Synthesis and biological evaluation of (E)-3-styrylpyridine derivatives as amyloid imaging agents for Alzheimer's disease

A new series of (E)-3-styrylpyridine derivatives as potential diagnostic imaging agents targeting amyloid plaques in Alzheimer's disease (AD) were synthesized and examined. When in vitro binding studies using AD brain homogenates were carried out with a series of styrylpyridine derivatives, (E)-2-Bromo-5-(4-dimethylaminostyryl)pyridine (7) with a dimethylamino group showed the highest binding affinity. Compound 7 intensely stained neuritic and diffused plaques and cerebrovascular amyloids on postmortem AD brain sections. (E)-2-Iodo-5-(4-dimethylaminostyryl)pyridine, the iodo derivative of compound 7, also stained senile plaques in human AD sections. The radioiodinated ligand [125I] was successfully prepared through an iododestannylation reaction from the corresponding tributyltin derivatives using hydrogen peroxide as the oxidant in high yields and with high radiochemical purity. A biodistribution study in normal mice after an intravenous injection of [125I] displayed high brain uptake and fast washout. Taken together, the data suggest that the new radio tracer, [125I], may be useful as a radioiodinated imaging agent for mapping A beta plaques in the brains of patients with AD.

Imaging beta-amyloid fibrils in Alzheimer's disease: a critical analysis through simulation of amyloid fibril polymerization

The polymerization of beta-amyloid (A beta) peptides into fibrillary plaques is implicated, in part, in the pathogenesis of Alzheimer's disease. A beta molecular imaging probes (A beta-MIPs) have been introduced in an effort to quantify amyloid burden or load, in subjects afflicted with AD by invoking the classic PET receptor model for the quantitation of neuronal receptor density. In this communication, we explore conceptual differences between imaging the density of amyloid fibril polymers and neuronal receptors. We formulate a mathematical model for the polymerization of A beta with parameters that are mapped to biological modulators of fibrillogenesis and introduce a universal measure for amyloid load to accommodate various interactions of A beta-MIPs with fibrils. Subsequently, we hypothesize four A beta-MIPs and utilize the fibrillogenesis model to simulate PET tissue time activity curves (TACs). Given the unique nature of polymer growth and resulting PET TAC, the four probes report differing amyloid burdens for a given brain pathology, thus complicating the interpretation of PET images. In addition, we introduce the notion of an MIP's resolution, apparent maximal binding site concentration, optimal kinetic topology and its resolving power in characterizing the pathological progression of AD and the effectiveness of drug therapy. The concepts introduced in this work call for a new paradigm that goes beyond the classic parameters B(max) and K(D) to include binding characteristics to polymeric peptide aggregates such as amyloid fibrils, neurofibrillary tangles and prions.

[The contribution of nuclear medicine procedures in the diagnosis of dementia of Alzheimer's type. Imaging beta amyloid plaques]

Pathogenesis of dementia of Alzheimer's type (DAT) is unknown. Progressive deposition of beta-amyloid plaques (beta-AlphaPs) is one of the major pathological features of DAT and precedes cognitive decline in these patients. Noninvasive detection of beta-AlphaPs could, therefore, may be a potential diagnostic test for the early detection of DAT patients. A series of derivatives that achieve high binding affinity for beta amyloid fibrils both for PET and SPET studies have been developed for imaging AlphaPs in the living human brain. Lesions at the very early but also at late stages of the disease can also be identified by nuclear medicine techniques.

