Bisquaternary ligands of the common allosteric site of M2 acetylcholine receptors: search for the minimum essential distances between the pharmacophoric elements.

Structurally diverse molecules, such as alcuronium, gallamine, and tubocurarine as well as W84 and WDUO, are known to interact allosterically with ligand binding to muscarinic M2 acetylcholine receptors. Preliminary molecular modeling studies revealed two positive charges in the middle and two lateral aromatic areas to be essential elements of a high allosteric potency. To find out the optimum distances between these pharmacophoric elements, a systematic variation of the spacer in the series of W84, WDUO, and IWDUO compounds was performed. The allosteric reduction of the rate of dissociation of the antagonist [3H]-N-methylscopolamine from porcine heart M2 receptors served as a test system. The minimal essential distance between the positive charges was found to be 10 A. The length of the peripheral spacers connecting the positive charge and the lateral aromatic moiety appears to depend on the chemical functionality; the peripheral spacers have to be long and flexible enough to position the aromatic skeletons in the spatial neighborhood of the alkane middle chain: in the case of an oxime ether containing peripheral spacer, six atoms are required, and in the case of an alkane chain, four carbon atoms are necessary to adopt the pharmacophoric S-shape conformation.

A stable and highly potent hexamethonium derivative which modulates muscarinic receptors allosterically in guinea-pig hearts.

W84 (N,N,N',N'-tetramethyl-N,N'-bis-(3-phthalimidopropyl)-N,N'-hexane- 1,6- diyl-bis-ammonium dibromide) is a potent stabilizer of antagonist binding to muscarinic receptors; however, W84 hydrolyses in aqueous buffered medium. The synthesis of the stable derivative CHIN3/6 is presented containing a 2-phenyl-quinazolinone instead of the labile phthalimide substituent. This compound retarded [3H]N-methylscopolamine-dissociation in guinea-pig cardiac membranes with slightly higher potency than W84, the EC50 values amounting to 7.5 x 10(-7) and 13 x 10(-7) M, respectively. Molecular modelling revealed differences in the electron density of the substituents and in their molecular shape. It is suggested that the derivatives use partially different sites of attachment when occupying the allosteric binding site of the receptor protein.