Positron-Emitting Radiopharmaceuticals for Diagnostic Applications
Positron emission tomography is one of the most important tools in medical imaging with many applications in neurology, oncology, cardiology, neuroscience and infection. Over the past 30 years, technological advances in radiochemistry have led to the development of many radiotracers at the present used in clinical routine.
In this chapter we will discuss the main physical property of PET radionuclide and the main pharmacokinetic characteristics of PET radiotracers commonly used for molecular imaging.
KeywordsPositron Emission Tomography Radiopharmaceuticals PET tracers PET Radionuclides
Amine precursor uptake and decarboxylation
United States Food and Drug Administration
Gigaelectron volt (109 volt)
Glucose transporter family
Kiloelectron volt (103 volt)
Megaelectron volt (106 volt)
Positron emission tomography
Positron emission tomography (PET) is based on the coincidence detection of the paired 511 keV γ rays produced in the annihilation process that takes place when a β− particle (the negative electron, or negatron) in the surrounding matter combines with a β+ particle (the positive electron, or positron) emitted during nuclear decay of a positron-emitting radionuclide. Neutron-deficient nuclei reduce their proton surplus by emitting positrons; atomic mass of these elements is usually one unit smaller than the most common form of the stable isotope (e.g., positron-emitting 15O, 13N, and 11C versus stable 16O, 14N, and 12C). The annihilation event between a β+ particle and a β− particle, which occurs at a certain distance from the decay point (positron range) over which the positron loses part of its energy, creates two 511 keV photons, coincidence detection of the photons constitutes the basis for PET imaging (see chapter “Instrumentation for Positron Emission Imaging” of this book).
Main physical characteristics of the β+ emitting radionuclides most frequently employed for clinical purposes (arranged in increasing order of atomic mass number)
Maximum specific activity (GBq/μmol)
Branching β+ emission (%)
Emax β+ (MeV)
Modality of production
Maximum β+ range in water (mm)
Carbon-11 is generally produced by the 14N(p,α)11C nuclear reaction and decays with a 99.8% positron emission and half-life of 20.4 min. 11C-labeled molecules of biological interest in which a native 12C atom has been replaced with an 11C atom behave chemically and biologically exactly as their unlabeled equivalents. 11C is often incorporated into PET radiopharmaceuticals through methylation reactions using [11C]methyl iodide or [11C]triflate. The most common precursors for synthesis are [11C]cyanide, [11C]carbon dioxide, and [11C]carbon monoxide.
Nitrogen-13 can be produced through different nuclear reactions. The 16O(p,α)13N reaction is the most commonly used, and it is performed using a solution of ethanol (5 mM) or water; ethanol serves to reduce the amount of [13N]nitrates or nitrites formed in situ. Similarly as for carbon, the stable isotope of nitrogen (14N) is ubiquitous in biologically active organic molecules. Nevertheless, the half-life of 13N (9.96 min) is too short to perform radiochemical synthesis of complex biological molecules. Thus, the most common application of 13N-labeled molecules is in the form of [13N]ammonia, which is routinely employed to assess myocardial blood flow. [13N]Ammonia can also been used to prepare [13N] cisplatin for investigational purposes.
Oxygen-15 is produced using either the 14N(d,n)15O or the 15N(p,n)15O nuclear reactions. The very short half-life (2.04 min) of this radionuclide restricts its use to label simple molecules such as water, gaseous oxygen, and carbon monoxide. [15O]Water is used to investigate cerebral and myocardial perfusion, as well as tumor perfusion, while gaseous 15O2 can be used to investigate oxygen metabolism; [15O]carbon monoxide are employed to image the blood pool.
Fluorine-18 is the most widely used radionuclide for clinical PET investigations. 18F (which decays to stable 18O with nearly 97% positron emission) can be produced either by the 20Ne(d,a)18F or by the 18O(p,n)18F nuclear reactions. The former reaction leads to the formation of [18F]F2, while the latter results in the formation of [18F]F−. The short range in tissues of positrons emitted by 18F (max 2.3 mm in water) contributes to the high-resolution PET images obtained with this radionuclide. The relatively long half-life of this radionuclide (almost 110 min) permits multistep labeling reactions for the synthesis of relatively complex molecules (with radiolabeling yields as high as 20–40%) and also allows for shipment from the production site to peripheral PET centers. Fluorine reacts with many organic and inorganic chemicals, since it is the most electronegative of all the elements; the chemistries most widely employed for incorporating 18F into organic molecules are nucleophilic substitution and electrophilic substitution. Since the electrophilic labeling approach (which uses [18F]F2) results in molecules with low specific activities, the nucleophilic approach (based on the use of [18F]F−) is most frequently employed, not only because of the higher specific radioactivity of the compounds so produced but also because of greater selectivity of the radiolabeling reactions.