Anti-Abeta antibody treatment promotes the rapid recovery of amyloid-associated neuritic dystrophy in PDAPP transgenic mice

Neuritic plaques are a defining feature of Alzheimer disease (AD) pathology. These structures are composed of extracellular accumulations of amyloid-beta peptide (Abeta) and other plaque-associated proteins, surrounded by large, swollen axons and dendrites (dystrophic neurites) and activated glia. Dystrophic neurites are thought to disrupt neuronal function, but whether this damage is static, dynamic, or reversible is unknown. To address this, we monitored neuritic plaques in the brains of living PDAPP;Thy-1:YFP transgenic mice, a model that develops AD-like pathology and also stably expresses yellow fluorescent protein (YFP) in a subset of neurons in the brain. Using multiphoton microscopy, we observed and monitored amyloid through cranial windows in PDAPP;Thy-1:YFP double-transgenic mice using the in vivo amyloid-imaging fluorophore methoxy-X04, and individual YFP-labeled dystrophic neurites by their inherent fluorescence. In vivo studies using this system suggest that amyloid-associated dystrophic neurites are relatively stable structures in PDAPP;Thy-1:YFP transgenic mice over several days. However, a significant reduction in the number and size of dystrophic neurites was seen 3 days after Abeta deposits were cleared by anti-Abeta antibody treatment. This analysis suggests that ongoing axonal and dendritic damage is secondary to Abeta and is, in part, rapidly reversible.

19F and 1H MRI detection of amyloid beta plaques in vivo

Formation of senile plaques composed of amyloid beta peptide, a pathological hallmark of Alzheimer disease, in human brains precedes disease onset by many years. Noninvasive detection of such plaques could be critical in presymptomatic diagnosis and could contribute to early preventive treatment strategies. Using amyloid precursor protein (APP) transgenic mice as a model of amyloid beta amyloidosis, we demonstrate here that an intravenously administered (19)F-containing amyloidophilic compound labels brain plaques and allows them to be visualized in living mice by magnetic resonance imaging (MRI) using (19)F and (1)H. Our findings provide a new direction for specific noninvasive amyloid imaging without the danger of exposure to radiation. This approach could be used in longitudinal studies in mouse models of Alzheimer disease to search for biomarkers associated with amyloid beta pathology as well as to track disease course after treatment with candidate medications.

Biphenyls labeled with technetium 99m for imaging β-amyloid plaques in the brain

Formation and accumulation of excess aggregates of beta-amyloid (Abeta) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Abeta plaques in the brain. The aim of this study was to develop potential technetium 99m (99mTc)-labeled diagnostic imaging agents specific for the detection of Abeta plaques. Based on previously obtained Abeta plaque-specific biphenyls containing a p-N, N-dimethylaminophenyl group, a series of 99mTc and Re-N2S2-biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [99mTc]19 and [99mTc]23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [99mTc]23A readily passed through the blood-brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [99mTc]19 (0.29% dose/brain at 2 min). Initial results suggested that [99mTc]23A showed selective binding to the Abeta plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Abeta plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates--the Re complexes.

Development of a PET/SPECT agent for amyloid imaging in Alzheimer's disease

In the search for a cure for Alzheimer's disease (AD), efforts have been focused on preventing or reversing amyloid deposition in the brain. Efficacy evaluation of these antiamyloid therapies would greatly benefit from development of a tool for the in vivo detection and quantitation of amyloid deposits in the brain. Toward this goal, we have developed a series of benzothiazole derivatives as amyloid-imaging agents for positron emission tomography (PET). To extend the potential of these amyloid-imaging agents for routine clinical studies, we also set out to develop iodinated benzothiazole derivatives that could be used as dual agents for either PET or the complementary single photon emission computed tomography (SPECT). Such dual agents would permit PET or SPECT studies using radiotracers with the same chemical identity but labeled with different radionuclides. This would facilitate the validation of clinical SPECT studies, based on quantitative PET studies. In this work we report the synthesis and biological evaluation of a potent, selective, and brain-permeable benzothiazole compound, 2-(3'-iodo-4'-methylaminophenyl)-6-hydroxy-benzothialzole, termed 6-OH-BTA-1-3'-I (4), which can be radiolabeled with either positron-emitting carbon-11 or single photon-emitting iodine-125/iodine-123. The synthesis and radiolabeling of [125I]4 or [11C]4 were achieved through direct iodination with sodium [125I]iodide in the presence of chloramine T or through radiomethylation with [11C]CH3I. In vitro amyloid binding assays indicated that [125I]4 bound to amyloid deposits in a saturable manner and exhibited affinities in the nanomolar concentration range. Binding studies of [125I]4 to postmortem human brain homogenates also showed preference of binding to frontal cortex in the AD homogenates relative to age-matched control homogenates or cerebellum from either AD or control. In vivo pharmacokinetic studies in normal mice following iv injection of [11C]4 indicated that the radioligand entered the brain readily at early time points and cleared from the brain rapidly at later time points with a 2- to 30-min ratio >3. These results suggest that the new radioiodinated benzothiazole ligand might be useful as a surrogate marker for the in vivo quantitation of amyloid deposition in human brain for use with either PET or SPECT.

