Analgesic effects mediated by muscarinic ligands acting at both orthosteric and allosteric receptor binding sites

Our interest in the research field on muscarinic acetylcholine receptor (mAChR) ligands has been recently focused on the study of derivatives designed to simultaneously interact with the well conserved transmitter binding site (orthosteric site) and an additional, less conserved, recognition site (allosteric site), which is localized at the extracellular entrance of the binding pocket (1,2). This approach, exploiting complementary characteristics of the orthosteric and allosteric site by a single ligand molecule, could improve mAChR subtype selectivity. The most interesting compounds investigated so far, represented by the general molecular skeleton A, are alkylbisammonio derivatives incorporating a) iperoxo, an oxotremorine-related unselective muscarinic superagonist (3,4), b) a polymethylene spacer chain, and c) a heteroaromatic fragment targeting the allosteric site. As an extension of the pharmacological studies on these muscarinic ligands, a group of iperoxo-containing derivatives, characterized in tissue preparations containing the M1, M2, and M3 mAChR subtypes, were assayed for their analgesic activity. As far as the writhing test results in mice are taken into account, the antinociceptive effects produced by the investigated bitopic ligands were less pronounced than those shown by the orthosteric reference compounds (iperoxo and oxotremorine). However, some of the most potent bitopic analgesic compounds were nearly devoid of common muscarinic side-effects, such as diarrhea, salivation, lacrimation, tremor, and hypothermia. The synthetic details along with and the most relevant results of the pharmacological investigation will be presented and commented. (1) Antony, J.; Kellershohn, K.; Mohr-Andrä, M.; Kebig, A.; Prilla, S.; Muth, M.; Heller, E.; Disingrini, T.; Dallanoce, C.; Bertoni, S.; Schrobang, J.; Tränkle, C.; Kostenis, E.; Christopoulos, A.; Höltje, H.-D.; Barocelli, E.; De Amici, M.; Holzgrabe, U.; Mohr, K. FASEB J. 2009, 23, 442-450. (2) Mohr, K.; Tränkle, C.; Kostenis, E.; Barocelli, E.; De Amici, M.; Holzgrabe, U. Br. J. Pharmacol. 2010, 159, 997-1008. (3) Dallanoce, C.; Conti, P.; De Amici, M.; De Micheli, C.; Barocelli, E.; Chiavarini, M.; Ballabeni, V.; Bertoni, S.; Impicciatore, M. Bioorg. Med. Chem. 1999, 7, 1539-1547. (4) Barocelli, E.; Ballabeni, V.; Bertoni, S.; De Amici, M.; Impicciatore, M. Life Sci. 2001, 68, 1775-1785.