Design and chemical synthesis of a magnetic resonance contrast agent with enhanced in vitro binding, high blood-brain barrier permeability, and in vivo targeting to Alzheimer's disease amyloid plaques

Molecular imaging is an important new direction in medical diagnosis; however, its success is dependent upon molecular probes that demonstrate selective tissue targeting. We report the design and chemical synthesis of a derivative of human amyloid-beta (Abeta) peptide that is capable of selectively targeting individual amyloid plaques in the brain of Alzheimer's disease transgenic mice after being intravenously injected. This derivative is based on the sequence of the first 30 amino acid residues of Abeta with asparagyl/glutamyl-4-aminobutane residues (N-4ab/Q-4ab) substituted at unique Asp and Glu positions and with Gd-DTPA-aminohexanoic acid covalently attached at the N-terminal Asp. The Gd[N-4ab/Q-4ab]Abeta30 peptide was homogeneous as shown by high-resolution analytical techniques with a mass of +/-4385 Da determined by electrospray ionization mass spectrometry. This diamine- and gadolinium-substituted derivative of Abeta is shown to have enhanced in vitro binding to Alzheimer's disease (AD) amyloid plaques and increased in vivo permeability at the blood-brain barrier because of the unique Asp/Glu substitutions. In addition, specific in vivo targeting to AD amyloid plaques is demonstrated throughout the brain of an APP, PS1 transgenic mouse after intravenous injection. Because of the magnetic resonance (MR) imaging contrast enhancement provided by gadolinium, this derivative should enable the in vivo MR imaging of individual amyloid plaques in the brains of AD animals or patients to allow for early diagnosis and also provide a direct measure of the efficacy of anti-amyloid therapies currently being developed.     

Aurones serve as probes of beta-amyloid plaques in Alzheimer's disease

A novel series of aurone derivatives for in vivo imaging of beta-amyloid plaques in the brain of Alzheimer's disease (AD) were synthesized and characterized. When in vitro binding studies using Abeta(1-42) aggregates were carried out with aurone derivatives, they showed high binding affinities for Abeta(1-42) aggregates at the K(i) values ranging from 1.2 to 6.8 nM. When in vitro plaque labeling was carried out using double transgenic mice brain sections, the aurone derivatives intensely stained beta-amyiloid plaques. Biodistribution studies in normal mice after i.v. injection of the radioiodinated aurones displayed high brain uptake (1.9-4.6% ID/g at 2 min) and rapid clearance from the brain (0.11-0.26% ID/g at 60 min), which is highly desirable for amyloid imaging agents. The results in this study suggest that novel radiolabeled aurones may be useful amyloid imaging agents for detecting beta-amyloid plaques in the brain of AD.     

Novel chalcones as probes for in vivo imaging of beta-amyloid plaques in Alzheimer's brains

A novel series of chalcone derivatives for in vivo imaging beta-amyloid plaques in the brain of Alzheimer's disease (AD) were synthesized and characterized. When in vitro binding studies using Abeta aggregates were carried out with chalcone derivatives, the binding affinities for Abeta aggregate varied from 3 to 105 nM. The radioiodinated chalcones were successfully prepared through an iododestannylation reaction from the corresponding tributyltin derivatives using hydrogen peroxide as the oxidant in high yields and with high radiochemical purities. Biodistribution studies in normal mice after iv injection of the radioiodinated chalcones displayed high brain uptake (2.0-4.7% ID/g at 2 min) and rapid clearance from the brain (0.2-0.6% ID/g at 30 min), which is highly desirable for amyloid imaging agents. The results in this study suggest that the novel radioiodinated chalcones may be useful amyloid imaging agents for detecting beta-amyloid plaques in the brain of AD.     

