Identification of positron emission tomography ligands for NPY Y5 receptors in the brain

A series of trans-3-oxospiro[(aza)isobenzofuran-1(3H),1′-cyclohexane]-4′-carboxamide derivatives were synthesized and profiled for NPY Y5 binding affinity, brain and CSF penetrability in rats, and susceptibility to human and mouse P-glycoprotein transporters in order to develop a PET ligand. Compound 12b exhibited an acceptable profile for a PET ligand, and [¹¹C]12b was successfully utilized in clinical settings as a Y5 PET ligand.     

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.     

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.     

Selective accumulation of [62Zn]-labeled glycoconjugated porphyrins as multi-functional positron emission tomography tracers in cancer cells

Positron-emission tomography (PET) can be used to visualize active stage cancer. Fluorine-18 ([¹⁸F])-labeled 2-([¹⁸F])2-deoxy-2-fluoroglucose (([¹⁸F])-FDG), which accumulates in glucose-dependent tissues, is a good cancer-targeting tracer. However, ([¹⁸F])-FDG is obscured in glucose-dependent normal tissues. In this study, we assessed the cancer-selective accumulation of zinc-labeled glycoconjugated 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (ZnGlc1–4), both in vitro and in vivo. Experiments using both normal and cancer cells confirmed the relationship between cancer cell-selective accumulation and the substitution numbers and orientations of glycoconjugated porphyrins. ZnGlctrans-2 accumulated at greater levels in cancer cells compared with other glycoconjugated porphyrins. PET imaging showed that ZnGlctrans-2 accumulated in tumor.     

Synthesis of fluorinated analogues of sphingosine-1-phosphate antagonists as potential radiotracers for molecular imaging using positron emission tomography

Sphingosine-1-phosphate (S1P) receptors play major roles in cardiovascular, immunological and neurological diseases. The recent approval of the sphingolipid drug Fingolimod (Gilenya®), a sphingosine-1-phosphate agonist for relapsing multiple sclerosis, in 2010 exemplifies the potential for targeting sphingolipids for the treatment of human disorders. Moreover, non-invasive in vivo imaging of S1P receptors that are not available till now would contribute to the understanding of their role in specific pathologies and is therefore of preclinical interest. Based on fluorinated analogues of the S1P₁ receptor antagonist W146 showing practically equal in vitro potency as the lead structure, the first S1P receptor antagonist [¹⁸F]-radiotracer has been synthesized and tested for in vivo imaging of the S1P₁ receptor using positron emission tomography (PET). Though the tracer is serum stable, initial in vivo images show fast metabolism and subsequent accumulation of free [¹⁸F]fluoride in the bones.     

[F]Fluorodeoxyglucose positron emission tomography in advanced thyroid cancer

In recent years, [¹⁸F]Fluorodéoxyglucose-PET imaging has emerged as an important oncological imaging modality. In metastatic thyroid carcinoma (M1), [¹⁸F]FDG-PET has been shown to have a high sensitivity in non iodine-avid metastases and/or dedifferentiated tumours and may therefore provide real-time prognostic information. The use of [¹⁸F]FDG-PET is more controversial in M0 patients with low residual serum Tg values but is very sensitive in aggressive histotypes such as tall cell variants of papillary thyroid carcinomas.     

Genetic alterations associated with 18F-fluorodeoxyglucose positron emission tomography/computed tomography in head and neck squamous cell carcinoma

BackgroundWe investigated the relationship between genetic alterations and ¹⁸F-FDG PET/CT findings in head and neck squamous cell carcinoma (HNSC).MethodsUsing mRNA-sequences of HNSC samples (480 patients) from the Cancer Genome Atlas (TCGA) portal, gene coexpression networks were constructed via a weighted correlation network analysis (WGCNA) algorithm, and their association with the tumor-to-blood signal ratio on ¹⁸F-FDG PET/CT data (21 patients) was explored. An elastic-net regression model was developed to estimate the PET tumor-to-blood ratio from the gene networks and to derive an FDG signature score (FDG_SS). The FDG_SS was evaluated with regard to clinical variables and general mutational profiles, as well as alterations to oncogenic signaling pathways.FindingsThe FDG_SS values differed across clinical stages (p = 0.027), HPV-status (p< 0.001), and molecular subtypes of HNSC (p< 0.001). Multivariate Cox regression demonstrated that FDG_SS was an independent predictor for overall (p = 0.019) and progression-free survival (p = 0.024). FDG_SS positively correlated with total mutation rate (p = 0.016), aneuploidy (p < 0.001), and somatic copy number alteration scores (p < 0.001). CDKN2A in the cell cycle pathway (q = 0.014) and the TP53 gene in the TP53 pathway (q = 0.005) showed significant differences between high and low FDG_SS patients.ConclusionFDG_SS based on the gene coexpression network was associated with the mutational landscape of HNSC. ¹⁸F-FDG PET/CT is therefore a valuable tool for the in vivo imaging of these cancers, being able to visualize the glucose metabolism of the tumor and allow inferences to be made on the underlying genetic alterations in the tumor.     

