHV) human brains, using thin-section in vitro autoradiography (ARG). Region of interest analysis was used to quantify radioligand binding density and determine whether the radioligands provide a signal-to-noise ratio optimal for showing changes in binding. Our preliminary study confirmed that all six radiotracers show specific binding in MCI and AD. An expected decrease in their respective target density in human AD hippocampus tissues compared to NHV was observed with [3H]UCB-J, [3H]SynVesT-1, [3H]JNJ-64511070, and [3H]MK-6884. This preliminary study will be used to guide human PET imaging of SV2A, TARP-γ8 and the mAChR M4 subtype for imaging in AD and related dementias.

Regulation of neuronal circHomer1 biogenesis by PKA/CREB/ERK-mediated pathways and effects of glutamate and dopamine receptor blockade

There are currently only very few efficacious drug treatments for SCZ and BD, none of which can significantly ameliorate cognitive symptoms. Thus, further research is needed in elucidating molecular pathways linked to cognitive function and antipsychotic treatment. Circular RNAs (circRNAs) are stable brain-enriched non-coding RNAs, derived from the covalent back-splicing of precursor mRNA molecules. CircHomer1 is a neuronal-enriched, activity-dependent circRNA, derived from the precursor of the long HOMER1B mRNA isoform, which is significantly downregulated in the prefrontal cortex of subjects with psychosis and is able to regulate cognitive function. Even though its relevance to psychiatric disorders and its role in brain function and synaptic plasticity have been well established, little is known about the molecular mechanisms that underlie circHomer1 biogenesis in response to neuronal activity and psychiatric drug treatment. Here we suggest that the RNA-binding protein (RBP) FUS positively regulates neuronal circHomer1 expression. Furthermore, we show that the MEK/ERK and PKA/CREB pathways positively regulate neuronal circHomer1 expression, as well as promote the transcription of Fus and Eif4a3, another RBP previously shown to activate circHomer1 biogenesis. We then demonstrate via both in vitro and in vivo studies that NMDA and mGluR5 receptors are upstream modulators of circHomer1 expression. Lastly, we report that in vivo D2R antagonism increases circHomer1 expression, whereas 5HT2AR blockade reduces circHomer1 levels in multiple brain regions. Taken together, this study allows us to gain novel insights into the molecular circuits that underlie the biogenesis of a psychiatric disease-associated circRNA.

Evolving views on the first two ligands of the angiotensin II type 2 receptor. From putative antagonists to potential agonists?

The renin-angiotensin system is one of the most complex regulatory systems that controls multiple organ functions. One of its key components, angiotensin II (Ang II), stimulates two G-protein coupled class A receptors: the Ang II type 1 (AT1) receptor and the Ang II type 2 (AT2) receptor. While stimulation of the AT1 receptor causes G-protein-dependent signaling and arrestin recruitment, the AT2 receptor seems to have a constitutively active-like conformation and appears to act via G-protein-dependent and -independent pathways. Overstimulation of the AT1 receptor may lead to unwanted effects like inflammation and fibrosis. In contrast, stimulation of the AT2 receptor leads to opposite effects thus restoring the balance. However, the role of the AT2 receptor has become controversial due to beneficial effects of putative AT2 receptor antagonists. The two first synthetic AT2 receptor-selective ligands, peptide CGP42112 and small molecule PD123319, were initially both considered antagonists. CGP42112 was subsequently considered a partial agonist and it was recently demonstrated to be a full agonist. Based on the search-term PD123319 in Pubmed, 1652 studies have investigated putative AT2 receptor antagonist PD123319. Here, we put forward literature that shows beneficial effects of PD123319 alone, even at doses too low for antagonist efficacy. These beneficial effects appear compatible with agonist-like activity via the AT2 receptor. Taken together, a more consistent image of a therapeutic role of stimulated AT2 receptor emerges which may clarify current controversies.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

The Lateral Metalation of Isoxazolo[3,4-d]pyridazinones towards Hit-to-Lead Development of Selective Positive Modulators of Metabotropic Glutamate Receptors

Isoxazolo[3,4-d] pyridazinones ([3,4-d]s) were previously shown to have selective positive modulation at the metabotropic glutamate receptor (mGluR) Subtypes 2 and 4, with no functional cross-reactivity at mGluR1a, mGluR5, or mGluR8. Additional analogs were prepared to access more of the allosteric pocket and achieve higher binding affinity, as suggested by homology modeling. Two different sets of analogs were generated. One uses the fully formed [3,4-d] with an N6-aryl with and without halogens. These underwent successful selective lateral metalation and electrophilic quenching (LM&EQ) at the C3 of the isoxazole. In a second set of analogs, a phenyl group was introduced at the C4 position of the [3,4-d] ring via a condensation of 4-phenylacetyl-3-ethoxcarbonyl-5-methyl isoxazole with the corresponding hydrazine to generate the 3,4-ds 2b and 2j to 2n.

Structural dynamics of chemokine receptors

Membrane proteins such as G protein-coupled receptors (GPCRs) are involved in awide range of physiological and pathological cellular processes. Binding of extracellular signals to GPCRs, including hormones, neurotransmitters, peptides and proteins, can activate intracellular signaling cascades via G protein interaction. Chemokine receptors are key GPCRs implicated in cancers, immune responses, cell migration and inflammation. Specifically, the CCR5 and CXCR4 chemokine receptors serve as important therapeutic targets against Human Immunodeficiency virus (HIV) entry into human cells. Maraviroc and Vicriviroc, two clinically used HIV entry inhibitors, are antagonists of the CCR5 receptor. These drugs block HIV entry, but ultimately resistance develops, due to emergence of viruses that can utilize the CXCR4 co-receptor. Unfortunately, development of chemokine receptor antagonists as selective drugs of HIV infection has been greatly hindered as their target orthosteric site is conserved among different receptor subtypes. Accordingly, it is important to understand the structural dynamics of these receptors to develop more effective therapeutics. In this chapter, we describe the latest advances in studies of these two key chemokine receptors with respect to their structures, dynamics and function.

