Discovery of N-Substituted (2-Phenylcyclopropyl)methylamines as Functionally Selective Serotonin 2C Receptor Agonists for Potential Use as Antipsychotic Medications

G protein-coupled receptors (GPCRs): advances in structures, mechanisms, and drug discovery

G protein-coupled receptors (GPCRs), the largest family of human membrane proteins and an important class of drug targets, play a role in maintaining numerous physiological processes. Agonist or antagonist, orthosteric effects or allosteric effects, and biased signaling or balanced signaling, characterize the complexity of GPCR dynamic features. In this study, we first review the structural advancements, activation mechanisms, and functional diversity of GPCRs. We then focus on GPCR drug discovery by revealing the detailed drug-target interactions and the underlying mechanisms of orthosteric drugs approved by the US Food and Drug Administration in the past five years. Particularly, an up-to-date analysis is performed on available GPCR structures complexed with synthetic small-molecule allosteric modulators to elucidate key receptor-ligand interactions and allosteric mechanisms. Finally, we highlight how the widespread GPCR-druggable allosteric sites can guide structure- or mechanism-based drug design and propose prospects of designing bitopic ligands for the future therapeutic potential of targeting this receptor family.

2-Phenylcyclopropylmethylamine (PCPMA) as a privileged scaffold for central nervous system drug design

The use of privileged scaffolds in medicinal chemistry is an effective way to accelerate the drug discovery process, especially at the hit/lead optimization stage. 2-Phenylcyclopropylmethylamine (PCPMA) is a less commonly used chemical scaffold in medicinal chemistry, but many PCPMA-containing compounds exert therapeutic effects for various diseases, in particular central nervous system (CNS) diseases such as depression, schizophrenia, sleep disorder, and Parkinson's disease. The backbone of the PCPMA scaffold enables a unique structure of an amino group linked to a benzene ring through an alkyl linker, making it a useful template for the design of bioactive compounds especially for CNS drug targets including aminergic GPCRs and transporters. This review summarizes the medicinal chemistry studies of PCPMA-containing drugs and drug-like molecules, their mechanisms of action, and biological activities. We conclude that PCPMA is a unique and useful privileged scaffold for CNS drug design.

Identification of Spiro[chromene-2,4'-piperidine]s as Potent, Selective, and Gq-Biased 5-HT2C Receptor Partial Agonists

A series of spiropiperidines was designed and synthesized by structural modifications based on our previous lead compound 1 and evaluated with cellular signaling assays for the discovery of 5-HT2C receptor (5-HT2CR) selective agonists with a Gq bias. Structure-activity relationship (SAR) studies of spiropiperidines uncovered spiro[chromene-2,4'-piperidine]s as a novel chemotype of 5-HT2CR selective agonists. Among this new series, the 7-chloro analogue 8 was identified as the most potent and selective 5-HT2CR partial agonist (Emax = 71.09%) with an EC50 value of 121.5 nM and no observed activity toward 5-HT2AR or 5-HT2BR. Moreover, compound 8 exhibited no recruitment activity for β-arrestin and showed low inhibition of hERG at 10 μM. These findings may pave the way to develop more potent Gq-biased 5-HT2CR partial agonists as useful pharmacological tool compounds or potential drug candidates.

Organocatalytic Reductive Amination of the Chiral Formylcyclopropanes: Scope and Applications

We developed a sustainable three-component reductive amination protocol for the chemoselective coupling of optically active functionally rich donor-acceptor carbonyl-cyclopropanes with various amines under 10 mol % of diphenyl phosphate in the presence of Hantzsch ester as a hydride source. The catalytic selective reductive C-N coupling has wide advantages like no epimerization, no ring opening, large substrate scope, generating only mono N-alkylation products and simultaneously resulting in chiral cyclopropane-containing amines possessing many applications in the medicinal chemistry. In this article, we have shown the synthetic applications of reductive C-N coupling reaction to make chiral α-carbonyl-cyclopropane containing amines 8, double C-N coupled cyclopropane-amines 10, unusual C-N/C-C coupled cyclopropane-amines 12, chiral tert-butylsulfinamide containing cyclopropanes 14/15, and functionally rich chiral cyclopropane-fused N-heterocycles 16/18/19. Many of these chiral cyclopropane-amines 5-19 can serve as building blocks for the synthesis of drug-like small molecules, natural products, pharmaceuticals, and their analogues.

