Cell-based assessment of radioiodinated immunoconjugate via a novel linker for enhancing residualizing power

Objectives As reported earlier, we have developed a new linker, (N-(4-isothiocyanatobenzyl)-2-(3-(tributyl stannyl) phenyl) acetamide (FCCS12026), for the preparation of more stable radioiodinated antibodies. Herein, we have evaluated utility of the FCCS12026 to cetuximab, an internalizing monoclonal antibody, on the aspect of target binding and internalizing characteristics.

Methods [¹²⁵I]-Cetuximab was prepared using the Chloramine T method. For indirect labeling of cetuximab, FCCS12026 was labeled with ¹²⁵I by chloramine T to give, N-(4-Isothiocyanatobenzyl)-2-(3-[¹²⁵I]phenyl)acetamide ([¹²⁵I]-FCCS12027), and conjugated with cetuximab. Cell binding and internalization assays were performed in 6 cell lines (FaDu, H1376, A431, NCI-H727, NCI-H1299, NCI-H460). And the immunoreactive fraction was estimated from the cell binding data by the Lindmo method.

Results [¹²⁵I]-FCCS12027-Cetuximab had 1.8~4.5 fold higher specific binding values than [¹²⁵I]-Cetuximab in 6 cell lines. Averaged immunoreactive fractions of [¹²⁵I]-Cetuximab and [¹²⁵I]-FCCS12027-Cetuximab were 0.4 and 0.7, respectively. The internalization ratio of [¹²⁵I]-FCCS12027-Cetuximab was similar to that of [¹²⁵I]-Cetuximab in each 6 cell lines.

Conclusions All the results indicate that [¹²⁵I]-FCCS12027-Cetuximab has great potential for specific radiolocalization and inactivation of tumor cells. Consequently, the use of a new linker might significantly improve the therapeutic efficacy and decrease in undesirable uptake of radiation to normal tissues during radioimmunotherapies.