Mengchao Cui

Smart near-infrared fluorescence probes with donor-acceptor structure for in vivo detection of β-amyloid deposits.

The deposition of β-amyloid (Aβ) plaques in the parenchymal and cortical brain is accepted as the main pathological hallmark of Alzheimers disease (AD); however, early detection of AD still presents a challenge. With the assistance of molecular imaging techniques, imaging agents specifically targeting Aβ plaques in the brain may lead to the early diagnosis of AD. Herein, we report the design, synthesis, and evaluation of a series of smart near-infrared fluorescence (NIRF) imaging probes with donor-acceptor architecture bridged by a conjugated π-electron chain for Aβ plaques. The chemical structure of these NIRF probes is completely different from Congo Red and Thioflavin-T. Probes with a longer conjugated π system (carbon-carbon double bond) displayed maximum emission in PBS (>650 nm), which falls in the best range for NIRF probes. These probes were proved to have affinity to Aβ plaques in fluorescent staining of brain sections from an AD patient and double transgenic mice, as well as in an in vitro binding assay using Aβ(1-42) aggregates. One probe with high affinity (K(i) = 37 nM, K(d) = 27 nM) was selected for in vivo imaging. It can penetrate the blood-brain barrier of nude mice efficiently and is quickly washed out of the normal brain. Moreover, after intravenous injection of this probe, 22-month-old APPswe/PSEN1 mice exhibited a higher relative signal than control mice over the same period of time, and ex vivo fluorescent observations confirmed the existence of Aβ plaques. In summary, this probe meets most of the requirements for a NIRF contrast agent for the detection of Aβ plaques both in vitro and in vivo.

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 Alzheimers 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.

Synthesis and structure-affinity relationships of novel dibenzylideneacetone derivatives as probes for β-amyloid plaques.

A new and extensive set of dibenzylideneacetone derivatives was synthesized and screened for affinity toward Aβ(1-42) aggregates. Structure-activity relationships revealed the binding of dibenzylideneacetones to be affected by various substituents. The introduction of a substituent group in the ortho position reduced or abolished the binding. However, the para position was highly tolerant of sterically demanding substitutions. Three radioiodinated ligands (6, 70, and 71) and two (18)F fluoro-pegylated (FPEG) ligands (83 and 85) were prepared, all of which displayed high affinity for Aβ(1-42) aggregates (K(i) ranging from 0.9 to 7.0 nM). In biodistribution experiments, they exhibited good initial penetration (1.59, 4.68, 4.56, 4.13, and 5.15% ID/g, respectively, at 2 min) of and fast clearance from the brain. Autoradiography with sections of postmortem AD brain and transgenic mouse brain confirmed the high affinity of these tracers. These preliminary results strongly suggest the dibenzylideneacetone structure to be a potential new scaffold for β-amyloid imaging probes.

Radioiodinated benzimidazole derivatives as single photon emission computed tomography probes for imaging of β-amyloid plaques in Alzheimer's disease

Five iodinated 2-phenyl-1H-benzo[d]imidazole derivatives were synthesized and evaluated as potential probes for β-amyloid (Aβ) plaques. One of the compounds, 4-(6-iodo-1H-benzo[d]imidazol-2-yl)-N,N-dimethylaniline (12), showed excellent affinity for Aβ(1-42) aggregates (K(i) = 9.8 nM). Autoradiography with sections of postmortem Alzheimers disease (AD) brain revealed that a radioiodinated probe [(125)I]12, labeled Aβ plaques selectively with low nonspecific binding. Biodistribution experiments with normal mice injected intravenously with [(125)I]12 showed high uptake [4.14 percent injected dose per gram (% ID/g) at 2 min] into and rapid clearance (0.15% ID/g at 60 min) from the brain, which may bring about a good signal-to-noise ratio and therefore achieve highly sensitive detection of Aβ plaques. In addition, [(125)I]12 labeled amyloid plaques in vivo in an AD transgenic model. The preliminary results strongly suggest that [(125)I]12 bears characteristics suitable for detecting amyloid plaques in vivo. When labeled with (123)I, it may be a useful SPECT imaging agent for Aβ plaques in the brain of living AD patients.

Synthesis and Evaluation of Novel 18F Labeled 2-Pyridinylbenzoxazole and 2-Pyridinylbenzothiazole Derivatives as Ligands for Positron Emission Tomography (PET) Imaging of β-Amyloid Plaques

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 Aβ(1-42) aggregates that varied from 2.7 to 101.6 nM. Seven ligands with high affinity were selected for (18)F 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, [(18)F]-5-(5-(2-fluoroethoxy)benzo[d]oxazol-2-yl)-N-methylpyridin-2-amine ([(18)F]32) (K(i) = 8.0 ± 3.2 nM) displayed a brain(2min)/brain(60min) ratio of 4.66, which is highly desirable for Aβ imaging agents. Target specific binding of [(18)F]32 to Aβ plaques was validated by ex vivo autoradiographic experiment with transgenic model mouse. Overall, [(18)F]32 is a promising Aβ imaging agent for PET and merits further evaluation in human subjects.

