Competition, Selectivity and Efficacy of Analogs of A-84543 for Nicotinic Acetylcholine Receptors with Repositioning of Pyridine Nitrogen

A Methyl Scan of the Pyrrolidinium Ring of Nicotine Reveals Significant Differences in Its Interactions with α7 and α4β2 Nicotinic Acetylcholine Receptors

The two major nicotinic acetylcholine receptors (nAChRs) in the brain are the α4β2 and α7 subtypes. A "methyl scan" of the pyrrolidinium ring was used to detect differences in nicotine's interactions with these two receptors. Each methylnicotine was investigated using voltage-clamp and radioligand binding techniques. Methylation at each ring carbon elicited unique changes in nicotine's receptor interactions. Replacing the 1'-N-methyl with an ethyl group or adding a second 1'-N-methyl group significantly reduced interaction with α4β2 but not α7 receptors. The 2'-methylation uniquely enhanced binding and agonist potency at α7 receptors. Although 3'- and 5'-trans-methylations were much better tolerated by α7 receptors than α4β2 receptors, 4'-methylation decreased potency and efficacy at α7 receptors much more than at α4β2 receptors. Whereas cis-5'-methylnicotine lacked agonist activity and displayed a low affinity at both receptors, trans-5'-methylnicotine retained considerable α7 receptor activity. Differences between the two 5'-methylated analogs of the potent pyridyl oxymethylene-bridged nicotine analog A84543 were consistent with what was found for the 5'-methylnicotines. Computer docking of the methylnicotines to the Lymnaea acetylcholine binding protein crystal structure containing two persistent waters predicted most of the changes in receptor affinity that were observed with methylation, particularly the lower affinities of the cis-methylnicotines. The much smaller effects of 1'-, 3'-, and 5'-methylations and the greater effects of 2'- and 4'-methylations on nicotine α7 nAChR interaction might be exploited for the design of new drugs based on the nicotine scaffold. SIGNIFICANCE STATEMENT: Using a comprehensive "methyl scan" approach, we show that the orthosteric binding sites for acetylcholine and nicotine in the two major brain nicotinic acetylcholine receptors interact differently with the pyrrolidinium ring of nicotine, and we suggest reasons for the higher affinity of nicotine for the heteromeric receptor. Potential sites for nicotine structure modification were identified that may be useful in the design of new drugs targeting these receptors.

Site-Selective Switching Strategies to Functionalize Polyazines

Many drug fragments and therapeutic compounds contain multiple pyridines and diazines. Developing site-selective reactions where specific C-H bonds can be transformed in polyazine structures would enable rapid access to valuable derivatives. We present a study that addresses this challenge by selectively installing a phosphonium ion as a versatile functional handle. Inherent factors that control site-selectivity are described along with mechanistically driven approaches for site-selective switching, where the C-+PPh3 group can be predictably installed at other positions in the polyazine system. Simple protocols, readily available reagents, and application to complex drug-like molecules make this approach appealing to medicinal chemists.

From pyrrolidinyl-benzodioxane to pyrrolidinyl-pyridodioxanes, or from unselective antagonism to selective partial agonism at α4β2 nicotinic acetylcholine receptor

Each of the four aromatic -CH= of (S,R)-2-pyrrolidinyl-1,4-benzodioxane [(S,R)-6] and of its epimer at the dioxane stereocenter (S,S)-6, previously reported as α4β2 nAChR ligands, was replaced with nitrogen. The resulting four diastereoisomeric pairs of pyrrolidinyl-pyridodioxanes were studied for the nicotinic affinity and activity at α4β2, α3β4 and α7 nAChR subtypes and compared to their common carbaisostere. It turned out that such isosteric substitutions are highly detrimental, but with the important exception of the S,R stereoisomer of the pyrrolidinyl-pyridodioxane with the pyridine nitrogen adjacent to the dioxane and seven atoms distant from the pyrrolidine nitrogen. Indeed, this stereo/regioisomer not only maintained the α4β2 affinity of [(S,R)-6], but also greatly improved in selectivity over the α3β4 and α7 subtypes and, most importantly, exhibited a highly selective α4β2 partial agonism. The finding that [(S,R)-6] is, instead, an unselective α4β2 antagonist indicates that the benzodioxane substructure confers affinity for the α4β2 nAChR binding site, but activation of this receptor subtype needs benzodioxane functionalization under strict steric requirements, such as the previously reported 7-OH substitution or the present isosteric modification.