Synthesis and Amyloid Binding Properties of Rhenium Complexes: Preliminary Progress Toward a Reagent for SPECT Imaging of Alzheimer’s Disease Brain Weiguo Zhen, †,‡,⊥,∇ Hogyu Han, ‡,⊥,§ Magdalena Anguiano, †,‡ Cynthia A. Lemere, † Cheon-Gyu Cho, †,‡,| and Peter T. Lansbury, Jr.* ,† Center for Neurologic Diseases, Brigham and Women’s Hospital and Harvard Medical School, Harvard Institutes of Medicine, Room 754, 77 Avenue Louis Pasteur, Boston, Massachusetts 02115, and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 8, 1999 The definitive diagnosis of Alzheimer’s disease (AD) requires the detection of amyloid plaques in postmortem brain. Although the amount of fibrillar amyloid roughly correlates with the severity of symptoms at the time of death, the temporal relationship between amyloid deposition, neuronal loss, and cognitive decline is unclear. To elucidate this relationship, a noninvasive, practical method for the quantitation of brain amyloid deposition is required. We describe herein the initial stages of a strategy to accomplish this goal by single photon computed tomographic imaging. The amyloid-binding dye Congo Red was modified to allow its conjugation to the monoamine-monoamide bis(thiol) ligand. This ligand complexes technetium(V) in its neutral oxo form. A biphenyl-containing building block was conjugated to the protected ligand, and the product was coupled to the relevant aromatic compounds. Rhenium oxo complexes, which are isosteric, but nonradioactive, analogues of the potential imaging agent technetium oxo complexes, were synthesized. These complexes bound to A amyloid fibrils produced in vitro and stained amyloid plaques and vascular amyloid in AD brain sections. Introduction Alzheimer’s disease (AD) is currently diagnosed based on the clinical observation of cognitive decline, coupled with the systematic elimination of other possible causes of those symptoms. 1,2 The confirmation of the clinical diagnosis of “probable AD” can only be made by exami- nation of the postmortem brain. 1,3,4 The AD brain is characterized by a loss of neurons in regions of the brain responsible for learning and memory (e.g., hippocampus) and by the appearance, in these regions, of two distinct abnormal proteinaceous deposits: extracellular amyloid plaques, which are characteristic of AD, and intracel- lular neurofibrillary tangles (NFTs), which are found in other neurodegenerative disorders. 1-4 The amount of amyloid deposits roughly correlates with the severity of symptoms at the time of death; 5 although synaptic count, a more downstream marker, correlates more closely. Amyloid plaques comprise dystrophic neurites and other altered astrocytes and microglia surrounding an insoluble fibrillar core. AD amyloid fibrils comprise a family of proteins known collectively as the amyloid -proteins (A), predominantly two variants: A40 and A42. 6 A is derived from the ubiquitously expressed cell surface amyloid precursor protein (APP). 3,4,7 Several lines of circumstantial evidence suggest that A amyloid fibril formation is an initiating event in the AD patho- genic cascade: (1) overexpression of APP is character- istic of Down syndrome, and early-onset AD is a virtual certainty in this population; 8,9 (2) missense mutations in APP cause early-onset AD; 3,7 (3) mutations in the presenilin proteins that also cause early-onset AD all increase the expression of the variant A42 that is known to fibrillize more rapidly than A40; 7,10-12 (4) the apoE variant encoded by the apoE4 allele, which confers susceptibility to late-onset AD, is more permissive of A amyloid formation than the other apoE variants; 13-16 and (5) transgenic mice that overexpress mutant APP develop AD-like neuropathology. 17,18 While these facts strongly suggest that amyloid formation precedes neu- rodegeneration, a direct proof is lacking. We sought to elucidate the relationship between amyloid formation and neurodegeneration by designing amyloid probes that could be used to measure brain amyloid noninvasively by single photon computed to- mography (SPECT). 19-26 Three approaches to the SPECT imaging of amyloid fibrils, all involving protein probes, have been reported. The amyloid-associated protein serum amyloid P component (SAP), labeled with 123 I, accumulates at low levels in the cerebral cortex, possibly in vessel walls, of patients with cerebral amyloidosis. 19 Two other approaches have been discussed but have not been reduced to practice. Iodinated A1-40, which binds AD amyloid plaque in tissue sections, can be transported across the blood-brain barrier (BBB) by conjugation to a protein that is actively transported. 25 In addition, antibodies to A have been proposed to be useful imaging probes, although a method to deliver these probes across the BBB has not been described. 24 These approaches suffer from some or all of these disadvantages: (1) 123 I is not an ideal radioisotope for SPECT applications, since it must be generated in a † Brigham and Women’s Hospital and Harvard Medical School. ‡ Massachusetts Institute of Technology. ⊥ These two authors contributed equally to this work. ∇ Current address: Perkin-Elmer Biosystems, Foster City, CA 94404. § Current address: Department of Chemistry, Korea University, 1-Anamdong, Seoul 136-701, Korea. | Current address: Department of Chemistry, Hangyang University, 17 Haengdang-Dong, Sungdong-Ku, Seoul 133-791, Korea. 2805 J. Med. Chem. 1999, 42, 2805-2815 10.1021/jm990103w CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999