A structure-activity analysis of biased agonism at the dopamine D2 receptor

Carbazole and tetrahydro-carboline derivatives as dopamine D3 receptor antagonists with the multiple antipsychotic-like properties

Dopamine D3 receptor (D3R) is implicated in multiple psychotic symptoms. Increasing the D3R selectivity over dopamine D2 receptor (D2R) would facilitate the antipsychotic treatments. Herein, novel carbazole and tetrahydro-carboline derivatives were reported as D3R selective ligands. Through a structure-based virtual screen, ZLG-25 (D3R Ki = 685 nmol/L; D2R Ki > 10,000 nmol/L) was identified as a novel D3R selective bitopic ligand with a carbazole scaffold. Scaffolds hopping led to the discovery of novel D3R-selective analogs with tetrahydro-β-carboline or tetrahydro-γ-carboline core. Further functional studies showed that most derivatives acted as hD3R-selective antagonists. Several lead compounds could dose-dependently inhibit the MK-801-induced hyperactivity. Additional investigation revealed that 23j and 36b could decrease the apomorphine-induced climbing without cataleptic reaction. Furthermore, 36b demonstrated unusual antidepressant-like activity in the forced swimming tests and the tail suspension tests, and alleviated the MK-801-induced disruption of novel object recognition in mice. Additionally, preliminary studies confirmed the favorable PK/PD profiles, no weight gain and limited serum prolactin levels in mice. These results revealed that 36b provided potential opportunities to new antipsychotic drugs with the multiple antipsychotic-like properties.

Optical Control of Dopamine D2-like Receptors with Cell-Specific Fast-Relaxing Photoswitches

Dopamine D2-like receptors (D2R, D3R, and D4R) control diverse physiological and behavioral functions and are important targets for the treatment of a variety of neuropsychiatric disorders. Their complex distribution and activation kinetics in the brain make it difficult to target specific receptor populations with sufficient precision. We describe a new toolkit of light-activatable, fast-relaxing, covalently taggable chemical photoswitches that fully activate, partially activate, or block D2-like receptors. This technology combines the spatiotemporal precision of a photoswitchable ligand (P) with cell type and spatial specificity of a genetically encoded membrane anchoring protein (M) to which the P tethers. These tools set the stage for targeting endogenous D2-like receptor signaling with molecular, cellular, and spatiotemporal precision using only one wavelength of light.

Dopamine D3/D2 Receptor Ligands Based on Cariprazine for the Treatment of Psychostimulant Use Disorders That May Be Dual Diagnosed with Affective Disorders

Highly selective dopamine D3 receptor (D3R) partial agonists/antagonists have been developed for the treatment of psychostimulant use disorders (PSUD). However, none have reached the clinic due to insufficient potency/efficacy or potential cardiotoxicity. Cariprazine, an FDA-approved drug for the treatment of schizophrenia and bipolar disorder, is a high-affinity D3R partial agonist (Ki = 0.22 nM) with 3.6-fold selectivity over the homologous dopamine D2 receptor (D2R). We hypothesized that compounds that are moderately D3R/D2R-selective partial agonists/antagonists may be effective for the treatment of PSUD. By systematically modifying the parent molecule, we discovered partial agonists/antagonists, as measured in bioluminescence resonance energy transfer (BRET)-based assays, with high D3R affinities (Ki = 0.14-50 nM) and moderate selectivity (<100-fold) over D2R. Cariprazine and two lead analogues, 13a and 13e, decreased cocaine self-administration (FR2; 1-10 mg/kg, i.p.) in rats, suggesting that partial agonists/antagonists with modest D3R/D2R selectivity may be effective in treating PSUD and potentially comorbidities with other affective disorders.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Dopamine D2L receptor density influences the recruitment of β-arrestin2 and Gi1 induced by antiparkinsonian drugs

INTRODUCTION

Brain imaging studies have highlighted that the density of dopamine D₂ receptors markedly fluctuates across the stages of Parkinson's disease and in response to pharmacological treatment. Moreover, receptor density constitutes a molecular determinant for the signaling profile of D₂ receptor ligands. We therefore hypothesized that variations in receptor expression could influence D₂ receptor response to antiparkinsonian drugs, most notably with respect to the recruitment bias between G_i1 and β-arrestin2.

METHODS

The recruitment bias of dopamine, pramipexole, ropinirole, and rotigotine was examined using a nanoluciferase-based biosensor for probing the interactions of the D_2L receptor with either G_i1 or β-arrestin2. The characterization of the functional selectivity of these D₂ receptor agonists was performed at two distinct D_2L receptor densities by taking advantage of a cell model carrying an inducible system that enables the overexpression of the D_2L receptor when exposed to doxycycline.

RESULTS

A high receptor density oriented the balanced signaling profile of dopamine towards a preferential recruitment of G_i1. It also moderated the marked G_i1 and β-arrestin2 biases of pramipexole and rotigotine, respectively. At variance, the G_i1 bias of ropinirole appeared as not being influenced by D_2L receptor density.

CONCLUSIONS

Taken together, these observations highlight receptor density as a key driver of the signaling transducer recruitment triggered by antiparkinsonian agents. Moreover, given the putative beneficial properties of β-arrestin2 in promoting locomotion, this study provides molecular insights that position the arrestin-biased ligand rotigotine as a putatively more beneficial D₂ receptor agonist for the treatment of early and late Parkinson's disease.

