Binding properties of radiolabeled cetuximab conjugates

The monoclonal antibody cetuximab (C225) binds with high affinity to the epidermal growth factor receptor (EGFR), which is a major molecular target for treatment of different types of cancer. Radiolabeled C225 has been proven to be appropriate for cancer imaging and treatment. This study comprises an affinity comparison of different C225 conjugates incorporating p-SCN-Bn-NOTA (1), p-SCN-Bn-dipicolyl-TACN (2) and p-SCN-Bn-CHX-A″-DTPA (3). Evaluation of the K_i values using homogenates of A431 cells (EGFR^high/Her2^high expression) revealed minimal loss of affinity for these conjugates compared to unchanged C225. Saturation assays have been applied to compare the binding properties of ([⁶⁴Cu]Cu-1)₃-C225, ([⁶⁴Cu]Cu-2)₂-C225, ([⁹⁰Y]Y-3)₃-C225 and ([¹¹¹In]In-3)₃-C225 on homogenates of different cancer cell lines. The labeled conjugates were found to bind with high specificity and affinity to both the A431 and FaDu (EGFR^medium/Her2^low expression) cells, however, the affinity for the FaDu was higher than for the A431 cells. The affinity of ([⁶⁴Cu]Cu-1)₃-C225 and ([⁶⁴Cu]Cu-2)₄-C225 for both EGFR expressing cell lines was somewhat higher than that displayed by ([⁹⁰Y]Y-3)₄-C225 and ([¹¹¹In]In-3)₃-C225. No specific binding was observed in the case of the EGFR-negative MDA-MB-453S cells. Moreover, immunoreactive fractions of more than 80% were determined, indicating that the conjugates are promising candidates for further in vivo evaluation.