Targeting epidermal growth factor receptor--are we missing the mark?

Aberrant signalling through the epidermal growth factor receptor (EGFR) is associated with neoplastic cell proliferation, migration, stromal invasion, resistance to apoptosis, and angiogenesis. The high frequency of abnormalities in EGFR signalling in human carcinomas and gliomas and laboratory studies showing that inhibition of EGFRcan impair tumour growth means that EGFR is an attractive target for the development of cancer therapeutics. Among the classes of agents targeting EGFR in clinical development are monoclonal antibodies against the extracellular ligand-binding domain of the receptor, and small molecules that inhibit activation of the receptor tyrosine kinase. Although there are pharmacological and mechanistic differences between the two classes of inhibitor, preclinical studies suggest they both inhibit cell proliferation and have additive or synergistic cytotoxicity with standard therapies. Results from early clinical trials indicate that these agents are well tolerated and have anti-tumour activity.

In May, 2003, the Australian Therapeutic Goods Administration and the US Food and Drug Administration approved the EGFR inhibitor gefitinib (ZD1839, Iressa) for the treatment of patients with advanced non-small-cell lung cancer (NSCLC) previously treated with chemotherapy. The US approval was based on results of a phase 2 study of 216 patients with NSCLC, including 142 patients with refractory disease. In this subgroup, the response rate was about 10%. The approval of the drug was granted despite negative results from two randomised controlled trials in over 2000 previously untreated patients with NSCLC, which showed no benefit in survival, objective tumour response, or time to progression when gefitinib was added to chemotherapy. WHERE NEXT? Research is needed to identify and validate predictive factors that can be used to select patients with disease likely to respond to EGFR inhibitors, and to elucidate the mechanism of interaction of these agents with standard therapies and other molecularly targeted agents. Appropriately designed clinical trials are required to define the optimum dose, schedule, and sequence for these agents in combination with conventional therapies and other targeted agents.