Preclinical Evaluation of a Novel TSPO PET Ligand 2-(7-Butyl-2-(4-(2-[18F]Fluoroethoxy)phenyl)-5-Methylpyrazolo[1,5-a]Pyrimidin-3-yl)-N,N-Diethylacetamide (18F-VUIIS1018A) to Image Glioma

Purpose

There is an urgent need for the development of novel positron emission tomography (PET) tracers for glioma imaging. In this study, we developed a novel PET probe ([¹⁸F]VUIIS1018A) by targeting translocator protein (TSPO), an imaging biomarker for glioma. The purpose of this preclinical study was to evaluate this novel TSPO probe for glioma imaging.

Procedures

In this study, we synthesized [¹⁹F]VUIIS1018A and the precursor for radiosynthesis of [¹⁸F]VUIIS1018A. TSPO binding affinity was confirmed using a radioligand competitive binding assay in C6 glioma cell lysate. Further, dynamic imaging studies were performed in rats using a microPET system. These studies include displacement and blocking studies for ligand reversibility and specificity evaluation, and compartment modeling of PET data for pharmacokinetic parameter measurement using metabolite-corrected arterial input functions and PMOD.

Results

Compared to previously reported TSPO tracers including [¹⁸F]VUIIS1008 and [¹⁸F]DPA-714, the novel tracer [¹⁸F]VUIIS1018A demonstrated higher binding affinity and BP_ND. Pretreatment with the cold analog [¹⁹F]VUIIS1018A could partially block tumor accumulation of this novel tracer. Further, compartment modeling of this novel tracer also exhibited a greater tumor-to-background ratio, a higher tumor binding potential and a lower brain binding potential when compared with other TSPO probes, such as [¹⁸F]DPA-714 and [¹⁸F]VUIIS1008.

Conclusions

These studies illustrate that [¹⁸F]VUIIS1018A can serve as a promising TSPO PET tracer for glioma imaging and potentially imaging of other solid tumors.

     

Evaluation of TSPO PET Ligands [<Superscript>18</Superscript>F]VUIIS1009A and [<Superscript>18</Superscript>F]VUIIS1009B: Tracers for Cancer Imaging

Purpose

Positron emission tomography (PET) ligands targeting translocator protein (TSPO) are potential imaging diagnostics of cancer. In this study, we report two novel, high-affinity TSPO PET ligands that are 5,7 regioisomers, [¹⁸F]VUIIS1009A ([¹⁸F]3A) and [¹⁸F]VUIIS1009B ([¹⁸F]3B), and their initial in vitro and in vivo evaluation in healthy mice and glioma-bearing rats.

Procedures

VUIIS1009A/B was synthesized and confirmed by X-ray crystallography. Interactions between TSPO binding pocket and novel ligands were evaluated and compared with contemporary TSPO ligands using 2D ¹H-¹⁵N heteronuclear single quantum coherence (HSQC) spectroscopy. In vivo biodistribution of [¹⁸F]VUIIS1009A and [¹⁸F]VUIIS1009B was carried out in healthy mice with and without radioligand displacement. Dynamic PET imaging data were acquired simultaneously with [¹⁸F]VUIIS1009A/B injections in glioma-bearing rats, with binding reversibility and specificity evaluated by radioligand displacement. In vivo radiometabolite analysis was performed using radio-TLC, and quantitative analysis of PET data was performed using metabolite-corrected arterial input functions. Imaging was validated with histology and immunohistochemistry.

Results

Both VUIIS1009A (3A) and VUIIS1009B (3B) were found to exhibit exceptional binding affinity to TSPO, with observed IC₅₀ values against PK11195 approximately 500-fold lower than DPA-714. However, HSQC NMR suggested that VUIIS1009A and VUIIS1009B share a common binding pocket within mammalian TSPO (mTSPO) as DPA-714 and to a lesser extent, PK11195. [¹⁸F]VUIIS1009A ([¹⁸F]3A) and [¹⁸F]VUIIS1009B ([¹⁸F]3B) exhibited similar biodistribution in healthy mice. In rats bearing C6 gliomas, both [¹⁸F]VUIIS1009A and [¹⁸F]VUIIS1009B exhibited greater binding potential (k ₃/k ₄)in tumor tissue compared to [¹⁸F]DPA-714. Interestingly, [¹⁸F]VUIIS1009B exhibited significantly greater tumor uptake (V _T) than [¹⁸F]VUIIS1009A, which was attributed primarily to greater plasma-to-tumor extraction efficiency.

