Development of a radioiodinated thioflavin-T-Congo-red hybrid probe for diagnosis of systemic amyloidosis.

Introduction: Systemic amyloidosis is a group of diseases characterized by the deposition of amyloid protein in multiple organs throughout the body and causing their dysfunction. As amyloid deposition is observed at an early phase and is highly specific to systemic amyloidosis, noninvasive detection of amyloid is considered useful for the early diagnosis of systemic amyloidosis. In this study, we designed and synthesized a novel radiolabeled amyloid imaging probe, sodium (E)-4-amino-3-((4-(6-iodobenzothiazol-2-yl)phenyl)diazenyl)naphthalene-1-sulfonate (1), which combines two amyloid-binding compounds, thioflavin-T and Congo-red, and evaluated its effectiveness in diagnosing amyloidosis.

Methods: A tributyltin precursor was synthesized through a 5-step reaction from 2-amino-6-bromobenzothiazole, and [I]1 was synthesized by an iododestannylation reaction with a tributyltin precursor. Mouse models of amyloid A (AA) amyloidosis, a type of systemic amyloidosis, were prepared by intraperitoneal injection of amyloid-enhancing factor into mice. An in vitro autoradiographic study was performed using spleen sections from normal mice and AA amyloidosis mice. Furthermore, [I]1 was intravenously injected into mice, and its distribution was evaluated. Finally, an ex vivo autoradiographic study was performed using AA amyloidosis mice.

Results: [I]1 was obtained with a radiochemical yield of 66% and a radiochemical purity of over 95%. In vitro autoradiography revealed specific binding of [I]1 to thioflavin-S-stained regions in the spleen. Normal mice showed relatively rapid clearance of [I]1 from the organs, whereas radioactivity was retained in the spleen, where amyloid deposition was observed in model mice. Furthermore, ex vivo autoradiography showed a heterogeneous distribution of [I]1, which was co-localized with thioflavin-S-stained regions in the spleen of model mice.

Conclusion: These results indicate the potential of radioiodinated 1 as a nuclear imaging probe for diagnosing AA amyloidosis.