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

Transformation of a Dopamine D2 Receptor Agonist to Partial Agonists as Novel Antipsychotic Agents

Designed ligands of G protein-coupled receptors can exert a spectrum of modulating effects, varying from full agonists and partial agonists to antagonists and inverse agonists. For the dopamine D₂ receptor (D₂R), partial agonist activity is the pharmacological feature of the third-generation antipsychotics, including aripiprazole, brexpiprazole, and cariprazine. Started from a benzofuran-derived D₂R full agonist O₄LE₆ (4), which was identified using a structure-based method by us in previous studies, a series of D₂R partial agonists were designed and synthesized by introducing different tail groups. Among them, compound 10b showed excellent activity in D₂R pharmacological assays. Further optimizations using a structural rigidification approach led to the discovery of brain-penetrant compounds 29c and 29d, which exhibited potent antipsychotic effects in the mouse hyperlocomotion model. Compound 29c also showed excellent drug-like pharmacokinetic properties in rats and qualifies as an antipsychotic agent that is worth further evaluations.

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.

Approach to the specificity and selectivity between D2 and D3 receptors by mutagenesis and binding experiments part I: Expression and characterization of D2 and D3 receptor mutants

D3/D2 sub-specificity is a complex problem to solve. Indeed, in the absence of easy structural biology of the G-protein coupled receptors, and despite key progresses in this area, the systematic knowledge of the ligand/receptor relationship is difficult to obtain. Due to these structural biology limitations concerning membrane proteins, we favored the use of directed mutagenesis to document a rational towards the discovery of markedly specific D3 ligands over D2 ligands together with basic binding experiments. Using our methodology of stable expression of receptors in HEK cells, we constructed the gene encoding for 24 mutants and 4 chimeras of either D2 or D3 receptors and expressed them stably. Those cell lines, expressing a single copy of one receptor mutant each, were stably constructed, selected, amplified and the membranes from them were prepared. Binding data at those receptors were obtained using standard binding conditions for D2 and D3 dopamine receptors. We generated 26 new molecules derived from D2 or D3 ligands. Using 8 reference compounds and those 26 molecules, we characterized their binding at those mutants and chimeras, exemplifying an approach to better understand the difference at the molecular level of the D2 and D3 receptors. Although all the individual results are presented and could be used for minute analyses, the present report does not discuss the differences between D2 and D3 data. It simply shows the feasibility of the approach and its potential.

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.

2-Phenylcyclopropylmethylamine Derivatives as Dopamine D2 Receptor Partial Agonists: Design, Synthesis, and Biological Evaluation

Partial agonist activity at the dopamine D₂ receptor (D₂R) is the primary pharmacological feature of the third-generation antipsychotics─aripiprazole, brexpiprazole, and cariprazine. However, all these drugs share a common phenyl-piperazine moiety as the primary pharmacophore. In this study, we designed and synthesized a series of novel compounds based on the 2-phenylcyclopropylmethylamine (PCPMA) scaffold and studied their pharmacological activity at the D₂R. A number of potent D₂R partial agonists were identified through binding affinity screening and functional activity profiling in both G protein and β-arrestin assays. The structure-functional activity relationship results showed that the spacer group is crucial for fine-tuning the intrinsic activity of these compounds. Compounds (+)-14j and (+)-14l showed good pharmacokinetic properties and an unexpected selectivity against the serotonin 2A (5-HT_2A) receptor. Preliminary suppressive effects in a mouse hyperlocomotion model proved that these PCPMA-derived D₂R partial agonists are effective as potential novel antipsychotics.

Potent and Subtype-Selective Dopamine D2 Receptor Biased Partial Agonists Discovered via an Ugi-Based Approach

Using a previously unexplored, efficient, and versatile multicomponent method, we herein report the rapid generation of novel potent and subtype-selective DRD₂ biased partial agonists. This strategy exemplifies the search for diverse and previously unexplored moieties for the secondary/allosteric pharmacophore of the common phenyl-piperazine scaffold. The pharmacological characterization of the new compound series led to the identification of several ligands with excellent DRD₂ affinity and subtype selectivity and remarkable functional selectivity for either the cAMP (22a and 24d) or the β-arrestin (27a and 29c) signaling pathways. These results were further interpreted on the basis of molecular models of these ligands in complex with the recent DRD₂ crystal structures, highlighting the critical role of the secondary/allosteric pharmacophore in modulating the functional selectivity profile.

New phosphosite-specific antibodies to unravel the role of GRK phosphorylation in dopamine D2 receptor regulation and signaling

The dopamine D₂ receptor (D₂R) is the target of drugs used to treat the symptoms of Parkinson’s disease and schizophrenia. The D₂R is regulated through its interaction with and phosphorylation by G protein receptor kinases (GRKs) and interaction with arrestins. More recently, D₂R arrestin-mediated signaling has been shown to have distinct physiological functions to those of G protein signalling. Relatively little is known regarding the patterns of D₂R phosphorylation that might control these processes. We aimed to generate antibodies specific for intracellular D₂R phosphorylation sites to facilitate the investigation of these mechanisms. We synthesised double phosphorylated peptides corresponding to regions within intracellular loop 3 of the hD₂R and used them to raise phosphosite-specific antibodies to capture a broad screen of GRK-mediated phosphorylation. We identify an antibody specific to a GRK2/3 phosphorylation site in intracellular loop 3 of the D₂R. We compared measurements of D₂R phosphorylation with other measurements of D₂R signalling to profile selected D₂R agonists including previously described biased agonists. These studies demonstrate the utility of novel phosphosite-specific antibodies to investigate D₂R regulation and signalling.

Targeting β-Arrestins in the Treatment of Psychiatric and Neurological Disorders

Therapies for psychiatric and neurological disorders have been in the development and refinement process for the past 5 decades. Yet, most of these therapies lack optimal therapeutic efficacy and have multiple debilitating side effects. Recent advances in understanding the pathophysiological processes of psychiatric and neurological disorders have revealed an important role for β-arrestins, which are important regulators of G-protein-coupled receptor (GPCR) function, including desensitization and intracellular signaling. These findings have pushed β-arrestins to the forefront as potential therapeutic targets. Here, we highlight current knowledge on β-arrestin functions in certain psychiatric and neurological disorders (schizophrenia, Parkinson's disease, and substance abuse disorders), and how this has been leveraged to develop new therapeutic strategies. Furthermore, we discuss the obstacles impacting the field of β-arrestin-based therapeutic development and future approaches that might help advance strategies to develop optimal β-arrestin-based therapies.

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.