Functional genomics and radioisotope-based imaging procedures

After the sequencing of the human genome has been completed, non-invasive imaging studies are needed to assess the function of new genes in living organisms. The evaluation of genetically manipulated animals or new designed biomolecules will require a thorough understanding of physiology, biochemistry and pharmacology, and the experimental approaches will involve many new technologies including in vivo imaging with single photon emission computed tomography (SPECT) and positron emission tomography (PET). Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in vivo reporter genes such as enzymes, receptors, antigens or transporters. Pharmacogenomics will identify new surrogate markers for therapy monitoring which may represent potential new tracers for imaging. Also, drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. Clinical gene therapy needs non-invasive tools to evaluate the efficiency of gene transfer. These informations can be used for therapy planning, follow-up studies in treated tumors and as an indicator of prognosis. Therapy planning is performed by the assessment of gene expression for example using radio-labeled specific substrates to determine the activity of suicide enzymes such as the Herpes Simplex Virus thymidine kinase. Follow-up studies with single photon emission tomography or positron emission tomography may be done to evaluate early or late effects of gene therapy on tumor metabolism or proliferation. Finally, new biomolecules will be developed by bioengineering methods which may be used for isotope-based diagnosis and treatment of disease.     

Non-invasive Imaging in Gene Therapy.

Several methods are available for non-invasive imaging of gene delivery and transgene expression, including magnetic resonance imaging (MRI), single photon emission tomography (SPECT)/positron emission tomography (PET), and fluorescence and bioluminescence imaging. However, these imaging modalities differ greatly in terms of their sensitivity, cost, and ability to measure the signal. Whereas MRI can produce a resolution of approximately 50 mum, optical imaging achieves only 3-5 mm but outperforms MRI in terms of the cost of the imaging device. Similarly, SPECT and PET give a resolution of only 1-2 mm but provide for relatively easy quantitation of the signal and need only nanograms of probe, compared with the microgram or milligram levels required for MRI and optical imaging. To develop safer and more efficient gene delivery vectors, it is essential to perform rigorous in vivo experiments, to image particle biodistribution and transduction patterns, and to quantify the transgene expression profile. Differences between modalities have a significant effect on the resultant imaging resolution for gene therapy. This review describes the methodologies in use and highlights recent key approaches using the latest imaging modalities in gene therapy. Future trends in gene therapy imaging are also discussed.     

Radiohalogenated nonnatural amino acids as PET and SPECT tumor imaging agents

Radiohalogenated α-amino acids are a diverse and useful class tumor imaging agents suitable for positron emission tomography and single photon emission computed tomography. These tracers target the increased rates of amino acid transport exhibited by many tumor cells. The most established clinical use for radiolabeled amino acids is imaging primary and recurrent gliomas, and there is growing evidence that they may also be useful for other oncologic applications, including neuroendocrine tumors and prostate cancer. This review focuses on the synthesis, radiolabeling, and preclinical evaluation of three series of nonnatural radiohalogenated amino acids: alicyclic, α,α-dialkyl, and 1H-[1,2,3]triazole amino acids which target system L, system A, and cationic amino acid transport systems, respectively.     

Current and Future Status of Blood Flow Tracers

Myocardial perfusion imaging is important for the management of patients with suspected or known coronary artery disease. Nuclear cardiology is the most widely used noninvasive approach for the assessment of myocardial perfusion. The available single-photon emission CT (SPECT) flow agents are characterized by a rapid myocardial extraction and by a cardiac uptake proportional to blood flow. In addition, different positron emission tomography (PET) tracers may be used for the absolute quantitative measurement of myocardial blood flow and coronary flow reserve. However, the available SPECT and PET tracers for myocardial perfusion imaging have some limitations that must be considered to maximize their clinical applications and there is still a well-recognized need for the development of new perfusion tracers with more ideal properties. This review illustrates the current status and the future perspectives of blood flow tracers for SPECT and PET myocardial perfusion imaging.     

