IMPY: an improved thioflavin-T derivative for in vivo labeling of beta-amyloid plaques.

Development of small molecular probes for in vivo labeling and detection of beta-amyloid (Abeta) plaques in patients of Alzheimer's disease (AD) is of significant scientific interest, and it may also assist the development of drugs targeting Abeta plaques for treatment of AD. A novel probe, [123I/(125)I]IMPY, 6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine, was successfully prepared with an iododestannylation reaction catalyzed by hydrogen peroxide. The modified thioflavin-T derivative displayed a good binding affinity for preformed synthetic Abeta40 aggregates in solution (K(i)=15+/-5 nM) and showed selective plaque labeling on postmortem AD brain sections. Biodistribution study in normal mice after an iv injection of [125I]IMPY exhibited excellent brain uptake (2.9% initial dose/brain at 2 min) and fast washout (0.2% initial dose/brain at 60 min). These properties are highly desirable for amyloid plaque imaging agents. In vivo plaque labeling was evaluated in a transgenic mouse model (Tg2576) engineered to produce excess amyloid plaques in the brain. Ex vivo autoradiograms of brain sections of the Tg 2576 mouse obtained at 4 h after an i.v. injection of [125I]IMPY clearly displayed a distinct plaque labeling with a low background activity. When the same brain section was stained with a fluorescent dye, thioflavin-S, the same Abeta plaques showed prominent fluorescent labeling consistent with the results of the autoradiogram. In conclusion, these findings clearly suggest that radioiodinated IMPY demonstrates desirable characteristics for in vivo labeling of Abeta plaques and it may be useful as a molecular imaging agent to study amyloidogenesis in the brain of living AD patients.