To expand the scope of D-π-A based near-infrared (NIR) probes for detecting β-amyloid (Aβ) plaques and to systematically explore the relationship among their structural characteristics, optical properties, and biological properties, three series of smart NIR probes with different aromatic rings and up to seven trans double bonds were synthesized and evaluated. Marked correlations between the conjugated π system and properties of these probes, such as optical data, binding ability, and brain uptake, were observed. One probe, PHC-4, displayed improved properties as a NIR probe for the in vivo detection of Aβ plaques.

•2-Arylbenzofurans with chiral side chain were synthesized and evaluated as Aβ probe.•They displayed moderate to high binding affinity towards Aβ aggregates.•(S)-[18F]17 displayed significant improved pharmacokinetics.A new class of optical isomers of 2-arylbenzofuran derivatives were synthesized and evaluated as potential β-amyloid plaques imaging agents. Both lipophilicity and signal-to-noise ratio were significantly improved by adding a chiral hydroxyl group to 1-fluoro-3-(oxidanyl)propan-2-ol side chain. These derivatives displayed moderate to high binding affinity towards Aβ1-42 aggregates. Four tracers possessing potent binding affinity (Ki < 30 nM) were chosen for further investigation. In in vitro autoradiography studies, the four selected probes showed effective binding to Aβ plaques in Tg mouse and AD human brain tissue after labeled by 18F. The purified enantiomers displayed apparent discrepancy in biodistribution experiments in normal mice, for (S)-enantiomers provided rather faster clearance than (R)-enantiomers. All in all, (S)-[18F]17 (Ki = 14.6 nM) with excellent pharmacokinetics (brain2 min = 8.60% ID/g, brain2 min/brain60 min = 14.1) deserves further evaluation.Download high-res image (192KB)Download full-size image

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).

Two neutral merocyanine-based near-infrared fluorescent probes were for the first time developed through rational engineering of the classical cationic cyanine scaffold IR-780 for in vivo imaging of amyloid-β plaques. In vivo NIRF imaging revealed that the probe could penetrate the blood–brain barrier and efficiently differentiate the living transgenic and wild-type mice.

We have designed and synthesized a series of indole-based σ2 receptor ligands containing 5,6-dimethoxyisoindoline or 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline as pharmacophore. In vitro competition binding assays showed that all ten ligands possessed low nanomolar affinity (Ki = 1.79–5.23 nM) for σ2 receptors and high subtype selectivity (Ki (σ2)/Ki (σ1) = 56–708). Moreover, they showed high selectivity for σ2 receptor over the vesicular acetylcholine transporter (>1000-fold). The corresponding radiotracers [18F]16 and [18F]21 were prepared by an efficient one-pot, two-step reaction sequence with a home-made automated synthesis module, with 10–15% radiochemical yield and radiochemical purity of >99%. Both radiotracers showed high brain uptake and σ2 receptor binding specificity in mice.Download high-res image (42KB)Download full-size image

The formation of extracellular amyloid-β (Aβ) plaques is a common molecular change that underlies several debilitating human conditions, including Alzheimer’s disease (AD); however, the existing near-infrared (NIR) fluorescent probes for the in vivo detection of Aβ plaques are limited by undesirable fluorescent properties and poor brain kinetics. In this work, we designed, synthesized, and evaluated a new family of efficient NIR probes that target Aβ plaques by incorporating hydroxyethyl groups into the ligand structure. Among these probes, DANIR 8c showed excellent fluorescent properties with an emission maximum above 670 nm upon binding to Aβ aggregates and also displayed a high sensitivity (a 629-fold increase in fluorescence intensity) and affinity (Kd = 14.5 nM). Because of the improved hydrophilicity that was induced by hydroxyls, 8c displayed increased initial brain uptake and a fast washout from the brain, as well as an acceptable biostability in the brain. In vivo NIR fluorescent imaging revealed that 8c could efficiently distinguish between AD transgenic model mice and normal controls. Overall, 8c is an efficient and veritable NIR fluorescent probe for the in vivo detection of Aβ plaques in the brain.