Performance and limitations of positron emission tomography (PET) scanners for imaging very low activity sources

Emerging applications for positron emission tomography (PET) may require the ability to image very low activity source distributions in the body. The performance of clinical PET scanners in the regime where activity in the field of view is <1 MBq has not previously been explored. In this study, we compared the counting rate performance of two clinical PET/CT scanners, the Siemens Biograph Reveal 16 scanner which is based on lutetium oxyorthosilicate (LSO) detectors and the GE Discovery-ST scanner which is based on bismuth germanate (BGO) detectors using a modified National Electrical Manufacturers Association (NEMA) NU 2-2007 protocol. Across the activity range studied (2–100 kBq/mL in a 5.5 mL line source in the NEMA scatter phantom), the BGO-based scanner significantly outperformed the LSO-based scanner. This was largely due to the effect of background counts emanating from naturally occurring but radioactive ¹⁷⁶Lu within the LSO detector material, which dominates the observed counting rate at the lowest activities. Increasing the lower energy threshold from 350 keV to 425 keV in an attempt to reduce this background did not significantly improve the measured NECR performance. The measured singles rate due to ¹⁷⁶Lu emissions within the scanner energy window was also found to be dependent on temperature, and to be affected by the operation of the CT component, making approaches to correct or compensate for the background more challenging. We conclude that for PET studies in a very low activity range, BGO-based scanners are likely to have better performance because of the lack of significant background.     

Radiolabelled proteins for positron emission tomography: Pros and cons of labelling methods

Background

Dynamic biomedical research is currently yielding a wealth of information about disease-associated molecular alterations on cell surfaces and in the extracellular space. The ability to visualize and quantify these alterations in vivo could provide important diagnostic information and be used to guide individually-optimized therapy. Biotechnology can provide proteinaceous molecular probes with highly specific target recognitions. Suitably labelled, these may be used as tracers for radionuclide-based imaging of molecular disease signatures. If the labels are positron-emitting radionuclides, the superior resolution, sensitivity and quantification capability of positron emission tomography (PET) can be exploited.

Scope of review

This article discusses different approaches to labelling proteins with positron-emitting nuclides with suggestions made depending on the biological features of the tracers.

Major conclusions

Factors such as matching biological and physical half-lives, availability of the nuclide, labelling yields, and influences of labelling on targeting properties (affinity, charge and lipophilicity, cellular processing and retention of catabolites) should be considered when selecting a labelling strategy for each proteinaceous tracer.

General significance

The labelling strategy used can make all the difference between success and failure in a tracer application. This review emphasises chemical, biological and pharmacological considerations in labelling proteins with positron-emitting radionuclides.     

Aspects of positron emission tomography radiochemistry as relevant for food chemistry

Summary. Positron emission tomography (PET) is a medical imaging technique using compounds labelled with short-lived positron emitting radioisotopes to obtain functional information of physiological, biochemical and pharmacological processes in vivo. The need to understand the potential link between the ingestion of individual dietary agents and the effect of health promotion or health risk requires the exact metabolic characterization of food ingredients in vivo. This exciting but rather new research field of PET would provide new insights and perspectives on food chemistry by assessing quantitative information on pharmocokinetics and pharmacodynamics of food ingredients and dietary agents. To fully exploit PET technology in food chemistry appropriately radiolabelled compounds as relevant for food sciences are needed. The most widely used short-lived positron emitters are ¹¹C (t_1/2 = 20.4 min) and ¹⁸F (t_1/2 = 109.8 min). Longer-lived radioisotopes are available by using ⁷⁶Br (t_1/2 = 16.2 h) and ¹²⁴I (t_1/2 = 4.12 d). The present review article tries to discuss some aspects for the radiolabelling of food ingredients and dietary agents either by means of isotopic labelling with ¹¹C or via prosthetic group labelling approaches using the positron emitting halogens ¹⁸F, ⁷⁶Br and ¹²⁴I.     

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.     