Development and evaluation of iodinated tracers targeting amyloid plaques for SPECT imaging

Successful development of iodinated ligands for various neurotransmitter receptors prompted us to explore the feasability of having iodinated ligands to target amyloid plaques of Alzheimer's disease. Several potential iodinated tracers based on various chemical backbone structures have been successfully prepared and evaluated toward this purpose. High binding affinities for Abeta aggregates were consistently observed for those ligands. However, the desirable in vivo properties were generally missing in the majority of those iodinated ligands. Only ligands with the promising in vitro and in vivo characteristics such as IMPY will likely warrant their success to be potential imaging agents mapping amyloid plaques in living human brain.

Iodinated tracers for imaging amyloid plaques in the brain

Excessive amounts of protein deposits, beta-amyloid (Abeta) plaques, are commonly detected in the postmortem brains of Alzheimer's disease (AD) patients. These Abeta plaques are believed to play an important role in the pathogenesis of the disease. Development of Abeta plaque-specific imaging agents for detecting and monitoring the changes of Abeta plaque deposition in living brains are reported. These agents, if successfully developed, may serve as potential biomarkers for the disease. Several iodinated derivatives based on variety of core structures are labeled with radioiodine as single photon emission computed tomography (SPECT) imaging agents. Thioflavin (or benzothiazole) derivatives displayed excellent in vitro characteristics with high binding affinities for Abeta aggregates (in subnanomolar to nanomolar range) and excellent plaque labeling of AD brain sections. However, the in vivo kinetic properties appeared unfavorable for further development. A novel series of imidazo-pyridine derivative, such as 2-(4'-dimethylaminophenyl)-6-iodo-imidazo[1,2-a]pyridine (IMPY), exhibited highly desirable in vivo properties. Additional structural modification resulting in stilbene derivatives displayed good binding affinities for Abeta aggregates. In addition, fluorene compounds with a rigid tricyclic structure showed in vitro and in vivo characteristics as potential SPECT imaging agents. Criteria for selection of radioiodinated tracers with suitable in vivo properties to detect amyloid plaques are discussed.

2-Dialkylamino-6-acylmalononitrile substituted naphthalenes (DDNP analogs): novel diagnostic and therapeutic tools in Alzheimer's disease

This article presents a comprehensive review of the in vitro and in vivo detection of neurofibrillary tangles (NFTs) and beta-amyloid senile plaques (SPs), neuropathological lesions found in the brains of the Alzheimer's disease (AD) patients, using FDDNP and its analogs. FDDNP and its analogs have excellent ability to bind to NFTs and SPs in vitro as shown by binding assays, confocal fluorescence microscopy with stained AD brain tissue and digital autoradiography with [18F]FDDNP. [18F]FDDNP-PET molecular imaging permits detection of these pathologies in living subjects. The discovery of a new binding site to Abeta(1-40) fibrils as a result of FDDNP binding also opens a therapeutic opportunity for early treatment of Alzheimer's disease. FDDNP shares a previously unrecognized common binding site on Abeta(1-40) fibrils and senile plaques with non-steroidal anti-inflammatory drugs (NSAIDs) (e.g., naproxen and ibuprofen). Naproxen, ibuprofen and even FDDNP significantly inhibit aggregation of the Abeta(1-40) peptide in the micromolar range. This new binding site on Abeta(1-40) fibrils also offers a molecular template for design of anti-aggregation drugs without the secondary effects of NSAIDs. Therefore it is anticipated that a new vision for prevention, early diagnosis and treatment of Alzheimer's disease would be rapidly developing.

