Ligandi dei siti recettoriali orto- e allosterici: caratterizzazione in vitro e in vivo di nuovi derivati dualsterici bisammonioalcanici

In this study we characterized in vitro and in vivo three different groups of dualsteric compounds, which were synthesized as M2 muscarinic agonists. These bitopic ligands, which incorporate in a unique molecule an orthosteric muscarinic agonist and a molecular fragment of different allosteric modulators, are expected to bind simultaneously to the orthosteric and the allosteric sites present in the M2 receptor discriminating this receptor subtype with respect to the other muscarinic receptors. The interest in these molecules derives from the hypothesis that compounds which display M2 subtype preference could be a promising strategy to develop novel analgesic agents as an improved approach alternative to the current pain therapy, today still inadequate because of the development of dependence and untoward effects or, sometimes, poorly effective. Indeed, by taking advantage of the structural heterogeneity between the allosteric sites of muscarinic receptors, dualsteric ligand could display M2 subtype preference to reach analgesic properties without relevant cholinergic side effects. The first two groups of hybrids here studied behaved as potent M2 full agonists endowed with slightly lower potency and intrinsic activity against M1 and M3 subtypes when studied in proper isolated organ preparations; instead the last group of compounds showed only weak muscarinic agonism and negligible ability to activate M2 receptors. In vivo, almost all the compounds showed dose-dependent analgesic effects in Writhing test but, nevertheless an improved tolerability profile compared with the respective orthosteric progenitor, only few agents deserved a deeper investigation on their effective analgesic activity. The final result was the identification of a selective M2 muscarinic bitopic agonist, 8b, with a remarkable and safe antinociceptive property that provides support to the strategy to generate dualsteric ligands as advantageous tools opening promising avenues toward novel pain therapy. Lastly, an exciting unexpected result arising from this research is the identification of analgesic compounds endowed with a multiple but muscarinic-independent mechanism of action whose pharmacology remains to be further investigated.