[18F]Atorvastatin Pharmacokinetics and Biodistribution in Healthy Female and Male Rats.

Statins are 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors that are widely used to prevent cardiovascular diseases. However, a series of pleiotropic mechanisms have been associated with statins, particularly with atorvastatin. Therefore, the assessment of [18F]atorvastatin kinetics with positron emission tomography (PET) may elucidate the mechanism of action of statins and the impact of sexual dimorphism, which is one of the most debated interindividual variations influencing the therapeutic efficacy. [18F]Atorvastatin was synthesized via a previously optimized 18F-deoxyfluorination strategy, used for preclinical PET studies in female and male Wistar rats (n = 7 for both groups), and for subsequent ex vivo biodistribution assessment. PET data were fitted to several pharmacokinetic models, which allowed for estimating relevant kinetic parameters. Both PET imaging and biodistribution studies showed negligible uptake of [18F]atorvastatin in all tissues compared with the primary target organ (liver), excretory pathways (kidneys and small intestine), and stomach. Uptake of [18F]atorvastatin was 38 ± 3% higher in the female liver than in the male liver. The irreversible 2-tissue compartment model showed the best fit to describe [18F]atorvastatin kinetics in the liver. A strong correlation (R2 > 0.93) between quantitative Ki (the radiotracer's unidirectional net rate of influx between compartments) and semi-quantitative liver's SUV (standard uptake value), measured between 40 to 90 min, showed potential to use the latter parameter, which circumvents the need for blood sampling as a surrogate of Ki for monitoring [18F]atorvastatin uptake. Preclinical assays showed faster uptake and clearance for female rats compared to males, seemingly related to a higher efficiency for exchanges between the arterial input and the hepatic tissue. Due to the slow [18F]atorvastatin kinetics, equilibrium between the liver and plasma concentration was not reached during the time frame studied, making it difficult to obtain sufficient and accurate kinetic information to quantitatively characterize the radiotracer pharmacokinetics over time. Nevertheless, the reported results suggest that the SUV can potentially be used as a simplified measure, provided all scans are performed at the same time point. Preclinical PET-studies with [18F]atorvastatin showed faster uptake and clearance in female compared to male rats, apparently related to higher efficiency for exchange between arterial blood and hepatic tissue.

Simultaneous measurements of single gamma ray of 131I and annihilation radiation of 18F with Compton PET hybrid camera.

In internal 131I therapy for thyroid cancer, a decision to continue treatment is made by comparing 131I scintigraphy and [18F]FDG-PET. However, with current SPECT and PET systems, simultaneous imaging of diagnostic PET nuclides and therapeutic 131I nuclides has not been achieved so far. Therefore, we demonstrated that the recently developed Compton PET hybrid camera with Ce:Gd3(Al,Ga)5O12 (GAGG)- Silicon Photomultiplier(SiPM) scintillation detectors can be used to simultaneously image 131I Compton image and 18F PET image.

Fully Automated Synthesis of Novel TSPO PET Imaging Ligand [18F]Fluoroethyltemazepam.

INTRODUCTION: Benzodiazepines, including temazepam are described as TSPO antagonists. In fact, TSPO was initially described as a peripheral benzodiazepine receptor (PBR) with a secondary binding site for diazepam. TSPO is a potential imaging target of neuroinflammation because there is an amplification of the expression of this receptor. OBJECTIVES: Herein, we developed a novel fluorinated benzodiazepine ligand, [18F]Fluoroethyltemazepam ([18F]F-FETEM), for positron emission tomography (PET) imaging of translocator protein (18 kDa). METHODS: [18F]F-FETEM was radiolabelled with an automated synthesizer via a one-pot procedure. We conducted a [18F]F-aliphatic nucleophilic substitution of a tosylated precursor followed by purification on C18 and Alumina N SPE cartridges. Quality control tests was also carried out. RESULTS: We obtained 2.0-3.0% decay-uncorrected radiochemical activity yield (3.7% decay-corrected) within the whole synthesis time about 33 min. The radiochemical purity of [18F]F-FETEM was over 90% by TLC analysis. CONCLUSIONS: This automated procedure may be used as basis for future production of [18F]F-FETEM for preclinical PET imaging studies.

