Imaging Alzheimer's amyloid

Chemical design of radioiodinated probes with a metabolizable linkage for target-selective imaging of systemic amyloidosis

INTRODUCTION

Systemic amyloidosis is a rare disease caused by the deposition of amyloid fibrils in various organs. Amyloid-targeted radiopharmaceuticals have been developed and applied to diagnose systemic amyloidosis peripherally; however, high-contrast imaging has not been achieved because of the high background signals in normal organs. To overcome this problem, we designed an amyloid-targeted radioiodinated probe 1 with a metabolizable linkage (ester bond) to release of radiolabeled metabolites (m-iodohippuric acid) in normal organs that could be rapidly excreted in the urine.

METHODS

Compound 1 was synthesized by conjugating 2-(4-(methylamino)phenyl)benzo[d]thiazol-6-ol, an amyloid-targeting compound, with m-iodohippuric acid. [125I]1 was synthesized via iododestannylation using a tributyltin precursor. Mouse models of amyloid A (AA) amyloidosis, a type of systemic amyloidosis, were prepared by administering amyloid-enhancing factor to mice and used for in vitro autoradiography using organ sections and in vivo evaluation.

RESULTS

[125I]1 was obtained with a radiochemical yield of 59% and radiochemical purity of over 95%. An in vitro autoradiographic study demonstrated that [125I]1 specifically binds to amyloid in the splenic tissue. Upon administration to normal mice, [125I]1 was distributed to organs throughout the body, followed by the rapid excretion of radioactivity in the urine as m-[125I]iodohippuric acid. Furthermore, ex vivo autoradiography showed that [125I]1 bound to the amyloid formed around the follicles in the spleens of AA amyloidosis model mice.

CONCLUSION

These results suggest that the interposition of a metabolizable linkage between an amyloid-targeting moiety and a radiolabeled hippuric acid would be useful in the design of radiopharmaceuticals for high-contrast imaging of systemic amyloidosis.

Design and Development of Benzothiazole-Based Fluorescent Probes for Selective Detection of Aβ Aggregates in Alzheimer's Disease

The formation and accumulation of amyloid beta (Aβ) peptide are considered the crucial events that are responsible for the progression of Alzheimer's disease (AD). Herein, we have designed and synthesized a series of fluorescent probes by using electron acceptor-donor end groups interacting with a π-conjugating system for the detection of Aβ aggregates. The chemical structure of these probes denoted as RMs, having a conjugated π-system (C═C), showed a maximum emission in PBS (>600 nm), which is the best range for a fluorescent imaging probe. Among all these probes, RM-28 showed an excellent fluorescence property with an emission maximum of >598 nm upon binding to Aβ aggregates. RM-28 also showed high sensitivity (7.5-fold) and high affinities toward Aβ aggregates (K_d = 175.69 ± 4.8 nM; K_a = 0.5 × 10⁷ M^-1). It can cross the blood-brain barrier of mice efficiently. The affinity of RM-28 toward Aβ aggregates was observed in 3xTg-AD brain sections of the hippocampus and cortex region using a fluorescent imaging technique, as well as an in vitro fluorescence-based binding assay with Aβ aggregates. Moreover, RM-28 is highly specific to Aβ aggregates and does not bind with intracellular proteins like bovine serum albumin (BSA) and α-synuclein (α-Syn) aggregates. The results indicate that the probe RM-28 emerges as an efficient and veritable highly specific fluorescent probe for the detection of Aβ aggregates in both in vitro and in vivo model systems.

Chronic vitamin D3 supplementation alleviates cognition impairment via inhibition of oxidative stress regulated by PI3K/AKT/Nrf2 in APP/PS1 transgenic mice

Oxidative stress plays essential role in the pathogenesis of Alzheimer's disease, and vitamin D₃ (VD₃) is a nutrient with neuroprotective and antioxidant activities. The present study aimed to confirm the neuroprotective effect and the ameliorative effect of cortical oxidative stress of VD₃ in APP/PS1 transgenic mice. APP/PS1 mice were treated with VD₃ for 20 weeks. After treatment, Morris Water Maze test was used to evaluate cognitive level. Western blotting was used to determine APP, p-tau, tau and PI3K/AKT/Nrf2 pathway-related protein expression levels. Immunohistochemical staining was performed to determine the levels of β amyloid peptide (Aβ) deposition. Enzyme linked immunosorbent assay was used to determine the 25(OH)D₃ levels and oxidative stress status. Our results showed that treatment with VD₃ ameliorated behavioral deficits of APP/PS1 mice. In addition, the administration of VD₃ significantly increased the cortical 25(OH)D₃ levels, while reducing the levels of cortical Aβ deposition and decreasing the expression levels of cortical APP, tau and p-tau in APP/PS1 mice. Moreover, VD₃ protected the cortex against oxidative stress by enhancing the levels of superoxide dismutase, glutathione and total antioxidant capacity, and downregulating the malondialdehyde levels. Furthermore, VD₃ clearly activated the PI3K/AKT/Nrf2 pathway, thereby elevating the expression levels of HO1 and NQO1. We concluded that VD₃ improved cognitive function and cortical Alzheimer-like pathology of APP/PS1 mice, which may be related to the inhibition of oxidative stress via activation the PI3K/AKT/Nrf2 pathway.