Ligand-based rational design, synthesis and evaluation of novel potential chemical chaperones for opsin

Inherited blinding diseases retinitis pigmentosa (RP) and a subset of Leber's congenital amaurosis (LCA) are caused by the misfolding and mistrafficking of rhodopsin molecules, which aggregate and accumulate in the endoplasmic reticulum (ER), leading to photoreceptor cell death. One potential therapeutic strategy to prevent the loss of photoreceptors in these conditions is to identify opsin-binding compounds that act as chemical chaperones for opsin, aiding its proper folding and trafficking to the outer cell membrane. Aiming to identify novel compounds with such effect, a rational ligand-based approach was applied to the structure of the visual pigment chromophore, 11-cis-retinal, and its locked analogue 11-cis-6mr-retinal. Following molecular docking studies on the main chromophore binding site of rhodopsin, 49 novel compounds were synthesized according to optimized one-to seven-step synthetic routes. These agents were evaluated for their ability to compete for the chromophore binding site of opsin, and their capacity to increase the trafficking of the P23H opsin mutant from the ER to the cell membrane. Different new molecules displayed an effect in at least one assay, acting either as chemical chaperones or as stabilizers of the 9-cis-retinal-rhodopsin complex. These compounds could provide the basis to develop novel therapeutics for RP and LCA.

Chiral Cyclic Aliphatic Linkers as Building Blocks for Selective Dopamine D2 or D3 Receptor Agonists

Linkers are emerging as a key component in regulating the pharmacology of bitopic ligands directed toward G-protein coupled receptors (GPCRs). In this study, the role of regio- and stereochemistry in cyclic aliphatic linkers tethering well-characterized primary and secondary pharmacophores targeting dopamine D2 and D3 receptor subtypes (D2R and D3R, respectively) is described. We introduce several potent and selective D2R (rel-trans-16b; D2R Ki = 4.58 nM) and D3R (rel-cis-14a; D3R Ki = 5.72 nM) agonists while modulating subtype selectivity in a stereospecific fashion, transferring D2R selectivity toward D3R via inversion of the stereochemistry around these cyclic aliphatic linkers [e.g., (-)-(1S,2R)-43 and (+)-(1R,2S)-42]. Pharmacological observations were supported with extensive molecular docking studies. Thus, not only is it an innovative approach to modulate the pharmacology of dopaminergic ligands described, but a new class of optically active cyclic linkers are also introduced, which can be used to expand the bitopic drug design approach toward other GPCRs.

C-N Bond Formation by Consecutive Continuous-Flow Reductions towards A Medicinally Relevant Piperazine Derivative

A new, continuous-flow consecutive reduction method was developed for the C-N bond formation in the synthesis of the key intermediate of the antipsychotic drug cariprazine. The two-step procedure consists of a DIBAL-H mediated selective ester reduction conducted in a novel, miniature alternating diameter reactor, followed by reductive amination using catalytic hydrogenation on 5% Pt/C. The connection of the optimized modules was accomplished using an at-line extraction to prevent precipitation of the aluminum salt byproducts.

Controlling receptor function from the extracellular vestibule of G-protein coupled receptors

Receptor function is traditionally controlled from the orthosteric binding site of G-protein coupled receptors. Here, we show that the functional activity and signalling of human dopamine D2 and D3 receptor ligands can be fine-tuned from the extracellular secondary binding pocket (SBP) located far from the signalling interface suggesting optimization of the SBP binding part of bitopic ligands might be a useful strategy to develop GPCR ligands with designed functional and signalling profile.

Big Data Challenges Targeting Proteins in GPCR Signaling Pathways; Combining PTML-ChEMBL Models and [35S]GTPγS Binding Assays

G-protein-coupled receptors (GPCRs), also known as 7-transmembrane receptors, are the single largest class of drug targets. Consequently, a large amount of preclinical assays having GPCRs as molecular targets has been released to public sources like the Chemical European Molecular Biology Laboratory (ChEMBL) database. These data are also very complex covering changes in drug chemical structure and assay conditions like c₀ = activity parameter (K_i, IC₅₀, etc.), c₁ = target protein, c₂ = cell line, c₃ = assay organism, etc., making difficult the analysis of these databases that are placed in the borders of a Big Data challenge. One of the aims of this work is to develop a computational model able to predict new GPCRs targeting drugs taking into consideration multiple conditions of assay. Another objective is to perform new predictive and experimental studies of selective 5-HTA2 receptor agonist, antagonist, or inverse agonist in human comparing the results with those from the literature. In this work, we combined Perturbation Theory (PT) and Machine Learning (ML) to seek a general PTML model for this data set. We analyzed 343 738 unique compounds with 812 072 end points (assay outcomes), with 185 different experimental parameters, 592 protein targets, 51 cell lines, and/or 55 organisms (species). The best PTML linear model found has three input variables only and predicted 56 202/58 653 positive outcomes (sensitivity = 95.8%) and 470 230/550 401 control cases (specificity = 85.4%) in training series. The model also predicted correctly 18 732/19 549 (95.8%) of positive outcomes and 156 739/183 469 (85.4%) of cases in external validation series. To illustrate its practical use, we used the model to predict the outcomes of six different 5-HT2A receptor drugs, namely, TCB-2, DOI, DOB, altanserin, pimavanserin, and nelotanserin, in a very large number of different pharmacological assays. 5-HT2A receptors are altered in schizophrenia and represent drug target for antipsychotic therapeutic activity. The model correctly predicted 93.83% (76 of 86) experimental results for these compounds reported in ChEMBL. Moreover, [³⁵S]GTPγS binding assays were performed experimentally with the same six drugs with the aim of determining their potency and efficacy in the modulation of G-proteins in human brain tissue. The antagonist ketanserin was included as inactive drug with demonstrated affinity for 5-HT2A/C receptors. Our results demonstrate that some of these drugs, previously described as serotonin 5-HT2A receptor agonists, antagonists, or inverse agonists, are not so specific and show different intrinsic activity to that previously reported. Overall, this work opens a new gate for the prediction of GPCRs targeting compounds.