Gallium-68 (half-life 68.3 min, maximum positron energy 1.9 MeV) is eluted from a 68Ge/68Ga radionuclide generator system, similar in concept to that of the 99Mo/99mTc generator (see chapter “Single-Photon Emitting Radiopharmaceuticals for Diagnostic Applications” of this book). 68Ge decays by electron capture with a physical half-life of 271 days; a single generator allows therefore continuous production of 68Ga for almost 1 year. 68Ga is an excellent positron emitter (88% branching emission of positrons), with a low photon emission (3.22% at 1.077 MeV). This metallic radionuclide is used mostly for labeling peptides, such as somatostatin analogs, and yields compounds with high radiochemical purity.
The ultrashort-lived 82Rb (half-life 76 s, with emission of 3.15 MeV positrons) is a potassium analog eluted from a 82Sr/82Rb generator for myocardial perfusion imaging. It decays to stable 82Kr by positron emission (nearly 96%) associated with only about 4% electron capture. Since the parent radionuclide 82Sr has a physical half-life of 25 days, the generator can be used for approximately 1 month.
Positron emission tomography is one of the most important tools in medical imaging with many applications in neurology, oncology, cardiology and neuroscience, and infection – just to mention the most frequent clinical applications. Over the past 30 years, advances in radiotracer chemistry have led to the development and evaluation in preclinical and clinical studies of many radiotracers with high specificity and suitable pharmacokinetic characteristics for molecular imaging.
Main mechanisms of in vivo distribution/localization of PET imaging agents in nuclear oncology
simple, facilitated, and active transport
Facilitated transport mediated by specific transport proteins: glucose transporters (GLUTs 1–5) expressed on the cell membrane as a function of energy demand
[18F]FDG for PET of tumors with enhanced glucose consumption
Enzyme-mediated intracellular trapping
Trapping into cells as the result of a specific interaction of the radiopharmaceutical with an enzyme: phosphorylated [18F]FDG not being the substrate for subsequent metabolic pathways
[18F]FDG for PET of tumors with high glucose demand
Competitive substrates for tumor cell structure
The radiopharmaceutical competes with the endogenous substrate for the same metabolic pathways for the synthesis of proteins or phospholipids
Various imaging agents for tumors: amino acids (protein synthesis) and choline-based tracers (phospholipid synthesis)
Competitive substrates for tumor cell functional processes
The radiopharmaceutical competes with endogenous catecholamine synthesis and storage in neurosecretory vesicles
Imaging agents for neuroendocrine tumors derived from the neural crest: 18F-DOPA
The radiopharmaceutical competes with the physiologic ligands for binding with the target receptor
Somatostatin analogs for neuroendocrine tumors expressing somatostatin receptors
The mechanisms leading to elevated glucose metabolism in cancers are multifactorial and include tumor-related components, biochemical and molecular alterations (e.g., glucose metabolic pathway, hypoxia), and nontumor-related constituents (e.g., inflammation). [18F]FDG is excreted through the kidneys, about 20% of injected activity being excreted in the urine within 2 h after administration. The uptake of [18F]FDG, which competes with endogenous glucose, by tumor tissue depends on plasma glucose levels; in case of high blood glucose levels (as well as low insulin level), the uptake of [18F]FDG is reduced. In the same way, some drugs that modify glucose level (e.g., valproic acid, glucocorticoids, carbamazepine) can alter radiotracer uptake. Many other factors (e.g., chemotherapy, radiation therapy, or recent administration of granulocyte-colony-stimulating factor) interfere with [18F]FDG uptake, thus possibly resulting in false-positive or false-negative imaging data.
[11C]Glucose, the perfect radioactive counterpart of endogenous glucose, has been synthesized, but the short physical half-life of the radionuclide and the production of labeled metabolites with fast washout from tissues and return to circulating blood limit the clinical application of this tracer.
Nitrogen-13 ammonia ([13N]ammonia) is used mostly for quantitative imaging of myocardial blood flow. In physiologic conditions, the majority of [13N]ammonia circulates in the form of the ammonium ion, NH4 +. Following i.v. injection, [13N]ammonia is rapidly cleared from the bloodstream and diffuses across the cell capillary membrane entering the cells via the ammonium transporter. Once in the cell, it is metabolized to 13 N-glutamine by glutamine synthetase and trapped within tissues such as the myocardium, liver, brain, and other organs. Its uptake in the tissues is proportional to blood flow. The main metabolite of [13N]ammonia is [13N]urea , which is excreted through the kidneys.
