2011-2012 ARDRAF Final Project Report

Small Molecules as Negative Allosteric Modulators of α7 nAChRs

Small Molecules as Negative Allosteric Modulators of α7 nAChRs

Malgorzata Dukat, PhD and Galia R. Abdrakhmanova, MD, PhD  

Virginia Commonwealth University

Both agonists and antagonists of α7 nAChRs (i.e., nicotinic acetylcholine receptors) have been shown to be of value in the treatment of AD.  Agonists might desensitize the action of ACh at these receptors, thereby reducing cholinergic transmission, and antagonists block ACh transmission. An entirely novel approach is to identify negative allosteric modulators (NAMs) of α7 nAChRs that can selectively, but indirectly, block the effect of ACh at α7 nAChRs without acting at α4β2 receptors.  The investigators previously identified one of the first small-molecule NAMs of α7 nAChRs, namely MD-354.  Because this compound is a 5-HT3 (serotonin) receptor agonist, the investigators modifed its structure to abolish that action, and thereby develop “selective” α7 nAChR allosteric modulators.  They synthesized a series of MD-354 analogs and evaluated them in functional assays to determine what structural features are required, and to optimize the pharmacological actions by eliminating affinity for 5-HT3 receptors.  The present study delivered proof of concept that small molecules, the guanidines, represent a novel class of α7 nAChR NAMs. Furthermore, the investigators demonstrated that small structural changes to MD-354 diminished or abolished its 5-HT3 receptor affinity, while retaining its α7 nAChR activity. All the NAMs examined display antagonism action at α7 nAChR with half maximal inhibitory concentration (IC50) values ranging from 1.3 – 34.8 µM. To eliminate possible competitive antagonism or channel blocking action, MD-354 and one of the newly synthesized agents were evaluated in voltage-dependence inhibition of ACh experiments and both proved to be α7 nAChR NAMs. In addition, they were assayed for antagonistic activity at α3β4 and α4β2 nAChRs and were found to be inactive, suggesting their selective action at α7 nAChRs. The most potent NAM, was further evaluated in radioligand binding assays for its selectivity among cloned nAChRs (i.e., α2β2, α2β4, α3β2, α4β2, α3β4, α4β2, α4β4) and was found to lack binding affinity at all seven (i.e., Ki > 10,000 nM).  This is the first study identifying guanidine analogs as small molecule α7 nAChR NAMs.  In contrast to current ACh inhibitors that are limited to symptomatic treatment of cognitive function, these new agents offer the potential for slowing the progression of AD.

(Dr. Dukat may be contacted at 804/828-5256; mdukat@vcu.edu and Dr. Abdrakhmanova may be contacted at 804/828-1797; gabdrakhmano@vcu.edu)


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