Molecular inhibition of epidermal growth factor receptor (EGFR/HER1) signaling is under active investigation as a promising cancer treatment strategy. We examined the potency of EGFR inhibition achieved by combining anti-EGFR monoclonal antibody and tyrosine kinase inhibitor, which target extracellular and intracellular domains of the receptor, respectively. We specifically studied the combination of cetuximab (Erbitux, C225; ImClone Systems, New York, NY) with either gefitinib (Iressa, ZD1839; AstraZeneca, Macclesfield, UK) or erlotinib (Tarceva, OSI-774; Genentech, South San Francisco, CA) across a variety of human cancer cells. The combination of cetuximab plus gefitinib or erlotinib enhanced growth inhibition over that observed with either agent alone. As measured by immunostaining, inhibition of EGFR phosphorylation with the combination of cetuximab plus gefitinib or erlotinib was augmented over that obtained with single-agent therapy in head and neck (H&N) cancer cell lines. Phosphorylation inhibition of downstream effector molecules [mitogen-activated protein kinase (MAPK) and AKT] also was enhanced in tumor cells treated with the combination of cetuximab plus gefitinib or erlotinib. Flow cytometry and immunoblot analysis demonstrated that treatment of H&N tumor cells with cetuximab in combination with either gefitinib or erlotinib amplified the induction of apoptosis. Following establishment of cetuximab-resistant cell lines, we observed that gefitinib or erlotinib retained the capacity to inhibit growth of lung and H&N tumor cells that were highly resistant to cetuximab. Treatment with gefitinib or erlotinib, but not cetuximab, also could further inhibit the activation of downstream effectors of EGFR signaling in cetuximab-resistant cells, including MAPK and AKT. These data suggest that tyrosine kinase inhibitors may further modulate intracellular signaling that is not fully blocked by extracellular anti-EGFR antibody treatment. Finally, animal studies confirmed that single EGFR inhibitor treatment resulted in partial and transient tumor regression in human lung cancer xenografts. In contrast, more profound tumor regression and regrowth delay were observed in mice treated with the combination of cetuximab and gefitinib or erlotinib. Immunohistochemical staining, which demonstrated significant reduction of the proliferative marker proliferating cell nuclear antigen in mice treated with dual EGFR inhibitors, further supported this in vivo observation. Together, these data suggest that combined treatment with distinct EGFR inhibitory agents can augment the potency of EGFR signaling inhibition. This approach suggests potential new strategies to maximize effective target inhibition, which may improve the therapeutic ratio for anti-EGFR-targeted therapies in developing clinical trials.
