Abstract Objective We wished to develop a highly selective positron emission tomography (PET) imaging agent targeting PHF-tau in human Alzheimer's disease (AD) brains. Methods To screen potential tau ...binders, human AD brain sections were used as a source of native paired helical filament (PHF)-tau and Aβ rather than synthetic tau aggregates or Aβ fibrils generated in vitro to measure the affinity and selectivity of 18 FT807 to tau and Aβ. Brain uptake and biodistribution of 18 FT807 in mice were also tested. Results In vitro autoradiography results show that 18 FT807 exhibits strong binding to PHF-tau-positive human brain sections. A dissociation constant ( Kd ) of 18 FT807 (14.6 nM) was measured using brain sections from the frontal lobe of AD patients. A comparison of autoradiography and double immunohistochemical staining of PHF-tau and Aβ on adjacent sections demonstrated that 18 FT807 binding colocalized with immunoreactive PHF-tau pathology, but did not highlight Aβ plaques. In vivo studies in mice demonstrated that 18 FT807 was able to cross the blood–brain barrier and washed out quickly. Conclusions 18 FT807 demonstrates high affinity and selectivity to PHF-tau as well as favorable in vivo properties, making this a promising candidate as an imaging agent for AD.
Senile plaques and neurofibrillary tangles are prominent neuropathological hallmarks in Alzheimer's disease and are considered to be targets for therapeutic intervention as well as biomarkers for ...diagnostic in vivo imaging agents. While there are a number of amyloid-β positron emission tomography (PET) tracers currently in different stages of clinical development and commercialization, there have been very few reports on imaging agents selectively targeting tau aggregates. In search of 18F-PET tracers that possess great binding affinity and selectivity toward tau tangles, we tested more than 900 compounds utilizing a unique screening process. A competitive autoradiography assay was set up to test compounds for binding to native tau tangles and amyloid-β plaques on human brain tissue sections. In our in vitro assays, the 18F labeled compound 18F-T808 displayed a high level of binding affinity and good selectivity for tau aggregates over amyloid-β plaques. 18F-T808 showed rapid uptake and washout in rodent brains. Our in vitro and preclinical in vivo studies suggest that 18F-T808 possesses suitable properties and characteristics to be a specific and selective PET probe for imaging of paired helical filament tau in human brains.
Target‐guided synthesis: Rather than making and screening thousands of compounds for lead discovery, in situ click chemistry employs the biological target itself to assemble its inhibitors by ...selectively binding and interconnecting reagents within the confines of its binding sites. Subnanomolar inhibitors of carbonic anhydrase II were produced by the enzyme from simple azide and acetylene precursors (see picture).
State‐of‐the‐art: Automated chemical reaction circuits have been demonstrated to serve as a miniaturized operation platform for the parallel screening of 32 in situ click chemistry reactions with ...reduced consumption of reagents. A proof‐of‐concept study was performed with the bovine carbonic anhydrase II (bCAII) click chemistry system.
The natural products colchicine and combretastatin A-4 (
) have provided inspiration for the discovery and development of a wide array of derivatives and analogues that inhibit tubulin polymerization ...through a binding interaction at the colchicine site on β-tubulin. A water-soluble phosphate prodrug salt of
(referred to as
) has demonstrated the ability to selectively damage tumor-associated vasculature and ushered in a new class of developmental anticancer agents known as vascular disrupting agents (VDAs). Through a long-term program of structure activity relationship (SAR) driven inquiry, we discovered that the dihydronaphthalene molecular scaffold provided access to small-molecule inhibitors of tubulin polymerization. In particular, a dihydronaphthalene analogue bearing a pendant trimethoxy aryl ring (referred to as
) and a similar aroyl ring (referred to as
) were potent inhibitors of tubulin polymerization (IC
= 1.0 and 1.2 μM, respectively) and displayed low nM cytotoxicity against human cancer cell lines. In order to enhance water-solubility for
evaluation, the corresponding phosphate prodrug salts (
and
, respectively) were synthesized. In a preliminary
study in a SCID-BALB/c mouse model bearing the human breast tumor MDA-MB-231-luc, a 99% reduction in signal was observed with bioluminescence imaging (BLI) 4 h after IP administration of
(200 mg kg
) indicating reduced tumor blood flow. In a separate study, disruption of tumor-associated blood flow in a Fischer rat bearing an A549-luc human lung tumor was observed by color Doppler ultrasound following administration of
(15 mg kg
).
Purpose
We identified and validated
18
F-CP18, a DEVD (the caspase 3 substrate recognition motif) containing substrate-based compound as an imaging tracer for caspase-3 activity in apoptotic cells.
...Procedures
CP18 was radiolabeled with fluorine-18 using click chemistry. The affinity and selectivity of CP18 for caspase-3 were evaluated
in vitro
. The biodistribution and metabolism pattern of
18
F-CP18 were assessed
in vivo
.
18
F-CP18 positron emission tomography (PET) scans were performed in a dexamethasone-induced thymic apoptosis mouse model. After imaging, the mice were sacrificed, and individual organs were collected, measured in a gamma counter, and tested for caspase-3 activity.
Results
In vitro
enzymatic caspase-3 assay demonstrated specific cleavage of CP18.