2016 Philip S. Portoghese Medicinal Chemistry Lectureship: Designing Bivalent or Bitopic Molecules for G-Protein Coupled Receptors. The Whole Is Greater Than the Sum of Its Parts

The genesis of designing bivalent or bitopic molecules that engender unique pharmacological properties began with Portoghese's work directed toward opioid receptors, in the early 1980s. This strategy has evolved as an attractive way to engineer highly selective compounds for targeted G-protein coupled receptors (GPCRs) with optimized efficacies and/or signaling bias. The emergence of X-ray crystal structures of many GPCRs and the identification of both orthosteric and allosteric binding sites have provided further guidance to ligand drug design that includes a primary pharmacophore (PP), a secondary pharmacophore (SP), and a linker between them. It is critical to note the synergistic relationship among all three of these components as they contribute to the overall interaction of these molecules with their receptor proteins and that strategically designed combinations have and will continue to provide the GPCR molecular tools of the future.

Designing Functionally Selective Noncatechol Dopamine D1 Receptor Agonists with Potent In Vivo Antiparkinsonian Activity

Dopamine receptors are important G protein-coupled receptors (GPCRs) with therapeutic opportunities for treating Parkinson's Disease (PD) motor and cognitive deficits. Biased D₁ dopamine ligands that differentially activate G protein over β-arrestin recruitment pathways are valuable chemical tools for dissecting positive versus negative effects in drugs for PD. Here, we reveal an iterative approach toward modification of a D₁-selective noncatechol scaffold critical for G protein-biased agonism. This approach provided enhanced understanding of the structural components critical for activity and signaling bias and led to the discovery of several novel compounds with useful pharmacological properties, including three highly G_S-biased partial agonists. Administration of a potent, balanced, and brain-penetrant lead compound from this series results in robust antiparkinsonian effects in a rodent model of PD. This study suggests that the noncatechol ligands developed through this approach are valuable tools for probing D₁ receptor signaling biology and biased agonism in models of neurologic disease.

D2 Dopamine Receptor G Protein-Biased Partial Agonists Based on Cariprazine

Functionally selective G protein-coupled receptor ligands are valuable tools for deciphering the roles of downstream signaling pathways that potentially contribute to therapeutic effects versus side effects. Recently, we discovered both G_i/o-biased and β-arrestin2-biased D₂ receptor agonists based on the Food and Drug Administration (FDA)-approved drug aripiprazole. In this work, based on another FDA-approved drug, cariprazine, we conducted a structure-functional selectivity relationship study and discovered compound 38 (MS1768) as a potent partial agonist that selectively activates the G_i/o pathway over β-arrestin2. Unlike the dual D₂R/D₃R partial agonist cariprazine, compound 38 showed selective agonist activity for D₂R over D₃R. In fact, compound 38 exhibited potent antagonism of dopamine-stimulated β-arrestin2 recruitment. In our docking studies, compound 38 directly interacts with S193^5.42 on TM5 but has no interactions with extracellular loop 2, which appears to be in contrast to the binding poses of D₂R β-arrestin2-biased ligands. In in vivo studies, compound 38 showed high D₂R receptor occupancy in mice and effectively inhibited phencyclidine-induced hyperlocomotion.

Defining Structure-Functional Selectivity Relationships (SFSR) for a Class of Non-Catechol Dopamine D1 Receptor Agonists

G protein-coupled receptors (GPCRs) are capable of downstream signaling through distinct noncanonical pathways such as β-arrestins in addition to the canonical G protein-dependent pathways. GPCR ligands that differentially activate the downstream signaling pathways are termed functionally selective or biased ligands. A class of novel non-catechol G protein-biased agonists of the dopamine D₁ receptor (D₁R) was recently disclosed. We conducted the first comprehensive structure-functional selectivity relationship study measuring G_S and β-arrestin2 recruitment activities focused on four regions of this scaffold, resulting in over 50 analogs with diverse functional selectivity profiles. Some compounds became potent full agonists of β-arrestin2 recruitment, while others displayed enhanced G_S bias compared to the starting compound. Pharmacokinetic testing of an analog with an altered functional selectivity profile demonstrated excellent blood-brain barrier penetration. This study provides novel tools for studying ligand bias at D₁R and paves the way for developing the next generation of biased D₁R ligands.

Therapeutic Potential of Targeting ß-Arrestin

ß-arrestins are multifunctional proteins that modulate heptahelical 7 transmembrane receptors, also known as G protein-coupled receptors (GPCRs), a superfamily of receptors that regulate most physiological processes. ß-arrestin modulation of GPCR function includes termination of G protein-dependent signaling, initiation of ß-arrestin-dependent signaling, receptor trafficking to degradative or recycling pathways, receptor transactivation, transcriptional regulation, and localization of second messenger regulators. The pleiotropic influence ß-arrestins exert on these receptors regulates a breadth of physiological functions, and additionally, ß-arrestins are involved in the pathophysiology of numerous and wide-ranging diseases, making them prime therapeutic targets. In this review, we briefly describe the mechanisms by which ß-arrestins regulate GPCR signaling, including the functional cellular mechanisms modulated by ß-arrestins and relate this to observed pathophysiological responses associated with ß-arrestins. We focus on the role for ß-arrestins in transducing cell signaling; a pathway that is complementary to the classical G protein-coupling pathway. The existence of these GPCR dual signaling pathways offers an immense therapeutic opportunity through selective targeting of one signaling pathway over the other. Finally, we will consider several mechanisms by which the potential of dual signaling pathway regulation can be harnessed and the implications for improved disease treatments.