Melanoma cell-derived vascular endothelial growth factor induces endothelial tubulogenesis within fibrin gels by a metalloproteinase-mediated mechanism

Angiogenesis is a process required not only for embryonal development but is encountered in wound healing and in pathological situations such as tumour growth. In vitro, formation of capillary-like structures can be induced by seeding human microvascular endothelial cells (HDMECs) on top of a fibrin matrix in the presence of phorbol 12-myristate 13-acetate (PMA) as a stimulating agent. In this study, we show that supernatants collected from high-invasive melanoma cells (BLM) induce the formation of tubular structures similar to PMA treatment whereas supernatants from low-invasive cells (WM164) did not. Analysis of proteins secreted into the supernatant of both melanoma cell lines identified differential expression of several pro-angiogenic proteins in high- and low-invasive melanoma cells. Vascular endothelial growth factor (VEGF) was strongly expressed by high- but not by low-invasive melanoma cells. Neutralisation of VEGF as well as inhibition of matrix metalloproteases (MMPs) using the broad spectrum MMP inhibitor 1,10-phenanthroline, both strongly reduced the melanoma-induced tube formation. PMA treatment of HDMECs on a fibrin matrix stimulated MT1-MMP synthesis, indicating that this protease is involved in PMA-induced angiogenesis. In addition, stimulation of HDMECs by supernatants of BLM melanoma cells resulted in a strong induction of ADAM-15, which is known to act as a metalloproteinase. In conclusion, these results show that VEGF released by melanoma cells is an important mediator of neo-vascularisation and that this process depends on the presence of metalloproteinases.     

In vitro studies on the existence of endothelial precursor cells in the subectodermal avascular region of quail wing buds

In vitro studies were performed to investigate the angiogenic capacity of different parts of the avian limb bud. Small pieces of wing mesenchyme of the vascularized core or of the avascular subectodermal region were obtained from quail embryos at stages 18 to 25, and were cultured. The identification of the avascular wing mesenchyme was made possible after injection of India ink via the vitelline vein or by bleeding control during in vivo dissection. Tissue cultures were treated with the QH-1 antibody or/and the endothelial cell marker DiI-Ac-LDL. Endothelial cells were found in cultures of the mesenchymal core and in those of the avascular subectodermal wing mesenchyme. Moreover, their appearance was independent of the stage of the donor embryo. Although there were no vessels, the subectodermal wing mesencyme was able to produce endothelial cells that proliferated and differentiated under in vitro conditions. Thus, endothelial precursor cells probably existed within the avascular wing mesenchyme. These cells might be identical with the QH-1-positive isolated cells that have been described in immunohistochemical studies of this region; they may contribute to the growing capillary plexus of the limb bud.     

Benefits and risks of transforming data from dynamic positron emission tomography, with an application to hepatic encephalopathy

Transforming data sets to bring out expected model features can be valuable within limits and misleading outside them. Here we establish such limits for the widely used Gjedde-Patlak representation of dynamic PET data, with an application to hepatic encephalopathy.     

Differentiation of human dermal fibroblasts towards endothelial cells

The ultimate goal of vascular tissue engineering is the production of functional grafts for clinical use. Difficulties acquiring autologous endothelial cells have motivated the search for alternative cell sources. Differentiation of dermal fibroblasts towards several mesenchymal lineages as well as endothelial cells has been proposed. The aim of the present study was to investigate the endothelial differentiation capacity of human dermal fibroblasts on a gene expression, protein expression and functional physiological level. Endothelial differentiation of fibroblasts was induced by culturing cells in 30% human serum, but not in fetal calf serum. Expression of proteins and genes relevant for endothelial function and differentiation was increased after induction. Furthermore, fibroblasts exposed to 30% human serum displayed increased uptake of low-density lipoprotein and formation of capillary-like networks. The results of this study may have an impact on cell sourcing for vascular tissue engineering, and the development of methods for vascularization of autologous tissue engineered constructs.     

Synthesis of 18F-labeled streptozotocin derivatives and an in-vivo kinetics study using positron emission tomography

Glucose transporter 2 (GLUT2) is involved in glucose uptake by hepatocytes, pancreatic beta cells, and absorptive cells in the intestine and proximal tubules in the kidney. Pancreatic GLUT2 also plays an important role in the mechanism of glucose-stimulated insulin secretion. In this study, novel Fluorine-18-labeled streptozotocin (STZ) derivatives were synthesized to serve as glycoside analogs for in-vivo GLUT2 imaging. Fluorine was introduced to hexyl groups at the 3′-positions of the compounds, and we aimed to synthesize compounds that were more stable than STZ. The nitroso derivatives exhibited relatively good stability during purification and purity analysis after radiosynthesis. We then evaluated the compounds in PET imaging and ex-vivo biodistribution studies. We observed high levels of radioactivity in the liver and kidney, which indicated accumulation in these organs within 5 min of administration. In contrast, the denitroso derivatives accumulated only in the kidney and bladder shortly after administration. Compounds with nitroso groups are thus expected to accumulate in GLUT2-expressing organs, and the presence of a nitroso group is essential for in-vivo GLUT2 imaging.     