A series of 2-arylbenzothiazole derivatives conjugated with bis(aminoethanethiol) (BAT) chelating groups were designed and synthesized. A competitive binding assay-based screening was used to select seven rhenium complexes with potent binding affinity toward Aβ1–42 aggregates (Ki < 50 nM) for 99mTc labeling and further evaluation. The 99mTc-labeled probes showed good affinity and specificity to Aβ plaques in Tg mouse brain tissue in in vitro autoradiography studies. Moreover, [99mTc]14b exhibited favorable brain pharmacokinetics in normal mice (2.11% ID/g at 2 min and 0.62% ID/g at 60 min). Ex vivo autoradiography revealed extensive labeling of Aβ plaques by [99mTc]14b in the brain of Tg mice. Furthermore, we performed the first single-photon emission computed tomography (SPECT) imaging study in nonhuman primates with 99mTc-labeled Aβ probes. The semiquantitative data showed that [99mTc]14b penetrated the brains of rhesus monkeys. These results indicate that [99mTc]14b could be utilized as a SPECT imaging probe for Aβ plaques.

This study describes an effective strategy to improve pharmacokinetics of Aβ imaging agents, offering a novel class of (R)- and (S)-18F-labeled 2-arylbenzoheterocyclic derivatives which bear an additional chiral hydroxyl group on the side chain. These ligands displayed binding abilities toward Aβ aggregates with Ki values ranging from 3.2 to 195.6 nM. Chirality-related discrepancy was observed in biodistribution, and (S)-2-phenylbenzoxazole enantiomers exhibited vastly improved brain clearance with washout ratios higher than 20. Notably, (S)-[18F]28 possessed high binding potency (Ki = 7.6 nM) and exceptional brain kinetics (9.46% ID/g at 2 min, brain2min/brain60min = 27.8) that is superior to well-established [18F]AV45. The excellent pharmacokinetics and low nonspecific binding of (S)-[18F]28 were testified by dynamic PET/CT scans in monkey brains. In addition, (S)-[18F]28 clearly labeled Aβ plaques both in vitro and ex vivo. These results might qualify (S)-[18F]28 to detect Aβ plaques with high signal-to-noise ratio.Keywords: Alzheimer’s disease; molecular imaging; positron emission tomography; β-amyloid plaque;

•99mTc/Re-labeled 2-arylbenzothiazoles were synthesized and evaluated as Aβ probes.•[99mTc]4h labeled Aβ deposits in the blood vessels on brain section of AD patient.•[99mTc]4h may be used to detect Aβ related disease other than AD.An array of complexes, 99mTc/Re-labeled cyclopentadienyl tricarbonyl and 2-phenyl/pyridylbenzothiazoles, conjugated through an ester linkage were tested as potential β-amyloid (Aβ) probes for SPECT imaging. The [CpRe/99mTc(CO)3] complexes were prepared by double ligand transfer reactions from ferrocene precursors, and the X-ray structure of one rhenium surrogate revealed a classical “piano stool” like geometry. Several of the rhenium complexes displayed high affinity for Aβ1-42 aggregates in in vitro inhibition assays, and they could intensely stain Aβ deposits on brain sections from transgenic mice and Alzheimer's disease (AD) patients. Complex [99mTc]4h strongly binds to Aβ deposits in blood vessels of the brain section of AD patients in in vitro autoradiography. Biodistribution experiments in normal mice revealed that 99mTc-labeled tracers exhibited moderate degree of initial brain uptake (0.54%–1.06% ID/g at 2 min). These tracers targeting Aβ plaques of AD patients warrant further structure optimization to improve their ability to penetrate blood-brain barrier, moreover, they may be suitable for developing imaging probes for targeting amyloid aggregates outside of the brain.