Characterization of a Novel M4 PAM PET Radioligand [11C]PF06885190 in Nonhuman Primates (NHP)

Muscarinic acetylcholine receptors (mAChR), including M4, draw attention as therapeutic targets for several neurodegenerative diseases including Alzheimer's disease (AD). PET imaging of M4 positive allosteric modulator (PAM) allows qualification of the distribution as well as the expression of this receptor under physiological conditions and thereby helps to assess the receptor occupancy (RO) of a drug candidate. In this study, our aims were (a) to synthesize a novel M4 PAM PET radioligand [11C]PF06885190 (b) to evaluate the brain distribution of [¹¹C]PF06885190 in nonhuman primates (NHP) and (c) to analyze its radiometabolites in the blood plasma of NHP. Radiolabeling of [¹¹C]PF06885190 was accomplished via N-methylation of the precursor. Six PET measurements were performed using two male cynomolgus monkeys, where three PET measurements were at baseline, two after pretreatment with a selective M4 PAM compound CVL-231 and one after pretreatment with donepezil. The total volume of distribution (V_T) of [¹¹C]PF06885190 was examined using Logan graphical analysis with arterial input function. Radiometabolites were analyzed in monkey blood plasma using gradient HPLC system. Radiolabeling of [¹¹C]PF06885190 was successfully accomplished and the radioligand was found to be stable in the formulation, with radiochemical purity exceeding 99% 1 h after the end of the synthesis. [¹¹C]PF06885190 was characterized in the cynomolgus monkey brain where a moderate brain uptake was found at the baseline condition. However, it showed fast wash-out as it dropped to half of the peak at around 10 min. Change of V_T from baseline was around -10% after pretreatment with a M4 PAM, CVL-231. Radiometabolite studies showed relatively fast metabolism. Although sufficient brain uptake of [¹¹C]PF06885190 was observed, these data suggest that [¹¹C]PF06885190 might have too low specific binding in the NHP brain to be further applied in PET imaging.

Research progress of indole-fused derivatives as allosteric modulators: Opportunities for drug development

Allosteric modulation is a direct and effective method for regulating the function of biological macromolecules, which play vital roles in various cellular activities. Unlike orthosteric modulators, allosteric modulators bind to sites distant from the protein's orthosteric/active site and can have specific effects on the protein's function or activity without competing with endogenous ligands. Compared to traditional orthosteric modulators, allosteric modulators offer several advantages, including reduced side effects, greater specificity, and lower toxicity, making them a promising strategy for developing novel drugs. Indole-fused architectures are widely distributed in natural products and bioactive drug leads, displaying diverse biological activities that attract the interest of both chemists and biologists in drug discovery. Currently, an increasing number of indole-fused compounds have exhibited potent activities in allosteric modulation. In this review, we provide a brief summary of examples of allosteric modulators based on the indole-fused complex architecture, highlighting the strategies for drug design/discovery and the structure-activity relationships of allosteric modulators from the perspective of medicinal chemistry.

The pyridazine heterocycle in molecular recognition and drug discovery

The pyridazine ring is endowed with unique physicochemical properties, characterized by weak basicity, a high dipole moment that subtends π-π stacking interactions and robust, dual hydrogen-bonding capacity that can be of importance in drug-target interactions. These properties contribute to unique applications in molecular recognition while the inherent polarity, low cytochrome P450 inhibitory effects and potential to reduce interaction of a molecule with the cardiac hERG potassium channel add additional value in drug discovery and development. The recent approvals of the gonadotropin-releasing hormone receptor antagonist relugolix (24) and the allosteric tyrosine kinase 2 inhibitor deucravacitinib (25) represent the first examples of FDA-approved drugs that incorporate a pyridazine ring. In this review, the properties of the pyridazine ring are summarized in comparison to the other azines and its potential in drug discovery is illustrated through vignettes that explore applications that take advantage of the inherent physicochemical properties as an approach to solving challenges associated with candidate optimization.

Graphical Abstract

Allosteric modulation of a human odorant receptor

Odor perception is first determined by how the myriad of environmental volatiles are detected at the periphery of the olfactory system. The combinatorial activation of dedicated odorant receptors generates enough encoding power for the discrimination of tens of thousands of odorants. Recent studies have revealed that odorant receptors undergo widespread inhibitory modulation of their activity when presented with mixtures of odorants, a property likely required to maintain discrimination and ensure sparsity of the code for complex mixtures. Here, we establish the role of human OR5AN1 in the detection of musks and identify distinct odorants capable of enhancing its activity in binary mixtures. Chemical and pharmacological characterization indicate that specific α-β unsaturated aliphatic aldehydes act as positive allosteric modulators. Sensory experiments show decreased odor detection threshold in humans, suggesting that allosteric modulation of odorant receptors is perceptually relevant and likely adds another layer of complexity to how odors are encoded in the peripheral olfactory system.

PET imaging studies to investigate functional expression of mGluR2 using [11C]mG2P001

Metabotropic glutamate receptor 2 (mGluR2) has been extensively studied for the treatment of various neurological and psychiatric disorders. Understanding of the mGluR2 function is pivotal in supporting the drug discovery targeting mGluR2. Herein, the positive allosteric modulation of mGluR2 was investigated via the in vivo positron emission tomography (PET) imaging using 2-((4-(2-[11C]methoxy-4-(trifluoromethyl)phenyl)piperidin-1-yl)methyl)-1-methyl-1H-imidazo[4,5-b]pyridine ([11C]mG2P001). Distinct from the orthosteric compounds, pretreatment with the unlabeled mG2P001, a potent mGluR2 positive allosteric modulator (PAM), resulted in a significant increase instead of decrease of the [11C]mG2P001 accumulation in rat brain detected by PET imaging. Subsequent in vitro studies with [3H]mG2P001 revealed the cooperative binding mechanism of mG2P001 with glutamate and its pharmacological effect that contributed to the enhanced binding of [3H]mG2P001 in transfected CHO cells expressing mGluR2. The in vivo PET imaging and quantitative analysis of [11C]mG2P001 in non-human primates (NHPs) further validated the characteristics of [11C]mG2P001 as an imaging ligand for mGluR2. Self-blocking studies in primates enhanced accumulation of [11C]mG2P001. Altogether, these studies show that [11C]mG2P001 is a sensitive biomarker for mGluR2 expression and the binding is affected by the tissue glutamate concentration.

