Vani P. Mocharla

[18F]T807, a novel tau positron emission tomography imaging agent for Alzheimer's disease

We wished to develop a highly selective positron emission tomography (PET) imaging agent targeting PHF‐tau in human Alzheimers disease (AD) brains.

A Highly Selective and Specific PET Tracer for Imaging of Tau Pathologies

Senile plaques and neurofibrillary tangles are prominent neuropathological hallmarks in Alzheimers 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.

Biodistribution and Radiation Dosimetry of the Integrin Marker 18F-RGD-K5 Determined from Whole-Body PET/CT in Monkeys and Humans

2-((2S,5R,8S,11S)-5-benzyl-8-(4-((2S,3R,4R,5R,6S)-6-((2-(4-(3-18F-fluoropropyl)-1H-1,2,3-triazol-1-yl)acetamido)methyl)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxamido)butyl)-11-(3-guanidinopropyl)-3,6,9,12,15-pentaoxo-1,4,7,10,13-pentaazacyclopentadecan-2-yl)acetic acid (18F-RGD-K5) has been developed as an αvβ3 integrin marker for PET. The purpose of this study was to determine the biodistribution and estimate the radiation dose from 18F-RGD-K5 using whole-body PET/CT scans in monkeys and humans. Methods: Successive whole-body PET/CT scans were obtained after intravenous injection of 18F-RGD-K5 in 3 rhesus monkeys (167 ± 19 MBq) and 4 healthy humans (583 ± 78 MBq). In humans, blood samples were collected between the PET/CT scans, and stability of 18F-RGD-K5 was assessed. Urine was also collected between the scans, to determine the total activity excreted in urine. The PET scans were analyzed to determine the radiotracer uptake in different organs. OLINDA/EXM software was used to calculate human radiation doses based on human and monkey biodistributions. Results: 18F-RGD-K5 was metabolically stable in human blood up to 90 min after injection, and it cleared rapidly from the blood pool, with a 12-min half-time. For both monkeys and humans, increased 18F-RGD-K5 uptake was observed in the kidneys, bladder, liver, and gallbladder, with mean standardized uptake values at 1 h after injection for humans being approximately 20, 50, 4, and 10, respectively. For human biodistribution data, the calculated effective dose was 31 ± 1 μSv/MBq, and the urinary bladder wall had the highest absorbed dose at 376 ± 19 μGy/MBq using the 4.8-h bladder-voiding model. With the 1-h voiding model, these doses reduced to 15 ± 1 μSv/MBq for the effective dose and 103 ± 4 μGy/MBq for the absorbed dose in the urinary bladder wall. For a typical injected activity of 555 MBq, the effective dose would be 17.2 ± 0.6 mSv for the 4.8-h model, reducing to 8.3 ± 0.4 mSv for the 1-h model. For monkey biodistribution data, the effective dose to humans would be 22.2 ± 2.4 mSv for the 4.8-h model and 12.8 ± 0.2 mSv for the 1-h model. Conclusion: The biodistribution profile of 18F-RGD-K5 in monkeys and humans was similar, with increased uptake in the bladder, liver, and kidneys. There was rapid clearance of 18F-RGD-K5 through the renal system. The urinary bladder wall received the highest radiation dose and was deemed the critical organ. Both whole-body effective dose and bladder dose can be reduced by more frequent voiding. 18F-RGD-K5 can be used safely for imaging αvβ3 integrin expression in humans.

A New Class of Highly Potent Matrix Metalloproteinase Inhibitors Based on Triazole-Substituted Hydroxamates: (Radio)Synthesis and in Vitro and First in Vivo Evaluation

In vivo imaging of MMPs is of great (pre)clinical interest and can potentially be realized with modern three-dimensional and noninvasive in vivo molecular imaging techniques such as positron emission tomography (PET). Consequently, MMP inhibitors (MMPIs) radiolabeled with positron emitting nuclides (e.g., (18)F) represent a suitable tool for the visualization of activated MMPs with PET. On the basis of our previous work and results regarding radiolabeled and unlabeled derivatives of the nonselective MMPIs, we discovered a new class of fluorinated MMPIs with a triazole-substituted hydroxamate substructure. These novel MMPIs are characterized by an increased hydrophilicity compared with the lead structures and excellent MMP inhibition potencies for MMP-2, MMP-8, MMP-9, and MMP-13 (IC(50) = 0.006-107 nM). Therefore, one promising fluorinated triazole-substituted hydroxamate (30b) was selected and resynthesised as its (18)F-labeled version to yield the potential PET radioligand [(18)F]30b. The biodistribution behavior of this novel compound was investigated with small animal PET.

Evaluation of [18F]-CP18 as a PET Imaging Tracer for Apoptosis

PurposeWe identified and validated [18F]-CP18, a DEVD (the caspase 3 substrate recognition motif) containing substrate-based compound as an imaging tracer for caspase-3 activity in apoptotic cells.ProceduresCP18 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 [18F]-CP18 were assessed in vivo. [18F]-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.ResultsIn vitro enzymatic caspase-3 assay demonstrated specific cleavage of CP18. In vivo, [18F]-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 [18F]-CP18 retention in the dexamethasone-induced thymus of treated versus control mice.ConclusionsWe report the use [18F]-CP18 as a PET tracer for imaging apoptosis. Our data support further development of this tracer for clinical PET applications.

In Vitro and In Vivo Evaluation of the Caspase-3 Substrate-Based Radiotracer [ 18 F]-CP18 for PET Imaging of Apoptosis in Tumors

PurposeA novel caspase-3 substrate-based probe [18F]-CP18 was evaluated as an in vivo positron emission tomography (PET) imaging agent for monitoring apoptosis in tumors.MethodsUptake of [18F]-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.ResultsThe in vitro cell assays showed caspase-3-dependent uptake of [18F]-CP18 in tumor cells when treated with an apoptosis inducer. The in vivo microPET imaging signal of [18F]-CP18 in xenograft tumors correlated with the ex vivo caspase-3/7 activities in these tumors. Furthermore, tumor autoradiographies of [18F]-CP18 in tumor sections matched adjacent sections stained by TUNEL and caspase-3 immunohistochemistry (IHC).Conclusions[18F]-CP18 demonstrated high affinity and selectivity for activated caspase-3 both in vitro and in vivo, and the results support [18F]-CP18 as a promising new PET imaging agent for apoptosis.

From In Situ to In Vivo: An In Situ Click-Chemistry-Derived Carbonic Anhydrase II Imaging Agent for Positron Emission Tomography

CA II makes a good PET: Discovering positron emission tomography (PET) probes with high target affinities is challenging. PET probe discovery using in situ click chemistry uses (19) F-bearing fragments as (18) F surrogates. This ensures that the lead hits and PET probes have equivalent chemical or biological characteristics, making PET probe discovery predictable and reliable.