In vivo characterization of myocardium muscarinic receptors by positron emission tomography

The feasibility of studying myocardium muscarinic receptors, non invasively, in a “live” being can be demonstrated using positron emission tomography (PET) and a ligand labelled by carbon 11, an externally detectable short lived radionuclide. Criteria necessary for in vitro characterization of muscarinic receptors by a specific ligand were verified in vivo by this method. This demonstration was carried out after injecting in a baboon, high specific activity ¹¹C-MQNB (the methiodide salt of quinuclidinyl benzylate) a muscarinic antagonist drug, and displacing the radioactive ligand by increasing amounts of atropine. Displacement was proportionnal to the dose of atropine and a correlation was observed between displacement and pharmacological activity (increase of heart rate). Stereospecificity of the binding was also demonstrated by using two stereoisomers of benzetimide : dexetimide and levetimide.     

Al18F labeled sulfonamide-conjugated positron emission tomography tracer in vivo tumor-targeted imaging

An ideal positron emission tomography (PET) tracer should be highly extractable by the tumor tissue or organ that contains low toxicity and can provide high-resolution images in vivo. In this work, the aim was to evaluate the application of Al¹⁸F-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid containing sulfonamide group (¹⁸F-Al-NOTA-SN) as a potential tumor-targeting signal-enhanced radioactive tracer in PET. SN as a tumor-targeting group was incorporated to NOTA to make a ligand. Subsequently, this ligand reacted with Na¹⁸F and AlCl₃ to produce a compound ¹⁸F-Al-NOTA-SN. This compound was further characterized and its property in regard to cell cytotoxicity assay, microPET imaging, biodistribution, cell uptake assay, and tumor selectivity in vitro and in vivo, was also investigated. ¹⁸F-Al-NOTA-SN possessed low cell cytotoxicity and uptake to COS-7 and 293T healthy cells and high cell cytotoxicity and uptake to MDA-MB-231, HepG2, and HeLa tumor cells in vitro. Moreover, ¹⁸F-Al-NOTA-SN showed good tumor-targeting property and high PET signal enhancement of HeLa tumors, liver, and kidneys in mice, as well as the uptake ratios of tumor to blood and tumor to muscle, were 4.98 and 3.87, respectively. ¹⁸F-Al-NOTA-SN can be accepted to be kidney and liver eliminated earlier and show a potential tumor-targeting signal-enhanced radioactive tracer in PET.     

Principles of quantitative positron emission tomography

Summary. The central distinguishing feature of positron emission tomography (PET) is its ability to investigate quantitatively regional cellular and molecular transport processes in vivo with good spatial resolution. This review wants to provide a concise overview of the established principles underlying quantitative data evaluations of the acquired PET images. Especially, the compartment modelling framework is discussed on which virtually all quantification methods utilized in PET are based. The aim of the review is twofold: first, to provide the reader with an idea of the theoretical framework and mathematical tools and second, to enable an intuitive grasp of the possibilities and limitations of a quantitative approach to PET data evaluation. This should facilitate an understanding of how PET measurements translate into quantities such as regional blood flow, volume of distribution, and metabolic rates of specific substrates.     

Application of positron emission tomography to neuroimaging in sports sciences

To investigate exercise-induced regional metabolic and perfusion changes in the human brain, various methods are available, such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI), near-infrared spectroscopy (NIRS) and electroencephalography (EEG). In this paper, details of methods of metabolic measurement using PET, [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) and [¹⁵O]radio-labelled water ([¹⁵O]H₂O) will be explained.Functional neuroimaging in the field of neuroscience was started in the 1970s using an autoradiography technique on experimental animals. The first human functional neuroimaging exercise study was conducted in 1987 using a rough measurement system known as ¹³³Xe inhalation. Although the data was useful, more detailed and exact functional neuroimaging, especially with respect to spatial resolution, was achieved by positron emission tomography. Early studies measured the cerebral blood flow changes during exercise. Recently, PET was made more applicable to exercise physiology and psychology by the use of the tracer [¹⁸F]FDG. This technique allowed subjects to be scanned after an exercise task is completed but still obtain data from the exercise itself, which is similar to autoradiography studies.In this report, methodological information is provided with respect to the recommended protocol design, the selection of the scanning mode, how to evaluate the cerebral glucose metabolism and how to interpret the regional brain activity using voxel-by-voxel analysis and regions of interest techniques (ROI).Considering the important role of exercise in health promotion, further efforts in this line of research should be encouraged in order to better understand health behavior. Although the number of research papers is still limited, recent work has indicated that the [¹⁸F]FDG-PET technique is a useful tool to understand brain activity during exercise.     

