A radiopharmaceutical is basically any substance which by virtue of its pharmaceutical form and quantity and quality of emitted radiation can be used for the diagnosis and treatment of diseases in living organisms.
Some of the most widely used clinical applications are the anatomical visualization of an organ or system, the evaluation of physiopathological behavior at tissue level, the study of the radiopharmaceuticals metabolism and biochemical behavior based on pharmacokinetic parameters, etc.
Article 48 of the Spanish Law 29/2006 of 26 July on guarantees and rational use of medicinal products and medical devices regulates radiopharmaceuticals, defining them as “any product prepared for diagnostic or therapeutic use containing one or more radionuclides (radioactive isotopes)”.
The vast majority of radiopharmaceuticals (gamma radiation emitters and β+ emitters) are used for medical diagnostic purposes and do not have concrete pharmacological effects, though some β+ emitters are used for therapeutic or palliative purposes. Radiation is a general property of all radiopharmaceuticals. Due evaluation of the safety and efficacy of all radiopharmaceuticals is required, in addition to consideration of the general parameters, radiopharmacological and radiation protection aspects and radiation dosimetry.
The use of radiopharmaceuticals must be justified by the obtainment of a net benefit offsetting the radiological risk inherent to the nature of the drug. In other words, the risk/benefit ratio must be adequate, just as with all non-radioactive drugs.
Positron emission tomography (PET) is a noninvasive diagnostic technique that provides images of the distribution of radiopharmaceuticals labeled with positron-emitting radionuclides inside the body, thereby making it possible to visualize different physiological or physiopathological processes in vivo.
The radiopharmaceuticals used in PET can act as substrates of metabolic pathways, as ligands that selectively interact in a neurotransmission process, or simply as radiopharmaceuticals for measuring regional blood flow. Thus, PET can be used to study, visualize and quantify many biochemical and physiological processes, such as glucose metabolism, the rate of protein synthesis, cell proliferation, enzyme activity, a compound’s affinity for a given receptor, oxygen consumption rate, intracellular pH, blood flow, receptor density in a given zone, gene expression and regulation, amino acid transport, etc.
The main indications of PET in clinical practice are in oncology, neurology and cardiology. Hitherto, a large number and variety of PET radiopharmaceuticals have been used, although most of them are employed in the research setting. A limited number of radiopharmaceuticals such as 18FDG, 18FDOPA, 11C-methionine, 15O-water and 13N-ammonia have become essential in routine clinical practice thanks to their excellent characteristics in the study of many diseases.
Carbon, N and O atoms are present in all biomolecules, and their use as radioisotopes is therefore ideal for labeling many compounds identical to naturally-occurring ones. The most suitable positron-emitting radionuclides for use in PET studies are 11C, 13N, 15O and 18F. The first three have a short radioactive half-life, which limits their possibility for use in centers located far from the isotope production site. In contrast, 18F is more suitable for distribution since it is more stable as a radioisotope. This feature has caused labeling with 18F to be the most widely-used option in the manufacture of radiopharmaceuticals labeled with positron-emitting radionuclides for PET.
The best radionuclide for clinical application in routine diagnostic PET procedures is 18F.
The 18F positron-emitting energy is only 0.64 MeV – the lowest of all the positron emitters used in PET. This implies lesser patient radiation exposure and improved diagnostic image resolution. Furthermore, 18F decay does not involve the emission of gamma radiation that can interfere with photon detection or of particles (β- or α) that may constitute an increase in the dose of radiation received by the patient. The radioactive half-life of 18F (110 minutes) permits the transport and distribution of 18F-labeled radiopharmaceuticals to satellite centers and hospitals distant from the production site. This half-life makes it possible to complete complex syntheses and PET protocols with a duration of several hours, thereby allowing pharmacokinetic studies and analyses of metabolites.
The F atom is slightly larger than the H atom, hence the possible replacement of an H atom with an F atom does not entail substantial changes in molecular structure. Nevertheless, the electronegativity of the F atom (4.0) is greater than that of the H atom (2.1) – which impacts the molecule physicochemical properties. Another feature that reinforces F suitability in the labeling of radiopharmaceuticals is that the C-F bond is stronger and more stable in vivo than the C-H bond. As a result, the inclusion of F in the structure of biological molecules involves a prolongation of their half-lives in the organism. Thus, the inclusion of an F atom in a biological molecule alters its biodistribution, metabolization, protein-binding characteristics, etc. Nevertheless, the analogy with the natural substrate makes it possible to initiate the corresponding metabolic pathways and the subsequent metabolic block to facilitate the diagnostic study.
