An Unusual Pattern of Ligand-Receptor Interactions for the α7 Nicotinic Acetylcholine Receptor, with Implications for the Binding of Varenicline

The alpha 7 nicotinic acetylcholine receptor shows broad pharmacology, complicating the development of subtype-specific nicotinic receptor agonists. Here we use unnatural amino acid mutagenesis to characterize binding to a7 by the smoking cessation drug varenicline (Chantix; Pfizer, Groton, CT), an alpha 4 beta 2-targeted agonist that shows full efficacy and modest potency at the alpha 7 receptor. We find that unlike binding to its target receptor, varenicline does not form a cation-pi interaction with TrpB, further supporting a unique binding mode for the cationic amine of nicotinic agonists at the alpha 7 receptor. We also evaluate binding to the complementary face of the receptor's binding site by varenicline, the endogenous agonist acetylcholine, and the potent nicotine analog epibatidine. Interestingly, we find no evidence for functionally important interactions involving backbone NH and CO groups thought to bind the canonical agonist hydrogen bond acceptor of the nicotinic pharmacophore, perhaps reflecting a lesser importance of this pharmacophore element for alpha 7 binding. We also show that the Trp55 and Leu119 side chains of the binding site's complementary face are important for the binding of the larger agonists epibatidine and varenicline, but dispensable for binding of the smaller, endogenous agonist acetylcholine.