SEARCH FOR LEAD STRUCTURES TO DEVELOP NEW ALLOSTERIC MODULATORS OF MUSCARINIC RECEPTORS

Various compounds are known to allosterically modulate the binding of ligands to muscarinic receptors. Most of these compounds have another predominant, pharmacological action. Identification of the potent repr esentatives should be useful far the development of allosteric modulat ors that are specific and highly active. For various reasons, a direct comparison of allosteric potencies on the basis of literature data is difficult. Therefore, a series of compounds was compared with regard to the allosteric delay of the dissociation of N-[H-3]methylscopolamin e from porcine heart M(2) receptors under the following assay conditio ns: ''Na,K,P-i buffer'', 4 mM Na2HPO4, 1 mM KH2PO4, pH 7.4, 23 degrees C; ''Mg,Tris,CI,P-i buffer'', 50 mM Tris-HCl, 3 mM MgHPO4, pH 7.3, 37 degrees C. Generally, the allosteric potency of the compounds was hig her in the Na,K,P-i buffer, compared with the Mg,Tris,Cl,P-i buffer, H owever, the extent of the potency shift differed, ranging from approxi mately 2-fold for tacrine to approximately 100-fold for gallamine. The concentration retarding radioligand dissociation to half of the contr ol rate (EC(50)) served as a measure of allosteric potency. Under both assay conditions, alcuronium was the most potent compound (EC(50,Na,K ,Pi) = 4 nM and EC(50,Mg,Tris,Cl,Pi) = 55 nM), followed by alkane-bisa mmonium and bispyridinium compounds containing phthalimido moieties. G allamine showed intermediate potency (EC(50) values of 180 nM and 16,0 00 nM in Na,K,P-i buffer and Mg,Tris,Cl,P-i buffer, respectively). Obi doxime and hexamethonium, both known to antagonize allosteric actions, revealed submaximal efficacy and low potency (EC(50,Na,K,P?i) of appr oximately 100,000 nM). The relevance of these results, regarding the i dentification of lead structures for the development of new allosteric modulators, is discussed.