Radioiodinated styrylbenzenes and thioflavins as probes for amyloid aggregates

We report for the first time that small molecule-based radiodiodinated ligands, showing selective binding to A beta aggregates, cross the intact blood-brain barrier by simple diffusion. Four novel ligands showing preferential labeling of amyloid aggregates of A beta (1-40) and A beta (1-42) peptides, commonly associated with plaques in the brain of people with Alzheimer's disease (AD), were developed. Two I-125-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 I-125-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 A beta (1-40) and 0.15, 0.73, 0.14, and 0.15 nM for aggregates of A beta (1-42), respectively. Interestingly, under a competitive-binding assaying condition, different binding sites on A beta (1-40) and A beta (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 A beta (1-42) aggregates in the brain showed that both [I-125]18a and [I-125]18b labeled these brain sections, but [I-125]13, selective for A beta (1-40) aggregates, exhibited very low labeling of the comparable brain section. Biodistribution studies in normal mice after an iv injection showed that [I-125]18a and [I-125]18b exhibited excellent brain uptake and retention, the levels of which were much higher than those of [I-125]12 and [I-125]13. These findings strongly suggest that the new radioiodinated ligands, [I-125]12 (ISB), [I-125]13 (IMSB), [I-125]18a, (TZDM), and [I-125]18b (TZPI), may be useful as biomarkers for studying A beta (1-40) as well as A beta (1-42) aggregates of amyloidogenesis in AD patients.