Synthesis and Initial Characterization of a Reversible, Selective 18F-Labeled Radiotracer for Human Butyrylcholinesterase.

PURPOSE: A neuropathological hallmark of Alzheimer's disease (AD) is the presence of amyloid-ß (Aß) plaques in the brain, which are observed in a significant number of cognitively normal, older adults as well. In AD, butyrylcholinesterase (BChE) becomes associated with Aß aggregates, making it a promising target for imaging probes to support diagnosis of AD. In this study, we present the synthesis, radiochemistry, in vitro and preliminary ex and in vivo investigations of a selective, reversible BChE inhibitor as PET-tracer for evaluation as an AD diagnostic. PROCEDURES: Radiolabeling of the inhibitor was achieved by fluorination of a respective tosylated precursor using K[18F]. IC50 values of the fluorinated compound were obtained in a colorimetric assay using recombinant, human (h) BChE. Dissociation constants were determined by measuring hBChE activity in the presence of different concentrations of inhibitor. RESULTS: Radiofluorination of the tosylate precursor gave the desired radiotracer in an average radiochemical yield of 20 ± 3 %. Identity and > 95.5 % radiochemical purity were confirmed by HPLC and TLC autoradiography. The inhibitory potency determined in Ellman's assay gave an IC50 value of 118.3 ± 19.6 nM. Dissociation constants measured in kinetic experiments revealed lower affinity of the inhibitor for binding to the acylated enzyme (K2 = 68.0 nM) in comparison to the free enzyme (K1 = 32.9 nM). CONCLUSIONS: The reversibly acting, selective radiotracer is synthetically easily accessible and retains promising activity and binding potential on hBChE. Radiosynthesis with 18F labeling of tosylates was feasible in a reasonable time frame and good radiochemical yield.

Synthesis and pharmacokinetic characterisation of a fluorine-18 labelled brain shuttle peptide fusion dimeric affibody.

Brain positron emission tomography (PET) imaging with radiolabelled proteins is an emerging concept that potentially enables visualization of unique molecular targets in the brain. However, the pharmacokinetics and protein radiolabelling methods remain challenging. Here, we report the performance of an engineered, blood-brain barrier (BBB)-permeable affibody molecule that exhibits rapid clearance from the brain, which was radiolabelled using a unique fluorine-18 labelling method, a cell-free protein radiosynthesis (CFPRS) system. AS69, a small (14 kDa) dimeric affibody molecule that binds to the monomeric and oligomeric states of α-synuclein, was newly designed for brain delivery with an apolipoprotein E (ApoE)-derived brain shuttle peptide as AS69-ApoE (22 kDa). The radiolabelled products 18F-AS69 and 18F-AS69-ApoE were successfully synthesised using the CFPRS system. Notably, 18F-AS69-ApoE showed higher BBB permeability than 18F-AS69 in an ex vivo study at 10 and 30 min post injection and was partially cleared from the brain at 120 min post injection. These results suggest that small, a brain shuttle peptide-fused fluorine-18 labelled protein binders can potentially be utilised for brain molecular imaging.

Quantification of Macrophage-Driven Inflammation During Myocardial Infarction with 18F-LW223, a Novel TSPO Radiotracer with Binding Independent of the rs6971 Human Polymorphism.