Exploring the LDS 821 dye as a potential NIR probe for the two photon imaging of amyloid fibrils

We report a commercially available benzothiazolium based dye LDS 821 (Styryl 9M) as a near infrared fluorescent probe for the detection of lysozyme amyloid fibrils. Change in the photophysical properties of the dye with respect to the change in viscosity of the environment is investigated. Increment in fluorescence lifetime and quantum yield with increment in viscosity proves the dye as a molecular rotor. The dye, upon binding with lysozyme fibrils, exhibits a red shift in the absorption spectrum with increased quantum yield. Strong fluorescence emission near the biological window as compared with Thioflavin T makes the LDS 821 dye a potential probe for imaging amyloid fibrils in vivo. Molecular docking studies were carried out to understand the mode of interaction between the dye and amyloid fibrils. Nonlinear optical properties of the dye upon incorporation with amyloid fibrils were explored, and they show a sizeable enhancement in two photon absorption with an increase in the concentration of amyloid fibrils. The findings suggest that the nonlinear optical absorption of the LDS 821 dye can be used as an alternative marker for amyloid fibrils.

Rational Structure-Based Design of Fluorescent Probes for Amyloid Folds

Amyloid fibrils are pathological hallmarks of various human diseases, including Parkinson's, Alzheimer's, amyotrophic lateral sclerosis (ALS or motor neurone disease), and prion diseases. Treatment of the amyloid diseases are hindered, among other factors, by timely detection and therefore, early detection of the amyloid fibrils would be beneficial for treatment against these disorders. Here, a small molecular fluorescent probe is reported that selectively recognize the fibrillar form of amyloid beta(1-42), α-synuclein, and HET-s(218-289) protein over their monomeric conformation. The rational design of the reporters relies on the well-known cross-β-sheet repetition motif, the key structural feature of amyloids.

A Pyridazine-Based Fluorescent Probe Targeting Aβ Plaques in Alzheimer's Disease

Accumulation of β-amyloid (Aβ) plaques comprising Aβ40 and Aβ42 in the brain is the most significant factor in the pathogenesis of Alzheimer's disease (AD). Thus, the detection of Aβ plaques has increasingly attracted interest in the context of AD diagnosis. In the present study, a fluorescent pyridazine-based dye that can detect and image Aβ plaques was designed and synthesized, and its optical properties in the presence of Aβ aggregates were evaluated. An approximately 34-fold increase in emission intensity was exhibited by the fluorescent probe after binding with Aβ aggregates, for which it showed high affinity (K_D  = 0.35 µM). Moreover, the reasonable hydrophobic properties of the probe (log P = 2.94) allow it to penetrate the blood brain barrier (BBB). In addition, the pyridazine-based probe was used in the histological costaining of transgenic mouse (APP/PS1) brain sections to validate the selective binding of the probe to Aβ plaques. The results suggest that the pyridazine-based compound has the potential to serve as a fluorescent probe for the diagnosis of AD.

Coordination Chemistry of Bifunctional Chemical Agents Designed for Applications in 64Cu PET Imaging for Alzheimer's Disease

Positron emission tomography (PET) is emerging as one of the most important diagnostic tools for brain imaging, yet the most commonly used radioisotopes in PET imaging, ¹¹C and ¹⁸F, have short half-lives, and their usage is thus somewhat limited. By comparison, the ⁶⁴Cu radionuclide has a half-life of 12.7 h, which is ideal for administering and imaging purposes. In spite of appreciable research efforts, high-affinity copper chelators suitable for brain imaging applications are still lacking. Herein, we present the synthesis and characterization of a series of bifunctional compounds (BFCs) based on macrocyclic 1,4,7-triazacyclononane and 2,11-diaza[3.3](2,6)pyridinophane ligand frameworks that exhibit a high affinity for Cu²⁺ ions. In addition, these BFCs contain a 2-phenylbenzothiazole fragment that is known to interact tightly with amyloid β fibrillar aggregates. Determination of the protonation constants (pK_a values) and stability constants (log β values) of these BFCs, as well as characterization of the isolated copper complexes using X-ray crystallography, electron paramagnetic resonance spectroscopy, and electrochemical studies, suggests that these BFCs exhibit desirable properties for the development of novel ⁶⁴Cu PET imaging agents for Alzheimer’s disease.

