Alzheimer's disease (AD) is a neurodegenerative disorder most often diagnosed 10 years after its onset and development. It is characterized by the accumulation of amyloid-β peptide (ABP) into amyloid plaques between nerve cells, which produces a massive local neurodegeneration. Molecular magnetic resonance imaging allows diagnosis of AD by showing ABP accumulation in the brain. The ultrasmall particles of iron oxide (USPIO) derivatives proposed in the present work were functionalized with peptides that present an affinity for ABP, independently of its state of aggregation. Their nanomolar Kd * confirms the high affinity of our vectorized contrast agents (VCA) for ABP and therefore their high labeling potential, specificity and sensitivity. Their lack of toxicity has been demonstrated, both by in vitro studies using the MTT method on several cell types, and by in vivo investigations with assessment of renal and hepatic biomarkers and by histopathology evaluation. The results of biodistribution studies corroborated by MRI demonstrate that USPIO-PHO (USPIO coupled to peptide C-IPLPFYN-C) are able to cross the blood-brain barrier without any facilitating strategy, and accumulates in the brain 90 min after its injection in NMRI mice. None of the USPIO derivatives were found in any organs one week after administration. To conclude, USPIO-PHO seems to have a genuine potential for labeling amyloid plaques in the brain; it has a nanomolar binding affinity, no toxic effects, and its elimination half-life is about 3 h. Further tests will be made on transgenic mice, aimed at confirming the potential of early AD diagnosis using our VCA.

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The diagnosis of Alzheimer's disease (AD) is a critical step in the management of patients. We have developed a non-invasive diagnosis tool based on magnetic resonance molecular imaging (MRMI) of amyloid-β peptide using ultra-small particles of iron oxide (USPIO) functionalized with a disulfide constrained cyclic heptapeptide (PHO) identified by phage display (USPIO-PHO). After previously demonstrating the optimal pharmacologic properties of USPIO-PHO and its capacity to cross the blood-brain barrier (BBB), the ability of USPIO-PHO to target amyloid plaques (AP) by MRMI has been validated in the present work on AD transgenic mice. The immunohistochemistry and immunofluorescent detection of USPIO-PHO on brain sections collected after in vivo MRMI studies enabled its colocalization with AP, confirming the BBB passage and specific targeting. The AP targeting by USPIO-PHO has been moreover corroborated by the good correlation between the number of AP detected with anti-amyloid β antibody and Perls'-DAB staining. Finally, the crossing mechanism of USPIO-PHO through the BBB was elucidated, revealing the involvement of non-degradation pathway of caveolae, while the control contrast agent USPIO-PEG was not endocytosed by the human brain endothelial cells.