Immunohistochemistry detection of epidermal growth factor receptor (EGFR) exons 19 and 21 mutations in non-small cell lung cancer (NSCLC) using novel mutation specific antibodies.

10542 Background: Activating EGFR mutations have become a backbone in the therapy alogrithm for NSCLC as mutations predict significantly better response and survival to EGFR TKIs. The most common EGFR mutations in NSCLC are exon 19 deletions and the exon 21 L858R point mutation. Among the exon 19 deletions, E746-A750 occurs in 61% of cases. In this study we compared novel IHC antibodies for E764-A750 and L858R to direct DNA sequencing in specimens from Japanese patients with NSCLC treated with gefitinib. Methods: IHC using the E746-A750 and L858R mutation specific antibodies (Cell Signaling Technologies, Danvers, MA) was performed on resected NSCLC Japanese patients who were subsequently treated with gefitinib at relapse. Extracted DNA was sequenced for mutational analysis of EGFR exons 18 to 21. Results: DNA sequencing of all 70 patients showed exon 19 deletions in 18 patients (25.7%), 11 had a deletion at E746-A750, exon 20 mutations in 18 patients, exon 21 mutations in 12 (17.1%), and exon 18 mutations in one patient. The mutation specific antibodies detected 9/11 of the E746-A750 mutations (2 false negatives, 0 false positive, sensitivity 81.8%, and specificity 100%) and 9/12 of the L858R point mutations (2 false positives, 3 false negatives, sensitivity 75%, and specificity 96.6%). Objective response rates (ORR) were associated significantly with exon 19 deletions (44.4%, p = 0.006) and with exon 21 mutations (36.4%, p = 0.009). Conclusions: The exon 19 and 21 mutation specific antibodies for IHC staining have high sensitivity and specificity for the pre-defined mutations. Such techniques would be very cost-effective for mutation screening in NSCLC. False positives could result from low-sensitivity sequencing, whereas false negative could result from small sample size of TMA relative to tumor size sequenced. For clinical decisions, negative IHC staining may require further EGFR mutation analyses. No significant financial relationships to disclose.