Design, synthesis and pharmacological properties of alkylbisammonio bitopic ligands of muscarinic acetylcholine receptors

Over the past two decades, novel opportunities for drug discovery have risen from a greater understanding of the complexity of GPCR signaling. A striking example of this is the appreciation that GPCRs possess functional allosteric binding sites.[1] The five muscarinic acetylcholine receptor (mAChR) subtypes bind their physiological transmitter in the highly conserved orthosteric site within the transmembrane domain of the receptors. Orthosteric muscarinic agonists have negligible binding selectivity and poor signaling specificity. A less conserved allosteric site has been also characterized at the extracellular entrance of the binding pocket of mAChRs. Our interest in the research field on mAChR ligands has been recently extended to the study of derivatives designed to simultaneously interact with the orthosteric and the allosteric site (bitopic ligands) of these receptors, thus exploiting the complementary characteristics of the two different binding sites by a single ligand molecule.[2,3] This approach is a variation of the more traditional bivalent ligand concept and shares some of the same challenges, including the choice and role of the linker between the two pharmacophores and the validation of mechanism of action.[4] The most interesting bitopic compounds which emerged from our investigation (represented by the general molecular skeleton A in Figure 1) are alkylbisammonio derivatives incorporating a) iperoxo, an oxotremorine-related unselective muscarinic superagonist, b) a polymethylene spacer chain, and c) a heteroaromatic fragment targeting the allosteric site. The synthetic approach of target compounds and the most relevant results of their pharmacological investigation will be presented. In particular, the analgesic effects observed for a group of the compounds under study will be illustrated in detail. References 1. Wang, L.; Martin, B.; Brenneman, R.; Luttrell, L. M.; Maudsley, S. Allosteric Modulators of G Protein-Coupled Receptors: Future Therapeutics for Complex Physiological Disorders. J. Pharmacol. Exp. Ther. 2009, 331, 340-348 2. Antony, J.; Kellershohn, K.; Disingrini, T.; Dallanoce, C.; Tränkle, C.; Christopoulos, A.; Barocelli, E.; De Amici, M.; Holzgrabe, U.; Mohr, K. et al. Dualsteric GPCR targeting: a novel route to binding and signaling pathway selectivity. FASEB J. 2009, 23, 442-450 3. Bock, A.; Merten, N.; Schrage, R.; Dallanoce, C.; Matera, C.; Hoffmann, C.; De Amici, M.; Holzgrabe, U.; Kostenis, E.; Mohr, K. et al. The allosteric vestibule of a seven transmembrane helical receptor controls G-protein coupling. Nat. Commun. 2012, 3, 1044 4. Valant, C.; Lane, J. R.; Sexton, P. M.; Christopoulos, A. The Best of Both Worlds? Bitopic Orthosteric/Allosteric Ligands of G Protein-Coupled Receptors. Annu. Rev. Pharmacol. Toxicol. 2012, 52, 153-178