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. 2023 May 17;14(10):1884-1895.
doi: 10.1021/acschemneuro.3c00148. Epub 2023 Apr 27.

Conformationally Selective 2-Aminotetralin Ligands Targeting the alpha2A- and alpha2C-Adrenergic Receptors

Conformationally Selective 2-Aminotetralin Ligands Targeting the alpha2A- and alpha2C-Adrenergic Receptors

Nicholas R Fragola et al. ACS Chem Neurosci. .

Abstract

Many important physiological processes are mediated by alpha2A- and alpha2C-adrenergic receptors (α2Rs), a subtype of class A G protein-coupled receptors (GPCRs). However, α2R signaling is poorly understood, and there are few approved medications targeting these receptors. Drug discovery aimed at α2Rs is complicated by the high degree of binding pocket homology between α2AR and α2CR, which confounds ligand-mediated selective activation or inactivation of signaling associated with a particular subtype. Meanwhile, α2R signaling is complex and it is reported that activating α2AR is beneficial in many clinical contexts, while activating α2CR signaling may be detrimental to these positive effects. Here, we report on a novel 5-substituted-2-aminotetralin (5-SAT) chemotype that, depending on substitution, has diverse pharmacological activities at α2Rs. Certain lead 5-SAT analogues act as partial agonists at α2ARs, while functioning as inverse agonists at α2CRs, a novel pharmacological profile. Leads demonstrate high potency (e.g., EC50 < 2 nM) at the α2AR and α2CRs regarding Gαi-mediated inhibition of adenylyl cyclase and production of cyclic adenosine monophosphate (cAMP). To help understand the molecular basis of 5-SAT α2R multifaceted functional activity, α2AR and α2CR molecular models were built from the crystal structures and 1 μs molecular dynamics (MD) simulations and molecular docking experiments were performed for a lead 5-SAT with α2AR agonist and α2CR inverse agonist activity, i.e., (2S)-5-(2'-fluorophenyl)-N,N-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine (FPT), in comparison to the FDA-approved (for opioid withdrawal symptoms) α2AR/α2CR agonist lofexidine. Results reveal several interactions between FPT and α2AR and α2CR amino acids that may impact the functional activity. The computational data in conjunction with experimental in vitro affinity and function results provide information to understand ligand stabilization of functionally distinct GPCR conformations regarding α2AR and α2CRs.

Keywords: 2-aminotetralin; adrenergic receptor; molecular modeling; α2A receptor; α2C receptor.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Lofexidine, clonidine, and guanfacine (from left).
Figure 2
Figure 2
Comparative assessment of functional activity elicited by 5-SAT ligands in cells stably expressing the α2A-adrenoreceptor (left) and α2C-adrenoreceptor (right). The y-axis displays the unit of receptor activation, which was determined as percent activation relative to lofexidine. Percent change in cAMP over forskolin-stimulated baseline was determined, and then normalized as a function of the percent change stimulated by lofexidine. A minimum of five independent experiments were performed for each compound displayed, tested in triplicate in each experiment; data displayed represent the mean value across all experiments.
Figure 3
Figure 3
Molecular models and ligand docking. (A) 2-D structure of S-FPT. (B) S-FPT docked at the α2AR model. (C) S-FPT docked at the α2CR model. (D) Superimposed docks of S-FPT at α2AR (green) and α2CR (light blue) models. (E) 2-D structure of lofexidine. (F) Lofexidine docked at the α2AR model. (G) Lofexidine docked at the α2CR model. (H) Superimposed docks of lofexidine at α2AR (green) and α2CR (light blue) models.
Figure 4
Figure 4
Molecular dynamics simulations showing the ionic lock distance, the distance between R3.50 and D6.30, when FPT is bound. This bond is engaged in α2CR, while it is broken in α2AR, supporting the observed in vitro profile.
Scheme 1
Scheme 1. Chiral Synthesis of R-2′-F-5-PAT (FPT)
Reagents and conditions. (a) HBr, 130 °C, 3.5 h; (b) CH2O, MeOH, reflux, 2 h then NaBH4, 0 °C to rt, 3 h; (c) N-(2-pyridyl)-bis(trifluoromethanesulfonimide), DIPEA, CH2Cl2, 0 °C to rt, 20 h; (d) 2-fluorophenylboronic acid, Pd(PPh3)4, K2CO3, 1,4-dioxane, 120 °C, 6 h.
Scheme 2
Scheme 2. Chiral Synthesis of (2-Pyrrolidinyl)-2′-F-5-PAT (PFPT)
Reagents and conditions. (a) 1,4-dibromobutane, TEA, ACN, reflux 4 h, then 40 °C, 30 min, or, 1,4-butandial, CH3COOH, NaBH3CN, ACN, rt, 4 h; (b) HBr,130 °C, 3.5 h; (c) N-(2-pyridyl)-bis(trifluoromethanesulfonimide), DIPEA, CH2Cl2, 0 °C to rt, 20 h; (d) 2-fluorophenylboronic acid, Pd(PPh3)4, K2CO3, 1,4-dioxane, 120 °C, 6 h.

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