Conclusions

The novel PET ligand [¹⁸F]VUIIS1009B exhibits promising characteristics for imaging glioma; its superiority over [¹⁸F]VUIIS1009A, a regioisomer, appears to be primarily due to improved plasma extraction efficiency. Continued evaluation of [¹⁸F]VUIIS1009B as a high-affinity TSPO PET ligand for precision medicine appears warranted.

     

PET Imaging of Stroke-Induced Neuroinflammation in Mice Using [18F]PBR06

Purpose

The purpose of this study is to evaluate the 18 kDa translocator protein (TSPO) radioligand [¹⁸F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([¹⁸F]PBR06) as a positron emission tomography (PET) imaging biomarker of stroke-induced neuroinflammation in a rodent model.

Procedures

Stroke was induced by transient middle cerebral artery occlusion in Balb/c mice. Dynamic PET/CT imaging with displacement and preblocking using PK111195 was performed 3 days later. PET data were correlated with immunohistochemistry (IHC) for the activated microglial markers TSPO and CD68 and with autoradiography.

Results

[¹⁸F]PBR06 accumulation peaked within the first 5 min postinjection, then decreased gradually, remaining significantly higher in infarct compared to noninfarct regions. Displacement or preblocking with PK11195 eliminated the difference in [¹⁸F]PBR06 uptake between infarct and noninfarct regions. Autoradiography and IHC correlated well spatially with uptake on PET.

Conclusions

[¹⁸F]PBR06 PET specifically images TSPO in microglial neuroinflammation in a mouse model of stroke and shows promise for imaging and monitoring microglial activation/neuroinflammation in other disease models.     

Translocator Protein PET Imaging in a Preclinical Prostate Cancer Model

Purpose

The identification and targeting of biomarkers specific to prostate cancer (PCa) could improve its detection. Given the high expression of translocator protein (TSPO) in PCa, we investigated the use of [¹⁸F]VUIIS1008 (a novel TSPO-targeting radioligand) coupled with positron emission tomography (PET) to identify PCa in mice and to characterize their TSPO uptake.

Procedures

Pten^pc?/?, Trp53^pc?/? prostate cancer-bearing mice (n = 9, 4–6 months old) were imaged in a 7T MRI scanner for lesion localization. Within 24 h, the mice were imaged using a microPET scanner for 60 min in dynamic mode following a retro-orbital injection of ~ 18 MBq [¹⁸F]VUIIS1008. Following imaging, tumors were harvested and stained with a TSPO antibody. Regions of interest (ROIs) were drawn around the tumor and muscle (hind limb) in the PET images. Time-activity curves (TACs) were recorded over the duration of the scan for each ROI. The mean activity concentrations between 40 and 60 min post radiotracer administration between tumor and muscle were compared.

Results

Tumor presence was confirmed by visual inspection of the MR images. The uptake of [¹⁸F]VUIIS1008 in the tumors was significantly higher (p < 0.05) than that in the muscle, where the percent injected dose per unit volume for tumor was 7.1 ± 1.6 % ID/ml and that of muscle was < 1 % ID/ml. In addition, positive TSPO expression was observed in tumor tissue analysis.

Conclusions

The foregoing preliminary data suggest that TSPO may be a useful biomarker of PCa. Therefore, using TSPO-targeting PET ligands, such as [¹⁸F]VUIIS1008, may improve PCa detectability and characterization.

     

6-[18F]Fluoro-l-DOPA Uptake in the Rat Pancreas is Dependent on the Tracer Metabolism

Purpose

6-[¹⁸F]fluoro-l-3,4-dihydroxyphenyl alanine ([¹⁸F]FDOPA) positron emission tomography (PET) is a diagnostic tool which can detect malignancies of the pancreas. We aimed to study whether the manipulation of the [¹⁸F]FDOPA metabolic pathway would change the ¹⁸F-behavior to provide a biochemical foundation for PET imaging of rat pancreas with [¹⁸F]FDOPA.

