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We report for the first time that small molecule-based radiodiodinated ligands, showing selective binding to Aβ aggregates, cross the intact blood−brain barrier by simple diffusion. Four novel ligands showing preferential labeling of amyloid aggregates of Aβ(1−40) and Aβ(1−42) peptides, commonly associated with plaques in the brain of people with Alzheimer's disease (AD), were developed. Two 125I-labeled styrylbenzenes, (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-hydroxy)styrylbenzene, 12 (ISB), and (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-methoxy)styrylbenzene, 13 (IMSB), and two 125I-labeled thioflavins, 2-[4‘-(dimethylamino)phenyl]-6-iodobenzothiazole, 18a (TZDM), and 2-[4‘-(4‘ ‘-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, 18b (TZPI), were prepared at a high specific activity (2200 Ci/mmol). In vitro binding studies of these ligands showed excellent binding affinities with K d values of 0.08, 0.13, 0.06, and 0.13 nM for aggregates of Aβ(1−40) and 0.15, 0.73, 0.14, and 0.15 nM for aggregates of Aβ(1−42), respectively. Interestingly, under a competitive-binding assaying condition, different binding sites on Aβ(1−40) and Aβ(1−42) aggregates, which are mutually exclusive, were observed for styrylbenzenes and thioflavins. Autoradiography studies of postmortem brain sections of a patient with Down's syndrome known to contain primarily Aβ(1−42) aggregates in the brain showed that both [125I]18a and [125I]18b labeled these brain sections, but [125I]13, selective for Aβ(1−40) aggregates, exhibited very low labeling of the comparable brain section. Biodistribution studies in normal mice after an iv injection showed that [125I]18a and [125I]18b exhibited excellent brain uptake and retention, the levels of which were much higher than those of [125I]12 and [125I]13. These findings strongly suggest that the new radioiodinated ligands, [125I]12 (ISB), [125I]13 (IMSB), [125I]18a (TZDM), and [125I]18b (TZPI), may be useful as biomarkers for studying Aβ(1−40) as well as Aβ(1−42) aggregates of amyloidogenesis in AD patients.

Several small molecule ligands for amyloid‐β (Aβ) fibrils deposited in brain have been developed to facilitate radiological diagnosis of Alzheimer’s disease (AD). Recently, the build‐up of Aβ oligomers (AβO) in brain has been recognized as an additional hallmark of AD and may play a more significant role in early stages. Evidence suggests that quantitative assessment of AβO would provide a more accurate index of therapeutic effect of drug trials. Therefore, there is an urgent need to develop methods for efficient identification as well as structural analysis of AβO. We found that some well established amyloid ligands, analogs of Congo red and thioflavin‐T (ThT), bind AβO with high affinity and detect AβO in vitro and in vivo. Binding studies revealed the presence of binding sites for Congo red‐ and thioflavin‐T‐analogs on AβO. Furthermore, these ligands can be used for imaging intracellular AβO in living cells and animals and as positive contrast agent for ultrastructural imaging of AβO, two applications useful for structural analysis of AβO in cells. We propose that by improving the binding affinity of current ligands, in vivo imaging of AβO is feasible by a ‘signal subtraction’ procedure. This approach may facilitate the identification of individuals with early AD.

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