The clinical value of cardiac sympathetic imaging in heart failure

The autonomic nervous system plays an important role in the pathology of heart failure. The single‐photon emission computed tomography tracer iodine‐123‐metaiodobenzylguanidine (¹²³I‐MIBG) can be used to investigate the activity of the predominant neurotransmitter of the sympathetic nervous system, norepinephrine. Also, positron emission tomography tracers are being developed for the same purpose. With ¹²³I‐MIBG as a starting point, this brief review introduces the modalities used for cardiac sympathetic imaging.     

Cardiac Imaging as a Guide for Revascularization and Medical Management of Chronic Coronary Artery Disease

There is a strong association between obstructive coronary artery disease (CAD) and adverse outcomes. There is an ongoing debate regarding the role of revascularization and maximal medical therapy in patients with obstructive CAD and noninvasive imaging is recognized as a key player in guiding downstream clinical decision making especially in stable patients with known or suspected CAD. Most often risk stratification is performed with noninvasive imaging techniques including echocardiography, coronary CT angiography, single photon emission computed tomography myocardial perfusion imaging (SPECT-MPI), positron emission computed tomography-based myocardial perfusion imaging (PET-MPI), and magnetic resonance imaging (MRI). Occasionally, risk stratification may be performed with invasive angiography. In this review, we will describe some of the evidence in favor of noninvasive anatomy (CCTA) and physiology-based techniques (SPECT-MPI, PET-MPI, MRI) in guiding the choice of treatment with revascularization vs medical therapy in patients with CAD.     

Role of nuclear cardiology for guiding device therapy in patients with heart failure

Heart failure is a dynamic condition with high morbidity and mortality and its prognosis should be reassessed frequently, particularly in patients for whom critical treatment decisions may depend on the results of prognostication. In patients with heart failure, nuclear cardiology techniques are useful to establish the etiology and the severity of the disease, while fewer studies have explored the potential capability of nuclear cardiology to guide cardiac resynchronization therapy(CRT) and to select patients for implantable cardioverter defibrillators(ICD). Left ventricular synchrony may be assessed by radionuclide angiography or gated singlephoton emission computed tomography myocardial perfusion scintigraphy. These modalities have shown promise as predictors of CRT outcome using phase analysis. Combined assessment of myocardial viability and left ventricular dyssynchrony is feasible using positron emission tomography and could improve conventional response prediction criteria for CRT. Preliminary data also exists on integrated positron emission tomography/computed tomography approach for assessing myocardial viability, identifying the location of biventricular pacemaker leads, and obtaining left ventricular functional data, including contractile phase analysis. Finally, cardiac imaging with autonomic radiotracers may be useful in predicting CRT response and for identifying patients at risk for sudden cardiac death, therefore potentially offering a way to select patients for both CRT and ICD therapy. Prospective trials where imaging is combined with image-test driven therapy are needed to better define the role of nuclear cardiology for guiding device therapy in patients with heart failure.     

BMIPP imaging to assess functional outcome in patients with acute and chronic left ventricular dysfunction

Assessment of myocardial viability is an important clinical issue for patient management during the acute and chronic stages of myocardial infarction. BMIPP (15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid) is a free fatty acid analogue which is trapped in the myocardium, thus permitting for metabolic imaging with single photon emission computerized tomography (SPECT). Less BMIPP than flow tracers that may be observed in the areas of infarction, may reflect the metabolic shift from fatty acid to glucose utilization in ischaemic myocardium. In this sense, the combined imaging of BMIPP and a flow tracer with SPECT may provide similar and important information as fluoro-18 deoxyglucose (FDG) and positron emission tomography (PET) regarding the assessment of myocardial viability. The purpose of this article is to review the clinical impact of BMIPP in patients with acute and with chronic left ventricular dysfunction for the identification of jeopardized but viable myocardium and the prediction of the functional outcome.     

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high-affinity state postulates that a certain subset of G-protein-coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high-affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high-affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high-affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high-affinity state in vivo are discussed.     