Myocardial infarction (MI) is one of the leading causes of death worldwide, and inflammation is central to tissue response and patient outcomes. The 18-kDa translocator protein (TSPO) has been used in PET as an inflammatory biomarker. The aims of this study were to screen novel, fluorinated, TSPO radiotracers for susceptibility to the rs6971 genetic polymorphism using in vitro competition binding assays in human brain and heart; assess whether the in vivo characteristics of our lead radiotracer, 18F-LW223, are suitable for clinical translation; and validate whether 18F-LW223 can detect macrophage-driven inflammation in a rat MI model. Methods: Fifty-one human brain and 29 human heart tissue samples were screened for the rs6971 polymorphism. Competition binding assays were conducted with 3H-PK11195 and the following ligands: PK11195, PBR28, and our novel compounds (AB5186 and LW223). Naïve rats and mice were used for in vivo PET kinetic studies, radiometabolite studies, and dosimetry experiments. Rats underwent permanent coronary artery ligation and were scanned using PET/CT with an invasive input function at 7 d after MI. For quantification of PET signal in the hypoperfused myocardium, K1 (rate constant for transfer from arterial plasma to tissues) was used as a surrogate marker of perfusion to correct the binding potential for impaired radiotracer transfer from plasma to tissue (BPTC). Results: LW223 binding to TSPO was not susceptible to the rs6971 genetic polymorphism in human brain and heart samples. In rodents, 18F-LW223 displayed a specific uptake consistent with TSPO expression, a slow metabolism in blood (69% of parent at 120 min), a high plasma free fraction of 38.5%, and a suitable dosimetry profile (effective dose of 20.5-24.5 µSv/MBq). 18F-LW223 BPTC was significantly higher in the MI cohort within the infarct territory of the anterior wall relative to the anterior wall of naïve animals (32.7 ± 5.0 vs. 10.0 ± 2.4 cm3/mL/min, P ≤ 0.001). Ex vivo immunofluorescent staining for TSPO and CD68 (macrophage marker) resulted in the same pattern seen with in vivo BPTC analysis. Conclusion:18F-LW223 is not susceptible to the rs6971 genetic polymorphism in in vitro assays, has favorable in vivo characteristics, and is able to accurately map macrophage-driven inflammation after MI.

Analytical GC-FID Method for the Determination of Organic Solvents in Radiopharmaceuticals.

BACKGROUND: Organic solvents play an indispensable role in most of the radiopharmaceutical production stages. It is almost impossible to remove them entirely in the final formulation of the product. OBJECTIVE: In this presented work, an analytical method by gas chromatography coupled with flame ionization detection (GC-FID) has been developed to determine organic solvents in radiopharmaceutical samples. The effect of injection holding time, temperature variation in the injection port, and the column temperature on the analysis time and resolution (R ≥ 1.5) of ethanol and acetonitrile was studied extensively. METHODS: The experimental conditions were optimized with the aid of further statistical analysis; thence, the proposed method was validated following the International Council for Harmonisation (ICH) Q2 (R1) guideline. RESULTS: The proposed analytical method surpassed the acceptance criteria including the linearity > 0.990 (correlation coefficient of R2), precision < 2%, LOD, and LOQ, accuracy > 90% for all solvents. The separation between ethanol and acetonitrile was acceptable with a resolution R > 1.5. Further statistical analysis of Oneway ANOVA revealed that the increment in injection holding time and variation of temperature at the injection port did not significantly affect the analysis time. Nevertheless, the variation in injection port temperature substantially influenced the resolution of ethanol and acetonitrile peaks (p < 0.05). CONCLUSION: The proposed analytical method has been successfully implemented to determine the organic solvent in the [18F]fluoro-ethyl-tyrosine ([18F]FET), [18F]fluoromisonidazole ([18F]FMISO), and [18F]fluorothymidine ([18F]FLT).

Radionuclide calibrator intercomparison study of clinical PET centres in England to a single traceable 68Ge syringe source.

OBJECTIVES: The aim of this study was to characterize national variation in radionuclide calibrator activity response to a single National Institute of Standards and Technology (NIST) traceable reference Ge source used as a surrogate for F at clinical PET centres in England using National Physical Laboratory approved techniques. METHODS: Readings from 20 instruments at 13 centres using local F and Ge factor settings were recorded with the source located in vial and syringe positions. Ten repeat measurements were conducted to investigate repeatability using % coefficient of variability (COV). Comparison ratios to investigate accuracy were made between calibrator responses and decay-corrected NISTref reference activity for syringe and vial position measurements. RESULTS: The maximum %COV was 0.79%, while 90, 95 and 80% of calibrators conformed to 5% accuracy for F syringe, Ge syringe and Ge vial position readings, respectively. We revealed a trend towards reduced bias in measurements using Veenstra devices for F and using Capintec devices for Ge factor settings. CONCLUSIONS: This study demonstrated good repeatability in local device measurements. In total, 70% of English calibrators tested and 88% of all measurements performed achieved 5% accuracy. While statistically significant bias was exhibited between different vendor equipment dependent upon radioisotope selected, our study recommends regular traceability checks for optimum instrument performance conducted within National Metrology Institutes guidelines.

A New γ-Glutamyltranspeptidase-Based Intracellular Self-Assembly of Fluorine-18 Labeled Probe for Enhancing PET Imaging in Tumors.

