Rationale and Objectives: As a neurodegenerative pathology, Alzheimer disease (AD) is the principal cause of dementia in the elderly (before cerebral stroke and Parkinson disease) and the fourth cause of mortality in developed countries (after cardiac diseases, cancer and cerebral stroke). Its diagnosis can only be confirmed by autopsy, since the symptomatology is common to other neurodegenerative pathologies. The senile plaques, constituted essentially by the amyloid peptide Ab 1-42 (Ab 1-42 ), represent the main morpho-pathological feature of Alzheimer disease. An early and non-invasive diagnosis of this pathology would allow a more efficient treatment and may help to prolong the life expectancy of patients suffering from it. Molecular imaging is a recent and growing discipline, which could help to differentiate AD from other dementias. We propose to attempt MRI detection of the amyloid plaques by using peptides selected by phage display and subsequently grafted to adequate magnetic reporters. Methods: To select peptides with high affinity for the senile plaques, a disulfide constrained phage display heptapeptide library (New England Biolabs, The Netherlands) was incubated with Ab 1-42 (Bachem, Switzerland), which was immobilized by hydrophobic interactions on a plastic surface. After four rounds of selection, 72 candidate phage clones were arbitrarily isolated for further screening of their affinity for the target. The ELISA tests of affinity highlighted 23 phage clones with an optimal affinity for Ab 1-42 . Their peptide structure was subsequently determined by analysis of the DNA sequence of the fusion insert, and their dissociation constants (K d ) for the target were estimated with the aim of identifying the most efficient peptides. The best one was obtained by solid-phase synthesis and its K d was again evaluated after biotinylation. Results: The peptide sequence analysis of the 23 selected phage clones evidenced a more frequent representation of the amino acids Leu, Pro, His and Phe. With the exception of His, these amino acids are hydrophobic. This observation shows the importance of hydrophobic interactions with the target molecule. Concerning the amino acid representation in the peptide structures, the hydrophobic amino acids frequently occupy the first positions whereas Asn and Gln are well represented ($70%) in the seventh position of the insert. The K d of the 23 selected phage clones ranged from 2.2Γ10 Γ10 to 2.0Γ10 Γ9 M.
Conclusion:
The peptide with the most important K d was selected for further evaluation and in vivo detection of the senile plaques in AD by MRI; it will be linked to optimal paramagnetic and superparamagnetic reporters and tested with in vivo models.