Improved in Vivo Stability of Radioiodinated Rituximab Using an Iodination Linker for Radioimmunotherapy

Purpose: Directly radioiodinated [¹³¹I]-rituximab has been developed as a radioimmunotherapeutic agent in patients with CD20-positive B cell non-Hodgkin's lymphoma. However, there are concerns over its in vivo catabolism and deiodination. A novel radioiodination linker, N-(4-isothiocyanatobenzyl)-2-(3-(tributylstannyl)phenyl) acetamide (IBPA), was synthesized for the preparation of stable radioiodinated proteins. Methods: The authors evaluated the potential of IBPA as a stable radioiodinated linker for rituximab. [¹²⁵I]-IBPA was purified and conjugated with rituximab, and in vitro stability testing was performed in serum and liver microsomes. In vivo studies were performed after i.v. injection of [¹²⁵I]-rituximab or [¹²⁵I]-IBPA-rituximab to nude mice. Results: In in vitro studies, [¹²⁵I]-IBPA-rituximab was stable in serum and liver microsomes. In static scans, high radioactivity was evident in the thyroid following injection of [¹²⁵I]-rituximab, but low radioactivity was seen in the thyroid following injection of [¹²⁵I]-IBPA-rituximab. In biodistribution studies, radioactivity uptake in thyroid glands of [¹²⁵I]-IBPA-rituximab was decreased by approximately sevenfold compared to [¹²⁵I]-rituximab. In pharmacokinetics, the half-life of [¹²⁵I]-rituximab was shorter than that of [¹²⁵I]-IBPA-rituximab in plasma of nude mice. Conclusions: The authors demonstrate that [¹²⁵I]-IBPA-rituximab is more stable to metabolic deiodination in vivo than is [¹²⁵I]-rituximab. Radioiodination of rituximab using IBPA is thus preferable to direct labeling in terms of in vivo stability.