Radioiodinated Styrylbenzenes and Thioflavins as Probes for Amyloid Aggregates

Abstract

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.