Curcumin inhibits formation of amyloid beta oligomers and fibrils, binds plaques, and reduces amyloid in vivo

Alzheimer's disease (AD) involves amyloid beta (Abeta) accumulation, oxidative damage, and inflammation, and risk is reduced with increased antioxidant and anti-inflammatory consumption. The phenolic yellow curry pigment curcumin has potent anti-inflammatory and antioxidant activities and can suppress oxidative damage, inflammation, cognitive deficits, and amyloid accumulation. Since the molecular structure of curcumin suggested potential Abeta binding, we investigated whether its efficacy in AD models could be explained by effects on Abeta aggregation. Under aggregating conditions in vitro, curcumin inhibited aggregation (IC(50) = 0.8 microM) as well as disaggregated fibrillar Abeta40 (IC(50) = 1 microM), indicating favorable stoichiometry for inhibition. Curcumin was a better Abeta40 aggregation inhibitor than ibuprofen and naproxen, and prevented Abeta42 oligomer formation and toxicity between 0.1 and 1.0 microM. Under EM, curcumin decreased dose dependently Abeta fibril formation beginning with 0.125 microM. The effects of curcumin did not depend on Abeta sequence but on fibril-related conformation. AD and Tg2576 mice brain sections incubated with curcumin revealed preferential labeling of amyloid plaques. In vivo studies showed that curcumin injected peripherally into aged Tg mice crossed the blood-brain barrier and bound plaques. When fed to aged Tg2576 mice with advanced amyloid accumulation, curcumin labeled plaques and reduced amyloid levels and plaque burden. Hence, curcumin directly binds small beta-amyloid species to block aggregation and fibril formation in vitro and in vivo. These data suggest that low dose curcumin effectively disaggregates Abeta as well as prevents fibril and oligomer formation, supporting the rationale for curcumin use in clinical trials preventing or treating AD.     

Amelioration of amyloid load by anti-Abeta single-chain antibody in Alzheimer mouse model

Parenteral immunization of transgenic mouse models of Alzheimer disease (AD) with synthetic amyloid beta-peptide (Abeta) prevented or reduced Abeta deposits and attenuated their memory and learning deficits. A clinical trial of immunization with synthetic Abeta, however, was halted due to brain inflammation, presumably induced by a toxic Abeta, T-cell- and/or Fc-mediated immune response. Another issue relating to such immunizations is that some AD patients may not be able to raise an adequate immune response to Abeta vaccination due to immunological tolerance or age-associated decline. Because peripheral administration of antibodies against Abeta also induced clearance of amyloid plaques in the model mice, injection of humanized Abeta antibodies has been proposed as a possible therapy for AD. By screening a human single-chain antibody (scFv) library for Abeta immunoreactivity, we have isolated a scFv that specifically reacts with oligomeric Abeta as well as amyloid plaques in the brain. The scFv inhibited Abeta amyloid fibril formation and Abeta-mediated cytotoxicity in vitro. We have tested the efficacy of the human scFv in a mouse model of AD (Tg2576 mice). Relative to control mice, injections of the scFv into the brain of Tg2576 mice reduced Abeta deposits. Because scFvs lack the Fc portion of the immunoglobulin molecule, human scFvs against Abeta may be useful to treat AD patients without eliciting brain inflammation.     

Practical assay and molecular mechanism of aggregation inhibitors of beta-amyloid.

Beta-Amyloid peptide (Abeta) is the main protein component of neuritic plaques in the brain of patients of Alzheimer's disease (AD), and its neurotoxicity would be exposed by the formation of aggregates. The aggregation inhibitors composed of an Abeta recognition element (KLVFF) and a hydrophilic moiety are evaluated by a novel fluorescence assay. These compounds inhibit growth of the model aggregates on the KLVFF immobilized surface. In addition, some compounds also possess disrupting activities of preformed aggregates. These compounds could be a key candidate for therapeutic drugs for AD by their novel molecular mechanisms.     

In vivo targeting of antibody fragments to the nervous system for Alzheimer's disease immunotherapy and molecular imaging of amyloid plaques

Targeting therapeutic or diagnostic proteins to the nervous system is limited by the presence of the blood-brain barrier. We report that a F(ab')(2) fragment of a monoclonal antibody against fibrillar human Abeta42 that is polyamine (p)-modified has increased permeability at the blood-brain barrier, comparable binding to the antigen, and comparable in vitro binding to amyloid plaques in Alzheimer's disease (AD) transgenic mouse brain sections. Intravenous injection of the pF(ab')(2)4.1 in the AD transgenic mouse demonstrated efficient targeting to amyloid plaques throughout the brain, whereas the unmodified fragment did not. Removal of the Fc portion of this antibody derivative will minimize the inflammatory response and cerebral hemorrhaging associated with passive immunization and provide increased therapeutic potential for treating AD. Coupling contrast agents/radioisotopes might facilitate the molecular imaging of amyloid plaques with magnetic resonance imaging/positron emission tomography. The efficient delivery of immunoglobulin G fragments may also have important applications to other neurodegenerative disorders or for the generalized targeting of nervous system antigens.     

