Effective specific activities determined by scintillation proximity counting for production runs of [(18)F]FES and 4F-M[(18)F]FES

INTRODUCTION: 16alpha-[(18)F]Fluoro-17beta-estradiol ([(18)F]FES) and various derivatives can be used to image noninvasively the expression of estrogen receptors in breast cancer. A high specific activity is required for successful visualization of ER expression in vivo, particularly for small animal imaging. We describe a simple method for effective specific activity (ESA) measurements of ER-binding ligands. METHODS: Scintillator-coated polystyrene microplates (FlashPlate) were coated with purified ER of the alpha subtype. [(18)F]FES and 4-fluoro-11beta-methoxy-16alpha-[(18)F]fluoroestradiol (4F-M[(18)F]FES) were prepared by stereoselective opening of their respective cyclic sulfate precursors. After decay of the radioactivity, samples at various dilutions were put in the wells of the FlashPlate along with buffer and [(3)H]estradiol. On the same FlashPlate, nonradioactive estradiol was placed at concentrations ranging from 10(-11) to 10(-6) M to provide a standard competition curve. RESULTS: The average effective specific activities of different batches of [(18)F]FES and 4F-M[(18)F]FES were 1169 (range, 49-6251) and 4695 (range, 413-15,261) Ci/mmol, respectively. CONCLUSION: Scintillation proximity technology allows for simple and reproducible measurements of the ESA of receptor-binding radiopharmaceutical for which purified receptors are available.     

Automated synthesis of 16alpha-[18F]fluoroestradiol-3,17beta-disulphamate.

After 16alpha-[15F]fluoroestradiol ([18F]FES) has been successfully prepared in an automated module, the synthesis of 16alpha-[18F]fluoroestradiol-3,17beta-disulphamate ([18F]FESDS) is described as a module-assisted one-pot procedure which can provide 10GBq [18F]FESDS with a radiochemical purity better than 99%. The procedure is reliable and reproducible and requires a time of about 90 min. Because of its high sulphatase-inhibitory effect [15F]FESDS is thought to be a new PET tracer to image sites of high sulphatase activity.     

Automatic synthesis of 16 alpha-[(18)F]fluoro-17beta-estradiol using a cassette-type [(18)F]fluorodeoxyglucose synthesizer

16 alpha-[(18)F]fluoro-17beta-estradiol ([(18)F]FES) is a radiotracer for imaging estrogen receptors by positron emission tomography. We developed a clinically applicable automatic preparation system for [(18)F]FES by modifying a cassette-type [(18)F]fluorodeoxyglucose synthesizer. Two milligrams of 3-O-methoxymethyl-16,17-O-sulfuryl-16-epiestriol in acetonitrile was heated at 105 degrees C for 10 min with dried [(18)F]fluoride. The resultant solution was evaporated and hydrolyzed with 0.2 N HCl in 90% acetonitrile/water at 95 degrees C for 10 min under pressurized condition. The neutralization was carried out with 2.8% NaHCO(3), and then the high-performance liquid chromatography (HPLC) purification was performed. The desired radioactive fraction was collected and the solvent was replaced by 10 ml of saline, and then passed through a 0.22-microm filter into a pyrogen-free vial as the final product. The HPLC purification data demonstrated that [(18)F]FES was synthesized with a yield of 76.4+/-1.9% (n=5). The yield as the final product for clinical use was 42.4+/-3.2% (n=5, decay corrected). The total preparation time was 88.2+/-6.4 min, including the HPLC purification and the solvent replacement process. The radiochemical purity of the final product was >99%, and the specific activity was more than 111 GBq/micromol. The final product was stable for more than 6 h in saline containing sodium ascorbate. This new preparation system enables us to produce [(18)F]FES safe for clinical use with high and reproducible yield.     

Interactions of 16alpha-[18F]-fluoroestradiol (FES) with sex steroid binding protein (SBP)

Fluorine-18 16alpha-Fluoroestradiol ([18F]-FES) is a positron-emitting tracer for the estrogen receptor that is used for positron emission tomography (PET) studies of tumor tissues rich in the estrogen receptor. The role of the sex steroid binding protein (SBP or SHBG) in the transport of the [18F]-FES to the estrogen-receptor-rich tissue in breast cancer patients in vivo was investigated. To determine the extent to which [18F]-FES is bound to SBP in the blood, we performed a series of studies using blood samples obtained from patients undergoing [18F]-FES PET scans. The binding of [18F]-FES to the SBP was measured using a simple protein precipitation assay. The binding of [18F]-FES metabolites to SBP was also measured. These measurements showed that the tracer was distributed between albumin and SBP, and the binding capacity of SBP was sufficient to ensure that the protein was not saturated when the tracer was fully mixed with the plasma; however, local saturation of SBP may occur when [18F]-FES is administered intravenously. Typically about 45% of [18F]-FES in circulating plasma was bound to SBP, but this fraction was dependent on the concentration of SBP in plasma. The transfer of the tracer between the two proteins was rapid, complete in less than 20 s at 0 degrees C, suggesting that the equilibrium was maintained under most circumstances and that local saturation resolved quickly when blood from the injection site entered the central circulation. These data suggest that SBP binding of [18F]-FES is significant and will affect the input function of the tracer for any model that is used for the quantitative evaluation of [18F]-FES uptake in PET studies. Estimates of equilibrium binding in blood samples are sufficient to characterize [18F]-FES binding to SBP in the circulation.     