Sortase A‐mediated site‐specific labeling of camelid single‐domain antibody‐fragments: a versatile strategy for multiple molecular imaging modalities

A generic site‐specific conjugation method that generates a homogeneous product is of utmost importance in tracer development for molecular imaging and therapy. We explored the protein‐ligation capacity of the enzyme Sortase A to label camelid single‐domain antibody‐fragments, also known as nanobodies. The versatility of the approach was demonstrated by conjugating independently three different imaging probes: the chelating agents CHX‐A"‐DTPA and NOTA for single‐photon emission computed tomography (SPECT) with indium‐111 and positron emission tomography (PET) with gallium‐68, respectively, and the fluorescent dye Cy5 for fluorescence reflectance imaging (FRI). After a straightforward purification process, homogeneous single‐conjugated tracer populations were obtained in high yield (30–50%). The enzymatic conjugation did not affect the affinity of the tracers, nor the radiolabeling efficiency or spectral characteristics. In vivo, the tracers enabled the visualization of human epidermal growth factor receptor 2 (HER2) expressing BT474M1‐tumors with high contrast and specificity as soon as 1 h post injection in all three imaging modalities. These data demonstrate Sortase A‐mediated conjugation as a valuable strategy for the development of site‐specifically labeled camelid single‐domain antibody‐fragments for use in multiple molecular imaging modalities. Copyright © 2016 John Wiley & Sons, Ltd.     

Tracing transgene expression in cancer gene therapy: a requirement for rational progress in the field.

This review summarizes the status of gene therapy in medicine and the role of molecular imaging in its development. In gene therapy, genetic material is introduced into cells in order to generate a specific biological effect. Natural (viruses) or artificial molecular constructs, named gene therapy vectors, are used to achieve efficient cell transduction. This new form of therapy can be used for treating a broad variety of conditions including hereditary diseases, infections, degenerative disorders and cancer. Monitoring transgene expression using noninvasive imaging techniques is a necessary complement for the development of clinical gene therapy. Recent developments in magnetic resonance imaging afford the possibility of detecting gene transfer in vivo, but the most promising results have been obtained with positron emission tomography (PET). PET allows imaging gene therapy products by administration of a labeled substrate when the transgene codes for an enzyme or by administration of a labeled ligand when the transgene codes for a receptor. In the latter strategy, a membrane molecule (somatostatin or dopamine receptors) is used to detect the selective trapping of its radiolabeled ligand in the transduced cells. One of the approaches for the genetic treatment of cancer consists in transferring the "suicide genes" into tumor cells, the most common being the thymidine kinase (tk) of herpes viruses. Different nucleoside analogs can be labeled for its use as PET reporter probes in order to visualize tk expression. The results of pre-clinical studies are extremely encouraging. Reliable methods for the in vivo tracing of transgene expression in humans have to be developed in order for the field of gene therapy to mature. PET has emerged as a powerful tool to assist in achieving this goal.     

Cardiac Positron Emission Tomography: a Clinical Perspective

Cardiac positron emission tomography is a powerful, quantitative, non-invasive imaging modality, which adds valuable diagnostic and prognostic information to the clinical work-up. Myocardial perfusion and viability imaging are, as a result of continuously growing evidence, established clinical indications that may be cost-effective, due to the high diagnostic accuracy of cardiac positron emission tomography, despite high single-test costs. In the field of inflammation imaging, new indications are entering the clinical arena, which may contribute to a better diagnosis and overall patient care, as for instance in patients with cardiac sarcoidosis, prosthetic valve endocarditis and cardiac device infections. This review will discuss the individual strengths and weaknesses of cardiac positron emission tomography and, hence, the resulting clinical usefulness based on the current evidence for an individualized, patient-centered imaging approach.     

Myocardial perfusion imaging to evaluate the efficacy of medical therapy in patients with coronary artery disease

Myocardial perfusion imaging is commonly used to risk-stratify patients based on the presence and extent of stress-induced myocardial ischemia. Recent studies have shown that both positron and single photon emission tomography techniques can be used to assess the effectiveness of coronary revascularization procedures as well as various anti-ischemic medical therapies on myocardial perfusion. In this regard, perfusion imaging may be used not only to assess initial risk but also to track subsequent risk based on changes in perfusion results following medical and/or interventional therapies.     