Novel bifunctional chelators (BFCs) containing 1,4,7-triazacyclononane or pyridinophane macrocycles and amyloid-binding 2-phenylbenzothiazole fragments have been synthesized, and their copper coordination properties have been characterized in detail. These BFCs are attractive candidates for the development of novel ⁶⁴Cu-labeled PET imaging agents for Alzheimer’s disease.

A High Affinity Red Fluorescence and Colorimetric Probe for Amyloid β Aggregates

A major challenge in the Alzheimer's disease (AD) is its timely diagnosis. Amyloid β (Aβ) aggregates have been proposed as the most viable biomarker for the diagnosis of AD. Here, we demonstrate hemicyanine-based benzothiazole-coumarin (TC) as a potential probe for the detection of highly toxic Aβ42 aggregates through switch-on, enhanced (~30 fold) red fluorescence (Emax = 654 nm) and characteristic colorimetric (light red to purple) optical outputs. Interestingly, TC exhibits selectivity towards Aβ42 fibrils compared to other abnormal protein aggregates. TC probe show nanomolar binding affinity (Ka = 1.72 × 10(7) M(-1)) towards Aβ42 aggregates and also displace ThT bound to Aβ42 fibrils due to its high binding affinity. The Aβ42 fibril-specific red-shift in the absorption spectra of TC responsible for the observed colorimetric optical output has been attributed to micro-environment change around the probe from hydrophilic-like to hydrophobic-like nature. The binding site, binding energy and changes in optical properties observed for TC upon interaction with Aβ42 fibrils have been further validated by molecular docking and time dependent density functional theory studies.

Access to 2-(Het)aryl and 2-Styryl Benzoxazoles via Palladium-Catalyzed Aminocarbonylation of Aryl and Vinyl Bromides

A sequential one-pot procedure for the synthesis of either 2-(hetero)aryl or 2-styryl benzoxazoles is reported, starting from aryl and vinyl bromides, respectively, involving an initial aminocarbonylation with 2-aminophenols as nucleophiles followed by an acid mediated ring closure to generate the heterocycle. The methodology displays a broad substrate scope in moderate to excellent yields and can be exploited for (13)C-isotope labeling. Finally, this carbonylative protocol was applied to the synthesis of a potential Alzheimer's plaque binder and a selective PPAR antagonist including site-specific labeling with (13)C-carbon monoxide.

Possible role of resveratrol targeting estradiol and neprilysin pathways in lipopolysaccharide model of Alzheimer disease

Alzheimer's disease (AD) is an irreversible, progressive neurodegenerative brain disease that slowly destroys memory and thinking skills. It is the most common cause of dementia among older people. One of the most important hallmarks of AD is the presence of amyloid beta (Aβ) peptide in the brain that suggests that it is the primary trigger for neuronal loss. Herbal extracts have been studied over the years for their potential therapeutic effect in AD. Resveratrol (RSV), one of the most important phytoestrogens, is considered to be useful as estrogen plays an important role in AD. One of the most important amyloid degrading enzymes is neprilysin (NEP), which plays a major role in degrading Aβ, and mainly affected by estrogen. So, the aim of the present study is investigating the possible role of resveratrol in lipopolysaccharide model of AD and the implication of its possible role in regulating the estradiol and neprilysin pathways. Mice were divided into four groups: Control group (0.9 % saline), LPS group (0.8 mg/kg i.p once), Treatment group with RSV (mice were once injected with LPS then after 30 min given a dose of {4 mg/kg} RSV for 7 days), and RSV group only (mice received 4 mg/kg i.p for 7 days only). After 7 days mice were subjected to different behavioral tests using Y-maze, object recognition test, and open field tests. Estradiol and NEP level were measured using ELISA kit. Results showed RSV was able to reverse the decline in different types of memory (working, nonspatial, and locomotor functions) caused by LPS induction in mice. Moreover RSV was able to significantly increase both the estradiol level and NEP level and that may have a great role to decrease Aβ deposition as it has been confirmed that there is a link between NEP and estradiol level; by upregulation of estradiol level this consequently leads to increase in the level of NEP level, and by increasing the NEP level in brain, this lead to decrease in Aβ deposition and enhancing its degradation by NEP.

