Jiapei Dai

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.

99mTc(CO)3-Labeled Benzothiazole Derivatives Preferentially Bind Cerebrovascular Amyloid: Potential Use as Imaging Agents for Cerebral Amyloid Angiopathy.

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 Alzheimers 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.

99mTc-labeled benzothiazole and stilbene derivatives as imaging agents for Aβ plaques in cerebral amyloid angiopathy

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 Alzheimers 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.

Radioiodinated Benzyloxybenzene Derivatives: A Class of Flexible Ligands Target to β-Amyloid Plaques in Alzheimer’s Brains

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.

Amyloid-β Deposits Target Efficient Near-Infrared Fluorescent Probes: Synthesis, in Vitro Evaluation and in Vivo Imaging

The formation of extracellular amyloid-β (Aβ) plaques is a common molecular change that underlies several debilitating human conditions, including Alzheimers 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.

99mTc-Labeled 2-Arylbenzothiazoles: Aβ Imaging Probes with Favorable Brain Pharmacokinetics for Single-Photon Emission Computed Tomography

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.

Novel 18F-labeled dibenzylideneacetone derivatives as potential positron emission tomography probes for in vivo imaging of β-amyloid plaques

A series of dibenzylideneacetones were synthesized and evaluated as imaging probes for β-amyloid plaques. They displayed high binding affinity to Aβ(1-42) aggregates (K(i) = 6.4 for 8, K(i) = 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 (18)F-labeling directly on the benzene ring. In biodistribution experiments, [(18)F]8 and [(18)F]9 displayed high initial uptakes (9.29 ± 0.41 and 5.38 ± 0.68% ID/g) and rapid washouts from the normal brain (brain(2 min)/brain(60 min) ratios of 21.6 and 13.4). These preliminary results suggest that [(18)F]8 and [(18)F]9 may be used as potential PET imaging agents for the detection of Aβ plaques in the brain.

Smart D-π-A Type Near-Infrared Aβ Probes: Effects of a Marked π Bridge on Optical and Biological Properties

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.

Preliminary Characterization and In Vivo Studies of Structurally Identical (18)F- and (125)I-Labeled Benzyloxybenzenes for PET/SPECT Imaging of β-Amyloid Plaques.

With the assistance of molecular docking and 3D-QSAR models established previously, structurally identical 18F- and 125I-labeled benzyloxybenzene derivatives were designed to achieve the early detection of Aβ plaques by PET/SPECT imaging. In competition binding assay, ligands 7a and 12a displayed high binding affinities to Aβ42 aggregates with Ki values of 19.5 nM and 23.9 nM, respectively. Specific plaque labeling was observed on the in vitro autoradiography of brain sections from AD patients and Tg mice. In biodistribution, [125I]7a, [18F]7a, [125I]12a and [18F]12a all exhibited high initial brain uptakes (>5% ID/g at 2 min). [125I]7a and [125I]12a cleared fast from the normal brain regions, while corresponding [18F]7a and [18F]12a showed slow washout rates. Dynamic microPET/CT and microSPECT/CT imaging data in normal ICR mice were in accordance with in vivo biodistribution results. In vivo metabolism results indicated that the different clearance profiles between the structurally identical 18F- and 125I-labeled tracers could be attributed to different biochemical characteristics of the radiometabolites. Radioiodinated benzyloxybenzene derivatives exhibited good in vivo biostability in brain. Ex vivo autoradiography further confirmed the strong in vivo Aβ labeling ability of [125I]7a. These new fluorinated and iodinated benzyloxybenzenes can develop into PET/SPECT dual imaging agents targeting Aβ plaques.