Procedures

Inhibitors of aromatic amino acid decarboxylase, catechol-O-methyltransferase, monoamine oxidases A and B, or their combinations on [¹⁸F]FDOPA uptake, metabolism, and the regional distribution in the rat pancreas was evaluated using in vivo PET/computed tomography imaging, chromatographic metabolite analyses, and autoradiography.

Results

Enzyme inhibition generally increased the uptake of [¹⁸F]FDOPA derived ¹⁸F-radioactivity in rat pancreas. Dependent on which enzymatic pathway is blocked (or a combination of pathways), different radiolabeled metabolites in pancreas are responsible for this increase in uptake.

Conclusions

Altering the metabolism of [¹⁸F]FDOPA by using various enzymatic inhibitors increased the radioactivity uptake and changed the radiometabolic profile in the pancreas allowing better discrimination between pancreas and surrounding tissues of rat. However, these manipulations did not separate islets from the exocrine pancreas. Elucidating the metabolic behavior of [¹⁸F]FDOPA provides a biochemical foundation of PET imaging of the rat pancreas.     

Imaging Tumour ATB0,+ Transport Activity by PET with the Cationic Amino Acid O-2((2-[18F]fluoroethyl)methyl-amino)ethyltyrosine

Purpose

The concentrative amino acid transporter ATB^0,+ (SLC6A14) is under evaluation as a target for anticancer therapy. An ATB^0,+-selective positron emission tomography (PET) probe could advance preclinical drug development. We characterised the cationic tyrosine analogue O-2((2-[¹⁸F]fluoroethyl)methyl-amino)ethyltyrosine ([¹⁸F]FEMAET) as a PET probe for ATB^0,+ activity.

Procedures

Cell uptake was studied in vitro. ATB^0,+ expression was quantified by real-time PCR. [¹⁸F]FEMAET accumulation in xenografts was investigated by small animal PET with mice.

Results

[¹⁸F]FEMAET accumulated in PC-3 and NCI-H69 cancer cells in vitro. As expected for ATB^0,+ transport, uptake was inhibited by LAT/ATB^0,+ inhibitors and dibasic amino acids, and [¹⁸F]FEMAET efflux was only moderately stimulated by extracellular amino acids. ATB^0,+ was expressed in PC-3 and NCI-H69 but not MDA-MB-231 xenografts. PET revealed accumulation in PC-3 and NCI-H69 xenografts and significant reduction by ATB^0,+ inhibition. Uptake was negligible in MDA-MB-231 xenografts.

Conclusion

ATB^0,+ activity can be imaged in vivo by PET with [¹⁸F]FEMAET.     

2-[18F]Fludarabine, a Novel Positron Emission Tomography (PET) Tracer for Imaging Lymphoma: a Micro-PET Study in Murine Models

Purpose

Fludarabine has proven to be of considerable efficacy in the treatment of low-grade lymphomas. We have developed the labeling of this drug with fluorine-18 and evaluated 2-[¹⁸F]fludarabine as a novel positron emission tomography (PET) probe for in vivo imaging.

Procedures

Preclinical studies were conducted with 2-[¹⁸F]fludarabine, in parallel with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG), in Swiss CD-1 and CB17 severely combined immunodeficient (SCID) mice, both as tumor-free control groups, and SCID mice bearing RL lymphomas.

Results

In Swiss mice, micro-PET studies with 2-[¹⁸F]fludarabine showed a distribution restricted to the organs of excretion and the spleen, the latter being less evident in SCID animals. In lymphoma-bearing SCID mice, 2-[¹⁸F]fludarabine demonstrated a rapid tumor uptake over the first 20 min which subsequently plateaued and provided an improved contrast than that of [¹⁸F]FDG.

Conclusion

This radiotracer merits further evaluation to establish its clinical usefulness to image low-grade lymphoma in humans in future clinical investigations.     

Differences in Transport Mechanisms of trans-1-Amino-3-[18F]Fluorocyclobutanecarboxylic Acid in Inflammation,Prostate Cancer,and Glioma Cells: Comparison with l-[Methyl-11C]Methionine and 2-Deoxy-2-[18F]Fluoro-d-Glucose

Purpose

We aimed to elucidate trans-1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid (anti-[¹⁸F]FACBC) uptake mechanisms in inflammatory and tumor cells, in comparison with those of l-[methyl-¹¹C]methionine ([¹¹C]Met) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG).

