Theoretical Basics of Radiopharmacy PDF

Theoretical basics of radiopharmacy Prepared by: Maryam Sawalha  Radiopharmacy  Radiopharmacy is the art of preparing high-quality, radioactive, medicinal products for use in diagnosis and therapy.  The production and handling of radiopharmaceuticals requires specific expertise.  Differen...

Theoretical basics of radiopharmacy Prepared by: Maryam Sawalha  Radiopharmacy  Radiopharmacy is the art of preparing high-quality, radioactive, medicinal products for use in diagnosis and therapy.  The production and handling of radiopharmaceuticals requires specific expertise.  Different aspects need to be taken into consideration, including:  correct usage,  storage of rapidly decaying diagnostic radionuclides,  disposal of radioactive waste when using longer-lived therapeutic radionuclides and  quality controls. In this chapter we will focus on the theoretical basics of radiopharmacy. prepared by: Maryam Sawalha  The starting point in the preparation of a radiopharmaceutical always depends on the goal, i.e. what we want to see or treat.  This can be the function of an organ/system, a characteristic biomarker of a disease or a hallmark of cancer.  Vector molecules have to be used against these targets, and have to demonstrate sufficient specificity to allow an appropriate contrast.  The vector molecule is labelled with a radioactive flag, i.e. a radionuclide.  This step in radiopharmaceutical production is commonly referred to as radiolabelling, and it starts with the choice of the most appropriate radionuclide. prepared by: Maryam Sawalha  CHOICE OF RADIONUCLIDE  The choice of radionuclide is predominantly based on three factors: a. The purpose for which the radiopharmaceutical is to be used b. The compatibility of the radionuclide with the vector molecule c. The availability and price of the radionuclide prepared by: Maryam Sawalha Purpose of using the radiopharmaceutical  Radionuclides are unstable atoms that attempt to attain a stable state through the release of ionising energy in the form of alpha (α), beta (β–/+) particles or gamma (γ).  The nature of each of these forms of emission and their characteristics determine the purpose (therapy or imaging) for which particular radionuclides are best suited.  A radionuclide can need one or multiple steps to reach a stable atom, leading to a decay scheme containing different emissions. prepared by: Maryam Sawalha  Alpha particles  Alpha particles are helium nuclei consisting of two protons and two neutrons.  They are heavy particles and as a consequence of their large size they interact very strongly with matter, more so than other particles, and deposit their energy on a short path length [linear energy transfer (LET )].  Accordingly, they are much more potent for therapeutic purposes than other emission types. prepared by: Maryam Sawalha  Alpha particles  With regard to radioprotection they are stopped without difficulty (a simple piece of paper will suffice), making them more difficult to detect.  The alpha-emitting radionuclides used in nuclear medicine also have gamma emission in their decay scheme, allowing detection.  To date only one alpha radiopharmaceutical has been approved for clinical use (radium-223 or Xofigo), but several others are either in clinical trials or under development. prepared by: Maryam Sawalha  Beta particles  Beta particles are smaller charged particles and are either electrons (β-) or positrons (β+).  Electrons are used for therapy and the positrons for imaging.  The smaller size of β- compared with alpha particles means: - that they have a lower LET and are thus less potent in causing cell damage. - this can also be an advantage in terms of toxicity. - allow the treatment of larger tumours compared with alpha particles.  To stop β- particles, a denser material than paper is needed, such as Plexiglas or aluminium. prepared by: Maryam Sawalha  Beta particles  β+ particles are used for imaging purposes.  In fact, however, it is not the β+ particles themselves that are imaged, but secondary emitted gamma rays:  When a β+ particle encounters an electron, a phenomenon termed annihilation takes place, whereby the mass of both particles is converted into energy and two gamma photons (511 keV) are emitted in diametrically opposed paths. These are the gamma photons that are detected by PET. prepared by: Maryam Sawalha  Gamma rays  Gamma rays are electromagnetic emissions that are used for imaging.  They have the least interaction with matter and thus can be detected outside the body, in contrast to the other types of emission.  To stop gamma rays, even denser material is needed, such as lead. prepared by: Maryam Sawalha  Compatibility of the radionuclide with the vector molecule  a radionuclide is characterized by: - the type of emission, - physical half-life (t1/2 ), i.e. the time needed for half of the atoms to decay and reach the stable state. prepared by: Maryam Sawalha  Compatibility of the radionuclide with the vector molecule  The physical t1/2 of a radionuclide has to be sufficiently long to allow (1) the radiolabelling process, (2) the performance of quality control testing (excluding sterility), (3) administration to the patient and (4) adequate distribution of the radiopharmaceutical. (determined by the biological t1/2 of the vector molecule).  