The synthesis of 11 beta-methoxy-[16 alpha-123I] iodoestradiol and its interaction with the estrogen receptor in vivo and in vitro

In order to produce an estrogen receptor mediated imaging agent, we have synthesized 11 beta-methoxy-16 alpha-iodoestradiol labeled with 123I, and have studied its interaction with the estrogen receptor and its distribution in rats and rabbits. This 123I-labeled steroid, 11 beta-methoxy-16 alpha-[123I]iodoestradiol, binds with high affinity, Ka = 6 x 10(9) M-1, and specificity to the estrogen receptor in uterine cytosol. When tested in vivo, this radiolabeled steroid concentrates by a receptor mediated mechanism, in the estrogen target tissue, the uterus, producing very high target to nontarget tissue ratios. The results of these experiments indicate that 11 beta-methoxy-16 alpha-[123I]iodoestradiol may be a useful imaging agent for clinically monitoring and detecting estrogen receptor containing tumors.     

Automated radiosynthesis of N-succinimidyl 3-(di-tert-butyl[18F]fluorosilyl)benzoate ([18F]SiFB) for peptides and proteins radiolabeling for positron emission tomography

Recently, silicon fluoride building blocks (SiFA) have emerged as valuable and promising tools to overcome challenges in the labeling of peptides and proteins for positron emission tomography (PET). Herein, we report a fully automated synthesis of N-succinimidyl 3-(di-tert-butyl[¹⁸F]fluorosilyl)benzoate ([¹⁸F]SiFB) by a commercially available Scintomics Hot Box 3 synthesis module, to be used as a prosthetic group for peptide and protein labeling. The drying of K2.2.2./K ¹⁸F complex was performed according to the Munich method modified by our group (avoiding azeotropic drying) using oxalic acid to neutralize the base from the ¹⁸F⁻ containing QMA eluent. This K2.2.2./K ¹⁸F complex was then used for SiFA ¹⁸F–¹⁹F isotopic exchange followed by a fast purification by a solid-phase-extraction (SPE) to afford [¹⁸F]SiFB with an average preparative radiochemical yield (RCY) of 24±1% (non-decay corrected (NDC)) within a synthesis time of 30 min. The [¹⁸F]SiFB produced by automated synthesis was then used for the ¹⁸F-labeling of rat serum albumin (RSA) as a proof of applicability.     

Automated synthesis of the generic peptide labelling agent N-succinimidyl 4-[(18)F]fluorobenzoate and application to (18)F-label the vasoactive transmitter urotensin-II as a ligand for positron emission tomography

INTRODUCTION: The objectives of this work were to develop an automated production of N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]-SFB) and to test whether the vasoactive peptide urotensin-II (U-II) could be labelled by conjugation with [(18)F]-SFB. METHODS: A TRACERlab MX(FDG) synthesizer including an HPLC unit was used. The MS Excel synthesis sequence and the standard disposable FDG cassette were modified to allow the synthesis of [(18)F]-SFB. U-II was subsequently conjugated with [(18)F]-SFB, and the resulting (18)F-labelled peptides were characterised using in vitro ligand binding assays. RESULTS: [(18)F]-SFB was successfully synthesised in the TRACERlab MX(FDG) in 44.3+/-2.5% (n=25) radiochemical yield in 98 min. [(18)F]-SFB (8-12 GBq) has been produced with specific activities in the range of 250-350 GBq/mumol and a radiochemical purity >95%. [(18)F]-SFB was subsequently used to label U-II. Two radiolabelled products, [(18)F]-(Glu(1))-U-II and [(18)F]-(Lys(8))-U-II, were formed in an isolated radiochemical yield from [(18)F]-SFB of 5.2+/-0.3% and 29.0+/-3.7%, respectively (n=7). Radioligand binding assays revealed that [(18)F]-(Glu(1))-U-II had retained subnanomolar affinity. Binding to human skeletal muscle (n=3) was concentration dependent and saturable with K(d)=0.84+/-0.51 nM, B(max)=0.69+/-0.14 fmol/mg protein and Hill slope (nH)=1.03+/-0.12. CONCLUSIONS: [(18)F]-SFB has been synthesised using the TRACERlab MX(FDG) module, allowing production of up to 8-12 GBq of [(18)F]-SFB with specific activities of 250-350 GBq/mumol. [(18)F]-SFB was used for the labelling of U-II. In vitro characterisation demonstrated that [(18)F]-(Glu(1))-U-II had retained desirable binding properties and may be suitable as a positron emission tomography radioligand for the imaging of the U-II receptor.     