[13N]ammonia is used clinically for myocardial perfusion studies and quantification of myocardial blood flow and flow reserve. The high net extraction fraction of 13NH3 by myocardial cells (80%) and its short positron range (0.4 mm) yield very high-quality PET images.
Oxygen-15 water ([15O]H2O) is excellent to trace blood flow. It is a freely diffusible agent, most frequently used to investigate cerebral and myocardial perfusion, as well as tumor perfusion. Thanks to the short physical half-life, perfusion studies can be repeated every 8–10 min. As a cyclotron product with a very short half-life (2.04 min), its use in the clinical setting is to institutions with an on-site cyclotron.
Rubidium-82 chloride (82RbCl) is a potassium analog used for PET myocardial perfusion imaging. Once administered intravenously, 82RbCl dissociates rapidly, and the 82Rb+ ion is rapidly extracted by the myocardium in a fashion which is proportional to blood flow and enters the myocardial cells through the Na+/K+ ATPase pump, similarly to 201Tl. Myocardial activity is noted within the first minute after the i.v. injection. The first-pass extraction fraction is about 50–60% at resting blood flow levels, and several studies have demonstrated that extraction decreases in a nonlinear manner at high blood flow rates. 82Rb decays to stable krypton-82 (82Kr), which is exhaled through the lungs.
Due to the short half-life of 82Rb (76 s) and the rapid replenishment of 82Sr/82Rb generator (making it possible to elute every 10 min), fast sequential resting and pharmacological stress myocardial perfusion studies can be performed, in order to quantitatively assess not only myocardial blood flow but also the coronary artery blood flow reserve.
Despite the ultrashort half-life and the high kinetic energy (3.15 MeV) resulting in a relatively long positron range, 82RbCl still provides very good-quality PET images, and it has been demonstrated that myocardial perfusion studies with 82Rb-PET have superior accuracy than scans based on the use of 99mTc-labeled agents.
[11C]Methionine and Other Tracers for Protein Synthesis
Tumor cells generally exhibit increased amino acid transport and increased protein synthesis rates. Several processes contribute to enhance amino acid transport per se in addition to overall protein synthesis, including transamination and transmethylation, a specific role of methionine in initiating protein synthesis, and the use of amino acids as glutamine for energy.
Many amino acids and analogs labeled with β+ emitters have been developed to measure either the rate of protein synthesis or the rate of uptake into the cells; these tracers differ in ease of synthesis, biodistribution, and formation of radiolabeled metabolites in vivo, but only a few have substantial impact in clinical oncology.
Nontumoral uptake of all radiolabeled amino acids (though generally less intense than in tumors) includes ischemic brain areas, scar tissue, sarcoidosis, recently irradiated areas, and hemangiomas. Whereas, their uptake in inflammatory tissue is much lower than [18F]FDG uptake.
Choline-Based Tracers for Phospholipid Synthesis
The synthesis of cell membrane is very rapid in neoplastic tissue, reflecting indirectly the rate of cell proliferation. Choline , an essential element of the phospholipid constituents in the cell membrane, is transported into cells and phosphorylated by choline kinase before being incorporated into the membrane phospholipids. Besides other molecular signatures of increased energy expenditure and accelerated proliferation, tumor cells have enhanced levels of choline kinase.
Fluorine-18 Sodium Fluoride
Fluorine-18 sodium fluoride ([18F]fluoride) was first introduced as a bone-imaging agent for skeletal scintigraphy in the early 1960s and approved by the FDA in 1972. It was then withdrawn from the approval list in 1984 because of technical limitations at that time, that is, the difficulty in imaging the 511 keV annihilation photons with gamma cameras optimized for 100–200 keV γ rays; the concomitant wide diffusion of 99mTc-labeled diphosphonates contributed to the declining interest in 18F-fluoride for bone scintigraphy.
Interest in the clinical use of [18F]fluoride as a bone-scanning agent has more recently been revived after the development of modern PET scanners. [18F]Fluoride is administered intravenously (185–370 MBq), and the intensity and extent of its uptake reflects blood flow and bone remodeling. This agent has high and rapid bone uptake combined with a fast, biexponential blood clearance that results in achievement of a high bone-to-background ratio within a short time; in fact, imaging can be performed less than 1 h after administration. 18F-ion exchanges for an OH− ion on the surface of the hydroxyapatite matrix of the bone and remains then chemisorbed into the crystalline matrix where it is retained until the bone is remodeled. Red blood cell uptake (30% of the injected activity) does not interfere with bone uptake. Within the first 1–2 h upon administration of Na18F, about 20% is excreted through the urinary tract.
