Semisynthetic Analogues of Toxiferine I and Their Pharmacological Properties at α7 nAChRs, Muscle-Type nAChRs, and the Allosteric Binding Site of Muscarinic M2 Receptors

A new series of analogues of the calabash curare alkaloid toxiferine I was prepared and pharmacologically evaluated at α7 and muscle-type nAChRs and the allosteric site of muscarinic M2 receptors. The new ligands differ from toxiferine I by the absence of one (2a-c) or two (3a-c) hydroxy groups, saturation of the exocyclic double bonds, and various N-substituents (methyl, allyl, 4-nitrobenzyl). At the muscle-type nAChRs, most ligands showed similar binding to the muscle relaxant alcuronium, indicating neuromuscular blocking activity, with the nonhydroxylated analogues 3b (Ki = 75 nM) and 3c (Ki = 82 nM) displaying the highest affinity. At α7 nAChRs, all ligands showed a moderate to low antagonistic effect, suggesting that the alcoholic functions are not necessary for antagonistic action. Compound 3c exerted the highest preference for the muscle-type nAChRs (Ki = 82 nM) over α7 (IC50 = 21 μM). As for the allosteric site of M2 receptors, binding was found to be dependent on N-substitution rather than on the nature of the side chains. The most potent ligands were the N-allyl analogues 2b and 3b (EC0.5,diss = 12 and 36 nM) and the N-nitrobenzyl derivatives 2c and 3c (EC0.5,diss = 32 and 49 nM). The present findings should help delineate the structural requirements for activity at different types of AChRs and for the design of novel selective ligands.