Valproic acid alleviates memory deficits and attenuates amyloid-β deposition in transgenic mouse model of Alzheimer's disease

In the brains of patients with Alzheimer's disease (AD) and transgenic AD mouse models, astrocytes and microglia activated by amyloid-β (Aβ) contribute to the inflammatory process that develops around injury in the brain. Valproic acid (VPA) has been shown to have anti-inflammatory function. The present study intended to explore the therapeutic effect of VPA on the neuropathology and memory deficits in APPswe/PS1ΔE9 (APP/PS1) transgenic mice. Here, we report that VPA-treated APP/PS1 mice markedly improved memory deficits and decreased Aβ deposition compared with the vehicle-treated APP/PS1 mice. Moreover, the extensive astrogliosis and microgliosis as well as the increased expression in interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the hippocampus and cortex of APP/PS1 transgenic mice were significantly reduced following administration of VPA, which attenuated neuronal degeneration. Concomitantly, VPA alleviated the levels of p65 NF-κB phosphorylation and enhanced the levels of acetyl-H3, Bcl-2, and phospho-glycogen synthase kinase (GSK)-3β that occurred in the hippocampus of APP/PS1 transgenic mice. These results demonstrate that VPA could significantly ameliorate spatial memory impairment and Aβ deposition at least in part via the inhibition of inflammation, suggesting that administration of VPA could provide a therapeutic approach for AD.

Initial clinical comparison of 18F-florbetapir and 18F-FDG PET in patients with Alzheimer disease and controls

The purpose of this study was to determine how clinical interpretations of the (18)F-amyloid tracer florbetapir compares diagnostically with (18)F-FDG PET when evaluating patients with Alzheimer disease (AD) and controls.

METHODS

Nineteen patients with a clinical diagnosis of AD and 21 elderly controls were evaluated with both (18)F-florbetapir and (18)F-FDG PET scans. Scans were interpreted together by 2 expert readers masked to any case information and were assessed for tracer binding patterns consistent with AD. The criteria for interpreting the (18)F-florbetapir scan as positive for AD was the presence of binding in the cortical regions relative to the cerebellum. (18)F-FDG PET scans were interpreted as positive if they displayed the classic pattern of hypometabolism in the temporoparietal regions. Scans were interpreted as either positive or negative for AD. In addition, a relative scoring system was used to assess the degree of either hypometabolism or amyloid binding in specified regions. The metabolism and amyloid binding scores for each region were compared across subjects. An overall ratio was calculated on the basis of values in regions expected to be affected by AD and those not expected to be affected. The metabolic ratio and amyloid ratio were then correlated with the mini-mental status examination (MMSE) score.

RESULTS

The sensitivity and specificity, compared with the clinical diagnosis of AD or controls, for the (18)F-florbetapir scans were 95% and 95%, respectively, and for the (18)F-FDG scans 89% and 86%, respectively. When a comparison with MMSE scores was made, (18)F-FDG significantly correlated with MMSE when both controls and AD patients were included (r = 0.79, P < 0.0001) and in AD patients alone (r = 0.70, P = 0.001). The (18)F-florbetapir scores significantly correlated with MMSE scores only when both controls and AD patients were included (r = 0.62, P < 0.001) but not in the AD group alone (r = 0.12, P = 0.66).

CONCLUSION

Overall, both scans performed well in detecting AD in patients with known clinical AD. Both scans correlated well with cognitive status as determined by MMSE when the entire cohort of controls and AD patients was evaluated. However, only the (18)F-FDG scans correlated with cognitive status when AD patients were evaluated separately.

BODIPY-based molecular probe for imaging of cerebral β-amyloid plaques

We designed and synthesized a BODIPY-based probe (BAP-1) for the imaging of β-amyloid plaques in the brain. In binding experiments in vitro, BAP-1 showed excellent affinity for synthetic Aβ aggregates. β-Amyloid plaques in Tg2576 mouse brain were clearly visualized with BAP-1. In addition, the labeling of β-amyloid plaques was demonstrated in vivo in Tg2576 mice. These results suggest BAP-1 to be a useful fluorescent probe for the optical imaging of cerebral β-amyloid plaques in patients with Alzheimer's disease.