Aluminium exposure induces Alzheimer's disease-like histopathological alterations in mouse brain

Aluminium (Al) is a neurotoxic metal and Al exposure may be a factor in the aetiology of various neurodegenerative diseases such as Alzheimer's disease (AD). The major pathohistological findings in the AD brain are the presence of neuritic plaques containing beta-amyloid (Abeta) which may interfere with neuronal communication. Moreover, it has been observed that GRP78, a stress-response protein induced by conditions that adversely affect endoplasmic reticulum (ER) function, is reduced in the brain of AD patients. In this study, we investigated the correlation between the expression of Abeta and GRP78 in the brain cortex of mice chronically treated with aluminium sulphate. Chronic exposure over 12 months to aluminium sulphate in drinking water resulted in deposition of Abeta similar to that seen in congophilic amyloid angiopathy (CAA) in humans and a reduction in neuronal expression of GRP78 similar to what has previously been observed in Alzheimer's disease. So, we hypothesise that chronic Al administration is responsible for oxidative cell damage that interferes with ER functions inducing Abeta accumulation and neurodegenerative damage.     

Abeta(1-40) peptide radiopharmaceuticals for brain amyloid imaging: (111)In chelation, conjugation to poly(ethylene glycol)-biotin linkers, and autoradiography with Alzheimer's disease brain sections

The amyloid plaques of Alzheimer's disease (AD) are formed by the neuropeptide Abeta(1)(-)(42/43), and carboxyl terminal truncated forms of this neuropeptide, designated Abeta(1)(-)(40), bind to amyloid plaques of AD autopsy tissue sections. Therefore, Abeta(1)(-)(40) is a potential peptide radiopharmaceutical that could be used for imaging brain amyloid in living subjects with AD, should this neuropeptide be made transportable through the blood-brain barrier (BBB). To accomplish this, the neuropeptide must be modified to enable (i) attachment to a BBB drug targeting system and (ii) labeling with a radionuclide, e.g., 111-indium, suitable for brain imaging by external detection modalities such as single photon emission computed tomography (SPECT). The present studies describe the synthesis of an Abeta(1)(-)(40) analogue that contains a biotin at the amino terminus and a diethylenetriaminepentaacetic acid (DTPA) moiety conjugated to one of the internal lysine residues. The DTPA-[N-biotin]-Abeta(1)(-)(40) was purified by gel filtration fast-protein liquid chromatography (FPLC) using two Superose 12HR columns in series, and the structure of the purified peptide was confirmed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry. The binding of the [(111)In]DTPA-[N-biotin]-Abeta(1)(-)(40) to amyloid plaques of AD autopsy tissue sections was demonstrated by film and emulsion autoradiography. A poly(ethylene glycol) (PEG) linker of 3400 Da molecular mass, designated PEG(3400), was inserted between the Abeta(1)(-)(40) and the biotin moiety, but this modification diminishes binding of Abeta(1)(-)(40) to the AD amyloid plaques. In summary, these studies describe a novel formulation of biotinylated Abeta(1)(-)(40) that allows radiolabeling with 111-indium. The peptide radiopharmaceutical may be conjugated to an avidin-based BBB drug targeting system to enable transport through the BBB and imaging of brain amyloid in vivo.     

Proteolytic antibody light chains alter beta-amyloid aggregation and prevent cytotoxicity

Beta-amyloid (Abeta), a peptide generated by proteolytic cleavage of the amyloid precursor protein (APP), is a major constituent of the neuritic plaques associated with Alzheimer's disease (AD). Up-regulation of alpha-secretase, which can hydrolyze Abeta between Lys16 and Leu17, has been proposed as a potential therapeutic strategy in the treatment of AD. Previously, we identified two light-chain antibody fragments that had proteolytic activity against Abeta, one with alpha-secretase-like activity and one with carboxypeptidase-like activity. Here we show that cleavage of Abeta40 by hk14, the light-chain antibody having carboxypeptidase-like activity, alters aggregation of Abeta and neutralizes any cytotoxic effects of the peptide. Cleavage of Abeta40 with c23.5, the light chain having alpha-secretase-like cleavage, substantially increases the aggregation rate of Abeta; however, it does not show any corresponding increase in cytotoxicity. The increase in aggregation resulting from hydrolysis by c23.5 can be attributed to the decreased solubility of the hydrolyzed products relative to the parent Abeta40, primarily the Abeta17-40 fragment. These results demonstrate that antibody fragment mediated proteolytic degradation of Abeta peptide can be a potential therapeutic route to control Abeta aggregation and toxicity in vivo. Our results also suggest that increasing alpha-secretase activity as a therapeutic route must be approached with some caution because this can lead to a substantial increase in aggregation.     

