Epidermal growth factor receptor (EGFR) is a transmembrane receptor with a cytoplasmic tyrosine kinase (TK) domain present on many solid tumors including non-small cell lung cancer (NSCLC). Once stimulated by ligand, the downstream pathway is activated leading to cell growth, survival, and carcinogenesis. There are several methods of EGFR inhibition including monoclonal antibodies directed against the external region and small molecule inhibitors of TK domain. Erlotinib and gefitinib are orally available small molecule EGFR TK inhibitors, with proven efficacy in NSCLC. The most common side effects are skin toxicity and diarrhea. Erlotinib has been shown to improve survival compared to placebo in second or third-line therapy for NSCLC. However, erlotinib in combination with chemotherapy failed to show a survival advantage in two first-line studies which could be due to the timing of chemotherapy administration. In general, patients with adenocarcinoma histology, female gender, Asian ethnicity, and never smokers have a better response when treated with erlotinib. This could be related to the presence of EGFR mutations, lack of KRAS mutations, or overexpression of EGFR as measured by fluorescent in-situ hybridization (FISH) analysis. Future studies should concentrate on further development of predictors of clinical benefit with erlotinib, overcoming resistance to erlotinib that develops in initial responders, as well as more effective sequencing of erlotinib with chemotherapy and combinations of the drug with other "targeted" therapeutic agents.
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