The highly rigid and planar scaffold with π-conjugated systems has been widely considered to be indispensable for Aβ binding probes. However, the flexible benzyloxybenzene derivative [125I]BOB-4 represents an excellent lead candidate for targeting Aβ in AD brains. Based on that, we designed two pairs of more flexible and optically pure diphenoxy derivatives with a chiral center as novel Aβ probes. These compounds possessed high affinity (Ki = 15.8–45.0 nM) for Aβ1–42 aggregates, and (R)-enantiomers showed slightly better binding ability than (S)-enantiomers. In addition, the competition binding assay implied that the optically pure diphenoxy derivatives with more flexible geometry shared the same binding site as IMPY, a classical rigid and planar Aβ probe. For 125I-radiolabeled enantiomers, (S)-[125I]5 and (R)-[125I]5, specific plaque labeling on brain sections of Tg mice and AD patients were observed in in vitro autoradiography, persuasively proving the excellent affinity of the probes. In biodistribution, (S)-[125I]5 and (R)-[125I]5 with relatively low lipophilicity exhibited moderate initial brain uptake (4.37% and 3.72% ID/g at 2 min, respectively) and extremely fast washout from normal mice brain (brain2min/brain60min = 19.0 and 17.7, respectively). In summary, the separate enantiomers displayed similar properties in vitro and in vivo, and (S/R)-[123I]5 may be potential SPECT probes for recognizing Aβ plaques in AD brains.Keywords: Alzheimer’s disease; Aβ plaque; benzyloxybenzene; enantiopure; imaging probe

In continuation of our investigation of the dibenzylideneacetone scaffold as Aβ imaging probes, a series of derivatives containing pyridine rings with lower lipophilicity was synthesized and evaluated. Some of these probes displayed high affinities to Aβ1–42 aggregates, which ranged from 18.0 to 8.2 nM. The high and specific binding of the radiolabeled tracers was further confirmed by in vitro autoradiography on brain sections from AD patients and transgenic mouse. In biodistribution experiments, two 18F-labeled tracers [18F]17a and [18F]18 displayed high initial brain uptake (5.05 and 6.24% ID g−1, respectively, at 2 min postinjection) and fast clearance from the brain with brain2 min/brain60 min ratios of 6.08 and 8.00. These results demonstrate that the probes [18F]17a and [18F]18 with a dibenzylideneacetone structure containing pyridine rings have great strength in imaging Aβ plaques in the brain.

A new array of near-infrared probes containing barbituric acid acceptors has been developed as Aβ imaging agents. These probes displayed long-emission wavelengths and large Stokes shifts, as well as high affinities for Aβ aggregates. In vivo and ex vivo studies demonstrated that BBTOM-3 could intensely label Aβ plaques in the brains of transgenic mice.

Alzheimer’s disease (AD) is pathologically characterized by the accumulation of β-amyloid (Aβ) deposits in the parenchymal and cortical brain. In this work, we designed, synthesized, and evaluated a series of near-infrared (NIR) probes with electron donor–acceptor end groups interacting through a π-conjugated system for the detection of Aβ deposits in the brain. Among these probes, 3b and 3c had excellent fluorescent properties (emission maxima > 650 nm and high quantum yields) and displayed high sensitivity and high affinities to Aβ aggregates (3b, Kd = 8.8 nM; 3c, Kd = 1.9 nM). Both 3b and 3c could readily penetrate the blood–brain barrier with high initial brain uptake and fast to moderate washout from the brain. In vivo NIR imaging revealed that 3b and 3c could efficiently differentiate transgenic and wild-type mice. In summary, our research provides new hints for developing smarter and more activatable NIR probes targeting Aβ.

Technetium-99m-labeled cyclopentadienyl tricarbonyl complexes conjugated with the 2-phenylbenzothiazole binding motif were synthesized. The rhenium surrogates 20, 21, 22 and 23 were demonstrated to have moderate to high affinities for Aβ1–42 aggregates with Ki values of 142, 76, 64 and 24 nM, respectively. During the fluorescence staining of brain sections of transgenic mice and patients with Alzheimer's disease, these rhenium complexes demonstrated perfect and intense labeling of Aβ plaques. Moreover, in in vitro autoradiography, 99mTc-labeled complexes clearly detected β-amyloid plaques on sections of brain tissue from transgenic mice, which confirmed the sufficient affinity of these tracers for Aβ plaques. However, these compounds did not show desirable properties in vivo, especially showing poor brain uptake (below 0.5% ID g−1), which will hinder the further development of these tracers as brain imaging agents. Nonetheless, it is encouraging that these 99mTc-labeled complexes designed by a conjugate approach displayed sufficient affinities for Aβ plaques.