Urokinase‐type plasminogen activator receptor (uPAR) as a promising new imaging target: potential clinical applications

Urokinase‐type plasminogen activator receptor (uPAR) has been shown to be of special importance during cancer invasion and metastasis. However, currently, tissue samples are needed for measurement of uPAR expression limiting the potential as a clinical routine. Therefore, non‐invasive methods are needed. In line with this, uPAR has recently been identified as a very promising imaging target candidate. uPAR consists of three domains attached to the cell membrane via a glycosylphosphatidylinositol (GPI) anchor and binds it natural ligand uPA with high affinity to localize plasminogen activation at the cell surface. Due to the importance of uPAR in cancer invasion and metastasis, a number of high‐affinity ligands have been identified during the last decades. These ligands have recently been used as starting point for the development of a number of ligands for imaging of uPAR using various imaging modalities such as optical imaging, magnetic resonance imaging, single photon emission computer tomography (SPECT) and positron emission topography (PET). In this review, we will discuss recent advances in the development of uPAR‐targeted imaging ligands according to imaging modality. In addition, we will discuss the potential future clinical application for uPAR imaging as a new imaging biomarker.     

Positron emission tomography and gene therapy: basic concepts and experimental approaches for in vivo gene expression imaging.

More than two decades of intense research have allowed gene therapy to move from the laboratory to the clinical setting, where its use for the treatment of human pathologies has been considerably increased in the last years. However, many crucial questions remain to be solved in this challenging field. In vivo imaging with positron emission tomography (PET) by combination of the appropriate PET reporter gene and PET reporter probe could provide invaluable qualitative and quantitative information to answer multiple unsolved questions about gene therapy. PET imaging could be used to define parameters not available by other techniques that are of substantial interest not only for the proper understanding of the gene therapy process, but also for its future development and clinical application in humans. This review focuses on the molecular biology basis of gene therapy and molecular imaging, describing the fundamentals of in vivo gene expression imaging by PET, and the application of PET to gene therapy, as a technology that can be used in many different ways. It could be applied to avoid invasive procedures for gene therapy monitoring; accurately diagnose the pathology for better planning of the most adequate therapeutic approach; as treatment evaluation to image the functional effects of gene therapy at the biochemical level; as a quantitative noninvasive way to monitor the location, magnitude and persistence of gene expression over time; and would also help to a better understanding of vector biology and pharmacology devoted to the development of safer and more efficient vectors.     

Application of MR/PET in Oncologic Imaging

The present review aims to depict the possibilities offered by hybrid imaging with magnetic resonance positron emission tomography (MR/PET). Recently, new whole-body MR/PET scanners were introduced allowing for the combination of both modalities outside the brain. This is a challenge for both modalities: For MRI, it is essential to provide anatomical images with high resolution. Additionally, diffusion-weighted imaging (DWI), proton spectroscopy, but also dynamic contrast-enhanced imaging plays an important role. With regard to PET, the technical challenge mainly consists of obtaining an appropriate MR-based attenuation correction for the PET data. Using MR/PET, it is possible to acquire morphological and functional data in one examination. In particular, children and young adults will benefit from this new hybrid technique, especially in oncologic imaging with multiple follow-up examinations. However, it is expected that PET/CT will not be replaced completely by MR/PET because PET/CT is less cost-intensive and more widely available. Moreover, in lung imaging, MRI limitations still have to be accepted. Concerning research, simultaneous MR/PET offers a variety of new possibilities, for example cardiac imaging, functional brain studies or the evaluation of new tracers in correlation with specific MR techniques.     

Review: comparison of PET rubidium‐82 with conventional SPECT myocardial perfusion imaging

Nuclear cardiology has for many years been focused on gamma camera technology. With ever improving cameras and software applications, this modality has developed into an important assessment tool for ischaemic heart disease. However, the development of new perfusion tracers has been scarce. While cardiac positron emission tomography (PET) so far largely has been limited to centres with on‐site cyclotron, recent developments with generator produced perfusion tracers such as rubidium‐82, as well as an increasing number of PET scanners installed, may enable a larger patient flow that may supersede that of gamma camera myocardial perfusion imaging.     

中枢神经系统对排尿的控制和调节

排尿是一项复杂的生理活动,受多级神经的控制。随着正电子发射断层扫描单光子发射计算机断层扫描以及功能磁共振成像技术的应用,人们可以在膀胱储尿和排尿的同时进行脑功能实时成像。本文针对排尿的生理、中枢的调控进行综述。