γ-Glutamyltranspeptidase (GGT) is a cell -membrane-associated enzyme which has been recognized as a promising biomarker for the diagnosis of many malignant tumors. Herein, we rationally designed a fluorine-18 labeled small-molecule probe, [18F]γ-Glu-Cys(StBu)-PPG(CBT)-AmBF3 (18F-1G), by applying a biocompatible CBT-Cys condensation reaction and ingeniously decorating it with a GGT-recognizable substrate, γ-glutamate (γ-Glu), for enhancing PET imaging to detect GGT level of tumors in living nude mice. The probe had exceptional stability at physiological conditions, but could be efficiently cleaved by GGT, followed by a reduction-triggered self-assembly and formation of nanoparticles (NPs) progressively that could be directly observed by transmission electron microscopy (TEM). In in vitro cell experiments, 18F-1G showed GGT-targeted uptake contrast of 2.7-fold to that of 18F-1 for the detection of intracellular GGT level. Moreover, the higher uptake in GGT overexpressed HCT116 tumor cells (∼4-fold) compared to GGT-deficient L929 normal cells demonstrated that 18F-1G was also capable of distinguishing some tumor cells from normal cells. In vivo PET imaging revealed enhanced and durable radioactive signal in tumor regions after 18F-1G coinjecting with 1G, thus allowing real-time detection of endogenous GGT level with high sensitivity and noninvasive effect. We anticipated that our probe could serve as a new tool to investigate GGT-related diseases in the near future.

Assessment of simplified methods for quantification of [18F]-DPA-714 using 3D whole-brain TSPO immunohistochemistry in a non-human primate.

The 18 kDa translocator protein (TSPO) is the main molecular target to image neuroinflammation by positron emission tomography (PET). However, TSPO-PET quantification is complex and none of the kinetic modelling approaches has been validated using a voxel-by-voxel comparison of TSPO-PET data with the actual TSPO levels of expression. Here, we present a single case study of binary classification of in vivo PET data to evaluate the statistical performance of different TSPO-PET quantification methods. To that end, we induced a localized and adjustable increase of TSPO levels in a non-human primate brain through a viral-vector strategy. We then performed a voxel-wise comparison of the different TSPO-PET quantification approaches providing parametric [18F]-DPA-714 PET images, with co-registered in vitro three-dimensional TSPO immunohistochemistry (3D-IHC) data. A data matrix was extracted from each brain hemisphere, containing the TSPO-IHC and TSPO-PET data for each voxel position. Each voxel was then classified as false or true, positive or negative after comparison of the TSPO-PET measure to the reference 3D-IHC method. Finally, receiver operating characteristic curves (ROC) were calculated for each TSPO-PET quantification method. Our results show that standard uptake value ratios using cerebellum as a reference region (SUVCBL) has the most optimal ROC score amongst all non-invasive approaches.

High-potency ligands for DREADD imaging and activation in rodents and monkeys.

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are a popular chemogenetic technology for manipulation of neuronal activity in uninstrumented awake animals with potential for human applications as well. The prototypical DREADD agonist clozapine N-oxide (CNO) lacks brain entry and converts to clozapine, making it difficult to apply in basic and translational applications. Here we report the development of two novel DREADD agonists, JHU37152 and JHU37160, and the first dedicated 18F positron emission tomography (PET) DREADD radiotracer, [18F]JHU37107. We show that JHU37152 and JHU37160 exhibit high in vivo DREADD potency. [18F]JHU37107 combined with PET allows for DREADD detection in locally-targeted neurons, and at their long-range projections, enabling noninvasive and longitudinal neuronal projection mapping.

Novel methods for the quantification of toxic, residual phase transfer catalyst in fluorine-18 labeled radiotracers.

