Formation of beta-amyloid plaques of Alzheimer's disease is initiated by intermolecular contact of the 5-amino acid sequence, KLVFF, in b-amyloid peptides ranging in size from 40 to 43 residues. Through optimization of binding avidity using structure/function studies, we have found that the retro-inverso peptide, ffvlk, binds artificial fibrils made from Abetal-40 with moderate affinity (Kd=5x10-7 M). Conjugates having 2 copies of this peptide, whether connected by a long poly(ethylene glycol) (PEG) spacer or just by a couple of amino acids, display about 100-fold greater affinity for fibrils. Placing 6 copies of ffvlk on a branched PEG resulted in a 10,000-fold greater affinity (Kd=lxl0-10 M) than the monomer peptide. This increased affinity was accompanied by more effective inhibition of the thioflavin T fluorescence signal, which correlates with neurotoxicity of plaques and fibrils. We propose that conjugates bearing several copies of ffvlk may be useful as diagnostic and therapeutic agents for Alzheimer's disease. However, these conjugates must be able to cross the blood-brain barrier (BBB). We have established a bovine brain microvessel endothelial cell (BMEC) assay that can be used to measure permeability of test compounds across the BBB. We will synthesize conjugates and test them in this model transport system. The transport ligand on these conjugates will include vitamins, penetrating peptides, sugars and lipids, either in single or multiple copy number. Once substantial transport across the BBB has been achieved, we can proceed with the task of inhibiting neurotoxic plaques. This R03 proposal, however, will focus only on the BBB transport process.