The role of VEGF and EGFR inhibition: implications for combining anti-VEGF and anti-EGFR agents.

Multiple cellular pathways influence the growth and metastatic potential of tumors. This creates heterogeneity, redundancy, and the potential for tumors to bypass signaling pathway blockade, resulting in primary or acquired resistance. Combining therapies that inhibit different signaling pathways has the potential to be more effective than inhibition of a single pathway and to overcome tumor resistance. Vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFR) inhibitors have become key therapies in several tumor types. Close relationships between these factors exist: VEGF signaling is up-regulated by EGFR expression and, conversely, VEGF up-regulation independent of EGFR signaling seems to contribute to resistance to EGFR inhibition. Therefore, inhibition of both pathways could improve antitumor efficacy and overcome resistance to EGFR inhibition. Preclinical studies have shown that VEGF and EGFR inhibitors can have additive effects and that combined inhibition is effective in EGFR inhibitor-resistant cell lines. Clinical trials have also produced promising data: combining the anti-VEGF monoclonal antibody bevacizumab with the anti-EGFR antibody cetuximab or the EGFR tyrosine kinase inhibitor erlotinib increases benefit compared with either of these anti-EGFR agents alone or combined with chemotherapy. The potential of this novel approach to anticancer therapy will be elucidated by large, ongoing clinical trials.