Age-dependent high-density clustering of GM1 ganglioside at presynaptic neuritic terminals promotes amyloid beta-protein fibrillogenesis

The deposition of amyloid beta-protein (Abeta) is an invariable feature of Alzheimer's disease (AD); however, the biological mechanism underlying Abeta assembly into fibrils in the brain remains unclear. Here, we show that a high-density cluster of GM1 ganglioside (GM1), which was detected by the specific binding of a novel peptide (p3), appeared selectively on synaptosomes prepared from aged mouse brains. Notably, the synaptosomes bearing the high-density GM1 cluster showed extraordinary potency to induce Abeta assembly, which was suppressed by an antibody specific to GM1-bound Abeta, an endogenous seed for AD amyloid. Together with evidence that Abeta deposition starts at presynaptic terminals in the AD brain and that GM1 levels significantly increase in amyloid-positive synaptosomes prepared from the AD brain, our results suggest that the age-dependent high-density GM1 clustering at presynaptic neuritic terminals is a critical step for Abeta deposition in AD.     

Antibodies to potato virus Y bind the amyloid beta peptide: immunohistochemical and NMR studies

Studies in transgenic mice bearing mutated human Alzheimer disease (AD) genes show that active vaccination with the amyloid beta (Abeta) protein or passive immunization with anti-Abeta antibodies has beneficial effects on the development of disease. Although a trial of Abeta vaccination in humans was halted because of autoimmune meningoencephalitis, favorable effects on Abeta deposition in the brain and on behavior were seen. Conflicting results have been observed concerning the relationship of circulating anti-Abeta antibodies and AD. Although these autoantibodies are thought to arise from exposure to Abeta, it is also possible that homologous proteins may induce antibody synthesis. We propose that the long-standing presence of anti-Abeta antibodies or antibodies to immunogens homologous to the Abeta protein may produce protective effects. The amino acid sequence of the potato virus Y (PVY) nuclear inclusion b protein is highly homologous to the immunogenic N-terminal region of Abeta. PVY infects potatoes and related crops worldwide. Here, we show through immunocytochemistry, enzyme-linked immunosorbent assay, and NMR studies that mice inoculated with PVY develop antibodies that bind to Abeta in both neuritic plaques and neurofibrillary tangles, whereas antibodies to material from uninfected potato leaf show only modest levels of background immunoreactivity. NMR data show that the anti-PVY antibody binds to Abeta within the Phe4-Ser8 and His13-Leu17 regions. Immune responses generated from dietary exposure to proteins homologous to Abeta may induce antibodies that could influence the normal physiological processing of the protein and the development or progression of AD.     

Reduction of Abeta accumulation in the Tg2576 animal model of Alzheimer's disease after oral administration of the phosphatidyl-inositol kinase inhibitor wortmannin.

The abnormal accumulation of the amyloid beta protein (Abeta) has been implicated as an early and critical event in the etiology and pathogenesis of Alzheimer's disease (AD). Compounds that reduce Abeta accumulation may therefore be useful therapeutically. In cell-based screens we detected a significant reduction in Abeta concentration after treatment with the phosphatidylinositol kinase inhibitors wortmannin and LY294002. To determine the effect of this class of compounds on in vivo Abeta accumulation, we administered wortmannin to the Tg2576 mouse model of AD. Oral administration of wortmannin over four months resulted in a significant, non-overlapping 40%-50% reduction in the number of senile plaques, one of the pathological hallmarks of AD. Sandwich ELISA analysis of formic acid extractable Abeta in the brain of treated animals indicates that both Abeta40 and the longer, more amyloidogenic form of the peptide, Abeta42, were significantly reduced. These data provide the first direct evidence that compounds identified by their ability to reduce Abeta concentration in vitro can reduce Abeta accumulation and deposition in the brain, thus establishing a basic paradigm for the identification and evaluation of additional compounds that lower Abeta accumulation.     

