Determination of unchanged [18F]dopamine in human and non-human primate plasma during positron emission tomography studies: a new solid-phase extraction method comparable to radio-thin-layer chromatography analysis

Routine determination of [¹⁸F]DOPA and its metabolites in plasma is essential for assessment and quantification of presynaptic dopamine function in vivo using a modeling approach with positron emission tomography (PET). The determination of unchanged [¹⁸F]DOPA from human and non-human primate plasma using solid-phase extraction (SPE) with Sep-Pak cartridges during PET dopaminergic studies is described here. The results from the studies showed that this new approach in comparsion to a method such as thin-layer chromatography (TLC) possessed a simplicity, rapidity and accuracy as well as good correlation between the two techniques (p<0.0001). A proposed procedure involving radio-analysis on alumina plates (Al₂O₃) was also developed with an excellent correlation compared to the conventional C₁₈ plates (r=0.96). Thus it could be concluded that the SPE on either C₁₈ or alumina cartridges (Waters) compared to radio-TLC analysis on C₁₈ and alumina systems, appears to be a useful analytical method suitable for correcting the input arterial function in routine clinical PET neurotransmission studies.     

Solid-phase analysis method for (S)-[F]fluorocarazolol and its metabolites

(S)-[¹⁸F]Fluorocarazolol is a radiopharmaceutical developed to quantitatively assess β-adrenergic receptors in vivo via positron emission tomography imaging. Since radioactive metabolites of (S)-[¹⁸F]fluorocarazolol rapidly appear in the plasma, methods for conveniently and reliably evaluating plasma for (S)-[¹⁸F]fluorocarazolol content are required. Here we present methods and validation of an approach using commercial extraction cartridges that is faster and more convenient than an approach using internal-surface reverse-phase chromatography but yields comparable results.     

In vivo evaluation of [18F]FECIMBI-36, an agonist 5-HT2A/2C receptor PET radioligand in nonhuman primate

We recently reported the radiosynthesis and in vitro evaluation of [¹⁸F]-2-(4-bromo-2,5-dimethoxyphenyl)-N-(2-(2-fluoroethoxy)benzyl)ethanamine, ([¹⁸F]FECIMBI-36) or ([¹⁸F]1), an agonist radioligand for 5HT_2A/2C receptors in postmortem samples of human brain. Herein we describe the in vivo evaluation of [¹⁸F]FECIMBI-36 in vervet/African green monkeys by PET imaging. PET images show that [¹⁸F]FECIMBI-36 penetrates the blood-brain barrier and a low retention of radioactivity is observed in monkey brain. Although the time activity curves indicate a somehow heterogeneous distribution of the radioligand in the brain, the low level of [¹⁸F]FECIMBI-36 in brain may limit the use of this tracer for quantification of 5-HT_2A/2C receptors by PET.     

Solid-phase analysis method for (S)-[18F]fluorocarazolol and its metabolites

(S)-[¹⁸F]Fluorocarazolol is a radiopharmaceutical developed to quantitatively assess β-adrenergic receptors in vivo via positron emission tomography imaging. Since radioactive metabolites of (S)-[¹⁸F]fluorocarazolol rapidly appear in the plasma, methods for conveniently and reliably evaluating plasma for (S)-[¹⁸F]fluorocarazolol content are required. Here we present methods and validation of an approach using commercial extraction cartridges that is faster and more convenient than an approach using internal-surface reverse-phase chromatography but yields comparable results.     