INTRODUCTION: Fluorine-18 labeled radiopharmaceuticals undergo quality control testing for residual phase-transfer-catalyst content. The almost universally used quality-control test is a silica plate spot-test comparison of the radiopharmaceutical beside a 50-ppm standard. Once developed by staining, the radiopharmaceutical spot must be of equal or less intensity to pass the test. There is currently a need for a quantitative, inexpensive, and less subjective quality control method that allows the automatic incorporation of the acquired measurement directly into electronic batch reports. RESULTS: In the developed method, a resazurin test solution is mixed with an aliquot of the radiopharmaceutical analyte along with dichloromethane (DCM). The mixture is vortexed. The potassium resazurin-phase transfer catalyst complex solubilizes into the DCM imparting a blue color. The organic layer is then removed for analysis. Three measurement methods were utilized: visual colorimetry against pre-prepared standards, spectrophotometric measurement of transmittance, and electrical conductance. A simple prototype spectrophotometer and an electrical test cell were constructed to acquire data. Sodium Resazurin dye was found to be a suitable test chromophore for residual phase transfer catalyst analysis of aqueous solutions. Quantitative spectrophotometric measurements are possible in the 0-100-ppm range (18-crown-6) and 0-150-ppm range (Kryptofix® or tetrabutylammonium). Electrical resistance measurements of the phase transfer-catalyst resazurin complex in DCM are also a viable method, allowing quantitative phase transfer catalyst measurements in the 0-100-ppm range. CONCLUSION: The methodologies developed are more quantitative alternatives to the current spot-test method. The spectrophotometric method was determined to be the most accurate method.

Alcohol-Supported Cu-Mediated 18F-Fluorination of Iodonium Salts under "Minimalist" Conditions.

In the era of personalized precision medicine, positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI gain recognition as indispensable tools of clinical diagnostics. A broader implementation of these imaging modalities in clinical routine is closely dependent on the increased availability of established and emerging PET-tracers, which in turn could be accessible by the development of simple, reliable, and efficient radiolabeling procedures. A further requirement is a cGMP production of imaging probes in automated synthesis modules. Herein, a novel protocol for the efficient preparation of 18F-labeled aromatics via Cu-mediated radiofluorination of (aryl)(mesityl)iodonium salts without the need of evaporation steps is described. Labeled aromatics were prepared in high radiochemical yields simply by heating of iodonium [18F]fluorides with the Cu-mediator in methanolic DMF. The iodonium [18F]fluorides were prepared by direct elution of 18F- from an anion exchange resin with solutions of the corresponding precursors in MeOH/DMF. The practicality of the novel method was confirmed by the racemization-free production of radiolabeled fluorophenylalanines, including hitherto unknown 3-[18F]FPhe, in 22-69% isolated radiochemical yields as well as its direct implementation into a remote-controlled synthesis unit.

NEUTRON FLUX DISTRIBUTION IN THE RADIOISOTOPES TARGET ROOMS AND MAZE IN SYRIAN CYCLOTRON.

Gold activation foils were used in this work to measure thermal, epithermal and fast neutron fluxes in the 18F, 123I, (201Tl, 67Ga) production units in Syrian cyclotron. The neutron flux distribution around the targets were determined. It shows that, the maze thermalizes the fast neutrons and reduces the flux about four orders of magnitudes. The results can be adopted by medical centers to identify radioactive hot spots and develop radiation protection.

Molecular Imaging of Vascular Calcification with 18F-Sodium-Fluoride in Patients Infected with Human Immunodeficiency Virus.

18F-Sodium Fluoride (NaF) accumulates in areas of active hydroxyapatite deposition and potentially unstable atherosclerotic plaques. We assessed the presence of atherosclerotic plaques in 50 adult patients with HIV (HIV+) who had undergone two cardiac computed tomography scans to measure coronary artery calcium (CAC) progression. CAC and its progression are predictive of an unfavorable prognosis. Tracer uptake was quantified in six arterial territories: aortic arch, innominate carotid artery, right and left internal carotid arteries, left coronary (anterior descending and circumflex) and right coronary artery. Thirty-one patients showed CAC progression and 19 did not. At least one territory with high NaF uptake was observed in 150 (50%) of 300 arterial territories. High NaF uptake was detected more often in non-calcified than calcified areas (68% vs. 32%), and in patients without than in those with prior CAC progression (68% vs. 32%). There was no correlation between clinical and demographic variables and NaF uptake. In clinically stable HIV+ patients, half of the arterial territories showed a high NaF uptake, often in the absence of macroscopic calcification. NaF uptake at one time point did not correlate with prior progression of CAC. Prospective studies will demonstrate the prognostic significance of high NaF uptake in HIV+ patients.