To obtain satisfactory contrast and thereby allow correct diagnosis or successful therapy >> the physical t1/2 of the radionuclide and the biological t1/2 of the radiotracer should be compatible. prepared by: Maryam Sawalha  Availability and price of the radionuclide  depend on their mode of production.  While there are several naturally occurring radionuclides, in the medical field radionuclides are synthetically produced by nuclear reactors, accelerators (cyclotrons) or generators. prepared by: Maryam Sawalha RADIOLABELLING METHODS TO OBTAIN RADIOPHARMACEUTICALS  The radiolabelling method used to obtain a radiopharmaceutical is mainly dictated by the choice of radionuclide.  Some radionuclides will allow direct radiolabelling of the vector molecule whereas with others only indirect radiolabelling is possible. prepared by: Maryam Sawalha  Direct radiolabelling of the vector molecule  Direct radiolabelling can be achieved through either substitution or chelation.  Substitution: An atom/group on the molecule is substituted by the radionuclide. This is achieved through an exchange between radioactive ions and non- radioactive groups in the molecule.  The exchange can be with either an anion or a cation.  The reaction itself is then called a nucleophile or an electrophile substitution, respectively.  A well-known example of nucleophile substitution is the production of 18Fluorodeoxyglucose (FDG) with fluorine-18 (18F). prepared by: Maryam Sawalha Direct radiolabelling of the vector molecule  Chelation: A molecule can have groups that can chelate cations.  These groups contain electron donors that have a free electron pair that enable coordination binding (O, S and N).  An example is the direct radiolabelling of molecules in kits with 99mTc.  It is to be noted that in order to enter such reactions the radioactive ion should first be in the correct oxidation state, through either reduction or oxidation. prepared by: Maryam Sawalha Indirect radiolabelling of the vector molecule  When there is no favourable way to radiolabel the vector molecule directly, the molecule first has to be modified with a bifunctional chelator (and a spacer) to allow radiolabelling through chelation.  The common traits required for a good bifunctional chelator are: Stable and kinetically inert: to avoid hydrolysis/trans-metalation/ chelation in vivo (e.g. binding of gallium-67 citrate to transferrin) Fast complexation: to allow the use of short-lived radionuclides Versatile conjugation chemistry Accessibility prepared by: Maryam Sawalha  METHODS FOR SYNTHESISING RADIOPHARMACEUTICALS  Indirect or direct radiolabelling methods can be performed by means of (a)Manual synthesis (b)Automatic synthesis (c)Kit-based synthesis. It is to be noted that, independent of the method, the vast majority of radiopharmaceuticals are administered intravenously and aseptic techniques should therefore be adopted. prepared by: Maryam Sawalha  Manual synthesis  Manual synthesis is less and less common these days, but manual approaches are still employed at the start of the process of developing new radiopharmaceuticals.  Although they have limitations in respect of radioprotection, they offer the flexibility to adapt and improve the synthesis through a trial and error approach.  Manual synthesis requires specific expertise, dexterity and constancy, and if these are lacking then the reproducibility of the synthesis outcome will be poor. prepared by: Maryam Sawalha  Automated synthesis  Automated synthesis is based on the use of synthesis modules to allow the automation of the radiolabelling, purification and sterile filtration of the radiopharmaceutical.  The systems used for automated synthesis are easily compatible with Good Manufacturing Practice.  Compared to manual synthesis, automated synthesis offers: - documentation of the manufacturing process and the use of disposable (sterile) cassettes, allowing full recording of the process and reducing the risk of cross-contamination. - higher reproducibility and lower radiation burden to the operator. prepared by: Maryam Sawalha Automated synthesis  the procedure for automated synthesis comprises the following key steps: 1. Preparation of the GMP cassettes: connect vials, perform self-tests and add precursor, reagents, buffer, etc. 2. Radionuclide transfer from cyclotron, generator or vial into the cassette 3. Radiolabelling: chemical reaction between the radionuclide and the vector molecule during an incubation at the desired temperature for a certain time. 4. Purification of the reaction mixture: separation of the radiopharmaceutical of interest from the rest of the components in the reaction mixture (free radionuclide, buffer, etc.) by means of solid-phase extraction or high-performance liquid chromatography. 5. Sterile filtration (0.22-µm filter) 6. Ready for patient injection prepared by: Maryam Sawalha  Kit-based synthesis  Kit-based synthesis consists in the reconstitution of proprietary single vials containing sterile, lyophilised precursor, buffer and scavenger.  This represents an all- in-one easy-to-use approach that avoids the need for expensive equipment and lengthy procedures.  Kit-based synthesis requires limited expertise, is very safe, is reproducible and is less cumbersome than the other radiolabelling approaches.  