Insights into the binding modes of human β3-adrenergic receptor agonists with ligand-based and receptor-based methods

Pharmacophore-guided Virtual Screening to Identify New β3 -adrenergic Receptor Agonists

The β3 -adrenergic receptor (β3 -AR) is found in several tissues such as adipose tissue and urinary bladder. It is a therapeutic target because it plays a role in thermogenesis, lipolysis, and bladder relaxation. Two β3 -AR agonists are used clinically: mirabegron 1 and vibegron 2, which are indicated for overactive bladder syndrome. However, these drugs show adverse effects, including increased blood pressure in mirabegron patients. Hence, new β3 -AR agonists are needed as starting points for drug development. Previous pharmacophore modeling studies of the β3 -AR did not involve experimental in vitro validation. Therefore, this study aimed to conduct prospective virtual screening and confirm the biological activity of virtual hits. Ligand-based pharmacophore modeling was performed since no 3D structure of human β3 -AR is yet available. A dataset consisting of β3 -AR agonists was prepared to build and validate the pharmacophore models. The best model was employed for prospective virtual screening, followed by physicochemical property filtering and a docking evaluation. To confirm the activity of the virtual hits, an in vitro assay was conducted, measuring cAMP levels at the cloned β3 -AR. Out of 35 tested compounds, 4 compounds were active in CHO-K1 cells expressing the human β3 -AR, and 8 compounds were active in CHO-K1 cells expressing the mouse β3 -AR.

Discovery of Novel Indazole Derivatives as Orally Available β3-Adrenergic Receptor Agonists Lacking Off-Target-Based Cardiovascular Side Effects

We previously discovered that indazole derivative 8 was a highly selective β₃-adrenergic receptor (β₃-AR) agonist, but it appeared to be metabolically unstable. To improve metabolic stability, further optimization of this scaffold was carried out. We focused on the sulfonamide moiety of this scaffold, which resulted in the discovery of compound 15 as a highly potent β₃-AR agonist (EC₅₀ = 18 nM) being inactive to β₁-, β₂-, and α_1A-AR (β₁/β₃, β₂/β₃, and α_1A/β₃ > 556-fold). Compound 15 showed dose-dependent β₃-AR-mediated responses in marmoset urinary bladder smooth muscle, had a desirable metabolic stability and pharmacokinetic profile (C_max and AUC), and did not obviously affect heart rate or mean blood pressure when administered intravenously (3 mg/kg) to anesthetized rats. Thus, compound 15 is a highly potent, selective, and orally available β₃-AR agonist, which may serve as a candidate drug for the treatment of overactive bladder without off-target-based cardiovascular side effects.

The essential role for aromatic cluster in the β3 adrenergic receptor

AIM

To explore the function of the conserved aromatic cluster F213(5.47), F308(6.51), and F309(6.52) in human β3 adrenergic receptor (hβ3AR).

METHODS

Point mutation technology was used to produce plasmid mutations of hβ3AR. HEK-293 cells were transiently co-transfected with the hβ3AR (wild-type or mutant) plasmids and luciferase reporter vector pCRE-luc. The expression levels of hβ3AR in the cells were determined by Western blot analysis. The constitutive signalling and the signalling induced by the β3AR selective agonist, BRL (BRL37344), were then evaluated. To further explore the interaction mechanism between BRL and β3AR, a three-dimensional complex model of β3AR and BRL was constructed by homology modelling and molecular docking.

RESULTS

For F308(6.51), Ala and Leu substitution significantly decreased the constitutive activities of β3AR to approximately 10% of that for the wild-type receptor. However, both the potency and maximal efficacy were unchanged by Ala substitution. In the F308(6.51)L construct, the EC(50) value manifested as a "right shift" of approximately two orders of magnitude with an increased E(max). Impressively, the molecular pharmacological phenotype was similar to the wild-type receptor for the introduction of Tyr at position 308(6.51), though the EC(50) value increased by approximately five-fold for the mutant. For F309(6.52), the constitutive signalling for both F309(6.52)A and F309(6.52)L constructs were strongly impaired. In the F309(6.52)A construct, BRL-stimulated signalling showed a normal E(max) but reduced potency. Leu substitution of F309(6.52) reduced both the E(max) and potency. When F309(6.52) was mutated to Tyr, the constitutive activity was decreased approximately three-fold, and BRL-stimulated signalling was significantly impaired. Furthermore, the double mutant (F308(6.51)A_F309(6.52)A) caused the total loss of β3AR function. The predicted binding mode between β3AR and BRL revealed that both F308(6.51) and F309(6.52) were in the BRL binding pocket of β3AR, while F213(5.47) and W305(6.48) were distant from the binding site.

CONCLUSION

These results revealed that aromatic residues, especially F308(6.51) and F309(6.52), play essential roles in the function of β3AR. Aromatic residues maintained the receptor in a partially activated state and significantly contributed to ligand binding. The results supported the common hypothesis that the aromatic cluster F[Y]5.47/F[Y]6.52/F[Y]6.51 conserved in class A G protein-coupled receptor (GPCR) plays an important role in the structural stability and activation of GPCRs.

Hierarchical virtual screening: identification of potential high-affinity and selective β(3)-adrenergic receptor agonists

The hierarchical virtual screening (HVS) study, consisting of pharmacophore modelling, docking and VS of the generated focussed virtual library, has been carried out to identify novel high-affinity and selective β(3)-adrenergic receptor (β-AR) agonists. The best pharmacophore model, comprising one H-bond donor, two hydrophobes, one positive ionizable and one negative ionizable feature, was developed based on a training set of 51 β(3)-AR agonists using the pharmacophore generation protocol implemented in Discovery Studio. The model was further validated with the test set, external set and ability of the pharmacophoric features to complement the active site amino acids of the homology modelled β(3)-AR developed using MODELLER software. The focussed virtual library was generated using the structure-based insights gained from our earlier reported comprehensive study focussing on the structural basis of β-AR subtype selectivity of representative agonists and antagonists. The HVS with the sequential use of the best pharmacophore model and homology modelled β(3)-AR in the screening of the generated focussed library has led to the identification of potential virtual leads as novel high-affinity and selective β(3)-AR agonists.

In silico investigation of interactions between human cannabinoid receptor-1 and its antagonists

Cannabinoid receptor-1 (CB(1)) is widely expressed in the central nervous system and plays a vital role in regulating food intake and energy expenditure. CB(1) antagonists such as Rimonabant have been used in clinic to inhibit food intake, and therefore reduce body weight in obese animals and humans. To investigate the binding modes of CB(1) antagonists to the receptor, both receptor- and ligand-based methods were implemented in this study. At first, a pharmacophore model was generated based on 31 diverse CB(1) antagonists collected from literature. A test set validation and a simulated virtual screening evaluation were then performed to verify the reliability and discriminating ability of the pharmacophore. Meanwhile, the homology model of CB(1) receptor was constructed based on the crystal structure of human β (2) adrenergic receptor (β (2)-AR). Several classical antagonists were then docked into the optimized homology model with induced fit docking method. A hydrogen bond between the antagonists and Lys192 on the third transmembrane helix of the receptor was formed in the docking study, which has proven to be critical for receptor-ligand interaction by biological experiments. The structure obtained from induced fit docking was then confirmed to be a reliable model for molecular docking from the result of the simulated virtual screening. The consistency between the pharmacophore and the homology structure further proved the previous observation. The built receptor structure and antagonists' pharmacophore should be useful for the understanding of inhibitory mechanism and development of novel CB(1) antagonists.