In vivo
,
18
F-CP18 is predominantly cleared through the kidneys and urine, and is rapidly eliminated from the bloodstream. There was a sixfold increase in caspase activity and a fourfold increase of
18
F-CP18 retention in the dexamethasone-induced thymus of treated versus control mice.
Conclusions
We report the use
18
F-CP18 as a PET tracer for imaging apoptosis. Our data support further development of this tracer for clinical PET applications.
Abstract Introduction Dysregulated MMP expression or activation is associated with several diseases. To study MMP activity in vivo by means of PET a radiolabeled MMP inhibitor (MMPI) functioning as ...radiotracer has been developed by our group based on the lead structure CGS 25966. Materials and Methods Aiming at the modification of the pharmacokinetics of this lipophilic model tracer a new class of MMPIs have been discovered, consisting of additional fluorinated hydrophilic substructures, such as mini- PEG and/or 1,2,3-triazole units. To identify the best candidate for further clinical applications, radiofluorinated compounds of each subgroup have been (radio)synthesized and evaluated regarding their biodistribution behavior and their metabolic stability. Results Radiosyntheses of different triazole based MMPIs could be realized using two step “click chemistry” procedures. Compared to lead structure 18 FFEtO-CGS 25966 ( 18 F1e , log D (exp) = 2.02, IC50 = 2 – 50 nM) all selected candidates showed increased hydrophilicities and inhibition potencies (log D (exp) = 0.23 – 1.25, IC50 = 0.006 – 6 nM). Interestingly, despite different hydrophilicities most triazole based MMPIs showed no significant differences in their in vivo biodistribution behavior and were cleared predominantly via the hepatobiliary excretion route. Biostability and metabolism studies in vitro and in vivo revealed significant higher metabolic stability for the triazole moiety compared to the benzyl ring in the lead structure. Cleavage of ethylene glycol subunits of the mini -PEG chain led to a faster metabolism of mini -PEG containing MMPIs. Conclusion The introduction of hydrophilic groups such as mini -PEG and 1,2,3-triazole units did not lead to a significant shift of the hepatobiliary elimination towards renal clearance. Particularly the introduction of mini -PEG chains led to an intense metabolic decomposition. Substitution of the benzyl moiety in lead structure 1e by a 1,2,3-trizole ring resulted in an increased metabolic stability. Therefore, the 1,2,3-triazole-1-yl-methyl substituted MMPI 18 F3a was found to be the most stable candidate in this series and should be chosen for further preclinical evaluation.
Purpose
A novel caspase-3 substrate-based probe
18
F-CP18 was evaluated as an
in vivo
positron emission tomography (PET) imaging agent for monitoring apoptosis in tumors.
Methods
Uptake of
18
...F-CP18 in cell assays and tumors was measured. Caspase-3/7 activities in cell lysates and tumor homogenates were determined. Autoradiography,Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and cleaved caspase-3 immunostaining were performed on adjacent tumor sections to identify areas of apoptosis.
Results
The
in vitro
cell assays showed caspase-3-dependent uptake of
18
F-CP18 in tumor cells when treated with an apoptosis inducer. The
in vivo
microPET imaging signal of
18
F-CP18 in xenograft tumors correlated with the
ex vivo
caspase-3/7 activities in these tumors. Furthermore, tumor autoradiographies of
18
F-CP18 in tumor sections matched adjacent sections stained by TUNEL and caspase-3 immunohistochemistry (IHC).
Conclusions
18
F-CP18 demonstrated high affinity and selectivity for activated caspase-3 both
in vitro
and
in vivo
, and the results support
18
F-CP18 as a promising new PET imaging agent for apoptosis.
Purpose
18
FVM4-037 has been developed as a positron emission tomography (PET) imaging marker to detect carbonic anhydrase IX (CA-IX) overexpression and is being investigated for use as a surrogate ...marker for tissue hypoxia. The purpose of this study was to determine the biodistribution and estimate the radiation dose from
18
FVM4-037 using whole-body PET/CT scans in healthy human volunteers.
Procedures
Successive whole-body PET/CT scans were performed after intravenous injection of
18
FVM4-037 in four healthy humans. The radiotracer uptakes in different organs were determined from the analysis of the PET scans. Human radiation doses were estimated using OLINDA/EXM software.
Results
High uptake of
18
FVM4-037 was observed in the liver and kidneys, with little clearance of activity during the study period, with mean standardized uptake values of ~35 in liver and ~22 in kidneys at ~1 h after injection. The estimated effective dose was 28 ± 1 μSv/MBq and the absorbed doses for the kidneys and liver were 273 ± 31 and 240 ± 68 μGy/MBq, respectively, for the adult male phantom. Hence, the effective dose would be 10 ± 0.5 mSv for the anticipated injected activity of 370 MBq, and the kidney and liver doses would be 101 ± 11 and 89 ± 25 mGy, respectively.
Conclusions
18
FVM4-037 displayed very high uptake in the liver and kidneys with little clearance of activity during the study period, resulting in these organs receiving the highest radiation doses among all bodily organs. Though the effective dose and the organ doses are within the limits considered as safe, the enhanced uptake of
18
FVM4-037 in the kidneys and liver will make the compound unsuitable for imaging overexpression of CA-IX in those two organs. However, the tracer may be suitable for imaging overexpression of CA-IX in lesions in other regions of the body such as in the lungs or head and neck region.
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