Cannabinoid Receptor 2 Signalling Bias Elicited by 2,4,6-Trisubstituted 1,3,5-Triazines

Cannabinoid receptor 2 (CB2) is predominantly distributed in immune tissues and cells and is a promising therapeutic target for modulating inflammation. In this study we designed and synthesised a series of 2,4,6-trisubstituted 1,3,5-triazines with piperazinylalkyl or 1,2-diethoxyethane (PEG2) chains as CB2 agonists, all of which were predicted to be considerably more polar than typical cannabinoid ligands. In this series, we found that triazines containing an adamantanyl group were conducive to CB2 binding whereas those with a cyclopentyl group were not. Although the covalent attachment of a PEG2 linker to the adamantyl triazines resulted in a decrease in binding affinity, some of the ligands produced very interesting hCB2 signalling profiles. Six compounds with notable hCB2 orthosteric binding were functionally characterised in three pathways; internalisation, cyclic adenosine monophosphate (cAMP) and ERK phosphorylation (pERK). These were predominantly confirmed to be hCB2 agonists, and upon comparison to a reference ligand (CP 55,940), four compounds exhibited signalling bias. Triazines 14 (UOSD017) and 15 were biased towards internalisation over cAMP and pERK, and 7 was biased away from pERK activation relative to cAMP and internalisation. Intriguingly, the triazine with an amino-PEG2-piperazinyl linker (13 [UOSD008]) was identified to be a mixed agonist/inverse agonist, exhibiting apparent neutral antagonism in the internalisation pathway, transient inverse agonism in the cAMP pathway and weak partial agonism in the pERK pathway. Both the cAMP and pERK signalling were pertussis toxin (PTX) sensitive, implying that 13 is acting as both a weak agonist and inverse agonist at CB2 via Gαi/o. Compound 10 (UOSD015) acted as a balanced high intrinsic efficacy agonist with the potential to produce greater hCB2-mediated efficacy than reference ligand CP 55,940. As 10 includes a Boc-protected PEG2 moiety it is also a promising candidate for further modification, for example with a secondary reporter or fluorophore. The highest affinity compound in this set of relatively polar hCB2 ligands was compound 16, which acted as a slightly partial balanced agonist in comparison with CP 55,940. The ligands characterised here may therefore exhibit unique functional properties in vivo and have the potential to be valuable in the future development of CB2-directed therapeutics.

Molecular dynamics simulation of biased agonists at the dopamine D2 receptor suggests the mechanism of receptor functional selectivity

The dopamine D2 receptor (D2R) is the primary target for antipsychotic drugs. Besides schizophrenia, this receptor is linked to dementia, Parkinson's disease, and depression. Recent studies have shown that β-arrestin biased agonists at this receptor treat schizophrenia with less side effects. Although the high resolution structure of this receptor exists, the mechanism of biased agonism at the receptor is unknown. In this study, dopamine, the endogenous unbiased G-protein agonist, MLS1547, a G-protein biased agonist, and UNC9975, a G-protein antagonist and a β-arrestin biased agonist, were docked to a homology model of the whole D2R including all flexible loops, and molecular dynamics simulations were conducted to study the potential mechanisms of biased agonism. Our thorough analysis on the protein-ligand interaction, secondary structure, tertiary structure, structure dynamics, and molecular switches of all three systems indicates that ligand binding to transmembrane 3 might be essential for G-protein recruitment, while ligand binding to transmembrane 6 might be essential for β-arrestin recruitment. Our analysis also suggests changes in both the secondary and the tertiary structures of TM5 and TM7, molecular switches and ICL3 flexibility are important in biased signaling. Communicated by Ramaswamy H. Sarma.

GPCR drug discovery: integrating solution NMR data with crystal and cryo-EM structures

The 826 G protein-coupled receptors (GPCRs) in the human proteome regulate key physiological processes and thus have long been attractive drug targets. With the crystal structures of more than 50 different human GPCRs determined over the past decade, an initial platform for structure-based rational design has been established for drugs that target GPCRs, which is currently being augmented with cryo-electron microscopy (cryo-EM) structures of higher-order GPCR complexes. Nuclear magnetic resonance (NMR) spectroscopy in solution is one of the key approaches for expanding this platform with dynamic features, which can be accessed at physiological temperature and with minimal modification of the wild-type GPCR covalent structures. Here, we review strategies for the use of advanced biochemistry and NMR techniques with GPCRs, survey projects in which crystal or cryo-EM structures have been complemented with NMR investigations and discuss the impact of this integrative approach on GPCR biology and drug discovery.

Cariprazine for acute and maintenance treatment of adults with schizophrenia: an evidence-based review and place in therapy

Cariprazine is an oral antipsychotic approved in the US and EU for the treatment of schizophrenia. Cariprazine differs from other antipsychotics in that it is a dopamine D3- and D2-receptor partial agonist, with tenfold higher affinity for D3 receptors than for D2 receptors. Cariprazine is metabolized in two steps by CYP3A4 to didesmethyl-cariprazine (DDCAR). DDCAR has a long half-life of 1-3 weeks and is the predominant circulating active moiety. Efficacy and safety in persons with acute schizophrenia were assessed in four similarly designed, short-term, randomized, placebo-controlled clinical trials in nonelderly adults, with three studies considered positive and yielding a number needed to treat vs placebo for response (change from baseline ≥30% in Positive and Negative Syndrome Scale total score) of ten for the approved dose range of cariprazine 1.5-6 mg/day. The most common adverse reactions were extrapyramidal symptoms (15% and 19% for 1.5-3 and 4.5-6 mg/day, respectively, vs 8% for placebo) and akathisia (9% and 12.5% for 1.5-3 and 4.5-6 mg/day, respectively, vs 4% for placebo). For the approved dose range, rates of discontinuation because of an adverse event were lower overall for patients receiving cariprazine vs placebo (9% vs 12%). Weight and metabolic profile appear favorable. Cariprazine does not increase prolactin levels or prolong the electrocardiographic QT interval. Cariprazine has also been found to be effective for the maintenance treatment of schizophrenia by delaying time to relapse when compared with placebo (HR 0.45). A 26-week randomized clinical trial evidenced superiority of cariprazine over risperidone for the treatment of predominantly negative symptoms in patients with schizophrenia. Cariprazine is also approved in the US for the acute treatment of manic or mixed episodes associated with bipolar I disorder in adults and is being studied for the treatment of bipolar I depression and major depressive disorder.