A Novel (18)F-Labeled Imidazo[2,1-b]benzothiazole (IBT) for High-Contrast PET Imaging of β-Amyloid Plaques

(18)F-labeled imidazo[2,1-b]benzothiazole ([(18)F]8) was synthesized and evaluated as a tracer for cerebral β-amyloid deposits (Aβ) by means of positron emission tomography (PET). [(18)F]8 exhibits a high affinity to Aβ and suitable brain uptake kinetics combined with a high metabolic stability in the brain. In a double transgenic APP/PS1 mouse model of Alzheimer's disease, we demonstrated a specific uptake of [(18)F]8 in Aβ-containing telencephalic brain regions. The specific binding of [(18)F]8 to Aβ was confirmed by regional brain biodistribution and autoradiography and correlated to immunohistochemistry staining. Analysis of brain sections of APP/PS1 mouse injected with a cocktail of [(18)F]8 and reference compound [(3)H]PiB revealed that the two tracers bind to Aβ plaques in the brain of mouse in a comparable binding pattern. [(18)F]8 represents the first high-contrast PET imaging agent for detection of Aβ plaques in transgenic mouse model of Alzheimer's disease and holds promise for transfer to a clinical evaluation.

Novel 18F-labeled benzofuran derivatives with improved properties for positron emission tomography (PET) imaging of β-amyloid plaques in Alzheimer's brains

In vivo imaging of β-amyloid plaques in the brain may lead to the early diagnosis of Alzheimer's disease (AD) and monitoring of the progression and effectiveness of treatment. In the present study, we report on the development of two potential PET probes, [(18)F]FPYBF-2 ([(18)F]10) and [(18)F]FPHBF-2 ([(18)F]21), for imaging of β-amyloid plaques in AD brain. In experiments in vitro, 10 and 21 displayed high affinity for Aβ(1-42) aggregates (K(i) = 2.41 and 3.85 nM, respectively). In biodistribution experiments using normal mice, they displayed high uptake in the brain (7.38 and 8.18% ID/g at 2 min postinjection, respectively), and the radioactivity washed out from the brain rapidly (3.15 and 3.87% ID/g at 60 min postinjection, respectively), which is highly desirable for β-amyloid imaging agents. In vivo, they clearly labeled β-amyloid plaques in Tg2576 mice. Furthermore, the specific labeling of β-amyloid plaques by 10 and 21 was observed in autoradiographs of sections of autopsied AD brain. These new fluorinated benzofuran derivatives are promising PET probes for imaging cerebral β-amyloid plaques.

Diphenylpropynone derivatives as probes for imaging β-amyloid plaques in Alzheimer's brains

A new series of diphenylpropynone (DPP) derivatives for use in vivo to image β-amyloid (Aβ) plaques in the brain of patients with Alzheimer's disease (AD) were synthesized and characterized. Binding experiments in vitro revealed high affinity for Aβ (1-42) aggregates at a K(i) value ranging from 6 to 326 nM. Furthermore, specific labeling of plaques was observed in sections of brain tissue from Tg2576 transgenic mice stained using one of the compounds, 1. In biodistribution experiments with normal mice, [(125)I]1 displayed moderate uptake (1.55%ID/g at 2 min) and clearance from the brain with time (0.76 ID/g at 60 min). Taken together, DPP can serve as a new molecular scaffold for developing novel Aβ imaging agents by introducing appropriate substituted groups.

Novel Benzofurans with (99m)Tc Complexes as Probes for Imaging Cerebral β-Amyloid Plaques

Two novel benzofuran derivatives coupled with (99m)Tc complexes were tested as probes for imaging cerebral β-amyloid plaques using single photon emission tomography. Although both derivatives bound to Aβ(1-42) aggregates, (99m)Tc-BAT-BF showed higher affinity than (99m)Tc-MAMA-BF. In sections of brain tissue from an animal model of AD, (99m)Tc-BAT-BF clearly labeled β-amyloid plaques. In biodistribution experiments using normal mice, (99m)Tc-BAT-BF displayed high uptake soon after its injection and washed out from the brain rapidly, a highly desirable feature for an imaging agent. (99m)Tc-BAT-BF may be a potential probe for imaging β-amyloid plaques in Alzheimer's brains.