Cerebral amyloid angiopathy (CAA) is a disorder affecting the elderly that is characterized by amyloid-β (Aβ) deposition in blood vessel walls of the brain. A series of 99mTc(CO)3-labeled benzothiazole derivatives as potential SPECT imaging probes for cerebrovascular Aβ deposition is reported. Rhenium surrogate displayed high affinities to Aβ aggregates with Ki values ranging from 106 to 42 nM, and they strongly stained Aβ deposits in transgenic mice (Tg) and Alzheimer’s disease (AD) patients. In vitro autoradiography on brain sections of Tg and AD patients confirmed that [99mTc]24 possessed sufficient affinity for Aβ plaques, and [99mTc]24 could only label Aβ deposition in blood vessels but not Aβ plaques in the parenchyma of the brain of AD patients. Moreover, [99mTc]24 possessed favorable initial uptake (1.21% ID/g) and fast blood washout (blood2 min/blood60 min = 23) in normal mice. These preliminary results suggest that [99mTc]24 may be used as an Aβ imaging probe for the detection of CAA.

•A series of flexible FPEG benzyloxybenzenes were evaluated to quantify Aβ plaques.•[18F]9a exhibited strong labeling of Aβ plaques and good kinetic profiles.•Ex vivo ARG of [18F]9a confirmed its in vivo binding abilities to Aβ plaques.A new series of fluoro-pegylated benzyloxybenzenes were designed, synthesized and evaluated as PET probes for early detection of Aβ plaques. Molecular docking revealed that all of the flexible benzyloxybenzenes inserted themselves into the hydrophobic Val18_Phe20 cleft on the flat spine of the Aβ fiber, in a manner similar to that of IMPY molecule. The most potent probe, [18F]9a, exhibited a combination of high binding affinity to Aβ aggregates (Ki = 21.0 ± 4.9 nM), high initial brain uptake (9.14% ID/g at 2 min), fast clearance from normal brain tissue (1.79% ID/g at 60 min), and satisfactory in vivo biostability in the brain (95% of intact form at 2 min). [18F]9a clearly labeled Aβ plaques in in vitro autoradiography of postmortem AD patients and Tg mice brain sections. Ex vivo autoradiography further demonstrated that [18F]9a did penetrate the intact BBB and specifically bind to Aβ plaques in vivo. Overall, [18F]9a may be a potential PET probe for imaging Aβ plaques in AD brains.

•Four 99mTc/Re-labeled 2-arylbenzoxazoles displayed affinity to Aβ1–42 aggregates.•[99mTc]20 displayed excellent plaque labeling on the brain sections of APP/PS1 mice.•[99mTc]20 displayed moderate uptake and rapid washout from the normal mice brain.Four neutral 99mTc/Re-labeled 2-arylbenzoxazole derivatives conjugated to bis (aminoethanethiol) (BAT) chelating ligand via a short propoxy spacer were synthesized and evaluated. In vitro binding assay showed that they displayed binding affinities to Aβ1–42 aggregates (Ki = 15.86–393.18 nM). In vitro autoradiography studies further confirmed the high and specific binding of [99mTc]20 to β-amyloid plaques on brain sections of transgenic mice. Biodistribution study of [99mTc]17–20 in normal mice displayed moderate initial brain uptake (0.96–1.55%ID/g at 2 min), and fast washed out from the brain (0.14–0.40%ID/g at 60 min), especially for [99mTc]20 with a brain2min/brain60min ratio of 8.86. Taken together, these preliminary data suggested that [99mTc]20 may be a potential imaging probe for detecting amyloid plaques in the brain.