Procedures

Using carbon-14-labeled tracers, in vitro time-course, pH dependence, and competitive inhibition uptake experiments were performed in rat inflammatory (T cells, B cells, granulocytes, macrophages), prostate cancer (MLLB2), and glioma (C6) cells.

Results

Anti-[¹⁴C]FACBC uptake ratios of T/B cells to tumor cells were comparable, while those of granulocytes/macrophages to tumor cells were lower than those for [¹⁴C]FDG. Over half of anti-[¹⁴C]FACBC uptake by T/B and tumor cells was mediated by Na⁺-dependent amino acid transporters (system ASC), whereas most [¹⁴C]Met transport in all cells was mediated by Na⁺-independent carriers (system L).

Conclusions

The low anti-[¹⁸F]FACBC accumulation in granulocytes/macrophages may be advantageous in discriminating inflamed regions from tumors. The significant anti-[¹⁸F]FACBC uptake in T/B cells may cause false-positives in some cancer patients who undergo FACBC-positron emission tomography (PET).     

Evaluation of PET Imaging Performance of the TSPO Radioligand [<Superscript>18</Superscript>F]DPA-714 in Mouse and Rat Models of Cancer and Inflammation

Purpose

Many radioligands have been explored for imaging the 18-kDa translocator protein (TSPO), a diagnostic and therapeutic target for inflammation and cancer. Here, we investigated the TSPO radioligand [¹⁸F]DPA-714 for positron emission tomography (PET) imaging of cancer and inflammation.

Procedures

[¹⁸F]DPA-714 PET imaging was performed in 8 mouse and rat models of breast and brain cancer and 4 mouse and rat models of muscular and bowel inflammation.

Results

[¹⁸F]DPA-714 showed different uptake levels in healthy organs and malignant tissues of mice and rats. Although high and displaceable [¹⁸F]DPA-714 binding is observed ex vivo, TSPO-positive PET imaging of peripheral lesions of cancer and inflammation in mice did not show significant lesion-to-background signal ratios. Slower [¹⁸F]DPA-714 metabolism and muscle clearance in mice compared to rats may explain the elevated background signal in peripheral organs in this species.

Conclusion

Although TSPO is an evolutionary conserved protein, inter- and intra-species differences call for further exploration of the pharmacological parameters of TSPO radioligands.

     

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

Purpose

We 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.

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 [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.

Results

In 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.

Conclusions

We report the use [¹⁸F]-CP18 as a PET tracer for imaging apoptosis. Our data support further development of this tracer for clinical PET applications.     

Pancreatic Beta Cell Mass PET Imaging and Quantification with [11C]DTBZ and [18F]FP-(+)-DTBZ in Rodent Models of Diabetes

Purpose

The aim of this study is to compare the utility of two positron emission tomography (PET) imaging ligands ((+)-[¹¹C]dihydrotetrabenazine ([¹¹C]DTBZ) and the fluoropropyl analog ([¹⁸F]FP-(+)-DTBZ)) that target islet ??-cell vesicular monoamine transporter type II to measure pancreatic ??-cell mass (BCM).

Procedures

[¹¹C]DTBZ or [¹⁸F]FP-(+)-DTBZ was injected, and serial PET images were acquired in rat models of diabetes (streptozotocin-treated and Zucker diabetic fatty) and ??-cell compensation (Zucker fatty). Radiotracer standardized uptake values (SUV) were correlated to pancreas insulin content measured biochemically and histomorphometrically.

Results

On a group level, a positive correlation of [¹¹C]DTBZ pancreatic SUV with pancreas insulin content and BCM was observed. In the STZ diabetic model, both [¹⁸F]FP-(+)-DTBZ and [¹¹C]DTBZ correlated positively with BCM, although only ??25% of uptake could be attributed to ??-cell uptake. [¹⁸F]FP-(+)-DTBZ displacement studies indicate that there is a substantial fraction of specific binding that is not to pancreatic islet ?? cells.

