Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs): simple drugs with a complex mechanism of action?

A range of target-based agents for the treatment of solid tumors are in development. The epidermal growth factor receptor (EGFR) has been identified as a relevant target as it is involved in regulating several cellular functions important in the proliferation and survival of cancer cells, is commonly expressed at high levels in a range of tumors, and high expression is often related to poor prognosis. EGFR is a member of the ErbB family of receptors which also includes ErbB-2, ErbB-3, and ErbB-4. These receptors form dimers of the same type (homodimers) or with other family members (heterodimers), each combination resulting in different downstream effects. Some of the most advanced targeted agents in development are the EGFR tyrosine kinase inhibitors (EGFR-TKIs), of which ZD1839 ('Iressa') is an example. In Phase II monotherapy trials, oral ZD1839 was well tolerated and demonstrated clinically meaningful antitumor activity and symptom relief in pretreated patients with advanced NSCLC. Preclinical studies have suggested that the antitumor activity of ZD1839 does not depend on the level of EGFR expression. Furthermore, in addition to an effect on EGFR signaling, treatment with ZD1839 as well as with other quinazoline EGFR-TKIs, may also affect signaling of other ErbB family members. EGFR-TKIs have been shown in preclinical studies to increase the efficacy of cytotoxic drugs and Phase III trials of such combinations are ongoing. On the basis that different signal transduction pathways contribute to the control of tumor growth, future therapeutic approaches are likely to involve combination of different targeted agents.