Rational Combination of Targeted Therapies As A Strategy to Overcome The Mechanisms of Resistance to Inhibitors of EGFR Signaling

The epidermal growth factor receptor (EGFR) has been widely used as a target for novel anticancer agents, such as blocking antibodies and small molecular weight tyrosine kinase compounds. In spite of recent advances in cancer cell biology, leading to the introduction of clinically active new drugs, such as cetuximab, panitumumab and erlotinib, unfortunately disease control remains unsuccessful due to the presence of constitutive resistance to EGFR inhibitors in most patients and the development of acquired resistance in the responders. A large number of molecular abnormalities in tumor cells seem to partly contribute to their resistance to anti-EGFR therapy: increased angiogenesis, constitutive activation of downstream mediators, overexpression of other tyrosine kinase receptors. Moreover, some mutations in the EGFR receptor kinase domain seem to play a crucial role in determining the sensitivity of cancer cells to specific inhibitors by altering the conformation of the receptor and its activity. The development of rational combinations of anticancer agents and EGFR inhibitors, able to exert synergistic cytotoxic interactions, has been widely accepted and used in both preclinical and clinical studies. Although the failure of large clinical trial based on empirical combination of anti-EGFR and classic chemotherapeutic agents, several preclinical data seems to support the hypothesis that combining EGFR inhibitors and other novel agents could efficiently inhibit tumor growth and overcome intrinsic resistance to a single-agent based therapy. This review focuses on the role of complementary signalling pathways in the development of resistance to EGFR targeting agents and the rationale to combine novel inhibitors as anticancer therapy.