18-Fludeoxyglucose (FDG) is the most important PET radiopharmaceutical by virtue of its applications in the study of a broad range of diseases. Like glucose, FDG crosses the blood-brain barrier and penetrates faster into the cells, where it only undergoes a first-pass metabolization through the glycolytic pathway, i.e., C6 phosphorylation catalyzed by a hexokinase enzyme. The resulting compound, 2-FDG-6-P, is not an adequate substrate for phosphoglucose isomerase due to the lack of a hydroxyl group in C2 – thus impeding the isomerization of glucose to fructose in glycolysis. Consequently, in all tissues (except the liver), the FDG molecules that enter the cells become confined inside them through metabolic trapping, since the conversion to 2-FDG-6-P does not permit diffusion through the cell membrane. Unlike what occurs in the rest of the body cells, no such metabolic trapping of FDG is observed in the liver cells. This is because the liver does not use glucose as a primary energy source, and this organ is moreover in charge of regulating blood glucose levels – releasing glucose obtained from the dephosphorylation of phosphorylated derivatives (glucose-6-P and 2-FDG-6-P) catalyzed by the enzyme glucose-6-phosphatase. This activity is only significant in the liver.
18-Fludeoxyglucose is the main radiopharmaceutical used in the study of tumors, since it is an indicator of cell carbohydrate metabolism and therefore of cell proliferation. The concentration of glucose (and of 18FDG) in tumor cells is a consequence of their greater demand for adenosine triphosphate (ATP), obtained from carbohydrate metabolism, in order to maintain a high cellular growth and proliferation rate, which results in increased glycolysis and glucose uptake by the cells.
Synthesis of 18FDG: The synthesis of 18FDG takes place through nucleophilic substitution (SN2-type reaction) with the 18F- ion on a mannose-derived precursor, mannose triflate (1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulfonyl-b-D-mannopyranose), permitting very rapid synthesis and under mild conditions with few or no collateral reactions.
The 11C radionuclide is a positron emitter with a maximum energy of 960 keV and a radioactive half-life of only 20.4 minutes. The possibility of substituting the carbon present in biological molecules with a positron-emitting isotope makes it possible to obtain labeled compounds with biochemical and pharmacological properties identical to those of the natural molecule.
The extremely short radioactive half-life of 11C means that radiopharmaceuticals labeled with this radionuclide do not involve a substantial radiation exposure for the patient and allows the conduct of repeat studies in the same individual in a short time interval. Compounds labeled with 11C must be manufactured on site at the time of use – requiring the availability of a cyclotron facility very close to the hospital where the study is to be performed.
The 15O radionuclide is a positron emitter with a radioactive half-life of only 2.05 minutes, meaning that it can be used in multiple repeat studies in the same individual in periods of 15-20 minutes between successive tests. The emission energy of the 15O positron is 1720 keV, i.e., practically threefold that of 18F, and it consequently offers poorer resolution compared with fluoridated radiopharmaceuticals. Due to this radionuclide’s short radioactive half-life, the synthesis of complex radiopharmaceuticals is not viable. The only 15O-labeled PET radiopharmaceuticals used in the clinical setting are O2, CO, CO2, H2O or butanol in oxygen metabolism and blood flow studies.
15O-labeled gases for use as PET radiopharmaceuticals can be produced directly in the target or through the production of 15O and posterior conversion to carbonic gases by means of chain reactions with different catalysts. The gas produced is administered directly to the patient. Due to the technical difficulties involved in their manufacture and application, 15O-labeled radiopharmaceuticals are only used in limited and highly specialized settings.
The radioactive half-life of 13N is only 9.97 minutes, which precludes its use in applications involving complex synthetic processes. In fact, the incorporation of 13N into organic molecules is not as direct as in the case of C or F, and makes its use for diagnostic purposes practically unviable. For this reason, 13N-labeled radiopharmaceuticals are exclusively limited to 13NH3.
An important number of the radiopharmaceuticals used in nuclear medicine are 99mTc- labeled compounds. The physical characteristics of the radiation emitted by 99mTc and its chemical reactivity make it very adequate for clinical indications. The production of 99mTc is based on generators of 99Mo/99mTc in the form of the pertechnetate ion (99mTcO4-).
The preparation of radiopharmaceuticals labeled with 99mTc is generally carried out extemporaneously from cold reaction systems. These cold systems contain all the reagents needed to obtain the final radiopharmaceutical, combining it with the precursor radionuclide from a generator.
Most of these reaction systems are designed to yield a radiopharmaceutical labeled with 99mTc through the binding of reduced species of 99mTc with agents or molecules (binding or chelating elements) that act as carriers for the radionuclide to the biological structures studied.
There is a wide range of reaction systems with different binding or chelating elements, such as MAG3/tartrate for renal imaging, EDTA/ECD for brain imaging, and MIBI/citrate or tetrofosmin/gluconate for myocardial imaging.
Radiopharmaceuticals based on autologous samples are prepared from a biological component of the patient undergoing the diagnostic study. Some examples of autologous radiopharmaceuticals are those obtained as a result of the radioisotopic labeling of blood cells such as leukocytes, erythrocytes and platelets, and certain plasma components such as albumin.
The use of labeled blood components provides a noninvasive technique that Leverages the specific biodistribution patterns of each cell population in order to conduct kinetic or imaging diagnostic studies. Different methods for the labeling of erythrocytes, leukocytes, granulocytes, lymphocytes and platelets are available. The radionuclides most widely used for such cellular labeling are 99mTc, 111In and 51Cr, among others.