Radiolabeling

1. PRINCE SATTAM BIN ABDUL AZIZ UNIVERSITY COLLEGE OF PHARMACY Nuclear Pharmacy (PHT 433 ) Radiolabeling • Dr. ShahidJamil L8- L9

2. Radiolabeling L8- L9

3. Radiolabeling • The use of compounds labeled with radionuclides has grown considerably in medical, biochemical, and other related fields. • In the medical field, compounds labeled with β- -emitting radionuclides are mainly restricted to in vitro experiments and therapeutic treatment, whereas those labeled with γ-emitting radionuclides have much wider applications for in vivo imaging of different organs. L8- L9

4. In a radiolabeled compound, atoms or groups of atoms of a molecule are substituted by similar or different radioactive atoms or groups of atoms. • In any labeling process, a variety of physicochemical conditions can be employed to achieve a specific kind of labeling. • There are six major methods employed in the preparation of labeled compounds for clinical use. L8- L9

5. General methods of radiolabeling. Isotope Exchange Reactions • In isotope exchange reactions, one or more atoms in a molecule are replaced by isotopes of the same element having different mass numbers. • Since the radiolabeled and parent molecules are identical except for the isotope effect, they are expected to have the same biologic and chemical properties. L8- L9

6. These labeling reactions are reversible and are useful for labeling iodine-containing material with iodine radioisotopes and for labeling many compounds with tritium. Examples: 125I-labeled triiodothyronine (T3), 125I-labeled thyroxine (T4), 14C-, 32S-, and 3H-labeled compounds. L8- L9

7. Introduction of a Foreign Label • In this type of labeling, a radionuclide is incorporated into a molecule that has a known biologic role, primarily by the formation of covalent or coordinate covalent bonds. • The tagging radionuclide is foreign to the molecule and does not label it by the exchange of one of its isotopes. Some examples are 99mTc-labeled albumin, 99mTc-DTPA, 51Cr-labeled red blood cells, and many iodinated proteins and enzymes. L8- L9

8. In several examples, the in vivo stability of the material becomes uncertain and one should be cautious about any alteration in the chemical and biologic properties of the labeled compound. • In some instances, a chemically analogous radionuclide can be substituted for an atom already present in the molecule; for example, 75Se can replace sulfur in methionine to form 75Se-selenomethionine. L8- L9

9. In many compounds of this category, the chemical bond is formed by chelation, that is, more than one atom donates a pair of electrons to the foreign acceptor atom, which is usually a transition metal. • Most of the 99mTc-labeled compounds used in nuclear medicine are formed by chelation. • Example, 99mTc binds to DTPA, gluceptate, and other ligands by chelation. L8- L9

10. Labeling with Bifunctional Chelates • Bifunctional chelates such as EDTA, DTPA, and desferoxamine have been used successfully in the labeling of various proteins. • In this method, proteins are allowed to form complexes with the bifunctional chelating agent and the complex is then labeled by chelation with an appropriate radionuclide. • Examples: 111In-labeled DTPA-albumin, 67 Ga-labeled desferoxamine-albumin, 99mTc-labeled DTPA-antibody. • Because of the presence of the chelate, the biological properties of the labeled protein may be altered and must be assessed before clinical use. L8- L9

11. Biosynthesis • In biosynthesis, a living organism is grown in a culture medium containing the radioactive tracer. • The tracer is incorporated into metabolites produced by the metabolic processes of the organism, and the metabolites are then chemically separated. • Example, vitamin B12 is labeled with 6OCo or 57Co by adding the tracer to a culture medium in which the organism Streptomyces griseus isgrown. • Other examples, 14C-labeled carbohydrates, proteins and fats, and L-75Se-selenomethionine. L8- L9

12. Recoil Labeling • Recoil (retreat) labeling is of limited interest because it is not used on a large scale for labeling. • In a nuclear reaction, when particles are emitted from a nucleus, recoil atoms or ions are produced that can form a bond with other molecules present in the target material. • The high energy of the recoil atoms results in poor yield and thus a low specific activity of the labeled product. Several tritiated compounds can be prepared in the reactor by the 6Li (n, α) 3H reaction. L8- L9

13. The compound to be labeled is mixed with a lithium salt and irradiated in the reactor. • Tritium produced in this reaction will label the compound by the isotope exchange mechanism and then the labeled compound is separated. L8- L9

14. Excitation Labeling • Excitation labeling entails the utilization of radioactive and highly reactive daughter ions produced in a nuclear decay process. • During βdecay or electron capture, energetic charged ions are produced that are capable of labeling various compounds of interest. • Krypton-77 decays to 77Br and, if the compound to be labeled is exposed to 77Kr, then energetic 77Br ions label the compound to form the brominated compound. Various proteins have been iodinated with 123I by exposing them to 123Xe, which decays to 123I. • The yield is low with this method. L8- L9

