Development of nicotinic receptor ligands for positron emission tomography - from in vitro to in vivo : Binding studies of neuronal nicotinic acetylcholine receptors using different ³H-, ¹1C- and `6Br - labeled ligands

Sammanfattning: Neuronal nicotinic acetylcholine receptors (nAChRs) are involved in central processes, such as cognitive functionand memory, and play a critical role in neuropathological changes in Alzheimer's disease (AD). The in vivocharacterization of nAChRs by positron emission tomography (PET) may increase our understanding of thefunctional implications of these receptors in AD and their early changes during this disease. So far, only (S)(-)[11C]nicotine has been available for human PET-studies, and the development of new nAChR ligands for PET hasthus been urged. Before new receptor Iigands can be used in PET-experiments, their in vitro properties should becharacterized. This requirement necessitated the establishment of methods for the in vitro characterization ofreceptor ligands. Thus, the Phosphor Imager system was applied and found to be an appropriated device formeasuring the fast-decaying PET-ligands, utilizing properties such as very high specific radioactivity and rapidityof the experiments.To obtain knowledge about the subregional distribution of nAChRs in the human cerebral cortex, in vitro autoradiography with (-)[3H]nicotine, [3H]epibatidine and [3H]cytisine was performed on human forebrainhemispheres and yielded for the three nAChR ligands essentially similar binding patterns with binding maxima inlayers I, III, and V, suggesting one common high-affinity nAChR binding site. In some cortical regions, an additional binding site was found for [3H](-)nicotine and [3H]epibatidine. In the medial temporal cortex a significant age-related decrease in the binding of the three 3HnAChR ligands was observed, whereas the binding of [3H]vesamicol representing vesicular acetylcholine transporter (VAChT) sites, did not change with aging. Further studies in temporal cortex of AD showed significant decreased binding of the 3H-nAChR ligands and [3H] vesamicol in all cortical layers; but nAChR binding was reduced twice as much as [3H]vesamicol binding. The less severe loss of [3H]vesamicol binding suggests that VAChT sites are more preserved in still existing presynaptic terminals having compensatory capacity to maintain cholinergic activity.Except for the 3H-ligands, the novel nAChR ligands for PET, (S)(-)[11C]nicotine, [11C]ABT-418, [11C]MPA,and [76Br]BAP were investigated and found to have high affinity for nAChRs, similar to the 3H-nAChR ligands. Theazetidine analogs [11C]MPA and [76Br]BAP showed slower binding dissociation and higher specific binding thanthe other two 11C-1igands; both have high selectivity for the α4β2 nAChR subtype that is abundant in the brain andprobably most vulnerable in AD. The similar in vitro binding properties of the new PET-ligands compared withthose of the 3H-nAChR ligands and the possibility to use the same tracer in vivo allow to test the new nAChR PET-ligands fast and easily in PET-studies. The [76Br]BAP uptake was blocked in vivo in rat brain up to 90% by (-)nicotine. In PET-studies with Rhesus monkeys, (-)nicotine displaced [76Br]BAP (60%) and [11C]MPA (30%) in thethalamus. The very high affinity, the high specific in vivo uptake, and the nAChR subtype selectivity of [76Br]BAPencourage its application in further PET-studies that might include human studies.