Conclusions

PET imaging with [¹⁸F]FP-(+)-DTBZ provides a noninvasive means to quantify insulin-positive BCM and may prove valuable as a diagnostic tool in assessing treatments to maintain or restore BCM.     

Fully Automated Radiosynthesis of 2-[18F]Fludarabine for PET Imaging of Low-Grade Lymphoma

Purpose

An efficient and fully automated radiosynthesis of 2-[¹⁸F]fluoro-9-β-d-arabinofuranosyl-adenine (2-[¹⁸F]fludarabine, [¹⁸F]-5) based on a GE TRACERlab? FX-FN module has been developed.

Procedures

A 2-nitro purine derivative 3 was developed as precursor for labeling with fluorine-18. The radiosynthesis of [¹⁸F]-5 was performed in two steps in a single reactor with an intermediary purification on Sep-Pak® silica which involved the addition of a three-way valve on the original module. After hydrolysis, [¹⁸F]-5 was purified by semi-preparative high-pressure liquid chromatography (HPLC) and a quality control was established.

Results

The labeling precursor 3 was obtained in 45 % overall yield. Nucleophilic substitution with K¹⁸F/K_2.2.2 afforded protected 2-[¹⁸F]fludarabine ([¹⁸F]-4) in 73?±?4 % , radiochemical yield (decay corrected to the end of bombardment (EOB)) and based on the initial [¹⁸F]F^? activity. An aqueous ammonia/methanol solution was used for the deprotection reaction and gave the desired [¹⁸F]-5 in 67?±?3 % yield after 20 min at 70 °C based on HPLC profile.

Conclusions

The process afforded pure 2-[¹⁸F]fludarabine in 48?±?3 % yield (decay corrected to the EOB) in 85 min, with a specific activity of 310?±?72 GBq/μmol at the end of synthesis (EOS) and a radiochemical purity up to 99 %.     

Increase of [18F]FLT Tumor Uptake In Vivo Mediated by FdUrd: Toward Improving Cell Proliferation Positron Emission Tomography

Purpose

3??-deoxy-3??-[¹⁸F]fluorothymidine ([¹⁸F]FLT), a cell proliferation positron emission tomography (PET) tracer, has been shown in numerous tumors to be more specific than 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) but less sensitive. We studied the capacity of a nontoxic concentration of 5-fluoro-2??-deoxyuridine (FdUrd), a thymidine synthesis inhibitor, to increase uptake of [¹⁸F]FLT in tumor xenografts.

Methods

The duration of the FdUrd effect in vivo on tumor cell cycling and thymidine analogue uptake was studied by varying FdUrd pretreatment timing and holding constant the timing of subsequent flow cytometry and 5-[¹²⁵I]iodo-2??-deoxyuridine biodistribution measurements. In [¹⁸F]FLT studies, FdUrd pretreatment was generally performed 1 h before radiotracer injection. [¹⁸F]FLT biodistributions were measured 1 to 3 h after radiotracer injection of mice grafted with five different human tumors and pretreated or not with FdUrd and compared with [¹⁸F]FDG tumor uptake. Using microPET, the dynamic distribution of [¹⁸F]FLT was followed for 1.5 h in FdUrd pretreated mice. High-field T2-weighted magnetic resonance imaging (MRI) and histology were used comparatively in assessing tumor viability and proliferation.

Results

FdUrd induced an immediate increase in tumor uptake of 5-[¹²⁵I]iodo-2??-deoxyuridine, that vanished after 6 h, as also confirmed by flow cytometry. Biodistribution measurements showed that FdUrd pretreatment increased [¹⁸F]FLT uptake in all tumors by factors of 3.2 to 7.8 compared with controls, while [¹⁸F]FDG tumor uptake was about fourfold and sixfold lower in breast cancers and lymphoma. Dynamic PET in FdUrd pretreated mice showed that [¹⁸F]FLT uptake in all tumors increased steadily up to 1.5 h. MRI showed a well-vascularized homogenous lymphoma with high [¹⁸F]FLT uptake, while in breast cancer, a central necrosis shown by MRI was inactive in PET, consistent with the histomorphological analysis.

Conclusion

We showed a reliable and significant uptake increase of [¹⁸F]FLT in different tumor xenografts after low-dose FdUrd pretreatment. These results show promise for a clinical application of FdUrd aimed at increasing the sensitivity of [¹⁸F]FLT PET.     