Identification and characterization of select oxysterols as ligands for GPR17

BACKGROUND AND PURPOSE

G-protein coupled receptor 17 (GPR17) is an orphan receptor involved in the process of myelination, due to its ability to inhibit the maturation of oligodendrocyte progenitor cells (OPCs) into myelinating oligodendrocytes. Despite multiple claims that the biological ligand has been identified, it remains an orphan receptor.

EXPERIMENTAL APPROACH

Seventy-seven oxysterols were screened in a cell-free [³⁵ S]GTPγS binding assay using membranes from cells expressing GPR17. The positive hits were characterized using adenosine 3',5' cyclic monophosphate (cAMP), inositol monophosphate (IP1) and calcium mobilization assays, with results confirmed in rat primary oligodendrocytes. Rat and pig brain extracts were separated by high-performance liquid chromatography (HPLC) and endogenous activator(s) were identified in receptor activation assays. Gene expression studies of GPR17, and CYP46A1 (cytochrome P450 family 46 subfamily A member 1) enzymes responsible for the conversion of cholesterol into specific oxysterols, were performed using quantitative real-time PCR.

KEY RESULTS

Five oxysterols were able to stimulate GPR17 activity, including the brain cholesterol, 24(S)-hydroxycholesterol (24S-HC). A specific brain fraction from rat and pig extracts containing 24S-HC activates GPR17 in vitro. Expression of Gpr17 during mouse brain development correlates with the expression of Cyp46a1 and the levels of 24S-HC itself. Other active oxysterols have low brain concentrations below effective ranges.

CONCLUSIONS AND IMPLICATIONS

Oxysterols, including but not limited to 24S-HC, could be physiological activators for GPR17 and thus potentially regulate OPC differentiation and myelination through activation of the receptor.

Preventing incubation of drug craving to treat drug relapse: from bench to bedside

In 1986, Gawin and Kleber reported a progressive increase in cue-induced drug craving in individuals with cocaine use disorders during prolonged abstinence. After years of controversy, as of 2001, this phenomenon was confirmed in rodent studies using self-administration model, and defined as the incubation of drug craving. The intensification of cue-induced drug craving after withdrawal exposes abstinent individuals to a high risk of relapse, which urged us to develop effective interventions to prevent incubated craving. Substantial achievements have been made in deciphering the neural mechanisms, with potential implications for reducing drug craving and preventing the relapse. The present review discusses promising drug targets that have been well investigated in animal studies, including some neurotransmitters, neuropeptides, neurotrophic factors, and epigenetic markers. We also discuss translational exploitation and challenges in the field of the incubation of drug craving, providing insights into future investigations and highlighting the potential of pharmacological interventions, environment-based interventions, and neuromodulation techniques.

Structure-activity relationship of pyrazol-4-yl-pyridine derivatives and identification of a radiofluorinated probe for imaging the muscarinic acetylcholine receptor M4

There is an accumulating body of evidence implicating the muscarinic acetylcholine receptor 4 (M₄) in schizophrenia and dementia with Lewy bodies, however, a clinically validated M₄ positron emission tomography (PET) radioligand is currently lacking. As such, the aim of this study was to develop a suitable M₄ PET ligand that allows the non-invasive visualization of M₄ in the brain. Structure-activity relationship studies of pyrazol-4-yl-pyridine derivates led to the discovery of target compound 12 - a subtype-selective positive allosteric modulator (PAM). The radiofluorinated analogue, [¹⁸F]12, was synthesized in 28 ± 10% radiochemical yield, >37 GBq/μmol and an excellent radiochemical purity >99%. Initial in vitro autoradiograms on rodent brain sections were performed in the absence of carbachol and showed moderate specificity as well as a low selectivity of [¹⁸F]12 for the M₄-rich striatum. However, in the presence of carbachol, a significant increase in tracer binding was observed in the rat striatum, which was reduced by >60% under blocking conditions, thus indicating that orthosteric ligand interaction is required for efficient binding of [¹⁸F]12 to the allosteric site. Remarkably, however, the presence of carbachol was not required for high specific binding in the non-human primate (NHP) and human striatum, and did not further improve the specificity and selectivity of [¹⁸F]12 in higher species. These results pointed towards significant species-differences and paved the way for a preliminary PET study in NHP, where peak brain uptake of [¹⁸F]12 was found in the putamen and temporal cortex. In conclusion, we report on the identification and preclinical development of the first radiofluorinated M₄ PET radioligand with promising attributes. The availability of a clinically validated M₄ PET radioligand harbors potential to facilitate drug development and provide a useful diagnostic tool for non-invasive imaging.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Membrane Lipids Are an Integral Part of Transmembrane Allosteric Sites in GPCRs: A Case Study of Cannabinoid CB1 Receptor Bound to a Negative Allosteric Modulator, ORG27569, and Analogs

A growing number of G-protein-coupled receptor (GPCR) structures reveal novel transmembrane lipid-exposed allosteric sites. Ligands must first partition into the surrounding membrane and take lipid paths to these sites. Remarkably, a significant part of the bound ligands appears exposed to the membrane lipids. The experimental structures do not usually account for the surrounding lipids, and their apparent contribution to ligand access and binding is often overlooked and poorly understood. Using classical and enhanced molecular dynamics simulations, we show that membrane lipids are critical in the access and binding of ORG27569 and its analogs at the transmembrane site of cannabinoid CB1 receptor. The observed differences in the binding affinity and cooperativity arise from the functional groups that interact primarily with lipids. Our results demonstrate the significance of incorporating membrane lipids as an integral component of transmembrane sites for accurate characterization, binding-affinity calculations, and lead optimization in drug discovery.