Classics in Neuroimaging: Radioligands for the Vesicular Monoamine Transporter 2.

Positron emission tomography (PET) studies of the monoamine neurotransmitter systems in the human brain employ a variety of radiotracers targeting the many receptors, transporters, and enzymes present in monoaminergic neurons. One of these is the vesicular monoamine transporter 2 (VMAT2), the protein responsible for the energy-dependent accumulation of monoamines into synaptic vesicles. The development of in vivo imaging radiotracers for VMAT2 is a story of starting with a well-characterized clinically used drug (tetrabenazine) which had a pharmacologically active metabolite: that metabolite that was in stepwise fashion refined and modified to provide both carbon-11 and fluorine-18 labeled VMAT2 radiotracers that are now used for human PET studies of neurodegenerative and psychiatric diseases. The design approach taken, which involved understanding the metabolism of the radiotracers and identification of the optimal ligand stereochemistry, are representative of important steps in the general concepts behind successful in vivo radiotracer design for brain imaging agents.

Experimental validation of estimated spatially variant radioisotope-specific point spread functions using published positron range simulations and fluorine-18 measurements.

In this work we compare spatially variant radioisotope-specific point spread functions (PSFs) derived from published positron range data with measured data using a high resolution research tomograph (HRRT). Spatially variant PSFs were measured on a HRRT for fluorine-18, carbon-11 and gallium-68 using an array of printed point sources. For gallium-68, this required modification of the original design to handle its longer positron range. Using the fluorine-18 measurements and previously published data from Monte-Carlo simulations of positron range, estimated PSFs for carbon-11 and gallium-68 were calculated and compared with experimental data. A double 3D Gaussian function was fitted to the estimated and measured data and used to model the spatially varying PSFs over the scanner field of view (FOV). Differences between the measured and estimated PSFs were quantified using the full-width-at-half-maximum (FWHM) and full-width-at-tenth-maximum (FWTM) in the tangential, radial and axial directions. While estimated PSFs were generally in agreement with the measured PSFs over the entire FOV better agreement was observed (FWHM and FWTM differences of less than 10%) when using one of the two sets of positron range simulations, especially for gallium-68 and for the FWTM. Spatially variant radioisotope specific PSFs can be accurately estimated from fluorine-18 measurements and published positron range data. We have experimentally validated this approach for carbon-11 and gallium-68, and such an approach may be applicable to other radioisotopes such as oxygen-15 for which measurements are not practical.

Incidental focal colonic uptake in studies 18F-FDG PET/CT. / Hallazgo incidental de captación focal del colon en estudios 18F-FDG PET/TC.

OBJECTIVES: To assess the frequency of focal colonic uptake as an incidental observation in 18F-FDG PET/CT studies, and to correlate this finding with histopathological results. MATERIAL AND METHODS: Out of a total of 3,176 PET/CT studies with 18F-FDG systematic analysis was carried out on 30 studies in which colonic focal uptake was observed. Patients with known colorectal neoplasia were excluded. The maximum standardised uptake values (SUVm) and the morphological findings provided by the CT were recorded. The studies were reported by a radiologist and a nuclear medicine doctor. The findings were compared with endoscopy and pathology findings. RESULTS: Of the 30 patients with focal hypermetabolic lesions of the colon (0.94%), 15 were men and 15 were women with ages between 27 and 73 (mean 55 years). The reasons for PET/CT were bronchopulmonary cancer (4), breast cancer (4), tumour of unknown origin (4), melanoma (3), renal carcinoma (3), cervical neoplasia (2), adenocarcinoma of ovary (2), and others (8). Of the 23 colonoscopies performed, 10 patients (43.4%) had malignant lesions, 6 (26.1%) had pre-malignant lesions, and in 7 patients (30.4%) no lesion was identified or was benign. No endoscopy was performed on 7 patients for various reasons (patient refusal to perform the study, advanced oncological disease). An analysis was performed with the SUVm, with no statistically significant differences being found between malignant-premalignant lesions and benign lesions. CONCLUSIONS: Focal uptake in the colon of 18F-FDG has clinical relevance, and is mainly associated with morphological lesions in CT. It should be evaluated, as it may be a second tumour or a pre-malignant lesion. It is recommended that all focal uptakes of the colon be evaluated with endoscopy.