Initial Experience with the Radiotracer Anti-1-amino-3-[18F]Fluorocyclobutane-1-Carboxylic Acid (Anti-[18F]FACBC) with PET in Renal Carcinoma

Purpose

Assessment of renal masses with conventional imaging may be challenging. Anti-1-amino-3-[¹⁸F]fluorocyclobutane-1-carboxylic acid (anti-[18F]FACBC) is a synthetic l-leucine analog with relatively little renal excretion. The present study examines anti-[¹⁸F]FACBC positron emission tomography uptake in patients with renal masses.

Procedures

Six patients with seven renal lesions were imaged dynamically for 2 h after injection of 10–10.9 mCi (370–403 MBq) anti-[¹⁸F]FACBC. Lesions were evaluated qualitatively and quantitatively and correlated with histology.

Results

Four clear cell and one Rosai–Dorfman lesion were hypo/isointense to normal cortex; two papillary lesions in the same patient were hyperintense. Mean SUV_max?±?SD at 30 min was 2.8?±?0.24 for clear cell carcinomas and 4.5?±?1.7 for papillary cell lesions. Mean SUV_max/SUV_mean ratios?±?SD of lesion to normal cortex at 30 min was 1.15?±?0.19 for the clear cell carcinomas and 2.3?±?0.84 for papillary cell.

Conclusions

In this small patient sample, relative amino acid transport compared with renal cortex is elevated in renal papillary cell carcinoma but not in clear cell carcinoma.     

Fusion of [18F]FDG PET with Fluorescence Diffuse Optical Tomography to Improve Validation of Probes and Tumor Imaging

Purpose

Given the progress of fluorescence diffuse optical tomography (fDOT) technology, here, we study the additional benefits provided by multimodal PET/fDOT imaging by comparing the biodistribution of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) in tumors with three fluorescent probes: a glucose analog, a protease activatable optical probe, and a ligand of αvβ3 integrin.

Procedures

Sequential fDOT/PET/computed tomography (CT) imaging of mice was performed with a custom multimodal mouse support that allows the subject to be transferred between the fDOT and the PET/CT scanners. Experiments were performed in xenografted tumor models derived from the human breast cancer line MDA-MB 231 and compared to ex vivo analysis.

Results

The three-dimensional signals showed that the fluorescent glucose analog is not colocalized with [¹⁸F]FDG, raising questions about its use as a surrogate probe of the PET tracer. Fusion of [¹⁸F]FDG with the other fluorescent probes showed evidence of high variability both for the protease activity and the αvβ3 integrin expression during tumor growth.

Conclusion

The added value of hybrid PET/fDOT over the two modalities was demonstrated for cross-validation of probes and for better characterization of tumor models.     

Preclinical Evaluation of 4-[<Superscript>18</Superscript>F]Fluoroglutamine PET to Assess ASCT2 Expression in Lung Cancer

Purpose

Alanine-serine-cysteine transporter 2 (ASCT2) expression has been demonstrated as a promising lung cancer biomarker. (2S,4R)-4-[¹⁸F]Fluoroglutamine (4-[¹⁸F]fluoro-Gln) positron emission tomography (PET) was evaluated in preclinical models of non-small cell lung cancer as a quantitative, non-invasive measure of ASCT2 expression.

Procedures

In vivo microPET studies of 4-[¹⁸F]fluoro-Gln uptake were undertaken in human cell line xenograft tumor-bearing mice of varying ASCT2 levels, followed by a genetically engineered mouse model of epidermal growth factor receptor (EGFR)-mutant lung cancer. The relationship between a tracer accumulation and ASCT2 levels in tumors was evaluated by IHC and immunoblotting.

Result

4-[¹⁸F]Fluoro-Gln uptake, but not 2-deoxy-2-[¹⁸F]fluoro-D-glucose, correlated with relative ASCT2 levels in xenograft tumors. In genetically engineered mice, 4-[¹⁸F]fluoro-Gln accumulation was significantly elevated in lung tumors, relative to normal lung and cardiac tissues.