Synthesis and biological evaluation of 2-(3,4-dimethoxyphenyl)-6-(2-[18F]fluoroethoxy)benzothiazole ([18F]FEDBT) for PET imaging of breast cancer

Given the ever-present demand for improved PET radiotracer in oncology imaging, we have synthesized 2-(3,4-dimethoxyphenyl)-6-(2-[¹⁸F]fluoroethoxy)benzothiazole ([¹⁸F]FEDBT), a fluorine-18-containing fluoroethylated benzothiazole to explore its utility as a PET imaging tracer. [¹⁸F]FEDBT was prepared via kryptofix-mediated nucleophilic substitution of the tosyl group precursor. Fractionated ethanol-based solid-phase (SPE cartridge-based) purification afforded [¹⁸F]FEDBT in 60% radiochemical yield (EOB), with radiochemical purity in excess of 98% and the specific activity was 35 GBq/μmol. The radiotracer displayed clearly higher cellular uptake ratio in various breast cancer cell lines MCF7, MDA-MB-468 and MDA-MB-231. However, both biodistribution and microPET studies have showed an higher abdominal accumulation of [¹⁸F]FEDMBT and the tumor/muscle ratio of 1.8 was observed in the MDA-MB-231 xenograft tumors mice model. Further the lipophilic improvement is needed for the reducement of hepatobilliary accumulation and to promote the tumor uptake for PET imaging of breast cancer.     

Studies with Differentially Labeled [11C]Cocaine, [11C]Norcocaine, [11C]Benzoylecgonine,and [11C]-and 4′-[18F]Fluorococaine to Probe the Extent to Which [11C]Cocaine Metabolites Contribute to PET Images of the Baboon Brain

Abstract: The psychostimulant drug of abuse, cocaine (benzoylecgonine methyl ester), is rapidly metabolized by cleavage of its two ester groups, to give benzoylecgonine (BE) and ecgonine methyl ester, and by N-demethylation, to give N-norcocaine (NC). The recent use of [N-methyl-¹¹CH₃]cocaine to image brain cocaine binding sites with positron emission tomography (PET) raises the question of whether PET images partially reflect the distribution and kinetics of labeled cocaine metabolites. We prepared [O-metty/-¹¹CH₃]cocaine by methylation of the sodium salt of BE with [¹¹C]CH₃l, and showed that PET baboon brain scans, as well as regional brain kinetics and plasma time-activity curves corrected for the presence of labeled metabolites, are nearly identical to those seen with [N-methyl-¹¹CH₃]cocaine. This strongly suggests that ¹¹C metabolites do not significantly affect PET images, because the metabolite pattern is different for the two labeled forms of cocaine. In particular, nearly half the ¹¹C in blood plasma at 30 min was [¹¹C]CO₂ when [N-methy/-¹¹CH₃]cocaine was administered, whereas [¹¹C]CO₂ was not formed from [O-methy/-¹¹CH₃]cocaine. Only a trace of [¹¹C]NC was detected in plasma after [O-methyl-¹¹CH₃]cocaine administration. Nearly identical brain PET data were also obtained when 4′-[N-methy/-¹¹CH₃]fluorococaine and 4′-[¹⁸F]fluoro-cocaine (prepared by nucleophilic aromatic substitution from [¹⁸F]fluoride-and 4′-nitrococaine) were compared with [N-methy/-¹¹CH₃]cocaine. In vitro assays with rat brain membranes showed that cocaine and 4′-fluoroco-caine were equipotent at the dopamine reuptake site, but that 4′-fluorococaine was about 100 times more potent at the 5-hydroxytryptamine reuptake site. The studies with positron-emitting 4′-fluorococaines thus support the lack of significance of labeled metabolites or of binding to 5-hydroxytryptamine reuptake sites to PET images taken with [N-methy/-¹¹CH₃]cocaine. [¹¹C]NC prepared by O-methylation of norbenzoylecgonine gave PET images with preferential uptake in striatum, but slower clearance from all brain regions than [O-methy/-¹¹CH₃]cocaine. [¹¹C]BE prepared by N-methylation of norbenzoylecgonine did not show brain uptake.     