Radiosynthesis and preclinical evaluation of [18F]FEM as a potential novel PET probe for tumor imaging

In the 21st century, the incidence and mortality of cancer, one of the most challenging diseases in the world, have rapidly increased. The purpose of this study was to develop 2-(2-[¹⁸F]fluoroethoxy)ethyl 4-methylbenzenesulfonate ([¹⁸F]FEM) as a positron emission tomography (PET) agent for tumor imaging. In this study, [¹⁸F]FEM was synthesized with a good radiochemical yield (45.4 ± 5.8%), high specific radioactivity (over 25 GBq/μmol), and commendable radiochemical purity (over 99%). The octanol/water partition coefficient of [¹⁸F]FEM was 1.44 ± 0.04. The probe demonstrated good stability in vitro (phosphate-buffered saline (PBS) and mouse serum (MS)), and binding specificity to five different tumor cell lines (A549, PC-3, HCC827, U87, and MDA-MB-231). PET imaging of tumor-bearing mice showed that [¹⁸F]FEM specifically accumulated at the tumor site of the five different tumor cell lines. The average tumor-to-muscle (T/M) ratio was over 2, and the maximum T/M values reached about 3.5. The biodistribution and dynamic PET imaging showed that most probes were metabolized by the liver, whereas a small part was metabolized by the kidney. Moreover, dynamic brain images and quantitative data showed [¹⁸F]FEM can quickly cross the blood brain barrier (BBB) and quickly fade out, thereby suggesting it may be a promising candidate probe for the imaging of brain tumors. The presented results demonstrated that [¹⁸F]FEM is a promising probe for tumor PET imaging.     

In vivo trafficking of endothelial progenitor cells their possible involvement in the tumor neovascularization

Circulating endothelial progenitor cell (EPCs) have been reported to contribute to vasculogenesis in adult organisms. To investigate the possible recruitment of EPCs and organization to form tumor vasculature, we investigated the in vivo real-time trafficking of EPCs non-invasively by using positron emission tomography (PET). A conditionally immortalized endothelial cell line derived from rat bone marrow (TR-BME1) was labeled with [2-(18)F] 2-fluoro-2-deoxy-D-glucose (FDG) and chased the accumulation in the rat tumor with PET. TR-BME1 cells were accumulated in the tumor tissues time-dependently. To investigate that the accumulation of the cells is specific or not, rats were previously irradiated with gamma-ray to suppress the influence of non-labeled EPCs derived from its bone marrow and used for PET analysis. The accumulation of TR-BME1 cells in the tumor was enhanced in gamma-ray-irradiated rats compared with that of non-irradiated ones, suggesting that TR-BME1 cells accumulated in the tumor specifically like as EPCs. Then the involvement of matrix metalloproteinases (MMPs) in EPC recruitment was examined. An inhibitor of MMP, MMI270, which suppressed invasion and tube formation abilities of TR-BME1 cells, only slightly suppressed the accumulation of TR-BME1 cells in the tumor of rats. These results suggest that EPCs are recruited in the tumor tissues for formation of tumor vasculature, and demonstrate the usefulness of TR-BME1 cells for studies on EPC related phenomena.     

Synthesis and evaluation of 18F-labeled tertiary benzenesulfonamides for imaging carbonic anhydrase IX expression in tumours with positron emission tomography

Three tertiary benzenesulfonamide inhibitors 4a–c were radiolabeled with ¹⁸F and evaluated for imaging carbonic anhydrase IX (CA IX) expression with positron emission tomography. All three inhibitors exhibit <10 nM affinity for CA IX with no measurable affinity for CA II. Despite good affinity/selectivity to CA IX and excellent stability in plasma, uptake of [¹⁸F]4a–c in CA IX-expressing HT-29 tumours was low without significant contrast. [¹⁸F]4a,b were excreted rapidly, while [¹⁸F]4c exhibited significant in vivo defluorination leading to high bone uptake. Due to minimal uptake in HT-29 tumours compared to normal organs/tissues, ¹⁸F-labeled benzenesulfonamides [¹⁸F]4a–c are not suitable as CA IX imaging agents.