Conclusions

4-[¹⁸F]Fluoro-Gln PET appears to provide a non-invasive measure of ASCT2 expression. Given the potential of ASCT2 as a lung cancer biomarker, this and other tracers reflecting ASCT2 levels could emerge as precision imaging diagnostics in this setting.

     

Dexamethasone-Induced Insulin Resistance: Kinetic Modeling Using Novel PET Radiopharmaceutical 6-Deoxy-6-[18F]fluoro-d-glucose

Purpose

An insulin-resistant rat model, induced by dexamethasone, was used to evaluate a Michaelis–Menten-based kinetic model using 6-deoxy-6-[¹⁸F]fluoro-d-glucose (6-[¹⁸F]FDG) to quantify glucose transport with PET.

Procedures

Seventeen, male, Sprague–Dawley rats were studied in three groups: control (Ctrl), control?+?insulin (Ctrl?+?I), and dexamethasone?+?insulin (Dex?+?I). PET scans were acquired for 2 h under euglycemic conditions in the Ctrl group and under hyperinsulinemic-euglycemic conditions in the Ctrl?+?I and Dex?+?I groups.

Results

Glucose transport, assessed according to the 6-[¹⁸F]FDG concentration, was highest in skeletal muscle in the Ctrl?+?I, intermediate in the Dex?+?I, and lowest in the Ctrl group, while that in the brain was similar among the groups. Modeling analysis applied to the skeletal muscle uptake curves yielded values of parameters related to glucose transport that were greatest in the Ctrl?+?I group and increased to a lesser degree in the Dex?+?I group, compared to the Ctrl group.

Conclusion

6-[¹⁸F]FDG and the Michaelis–Menten-based model can be used to measure insulin-stimulated glucose transport under basal and an insulin resistant state in vivo.     

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

Purpose

A 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.

Methods

Uptake 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.

Results

The 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.     

Improved Derivation of Input Function in Dynamic Mouse [18F]FDG PET Using Bladder Radioactivity Kinetics

Purpose

Accurate determination of the plasma input function (IF) is essential for absolute quantification of physiological parameters in positron emission tomography (PET). However, it requires an invasive and tedious procedure of arterial blood sampling that is challenging in mice because of the limited blood volume. In this study, a hybrid modeling approach is proposed to estimate the plasma IF of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) in mice using accumulated radioactivity in urinary bladder together with a single late-time blood sample measurement.

Methods

Dynamic PET scans were performed on nine isoflurane-anesthetized male C57BL/6 mice after a bolus injection of [¹⁸F]FDG at the lateral caudal vein. During a 60- or 90-min scan, serial blood samples were taken from the femoral artery. Image data were reconstructed using filtered backprojection with computed tomography-based attenuation correction. Total accumulated radioactivity in the urinary bladder at late times was fitted to a renal compartmental model with the last blood sample and a one-exponential function that described the [¹⁸F]FDG clearance in blood. Multiple late-time blood sample estimates were calculated by the blood [¹⁸F]FDG clearance equation. A sum of four-exponentials was assumed for the plasma IF that served as a forcing function to all tissues. The estimated plasma IF was obtained by simultaneously fitting the [¹⁸F]FDG model to the time–activity curves (TACs) of liver and muscle and the forcing function to early (0–1 min) left-ventricle data (corrected for delay, dispersion, partial-volume effects, and erythrocyte uptake) and the late-time blood estimates. Using only the blood sample collected at the end of the study to estimate the IF and the use of liver TAC as an alternative IF were also investigated.

Results

The area under the plasma IFs calculated for all studies using the hybrid approach was not significantly different from that using all blood samples. [¹⁸F]FDG uptake constants in brain, myocardium, skeletal muscle, and liver computed by the Patlak analysis using estimated and measured plasma IFs were in excellent agreement (slope?～?1; R ²?>?0.983). The IF estimated using only the last blood sample drawn at the end of the study and the use of liver TAC as the plasma IF provided less reliable results.

Conclusions

The estimated plasma IFs obtained with the hybrid method agreed well with those derived from arterial blood sampling. Importantly, the proposed method obviates the need of arterial catheterization, making it possible to perform repeated dynamic [¹⁸F]FDG PET studies on the same animal. Liver TAC is unsuitable as an input function for absolute quantification of [¹⁸F]FDG PET data.