Current Concepts and Treatments of Schizophrenia

Schizophrenia is a debilitating mental illness which involves three groups of symptoms, i.e., positive, negative and cognitive, and has major public health implications. According to various sources, it affects up to 1% of the population. The pathomechanism of schizophrenia is not fully understood and current antipsychotics are characterized by severe limitations. Firstly, these treatments are efficient for about half of patients only. Secondly, they ameliorate mainly positive symptoms (e.g., hallucinations and thought disorders which are the core of the disease) but negative (e.g., flat affect and social withdrawal) and cognitive (e.g., learning and attention disorders) symptoms remain untreated. Thirdly, they involve severe neurological and metabolic side effects and may lead to sexual dysfunction or agranulocytosis (clozapine). It is generally agreed that the interactions of antipsychotics with various neurotransmitter receptors are responsible for their effects to treat schizophrenia symptoms. In particular, several G protein-coupled receptors (GPCRs), mainly dopamine, serotonin and adrenaline receptors, are traditional molecular targets for antipsychotics. Comprehensive research on GPCRs resulted in the exploration of novel important signaling mechanisms of GPCRs which are crucial for drug discovery: intentionally non-selective multi-target compounds, allosteric modulators, functionally selective compounds and receptor oligomerization. In this review, we cover current hypotheses of schizophrenia, involving different neurotransmitter systems, discuss available treatments and present novel concepts in schizophrenia and its treatment, involving mainly novel mechanisms of GPCRs signaling.

Making Sense of Pharmacology: Inverse Agonism and Functional Selectivity

Constitutive receptor activity/inverse agonism and functional selectivity/biased agonism are 2 concepts in contemporary pharmacology that have major implications for the use of drugs in medicine and research as well as for the processes of new drug development. Traditional receptor theory postulated that receptors in a population are quiescent unless activated by a ligand. Within this framework ligands could act as agonists with various degrees of intrinsic efficacy, or as antagonists with zero intrinsic efficacy. We now know that receptors can be active without an activating ligand and thus display "constitutive" activity. As a result, a new class of ligand was discovered that can reduce the constitutive activity of a receptor. These ligands produce the opposite effect of an agonist and are called inverse agonists. The second topic discussed is functional selectivity, also commonly referred to as biased agonism. Traditional receptor theory also posited that intrinsic efficacy is a single drug property independent of the system in which the drug acts. However, we now know that a drug, acting at a single receptor subtype, can have multiple intrinsic efficacies that differ depending on which of the multiple responses coupled to a receptor is measured. Thus, a drug can be simultaneously an agonist, an antagonist, and an inverse agonist acting at the same receptor. This means that drugs have an additional level of selectivity (signaling selectivity or "functional selectivity") beyond the traditional receptor selectivity. Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools.

Biased Ligands of G Protein-Coupled Receptors (GPCRs): Structure-Functional Selectivity Relationships (SFSRs) and Therapeutic Potential

G protein-coupled receptors (GPCRs) signal through both G-protein-dependent and G-protein-independent pathways, and β-arrestin recruitment is the most recognized one of the latter. Biased ligands selective for either pathway are expected to regulate biological functions of GPCRs in a more precise way, therefore providing new drug molecules with superior efficacy and/or reduced side effects. During the past decade, biased ligands have been discovered and developed for many GPCRs, such as the μ opioid receptor, the angiotensin II receptor type 1, the dopamine D₂ receptor, and many others. In this Perspective, recent advances in this field are reviewed by discussing the structure-functional selectivity relationships (SFSRs) of GPCR biased ligands and the therapeutic potential of these molecules. Further understanding of the biological functions associated with each signaling pathway and structural basis for biased signaling will facilitate future drug design in this field.

Synthesis of 3-(3-hydroxyphenyl)pyrrolidine dopamine D3 receptor ligands with extended functionality for probing the secondary binding pocket

A series of 3-(3-hydroxyphenyl)pyrrolidine analogues which incorporate N-alkyl groups and N-butylamide-linked benzamide functionality have been synthesized and their in vitro binding affinities at human dopamine receptors have been evaluated. Our ligand design strategy was to take the 3-(3-hydroxyphenyl)pyrrolidine scaffold and extend functionality from the orthosteric binding site to the secondary binding pocket for enhancing affinity and selectivity for the D₃ receptor. The N-alkyl analogues constitute a homologous series from N-pentyl to N-decyl to probe the length/bulk tolerance of the secondary binding pocket of the D₃ receptor. Enantiomeric 3-(3-hydroxyphenyl)pyrrolidine analogues were also prepared in order to test the chirality preference of the orthosteric binding site for this scaffold. Benzamide analogues were prepared to enhance affinity and/or selectivity based upon the results of the homologous series.

Control of insulin secretion by GLP-1

Stimulation of insulin secretion by glucagon-like peptide-1 (GLP-1) and other gut-derived peptides is central to the incretin response to ingesting nutriments. Analogues of GLP-1, and inhibitors of its breakdown, have found widespread clinical use for the treatment of type 2 diabetes (T2D) and obesity. The release of these peptides underlies the improvements in glycaemic control and disease remission after bariatric surgery. Given therapeutically, GLP-1 analogues can lead to side effects including nausea, which limit dosage. Greater understanding of the interactions between the GLP-1 receptor (GLP-1R) and both the endogenous and artificial ligands therefore holds promise to provide more efficacious compounds. Here, we discuss recent findings concerning the signalling and trafficking of the GLP-1R in pancreatic beta cells. Leveraging "bias" at the receptor towards cAMP generation versus the recruitment of β-arrestins and extracellular signal-regulated kinases (ERK1/2) activation may allow the development of new analogues with significantly improved clinical efficacy. We describe how, unexpectedly, relatively low-affinity agonists, which prompt less receptor internalisation than the parent compound, provoke greater insulin secretion and consequent improvements in glycaemia.

