Abstract
Radioimmunotherapy (RIT) uses an antibody labeled with a radionuclide to deliver cytotoxic radiation to a target tumor cells. Radioiodine is most commonly employed to prepare radiolabeled proteins (antibodies, peptides) for in vitro and in vivo applications. A major shortcoming of radioiodinated proteins prepared by direct labeling methods is their deiodination in vivo. For the preparation of more stable radioiodinated antibodies, we developed a new linker (N-(4-isothiocyanatobenzyl)-2-(3-(tributylstannyl)phenyl) acetamide (IBPA). This study evaluated the usefulness of IBPA as a linker for the stable radioiodinated internalizing antibody, cetuximab. Directly labeled cetuximab ([125I]-cetuximab) was prepared by the chloramine T method. To prepare indirectly labeled cetuximab using IBPA ([125I]-IBPA-cetuximab), IBPA was radioiodinated using chloramine-T to give N-(4-isothiocyanatobenzyl)-2-(3-[125I]phenyl)acetamide ([125I]‑IBPA), which was purified by high performance liquid chromatography. [125I]-IBPA was then conjugated to cetuximab. In vitro target binding and internalizing assays were performed in PC9, LS174T, and FaDu cell lines. In vivo planar images were obtained using an Inveon SPECT scanner 3, 24, 48, and 168 h after i.v. injection of [125I]-cetuximab or [125I]-IBPA-cetuximab in athymic mice bearing LS174T tumor xenografts. Specific binding and internalized radioactivity of [125I]-IBPA-cetuximab were higher than those of [125I]-cetuximab in PC9, LS174T, and FaDu cell lines. In planar images scant radioactivity was evident in thyroid glands after injection of [125I]-IBPA-cetuximab, while a high level of radioactivity was present in thyroid glands after injection of [125I]-cetuximab. Tumor uptake value of [125I]-IBPA-cetuximab was higher than that of [125I]-cetuximab for up to 168 h. [125I]-IBPA-cetuximab is stable and resistant to deiodination in vivo. IBPA is a promising bi-functional linker for radioiodination of internalizing monoclonal antibodies for in vivo applications including radioimmunotherapy.