[1781] Assessment of EGFR Mutation Status in Lung Adenocarcinoma by Immunohistochemistry Using Antibodies Specific to the Two Major Forms of Mutant EGFR

M Brevet, M Arcila, M Ladanyi. Memorial Sloan-Kettering Cancer Center, New York, NY

Background: Somatic mutations within the tyrosine kinase domain of EGFR have emerged as the most reliable predictors of response to EGFR tyrosine kinase inhibitors (TKI) in patients with lung adenocarcinoma. Deletions in exon 19 and the L858R substitution in exon 21, accounting for approximately 90% of all mutations, are the best characterized sensitizing mutations. At present, mutations are detected by molecular methods but alternative assays that could be performed more widely in clinical laboratories remain of substantial interest.
Design: We evaluated two mutation-specific monoclonal antibodies for the detection of EGFR mutations by immunohistochemistry (IHC), generated respectively against the L858R mutant and the exon 19 mutant with the common 15bp/5AA deletion (Cell Signaling Technology). IHC staining performed on 218 paraffin-embedded lung adenocarcinomas was assessed on a 0 to 3+ scale, and positivity cutoffs of 1+ and 2+ were compared. All cases were studied by standard molecular methods for these two mutations and selected cases were also studied using higher sensitivity molecular assays. These molecular studies confirmed 21 cases with EGFR L858R and 55 cases with EGFR exon 19 deletions, both reflecting enrichment of these mutations for this study.
Results: Both antibodies showed a cytoplasmic and membranous staining which was usually homogeneous in cases with moderate to strong staining, but was more heterogeneous in cases with faint staining. The EGFR L858R mutant antibody showed a sensitivity of 95% and a positive predictive value (PPV) of 99% with a positivity cutoff of 1+ and a sensitivity of 76% and a PPV of 100% with a positivity cutoff of 2+. The EGFR exon 19 mutant-specific antibody showed reduced sensitivity for exon 19 deletions other than 15bp. A positivity cutoff of 1+ resulted in a sensitivity of 85% and a PPV of 99% while a 2+ cutoff gave a sensitivity of 67% and a PPV of 100%.
Conclusions: These two mutant specific antibodies could be incorporated into the routine IHC work-up of lung adenocarcinomas to reduce the molecular testing volume for EGFR mutations while allowing faster treatment initiation for some patients with sensitizing EGFR mutations.
Category: Pulmonary

Tuesday, March 23, 2010 1:15 PM

Platform Session: Section F, Tuesday Afternoon


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