Radioiodinated clioquinol as a biomarker for β‐amyloid: Zn2+ complexes in Alzheimer's disease

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Abstract
Neocortical β‐amyloid (Aβ) aggregates in Alzheimer's disease (AD) are enriched in transition metals that mediate assembly. Clioquinol (CQ) targets metal interaction with Aβ and inhibits amyloid pathology in transgenic mice. Here, we investigated the binding properties of radioiodinated CQ ([125I]CQ) to different in vitro and in vivo Alzheimer models. We observed saturable binding of [125I]CQ to synthetic Aβ precipitated by Zn2+ (Kd = 0.45 and 1.40 nm for Aβ1‐42 and Aβ1‐40, respectively), which was fully displaced by free Zn2+, Cu2+, the chelator DTPA (diethylene triamine pentaacetic acid) and partially by Congo red. Sucrose density gradient of post‐mortem AD brain indicated that [125I]CQ concentrated in a fraction enriched for both Aβ and Zn, which was modulated by exogenous addition of Zn2+ or DTPA. APP transgenic (Tg2576) mice injected with [125I]CQ exhibited higher brain retention of tracer compared to non‐Tg mice. Autoradiography of brain sections of these animals confirmed selective [125I]CQ enrichment in the neocortex. Histologically, both thioflavine‐S (ThS)‐positive and negative structures were labeled by [125I]CQ. A pilot SPECT study of [123I]CQ showed limited uptake of the tracer into the brain, which did however, appear to be more rapid in AD patients compared to age‐matched controls. These data support metallated Aβ species as the neuropharmacological target of CQ and indicate that this drug class may have potential as in vivo imaging agents for Alzheimer neuropathology.