The Impact of the Secondary Binding Pocket on the Pharmacology of Class A GPCRs

G-protein coupled receptors (GPCRs) are considered important therapeutic targets due to their pathophysiological significance and pharmacological relevance. Class A receptors represent the largest group of GPCRs that gives the highest number of validated drug targets. Endogenous ligands bind to the orthosteric binding pocket (OBP) embedded in the intrahelical space of the receptor. During the last 10 years, however, it has been turned out that in many receptors there is secondary binding pocket (SBP) located in the extracellular vestibule that is much less conserved. In some cases, it serves as a stable allosteric site harbouring allosteric ligands that modulate the pharmacology of orthosteric binders. In other cases it is used by bitopic compounds occupying both the OBP and SBP. In these terms, SBP binding moieties might influence the pharmacology of the bitopic ligands. Together with others, our research group showed that SBP binders contribute significantly to the affinity, selectivity, functional activity, functional selectivity and binding kinetics of bitopic ligands. Based on these observations we developed a structure-based protocol for designing bitopic compounds with desired pharmacological profile.

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.

Design and Synthesis of Bitopic 2-Phenylcyclopropylmethylamine (PCPMA) Derivatives as Selective Dopamine D3 Receptor Ligands

2-Phenylcyclopropylmethylamine (PCPMA) analogues have been reported as selective serotonin 2C agonists. On the basis of the same scaffold, we designed and synthesized a series of bitopic derivatives as dopamine D3R ligands. A number of these new compounds show a high binding affinity for D3R with excellent selectivity. Compound (1R,2R)-22e and its enantiomer (1S,2S)-22e show a comparable binding affinity for the D3R, but the former is a potent D3R agonist, while the latter acts as an antagonist. Molecular docking studies revealed different binding poses of the PCPMA moiety within the orthosteric binding pocket of the D3R, which might explain the different functional profiles of the enantiomers. Compound (1R,2R)-30q shows a high binding affinity for the D3R (K_i = 2.2 nM) along with good selectivity, as well as good bioavailability and brain penetration properties in mice. These results reveal that the PCPMA scaffold may serve as a privileged scaffold for the design of aminergic GPCR ligands.

A Novel G Protein-Biased and Subtype-Selective Agonist for a G Protein-Coupled Receptor Discovered from Screening Herbal Extracts

Subtype selectivity and functional bias are vital in current drug discovery for G protein-coupled receptors (GPCRs) as selective and biased ligands are expected to yield drug leads with optimal on-target benefits and minimal side-effects. However, structure-based design and medicinal chemistry exploration remain challenging in part because of highly conserved binding pockets within subfamilies. Herein, we present an affinity mass spectrometry approach for screening herbal extracts to identify active ligands of a GPCR, the 5-HT_2C receptor. Using this method, we discovered a naturally occurring aporphine 1857 that displayed strong selectivity for activating 5-HT_2C without activating the 5-HT_2A or 5-HT_2B receptors. Remarkably, this novel ligand exhibited exclusive bias toward G protein signaling for which key residues were identified, and it showed comparable in vivo efficacy for food intake suppression and weight loss as the antiobesity drug, lorcaserin. Our study establishes an efficient approach to discovering novel GPCR ligands by exploring the largely untapped chemical space of natural products.

Identification of Novel 1-O-Substituted Aporphine Analogues as Potent 5-HT2C Receptor Agonists

The 5-HT_2C receptor has emerged as a promising target in the treatment of a variety of central nervous system disorders. We have first identified aporphines as a new class of 5-HT_2C receptor agonists. Structure-activity relationship results indicate that the aporphine core may be required for 5-HT_2C receptor activity, and substitutions at its C1 position are important for 5-HT_2C receptor activity. Our efforts to optimize our hit 15781 lead to the identification of the highly potent and selective 5-HT_2C agonist 18b (MQ02-439) with an EC₅₀ value of 104 nM and weak antagonism at the 5-HT_2A and 5-HT_2B receptors. The findings may serve as good starting points for the development of more potent and selective 5-HT_2C agonists as valuable pharmacological tools or potential drug candidates.