Biased signalling: from simple switches to allosteric microprocessors

G protein-coupled receptors (GPCRs) are the largest class of receptors in the human genome and some of the most common drug targets. It is now well established that GPCRs can signal through multiple transducers, including heterotrimeric G proteins, GPCR kinases and β-arrestins. While these signalling pathways can be activated or blocked by 'balanced' agonists or antagonists, they can also be selectively activated in a 'biased' response. Biased responses can be induced by biased ligands, biased receptors or system bias, any of which can result in preferential signalling through G proteins or β-arrestins. At many GPCRs, signalling events mediated by G proteins and β-arrestins have been shown to have distinct biochemical and physiological actions from one another, and an accurate evaluation of biased signalling from pharmacology through physiology is crucial for preclinical drug development. Recent structural studies have provided snapshots of GPCR-transducer complexes, which should aid in the structure-based design of novel biased therapies. Our understanding of GPCRs has evolved from that of two-state, on-and-off switches to that of multistate allosteric microprocessors, in which biased ligands transmit distinct structural information that is processed into distinct biological outputs. The development of biased ligands as therapeutics heralds an era of increased drug efficacy with reduced drug side effects.

Structure-Guided Screening for Functionally Selective D2 Dopamine Receptor Ligands from a Virtual Chemical Library

Functionally selective ligands stabilize conformations of G protein-coupled receptors (GPCRs) that induce a preference for signaling via a subset of the intracellular pathways activated by the endogenous agonists. The possibility to fine-tune the functional activity of a receptor provides opportunities to develop drugs that selectively signal via pathways associated with a therapeutic effect and avoid those causing side effects. Animal studies have indicated that ligands displaying functional selectivity at the D₂ dopamine receptor (D₂R) could be safer and more efficacious drugs against neuropsychiatric diseases. In this work, computational design of functionally selective D₂R ligands was explored using structure-based virtual screening. Molecular docking of known functionally selective ligands to a D₂R homology model indicated that such compounds were anchored by interactions with the orthosteric site and extended into a common secondary pocket. A tailored virtual library with close to 13 000 compounds bearing 2,3-dichlorophenylpiperazine, a privileged orthosteric scaffold, connected to diverse chemical moieties via a linker was docked to the D₂R model. Eighteen top-ranked compounds that occupied both the orthosteric and allosteric site were synthesized, leading to the discovery of 16 partial agonists. A majority of the ligands had comparable maximum effects in the G protein and β-arrestin recruitment assays, but a subset displayed preference for a single pathway. In particular, compound 4 stimulated β-arrestin recruitment (EC₅₀ = 320 nM, E_max = 16%) but had no detectable G protein signaling. The use of structure-based screening and virtual libraries to discover GPCR ligands with tailored functional properties will be discussed.

Biased Signaling by Agonists of Protease Activated Receptor 2

Protease activated receptor 2 (PAR2) is associated with metabolism, obesity, inflammatory, respiratory and gastrointestinal disorders, pain, cancer, and other diseases. The extracellular N-terminus of PAR2 is a common target for multiple proteases, which cleave it at different sites to generate different N-termini that activate different PAR2-mediated intracellular signaling pathways. There are no synthetic PAR2 ligands that reproduce the same signaling profiles and potencies as proteases. Structure-activity relationships here for 26 compounds spanned a signaling bias over 3 log units, culminating in three small ligands as biased agonist tools for interrogating PAR2 functions. DF253 (2f-LAAAAI-NH₂) triggered PAR2-mediated calcium release (EC₅₀ 2 μM) but not ERK1/2 phosphorylation (EC₅₀ > 100 μM) in CHO cells transfected with hPAR2. AY77 (Isox-Cha-Chg-NH₂) was a more potent calcium-biased agonist (EC₅₀ 40 nM, Ca²⁺; EC₅₀ 2 μM, ERK1/2), while its analogue AY254 (Isox-Cha-Chg-A-R-NH₂) was an ERK-biased agonist (EC₅₀ 2 nM, ERK1/2; EC₅₀ 80 nM, Ca²⁺). Signaling bias led to different functional responses in human colorectal carcinoma cells (HT29). AY254, but not AY77 or DF253, attenuated cytokine-induced caspase 3/8 activation, promoted scratch-wound healing, and induced IL-8 secretion, all via PAR2-ERK1/2 signaling. Different ligand components were responsible for different PAR2 signaling and functions, clues that can potentially lead to drugs that modulate different pathway-selective cellular and physiological responses.

Pharmacological property optimization for allosteric ligands: A medicinal chemistry perspective

New strategies to potentially improve drug safety and efficacy emerge with allosteric programs. Biased allosteric modulators can be designed with high subtype selectivity and defined receptor signaling endpoints, however, selecting the most meaningful parameters for optimization can be perplexing. Historically, "potency hunting" at the expense of physicochemical and pharmacokinetic optimization has led to numerous tool compounds with excellent pharmacological properties but no path to drug development. Conversely, extensive physicochemical and pharmacokinetic screening with only post hoc bias and allosteric characterization has led to inefficacious compounds or compounds with on-target toxicities. This field is rapidly evolving with new mechanistic understanding, changes in terminology, and novel opportunities. The intent of this digest is to summarize current understanding and debates within the field. We aim to discuss, from a medicinal chemistry perspective, the parameter choices available to drive SAR.