Allostery: Allosteric Cancer Drivers and Innovative Allosteric Drugs

Here, we discuss the principles of allosteric activating mutations, propagation downstream of the signals that they prompt, and allosteric drugs, with examples from the Ras signaling network. We focus on Abl kinase where mutations shift the landscape toward the active, imatinib binding-incompetent conformation, likely resulting in the high affinity ATP outcompeting drug binding. Recent pharmacological innovation extends to allosteric inhibitor (GNF-5)-linked PROTAC, targeting Bcr-Abl1 myristoylation site, and broadly, allosteric heterobifunctional degraders that destroy targets, rather than inhibiting them. Designed chemical linkers in bifunctional degraders can connect the allosteric ligand that binds the target protein and the E3 ubiquitin ligase warhead anchor. The physical properties and favored conformational state of the engineered linker can precisely coordinate the distance and orientation between the target and the recruited E3. Allosteric PROTACs, noncompetitive molecular glues, and bitopic ligands, with covalent links of allosteric ligands and orthosteric warheads, increase the effective local concentration of productively oriented and placed ligands. Through covalent chemical or peptide linkers, allosteric drugs can collaborate with competitive drugs, degrader anchors, or other molecules of choice, driving innovative drug discovery.

Advances in translating mGlu2 and mGlu3 receptor selective allosteric modulators as breakthrough treatments for affective disorders and alcohol use disorder

Metabotropic glutamate (mGlu) receptors are promising targets for the treatment of affective disorders and alcohol use disorder (AUD). Nonspecific ligands for Group II (mGlu2 and mGlu3) mGlu receptors have demonstrated consistent therapeutic potential for affective disorders in preclinical models. Disentangling the specific roles of mGlu2 versus mGlu3 receptors in these effects has persisted as a major challenge, in part due to pharmacological limitations. However, the recent development of highly specific allosteric modulators for both mGlu2 and mGlu3 receptors have enabled straightforward and rigorous investigations into the specific function of each receptor. Here, we review recent experiments using these compounds that have demonstrated both similar and distinct receptor functions in behavioral, molecular, and electrophysiological measures associated with basal function and preclinical models of affective disorders. Studies using these selective drugs have demonstrated that mGlu2 is the predominant receptor subclass involved in presynaptic neurotransmitter release in prefrontal cortex. By contrast, the activation of postsynaptic mGlu3 receptors induces a cascade of cellular changes that results in AMPA receptor internalization, producing long-term depression and diminishing excitatory drive. Acute stress decreases the mGlu3 receptor function and dynamically alters transcript expression for both mGlu2 (Grm2) and mGlu3 (Grm3) receptors in brain areas involved in reward and stress. Accordingly, both mGlu2 and mGlu3 negative allosteric modulators show acute antidepressant-like effects and potential prophylactic effects against acute and traumatic stressors. The wide array of effects displayed by these new allosteric modulators of mGlu2 and mGlu3 receptors suggest that these drugs may act through improving endophenotypes of symptoms observed across several neuropsychiatric disorders. Therefore, recently developed allosteric modulators selective for mGlu2 or mGlu3 receptors show promise as potential therapeutics for affective disorders and AUD.

2-(Fluoromethoxy)-4'-(S-methanesulfonimidoyl)-1,1'-biphenyl (UCM-1306), an Orally Bioavailable Positive Allosteric Modulator of the Human Dopamine D1 Receptor for Parkinson's Disease

Tolerance development caused by dopamine replacement with l-DOPA and therapeutic drawbacks upon activation of dopaminergic receptors with orthosteric agonists reveal a significant unmet need for safe and effective treatment of Parkinson’s disease. In search for selective modulators of the D₁ receptor, the screening of a chemical library and subsequent medicinal chemistry program around an identified hit resulted in new synthetic compound 26 [UCM-1306, 2-(fluoromethoxy)-4′-(S-methanesulfonimidoyl)-1,1′-biphenyl] that increases the dopamine maximal effect in a dose-dependent manner in human and mouse D₁ receptors, is inactive in the absence of dopamine, modulates dopamine affinity for the receptor, exhibits subtype selectivity, and displays low binding competition with orthosteric ligands. The new allosteric modulator potentiates cocaine-induced locomotion and enhances l-DOPA recovery of decreased locomotor activity in reserpinized mice after oral administration. The behavior of compound 26 supports the interest of a positive allosteric modulator of the D₁ receptor as a promising therapeutic approach for Parkinson’s disease.

G protein coupling and activation of the metabotropic GABAB heterodimer

Metabotropic γ-aminobutyric acid receptor (GABA_BR), a class C G protein-coupled receptor (GPCR) heterodimer, plays a crucial role in the central nervous system. Cryo-electron microscopy studies revealed a drastic conformational change upon activation and a unique G protein (GP) binding mode. However, little is known about the mechanism for GP coupling and activation for class C GPCRs. Here, we use molecular metadynamics computations to predict the mechanism by which the inactive GP induces conformational changes in the GABA_BR transmembrane domain (TMD) to form an intermediate pre-activated state. We find that the inactive GP first interacts with TM3, which further leads to the TMD rearrangement and deeper insertion of the α5 helix that causes the Gα subunit to open, releasing GDP, and forming the experimentally observed activated structure. This mechanism provides fresh insights into the mechanistic details of class C GPCRs activation expected to be useful for designing selective agonists and antagonists.

Despite its crucial role in the central nervous system, little is known about the activation mechanism of GABA_B receptor. Here, the authors predict that the inactive G protein induces conformational changes of the receptor to form an intermediate state.

Opioid ligands addressing unconventional binding sites and more than one opioid receptor subtype

Opioid receptors (ORs) represent one of the most significant groups of G-protein coupled receptor (GPCR) drug targets and also act as prototypical models for GPCR function. In a constant effort to develop drugs with less side effects, and tools to explore the ORs nature and function, various (poly)pharmacological ligand design approaches have been performed. That is, besides classical ligands, a great number of bivalent ligands (i.e. aiming on two distinct OR subtypes), univalent heteromer-selective ligands and bitopic and allosteric ligands have been synthesized for the ORs. The scope of our review is to present the most important of the aforementioned ligands, highlight their properties and exhibit the current state-of-the-art pallet of promising drug candidates or useful molecular tools for the ORs.