Targeting the 5-HT2C Receptor in Biological Context and the Current State of 5-HT2C Receptor Ligand Development

Serotonin (5-HT) 5-HT2C receptor (5-HT2CR) is recognized as a critical mediator of diseaserelated pathways and behaviors based upon actions in the central nervous system (CNS). Since 5-HT2CR is a class A G protein-coupled receptor (GPCR), drug discovery efforts have traditionally pursued the activation of the receptor through synthetic ligands with agonists proposed for the treatment of obesity, substance use disorders and impulse control disorders while antagonists may add value for the treatment of anxiety, depression and schizophrenia. The most significant agonist discovery to date is the FDAapproved anti-obesity medication lorcaserin. In recent years, efforts towards developing other mechanisms to enhance receptor function have resulted in the discovery of Positive Allosteric Modulators (PAMs) for the 5-HT2CR, with several molecule series now reported. The biological significance and context for signaling and function of the 5-HT2CR, and the current status of 5-HT2CR agonists and PAMs are discussed in this review.

Catalytic Asymmetric Carbene Transfer Reactions of Diazo Oxime Ethers with Olefins and Their Synthetic Applications

The first catalytic asymmetric cyclopropanation of diazo oxime ethers with olefins was developed. In the presence of a Ru(II)-Pheox catalyst, various optically active cyclopropyl oxime derivatives were obtained in high yields (up to 99%) with high enantioselectivities (up to 98% ee). Furthermore, optically active cyclopropyl oxime ethers could be successfully converted into the corresponding cyclopropyl methylamine derivatives via metal hydride and Grignard reagent mediated Beckmann rearrangement, which are potential candidates for the assessment of biological and pharmaceutical activities.

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.

Design of fluorinated cyclopropane derivatives of 2-phenylcyclopropylmethylamine leading to identification of a selective serotonin 2C (5-HT2C) receptor agonist without 5-HT2B agonism

A new series of fluorinated 5-HT_2C agonists were designed and synthesized on the basis of our previous work on 2-phenylcyclopropylmethylamines as a potential approach for the treatment of central nervous system disorders. Key fluorinated cyclopropane moieties were constructed through transition metal catalyzed [2 + 1]-cycloaddition of aromatic vinyl fluorides, and the absolute stereochemistry of the representative compound (-)-21a was established. Functional activity measuring calcium flux at 5-HT₂ receptors reveals high potency for compounds (+)-21a-d. In particular, (+)-21b had no detectable 5-HT_2B agonism and displayed reasonable selectivity against 5-HT_2A. Molecular docking studies were further performed to explain the compounds' possible binding poses to the 5-HT_2C receptor.

Identification of dual role of piperazine-linked phenyl cyclopropyl methanone as positive allosteric modulator of 5-HT2C and negative allosteric modulator of 5-HT2B receptors

Allosteric modulators of G-protein-coupled receptors have lately gained significant traction in drug discovery. Recent studies have shown that allosteric modulation of serotonin 2C receptor (5-HT_2C) as a viable strategy for the treatment of various central nervous system (CNS) disorders. Considering the critical role of 5-HT_2C in the modulation of appetite, a selective positive allosteric modulator (PAM) of 5-HT_2C offers a new opportunity for anti-obesity therapeutic development. In this study, phenyl cyclopropyl-linked N-heterocycles were synthesized and evaluated at 5-HT_2C for agonist and PAM activity. Our study shows that imidazole linked phenyl cyclopropyl methanones has PAM activity on both 5-HT_2C and serotonin 2B receptor (5-HT_2B). Interestingly, piperazine linked phenyl cyclopropyl methanones (58) was active as PAM of 5-HT_2C (increased the E_max of 5-HT to 139%), and as negative allosteric modulator (NAM) of 5-HT_2B (decreases EC₅₀ of 5-HT 10 times without affecting E_max). Similar effect of compound 58 was observed with synthetic orthosteric agonist lorcaserin on 5-HT_2B. Molecular docking study revealed that all active compounds were binding to the predicted allosteric site on 5-HT_2C and shared a common interacting residues. Finally, compound 58 suppressed food intake in Sprague Dawley (SD) rats similar to lorcaserin after i.c.v. administration. Therefore, these results suggest that piperazine moiety is essential for dual activity (PAM & NAM) of compounds 58, and supports the hypothesis of 5-HT_2C PAM for the treatment of obesity similar to the full agonist.

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.