Iodine in PET studies
Monoclonal antibodies in immuno-PET and other large-molecule imaging agents have relatively slow pharmacokinetics; at least hours or even several days after injection are required to reach optimal biodistribution and image contrast. Radionuclides 124I and 89Zr have optimal half-lives, 4.18 and 3.27 days, respectively, for labelling these compounds. Additionally, the radiochemistry and in vivo behaviour of iodine (including isotopes 123I, 125I, and 131I) is well-investigated. In addition, the chemistry of iodine is very similar to that of bromine (Braghirolli et al., 2014).
Problems in using 124I include the high fraction of non-positron decays, leading to co-emission of photons with similar energy range than the annihilation photons, and the high energy of the emitted positrons, leading to long positron range and decreased image resolution, affecting especially preclinical PET scans (Belov et al., 2011; Lubberink and Herzog, 2011; Kuker et al., 2017).
Iodine label is usually added to activated aromatic rings, mostly phenols. In proteins, tyrosine is usually iodinated, and it is relatively stable in extracellular environments (Belov et al., 2011), although there are concerns on the in vivo dehalogenation of 124I-labelled antibodies. If iodinated compounds are transported into intracellular space, oxido-reductases, including iodotyrosine deiodinase (Friedman et al., 2006), eventually release iodide as a free ion, which is then rapidly cleared from cells and further from plasma into the thyroid gland and urine by the kidneys and into the stomach (Ullberg and Ewaldsson, 1964; Hays and Solomon, 1965). The transport of halides across plasma membranes is very fast.
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Updated at: 2018-12-17
Created at: 2017-06-22
Written by: Vesa Oikonen