Synthesis of [¹¹C]PBR06 and [¹⁸F]PBR06 as agents for positron emission tomographic (PET) imaging of the translocator protein (TSPO)

The translocator protein 18 kDa (TSPO) is an attractive target for molecular imaging of neuroinflammation and tumor progression. [¹⁸F]PBR06, a fluorine-18 labeled form of PBR06, is a promising PET TSPO radioligand originally developed at NIMH. [¹¹C]PBR06, a carbon-11 labeled form of PBR06, was designed and synthesized for the first time. The standard PBR06 was synthesized from 2,5-dimethoxybenzaldehyde in three steps with 71% overall chemical yield. The radiolabeling precursor desmethyl-PBR06 was synthesized from 2-hydroxy-5-methoxybenzaldehyde in five steps with 12% overall chemical yield. The target tracer [¹¹C]PBR06 was prepared by O-[¹¹C]methylation of desmethyl-PBR06 with [¹¹C]CH₃OTf in CH₃CN at 80 °C under basic condition and isolated by HPLC combined with SPE purification with 40–60% decay corrected radiochemical yield and 222–740 GBq/μmol specific activity at EOB. On the similar grounds, [¹⁸F]PBR06 was also designed and synthesized. The previously described Br-PBR06 precursor was synthesized from 2,5-dimethoxybenzaldehyde in two steps with 78% overall chemical yield. A new radiolabeling precursor tosyloxy-PBR06, previously undescribed tosylate congener of PBR06, was designed and synthesized from ethyl 2-hydroxyacetate, 4-methylbenzene-1-sulfonyl chloride, and N-(2,5-dimethoxybenzyl)-2-phenoxyaniline in four steps with 50% overall chemical yield. [¹⁸F]PBR06 was prepared by the nucleophilic substitution of either new tosyloxy-PBR06 precursor or known Br-PBR06 precursor in DMSO at 140 °C with K[¹⁸F]F/Kryptofix 2.2.2 for 15 min and HPLC combined with SPE purification in 20–60% decay corrected radiochemical yield, >99% radiochemical purity, 87–95% chemical purity, and 37–222 GBq/μmol specific activity at EOB. Radiosynthesis of [¹⁸F]PBR06 using new tosylated precursor gave similar radiochemical purity, and higher specific activity, radiochemical yield and chemical purity in comparison with radiosynthesis using bromine precursor.     

[18F]FEAC and [18F]FEDAC: Two novel positron emission tomography ligands for peripheral-type benzodiazepine receptor in the brain

[¹⁸F]FEAC ([¹⁸F]4a) and [¹⁸F]FEDAC ([¹⁸F]4b) were developed as two novel positron emission tomography (PET) ligands for peripheral-type benzodiazepine receptor (PBR). [¹⁸F]4a and [¹⁸F]4b were synthesized by fluoroethylation of precursors 8a and 8b with [¹⁸F]FCH₂CH₂Br ([¹⁸F]9), respectively. Small-animal PET scan for a neuroinflammatory rat model showed that the two radioligands had high uptakes of radioactivity in the kainic acid-infused striatum, a brain region where PBR density was increased.     

Construction and Evaluation of Quantitative Small-Animal PET Probabilistic Atlases for [18F]FDG and [18F]FECT Functional Mapping of the Mouse Brain

Automated voxel-based or pre-defined volume-of-interest (VOI) analysis of small-animal PET data in mice is necessary for optimal information usage as the number of available resolution elements is limited. We have mapped metabolic ([¹⁸F]FDG) and dopamine transporter ([¹⁸F]FECT) small-animal PET data onto a 3D Magnetic Resonance Microscopy (MRM) mouse brain template and aligned them in space to the Paxinos co-ordinate system. In this way, ligand-specific templates for sensitive analysis and accurate anatomical localization were created. Next, using a pre-defined VOI approach, test-retest and intersubject variability of various quantification methods were evaluated. Also, the feasibility of mouse brain statistical parametric mapping (SPM) was explored for [¹⁸F]FDG and [¹⁸F]FECT imaging of 6-hydroxydopamine-lesioned (6-OHDA) mice.