Functional Characterization of a Novel Series of Biased Signaling Dopamine D3 Receptor Agonists

Dopamine receptors play an integral role in controlling brain physiology. Importantly, subtype selective agonists and antagonists of dopamine receptors with biased signaling properties have been successful in treating psychiatric disorders with a low incidence of side effects. To this end, we recently designed and developed SK609, a dopamine D3 receptor (D3R) selective agonist that has atypical signaling properties. SK609 has shown efficacy in reversing akinesia and reducing L-dopa-induced dyskinesia in a hemiparkinsonian rats. In the current study, we demonstrate that SK609 has high selectivity for D3R with no binding affinity on D2R high- or low-affinity state when tested at a concentration of 10 μM. In addition, SK609 and its analogues do not induce desensitization of D3R as determined by repeated agonist treatment response in phosphorylation of ERK1/2 functional assay. Most significantly, SK609 and its analogues preferentially signal through the G-protein-dependent pathway and do not recruit β-arrestin-2, suggesting a functional bias toward the G-protein-dependent pathway. Structure-activity relationship (SAR) studies using analogues of SK609 demonstrate that the molecules bind at the orthosteric site by maintaining the conserved salt bridge interactions with aspartate 110 on transmembrane 3 and aryl interactions with histidine 349 on transmembrane 6, in addition to several hydrophobic interactions with residues from transmembranes 5 and 6. The compounds follow a strict SAR with reference to the three pharmacophore elements: substituted phenyl ring, length of the linker connecting phenyl ring and amine group, and orientation and hydrophobic branching groups at the amine among SK609 analogues for efficacy and functional selectivity. These features of SK609 and the analogues suggest that biased signaling is an inherent property of this series of molecules.

Discovery of G Protein-Biased D2 Dopamine Receptor Partial Agonists

Biased ligands (also known as functionally selective ligands) of G protein-coupled receptors are valuable tools for dissecting the roles of G protein-dependent and independent signaling pathways in health and disease. Biased ligands have also been increasingly pursued by the biomedical community as promising therapeutics with improved efficacy and reduced side effects compared with unbiased ligands. We previously discovered first-in-class β-arrestin-biased agonists of dopamine D2 receptor (D2R) by extensively exploring multiple regions of aripiprazole, a balanced D2R agonist. In our continuing efforts to identify biased agonists of D2R, we unexpectedly discovered a G protein-biased agonist of D2R, compound 1, which is the first G protein-biased D2R agonist from the aripiprazole scaffold. We designed and synthesized novel analogues to explore two regions of 1 and conducted structure-functional selectivity relationship (SFSR) studies. Here we report the discovery of 1, findings from our SFSR studies, and characterization of novel G protein-biased D2R agonists.

Isoform-Specific Biased Agonism of Histamine H3 Receptor Agonists

The human histamine H₃ receptor (hH₃R) is subject to extensive gene splicing that gives rise to a large number of functional and nonfunctional isoforms. Despite the general acceptance that G protein-coupled receptors can adopt different ligand-induced conformations that give rise to biased signaling, this has not been studied for the H₃R; further, it is unknown whether splice variants of the same receptor engender the same or differential biased signaling. Herein, we profiled the pharmacology of histamine receptor agonists at the two most abundant hH₃R splice variants (hH₃R₄₄₅ and hH₃R₃₆₅) across seven signaling endpoints. Both isoforms engender biased signaling, notably for 4-[3-(benzyloxy)propyl]-1H-imidazole (proxyfan) [e.g., strong bias toward phosphorylation of glycogen synthase kinase 3β (GSK3β) via the full-length receptor] and its congener 3-(1H-imidazol-4-yl)propyl-(4-iodophenyl)-methyl ether (iodoproxyfan), which are strongly consistent with the former's designation as a "protean" agonist. The 80 amino acid IL3 deleted isoform hH₃R₃₆₅ is more permissive in its signaling than hH₃R₄₄₅: 2-(1H-imidazol-5-yl)ethyl imidothiocarbamate (imetit), proxyfan, and iodoproxyfan were all markedly biased away from calcium signaling, and principal component analysis of the full data set revealed divergent profiles for all five agonists. However, most interesting was the identification of differential biased signaling between the two isoforms. Strikingly, hH₃R₃₆₅ was completely unable to stimulate GSK3β phosphorylation, an endpoint robustly activated by the full-length receptor. To the best of our knowledge, this is the first quantitative example of differential biased signaling via isoforms of the same G protein-coupled receptor that are simultaneously expressed in vivo and gives rise to the possibility of selective pharmacological targeting of individual receptor splice variants.

Extracellular Loop 2 of the Adenosine A1 Receptor Has a Key Role in Orthosteric Ligand Affinity and Agonist Efficacy

The adenosine A₁ G protein-coupled receptor (A₁AR) is an important therapeutic target implicated in a wide range of cardiovascular and neuronal disorders. Although it is well established that the A₁AR orthosteric site is located within the receptor's transmembrane (TM) bundle, prior studies have implicated extracellular loop 2 (ECL2) as having a significant role in contributing to orthosteric ligand affinity and signaling for various G protein-coupled receptors (GPCRs). We thus performed extensive alanine scanning mutagenesis of A₁AR-ECL2 to explore the role of this domain on A₁AR orthosteric ligand pharmacology. Using quantitative analytical approaches and molecular modeling, we identified ECL2 residues that interact either directly or indirectly with orthosteric agonists and antagonists. Discrete mutations proximal to a conserved ECL2-TM3 disulfide bond selectively affected orthosteric ligand affinity, whereas a cluster of five residues near the TM4-ECL2 juncture influenced orthosteric agonist efficacy. A combination of ligand docking, molecular dynamics simulations, and mutagenesis results suggested that the orthosteric agonist 5'-N-ethylcarboxamidoadenosine binds transiently to an extracellular vestibule formed by ECL2 and the top of TM5 and TM7, prior to entry into the canonical TM bundle orthosteric site. Collectively, this study highlights a key role for ECL2 in A₁AR orthosteric ligand binding and receptor activation.

