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 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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References:

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Cascini GL, Niccoli Asabella A, Notaristefano A, Restuccia A, Ferrari C, Rubini D, Altini C, Rubini G. 124Iodine: a longer-life positron emitter isotope - new opportunities in molecular imaging. Biomed Res Int. 2014: 672094.

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Friedman JE, Watson JA Jr, Lam DW-H, Rokita SE. Iodotyrosine deiodinase is the first mammalian member of the NADH oxidase/flavin reductase superfamily. J Biol Chem. 2006; 281: 2812-2819.

Hays MT, Solomom DH. Influence of the gastrointestinal iodide cycle on the early distribution of radioactive Iodide in man. J Clin Invest. 1965; 44: 117-127.

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Lubberink M, Herzog H. Quantitative imaging of 124I and 86Y with PET. Eur J Nucl Med Mol Imaging 2011;38(Suppl 1): S10-18.

Pentlow KS, Graham MC, Lambrecht RM, Cheung NK, Larson SM. Quantitative imaging of I-124 using positron emission tomography with applications to radioimmunodiagnosis and radioimmunotherapy. Med Phys. 1991; 18(3): 357-366.

Surti S, Scheuermann R, Karp JS. Correction technique for cascade gammas in I-124 imaging on a fully-3D, time-of-flight PET scanner. IEEE Trans Nucl Sci. 2009; 56(3): 653-660.

Ullberg S, Ewaldsson B. Distribution of radio-iodine studied by whole-body autoradiography. Acta Radiol Ther Phys Biol. 1964; 2: 24-32.

Verel I, Visser GW, Boerman OC, van Eerd JE, Finn R, Boellaard R, Vosjan MJ, Stigter-van Walsum M, Snow GB, van Dongen GA. Long-lived positron emitters zirconium-89 and iodine-124 for scouting of therapeutic radioimmunoconjugates with PET. Cancer Biother Radiopharm. 2003; 18(4): 655-661.



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Created at: 2017-06-22
Updated at: 2018-01-21
Written by: Vesa Oikonen