Targeting chemokine receptors from the inside-out: discovery and development of small-molecule intracellular antagonists

Ever since the first biologically active chemokines were discovered in the late 1980s, these messenger proteins and their receptors have been the target for a plethora of drug discovery efforts in the pharmaceutical industry, as well as in academia. Owing to the publication of several chemokine receptor X-ray crystal structures, a highly druggable, intracellular, allosteric binding site which partially overlaps with the G protein binding site was discovered. This intriguing, new approach for chemokine receptor antagonism has captured researchers around the world, pushing the exploration of this intracellular binding site and new antagonists thereof. In this review, we have highlighted the past two decades of research on small-molecule chemokine receptor antagonists that modulate receptor function at the intracellular binding site.

Discovery, X-ray Crystallography, and Anti-inflammatory Activity of Bromodomain-containing Protein 4 (BRD4) BD1 Inhibitors Targeting a Distinct New Binding Site

Bromodomain-containing protein 4 (BRD4) is an emerging epigenetic drug target for intractable inflammatory disorders. The lack of highly selective inhibitors among BRD4 family members has stalled the collective understanding of this critical system and the progress toward clinical development of effective therapeutics. Here we report the discovery of a potent BRD4 bromodomain 1 (BD1)-selective inhibitor ZL0590 (52) targeting a unique, previously unreported binding site, while exhibiting significant anti-inflammatory activities in vitro and in vivo. The X-ray crystal structural analysis of ZL0590 in complex with human BRD4 BD1 and the associated mutagenesis study illustrate a first-in-class nonacetylated lysine (KAc) binding site located at the helix αB and αC interface that contains important BRD4 residues (e.g., Glu151) not commonly shared among other family members and is spatially distinct from the classic KAc recognition pocket. This new finding facilitates further elucidation of the complex biology underpinning bromodomain specificity among BRD4 and its protein-protein interaction partners.

Positive allosteric modulation of endogenous delta opioid receptor signaling in the enteric nervous system is a potential treatment for gastrointestinal motility disorders

Allosteric modulators (AMs) are molecules that can fine-tune signaling by G protein-coupled receptors (GPCRs). Although they are a promising therapeutic approach for treating a range of disorders, allosteric modulation of GPCRs in the context of the enteric nervous system (ENS) and digestive dysfunction remains largely unexplored. This study examined allosteric modulation of the delta opioid receptor (DOR) in the ENS and assessed the suitability of DOR AMs for the treatment of irritable bowel syndrome (IBS) symptoms using mouse models. The effects of the positive allosteric modulator (PAM) of DOR, BMS-986187, on neurogenic contractions of the mouse colon and on DOR internalization in enteric neurons were quantified. The ability of BMS-986187 to influence colonic motility was assessed both in vitro and in vivo. BMS-986187 displayed DOR-selective PAM-agonist activity and orthosteric agonist probe dependence in the mouse colon. BMS-986187 augmented the inhibitory effects of DOR agonists on neurogenic contractions and enhanced reflex-evoked DOR internalization in myenteric neurons. BMS-986187 significantly increased DOR endocytosis in myenteric neurons in response to the weakly internalizing agonist ARM390. BMS-986187 reduced the generation of complex motor patterns in the isolated intact colon. BMS-986187 reduced fecal output and diarrhea onset in the novel environment stress and castor oil models of IBS symptoms, respectively. DOR PAMs enhance DOR-mediated signaling in the ENS and have potential benefit for the treatment of dysmotility. This study provides proof of concept to support the use of GPCR AMs for the treatment of gastrointestinal motility disorders.NEW & NOTEWORTHY This study assesses the use of positive allosteric modulation as a pharmacological approach to enhance opioid receptor signaling in the enteric nervous system. We demonstrate that selective modulation of endogenous delta opioid receptor signaling can suppress colonic motility without causing constipation. We propose that allosteric modulation of opioid receptor signaling may be a therapeutic strategy to normalize gastrointestinal motility in conditions such as irritable bowel syndrome.

Discovery of "Molecular Switches" within a Series of mGlu5 Allosteric Ligands Driven by a "Magic Methyl" Effect Affording Both PAMs and NAMs with In Vivo Activity, Derived from an M1 PAM Chemotype

In the course of optimizing an M₁ PAM chemotype, introduction of an ether moiety unexpectedly abolished M₁ PAM activity while engendering a "molecular switch" to afford a weak, pure mGlu₅ PAM. Further optimization was able to deliver a potent (mGlu₅ EC₅₀ = 520 nM, 63% Glu Max), centrally penetrant (K_p = 0.83), MPEP-site binding mGlu₅ PAM 17a (VU6036486) that reversed amphetamine-induced hyperlocomotion. A pronounced "magic methyl" effect was noted with a regioisomeric methyl congener, leading to a change in pharmacology to afford a potent (mGlu₅ IC₅₀ = 110 nM, 3% Glu Min), centrally penetrant (K_p = 0.94), MPEP-site binding NAM 28d (VU6044766) that displayed anxiolytic activity in a mouse marble burying assay. These data further support the growing body of literature concerning the existence of G protein-coupled receptor (GPCR) allosteric privileged structures, and the value and impact of subtle methyl group walks, as well as the highly productive fluorine walk, around allosteric ligand cores to stabilize unique GPCR conformations.

Allosteric ligands control the activation of a class C GPCR heterodimer by acting at the transmembrane interface

G protein-coupled receptors (GPCRs) are among the most promising drug targets. They often form homo- and heterodimers with allosteric cross-talk between receptor entities, which contributes to fine-tuning of transmembrane signaling. Specifically controlling the activity of GPCR dimers with ligands is a good approach to clarify their physiological roles and validate them as drug targets. Here, we examined the mode of action of positive allosteric modulators (PAMs) that bind at the interface of the transmembrane domains of the heterodimeric GABA_B receptor. Our site-directed mutagenesis results show that mutations of this interface impact the function of the three PAMs tested. The data support the inference that they act at the active interface between both transmembrane domains, the binding site involving residues of the TM6s of the GABA_B1 and the GABA_B2 subunit. Importantly, the agonist activity of these PAMs involves a key region in the central core of the GABA_B2 transmembrane domain, which also controls the constitutive activity of the GABA_B receptor. This region corresponds to the sodium ion binding site in class A GPCRs that controls the basal state of the receptors. Overall, these data reveal the possibility of developing allosteric compounds able to specifically modulate the activity of GPCR homo- and heterodimers by acting at their transmembrane interface.