Adult mouse astrocytes degrade amyloid-beta in vitro and in situ

Alzheimer disease (AD) is a progressive neurodegenerative disorder characterized by excessive deposition of amyloid-beta (Abeta) peptides in the brain. One of the earliest neuropathological changes in AD is the accumulation of astrocytes at sites of Abeta deposition, but the cause or significance of this cellular response is unclear. Here we show that cultured adult mouse astrocytes migrate in response to monocyte chemoattractant protein-1 (MCP-1), a chemokine present in AD lesions, and cease migration upon interaction with immobilized Abeta(1-42). We also show that astrocytes bind and degrade Abeta(1-42). Astrocytes plated on Abeta-laden brain sections from a mouse model of AD associate with the Abeta deposits and reduce overall Abeta levels in these sections. Our results suggest a novel mechanism for the accumulation of astrocytes around Abeta deposits, indicate a direct role for astrocytes in degradation of Abeta and implicate deficits in astroglial clearance of Abeta in the pathogenesis of AD. Treatments that increase removal of Abeta by astrocytes may therefore be a critical mechanism to reduce the neurodegeneration associated with AD.     

Role of cdk5 in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is characterized by two pathological hallmarks, namely, senile plaques and neurofibrillary tangles (NFTs). The former are mainly composed of amyloid-beta peptides (Abeta) while the latter consists mainly of filaments of hyperphosphorylated tau. Cyclin-dependent kinase 5 (cdk5) has been implicated not only in the tangle pathology, but recent data also implicate cdk5 in the generation of Abeta peptides. Since both Abeta peptides and NFTs are believed to play a role in neurodegeneration in AD, this proline-directed serine/threonine protein kinase is likely to contribute to the pathogenesis of AD. In vitro and in vivo animal data demonstrate the ability of cdk5 to induce phosphorylation and aggregation of tau, and NFT deposition and neurodegeneration. Findings from AD brain samples also show an elevated cdk5 activity and conditions that support the activation of cdk5. Evidence for the role of cdk5 in regulating Abeta production is just emerging. The mechanisms for this potentially damaging activity of cdk5 are largely unknown although amyloid precursor protein and presenilin-1 are both cdk5 substrates.     

The novel beta-secretase inhibitor KMI-429 reduces amyloid beta peptide production in amyloid precursor protein transgenic and wild-type mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. The major component of the plaques, amyloid beta peptide (Abeta), is generated from amyloid precursor protein (APP) by beta- and gamma-secretase-mediated cleavage. Because beta-secretase/beta-site APP cleaving enzyme 1 (BACE1) knockout mice produce much less Abeta and grow normally, a beta-secretase inhibitor is thought to be one of the most attractive targets for the development of therapeutic interventions for AD without apparent side-effects. Here, we report the in vivo inhibitory effects of a novel beta-secretase inhibitor, KMI-429, a transition-state mimic, which effectively inhibits beta-secretase activity in cultured cells in a dose-dependent manner. We injected KMI-429 into the hippocampus of APP transgenic mice. KMI-429 significantly reduced Abeta production in vivo in the soluble fraction compared with vehicle, but the level of Abeta in the insoluble fraction was unaffected. In contrast, an intrahippocampal injection of KMI-429 in wild-type mice remarkably reduced Abeta production in both the soluble and insoluble fractions. Our results indicate that the beta-secretase inhibitor KMI-429 is a promising candidate for the treatment of AD.     

Alzheimer vaccine: amyloid-beta on trial

A new therapeutic approach is being developed for the treatment of Alzheimer's disease (AD). This approach involves the deliberate induction of an autoimmune response to amyloid-beta (Abeta) peptide, the constituent of neuritic plaques that is thought to cause the neurodegeneration and dementia in AD. If this approach is to be effective, antibodies must be produced that can selectively target the toxic forms of Abeta, while leaving the functionally-relevant forms of Abeta and its precursor protein untouched. Furthermore, an approach needs to be found that avoids provoking an acute neuroinflammatory response. The situation is made even more challenging by uncertainty regarding which isoforms of Abeta contribute to the pathogenesis of AD.     

