Cetuximab-mediated antibody-dependent cellular cytotoxicity (ADCC) against tumor cell lines characterized for EGFR expression and K-ras mutation

e14583 Background: Mutated K-ras protein is a strong predictive factor of cetuximab resistance, bypassing the classical direct inhibitory effect on epidermal growth factor receptor (EGFR) signaling. However, cetuximab is also able to mediate ADCC, which may be part of the clinical response. The aim of this ex-vivo study was to quantify cetuximab-mediated ADCC on various human cancer cell lines characterized for EGFR-expression and K-ras mutation. Methods: Two K-ras mutated cell lines over-expressing EGFR and resistant to anti-EGFR tyrosine kinase inhibitor were tested (Capan-1 and Capan-2, pancreatic), along with 2 K-ras wild-type cell lines over- expressing EGFR (CAL166, head and neck; A431, epidermoid carcinoma) and an EGFR-negative cell line (OCM1, uveal melanoma). The tested monoclonal antibodies (mAbs) were: cetuximab (Merck, anti-EGFR IgG1 mAb), panitumumab (Amgen, anti-EGFR IgG2 mAb), and as a negative control, rituximab (Roche, IgG1 anti-CD20 mAb). ADCC (51Cr release assay) was performed using freshly- isolated peripheral blood mononuclear cells from a healthy donor. Results were expressed as % of potentially maximum 51Cr release. Results: Cetuximab mediates ADCC against EGFR-over-expressing cell lines CAL166 (38.4 ± 3.1 %), A431 (13.5 ± 1.7 %), Capan-1 (31.2 ± 0.8 %) and Capan-2 (27.8 ± 8.6 %) irrespective of the K-ras mutational status, but not against EGFR-negative OCM-1 (6.2 ± 1 %). Conversely, unlike IgG1 cetuximab, the anti-EGFR IgG2 panitumumab and the irrelevant antibody rituximab were both unable to induce significant ADCC (< 10 % on all tested cell lines). Conclusions: Cetuximab-mediated ADCC is independent of the K-ras mutational status of the tumor cell lines. Present data suggest that cetuximab may remain of clinical interest in K-ras-mutated patients. Immunostimulation, as well as new generation anti-EGFR mAbs with improved ability to induce ADCC, may be promising in the management of K-ras-mutated patients. No significant financial relationships to disclose.