Habenula orphan G-protein coupled receptors in the pathophysiology of fear and anxiety

The phasic emotion, fear, and the tonic emotion, anxiety, have been conventionally inspected in clinical frameworks to epitomize memory acquisition, storage, and retrieval. However, inappropriate expression of learned fear in a safe environment and its resistance to suppression is a cardinal feature of various fear-related disorders. A significant body of literature suggests the involvement of extra-amygdala circuitry in fear disorders. Consistent with this view, the present review underlies incentives for the association between the habenula and fear memory. G protein-coupled receptors (GPCRs) are important to understand the molecular mechanisms central to fear learning due to their neuromodulatory role. The efficacy of a pharmacological strategy aimed at exploiting habenular-GPCR desensitization machinery can serve as a therapeutic target combating the pathophysiology of fear disorders. Originating from this milieu, the conserved nature of orphan GPCRs in the brain, with some having the highest expression in the habenula can lead to recent endeavors in understanding its functionality in fear circuitry.

Exploring the binding mechanism of positive allosteric modulators in human metabotropic glutamate receptor 2 using molecular dynamics simulations

Positive allosteric modulators (PAMs) of human metabotropic glutamate receptor 2 (hmGlu₂) are well-known in the treatment of psychiatric disorders for their higher selectivity and lower tolerance risk. A variety of PAMs have been reported over the last decade and two compounds were in Phase II clinical trials for schizophrenia and anxiety. These trials were discontinued on account of the unsatisfactory therapeutic efficacy, but PAMs were explored as novel treatments for addiction and epilepsy. Thus, it is still important to explore novel hmGlu₂ PAMs in the near future. Nowadays, the challenges in optimizing drug potency and improving scaffold diversity for PAMs are the noncomprehensive character analyses of multiple scaffolds; the exploration of the binding modes of PAMs in the allosteric binding site have been proposed to reduce this difficulty. However, there has been no comprehensive research about the binding profiles of PAMs in the hmGlu₂ receptor. To address this issue, this work explores the binding characters of eight PAMs representing five chemical series by multiple computational methods. As a result, the shared binding modes of the eight studied PAMs interacting with 15 residues in the allosteric binding site were defined. In addition, the reduced hydrophobicity with low electronegativity of R₁, increased hydrophobicity with low negative electron density of R₂ and the electronegativity of the linker were identified as indicators that regulate the affinity of PAMs. This finding agrees well with the physicochemical properties of reported multiple series PAMs. This comprehensive work sheds additional light on the binding mechanism and physicochemical regularity underlining PAMs affinity and could be further utilized as a structural and energetic blueprint for discovering and assessing novel PAMs for hmGlu₂.

A Genetically Encoded F-19 NMR Probe Reveals the Allosteric Modulation Mechanism of Cannabinoid Receptor 1

Due to the lack of genetically encoded probes for fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR), its utility for probing eukaryotic membrane protein dynamics is limited. Here we report an efficient method for the genetic incorporation of an unnatural amino acid (UAA), 3'-trifluoromenthyl-phenylalanine (mtfF), into cannabinoid receptor 1 (CB1) in the Baculovirus Expression System. The probe can be inserted at any environmentally sensitive site, while causing minimal structural perturbation to the target protein. Using 19F NMR and X-ray crystallography methods, we discovered that the allosteric modulator Org27569 and agonists synergistically stabilize a previously unrecognized pre-active state. An allosteric modulation model is proposed to explain Org27569's distinct behavior. We demonstrate that our site-specific 19F NMR labeling method is a powerful tool in decoding the mechanism of GPCR allosteric modulation. This new method should be broadly applicable for uncovering conformational states for many important eukaryotic membrane proteins.

Rational Design of Allosteric Nanodevices Based on DNA Triple Helix

Inspired by allosteric regulation of natural molecules, we present a rational design scheme to build synthetic nucleic acid allosteric nanodevices. The clearly specified conformational states of switches obtained from systematic screening and analyses make the ON-OFF transition clear-cut and quantification ready. Under the rational design scheme, we have developed a series of DNA switches with triplex-forming oligos as allosteric modulators and implemented designated allosteric transitions, allosteric coregulation, and reaction pathway control. In conjunction with toehold-mediated strand displacement, our design scheme has also been applied to synthetic nucleic acid computing including a set of logic operations and complex algorithm.

Affinity Mass Spectrometry-Based Fragment Screening Identified a New Negative Allosteric Modulator of the Adenosine A2A Receptor Targeting the Sodium Ion Pocket

Allosteric ligands provide new opportunities to modulate G protein-coupled receptor (GPCR) function and present therapeutic benefits over orthosteric molecules. Negative allosteric modulators (NAMs) can inhibit the activation of a receptor and downstream signal transduction. Screening NAMs for a GPCR target is particularly challenging because of the difficulty in distinguishing NAMs from antagonists bound to the orthosteric site as they both show inhibitory effects in receptor signaling assays. Here we report an affinity mass spectrometry (MS)-based approach tailored to screening potential NAMs of a GPCR target especially from fragment libraries. Compared to regular surface plasmon resonance or NMR-based methods for fragment screening, our approach features a reduction of the protein and compound consumption by 2-4 orders of magnitude and an increase in the data acquisition speed by 2-3 orders of magnitude. Our affinity MS-based fragment screening led to the identification of a new NAM of the adenosine A_2A receptor (A_2AAR) bearing an unprecedented azetidine moiety predicted to occupy the allosteric sodium binding site. Molecular dynamics simulations, ligand structure-activity relationship (SAR) studies, and in-solution NMR analyses further revealed the unique binding mode and antagonistic property of this compound that differs considerably from HMA (5-(N,N-hexamethylene)amiloride), a well-characterized NAM of A_2AAR. Taken together, our work would facilitate fragment-based screening of allosteric modulators, as well as guide the design of novel NAMs acting at the sodium ion pocket of class A GPCRs.