It also results in important reductions in investment and production costs.  To this day, the lion’s share of radiopharmaceutical preparation in conventional nuclear medicine is by means of kit-based synthesis. prepared by: Maryam Sawalha QUALITY OF RADIOPHARMACEUTICALS  Workplace Within the workplace, the basic requirement is to ensure that the radio pharmaceuticals are prepared under the best conditions. In particular: The working space should be clean. Machines should be well maintained and calibrated. Precursors and other materials should be conserved appropriately. The laminar air flow should be maintained and tested regularly. prepared by: Maryam Sawalha  QUALITY OF RADIOPHARMACEUTICALS  Reception All incoming goods should pass a basic quality check at reception: Packaging should be inspected. Conformity assessment: Accuracy of the information on the documentation of incoming goods should be assessed (name, quantity, activity, certificate of analysis, leaflet, etc.). A swab/wipe test should be performed on delivered radionuclides or generators to ensure their integrity. prepared by: Maryam Sawalha  QUALITY OF RADIOPHARMACEUTICALS  Radiopharmaceutical  Quality control of drugs is of extreme importance and is performed thoroughly by the pharma industry.  Most radiopharmaceuticals are prepared in hospital settings, outside of the pharma industry.  Similarly to other drugs for intravenous injection, the appearance and the pH of the radiopharmaceutical have to be assessed. prepared by: Maryam Sawalha prepared by: Maryam Sawalha  PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS  Radiopharmaceuticals are used in nuclear medicine to study specific targets and biological functions.  The mechanisms underlying their accumulation in the human body are based on ten fundamental principles: prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS a. Diffusion: The radiopharmaceutical crosses the cell membrane passively.  Diffusion is driven by the concentration difference between the two sides of the cell membrane.  Example: Technegas accumulation in the lungs during a ventilation study. prepared by: Maryam Sawalha  PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS b. Ion exchange and transport: The radiopharmaceutical accumulates in cells passively, via protein channels and carrier proteins present on the cell membrane.  This phenomenon can also be referred to as facilitated diffusion.  Example: Technetium pertechnetate (99mTcO4-) accumulation during examination of the thyroid gland. prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS c. Active transport: In contrast to the passive use of protein channels and carrier proteins, active transport requires energy (ATP) to allow the radiopharmaceutical to cross the cell membrane.  Example: Radioactive iodine accumulation in the thyroid gland for imaging or therapy. prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS d. Compartmentalisation: The radiopharmaceutical is restricted to the vascular compartment.  Example: Use of radiolabelled red blood cells (RBCs) for determination of the blood pool or the investigation of occult bleeding. prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS e. Cell trapping:  The radiopharmaceutical is entrapped in the organ.  Example: While passing the spleen, RBCs will endure a stress test to ensure their integrity; if they fail the test, they will remain in the spleen to die.  Radiolabelled RBCs damaged by heat are sequestered into the spleen and thus can be used to image residual or ectopic spleen after splenectomy. prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS f. Phagocytosis:  The radiopharmaceutical is phagocytosed in the reticuloendothelial system (RES; liver, spleen and bone marrow).  Example: Radiolabelled colloids for imaging of the bone marrow. prepared by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS g. Capillary blockage:  The radiopharmaceutical is entrapped in the capillaries owing to its size.  Example: Macro-aggregated albumin radiolabelled with 99mTc (99mTc-MAA) for lung perfusion prepared by: Maryam Sawalha  PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS h. Metabolic trapping: Accumulation of the radiopharmaceutical is based on the enhanced metabolism of the cell.  While the radiopharmaceutical mimicking a building block/energy source of the cell enters the cell and undergoes the first step of metabolisation (e.g. phosphorylation), it is not completely metabolised and accumulates in the cell.  The cell can demonstrate enhanced glucose, protein or lipid metabolism.  Example: 18F-FDG PET to investigate the enhanced glucose metabolism of prepared cancer cells. by: Maryam Sawalha PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS i. Ligand binding:  The radiopharmaceutical is a ligand that will specifically bind to a determined receptor/enzyme.  Example: 68Ga/177Lu-DOTATATE (somatostatin analogue) for imaging/ treatment of neuroendocrine tumours. prepared by: Maryam Sawalha  PRINCIPLES OF ACCUMULATION OF RADIOPHARMACEUTICALS j. Ab-Ag complexes: The radiopharmaceutical is an antibody or antibody fragment/derivative that will recognise and bind an antigen with high specificity.  Examples: ImmunoPETwith 89Zr-trastuzumab against HER2in a patient with HER2-positive breast cancer. prepared by: Maryam Sawalha