Radiopharmaceutical chemistry for positron emission tomography

Radiopharmaceutical chemistry includes the selection, preparation, and preclinical evaluation of radiolabeled compounds. This paper describes selection criteria for candidates for positron emission tomography (PET) investigations. Practical aspects of nucleophilic and electrophilic (18)F-fluorinations and (11)C-methylations are described. These aspects include production of fluorine-18 and carbon-11, workup of fluorine-18, (18)F radiochemistry, production of [(11)C]methyl iodide and triflate, and (11)C-methylation radiochemistry.     

Targeting peptides and positron emission tomography

Biologically active peptides have during the last decades made their way into conventional nuclear medicine diagnosis using single photon emission computed tomography (SPECT) and gamma-camera. Several clinical trails are also investigating the role of radiolabeled peptides for targeting radionuclide therapy. This has raised the question as to whether positron emission tomography (PET) can be used in order to obtain better quantitative information of the peptide distribution in tumor and healthy organs, i.e., to get a better dosimetry. Positron emitting analogs of the therapeutic radionuclides used have been produced and successfully applied in peptide pharmacokinetic measurements with PET. But the recent boom in (18)FDG-PET ((18)FDG = [(18)F]2-deoxy-2-fluoro-D-glucose), and with this a worldwide increasing number of PET systems, has also inspired several research groups to hunt for alternative labels to be used for peptide diagnostics and PET. The rapid kinetic of short peptides agrees well with the short half-lives of standard PET nuclides like (11)C and (18)F. Especially, (18)F appears to be excellent for labeling bioactive peptides due to its favorable physical and nuclear characteristics. However, with present techniques labeling peptides with (18)F is laborious and time-consuming, and is not yet a clinical alternative. Other halogens like (75, 76)Br and (124)I are, from the chemical point of view, easier to apply. But an even better labeling alternative may be positron emitting metal ions like (55)Co, (68)Ga, and (110m)In since they tend to give better intracellular retention and thus a better signal-to-background ratio than the halogen labels. The main drawback with these radionuclides is that they are not readily available. Some of these radionuclides also emit gamma in their decay that may affect the measuring properties of the PET equipment. This article reviews mainly the present situation of production and use of nonconventional positron emitters for peptide labeling.     

Small animal positron emission tomography in food sciences

Summary. Positron emission tomography (PET) is a 3-dimensional imaging technique that has undergone tremendous developments during the last decade. Non-invasive tracing of molecular pathways in vivo is the key capability of PET. It has become an important tool in the diagnosis of human diseases as well as in biomedical and pharmaceutical research. In contrast to other imaging modalities, radiotracer concentrations can be determined quantitatively. By application of appropriate tracer kinetic models, the rate constants of numerous different biological processes can be determined. Rapid progress in PET radiochemistry has significantly increased the number of biologically important molecules labelled with PET nuclides to target a broader range of physiologic, metabolic, and molecular pathways. Progress in PET physics and technology strongly contributed to better scanners and image processing. In this context, dedicated high resolution scanners for dynamic PET studies in small laboratory animals are now available. These developments represent the driving force for the expansion of PET methodology into new areas of life sciences including food sciences. Small animal PET has a high potential to depict physiologic processes like absorption, distribution, metabolism, elimination and interactions of biologically significant substances, including nutrients, ‘nutriceuticals’, functional food ingredients, and foodborne toxicants. Based on present data, potential applications of small animal PET in food sciences are discussed.     

Synthesis of multi-galactose-conjugated 2′-O-methyl oligoribonucleotides and their in vivo imaging with positron emission tomography

⁶⁸Ga labelled 2′-O-methyl oligoribonucleotides (anti-miR-15b) bearing one, three or seven d-galactopyranoside residues have been prepared and their distribution in healthy rats has been studied by positron emission tomography (PET). To obtain the heptavalent conjugate, an appropriately protected 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) precursor bearing a 4-[4-(4,4′-dimethoxytrityloxy)butoxy]phenyl side arm was first immobilized via a base labile linker to the support and the oligonucleotide was assembled on the detritylated hydroxyl function of this handle. A phosphoramidite building block bearing two phthaloyl protected aminooxy groups and one protected hydroxyl function was introduced into the 5′-terminus. One acetylated galactopyranoside was coupled as a phosphoramidite to the hydroxyl function, the phthaloyl protections were removed on-support and two trivalent galactopyranoside clusters were attached as aldehydes by on-support oximation. A two-step cleavage with aqueous alkali and ammonia released the conjugate in a fully deprotected form, allowing radiolabelling with ⁶⁸Ga in solution. The mono- and tri-galactose conjugates were obtained in a closely related manner. In vivo imaging in rats with PET showed remarkable galactose-dependent liver targeting of the conjugates.     

