Abundant tyrosine residues in the antigen binding site in anti-osteosarcoma monoclonal antibodies TP-1 and TP-3: Application to radiolabeling

Enhanced tumor retention of radioiodinated anti-epidermal growth factor receptor antibody using novel bifunctional iodination linker for radioimmunotherapy

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.

Methods for radiolabelling of monoclonal antibodies

The use of radionuclide labels allows to study the pharmacokinetics of monoclonal antibodies, to control the specificity of their targeting and to monitor the response to an antibody treatment with high accuracy. Selection of label depends on the processing of an antibody after binding to an antigen, and on the character of information to be derived from the study (distribution of antibody in the extracellular space, target occupancy or determination of sites of metabolism). This chapter provides protocols for labelling of antibodies with iodine-125 (suitable also for other radioisotopes of iodine) and with indium-111. Since radiolabelling might damage and reduce the immunoreactive fraction and/or affinity of an antibody, the methods for assessment of these characteristics of an antibody are provided for control.

Preparation of N-succinimidyl 3-[*I]iodobenzoate: an agent for the indirect radioiodination of proteins

A procedure for the synthesis of N-succinimidyl 3-iodobenzoate labeled with any iodine isotope ([*I]SIB), which is an agent used in the radioiodination of proteins and peptides, from its tin precursor N-succinimidyl 3-(tri-n-butylstannyl)benzoate (STB) is described. Also included are protocols for the synthesis of an unlabeled standard of SIB and the tin precursor. Radioiododestannylation of STB using tert-butylhydroperoxide as the oxidant gives [*I]SIB in 80% radiochemical yields. The total time for the synthesis of [*I]SIB from STB is approximately 95 min. Use of [*I]SIB yields radioiodinated proteins that are considerably more stable in vivo than those radioiodinated by the direct electrophilic method.

Covalent disulfide-linked anti-CEA diabody allows site-specific conjugation and radiolabeling for tumor targeting applications

An engineered anti-carcinoembryonic antigen (CEA) diabody (scFv dimer, 55 kDa) was previously constructed from the murine anti-CEA T84.66 antibody. Tumor targeting, imaging and biodistribution studies in nude mice bearing LS174T xenografts with radiolabeled anti-CEA diabody demonstrated rapid tumor uptake and fast blood clearance, which are favorable properties for an imaging agent. Current radiolabeling approaches result in random modification of the protein surface, which may impair immunoreactivity especially for smaller antibody fragments. Site-specific conjugation approaches can direct modifications to reactive groups located away from the binding site. Here, cysteine residues were introduced into the anti-CEA diabody at three different locations, to provide specific thiol groups for chemical modification. One version (with a C-terminal Gly-Gly-Cys) existed exclusively as a disulfide-bonded dimer. This cysteine-modified diabody (Cys-diabody) retained high binding to CEA and demonstrated tumor targeting and biodistribution properties identical to the non-covalent diabody. Furthermore, following reduction of the disulfide bond, the Cys-diabody could be chemically modified using a thiol-specific bifunctional chelating agent, for radiometal labeling. Thus, the Cys-diabody provides a covalently linked alternative to conventional diabodies, which can be reduced and modified site-specifically. This format will provide a versatile platform for targeting a variety of agents to CEA-positive tumors.