Background: The monoclonal antibody cetuximab (IMC-225, Erbitux) inhibits epidermal growth factor receptor (EGFR) signaling and has been approved for metastatic colon cancer therapy. However, to achieve effective titers, passive antibody therapies must be repeatedly administered over long periods. To overcome this limitation, we aimed to generate a vaccine inducing continuously available "cetuximab-like" antibodies in vivo using the mimotope approach.
Methods: We used the phage display technique to identify four peptides structurally mimicking the cetuximab epitope. We coupled two of these peptides to an immunogenic carrier protein, and we vaccinated four groups (n = 8) of BALB/c mice intraperitoneally with 10 microg of the mimotope conjugates, a control peptide conjugate, or the carrier protein alone. We assessed antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity mediated by the induced antibodies against EGFR-overexpressing human A431 carcinoma cells. We then tested receptor internalization capacity of the induced antibodies with fluorescently labeled EGFR, and we assayed their growth inhibitory potential toward A431 cells with a [3H]thymidine proliferation assay.
Results: Mimotope-induced antibodies recognized EGFR, and both types of antibody-mediated cytotoxic effects were elicited by these antibodies. In both cellular cytotoxicity assays, the mimotope-induced antibodies exhibited specific lysis of more than 50%. The induced antibodies caused internalization of the receptor from the cell surface into endocytic vesicles and inhibited growth of EGFR-expressing cells to a similar extent as cetuximab [67% (95% confidence interval {CI} = 55% to 79%) and 69% (95% CI = 55% to 84%), respectively].
Conclusions: Epitope-specific immunization is feasible for active anti-EGFR immunotherapy. The in vitro biologic features of mimotope-induced antibodies are similar to those of the monoclonal antibody cetuximab.