Head-to-head comparison of tau positron emission tomography tracers [18F]flortaucipir and [18F]RO948

European Journal of Nuclear Medicine and Molecular Imaging - [18F]flortaucipir binds to paired helical filament tau and accurately identifies tau in Alzheimer’s disease (AD). However,...     

Evaluation of O-[18F]fluoromethyl-d-tyrosine as a radiotracer for tumor imaging with positron emission tomography

O-[¹⁸F]Fluoromethyl-d-tyrosine (d-[¹⁸F]FMT) has been reported as a potential tumor-detecting agent for positron emission tomography (PET). However, the reason why d-[¹⁸F]FMT is better than l-[¹⁸F]FMT is unclear. To clarify this point, we examined the mechanism of their transport and their suitability for tumor detection. The stereo-selective uptake and release of enantiomerically pure d- and l-[¹⁸F]FMT by rat C6 glioma cells and human cervix adenocarcinoma HeLa cells were examined. The results of a competitive inhibition study using various amino acids and a selective inhibitor for transport system L suggested that d-[¹⁸F]FMT, as well as l-[¹⁸F]FMT, was transported via system L, the large neutral amino acid transporter, possibly via LAT1. The in vivo distribution of both [¹⁸F]FMT and [¹⁸F]FDG in tumor-bearing mice and rats was imaged with a high-resolution small-animal PET system. In vivo PET imaging of d-[¹⁸F]FMT in mouse xenograft and rat allograft tumor models showed high contrast with a low background, especially in the abdominal and brain region. The results of our in vitro and in vivo studies indicate that l-[¹⁸F]FMT and d-[¹⁸F]FMT are specifically taken up by tumor cells via system L. d-[¹⁸F]FMT, however, provides a better tumor-to-background contrast with a tumor/background (contralateral region) ratio of 2.741 vs. 1.878 with the l-isomer, whose difference appears to be caused by a difference in the influence of extracellular amino acids on the uptake and excretion of these two isomers in various organs. Therefore, d-[¹⁸F]FMT would be a more powerful tool as a tumor-detecting agent for PET, especially for the imaging of a brain cancer and an abdominal cancer.     

Monitoring liver tumor therapy with [18F]FDG positron emission tomography

Positron emission tomography (PET) with [18F]-2-flurodeoxy-glucose (FDG) can be utilized as a functional imaging modality for monitoring liver tumor therapy. We report three cases in which PET-FDG was more useful for this purpose than other imaging methods and tumor markers.     

[18F]-labeled 2-methoxyphenylpiperazine derivative as a potential brain positron emission tomography imaging agent

This study reports the synthesis and characterization of N-(3-(4-(2-methoxyphenyl)piperazin-1-yl)propyl-4-[¹⁸F]fluorobenzamide ([¹⁸F]MPP3F). The total reaction time for [¹⁸F]MPP3F, including final high-performance liquid chromatography purification, was about 3 h. Typical decay-corrected radiochemical yield was 18.4±3.1%. The radiochemical purity was >98%. Biodistribution in mice showed that [¹⁸F]MPP3F is a potential brain imaging agent for positron emission tomography. The brain uptake of [¹⁸F]MPP3F was 6.59±0.77% Injected Dose/g at 2 min post-injection time. A brain-to-blood ratio of 3.67 was reached at 15 min after injection.     

Imaging DNA synthesis with [18F]FMAU and positron emission tomography in patients with cancer

Purpose FMAU (1-(2-deoxy-2-fluoro--D-arabinofuranosyl)thymine) is a thymidine analog that can be phosphorylated by thymidine kinase and incorporated into DNA. This first-in-human study of [¹⁸F]FMAU was conducted as a pilot in patients to determine its biodistribution and suitability for imaging DNA synthesis in tumors using positron emission tomography (PET).Methods Fourteen patients with diverse cancers (brain, prostate, colorectal, lung, and breast) were imaged with [¹⁸F]FMAU. We obtained dynamic PET images for 60 min and a whole-body image. Blood and urine samples were analyzed by high-performance liquid chromatography to measure metabolites and clearance.Results Active tumors in the breast, brain, lung and prostate were clearly visualized with standardized uptake values (SUVs) of 2.19, 1.28, 2.21, and 2.27–4.42, respectively. Unlike with other tracers of proliferation, low uptake of [¹⁸F]FMAU was seen in the normal bone marrow (SUV_mean 0.7), allowing visualization of metastatic prostate cancer (SUV 3.07). Low background was also observed in the brain, pelvis, and thorax, aside from heart uptake (SUV 3.36–8.78). In the abdomen, increased physiological uptake was seen in the liver (SUV 10.07–20.88) and kidneys (SUV 7.18–15.66) due to metabolism and/or excretion, but the urinary bladder was barely visible (SUV_mean 2.03). On average, 95% of the activity in the blood was cleared within 10 min post injection and an average of 70% of the activity in the urine was intact FMAU at 60 min post injection.Conclusion Tumors in the brain, prostate, thorax, and bone can be clearly visualized with FMAU. In the upper abdomen, visualization is limited by the physiological uptake by the liver and kidneys.     