Role of the Second Extracellular Loop of the Adenosine A1 Receptor on Allosteric Modulator Binding, Signaling, and Cooperativity

Allosteric modulation of adenosine A1 receptors (A1ARs) offers a novel therapeutic approach for the treatment of numerous central and peripheral disorders; however, despite decades of research, there is a relative paucity of structural information regarding the A1AR allosteric site and mechanisms governing cooperativity with orthosteric ligands. We combined alanine-scanning mutagenesis of the A1AR second extracellular loop (ECL2) with radioligand binding and functional interaction assays to quantify effects on allosteric ligand affinity, cooperativity, and efficacy. Docking and molecular dynamics (MD) simulations were performed using an A1AR homology model based on an agonist-bound A2AAR structure. Substitution of E172ECL2 for alanine reduced the affinity of the allosteric modulators PD81723 and VCP171 for the unoccupied A1AR. Residues involved in cooperativity with the orthosteric agonist NECA were different in PD81723 and VCP171; positive cooperativity between PD81723 and NECA was reduced on alanine substitution of a number of ECL2 residues, including E170ECL2 and K173ECL2, whereas mutation of W146ECL2 and W156ECL2 decreased VCP171 cooperativity with NECA. Molecular modeling localized a likely allosteric pocket for both modulators to an extracellular vestibule that overlaps with a region used by orthosteric ligands as they transit into the canonical A1AR orthosteric site. MD simulations confirmed a key interaction between E172ECL2 and both modulators. Bound PD81723 is flanked by another residue, E170ECL2, which forms hydrogen bonds with adjacent K168ECL2 and K173ECL2. Collectively, our data suggest E172ECL2 is a key allosteric ligand-binding determinant, whereas hydrogen-bonding networks within the extracellular vestibule may facilitate the transmission of cooperativity between orthosteric and allosteric sites.

Structure-Activity Analysis of Biased Agonism at the Human Adenosine A3 Receptor

Biased agonism at G protein-coupled receptors (GPCRs) has significant implications for current drug discovery, but molecular determinants that govern ligand bias remain largely unknown. The adenosine A3 GPCR (A3AR) is a potential therapeutic target for various conditions, including cancer, inflammation, and ischemia, but for which biased agonism remains largely unexplored. We now report the generation of bias "fingerprints" for prototypical ribose containing A3AR agonists and rigidified (N)-methanocarba 5'-N-methyluronamide nucleoside derivatives with regard to their ability to mediate different signaling pathways. Relative to the reference prototypical agonist IB-MECA, (N)-methanocarba 5'-N-methyluronamide nucleoside derivatives with significant N(6) or C2 modifications, including elongated aryl-ethynyl groups, exhibited biased agonism. Significant positive correlation was observed between the C2 substituent length (in Å) and bias toward cell survival. Molecular modeling suggests that extended C2 substituents on (N)-methanocarba 5'-N-methyluronamide nucleosides promote a progressive outward shift of the A3AR transmembrane domain 2, which may contribute to the subset of A3AR conformations stabilized on biased agonist binding.

Biased ligand quantification in drug discovery: from theory to high throughput screening to identify new biased μ opioid receptor agonists

BACKGROUND AND PURPOSE

Biased GPCR ligands are able to engage with their target receptor in a manner that preferentially activates distinct downstream signalling and offers potential for next generation therapeutics. However, accurate quantification of ligand bias in vitro is complex, and current best practice is not amenable for testing large numbers of compound. We have therefore sought to apply ligand bias theory to an industrial scale screening campaign for the identification of new biased μ receptor agonists.

EXPERIMENTAL APPROACH

μ receptor assays with appropriate dynamic range were developed for both Gαi -dependent signalling and β-arrestin2 recruitment. Δlog(Emax /EC50 ) analysis was validated as an alternative for the operational model of agonism in calculating pathway bias towards Gαi -dependent signalling. The analysis was applied to a high throughput screen to characterize the prevalence and nature of pathway bias among a diverse set of compounds with μ receptor agonist activity.

KEY RESULTS

A high throughput screening campaign yielded 440 hits with greater than 10-fold bias relative to DAMGO. To validate these results, we quantified pathway bias of a subset of hits using the operational model of agonism. The high degree of correlation across these biased hits confirmed that Δlog(Emax /EC50 ) was a suitable method for identifying genuine biased ligands within a large collection of diverse compounds.

CONCLUSIONS AND IMPLICATIONS

This work demonstrates that using Δlog(Emax /EC50 ), drug discovery can apply the concept of biased ligand quantification on a large scale and accelerate the deliberate discovery of novel therapeutics acting via this complex pharmacology.

The role of kinetic context in apparent biased agonism at GPCRs

Biased agonism describes the ability of ligands to stabilize different conformations of a GPCR linked to distinct functional outcomes and offers the prospect of designing pathway-specific drugs that avoid on-target side effects. This mechanism is usually inferred from pharmacological data with the assumption that the confounding influences of observational (that is, assay dependent) and system (that is, cell background dependent) bias are excluded by experimental design and analysis. Here we reveal that ‘kinetic context', as determined by ligand-binding kinetics and the temporal pattern of receptor-signalling processes, can have a profound influence on the apparent bias of a series of agonists for the dopamine D₂ receptor and can even lead to reversals in the direction of bias. We propose that kinetic context must be acknowledged in the design and interpretation of studies of biased agonism.

Biased agonists act at a receptor to preferentially induce distinct intracellular signalling responses over others. Here the authors show how kinetics of ligand binding and signaling responses greatly influence observed bias profiles, and hence must be considered when studying biased agonists.