In-vivo imaging of Alzheimer disease beta-amyloid with [11C]SB-13 PET

OBJECTIVE

In-vivo imaging of beta-amyloid plaques (Abeta) may improve both early detection of Alzheimer disease (AD) and efficacy assessment of new treatments for AD. The authors' aim was to develop a novel Abeta-specific positron-emission tomography (PET) tracer.

METHODS

Five female AD patients (54-77 years old) and six healthy female comparison subjects (53-74 years old), completed 2-hour PET scans after intravenous injection of 10 mCi of both the stilbene [11C]SB-13 and the benzothiazole [11C]6-OH-BTA-1 (also known as [11C]PIB). Kinetic analyses were performed on the resulting time-activity curves to derive Abeta binding-potential estimates, using as input function either the unmetabolized tracer concentration in venous plasma from a two-tissue compartment model or the density of radioactivity in the cerebellum. Authors compared the binding characteristics of the two radiotracers.

RESULTS

The two radiotracers demonstrated similar binding properties with respect to regional distribution of retention (increased retention in the frontal and posterior temporal-inferior parietal association cortices in the AD patients, but not in the comparison subjects). Our preliminary PET data indicate that [11C]SB-13 may be similar to [11C]PIB in discriminating AD patients from comparison subjects.

CONCLUSIONS

[11C]SB-13 is an effective PET tracer for fibrillar Abeta imaging in vivo, with similar performance as [11C]PIB. Future research directions include evaluation of tracer in larger AD patient samples and in subjects with amnestic mild cognitive impairment, evaluation of arterial input function, and comparison with other tracers, such as [18F]FDG as they relate to cognitive functioning.

PET imaging of amyloid in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and is characterised by progressive impairment in cognitive function and behaviour. The pathological features of AD include neuritic plaques composed of amyloid-beta peptide (Abeta) fibrils, neurofibrillary tangles of hyperphosphorylated tau, and neurotransmitter deficits. Increases in the concentration of Abeta in the course of the disease with subtle effects on synaptic efficacy will lead to gradual increase in the load of amyloid plaques and progression in cognitive impairment. Direct imaging of amyloid load in patients with AD in vivo would be very useful for the early diagnosis of AD and the development and assessment of new treatment strategies. Three different strategies are being used to develop compounds suitable for in vivo imaging of amyloid deposits in human brains. Monoclonal antibodies against Abeta and peptide fragments have had limited uptake by the brain when tested in patients with AD. When putrescine-gadolinium-Abeta has been injected into transgenic mice overexpressing amyloid, labelling has been observed with MRI. The small molecular approach for amyloid imaging has so far been most successful. The binding of different derivatives of Congo red and thioflavin has been studied in human autopsy brain tissue and in transgenic mice. Two compounds, fluorine-18-labelled-FDDNP and carbon-11-labelled-PIB, both show more binding in the brains of patients with AD than in those of healthy people. Additional compounds will probably be developed that are suitable not only for PET but also for single photon emission CT (SPECT).