Methods

Twenty-three adult C57BL6 mice were scanned with [¹⁸F]FDG and [¹⁸F]FECT. Registrations and affine spatial normalizations were performed using SPM8. [¹⁸F]FDG data were quantified using (1) an image-derived-input function obtained from the liver (cMR_glc), using (2) standardized uptake values (SUV_glc) corrected for blood glucose levels and by (3) normalizing counts to the whole-brain uptake. Parametric [¹⁸F]FECT binding images were constructed by reference to the cerebellum. Registration accuracy was determined using random simulated misalignments and vectorial mismatch determination.

Results

Registration accuracy was between 0.21–1.11 mm. Regional intersubject variabilities of cMR_glc ranged from 15.4% to 19.2%, while test-retest values were between 5.0% and 13.0%. For [¹⁸F]FECT uptake in the caudate-putamen, these values were 13.0% and 10.3%, respectively. Regional values of cMR_glc positively correlated to SUV_glc measured within the 45–60 min time frame (spearman r = 0.71). Next, SPM analysis of 6-OHDA-lesioned mice showed hypometabolism in the bilateral caudate-putamen and cerebellum, and an unilateral striatal decrease in DAT availability.

Conclusion

MRM-based small-animal PET templates facilitate accurate assessment and spatial localization of mouse brain function using VOI or voxel-based analysis. Regional intersubject- and test-retest variations indicate that for these targets accuracy comparable to humans can be achieved.     

An Intra-Individual Comparison of MRI, [18F]-FET and [18F]-FLT PET in Patients with High-Grade Gliomas

Objectives

Intra-individual spatial overlap analysis of tumor volumes assessed by MRI, the amino acid PET tracer [¹⁸F]-FET and the nucleoside PET tracer [¹⁸F]-FLT in high-grade gliomas (HGG).

Methods

MRI, [¹⁸F]-FET and [¹⁸F]-FLT PET data sets were retrospectively analyzed in 23 HGG patients. Morphologic tumor volumes on MRI (post-contrast T1 (cT1) and T2 images) were calculated using a semi-automatic image segmentation method. Metabolic tumor volumes for [¹⁸F]-FET and [¹⁸F]-FLT PETs were determined by image segmentation using a threshold-based volume of interest analysis. After co-registration with MRI the morphologic and metabolic tumor volumes were compared on an intra-individual basis in order to estimate spatial overlaps using the Spearman''s rank correlation coefficient and the Mann-Whitney U test.

Results

[¹⁸F]-FLT uptake was negative in tumors with no or only moderate contrast enhancement on MRI, detecting only 21 of 23 (91%) HGG. In addition, [¹⁸F]-FLT uptake was mainly restricted to cT1 tumor areas on MRI and [¹⁸F]-FLT volumes strongly correlated with cT1 volumes (r = 0.841, p<0.001). In contrast, [¹⁸F]-FET PET detected 22 of 23 (96%) HGG. [¹⁸F]-FET uptake beyond areas of cT1 was found in 61% of cases and [¹⁸F]-FET volumes showed only a moderate correlation with cT1 volumes (r = 0.573, p<0.001). Metabolic tumor volumes beyond cT1 tumor areas were significantly larger for [¹⁸F]-FET compared to [¹⁸F]-FLT tracer uptake (8.3 vs. 2.7 cm³, p<0.001).

Conclusion

In HGG [¹⁸F]-FET but not [¹⁸F]-FLT PET was able to detect metabolic active tumor tissue beyond contrast enhancing tumor on MRI. In contrast to [¹⁸F]-FET, blood-brain barrier breakdown seems to be a prerequisite for [¹⁸F]-FLT tracer uptake.     

