Synthesis and Pharmacological Characterization of Novel Analogues of the Nicotinic Acetylcholine Receptor Agonist (±)-UB-165

Abstract

(+/-)-UB-165 (1) is a potent neuronal nicotinic acetylcholine receptor (nAChR) ligand, which displays functional selectivity between nAChR subtypes. Using UB-165 as a lead structure, two classes of racemic ligands were synthesized and assessed in binding assays for three major nAChR subtypes (alpha4beta2, alpha3beta4, and alpha7). The first class of compounds comprises the three pyridine isomers 4-6, corresponding to the 3-, 2-, and 4-substituted pyridine isomers, respectively. Deschloro UB-165 (4) displayed a 2-3-fold decrease in affinity at alpha4beta2 and alpha3beta4 nAChR subtypes, as compared with (+/-)-UB-165, while at the alpha7 subtype a 31-fold increase in affinity was observed. At each of the nAChR subtypes, high affinity binding was dependent on the presence of a 3-substituted pyridine, and the other isomers, 5 and 6, resulted in marked decreases in binding affinities. The second class of compounds is based on replacing the pyridyl unit of 1 with a diazine moiety, giving pyridazine (7), pyrimidine (8), and pyrazine (9), which retain the "3-pyridyl" substructure. Modest reductions in binding affinity were observed for all of the diazine ligands at all nAChR subtypes, with the exception of 7, which retained potency comparable to that of 4 in binding to alpha7 nAChR. In functional assays at the alpha3beta4 nAChR, all analogues 4-9 were less potent, as compared with 1, and the rank order of functional potencies correlated with that of binding potencies. Computational studies indicate that the 3-substituted pyridine 4 and 2-substituted pyridine 5, as well as the diazine analogues 7-9, all conform to a distance-based pharmacophore model recently proposed for the alpha4beta2 receptor. However, the nicotinic potencies of these ligands vary considerably and because 5 lacks appreciable nicotinic activity, it is clear that further refinements of this model are necessary in order to describe adequately the structural and electronic demands associated with this nAChR subtype. This rational series of compounds based on UB-165 presents a systematic approach to defining subtype specific pharmacophores.

The Allosteric Potentiation of Nicotinic Acetylcholine Receptors by Galantamine Is Transduced into Cellular Responses in Neurons: Ca2+Signals and Neurotransmitter Release.

“…Competition binding assays were performed as described previously (Sharples et al, 2000(Sharples et al, , 2002 (Cheng and Prusoff, 1973), assuming K d values determined in the corresponding saturation binding assays.…”

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

“…Affinity values (K i , nanomolar) are based on inhibition of ligand binding to nicotinic (α 4 β 2 /α 3 β 4 * /α 7 ) receptors. Binding and/or functional data has been reported for epiboxidine (Badio et al, 1997b); homoepiboxidine (Fitch et al, 2004); homoepibatidine (Cox et al, 2001;Malpass et al, 2001;Spang et al, 2000;Xu et al, 1996); mesohomoepibatidine (Olivo et al, 1999); dehydro-analog (Rádl et al, 2000); oxadiazole analog (Fitch et al, 2004); bishomoepibatidine ; isohomoepibatidine (Zhang et al, 1997); CMI 934 (Ellis et al, 1999); UB-165 (Sharples et al, 2002;Wright et al, 1997) and pyrimidine analogs (Seerden et al, 1998). receptors primarily with influx of sodium-22 or with efflux of rubidium-86 in cultured cells. (4) Neurotransmitter release elicited by nicotinic agonists has been primarily studied for release of acetylcholine or catecholamines with brain synaptosomes or slices (see Grady et al, 2001 and references therein).…”

Nicotinic Acetylcholine Receptor-Mediated [3H]Dopamine Release from Hippocampus

“…Although ABT-594 was equally efficacious in both assays, it was ϳ4. (Sharples et al, 2000) and rat (Sharples et al, 2002) recombinant ␣3␤4* nAChRs. If the pharmacologic profile of (Ϯ)-UB-165 at rat recombinant and native nAChRs is similar, the partial efficacy of (Ϯ)-UB-165 suggests that other subtypes that are not significantly activated by (Ϯ)-UB-165 in humans, such as ␣4␤2, ␣2␤2, and/or an nAChR(s) composed of multiple ␣ and ␤ subunits, also may function in nAChR-mediated rat hippocampal […”

“…Although little is known regarding its selectivity for specific nAChRs in the rodent brain, MG 624 has been reported to display nanomolar affinity and antagonist activity against rat ␣7 receptors expressed in Xenopus oocytes (Di Angelantonio et al, 2000). (Ϯ)-UB-165, a hybrid of anatoxin-a and epibatidine, has been demonstrated to functionally activate recombinant rat ␣3␤4 expressed in a mouse fibroblast cell (Sharples et al, 2002).…”