Integrated Multi-Class Classification and Prediction of GPCR Allosteric Modulators by Machine Learning Intelligence

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of cell surface receptors that respond to various extracellular signals. The allosteric modulation of GPCRs has emerged in recent years as a promising approach for developing target-selective therapies. Moreover, the discovery of new GPCR allosteric modulators can greatly benefit the further understanding of GPCR cell signaling mechanisms. It is critical but also challenging to make an accurate distinction of modulators for different GPCR groups in an efficient and effective manner. In this study, we focus on an 11-class classification task with 10 GPCR subtype classes and a random compounds class. We used a dataset containing 34,434 compounds with allosteric modulators collected from classical GPCR families A, B, and C, as well as random drug-like compounds. Six types of machine learning models, including support vector machine, naïve Bayes, decision tree, random forest, logistic regression, and multilayer perceptron, were trained using different combinations of features including molecular descriptors, Atom-pair fingerprints, MACCS fingerprints, and ECFP6 fingerprints. The performances of trained machine learning models with different feature combinations were closely investigated and discussed. To the best of our knowledge, this is the first work on the multi-class classification of GPCR allosteric modulators. We believe that the classification models developed in this study can be used as simple and accurate tools for the discovery and development of GPCR allosteric modulators.

Biased Allosteric Modulators: New Frontiers in GPCR Drug Discovery

G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors in the genome and the most successful family of targets of FDA-approved drugs. New frontiers in GPCR drug discovery remain, however, as achieving receptor subtype selectivity and controlling off- and on-target side effects are not always possible with classic agonist and antagonist ligands. These challenges may be overcome by focusing development efforts on allosteric ligands that confer signaling bias. Biased allosteric modulators (BAMs) are an emerging class of GPCR ligands that engage less well-conserved regulatory motifs outside the orthosteric pocket and exert pathway-specific effects on receptor signaling. The unique ways that BAMs texturize receptor signaling present opportunities to fine-tune physiology and develop safer, more selective therapeutics. Here, we provide a conceptual framework for understanding the pharmacology of BAMs, explore their therapeutic potential, and discuss strategies for their discovery.

Targeting mGlu5 for Methamphetamine Use Disorder

Methamphetamine abuse leads to devastating consequences, including addiction, crime, and death. Despite decades of research, no medication has been approved by the U.S. Food and Drug Administration for the treatment of Methamphetamine Use Disorder. Thus, there is a need for new therapeutic approaches. Animal studies demonstrate that methamphetamine exposure dysregulates forebrain function involving the Group-I metabotropic glutamate receptor subtype 5 (mGlu₅), which is predominantly localized to postsynaptic sites. Allosteric modulators of mGlu₅ offer a unique opportunity to modulate glutamatergic neurotransmission selectively, thereby potentially ameliorating methamphetamine-induced disruptions. Negative allosteric modulators of mGlu₅ attenuate the effects of methamphetamine, including rewarding/reinforcing properties of the drug across animal models, and have shown promising effects in clinical trials for Anxiety Disorder and Major Depressive Disorder. Preclinical studies have also sparked great interest in mGlu₅ positive allosteric modulators, which exhibit antipsychotic and anxiolytic properties, and facilitate extinction learning when access to methamphetamine is removed, possibly via the amelioration of methamphetamine-induced cognitive deficits. Clinical research is now needed to elucidate the mechanisms underlying the mGlu₅ receptor-related effects of methamphetamine and the contributions of these effects to addictive behaviors. The growing array of mGlu₅ allosteric modulators provides excellent tools for this purpose and may offer the prospect of developing tailored and effective medications for Methamphetamine Use Disorder.

The Effect of CB1 Antagonism on Hepatic Oxidative/Nitrosative Stress and Inflammation in Nonalcoholic Fatty Liver Disease

Dysfunction of the endocannabinoid system (ES) has been identified in nonalcoholic fatty liver disease (NAFLD) and associated metabolic disorders. Cannabinoid receptor type 1 (CB1) expression is largely dependent on nutritional status. Thus, individuals suffering from NAFLD and metabolic syndrome (MS) have a significant increase in ES activity. Furthermore, oxidative/ nitrosative stress and inflammatory process modulation in the liver are highly influenced by the ES. Numerous experimental studies indicate that oxidative and nitrosative stress in the liver is associated with steatosis and portal inflammation during NAFLD. On the other hand, inflammation itself may also contribute to reactive oxygen species (ROS) production due to Kupffer cell activation and increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. The pathways by which endocannabinoids and their lipid-related mediators modulate oxidative stress and lipid peroxidation represent a significant area of research that could yield novel pharmaceutical strategies for the treatment of NAFLD. Cumulative evidence suggested that the ES, particularly CB1 receptors, may also play a role in inflammation and disease progression toward steatohepatitis. Pharmacological inactivation of CB1 receptors in NAFLD exerts multiple beneficial effects, particularly due to the attenuation of hepatic oxidative/nitrosative stress parameters and significant reduction of proinflammatory cytokine production. However, further investigations regarding precise mechanisms by which CB1 blockade influences the reduction of hepatic oxidative/nitrosative stress and inflammation are required before moving toward the clinical phase of the investigation.

Lead optimization of the VU0486321 series of mGlu1 PAMs. Part 4: SAR reveals positive cooperativity across multiple mGlu receptor subtypes leading to subtype unselective PAMs

Further optimization of the VU0486321 series of highly selective and CNS-penetrant mGlu₁ PAMs identified unique 'molecular switches' on the central aromatic ring that engendered positive cooperativity with multiple mGlu subtypes across the receptor family, resulting in compounds with comparable activity at Group I (mGlu_1/5) and Group III (mGlu_4/6/7/8) mGlu receptors, receptors. These exciting data suggests this PAM chemotype appears to bind to multiple mGlu receptors, and that subtype selectivity is dictated by the degree of cooperativity, not a subtype selective, unique allosteric binding site. Moreover, there is interesting therapeutic potential for mGlu_1/4/7/8 PAMs, as well as the first report of a GPCR allosteric 'privileged structure'.