Development of a high throughput 96-well plate sample preparation method for the determination of trileptal (oxcarbazepine) and its metabolites in human plasma

A high throughput preparation method for the determination of trileptal (oxcarbazepine, OXC) and its mono (MHD) and dihydroxy (DHD) metabolites in human plasma, using 96-well plate technology, has been developed and validated according to international regulatory requirements. Preparation of plasma samples (50 μl) containing the compounds to be analysed involved solid-phase extraction (SPE) on Empore C₁₈ 96-well SPE plates. Eluates from the plate were injected onto a reversed-phase column (Hypersil C₁₈, 3 μm) with UV detection at 210 nm. Detector response was linear over the ranges 0.2–10, 0.1–200 and 0.1–20 μmol/l, for OXC, MHD and DHD, respectively, with relative standard deviations from 1 to 10% and mean accuracies within 4% of the nominal values (number of standard curves=3 in duplicate). The limits of quantitation were 0.2, 0.1 and 0.1 μmol/l, respectively. The overall mean accuracies ranged from 96 to 106% and precision was in the range 4 to 11%. Cross validation indicated no significant difference between plasma concentrations obtained using the 96-well method and the previous method using a traditional SPE method with a 50 mg C₁₈ cartridge. About a threefold increase in sample throughput and a twofold decrease of plasma volume required for the assays, were the main advantages obtained from the previous method. The method was applied for the determination of 3000 plasma samples from clinical studies.     

Synthesis and bioevaluation of [18F]4-fluoro-m-hydroxyphenethylguanidine ([18F]4F-MHPG): A novel radiotracer for quantitative PET studies of cardiac sympathetic innervation

A new cardiac sympathetic nerve imaging agent, [¹⁸F]4-fluoro-m-hydroxyphenethylguanidine ([¹⁸F]4F-MHPG), was synthesized and evaluated. The radiosynthetic intermediate [¹⁸F]4-fluoro-m-tyramine ([¹⁸F]4F-MTA) was prepared and then sequentially reacted with cyanogen bromide and NH₄Br/NH₄OH to afford [¹⁸F]4F-MHPG. Initial bioevaluations of [¹⁸F]4F-MHPG (biodistribution studies in rats and kinetic studies in the isolated rat heart) were similar to results previously reported for the carbon-11 labeled analog [¹¹C]4F-MHPG. The neuronal uptake rate of [¹⁸F]4F-MHPG into the isolated rat heart was 0.68 ml/min/g wet and its retention time in sympathetic neurons was very long (T_1/2 >13 h). A PET imaging study in a nonhuman primate with [¹⁸F]4F-MHPG provided high quality images of the heart, with heart-to-blood ratios at 80–90 min after injection of 5-to-1. These initial kinetic and imaging studies of [¹⁸F]4F-MHPG suggest that this radiotracer may allow for more accurate quantification of regional cardiac sympathetic nerve density than is currently possible with existing neuronal imaging agents.     

Synthesis and biological evaluation of F-18 labeled tetrahydroisoquinoline derivatives targeting orexin 1 receptor

Orexin 1 receptor (OX₁R) is thought to be involved in various body functions, including arousal maintenance and emotional control, but the full details of its function remain unknown. OX₁R imaging with positron emission tomography (PET) would be useful in elucidating the orexin system including OX₁R, but no PET probes targeting OX₁R have been reported. We, therefore, designed and synthesized tetrahydroisoquinoline (THIQ) derivatives as novel PET probes targeting OX₁R, and evaluated their utility. In an in vitro competitive binding assay, THIQ-1 and THIQ-2 showed significantly higher binding to OX₁R (IC₅₀ = 30 and 31 nM, respectively) than OX₂R (IC₅₀ = 160 and 332 nM, respectively). These features were also observed in a cell binding assay using [¹⁸F]THIQ-1 and [¹⁸F]THIQ-2, demonstrating their OX₁R-specific binding property in vitro. In a biodistribution study using normal mice, the brain uptake of [¹⁸F]THIQ-1 was higher than that of [¹⁸F]THIQ-2, but further improvement is required for in vivo imaging with PET. Taken together, [¹⁸F]THIQ-1 and [¹⁸F]THIQ-2 have the potential to become useful imaging probes for PET targeting the OX₁R, but require additional structural changes to improve their brain uptake.     

