SNaPshot multigene assay to detect mechanisms of acquired resistance to EGFR tyrosine kinase inhibitors (TKIs).

Abstract

7554 Background: NSCLCs harboring activating EGFR mutations often respond to EGFR TKIs, but drug resistance invariably occurs, usually after 9-12 months. Although two validated resistance mechanisms include T790M (50%) and MET amplification (10-15%), the mechanisms of drug resistance (MODR) in the remaining 35-40% of pts remain unknown. We recently implemented an innovative multiplexed PCR-based assay (SNaPshot) to genotype cancer pts more comprehensively. SNaPshot is performed in a CLIA-certified lab with tumor DNA from paraffin-embedded samples; it tests for common mutations in 13 key oncogenes such as EGFR, KRAS, PIK3CA, BRAF, and TP53. We examined SNaPshot results in EGFR mutant cancers with acquired TKI-resistance. Methods: We reviewed all NSCLC samples sent for prospective SNaPshot analysis from 03/09 to determine the range of tumor genotypes. A simultaneous chart review identified EGFR mutant pts undergoing repeat biopsy after the development of TKI-resistance in which a corresponding pre-treatment tumor sample was available for SNaPshot analysis and MET FISH testing. Results: A total of 271 patients with NSCLC underwent SNaPshot testing between 03/09 and 11/09; 164 (61%) were found to have at least one mutation. We identified 27 EGFR mutant pts who developed TKI-resistance and had paired samples available. SNaPshot results are summarized in the Table. Interestingly, 3 pts demonstrated a major histological change from NSCLC to SCLC in the TKI-resistant biopsy (confirmed by IHC), while maintaining the original EGFR mutation. Conclusions: Here we introduce SNaPshot as a comprehensive tumor genotyping assay that is valuable for examining MODR in the clinic. Among 27 EGFR mutant pts with TKI-resistance, T790M was found in 48% and MET amplification in 4%. Notably, 1 pt acquired a PIK3CA mutation, which has been described as a MODR in vitro. Three pts experienced a fundamental transformation from NSCLC to SCLC harboring the same EGFR mutation. Investigation into the significance of the SCLC evolution and the remaining pts without identified MODR is ongoing. Observed resistance mechanisms Number Total pts 27 T790M (2 T790M + TP53) 13 MET amplification 1 PIK3CA 1 No identified mutations 12 Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Gatekeeper Biogen Idec, Bristol-Myers Squibb, Daiichi Sankyo, GlaxoSmithKline, MedImmune, Millennium, Pfizer, Roche, Schering-Plough, Telik Novartis I participated in the development of the SNaPshot assay, which is the subject of a patent application and licensing discussions.