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

Many important physiological processes are mediated by alpha2A- and alpha2C-adrenergic receptors (alpha 2Rs), a subtype of class A G protein-coupled receptors (GPCRs). However, alpha 2R signaling is poorly understood, and there are few approved medications targeting these receptors. Drug discovery aimed at alpha 2Rs is complicated by the high degree of binding pocket homology between alpha 2AR and alpha 2CR, which confounds ligand-mediated selective activation or inactivation of signaling associated with a particular subtype. Meanwhile, alpha 2R signaling is complex and it is reported that activating alpha 2AR is beneficial in many clinical contexts, while activating alpha 2CR 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 alpha 2Rs. Certain lead 5-SAT analogues act as partial agonists at alpha 2ARs, while functioning as inverse agonists at alpha 2CRs, a novel pharmacological profile. Leads demonstrate high potency (e.g., EC50 < 2 nM) at the alpha 2AR and alpha 2CRs regarding Gai-mediated inhibition of adenylyl cyclase and production of cyclic adenosine monophosphate (cAMP). To help understand the molecular basis of 5-SAT alpha 2R multifaceted functional activity, alpha 2AR and alpha 2CR molecular models were built from the crystal structures and 1 mu s molecular dynamics (MD) simulations and molecular docking experiments were performed for a lead 5-SAT with alpha 2AR agonist and alpha 2CR 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) alpha 2AR/alpha 2CR agonist lofexidine. Results reveal several interactions between FPT and alpha 2AR and alpha 2CR 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 alpha 2AR and alpha 2CRs.

Automated summary

Many important physiological processes are mediated by alpha2A- and alpha2C-adrenergic receptors, but the signaling mechanisms are poorly understood and there are few medications targeting these receptors. The complex and poorly understood nature of alpha 2R signaling, along with the high degree of binding pocket homology, makes drug discovery aimed at alpha 2Rs challenging. However, a novel 5-SAT chemotype has shown diverse pharmacological activities, acting as partial agonists at alpha 2ARs and inverse agonists at alpha 2CRs.