Six Methods of Radiolabeling

Method of Labeling

Examples
Isotope Exchange
14C-, 35S-, and 3 H-labeled Compounds
Introduction of a Foreign Label
All 99mTc-compounds
111In-labeled compounds
18FDG
Labeling Bi-functional Chelating
111In-DTPA-Octreotide
99mTc-DTPA-MoAb
Biosynthesized
57Co-cyanocobalamin
14C-labeled compounds
Recoiling Labeling
Iodinated compounds
Excitation Labeling
123I-labeled compounds
  1. General comments
    1. Looks at the isotopes of iodine
      1. 123I, 124I, 125I, 127I, and 131I
      2. There is no biological difference when replacing one isotope from the other because they "behave" the same. (Question - Do the different iodines behave differently (metabolically) when injected into a human system?
      3. However, the main consideration with the different iodines relates to: the type of radiation being emitted, the amount of energy associated with it, and its T1/2
      4. Consider - Therapy or diagnostic? Which of the above iodines would you use?
      5. Should radiation dosimetry be considered?
  2. Isotope Exchange Reactions
    1. This is where a radioactive isotope exchanges with a stable isotope (Example - 123I, 124I, 125I, 127I, and 131I)
      1. Example of this process123I → 127I
      2. If you had a 1:1 ratio then the exchange that would occur would be 50%. A 20:1 ratio would yield a 95% exchange
      3. Example of the exchange where one of the paired iodines is radioactive: R-H + I2 → R-I + HI
      4. Example of radiopharmaceutical - I123 or I131MIBG
      5. This technique also applies to areas of invitro nuclear medicine. Example 125I(T3/T4)
  3. Introduction to foreign label
    1. A radionuclide is labeled to a molecule that has a known biological function
    2. Usually has a covalent or coordinate covalent bond
    3. Tagging of the radionuclide to the molecule of interest is unusual (doesn't occur in nature)

    4. Chelation - is a process in which one atom donates a pair of elections to the other (foreign) atom. Usually a transition metal
    5. Examples include:51Cr-labeled RBCs, 18FDG, 99mTcDTPA, and most 99mTc- labeled compounds
  4. Labeling with Bi-functional chelating agent
    1. Bi-functional chelating agent occurs when a radionuclide shares a union with another macromolecule (ex. Protein, MoAb)
    2. Either of two methods can done meet this process
      1. 99mTc chelate method - a chemical is used to establish the chelation. Diamidoldithiol, cyclam are examples of these types of agents, where a bond is established between the chelating agent and the protein
      2. Indirect chelate-antibody method
        1. Bi-functional chelating agent is conjugated with a macromolecule (Ex. MoAb), reacts with a metal-chelate-macromolecule complex
        2. Example 111In-DTPA-MoAb or 111In-DTPA-Octreotide
  5. Biosynthesis
    1. Radioactive material is introduced into a growth medium in which (Ex) bacteria grows
    2. Radioactive metabolites are produced as a byproduct
    3. Radioactive metabolites are chemically removed and can be applied
    4. 57Co-B12 as an example
  6. Recoil labeling
    1. Recoiled atoms or ions are produced in a target material by particles that are initially emitted from a radioactive material. These particles cause the atoms in the surrounding medium to recoil and rebonds generating a different molecule
    2. This process is rarely done, has a low specific activity of the bound product
  7. Excitation labeling
    1. Occurs in a highly energetic process, (Ex) β
    2. Particle ionize surrounding atoms causing ions to form resulting in new new bonds with other compounds
    3. Low yield

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