Cetuximab inhibits thymidylate synthase in colorectal cells expressing epidermal growth factor receptor

The monoclonal antibody cetuximab directed against the epidermal growth factor receptor (EGFR) is an attractive agent for targeted therapy in advanced colorectal cancer (CRC), especially when combined with 5‐fluorouracil (5‐FU)‐based chemotherapy. However, the mechanisms of cetuximab activity as chemosensitizer remain poorly understood. Using proteome‐fluorescence‐based technology, we found that cetuximab is able to suppress the expression of thymidylate synthase (TS), which is involved in the mechanism of 5‐FU action. Caco‐2, HRT‐18, HT‐29, WiDr and SW‐480 CRC cells were found to express EGFR. SW‐620 was used as EGFR‐negative cell line. Only in EGFR‐expressing cells cetuximab is able to inhibit TS expression. Combined treatment with cetuximab and 5‐FU revealed a synergistic anti‐tumor response that is closely correlated with functional activity of EGFR/mitogen‐activated protein kinase (MAPK) pathway. Moreover, no correlation was seen between constitutive TS protein expression, level of cetuximab‐induced TS down‐regulation and response either to 5‐FU alone or in combination with cetuximab. We demonstrated that only EGFR expression with high functional activity of EGFR/MAPK pathway is important for the synergistic effects between cetuximab and 5‐FU in the investigated cell lines.