Allosteric effect on muscarinic M2-receptors of derivatives of the alkane-bis-ammonium compound W84: comparison with bispyridinium-type allosteric modulators

Abstract The symmetrically shaped W84 = N,N,N′,N′-tetramethyl-N,N′-bis-(3-phthalimido-propyl)-N,N′-hexane-1,6-diyl-bis-ammonium dibromide is a potent allosteric stabilizer of antagonist-binding to cardiac muscarinic M2-receptors. The ability of unilaterally shortened W84 derivatives to allosterically retard the dissociation of [3H]N-methylscopolamine ([3H]NMS) from M2-receptors was determined in porcine cardiac membranes (3 mM MgHPO4, 50 mM TrisHCl, pH 7.3, 37°C). Shortening was accompanied by a reduction of the allosteric activity. For instance, W84 prolonged the half-life of [3H]NMS dissociation (control t 1 2 = 2.0 min) by a factor of 2 at a concentration of EC50 = 1.3 μM, whereas a derivative unilaterally lacking phthalimidopropyldimethylammonium was 40-fold less potent. It is concluded that the whole W84 molecule interacts with the allosteric site of the receptor. The structure-activity relationships found with this series of agents did not parallel findings made previously with similarly modified derivatives of the bispyridinium compound DUO 3, (E,E)-1,1′- (1,3-propanediyl)-bis[4-[[(2,6-dichlorobenzoxyl)imino]-methyl]pyridinium] dibromide, despite considerable similarity with respect to molecular shape and charge distribution.