Fluorinated Benzofuran Derivatives for PET Imaging of β-Amyloid Plaques in Alzheimer's Disease Brains

A series of fluorinated benzofuran derivatives as potential tracers for positron emission tomography (PET) targeting β-amyloid plaques in the brains of patients with Alzheimer's disease (AD) were synthesized and evaluated. The derivatives were produced using an intramolecular Wittig reaction. In experiments in vitro, all displayed high affinity for Aβ(1-42) aggregates with K i values in the nanomolar range. Radiofluorinated 17, [(18)F]17, in particular labeled β-amyloid plaques in sections of Tg2576 mouse brain and displayed high uptake (5.66% ID/g) at 10 min postinjection, sufficient for PET imaging. In addition, in vivo β-amyloid plaque labeling can be clearly demonstrated with [(18)F]17 in Tg2576 mice. In conclusion, [(18)F]17 may be useful for detecting β-amyloid plaques in patients with AD.

Synthesis and evaluation of novel chalcone derivatives with (99m)Tc/Re complexes as potential probes for detection of β-amyloid plaques

Four (99m)Tc-labeled chalcone derivatives and their corresponding rhenium analogues were tested as potential probes for imaging β-amyloid plaques. The chalcones showed higher affinity for Aβ(1-42) aggregates than did (99m)Tc complexes. In sections of brain tissue from an animal model of AD, the four Re chalcones intensely stained β-amyloid plaques. In biodistribution experiments using normal mice, (99m)Tc-BAT-chalcone ([(99m)Tc]17) displayed high uptake in the brain (1.48% ID/g) at 2 min postinjection. The radioactivity washed out from the brain rapidly (0.17% ID/g at 60 min), a highly desirable feature for an imaging agent. [(99m)Tc]17 may be a potential probe for imaging β-amyloid plaques in Alzheimer's brains.

A novel 18F-labeled pyridyl benzofuran derivative for imaging of β-amyloid plaques in Alzheimer's brains

A potential probe for PET targeting β-amyloid plaques in Alzheimer's disease (AD) brain, FPYBF-1 (5-(5-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)benzofuran-2-yl)-N,N-dimethylpyridin-2-amine), was synthesized and evaluated. In experiments in vitro, FPYBF-1 displayed high affinity for Aβ(1-42) aggregates (K(i)=0.9 nM), and substantial labeling of β-amyloid plaques in sections of postmortem AD brains but not control brains. In experiments in vivo, [(18)F]FPYBF-1 displayed good initial uptake (5.16%ID/g at 2 min postinjection) and rapid washout from the brain (2.44%ID/g at 60 min postinjection) in normal mice, and excellent binding to β-amyloid plaques in a murine model of AD. Furthermore, the specific labeling of plaques labeling was observed in autoradiographs of autopsied AD brain sections. [(18)F]FPYBF-1 may be a useful probe for imaging β-amyloid plaques in living brain tissue.

99mTc/Re complexes based on flavone and aurone as SPECT probes for imaging cerebral β-amyloid plaques

Two (99m)Tc/Re complexes based on flavone and aurone were tested as potential probes for imaging β-amyloid plaques using single photon emission computed tomography. Both (99m)Tc-labeled derivatives showed higher affinity for Aβ(1-42) aggregates than did (99m)Tc-BAT. In sections of brain tissue from an animal model of AD, the Re-flavone derivative 9 and Re-aurone derivative 19 intensely stained β-amyloid plaques. In biodistribution experiments using normal mice, (99m)Tc-labeled flavone and aurone displayed similar radioactivity pharmacokinetics. With additional modifications to improve their brain uptake, (99m)Tc complexes based on the flavone or aurone scaffold may serve as probes for imaging cerebral β-amyloid plaques.

GMP-compliant automated synthesis of [(18)F]AV-45 (Florbetapir F 18) for imaging beta-amyloid plaques in human brain

We report herein the Good Manufacturing Practice (GMP)-compliant automated synthesis of (18)F-labeled styrylpyridine, AV-45 (Florbetapir), a novel tracer for positron emission tomography (PET) imaging of beta-amyloid (Abeta) plaques in the brain of Alzheimer's disease patients. [(18)F]AV-45 was prepared in 105 min using a tosylate precursor with Sumitomo modules for radiosynthesis under GMP-compliant conditions. The overall yield was 25.4+/-7.7% with a final radiochemical purity of 95.3+/-2.2% (n=19). The specific activity of [(18)F]AV-45 reached as high as 470+/-135 TBq/mmol (n=19). The present studies show that [(18)F]AV-45 can be manufactured under GMP-compliant conditions and could be widely available for routine clinical use.