Styrylbenzoxazole derivatives for in vivo imaging of amyloid plaques in the brain

Progressive deposition of senile plaques (SPs) is one of the major neuropathological features of Alzheimer's disease (AD) that precedes cognitive decline. Noninvasive detection of SPs could, therefore, be a potential diagnostic test for early detection of AD patients. For imaging SPs in the living brain, we have developed a series of styrylbenzoxazole derivatives that achieve high binding affinity for amyloid-beta (Abeta) fibrils. One of these compounds, 6-(2-Fluoroethoxy)-2-[2-(4-methylaminophenil) ethenyl]benzoxazole (BF-168), selectively binds SPs in AD brain sections and recognizes Abeta1-42-positive diffuse plaques as well as neuritic plaques in AD brain sections. Intravenous injection of BF-168 in PS1/APP and APP23 transgenic mice resulted in specific in vivo labeling to both compact and diffuse amyloid deposits in the brain. In addition, (18)F-radiolabeled BF-168 demonstrated abundant initial brain uptake (3.9% injected dose/gm at 2 min after injection) and fast clearance (t(1/2) = 24.7 min) after intravenous administration in normal mice. Furthermore, autoradiograms of brain sections from APP23 transgenic mice at 180 min after intravenous injection of [(18)F]BF-168 showed selective labeling of brain amyloid deposits with little nonspecific binding. These findings strongly suggest that styrylbenzoxazole derivatives are promising candidate probes for positron emission tomography and single-photon emission computed tomography imaging for early detection of amyloid plaque formation.

[In vivo imaging of amyloid plaques in the brain]

The pathological hallmark of Alzheimer's disease (AD) is the deposition of senile plaques (SPs) and neurofibrillary tangles. The deposition of SPs, which initially occurs in the neocortex, is regarded as the critical event for the pathogenesis of AD. Previous pathological studies indicated that the earliest deposition of SPs is suspected to occur before any detectable cognitive decline. Therefore, noninvasive imaging of brain SPs is considered to be an ideal diagnostic method for presymptomatic detection of AD. For in vivo detection of SPs in the brain, a lipophilic probe that can selectively binds SPs is indispensable to the examination using positron emission tomography (PET) or single photon emission computed tomography (SPECT). For this purpose, many kinds of amyloid binding agents have been developed, such as FDDNP, BTA-1, IMPY. We have also developed a series of promising compounds for in vivo imaging of amyloid deposits. One of these compounds, compound-A, achieves high binding affinity for A beta fibrils. This agent also showed abundant initial brain uptake and short brain clearance half-life in normal mice. Furthermore, to investigate the in vivo binding property of compound-A to amyloid plaques, compound-A was intravenously administered to APP transgenic mice. Brain slices at 120 min post injection demonstrated specific binding of compound-A to amyloid plaques in the brain. These data emphasize the potential usefulness of compound-A as in vivo imaging probe for early diagnosis of AD.

Imaging the pathology of Alzheimer's disease: amyloid-imaging with positron emission tomography

The steep rise in the incidence of Alzheimer's disease (AD) has further added to the considerable public health burden caused by aging of the United States population. Among the most characteristic pathologic hallmarks of AD are neuritic plaques and neurofibrillary tangles. The capability to use positron emission tomography and selective markers for amyloid protein deposition promises to substantially alter the way we diagnosis and manage patients who have AD.

Characterization of IMPY as a potential imaging agent for ?-amyloid plaques in double transgenic PSAPP mice

Deposition of beta-amyloid (Abeta) plaques in the brain is likely linked to the pathogenesis of Alzheimer's disease (AD). Developing specific Abeta aggregate-binding ligands as in vivo imaging agents may be useful for diagnosis and monitoring the progression of AD. We have prepared a thioflavin derivative, 6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine, IMPY, which is readily radiolabeled with 125I/123I for binding or single-photon emission computerized tomography (SPECT) imaging studies. Characterization of [125I]IMPY binding to plaque-like structures was evaluated in double transgenic PSAPP mice. [125I]IMPY labeled Abeta plaques in transgenic mouse brain sections, and the labeling was consistent with fluorescent staining and Abeta-specific antibody labeling. Significant amounts of Abeta plaques present in the cortical, hippocampal, and entorhinal regions of the transgenic mouse brain were clearly detected with [125I]IMPY via ex vivo autoradiography. In contrast, [125I]IMPY showed little labeling in the age-matched control mouse brain. Tissue homogenate binding further corroborated the Abeta plaque-specific distribution in various brain regions of transgenic mouse, and correlated well with the known density of Abeta deposition. Using a tissue dissection technique, [125I]IMPY showed a moderate increase in the cortical region of transgenic mice as compared to the age-matched controls. In vitro blocking of [125I]IMPY by "carrier" observed via autoradiography in mouse brain sections was not replicated by an in vivo blocking experiment in living TT mouse brain. The failure was most likely due to a significant carrier effect, which slows down the tracer in vivo metabolism, leading to an increased brain uptake. Taken together, these data indicate that [123I]IMPY is a potentially useful SPECT imaging agent for in vivo labeling of Abeta plaques in the living brain.

