5-iodo-A-85380, an α4β2 subtype-selective ligand for nicotinic acetylcholine receptors

In an effort to develop selective radioligands for in vivo imaging of neuronal nicotinic acetylcholine receptors (nAChRs), we synthesized 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-iodo-A-85380) and labeled it with I-125 and I-123. Here we present the results of experiments characterizing this radioiodinated ligand in vitro. The affinity of 5-[I-125]iodo-A-85380 for alpha 4 beta 2 nAChRs in rat and human brain is defined by K-d values of 10 and 12 pM, respectively, similar to that of epibatidine (8 pM). In contrast to epibatidine, however, 5-iodo-A-85380 is more selective in binding to the alpha 4 beta 2 subtype than to other nAChR subtypes. In rat adrenal glands, 5-iodo-A-85380 binds to nAChRs containing alpha 3 and beta 4 subunits with 1/1000th the affinity of epibatidine, and exhibits 1/60th and 1/190th the affinity of epibatidine at alpha 7 and muscle-type nAChRs, respectively. Moreover, unlike epibatidine and cytisine, 5-[I-125]iodo-A-85380 shows no binding in any brain regions in mice homozygous for a mutation in the beta 2 subunit of nAChRs. Binding of 5-[I-125]iodo-A-85380 in rat brain is reversible, and is characterized by high specificity and a slow rate of dissociation of the receptor-ligand complex (t(1/2) for dissociation similar to 2 h). These properties, along with other features observed previously in in vivo experiments (low toxicity, rapid penetration of the blood-brain barrier, and a high ratio of specific to nonspecific binding), suggest that this compound, labeled with I-125 or I-123, is superior to other radioligands available for in vitro and in vivo studies of alpha 4 beta 2 nAChRs, respectively.