Optimization of automated radiosynthesis of [18F]AV-45: a new PET imaging agent for Alzheimer's disease

INTRODUCTION

Accumulation of β-amyloid (Aβ) aggregates in the brain is linked to the pathogenesis of Alzheimer's disease (AD). Imaging probes targeting these Aβ aggregates in the brain may provide a useful tool to facilitate the diagnosis of AD. Recently, [(18)F]AV-45 ([(18)F]5) demonstrated high binding to the Aβ aggregates in AD patients. To improve the availability of this agent for widespread clinical application, a rapid, fully automated, high-yield, cGMP-compliant radiosynthesis was necessary for production of this probe. We report herein an optimal [(18)F]fluorination, de-protection condition and fully automated radiosynthesis of [(18)F]AV-45 ([(18)F]5) on a radiosynthesis module (BNU F-A2).

METHODS

The preparation of [(18)F]AV-45 ([(18)F]5) was evaluated under different conditions, specifically by employing different precursors (-OTs and -Br as the leaving group), reagents (K222/K(2)CO(3) vs. tributylammonium bicarbonate) and deprotection in different acids. With optimized conditions from these experiments, the automated synthesis of [(18)F]AV-45 ([(18)F]5) was accomplished by using a computer-programmed, standard operating procedure, and was purified on an on-line solid-phase cartridge (Oasis HLB).

RESULTS

The optimized reaction conditions were successfully implemented to an automated nucleophilic fluorination module. The radiochemical purity of [(18)F]AV-45 ([(18)F]5) was >95%, and the automated synthesis yield was 33.6 ± 5.2% (no decay corrected, n=4), 50.1 ± 7.9% (decay corrected) in 50 min at a quantity level of 10-100 mCi (370-3700 MBq). Autoradiography studies of [(18)F]AV-45 ([(18)F]5) using postmortem AD brain and Tg mouse brain sections in the presence of different concentration of "cold" AV-136 showed a relatively low inhibition of in vitro binding of [(18)F]AV-45 ([(18)F]5) to the Aβ plaques (IC50=1-4 μM, a concentration several order of magnitude higher than the expected pseudo carrier concentration in the brain).

CONCLUSIONS

Solid-phase extraction purification and improved labeling conditions were successfully implemented into an automated synthesis module, which is more convenient, highly efficient and simpler in operation than using a semipreparative high-performance liquid chromatography method. This new, automated procedure for preparation of [(18)F]AV-45 ([(18)F]5) is suitable for routine clinical application.

Whole-body biodistribution and brain PET imaging with [18F]AV-45, a novel amyloid imaging agent--a pilot study

PURPOSE

The compound (E)-4-(2-(6-(2-(2-(2-(18)F-fluoroethoxy)ethoxy)ethoxy) pyridin-3-yl)vinyl)-N-methylbenzenamine ([(18)F]AV-45) is a novel radiopharmaceutical capable of selectively binding to beta-amyloid (A beta) plaques. This pilot study reports the safety, biodistribution, and radiation dosimetry of [(18)F]AV-45 in human subjects.

METHODS

In vitro autoradiography and fluorescent staining of postmortem brain tissue from patients with Alzheimer's disease (AD) and cognitively healthy subjects were performed to assess the specificity of the tracer. Biodistribution was assessed in three healthy elderly subjects (mean age: 60.0+/-5.2 years) who underwent 3-h whole-body positron emission tomography (PET)/computed tomographic (CT) scans after a bolus injection of 381.9+/-13.9 MBq of [(18)F]AV-45. Another six subjects (three AD patients and three healthy controls, mean age: 67.7+/-13.6 years) underwent brain PET studies. Source organs were delineated on PET/CT. All subjects underwent magnetic resonance imaging (MRI) for obtaining structural information.

RESULTS

In vitro autoradiography revealed exquisitely high specific binding of [(18)F]AV-45 to postmortem AD brain sections, but not to the control sections. There were no serious adverse events throughout the study period. The peak uptake of the tracer in the brain was 5.12+/-0.41% of the injected dose. The highest absorbed organ dose was to the gallbladder wall (184.7+/-78.6 microGy/MBq, 4.8 h voiding interval). The effective dose equivalent and effective dose values for [(18)F]AV-45 were 33.8+/-3.4 microSv/MBq and 19.3+/-1.3 microSv/MBq, respectively.

CONCLUSION

[(18)F]AV-45 binds specifically to A beta in vitro, and is a safe PET tracer for studying A beta distribution in human brain. The dosimetry is suitable for clinical and research application.