Synthesis and evaluation of an 18F-labeled boramino acid analog of aminosuberic acid for PET imaging of the antiporter system xC−

In this study, we synthesized ¹⁸F-ASu-BF₃, a close boramino acid analog of 5-[¹⁸F]fluoro-aminosuberic acid (¹⁸F-ASu), via ¹⁸F-¹⁹F isotope exchange reaction and evaluated its potential for imaging with positron emission tomography (PET). ¹⁸F-ASu-BF₃ was stable in mouse plasma and taken up into PC3 prostate cancer cells via the system x_C^? amino acid transporter. The continuous use of isoflurane for anesthesia during dynamic imaging acquisition slowed down the excretion of ¹⁸F-ASu-BF₃ and enabled visualization of PC3 tumor xenografts in mice. In contrast, no tumor visualization was observed from static images of ¹⁸F-BF₃-ASu due to its rapid renal excretion mediated in part by the organic anion transporter. Our data indicate that the pharmacokinetics of amino acids could be altered after being converted into their boramino acid analogs. Therefore, care should be taken when using the boramino acid strategy to design and prepare ¹⁸F-labeled tracers for imaging amino acid transporters/receptors with PET.     

Radiosynthesis,biological evaluation and preliminary microPET study of 18F-labeled 5-resorcinolic triazolone derivative based on ganetespib targeting HSP90

Heat-shock protein 90 (HSP90) is a molecular chaperone that activates oncogenic transformation in several solid tumors, including lung and breast cancers. Ganetespib, a most promising candidate among several HSP90 inhibitors under clinical trials, has entered Phase III clinical trials for cancer therapy. Despite numerous evidences validating HSP90 as a target of anticancer, there are few studies on PET agents targeting oncogenic HSP90. In this study, we synthesized and biologically evaluated a novel ¹⁸F-labeled 5-resorcinolic triazolone derivative (1, [¹⁸F]PTP-Ganetespib) based on ganetespib. [¹⁸F]PTP-Ganetespib was labeled by click chemistry of Ganetespib-PEG-Alkyne (10) and [¹⁸F]PEG-N₃ (11) with 37.3?±?5.11% of radiochemical yield and 99.7?±?0.09% of radiochemical purity. [¹⁸F]PTP-Ganetespib showed proper LogP (0.96?±?0.06) and good stability in human serum over 97% for 2?h. [¹⁸F]PTP-Ganetespib showed high uptakes in breast cancer cells containing triple negative breast cancer (TNBC) MDA-MB-231 and Her2-negative MCF-7 cells, which are target breast cancer cell lines of HSP90 inhibitor, ganetespib, as an anticancer. Blocking of HSP90 by the pretreatment of ganetespib exhibited significantly decreased accumulation of [¹⁸F]PTP-Ganetespib in MDA-MB-231 and MCF-7 cells, indicating the specific binding of [¹⁸F]PTP-Ganetespib to MDA-MB-231 and MCF-7 cells with high HSP90 expression. In the biodistribution and microPET imaging studies, the initial uptake into tumor was weaker than in other thoracic and abdominal organs, but [¹⁸F]PTP-Ganetespib was retained relatively longer in the tumor than other organs. The uptake of [¹⁸F]PTP-Ganetespib in tumors was not sufficient for further development as a tumor-specific PET imaging agent by itself, but this preliminary PET imaging study of [¹⁸F]PTP-Ganetespib can be basis for developing new PET imaging agents based on HSP90 inhibitor, ganetespib.     