Development of Novel 4-Arylpyridin-2-one and 6-Arylpyrimidin-4-one Positive Allosteric Modulators of the M1 Muscarinic Acetylcholine Receptor

This study investigated the structure-activity relationships of 4-phenylpyridin-2-one and 6-phenylpyrimidin-4-one M1 muscarinic acetylcholine receptor (M1 mAChRs) positive allosteric modulators (PAMs). The presented series focuses on modifications to the core and top motif of the reported leads, MIPS1650 (1) and MIPS1780 (2). Profiling of our novel analogues showed that these modifications result in more nuanced effects on the allosteric properties compared to our previous compounds with alterations to the biaryl pendant. Further pharmacological characterisation of the selected compounds in radioligand binding, IP1 accumulation and β-arrestin 2 recruitment assays demonstrated that, despite primarily acting as affinity modulators, the PAMs displayed different pharmacological properties across the two cellular assays. The novel PAM 7 f is a potential lead candidate for further development of peripherally restricted M1 PAMs, due to its lower blood-brain-barrier (BBB) permeability and improved exposure in the periphery compared to lead 2.

Allosteric Molecular Switches in Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGlu) are class C G protein-coupled receptors of eight subtypes that are omnipresently expressed in the central nervous system. mGlus have relevance in several psychiatric and neurological disorders, therefore they raise considerable interest as drug targets. Allosteric modulators of mGlus offer advantages over orthosteric ligands owing to their increased potential to achieve subtype selectivity, and this has prompted discovery programs that have produced a large number of reported allosteric mGlu ligands. However, the optimization of allosteric ligands into drug candidates has proved to be challenging owing to induced-fit effects, flat or steep structure-activity relationships and unexpected changes in theirpharmacology. Subtle structural changes identified as molecular switches might modulate the functional activity of allosteric ligands. Here we review these switches discovered in the metabotropic glutamate receptor family..

Explicit Treatment of Non-Michaelis-Menten and Atypical Kinetics in Early Drug Discovery*

Biological systems are highly regulated. They are also highly resistant to sudden perturbations enabling them to maintain the dynamic equilibrium essential to sustain life. This robustness is conferred by regulatory mechanisms that influence the activity of enzymes/proteins within their cellular context to adapt to changing environmental conditions. However, the initial rules governing the study of enzyme kinetics were mostly tested and implemented for cytosolic enzyme systems that were easy to isolate and/or recombinantly express. Moreover, these enzymes lacked complex regulatory modalities. Now, with academic labs and pharmaceutical companies turning their attention to more-complex systems (for instance, multiprotein complexes, oligomeric assemblies, membrane proteins and post-translationally modified proteins), the initial axioms defined by Michaelis-Menten (MM) kinetics are rendered inadequate, and the development of a new kind of kinetic analysis to study these systems is required. This review strives to present an overview of enzyme kinetic mechanisms that are atypical and, oftentimes, do not conform to the classical MM kinetics. Further, it presents initial ideas on the design and analysis of experiments in early drug-discovery for such systems, to enable effective screening and characterisation of small-molecule inhibitors with desirable physiological outcomes.

Allosteric Modulators of G Protein-Coupled Dopamine and Serotonin Receptors: A New Class of Atypical Antipsychotics

Schizophrenia was first described by Emil Krapelin in the 19th century as one of the major mental illnesses causing disability worldwide. Since the introduction of chlorpromazine in 1952, strategies aimed at modifying the activity of dopamine receptors have played a major role for the treatment of schizophrenia. The introduction of atypical antipsychotics with clozapine broadened the range of potential targets for the treatment of this psychiatric disease, as they also modify the activity of the serotoninergic receptors. Interestingly, all marketed drugs for schizophrenia bind to the orthosteric binding pocket of the receptor as competitive antagonists or partial agonists. In recent years, a strong effort to develop allosteric modulators as potential therapeutic agents for schizophrenia was made, mainly for the several advantages in their use. In particular, the allosteric binding sites are topographically distinct from the orthosteric pockets, and thus drugs targeting these sites have a higher degree of receptor subunit specificity. Moreover, “pure” allosteric modulators maintain the temporal and spatial fidelity of native orthosteric ligand. Furthermore, allosteric modulators have a “ceiling effect”, and their modulatory effect is saturated above certain concentrations. In this review, we summarize the progresses made in the identification of allosteric drugs for dopamine and serotonin receptors, which could lead to a new generation of atypical antipsychotics with a better profile, especially in terms of reduced side effects.

Novel Positive Allosteric Modulators of µ Opioid Receptor-Insight from In Silico and In Vivo Studies

Opioids are the drugs of choice in severe pain management. Unfortunately, their use involves serious, potentially lethal side effects. Therefore, efforts in opioid drug design turn toward safer and more effective mechanisms, including allosteric modulation. In this study, molecular dynamics simulations in silico and ‘writhing’ tests in vivo were used to characterize potential allosteric mechanism of two previously reported compounds. The results suggest that investigated compounds bind to μ opioid receptor in an allosteric site, augmenting action of morphine at subeffective doses, and exerting antinociceptive effect alone at higher doses. Detailed analysis of in silico calculations suggests that first of the compounds behaves more like allosteric agonist, while the second compound acts mainly as a positive allosteric modulator.

The GABAB Receptor-Structure, Ligand Binding and Drug Development

The γ-aminobutyric acid (GABA) type B receptor (GABA_B-R) belongs to class C of the G-protein coupled receptors (GPCRs). Together with the GABA_A receptor, the receptor mediates the neurotransmission of GABA, the main inhibitory neurotransmitter in the central nervous system (CNS). In recent decades, the receptor has been extensively studied with the intention being to understand pathophysiological roles, structural mechanisms and develop drugs. The dysfunction of the receptor is linked to a broad variety of disorders, including anxiety, depression, alcohol addiction, memory and cancer. Despite extensive efforts, few compounds are known to target the receptor, and only the agonist baclofen is approved for clinical use. The receptor is a mandatory heterodimer of the GABA_B1 and GABA_B2 subunits, and each subunit is composed of an extracellular Venus Flytrap domain (VFT) and a transmembrane domain of seven α-helices (7TM domain). In this review, we briefly present the existing knowledge about the receptor structure, activation and compounds targeting the receptor, emphasizing the role of the receptor in previous and future drug design and discovery efforts.