(M1130-02-07) Comparative In Vivo Stability of Tumor Targeting Antibodies Labeled Using Three Distinct Radioiodination Methods

Purpose: Biodistribution of large molecule therapeutics is often characterized in preclinical models using radionuclides. One of the most common methods to radiolabel is indirect iodination of tyrosines by aromatic electrophilic halogenation after iodinium (I+) is generated from iodide (I-) by the oxidizing agent, Iodogen. However, iodotyrosine (IT) modifications are subject to dehalogenation. Dehalogenases are predominantly enriched in the thyroid, but have also been found in the liver and kidney (1-4). In an effort to reduce susceptibility to in vivo dehalogenation, other methods of iodination have been developed wherein radiolabeled prosthetic groups such as Bolton-Hunter (BH) and N-succinimidyl 3-[125 I]iodobenzoate (SIB) can be conjugated through lysine residues. While these alternative iodination methods have been available for some time, they have never been directly compared to IT labeling in a systematic study.

Methods: We used a dual (¹²⁵I/ ¹¹¹In) isotope approach to evaluate the impact of iodination of HER2 targeting antibody in a KPL4 tumor xenograft mouse model.

Results: A direct comparison of IT, BH and SIB iodinated antibodies shows consistent systemic exposure between iodination methods. Tumor catabolism was comparable across all groups, indicating that efflux of ¹²⁵I-containing catabolites following receptor mediated catabolism was similar across all groups. Analysis of thyroid showed a persistent increase in ¹²⁵I enrichment only for the IT group. We confirmed this process to be independent of receptor mediated catabolism by showing the same trend for a non-binding antibody labeled analogously.

Conclusion: Overall, although the iodination methods evaluated did not affect exposure of the iodinated antibodies, SIB and BH methods were more resistant to dehalogenation and subsequent thyroid accumulation, but the value gained over IT labeling depends on the context of the study and tissue to be explored.

References: 1) Roche, J., Michel, R., Michel, O., and Lissitzky, S. 1952 Enzymatic dehalogenation of iodotyrosine by thyroid tissue; on its physiological role Biochem. Biophys. Acta 9, 161– 169.
2) Tong, W., Taurog, A., and Chaikoff, I. L. 1954 The metabolism of I131-labeled diiodotyrosine. J. Biol. Chem. 207, 59– 76.
3) Stanbury, J. B. 1957 The requirement of monoiodotyrosine deiodinase for triphosphopyridine nucleotide. J. Biol. Chem. 228, 801– 811.
4) Stanbury, J. B., and Morris, M. L. 1958 Deiodination of diiodotyrosine by cell-free systems. J. Biol. Chem. 233, 106– 108.