A novel class of near-infrared molecules based on the donor–acceptor architecture were synthesized and evaluated as Aβ imaging probes. In vivo imaging studies suggested that MCAAD-3 could penetrate the blood–brain barrier and label Aβ plaques in the brains of transgenic mice. Computational studies could reproduce the experimental trends well.

Benzyloxybenzene, as a novel flexible scaffold without rigid planarity, was synthesized and evaluated as ligand toward Aβ plaques. The binding site calculated for these flexible ligands was the hydrophobic Val18_Phe20 channel on the flat surface of Aβ fiber. Structure–activity relationship analysis generated a common trend that binding affinities declined significantly from para-substituted ligands to ortho-substituted ones, which was also quantitatively illustrated by 3D-QSAR modeling. Autoradiography in vitro further confirmed the high affinities of radioiodinated ligands [125I]4, [125I]24, and [125I]22 (Ki = 24.3, 49.4, and 17.6 nM, respectively). In biodistribution, [125I]4 exhibited high initial uptake and rapid washout property in the brain with brain2 min/brain60 min ratio of 16.3. The excellent in vitro and in vivo biostability of [125I]4 enhanced its potential for clinical application in SPECT imaging of Aβ plaques. This approach could also allow the design of a new generation of Aβ targeting ligands without rigid and planar framework.

•Two dibenzylideneacetones derivatives were labeled with 18F on the benzene ring.•High affinity to Aβ1–42 aggregates was confirmed by binding assay & autoradiography.•[18F]8/9 displayed high initial uptakes and rapid washouts from the normal brain.A series of dibenzylideneacetones were synthesized and evaluated as imaging probes for β-amyloid plaques. They displayed high binding affinity to Aβ1–42 aggregates (Ki = 6.4 for 8, Ki = 3.0 for 9), and the high binding were confirmed by in vitro autoradiography with AD human and transgenic mouse brain sections. Two of them were selected for 18F-labeling directly on the benzene ring. In biodistribution experiments, [18F]8 and [18F]9 displayed high initial uptakes (9.29 ± 0.41 and 5.38 ± 0.68% ID/g) and rapid washouts from the normal brain (brain2 min/brain60 min ratios of 21.6 and 13.4). These preliminary results suggest that [18F]8 and [18F]9 may be used as potential PET imaging agents for the detection of Aβ plaques in the brain.

•We summarized the progress of radiolabeled benzoheterocycles for imaging Aβ plaques.•Synthetic approaches, binding potencies and brain pharmacokinetics were discussed.•According to the type of scaffolds, the discussion was made in divided sections.Alzheimer's disease (AD) is a debilitating neurodegenerative dementia that involves substantial neuronal loss. Extracellular deposition of neurotoxic β-amyloid (Aβ) plaques in the brain has been recognized as the central histological characteristic of AD. In the past decade, precise detection of the Aβ plaques at preclinical AD with positron emission tomography (PET) or single photon emission computed tomography (SPECT) has achieved continued development. A big category of Aβ imaging agents was benzoheterocycles which derived from Thioflavin-T (ThT), a traditional amyloid binding dye. This review summarizes the past and current status of radioactive benzoheterocycles designed to selectively bind to Aβ plaques. Separate sections discuss the chemical synthesis, in vitro and in vivo investigations of radiolabeled benzothiazole, benzoxazole, benzofuran, benzothiophene, indole, imidazopyridine and quinoxaline analogs to act as PET/SPECT candidates for imaging Aβ plaques.

β-Amyloid (Aβ) plaques in the blood vessels of the brain are associated with cerebral amyloid angiopathy (CAA), which is a common cause of stroke and vascular diseases. Imaging agents that can differentiate between Aβ plaques in the brain and those on the walls of cerebrovascular vessels will provide non-invasive biomarkers to interrogate the pathogenesis of CAA and give insights into the mechanisms, significance and impact of Aβ-CAA for developing effective therapies for CAA and stroke. A new series of 99mTc-labeled benzothiazole and stilbene derivatives with positive charge were developed and evaluated for selectively targeting Aβ plaques in the blood vessels of the brain. The rhenium complexes 6, 7, 13 and 14 displayed medium binding affinity to Aβ1–42 aggregates with Ki values of 162, 37, 366 and 78 nM, respectively. In vitro fluorescence staining of 7 and 14 demonstrated an intense labeling of Aβ plaques associated with blood vessel walls on brain sections of a patient with Alzheimer's diseases. A relatively low initial brain uptake for [99mTc]7 and [99mTc]14, 0.18 and 0.24 ID% per gram, respectively, suggests that they may be useful SPECT imaging agents for selectively detecting Aβ plaques associated with cerebral vessels in the living human brain.

