Inactivation of HNSCC Cells by 90Y-Labeled Cetuximab Strictly Depends on the Number of Induced DNA Double-Strand Breaks

Radioimmunotherapy is considered to have great potential for efficient and highly specific treatment of tumors. The aim of this study was to determine the efficacy of radioimmunotherapy when using 90Y-labeled cetuximab and to determine to what degree induction and repair of DNA double-strand breaks (DSBs) are decisive for this approach. Methods: This study was performed with 9 cell lines of squamous cell carcinoma of the head and neck (HNSCC) differing strongly in epidermal growth factor receptor (EGFR) expression. The radionuclide 90Y was coupled by the chelator trans-cyclohexyl-diethylene-triaminepentaacetic acid (CHX-A"-DTPA)/linker construct to the EGFR-directed antibody cetuximab to yield 90Y-Y-CHX-A"-DTPA-cetuximab with a specific activity of approximately 1.2 GBq/mg. EGFR expression was determined by immunofluorescence and Western blotting, cetuximab binding by fluorescence-activated cell sorter analysis, the number of DSBs by immunofluorescence staining Gamma-H2AX/53BP1-positive repair foci, and cell survival by colony formation. Results: For the 9 HNSCC cell lines, cetuximab binding correlated with the amount of EGFR present in the cell membrane (r2 = 0.967, P < 0.001). When cells were exposed to 90Y-Y-CHX-A"-DTPA-cetuximab, the number of induced DSBs increased linearly with time (r2 = 0.968, P 5 0.016). This number was found to correlate with the amount of membranous EGFR (r2 = 0.877, P = 0.006). Most DSBs were repaired during incubation at 37C, but the small number of remaining DSBs still correlated with the amount of membranous EGFR (24 h: r2 = 0.977, P < 0.001; 48 h: r2 = 0.947, P < 0.001). Exposure to 90Y-Y-CHX-A"-DTPA-cetuximab also resulted in efficient cell killing, whereby the extent of cell killing correlated strongly with the respective number of remaining DSBs (r2 = 0.989, P < 0.001) and with the amount of membranous EGFR (r<S