Orthosteric and allosteric ligands selectively acting at cholinergic receptor subtypes

The presentation will focus on the results recently achieved by our research group in the design, synthesis and pharmacological evaluation of selective ligands which target cholinergic receptor subtypes, belonging to both the nicotinic (nAChR) and the muscarinic (mAChR) acetylcholine receptor families. A set of spirocyclic derivatives will be illustrated, in which the simultaneous presence of the quinuclidinyl and Delta2-isoxazolinyl moieties, coupled with suitable stereoelectronic features of the substituent at position 3 of the spirocyclic ring, engendered a selective agonist profile at the homomeric neuronal alpha7 nAChRs [1]. The most promising compound in the series has been further investigated in preclinical studies and in in vivo models of CNS disorders and neuropathic pain. A group of novel hybrid peptides structurally related to natural alpha-conotoxins MII and PIA will be also presented, which behave as competitive antagonists able to discriminate alpha6beta2* and alpha3beta2* nAChR subtypes [2]. The five mAChR subtypes bind their physiological transmitter in the highly conserved orthosteric site within the transmembrane domains 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. To gain subtypeselective M2 receptor activation, we designed a group of putative bitopic compounds, i. e. hybrid derivatives fusing highly potent, unselective oxotremorinelike orthosteric activators with M2-selective bis(ammonio)alkane-type allosteric fragments. The new ligands interacted simultaneously with both recognition areas of the receptor protein, thus allowing the exploitation of favorable features of the orthosteric and the allosteric site by a single ligand molecule. The orthosteric interaction provided high affinity binding and activation of M2 mAChRs. The allosteric interaction yielded receptor subtype-selectivity and, in addition, could modulate efficacy and activate pathway-specific intracellular signaling [3]. [1] C.Dallanoce, P.Magrone, C.Matera, F.Frigerio, G.Grazioso, M.De Amici, S.Fucile, V.Piccari, K.Frydenvang, L.Pucci, C.Gotti, F.Clementi, C.De Micheli, ChemMedChem, 6, 2011, 889-903. [2] M.De Amici, G.Grazioso, C.Dallanoce, C.De Micheli et al., submitted. [3] K.Mohr, C.Tränkle, E.Kostenis, E.Barocelli, M.De Amici, U.Holzgrabe, Br.J.Pharmacol., 159, 2010, 997-1008.