Novel EGFR mutation-specific antibodies for lung adenocarcinoma: Highly specific but not sensitive detection of an E746_A750 deletion in exon 19 and an L858R mutation in exon 21 by immunohistochemistry

ObjectivesActivating mutations in the epidermal growth factor receptor (EGFR) kinase domain are correlated with dramatic clinical responses in non-small cell lung cancer patients treated with EGFR-tyrosine kinase inhibitors (TKIs). The two most common EGFR mutations, representing 85–90% of EGFR mutations, are the E746_A750 deletion in exon 19 and the L858R point mutation in exon 21. We conducted this study to evaluate the suitability of mutation-specific antibodies that can detect E746_A750 deletion and L858R mutant EGFR proteins by immunohistochemistry (IHC).Materials and methodsIn a cohort of consecutive patients with surgically resected lung adenocarcinomas (n = 240), mutant EGFR protein expression was assessed by IHC using specific antibodies (clone SP111 and SP125) to the 2 major forms of EGFR mutations. Immunoreactivity was scored as 0–3, and the results were compared with the EGFR-mutational status.ResultsWith a cutoff value of IHC 2+ for SP 111 (anti-EGFR E746_A750 del antibody) and SP 125 (anti-EGFR L858R antibody), both antibodies showed high specificity (99.0% and 89.7%, respectively) and sensitivity (70.6% and 80.4%, respectively). While cases with IHC scores of 3 using these 2 antibodies positively correlated with the EGFR-mutational status, cases with IHC scores lower than 3+ showed variable results regarding EGFR-mutational status.ConclusionAlthough each antibody showed relatively high specificity, some EGFR-mutant cases were not detected by the mutation-specific antibodies. Various forms of exon 19 deletions, except E746_A750, were rarely detected by the mutant-specific antibody. Therefore, IHC-negative cases require further molecular analysis to confirm the presence of EGFR mutations.