MRI in dementia

With cognitive disorders increasingly common, clinicians urgently need faster and more accurate tools to classify such disorders and to noninvasively monitor therapeutic interventions. In this review, we provide information on MRI techniques that enable the study of the morphology, neuronal integrity, and metabolism of dementing illnesses. In addition, we explore the usefulness of such techniques as surrogate markers of these diseases.

Amyloid imaging in Alzheimer's disease and other dementias

With the advent of new therapeutic strategies aimed at reducing β-amyloid (Aβ) burden in the brain to potentially prevent or delay functional and irreversible cognitive loss, there is increased interest in developing agents that allow assessment of Aβ burden in vivo. Molecular neuroimaging techniques such as positron emission tomography (PET), in conjunction with related biomarkers in plasma and cerebrospinal fluid, are proving valuable in the early and differential diagnosis of Alzheimer's disease (AD). (11)C-PiB PET has proven useful in the discrimination of dementias, showing significantly higher PiB retention in grey matter of AD patients when compared with healthy controls or patients with frontotemporal dementia. (11)C-PiB PET also appears to be more accurate than FDG for the diagnosis of AD. Despite apparently underestimating the Aβ burden in the brain, (11)C-PiB PET is an optimal method to differentiate healthy controls from AD, matching histopathological reports in aging and dementia and reflecting the true regional density of Aβ plaques in cortical areas. High striatal Aβ deposition seems to be typical for carriers of familial forms of AD, whilst ApoE ε4 carriers, independent of diagnosis or disease severity, present with higher Aβ burden than non- ε4 carriers. Characterization of the binding properties of PiB has shown that despite binding to other misfolded proteins in vitro, PiB is extremely selective for Aβ at the concentrations achieved during a PET scan. Aβ burden as assessed by PET does not correlate with measures of cognition or cognitive decline in AD. Approximately 30% of apparently healthy older people, and 50-60% of people with mild cognitive impairment, present with cortical (11)C-PiB retention. In these groups, Aβ burden does correlate with episodic memory and rate of memory decline. These observations suggest that Aβ deposition is not part of normal ageing, supporting the hypothesis that Αβ deposition occurs well before the onset of symptoms and is likely to represent preclinical AD. Further longitudinal observations, coupled with different disease-specific tracers and biomarkers are required not only to confirm this hypothesis, but also to better elucidate the role of Αβ deposition in the course of Alzheimer's disease.

A versatile approach to beta-amyloid fibril-binding compounds exploiting the Shirakawa/Hayashi protocol for trans-alkene synthesis

Application of the Sonogashira coupling reaction followed by a trans-selective alkyne reduction proved highly adaptable for the efficient synthesis of a class of beta-amyloid fibril binding compounds possessing a styrylbenzene motif such as FSB, an FSB dimer, and (19)F-BAY94-9172.

Novel radioiodinated aurones as probes for SPECT imaging of beta-amyloid plaques in the brain

We report a novel series of radioiodinated aurone derivatives as probes for imaging Abeta plaques in the brains of patients with Alzheimer's disease (AD) using single photon emission computed tomography (SPECT). In binding experiments in vitro, aurone derivatives showed very good affinity for Abeta aggregates (K(i) = 1.1 to 3.4 nM). No-carrier-added radioiodinated aurones were successfully prepared through an iododestannylation reaction from the corresponding tributyltin derivatives. In biodistribution experiments using normal mice, aurone derivatives displayed high brain uptake (1.7-4.5% ID/g at 2 min) and rapid clearance from the brain (0.1-0.4% ID/g at 30 min), especially [125I]15. Furthermore, a specific plaque labeling signal was observed in in vitro autoradiography of postmortem AD brain sections using [125I]15. [125I]15 may be a useful SPECT imaging agent for detecting Abeta plaques in the brain of AD.

SPECT imaging in dementia

Single photon emission computed tomography (SPECT) is a non-invasive functional neuroimaging technique that can be used in the diagnosis of dementia. This review describes some of the SPECT radiotracers available for imaging dementia patients and discusses recommendations for the clinical use of this imaging technique.

An expedient synthesis of the fibril binding compound FSB via sequential Pd-catalyzed coupling reactions

The styryl benzene derivative (E, E)-1-fluoro-2,5-bis(3-hydroxycarbonyl-4-hydroxy)styrylbenzene (FSB), well-known for its binding to beta-amyloid peptide fibrils, was synthesized in an efficient manner exploiting two sequential palladium(0)-catalyzed coupling reactions in a 34% overall yield. This is a substantial improvement to the previously reported synthesis of FSB in 1.1%.

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.