Development of a novel linker for the preparation of stable radioiodinated antibody

Objectives Radioiodine is most commonly employed to prepare radiolabeled proteins with high specific activity for in vitro and in vivo applications. However, a major shortcoming of radioiodinated proteins prepared by direct labeling methods is deiodination in vivo. A new bifunctional linker for radiohalogenation of proteins, N-(4-Isothiocyanatobenzyl)-2-(3-(tributylstannyl)phenyl)acetamide(FCCS12026), was designed and synthesized. Herein we present our evaluative studies on using FCCS12026 for the radioiodination of an antibody and its comparison with directly labeled antibody.

Methods [125I]-Rituximab was prepared using the Chloramine T method. For indirect labeling FCCS12026 was radioiodinated using chloramine-T to give, N-(4-Isothiocyanatobenzyl)-2-(3-[125I]phenyl)acetamide([125I]-FCCS12027) which was purified by HPLC and conjugated with Rituximab. After injection of [125I]-Rituximab or [125I]-FCCS12027-Rituximab to nude mice, the static images were obtained at 24 and 48 h. Biodistribution studies of [125I]-Rituximab and [125I]-FCCS12027-Rituximab in nude mice were performed at 48 h postinjection.

Results The specific activities of [125I]-Rituximab and [125I]-FCCS12027-Rituximab were averaged each 51.6 MBq/nmole and 14.6 MBq/nmole. In planar images of mice, a high level of radioactivity was shown in thyroid after injection of [125I]-Rituximab but only few radioactivity was shown in thyroid after injection of [125I]-FCCS12027-Rituximab. The biodistribution studies also showed that the thyroid uptake was obviously decreased by injection of [125I]-FCCS12027-Rituximab instead of [125I]-Rituximab. The thyroid uptake value(%ID) after injection of [125I]-FCCS12027-Rituximab(0.12±0.01) was approximately 7-fold lower than that after injection of 125I-Rituximab(0.79±0.23).

Conclusions All the results indicate that the [125I]-FCCS12027-Rituximab is considerably more stable and resistant to deiodination in vivo. Therefore FCCS12027 is a promising bifunctional linker for radioiodination of proteins for in vivo applications including radioimmuno -imaging and -therapy.