15. L8- L9

16. Important Factors in Labeling The majority of radiopharmaceuticals used in clinical practice are relatively easy to prepare in ionic, colloidal, macroaggregated, or chelated forms. Many of them can be made using commercially available kits. L8- L9

17. Several factors that influence the integrity of labeled compounds are: Efficiency of the Labeling Process Chemical Stability of the Product Denaturation or Alteration Isotope Effect Carrier-Free or No-Carrier-Added (NCA) State Storage Conditions Radiolysis Purification and Analysis Shelf Life L8- L9

18. Efficiency of the Labeling Process • A high labeling yield is always desirable, although it may not be possible in many cases. • The higher the yield, the better the method of labeling. But a lower yield is sometimes acceptable if the product is pure and not damaged by the labeling method, the expense involved is minimal, and no better method of labeling is available. L8- L9

19. Chemical Stability of the Product • Stability is related to the type of bond between the radionuclide and the compound. • Compounds with covalent bonds are relatively stable under various physicochemical conditions. The stability constant of the labeled product should be large for greater stability. L8- L9

20. Denaturation or Alteration • The structure and/or the biologic properties of a labeled compound can be altered by various physicochemical conditions during a labeling procedure. • Example, proteins are denatured by heating, at pH below 2 and above 10, and by excessive iodination. Red blood cells are denatured by heating. L8- L9

21. Isotope Effect • The isotope effect results in different physical and biologic properties due to differences in isotope weights. • Example: In tritiated compounds, H atoms are replaced by 3H atoms and the difference in mass numbers of 3H and H may alter the property of the labeled compounds. • Also the physiologic behavior of tritiated water is different from that of normal water in the body. The isotope effect is not serious when the isotopes are heavier. L8- L9

22. Carrier-Free or No-Carrier-Added (NCA) State Radiopharmaceuticals tend to be adsorbed on glassware if they are in a carrier-free or NCA state. The molar concentration of carrier-free compounds is in the range of nanomolar or less, and it is very difficult to study their chemical behavior in such a low concentration. L8- L9

23. Storage Conditions • Many labeled compounds are susceptible to decomposition at higher temperatures. • Proteins and labeled dyes are degraded by heat and therefore should be stored at proper temperatures; • Example: Albumin should be stored under refrigeration. • Light may also break down some labeled compounds such as radioiodinated rose bengal thus should be stored in the dark. • The loss of carrier-free tracers by adsorption on the walls of the container can be prevented by the use of silicon-coated vials. L8- L9

24. Radiolysis • Many labeled compounds are decomposed by radiations emitted by the radionuclides present in them. • This kind of decomposition is called radiolysis. • The higher the specific activity, the greater the effect of radiolysis. • When the chemical bond breaks down by radiations from its own molecule, the process is termed "autoradiolysis". • Radiations may also decompose the solvent, producing free radicals that can break down the chemical bond of the labeled compounds; this process is indirect radiolysis. L8- L9

25. Example: radiations from a labeled molecule can decompose water to produce hydrogen peroxide or perhydroxyl free radical, which then oxidizes another labeled molecule. • To prevent indirect radiolysis, the pH of the solvent should be neutral because more reactions of this nature can occur at alkaline or acidic pH. • The longer the half-life of the radionuclide, the more extensive is the radiolysis, and the more energetic the radiations, the greater is the radiolysis. L8- L9

26. Radiolysis introduces a number of radiochemical impurities in the labeled material and one should be cautious about these unwanted products. • These factors set the guidelines for the expiration date of a radiopharmaceutical. L8- L9

27. Purification and Analysis Radionuclide impurities are radioactive contaminants arising from the method of production of radionuclides. • Fission method produce more impurities than nuclear reactions in a cyclotron or reactor because there are numerous modes of fission of the heavy nuclei. • Target impurities also add to the radionuclidic contaminants. • Radiochemical and chemical impurities arise from incomplete labeling of compounds. L8- L9

28. Often these impurities arise after labeling from natural degradation as well as from radiolysis. • Radionuclide impurities can be estimated by various analytical methods such as solvent extraction, ion exchange, paper, gel, or thin-layer chromatography, and electrophoresis. • The removal of radioactive contaminants can be accomplished by various chemical separation methods, usually at the radionuclide production stage. L8- L9

29. Shelf Life A labeled compound has a shelf life during which it can be used safely for intended purpose. The loss of efficacy of a labeled compound over a period of time may result from radiolysis and depends on: • The physical half-life of the radionuclide. • The solvent • Additives • The labeled molecule • The nature of emitted radiations • The nature of the chemical bond between the radionuclide, and the labeled compound. L8- L9

30. Usually a period of three half-lives or a maximum of 6 months is suggested as the limit for the shelf life of a labeled compound. The shelf-life of 99mTc-compounds varies between 0.5 and 18 hr. L8- L9