Structure−Activity Relationships of Dimethindene Derivatives as New M2-Selective Muscarinic Receptor Antagonists

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Abstract
A series of 2,3-disubstituted indenes, which are analogues of the widely used histamine H1 receptor antagonist dimethindene, have been synthesized and studied as muscarinic and histamine receptor antagonists. The affinities of these compounds for the five human muscarinic receptor subtypes (M1−M5) and for human histamine H1 receptors were determined in radioligand binding studies using membranes from transfected Chinese hamster ovary (CHO) cells and [3H]N-methylscopolamine ([3H]NMS). The results demonstrate that the diisopropyl analogue 19 has a similar high affinity as (S)-dimethindene at M2 receptors ((S)-dimethindene: pKi = 7.52; (−)-19: pKi = 7.37) with an improved selectivity pattern ((S)-dimethindene: M2/M1 = 6-fold, M2/M3 = 5-fold, M2/M4 = 10-fold, M2/M5 = 25-fold; (−)-19: M2/M1 = 36-fold, M2/M3 = 96-fold, M2/M4 = 42-fold, M2/M5 = 275-fold). In addition, compound (−)-19 showed 35-fold lower affinity at histamine H1 receptors (pKi = 5.61) than (S)-dimethindene (pKi = 7.16). Another interesting compound is the fluoroethyl derivative 20 (pKi/M2 = 7.49), which also exhibits a higher M2 selectivity (M2/M1 = 19-fold; M2/M3 = 22-fold; M2/M4 = 13-fold; M2/M5 = 62-fold) than (S)-dimethindene. Unfortunately, compound 20 also shows a high affinity for histamine H1 receptors (pKi = 8.14). The compound with the highest affinity for M2 receptors (pKi = 7.91), the dimethylaminomethylene analogue 31, displayed only a small preference for M2 receptors. In conclusion, compound (−)-19 might be useful to test the hypothesis that blockade of muscarinic M2 receptors in the brain is a viable mechanism by which to produce improved cognition. This second-generation dimethindene analogue might also be the starting point for the development of M2-selective muscarinic antagonists useful for quantifying M2 receptors in the central nervous system with positron emission tomography imaging.