Synthesis and biological evaluation of positron emission tomography radiotracers targeting serotonin 4 receptors in brain: [18F]MNI-698 and [18F]MNI-699

Two new benzodioxane derivatives were synthesized as candidates to image the serotonin 4 receptors by positron emission tomography (PET) and radiolabeled with fluorine-18 via a two-step procedure. Competition binding assays demonstrated that MNI-698 and MNI-699 had sub-nanomolar binding affinities against rat striatal 5-HT₄ receptors (K_i of 0.20 and 0.07 nM, respectively). PET imaging in rhesus monkey showed that the regional brain distribution of [¹⁸F]MNI-698 and [¹⁸F]MNI-699 were consistent with the known densities of 5-HT₄ in brain. [¹⁸F]MNI-698 and [¹⁸F]MNI-699 are among the first fluorine-18 radiotracers developed for imaging the 5-HT₄ receptors in vivo and are currently under preclinical investigation in primates for future human use.     

[18F]DAA1106: Automated radiosynthesis using spirocyclic iodonium ylide and preclinical evaluation for positron emission tomography imaging of translocator protein (18 kDa)

DAA1106 (N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide), is a potent and selective ligand for the translocator protein (18?kDa, TSPO) in brain mitochondrial fractions of rats and monkey (K_i?=?0.043 and 0.188?nM, respectively). In this study, to translate [¹⁸F]DAA1106 for clinical studies, we performed automated syntheses of [¹⁸F]DAA1106 using the spirocyclic iodonium ylide (1) as a radiolabelling precursor and conducted preclinical studies including positron emission tomography (PET) imaging of TSPO in ischemic rat brains. Radiofluorination of the ylide precursor 1 with [¹⁸F]F^?, followed by HPLC separation and formulation, produced the [¹⁸F]DAA1106 solution for injection in 6% average (n?=?10) radiochemical yield (based on [¹⁸F]F^?) with >98% radiochemical purity and molar activity of 60–100?GBq/μmol at the end of synthesis. The synthesis time was 87?min from the end of bombardment. The automated synthesis achieved [¹⁸F]DAA1106 with sufficient radioactivity available for preclinical and clinical use. Biodistribution study of [¹⁸F]DAA1106 showed a low uptake of radioactivity in the mouse bones. Metabolite analysis showed that >96% of total radioactivity in the mouse brain at 60?min after the radiotracer injection was unmetabolized [¹⁸F]DAA1106. PET study of ischemic rat brains visualized ischemic areas with a high uptake ratio (1.9?±?0.3) compared with the contralateral side. We have provided evidence that [¹⁸F]DAA1106 could be routinely produced for clinical studies.     

The norepinephrine transporter (NET) radioligand (S,S)-[18F]FMeNER-D2 shows significant decreases in NET density in the human brain in Alzheimer's disease: A post-mortem autoradiographic study

Earlier post-mortem histological and autoradiographic studies have indicated a reduction of cell numbers in the locus coeruleus (LC) and a corresponding decrease in norepinephrine transporter (NET) in brains obtained from Alzheimer's disease (AD) patients as compared to age-matched healthy controls. In order to test the hypothesis that the regional decrease of NET is a disease specific biomarker in AD and as such, it can be used in PET imaging studies for diagnostic considerations, regional differences in the density of NET in various anatomical structures were measured in whole hemisphere human brain slices obtained from AD patients and age-matched control subjects in a series of autoradiographic experiments using the novel selective PET radioligand for NET (S,S)-[¹⁸F]FMeNER-D₂. (S,S)-[¹⁸F]FMeNER-D₂ appears to be a useful imaging biomarker for quantifying the density of NET in various brain structures, including the LC and the thalamus wherein the highest densities are found in physiological conditions. In AD significant decreases of NET densities can be demonstrated with the radioligand in both structures as compared to age-matched controls. The decreases in AD correlate with the progress of the disease as indicated by Braak grades. As the size of the LC is below the spatial resolution of the PET scanners, but the size of the thalamus can be detected with appropriate spatial accuracy in advanced scanners, the present findings confirm our earlier observations with PET that the in vivo imaging of NET with (S,S)-[¹⁸F]FMeNER-D₂ in the thalamus is viable. Nevertheless, further studies are warranted to assess the usefulness of such an imaging approach for the early detection of changes in thalamic NET densities as a disease-specific biomarker and the possible use of (S,S)-[¹⁸F]FMeNER-D₂ as a molecular imaging biomarker in AD.     