Detection of Neuritic Plaques in Alzheimer's Disease Mouse Model

Alzheimer''s disease (AD) is the most common neurodegenerative disorder leading to dementia. Neuritic plaque formation is one of the pathological hallmarks of Alzheimer''s disease. The central component of neuritic plaques is a small filamentous protein called amyloid β protein (Aβ)¹, which is derived from sequential proteolytic cleavage of the beta-amyloid precursor protein (APP) by β-secretase and γ-secretase. The amyloid hypothesis entails that Aγ-containing plaques as the underlying toxic mechanism in AD pathology². The postmortem analysis of the presence of neuritic plaque confirms the diagnosis of AD. To further our understanding of Aγ neurobiology in AD pathogenesis, various mouse strains expressing AD-related mutations in the human APP genes were generated. Depending on the severity of the disease, these mice will develop neuritic plaques at different ages. These mice serve as invaluable tools for studying the pathogenesis and drug development that could affect the APP processing pathway and neuritic plaque formation. In this protocol, we employ an immunohistochemical method for specific detection of neuritic plaques in AD model mice. We will specifically discuss the preparation from extracting the half brain, paraformaldehyde fixation, cryosectioning, and two methods to detect neurotic plaques in AD transgenic mice: immunohistochemical detection using the ABC and DAB method and fluorescent detection using thiofalvin S staining method.     

Development of an anti-Aβ monoclonal antibody for in vivo imaging of amyloid angiopathy in Alzheimer's disease

We evaluated the efficacy of murine monoclonal antibodies (MAbs) targeted to the Aβ amyloid of Alzheimer's disease for development of procedures for the in vivo identification of amyloid angiopathy (AA). MAbs to Aβ were prepared and screened for effectiveness in visualizing AA and neuritic plaques in postmortem AD brain sections. They were assessed again after enzymatic cleavage to produce Fab fragments and after labeling with technetium-99m (^99mTc) using a diamide dimercaptide ligand system. Modified and radiolabeled Fab fragments retained activity and specificity toward amyloid-laden blood vessels and neuritic plaques. A highly specific murine MAb, 10H3, was identified and characterized that fulfills criteria necessary for the development of an in vivo diagnostic imaging agent. Toxicity studies in rats showed the MAb to be safe. Biodistribution studies in mice demonstrated desirable properties for use as an imaging agent. Expansion and adaptation of these strategies may provide the methods and materials for the noninvasive analysis of AA in living patients, and permit assessment of the contribution of AA to the clinical and pathological features of AD.     

Synthesis and characterization of styrylchromone derivatives as beta-amyloid imaging agents

Several promising agents have been synthesized and evaluated for in vivo imaging probes of beta-amyloid plaques in Alzheimer's disease (AD) brain. Recently, we have developed flavone derivatives, which possess the basic structure of the 2-phenylchromone, as useful candidates for amyloid imaging agents. In an attempt to further develop novel tracers, we synthesized and evaluated a series of 2-styrylchromone derivatives, which replace the 2-phenyl substituent of flavone backbone with the 2-styryl. A series of radioiodinated styrylchromone derivatives were designed and synthesized. The binding affinities for amyloid plaques were assessed by in vitro binding assay using pre-formed synthetic Abeta(1-40) aggregates. The new series of styrylchromone derivatives showed high binding affinity to Abeta aggregates at the K(d) values of 32.0, 17.5 and 8.7nM for [(125)I]6, [(125)I]9, and [(125)I]12, respectively. In biodistribution studies using normal mice, [(125)I]6 and [(125)I]9 examined in normal mice displayed high brain uptakes with 4.9 and 2.8%ID/g at 2min post injection. The radioactivity washed out from the brain rapidly (1.6 and 1.0%ID/g at 60min post injection for [(125)I]6 and [(125)I]9, respectively). But [(125)I]12 did not show marked brain uptake, and the washout rate from the brain was relatively slow throughout the time course (1.1 and 1.4%ID/g at 2 and 30min post injection, respectively). Although additional modifications are necessary to improve the brain uptake and rapid clearance of non-specifically bound radiotracer, the styrylchromone backbone may be useful as a backbone structure to develop novel beta-amyloid imaging agents.