The normal biodistribution of Na18F is in general rather uniform, but the axial skeleton has a higher uptake than the appendicular skeleton, and the bone around joints has higher uptake than the shafts of long bones.
Increased fluoride uptake has been reported in both sclerotic and lytic metastases. As for 99mTc-labeled diphosphonates, purely lytic lesions have lower or no uptake at all. 18F-Fluoride is not a tumor-specific tracer, and its clinical usefulness has been demonstrated also for a wide range of benign diseases of the bone.
Fluorine-18 Misonidazole and Other Hypoxia Tracers
PET imaging with 18F-MISO is used to quantitatively assess tumor hypoxia in lung, brain, and head-and-neck cancers (as well as for the evaluation of myocardial ischemia).
Fluorine-18-labeled fluoroetanidazole (18F-FETA), a fluorinated analog of etanidazole, has similar oxygen-dependent binding and tumor retention compared to 18F-MISO, but it releases fewer metabolites in plasma and urine. 18F-Fluoroerythronitroimidazole (18F-ETNIM) is an additional novel promising radiopharmaceutical with faster elimination from well-oxygenated tissues, because it is more hydrophilic.
Fluorine-18 DOPA and Other Amine Precursors
After i.v. administration, up to 1% of the injected 18F-DOPA crosses the blood–brain barrier and is transported into presynaptic neurons via the amino acid transport system, where an amino acid decarboxylase converts it to 18F-fluorodopamine, which is stored in intraneuronal vesicles within the striatum.
In peripheral tissues, 18F-DOPA is metabolized to 18F-fluorodopamine or to 3-O-methyl-6-fluoro-l-DOPA and excreted mainly through the kidneys. The renal excretion may be reduced by premedication with the decarboxylase inhibitor carbidopa, with the goal of improving in general the tumor uptake, except in patients with insulinomas or β-cell hyperplasia.
[11C]hydroxytryptophan is another PET tracer used for the localization of NETs, including insulinomas; this tracer is taken up by tumor cells, decarboxylated, and then stored and irreversibly trapped in vesicles as [11C]serotonin.
Gallium-68 DOTATOC (Edotreotide) and Other Somatostatin Analogs
Neuroendocrine tumors (NETs) are characterized by the presence of peptide receptors on the cell membrane, particularly the somatostatin (SS) receptors. Somatostatin analogs have been radiolabeled with diagnostic radionuclides as well as with emitters of β− particles for targeted radionuclide therapy. Five specific SS receptor subtypes (SSTR1–SSTR5) with different tissue distribution and affinity for somatostatin analogs have been identified. They are expressed also in peritumoral vessels and in inflammatory and immune cells, and this can account for visualization of those tumors whose cells do not express the receptors. The most commonly used somatostatin analog is octreotide that has high binding affinity to somatostatin receptor subtypes 2 and 5 but binds varyingly to the SSTR3 and SSTR4 subtypes. DOTA-Tyr3-octreotide (DOTA-TOC, or DOTATOC) can be variously labeled with either γ or β+ emitters (for imaging), as well as with β− emitters (such as 90Y or 177Lu, for therapy) (see also chapter “Novel Radiopharmaceuticals for Therapy” of this book). DOTA (1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid) is a macrocyclic chelating agent that forms stable complexes with the metal radionuclide of interest. At variance with DTPA (which is an open structure enabling to label radioconjugates at room temperature), DOTA is a closed structure, and labeling must be performed at high temperature (around 100 °C) in order to open the ring circumscribed by the four nitrogen atoms; in turn, the radioconjugates obtained with DOTA derivatives are much more stable in vivo than those obtained with DTPA. The time required to recover 68Ga from the 68Ge/68Ga generator, to synthesize, and to purify the 68Ga-labeled DOTA-conjugated peptides is less than 20 min, resulting in an overall simple and radiochemically efficient processing.
Other radiolabeled somatostatin analogs for PET imaging include 68Ga-DOTA-1-NaI3-octreotide (68Ga-DOTANOC, with better binding affinities for SSTR2, SSTR3, and SSTR5) and 68Ga-DOTA-Tyr3,Thr8-octreotide (68Ga-DOTATATE, with predominant affinity for SSTR2).
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