Approaches to the design of biochemical probes for positron emission tomography

Since the development of the 2-deoxy-D-glucose procedure by L. Sokoloff considerable advances have been made in the design of radiotracers for estimation of in-vivo biochemical parameters. Many of these advances are due to the development of positron emission tomography. As a result key biochemical processes can now be evaluated with newly developed positron-emitting labeled enzyme probes in man, in-vivo, allowing the study of a wide range of specific cellular processes in health and disease states.Special issue dedicated to Dr. Louis Sokoloff.     

Kinetics of in vivo binding of antagonist to muscarinic cholinergic receptor in the human heart studied by positron emission tomography

Positron Emission Tomography (PET) was used to analyse $\text{in vivo}$ antagonist binding to human myocardial muscarinic cholinergic receptor. The methiodide salt of the muscarinic antagonist, quinuclidinyl benzilate (MQNB), was labeled with the positron emitter, Carbon-11, and injected intravenously to 8 normal subjects. ¹¹C-MONB concentration was determined $\text{in vivo}$ in the ventricular septum from 40 cross-sectional images acquired at the same transverse level over a period of 70 minutes. In 4 subjects, various amounts of unlabeled atropine were rapidly injected at 20 minutes to study whether atropine competitively inhibited MQNB.The kinetics of binding of ¹¹C-MQNB were not the same $\text{in vivo}$ and $\text{in vitro}$. The apparent dissociation rate of ¹¹C-MQNB $\text{in vivo}$ was much slower (by 1 to 2 orders of magnitude) than that observed $\text{in vitro}$ with ³H-QNB. After atropine injection, ¹¹C-MNQB dissociated from its binding sites at a rate that apparently depended on the amount of atropine present. ¹¹C-MQNB kinetics were analysed with a mathematical model which assumes the existence of a boundary layer containing free ligand in the vicinity of the binding sites. The dissociation rate of the radioligand depends on the probability of its rebinding to a free receptor site.     

Caffeine and human cerebral blood flow: a positron emission tomography study

Positron emission tomography (PET) was used to quantify the effect of caffeine on whole brain and regional cerebral blood flow (CBF) in humans. A mean dose of 250 mg of caffeine produced approximately a 30% decrease in whole brain CBF; regional differences in caffeine effect were not observed. Pre-caffeine CBF strongly influenced the magnitude of the caffeine-induced decrease. Caffeine decreased paCO2 and increased systolic blood pressure significantly; the change in paCO2 did not account for the change in CBF. Smaller increases in diastolic blood pressure, heart rate, plasma epinephrine and norepinephrine, and subjectively reported anxiety were also observed.     

Organ-specific dietary fatty acid uptake in humans using positron emission tomography coupled to computed tomography

A noninvasive method to determine postprandial fatty acid tissue partition may elucidate the link between excess dietary fat and type 2 diabetes. We hypothesized that the positron-emitting fatty acid analog 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid ((18)FTHA) administered orally during a meal would be incorporated into chylomicron triglycerides, allowing determination of interorgan dietary fatty acid uptake. We administered (18)FTHA orally at the beginning of a standard liquid meal ingested in nine healthy men. There was no significant (18)FTHA uptake in the portal vein and the liver during the 1st hour. Whole body PET/CT acquisition revealed early appearance of (18)FTHA in the distal thoracic duct, reaching a peak at time 240 min. (18)FTHA mean standard uptake value increased progressively in the liver, heart, quadriceps, and subcutaneous and visceral adipose tissues between time 60 and 240 min. Most circulating (18)F activity between time 0 and 360 min was recovered into chylomicron triglycerides. Using Triton WR-1339 treatment in rats that received (18)FTHA by gavage, we confirmed that >90% of this tracer reached the circulation as triglycerides. This novel noninvasive method to determine tissue dietary fatty acid distribution in humans should prove useful in the study of the mechanisms leading to lipotoxicity.     

