Radioimaging on the ASK

• Overview
• Products and catalog numbers
• Labeling with Zr-89
• Labeling with I-124
• FAQs
• Citations

Overview

Positron emission tomography can be used to visualize antibodies and other biologics non-invasively, in vivo. PerkinElmer offers radionuclides for labeling biologics for use in in vivo imaging research. Two positron emitters, Zr-89 and I-124, are offered for molecular imaging using immuno-positron emission tomography (immuno-PET) or Cerenkov luminescence imaging (CLI) techniques. Both Zr-89 and I-124 are long-living positron emitters, providing an advantage over other emitters such as ¹⁸F and ⁸⁶Y when labeling large biomolecules (including therapeutic monoclonal antibodies) that accumulate more-slowly in vivo. Immuno-PET with Zr-89 and I-124 radionuclides enables noninvasive imaging of whole, intact antibodies.

Products and catalog numbers

For immuno-PET research applications, Zr-89 is commonly used for internalizing monoclonal antibodies because it is trapped inside the cell after internalization of the antibody (residualization). I-124 is commonly used for labeling non-internalizing antibodies, as it is non-residualizing. The target antibody is labeled with I-124 using direct labeling methods. Both ¹²⁴I and ⁸⁹Zr have a relatively long half-life (beyond that of 18F, Gallium-68, ⁶⁴Cu, or ⁸⁶Y), enabling imaging at later timepoints needed for larger research biologics.

Labeling antibodies with Zr-89

Antibodies are typically labeled indirectly with Zr-89 using a chelator, though direct labeling approaches have been described for labeling other biomolecules with Zr-89. The most common method for labeling is using Desferrioxamine B  (DFO). DFO is available as a salt, or in reactive forms such as isothiocyanate (ITC; labels lysine residues), maleimide (labels thiol groups/cysteine residues), and tetrafluorophenolic ester/Fe³⁺ derivatives (labels lysine residues). 

Labeling antibodies with I-124

Antibodies are typically labeled directly via radioiodination of tyrosine residues with electropositive radioiodine. Chloramine T and Iodogen^® methods can be used for oxidation of the ¹²⁴I radioiodide, for direct antibody labeling.

FAQs

Products are for research use only. Not for use in diagnostic procedures.

Q. Which PET isotope is the most suitable for my application?
A. This depends on the antibody you want to image. If it’s an internalizing antibody, then use residualizing Zr-89.

Q. Is Desferal (Df) the only suitable chelator for stable incorporation of Zr-89?
A. Yes, DOTA, NOTA or DTPA are not suitable for stable incorporation of Zr-89.

Q. Which Df chelator is best option for my research?
A. This depends on the type of research. The TFP-nSuc-Df chelator is used in clinical trials safely for many years and is commercially available at ABX (www.ABX.com), but requires the extra Fe-removal step at pH 4. The Df-Bz-NCS chelator was recently introduced in the clinic and is commercially available at Macrocyclics. Both of these are suitable for use in research applications.

Q. What are the minimum quantities of mAb to obtain reliable labelling yields?
A.

• Zr-89: we prefer to start the conjugation of the Df-chelator to the antibody with at least 2 mg mAb, and then subsequently radiolabel at least 0.5 mg mAb-Df.
• I-124: with the Iodogen method it is possible to radiolabel small amounts of mAb (starting with 50 µg) up to 5 mg.

References

1. Fischer, G., Seibold, U., Schirrmacher, R., Wängler, B. & Wängler, C. (89)Zr, a radiometal nuclide with high potential for molecular imaging with PET: chemistry, applications and remaining challenges. Molecules18, 6469–6490 (2013). Link
   
2. Treglia, G. & Salsano, M. PET imaging using radiolabelled antibodies: future direction in tumor diagnosis and correlate applications. Research and Reports in Nuclear Medicine 9 (2013).
   
3. Verel, I. et al. Long-lived positron emitters zirconium-89 and iodine-124 for scouting of therapeutic radioimmunoconjugates with PET. Cancer Biother. Radiopharm.18, 655–661 (2003). Link
   
4. Wu, A. M. & Olafsen, T. Antibodies for molecular imaging of cancer. Cancer J14, 191–197 (2008). Link
   
5. Koehler, L., Gagnon, K., McQuarrie, S. & Wuest, F. Iodine-124: a promising positron emitter for organic PET chemistry. Molecules15, 2686–2718 (2010). Link