Abstract
An exon 19 deletion and a L858R mutation in exon 21 of the epidermal growth factor receptor (EGFR) are the two most common mutations that predict favorable efficacy of EGFR tyrosine kinase inhibitors (TKIs) in patients with non-small cell lung cancer (NSCLC). Many retrospective and prospective studies, as well as meta-analyses including patients with NSCLC with various lines of EGFR TKI treatment, have demonstrated longer progression-free survival and sometimes more favorable overall survival in patients with an exon 19 deletion than those with the L858R or other mutations. In contrast, some clinical studies, including phase III trials, have demonstrated no difference in the efficacy of EGFR TKIs according to the EGFR mutation type. Therefore, the existence of clinically significant differences in sensitivity to EGFR-TKIs among different EGFR mutation subtypes remains controversial. In this review, we summarize the evidence suggesting different outcomes according to the type of EGFR mutation in patients with advanced NSCLC who were treated with EGFR-TKIs, along with their clinical significance. We also discuss possible mechanisms that can explain the different sensitivities to EGFR TKIs between cases with an exon 19 deletion and those with the L858R mutation.