Rhenium and technetium-99m cyclopentadienyl tricarbonyl complexes mimicking the chalcone structure were prepared. These complexes were proved to have affinity to β-amyloid (Aβ) in fluorescent staining on brain sections of Alzheimer’s Disease (AD) patient and binding assay using Aβ1–42 aggregates, with Ki values ranging from 899 to 108 nM as the extension of conjugated π system. In vitro autoradiograpy on sections of transgenic mouse brain confirmed the affinity of [99mTc]5 (Ki = 108 nM). In biodistribution, all compounds showed good initial uptakes into the brain and fast blood clearance, while the decreasing of initial brain uptakes correspond to increasing of conjugation length, from 4.10 ± 0.38% ID/g ([99mTc]3) to 1.11 ± 0.34% ID/g ([99mTc]5). These small technetium-99m complexes (<500 Da) designed by an integrated approach provide encouraging evidence that development of a promising 99mTc-labeled agent for imaging Aβ plaques in the brain may be feasible.

A novel series of fluorinated 2-phenylindole derivatives were synthesized and evaluated as β-amyloid imaging probes for PET. The in vitro inhibition assay demonstrated that their binding affinities for Aβ1–42 aggregates ranged from 28.4 to 1097.8 nM. One ligand was labeled with 18F ([18F]1a) for its high affinity (Ki = 28.4 nM), which was also confirmed by in vitro autoradiography experiments on brain sections of transgenic mouse (C57BL6, APPswe/PSEN1, 11 months old, male). In vivo biodistribution experiments in normal mice showed that this radiotracer displayed high initial uptake (5.82 ± 0.51% ID/g at 2 min) into and moderate washout (2.77 ± 0.31% ID/g at 60 min) from the brain. [18F]1a could be developed as a promising new PET imaging probe for Aβ plaques although necessary modifications are still needed.

A series of fluoro-pegylated (FPEG) 2-pyridinylbenzoxazole and 2-pyridinylbenzothiazole derivatives were synthesized and evaluated as novel β-amyloid (Aβ) imaging probes for PET. They displayed binding affinities for Aβ1–42 aggregates that varied from 2.7 to 101.6 nM. Seven ligands with high affinity were selected for 18F labeling. In vitro autoradiography results confirmed the high affinity of these radiotracers. In vivo biodistribution experiments in normal mice indicated that the radiotracers with a short FPEG chain (n = 1) displayed high initial uptake into and rapid washout from the brain. One of the 2-pyridinylbenzoxazole derivatives, [18F]-5-(5-(2-fluoroethoxy)benzo[d]oxazol-2-yl)-N-methylpyridin-2-amine ([18F]32) (Ki = 8.0 ± 3.2 nM) displayed a brain2min/brain60min ratio of 4.66, which is highly desirable for Aβ imaging agents. Target specific binding of [18F]32 to Aβ plaques was validated by ex vivo autoradiographic experiment with transgenic model mouse. Overall, [18F]32 is a promising Aβ imaging agent for PET and merits further evaluation in human subjects.