In vitro detection of (S)-naproxen and ibuprofen binding to plaques in the Alzheimer's brain using the positron emission tomography molecular imaging probe 2-(1-[6-[(2-[(18)F]fluoroethyl)(methyl)amino]-2-naphthyl]ethylidene)malononitrile

Epidemiological studies have suggested that the chronic use of non-steroidal anti-inflammatory drugs (NSAIDs) reduces the relative risk of Alzheimer's disease (AD). The possible neuroprotection by NSAIDs in AD is generally attributed to anti-inflammatory activity. An additional mode of drug action may involve anti-aggregation of beta-amyloid (Abeta) peptides by commonly used NSAIDs. We utilized in vitro competition assays, autoradiography, and fluorescence microscopy with AD brain specimens to demonstrate concentration-dependent decreases in the binding of the in vivo molecular imaging probe, 2-(1-[6-[(2-[(18)F]fluoroethyl)(methyl)amino]-2-naphthyl]ethylidene)malononitrile ([(18)F]FDDNP), against (S)-naproxen and (R)- and (S)-ibuprofen (but not diclofenac) to Abeta fibrils and ex vivo Abeta senile plaques. Conversely, in vitro amyloid dyes Congo Red and Thioflavine T were demonstrated in the same experiments not to bind to the FDDNP binding site. FDDNP and the NSAIDs that share the same binding site also exhibit anti-aggregation effects on Abeta peptides, suggesting that the shared binding site on Abeta fibrils and plaques may be a site of anti-aggregation drug action. Our results indicate for the first time the binding of select NSAIDs to plaques, specifically to the binding site of the molecular imaging probe [(18)F]FDDNP. Our understanding of the molecular requirements of FDDNP binding may help in the optimization of the Abeta anti-aggregation potency of experimental drugs. [(18)F]FDDNP has been used to image plaques in vivo with positron emission tomography (PET), and investigations into the influence of Abeta anti-aggregation on the risk-reduction effects of NSAIDs on AD could utilize [(18)F]FDDNP and PET in determining the occupancy rate of NSAIDs and experimental drugs in plaques in the living brain of AD patients.

Detection of amyloid plaques by radioligands for Abeta40 and Abeta42: potential imaging agents in Alzheimer's patients

Alzheimer s disease (AD) is linked to increased brain deposition of amyloid-beta (Abeta) peptides in senile plaques (SPs), and recent therapeutic efforts have focused on inhibiting the production or enhancing the clearance of Abeta in brain. However, it has not been possible to measure the burden of SPs or assess the effect of potential therapies on brain Abeta levels in patients. Toward that end, we have developed a novel radioligand, [(125)I]TZDM, which binds Abeta fibrils with high affinity, crosses the blood-brain barrier (BBB), and labels amyloid plaques in vivo. Compared to a styrylbenzene probe, [(125)I]IMSB, [(125)I]TZDM showed a 10-fold greater brain penetration and labeled plaques with higher sensitivity for in vivo imaging. However, this ligand also labels white matter, which contributes to undesirable high background regions of the brain. Interestingly, parallel to their differential binding characteristics onto fibrils composed of 40 (Abeta40)- or 42 (Abeta42)-amino-acid-long forms of Abeta peptides, these radioligands displayed differential labeling of SPs in AD brain sections under our experimental conditions. It was observed that [(125)I]IMSB labeled SPs containing Abeta40, amyloid angiopathy (AA), and neurofibrillary tangles, whereas [(125)I]TZDM detected only SPs and Abeta42-positive AA. Since increased production and deposition of Abeta42 relative to Abeta40 may be crucial for the generation of SPs, [(125)I]TZDM and related derivatives may be more attractive probes for in vivo plaque labeling. Further structural modifications of TZDM to lower the background labeling will be needed to optimize the plaque-labeling property.

