Synthesis and 5-HT-3 Receptor Binding of 2- and 3-(Halo)alkoxyl Substituted ( S)- N-(1-Azabicyclo [2.2.2]oct-3-yl)-5-chlorobenzamides as Potential Radioligands

In an effort to develop selective, high-affinity radioligands for the 5-HT-3 receptor, a series of homologues of 5-chloro-2,3-dimethoxy- N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide (2b) was prepared in which individual methoxy groups were replaced by ethoxyl, (2-fluoroethoxyl), allyloxyl, propargyloxyl, or (3-iodoallyl)oxyl groups. Affinities for the 5-HT-3 receptor were determined by displacement of the binding of [ 125I]MIZAC (2a), a selective 5-HT-3 receptor antagonist radioligand, in rat brain homogenates. The 3-substituted homologues were more potent than the lead compound, 2b. The homologue having the largest 3-substituent, i.e., E-(S)- N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-3-(3-iodo-2-propenyl)oxy-2-methoxybenzamide (3b, THIZAC), had one of the highest affinities, K i 0.08 nM. The 2-substituted homologues were equipotent with 2b, having K i 0.2–0.3 nM, regardless of the size of the substituent. The corresponding iodoallyl derivative, E-(S)- N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-(3-iodo-2-propenyl)oxy-3-methoxybenzamide (4, LIZAC), displayed a K i of 0.29 nM. Saturation binding of [ 125I]-4 gave a K D of 0.31 ± 0.04 nM and a B max of 2.36 ± 0.10 fmol/mg of entorhinal cortex. In vivo biodistribution of [ 125I]-4 in the rat brain showed increased accumulation in hippocampus relative to that in cerebellum. Both the high-affinity ligands [ 125I]-3b and [ 125I]-4 are potentially useful radioligands for studying the 5-HT-3 receptor.