Regional lung hematocrit in humans using positron emission tomography

Regional lung hematocrit ratio (R) was measured in five normal subjects and five patients (2 with pneumonia, 2 with nephrotic syndrome with anemia, and 1 with pancreatitis) using positron emission tomography, a red cell marker 11CO, and a plasma marker [methyl-11C]albumin). The measurements were made in a transaxial thoracic section at midheart level with the subject in supine posture and with a spatial resolution of 1.7 cm. The normal regional hematocrit ratio (means +/- SE) calculated for the lung was 0.90 +/- 0.014, 0.94 +/- 0.023 for the thoracic wall, and 1.00 +/- 0.003 for the heart chambers. The regional lung hematocrit ratio in the patients ranged between 0.81 and 0.86. No correlation was found among the regional lung hematocrit ratio and regional blood volume, lung extravascular density, and the peripheral hematocrit (obtained from venous blood samples). To the extent that 70% of the pulmonary blood in the field of view is in larger vessels with normal hematocrit, the hematocrit in the capillary bed is approximately two-thirds that of the peripheral venous value. Blood volume measurements on the basis of single vascular tracers need to take account of these results.     

Visualizing lung function with positron emission tomography.

Positron emission tomography (PET) provides three-dimensional images of the distributions of radionuclides that have been inhaled or injected into the lungs. By using radionuclides with short half-lives, the radiation exposure of the subject can be kept small. By following the evolution of the distributions of radionuclides in gases or compounds that participate in lung function, information about such diverse lung functions as regional ventilation, perfusion, shunt, gas fraction, capillary permeability, inflammation, and gene expression can be inferred. Thus PET has the potential to provide information about the links between cellular function and whole lung function in vivo. In this paper, recent advancements in PET methodology and techniques and information about lung function that have been obtained with these techniques are reviewed.     

Measuring drug-related receptor occupancy with positron emission tomography

Several techniques can be used to measure indirectly the effect of drugs (e.g., EEG, fMRI) in healthy volunteers and in patients. Although each technique has its merits, a direct link between drug efficacy and site of action in vivo usually cannot be established. In addition, when the specific mode of action of a drug has been determined from preclinical studies, it is often not known whether the administered dose is optimal for humans. Both industry and academia are becoming more and more interested in determining the dose-related occupancy of specific targets caused by administration of drugs under test. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are noninvasive imaging techniques that can give insight into the relationship between target occupancy and drug efficacy, provided a suitable radioligand is available. Although SPECT has certain advantages (e.g., a long half-life of the radionuclides), the spatial and temporal resolution as well as the labeling possibilities of this technique are limited. This review focuses on PET methodology for conducting drug occupancy studies in humans.     

Small-animal imaging using clinical positron emission tomography/computed tomography and super-resolution

Considering the high cost of dedicated small-animal positron emission tomography/computed tomography (PET/CT), an acceptable alternative in many situations might be clinical PET/CT. However, spatial resolution and image quality are of concern. The utility of clinical PET/CT for small-animal research and image quality improvements from super-resolution (spatial subsampling) were investigated. National Electrical Manufacturers Association (NEMA) NU 4 phantom and mouse data were acquired with a clinical PET/CT scanner, as both conventional static and stepped scans. Static scans were reconstructed with and without point spread function (PSF) modeling. Stepped images were postprocessed with iterative deconvolution to produce super-resolution images. Image quality was markedly improved using the super-resolution technique, avoiding certain artifacts produced by PSF modeling. The 2 mm rod of the NU 4 phantom was visualized with high contrast, and the major structures of the mouse were well resolved. Although not a perfect substitute for a state-of-the-art small-animal PET/CT scanner, a clinical PET/CT scanner with super-resolution produces acceptable small-animal image quality for many preclinical research studies.     

Assessing skeletal muscle glucose metabolism with positron emission tomography

Insulin has a marked effect to stimulate the transport and metabolism of glucose in skeletal muscle in healthy individuals, whereas an impaired response, termed insulin resistance, is a major risk factor for diabetes mellitus and other metabolic diseases. Studies of the molecular physiology of insulin action in skeletal muscle indicate that a principal loci of control resides within the proximal steps of glucose transport and phosphorylation. Deoxyglucose, the metabolism of which is limited to these proximal steps, is widely used for in vitro studies of insulin action on glucose transport. The technologies of PET imaging provide a unique opportunity to carry out similar studies in vivo in human skeletal muscle. In this instance, a short-lived positron labeled tracer, [18F] FDG, can be given at sufficiently high specific activity to image not only glucose uptake, but by dynamic PET imaging, by monitoring the time course of [18F] FDG tissue activity, data can be generated to examine the kinetics of glucose transport and phosphorylation. The experimental procedures of this approach, including an overview of the mathematical modeling, are described in this review, along with some of the key findings of the initial applications of PET for the study of glucose metabolism in human skeletal muscle.