Synthesis of 8-[(18)F]fluoroguanine derivatives: in vivo probes for imaging gene expression with positron emission tomography

A new method for the preparation of 8-[(18)F]fluoroguanine derivatives based on a direct radiofluorination reaction has been developed. The radiofluorination of ganciclovir (1a) with [(18)F]F(2) was carried out in absolute ethanol in the presence of tetraethylammonium hydroxide at room temperature to give 8-[(18)F]fluoroganciclovir (3a) in an approximately 1% radiochemical yield. Similarly, 8-[(18)F]fluoropenciclovir (3b), 8-[(18)F]fluoroacyclovir (3c), and 8-[(18)F]fluoroguanosine (3d) were synthesized from penciclovir (1b), acyclovir (1c), and guanosine (1d), respectively, using [(18)F]F(2). The structural analyses of the final products (3a, 3b, 3c, and 3d) were carried out after (18)F decay by (1)H, (13)C, and (19)F nuclear magnetic resonance and high resolution mass spectroscopy.     

18F]fluorodeoxyglucose positron emission tomography imaging in a case of relapsing polychondritis

Relapsing polychondritis is a rare multisystemic disease that is characterized by recurrent inflammation of the cartilaginous structures of the external ear, nose, joint, larynx, and tracheobronchial tree. Airway involvement is present in up to 50% of patients with the disease and is a major cause of morbidity and mortality. We describe a patient with relapsing polychondritis presenting with tracheal and bronchial abnormalities that were identified by an increased uptake on [18F]fluorodeoxyglucose positron emission tomography.     

Infection imaging using [18F]FDG-labelled white blood cell positron emission tomography–computed tomography

Localizing the sites of infection in the body is possible in nuclear medicine using a variety of radiopharmaceuticals that target different components of the infective and inflammatory cascade. Gamma(γ)-emitting agents such as [67Ga]gallium citrate were among the first tracers used, followed by development of positron-emitting tracers like 2-deoxy-2-[18F]fluoro-D-glucose (¹⁸F-FDG). Though these tracers are quite sensitive, they have limited specificity for infection due to their concentration in sites of non-infective inflammation. White blood cells (WBC) labelled with γ or positron emitters have higher accuracy for differentiating the infective processes from the non-infective conditions that may show positivity with tracers such as ¹⁸F-FDG. We present a pictorial review of potential clinical applications of PET/CT using ¹⁸F-FDG labelled WBC.     

11C]-MeJDTic: a novel radioligand for kappa-opioid receptor positron emission tomography imaging

INTRODUCTION: Radiopharmaceuticals that can bind selectively the kappa-opioid receptor may present opportunities for staging clinical brain disorders and evaluating the efficiency of new therapies related to stroke, neurodegenerative diseases or opiate addiction. The N-methylated derivative of JDTic (named MeJDTic), which has been recently described as a potent and selective antagonist of kappa-opioid receptor in vitro, was labeled with carbon-11 and evaluated for in vivo imaging the kappa-opioid receptor in mice. METHODS: [(11)C]-MeJDTic was prepared by methylation of JDTic with [(11)C]-methyl triflate. The binding of [(11)C]-MeJDTic to kappa-opioid receptor was investigated ex vivo by biodistribution and competition studies using nonfasted male CD1 mice. RESULTS: [(11)C]-MeJDTic exhibited a high and rapid distribution in peripheral organs. The uptake was maximal in lung where the kappa receptor is largely expressed. [(11)C]-MeJDTic rapidly crossed the blood-brain barrier and accumulated in the brain regions of interest (hypothalamus). The parent ligand remained the major radioactive compound in brain during the experiment. Chase studies with U50,488 (a kappa referring agonist), morphine (a mu agonist) and naltrindole (a delta antagonist) demonstrated that this uptake was the result of specific binding to the kappa-opioid receptor. CONCLUSION: These findings suggested that [(11)C]-MeJDTic appeared to be a promising selective "lead" radioligand for kappa-opioid receptor PET imaging.