[68Ga]Ga-DO2A-(OBu-l-tyr)2: Synthesis, 68Ga-radiolabeling and in vitro studies of a novel 68Ga-DO2A-tyrosine conjugate as potential tumor tracer for PET

The synthesis, ⁶⁸Ga-labeling and in vitro study of the novel tyrosine chelate derivative [⁶⁸Ga]Ga-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid-4,10-di-(O-butyl)-l-tyrosine ([⁶⁸Ga]Ga-DO₂A-(OBu-l-tyr)₂) as a potential tracer for imaging tumor metabolism by positron emission tomography (PET) is presented. This approach combines the biological amino acid transporter targeting properties of l-tyrosine with the outstanding availability of ⁶⁸Ga^III via the ⁶⁸Ge/⁶⁸Ga generator. In vitro studies utilizing the F98-glioblastoma cell line revealed specific uptake of [⁶⁸Ga]Ga-DO2A-(OBu-l-tyr)₂ that was comparable to that of the reference O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (FET). These promising results indicate a high potential of [⁶⁸Ga]Ga-DO₂A-(OBu-l-tyr)₂ for molecular imaging of tumor-driven amino acid uptake by PET.     

In Vivo Measurement of Hippocampal GABAA/cBZR Density with [18F]-Flumazenil PET for the Study of Disease Progression in an Animal Model of Temporal Lobe Epilepsy

Purpose

Imbalance of inhibitory GABAergic neurotransmission has been proposed to play a role in the pathogenesis of temporal lobe epilepsy (TLE). This study aimed to investigate whether [¹⁸F]-flumazenil ([¹⁸F]-FMZ) PET could be used to non-invasively characterise GABA_A/central benzodiazepine receptor (GABA_A/cBZR) density and affinity in vivo in the post-kainic acid status epilepticus (SE) model of TLE.

Methods

Dynamic [¹⁸F]-FMZ -PET scans using a multi-injection protocol were acquired in four male wistar rats for validation of the partial saturation model (PSM). SE was induced in eight male Wistar rats (10 weeks of age) by i.p. injection of kainic acid (7.5–25 mg/kg), while control rats (n = 7) received saline injections. Five weeks post-SE, an anatomic MRI scan was acquired and the following week an [¹⁸F]-FMZ PET scan (3.6–4.6 nmol). The PET data was co-registered to the MRI and regions of interest drawn on the MRI for selected structures. A PSM was used to derive receptor density and apparent affinity from the [¹⁸F]-FMZ PET data.

Key Findings

The PSM was found to adequately model [¹⁸F]-FMZ binding in vivo. There was a significant decrease in hippocampal receptor density in the SE group (p<0.01), accompanied by an increase in apparent affinity (p<0.05) compared to controls. No change in cortical receptor binding was observed. Hippocampal volume reduction and cell loss was only seen in a subset of animals. Histological assessment of hippocampal cell loss was significantly correlated with hippocampal volume measured by MRI (p<0.05), but did not correlate with [¹⁸F]-FMZ binding.

Significance

Alterations to hippocampal GABA_A/cBZR density and affinity in the post-kainic acid SE model of TLE are detectable in vivo with [¹⁸F]-FMZ PET and a PSM. These changes are independent from hippocampal cell and volume loss. [¹⁸F]-FMZ PET is useful for investigating the role that changes GABA_A/cBZR density and binding affinity play in the pathogenesis of TLE.