In continuation of our study on the 2-phenylquinoxaline scaffold as potential β-amyloid imaging probes, two [18F]fluoro-pegylated 2-phenylquinoxaline derivatives, 2-(4-(2-[18F]fluoroethoxy)phenyl)-N-methylquinoxalin-6-amine ([18F]4a) and 2-(4-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)phenyl)-N-methylquinoxalin-6-amine ([18F]4b) were prepared. Both of them displayed high binding affinity to Aβ1-42 aggregates (Ki = 10.0 ± 1.4 nM for 4a, Ki = 5.3 ± 3.2 nM for 4b). The specific and high binding of [18F]4a and [18F]4b to Aβ plaques was confirmed by in vitro autoradiography on brain sections of AD human and transgenic mice. In biodistribution in normal mice, [18F]4a displayed high initial brain uptake (8.17% ID/g at 2 min) and rapid washout from the brain. These preliminary results suggest [18F]4a may be a potential PET imaging agent for Aβ plaques in the living human brain.Graphical abstractHighlights► Two novel 18F-labeled 2-phenylquinoxaline derivatives were synthesized. ► Both of them displayed high affinity to Aβ1-42 aggregates. ► They showed excellent plaque labeling on brain sections of AD human and Tg mice. ► [18F]4a showed high initial uptake (8.17% ID/g) and rapid washout from the brain.

Two uncharged 99mTc-labeled phenylbenzoxazole derivatives were biologically evaluated as potential imaging probes for β-amyloid plaques. The 99mTc and corresponding rhenium complexes were synthesized by coupling monoamine–monoamide dithiol (MAMA) and bis(aminoethanethiol) (BAT) chelating ligand via a pentyloxy spacer to phenylbenzoxazole. The fluorescent rhenium complexes 6 and 9 selectively stainined the β-amyloid plaques on the sections of transgenic mouse, and showed high affinity for Aβ(1–42) aggregates (Ki = 11.1 nM and 14.3 nM, respectively). Autoradiography in vitro indicated that [99mTc]6 clearly labeled β-amyloid plaques on the sections of transgenic mouse. Biodistribution experiments in normal mice revealed that [99mTc]6 displayed moderate initial brain uptake (0.81% ID/g at 2 min), and quickly washed out from the brain (0.25% ID/g at 60 min). The preliminary results indicate that the properties of [99mTc]6 are promising, although additional refinements are needed to improve the ability to cross the blood–brain barrier.

A novel class of near-infrared molecules based on the donor–acceptor architecture were synthesized and evaluated as Aβ imaging probes. In vivo imaging studies suggested that MCAAD-3 could penetrate the blood–brain barrier and label Aβ plaques in the brains of transgenic mice. Computational studies could reproduce the experimental trends well.

Three fluorinated (19F or 18F) small molecules were evaluated as fluorescent or radiolabeled probes for Aβ deposits in the brain. In vivo near-infrared imaging suggested that probe 4c could efficiently distinguish between transgenic mice and wild-type controls. Furthermore, the in vivo Cerenkov luminescence imaging with [18F]4c showed modest differences between the two groups.

Technetium-99m-labeled cyclopentadienyl tricarbonyl complexes conjugated with the 2-phenylbenzothiazole binding motif were synthesized. The rhenium surrogates 20, 21, 22 and 23 were demonstrated to have moderate to high affinities for Aβ1–42 aggregates with Ki values of 142, 76, 64 and 24 nM, respectively. During the fluorescence staining of brain sections of transgenic mice and patients with Alzheimer's disease, these rhenium complexes demonstrated perfect and intense labeling of Aβ plaques. Moreover, in in vitro autoradiography, 99mTc-labeled complexes clearly detected β-amyloid plaques on sections of brain tissue from transgenic mice, which confirmed the sufficient affinity of these tracers for Aβ plaques. However, these compounds did not show desirable properties in vivo, especially showing poor brain uptake (below 0.5% ID g−1), which will hinder the further development of these tracers as brain imaging agents. Nonetheless, it is encouraging that these 99mTc-labeled complexes designed by a conjugate approach displayed sufficient affinities for Aβ plaques.

A new array of near-infrared probes containing barbituric acid acceptors has been developed as Aβ imaging agents. These probes displayed long-emission wavelengths and large Stokes shifts, as well as high affinities for Aβ aggregates. In vivo and ex vivo studies demonstrated that BBTOM-3 could intensely label Aβ plaques in the brains of transgenic mice.