Effects of Lipophilicity on the Affinity and Nonspecific Binding of Iodinated Benzothiazole Derivatives

Aseries of novel 2-aryl benzothiazole derivates substituted with iodine in different positions have been synthesized as amyloid-binding ligands. The affinity of these compounds for synthetic amyloid beta (1-40) (Abeta[1-40]) fibrils was determined. Introduction of the iodo group in the position ortho to an amino group increased the binding affinity, whereas the iodination ortho to a hydroxyl group decreased the binding affinity. Selected compounds with high binding affinity and moderate lipophilicity (logP values, 1.65-3.90) were radiolabeled and evaluated in normal mice for brain uptake and clearance. Structure-activity relationship (SAR) studies showed a strong correlation between the lipophilicity of the iodinated compounds and the binding affinity as well as nonspecific binding. As the lipophilicity increased, the affinity for Abeta(1-40) fibrils improved; however, nonspecific binding in mouse brain reflected by low brain clearance also increased with increasing lipophilicity. These results provide important SAR information to guide the development of novel amyloid-binding agents and provide further insights into the molecular interaction between 2-aryl benzothiazole ligands and Abeta fibrils.

Characterization of Radioiodinated Ligand Binding to Amyloid β Plaques

Several novel series of iodinated compounds based on the thioflavin backbone structure have been developed and characterized. These iodinated compounds showed high specific binding to amyloid beta (Abeta) aggregates with subnanomolar to nanomolar affinities. Probes like IMPY and MIPA display high brain uptakes and fast washout in normal mice, resulting in low background signals (presumably no amyloid plaques present in normal mouse brain), whereas TZDM shows long brain retention in normal mice suggesting high nonspecific in vivo binding. It is likely that tracers, that is, IMPY or MIPA, with desirable in vivo properties, will provide the highest target to non-target ratio; therefore, they are most likely to be successful as imaging agents targeting Abeta plaques in the brain.

Radioiodinated styrylbenzene derivatives as potential SPECT imaging agents for amyloid plaque detection in Alzheimer's disease

Development of probes for beta-amyloid (A beta) plaques, a critical factor associated with Alzheimer's disease (AD), provides important tools for studying their role in AD. Previously, we reported [125I]IMSB and [125I]ISB as excellent probes for A beta plaque labeling. Despite their exquisite in vitro binding characteristics, low brain uptakes (likely due to two ionizable carboxylic acid groups) limited their potential as in vivo imaging agents. To improve brain penetration, we have successfully prepared a neutral radioiodinated probe [125I]3. The improved probe displayed good binding affinity for A beta aggregates (Ki = 2.0 +/- 0.2 using A beta40 aggregates). In addition, the brominated counterpart displayed fluorescent-staining properties of A beta plaques in postmortem AD brain sections similar to BSB, a fluoroscent probe reported previously. [125I]3 gave excellent plaque labeling by film autoradiography of AD brain sections. Unlike [125I]IMSB (which preferentially detects A beta40 plaques), the improved radioioinated probe, [125I]3, can readily detect plaques containing aggregates of both A beta40 and A beta42. The initial brain uptake of [125I]3 in normal mice at 2 min p.i. was moderate (0.18% ID) and displayed a very slow washout from the brain (0.11%.ID at 4 h p.i). Taken together, these data suggest that [125I]3 is useful for in vitro plaque detection, it may not be suitable for in vivo monitoring of A beta progression and deposition.