Highly Sensitive Near-Infrared Fluorophores for in Vivo Detection of Amyloid-β Plaques in Alzheimer’s Disease

Alzheimers 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.

Novel Cyclopentadienyl Tricarbonyl Complexes of 99mTc Mimicking Chalcone as Potential Single-Photon Emission Computed Tomography Imaging Probes for β-Amyloid Plaques in 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 Alzheimers Disease (AD) patient and binding assay using Aβ(1-42) aggregates, with K(i) 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 [(99m)Tc]5 (K(i) = 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 ([(99m)Tc]3) to 1.11 ± 0.34% ID/g ([(99m)Tc]5). These small technetium-99m complexes (<500 Da) designed by an integrated approach provide encouraging evidence that development of a promising (99m)Tc-labeled agent for imaging Aβ plaques in the brain may be feasible.

Development of dual functional SPECT/fluorescent probes for imaging cerebral β-amyloid plaques

The imaging of beta-amyloid (Abeta) aggregates in the brain may lead to the early detection of Alzheimers disease (AD) and monitoring of the progression and effectiveness of treatment. The purpose of this study was to develop dual modality SPECT and fluorescent probes based on boron dipyrromethane (BODIPY) as a core structure. We designed and synthesized an (125)I-labeled derivative of BODIPY (BODIPY7). BODIPY7 had a K(i) value of 108nM for Abeta(1-42) aggregates and exhibited peaks of absorption/emission at 606/613nm. It detected Abeta plaques in sections of brain tissue from an animal model of AD and displayed low uptake in the brain and high uptake in the liver in normal mice. Although additional modifications of the BODIPY scaffold are necessary to improve brain uptake, these results should aid the development of dual functional SPECT/fluorescent probes for the imaging of Abeta plaques in the brain.

Synthesis and biological evaluation of indole-chalcone derivatives as β-amyloid imaging probe.

A series of chaclone derivatives containing an indole moiety were evaluated in competitive binding assays with Aβ(1-42) aggregates versus [(125)I]IMPY. The affinity of these compounds ranged from 4.46 to >1008 nM, depending on the substitution on the phenyl ring. Fluorescent staining in vitro showed that one compound with a N,N-dimethylamino group intensely stained Aβ plaques within brain sections of AD transgenic mice. The radioiodinated probe [(125)I]-(E)-3-(1H-indol-5-yl)-1-(4-iodophenyl)prop-2-en-1-one, [(125)I]4, was prepared and autoradiography in sections of brain tissue from an animal model of AD showed that it labeled Aβ plaques specifically. However, experiments with normal mice indicated that [(125)I]4 exhibited a low uptake into the brain in vivo (0.41% ID/g at 2 min). Additional chemical modifications of this indole-chalcone structure may lead to more useful imaging agents for detecting β-amyloid plaques in the brains of AD patients.

Novel 18F-Labeled Benzoxazole Derivatives as Potential Positron Emission Tomography Probes for Imaging of Cerebral β-Amyloid Plaques in Alzheimer’s Disease

Two radiofluoro-pegylated phenylbenzoxazole derivatives, 4-(5-(2-(2-(2-[(18)F]fluoroethoxy)ethoxy)ethoxy)benzo[d]oxazol-2-yl)-N-methylaniline ([(18)F]24) and 4-(5-(2-(2-(2-[(18)F]fluoroethoxy)ethoxy)ethoxy)benzo[d]oxazol-2-yl)-N,N-dimethylaniline ([(18)F]32), were synthesized and evaluated as probes for imaging cerebral β-amyloid (Aβ) plaques in living brain tissue by PET. [(18)F]24 and [(18)F]32 displayed high affinity for Aβ(1-42) aggregates (K(i) = 9.3 and 3.9 nM, respectively). In vitro autoradiography with sections of post-mortem AD brain and transgenic mouse brain confirmed the affinity of these tracers. Initial high uptake into and rapid washout from the brain in normal mice were observed. [(18)F]24 also displayed excellent binding to Aβ plaques in ex vivo autoradiographic experiments with Tg2576 mice. Furthermore, small-animal PET studies demonstrated significant differences in the clearance profile after the administration of [(18)F]24 between Tg2576 and wild-type mice. The results suggest [(18)F]24 to be a useful PET agent for detecting Aβ plaques in the living human brain.