Abstract C80: Development of EGFR C797S mutation in serial liquid biopsy assessments in the clinical practice setting
Authors: Mileham Kathryn F. , Zhang Qing , Farhangfar Carol J. , Haggstrom Daniel E. , Fairclough Stephen , Zill Oliver A. , Carrizosa Daniel R. , Lanman Richard B. , Kim Edward S. 
Published: May 2016

Abstract
Introduction: EGFR T790M has emerged as an important biomarker to predict response to 3rd-generation tyrosine kinase inhibitors (TKI). Therapies such as AZD 9291 and Rociletinib demonstrated impressive results in the clinical setting in patients with non-small cell lung cancer (NSCLC). However just as this resistance biomarker (T790M) has developed as a result of treatment with prior EGFR TKIs, EGFR C797S mutation is an acquired resistance mechanism to EGFR T790M inhibitors. This mutation was recently reported for the first time in clinical practice utilizing tissue-biopsy based next generation sequencing (NGS) and in the clinical trial setting in cell-free circulating DNA (cfDNA). Here we report the first case of a patient with NSCLC and emergence of C797S mutation utilizing a cfDNA biopsy-free NGS panel in serial assessments in the clinical setting.
Methods: Patients with NSCLC were seen at Levine Cancer Institute and had serial blood draws to assess molecular aberrations in cfDNA over time for comparison with radiographic reviews. Blood samples were sent to Guardant360 for assessment by NGS with a targeted cfDNA NGS panel for 68 genes with complete exon sequencing for all 28 exons in EGFR, and other genes in the panel. Mutant allele fractions (MAF) are reported as% of mutant DNA molecules divided by total molecules (mostly leukocyte DNA-derived) overlapping the same mutated nucleotide base position.
Results: A 50 year old African-American female never-smoker with lung adenocarcinoma developed a tissue-biopsy confirmed EGFR T790M mutation after 12 months on erlotinib for EGFR exon 19 deletion. She had significant clinical and radiographic responses to a 3rd generation TKI started May 2014. cfDNA NGS tests in March and April 2015 showed no genomic alterations detectable. In late May 2015, she developed clinical symptoms of cough, dyspnea and fatigue but no definitive radiographic evidence of progression on scans June 2015. She received steroids and antibiotics and continued treatment with the 3rd generation TKI. However, corresponding cfDNA NGS testing revealed emergence of multiple low level mutations and the original deletion in EGFR (EGFR L747_P753 DelInsS, T790M, C797S, and A755G variant of uncertain significance) and TP53 (all &lt;1.1%). Repeat scans in July 2015 demonstrated clear progression of disease and a fourth cfDNA NGS test showed a rise in all low level aberrations previously detected in cfDNA: EGFR A755G 4.2%, TP53 P278R 3.6%, EGFR T790M 3.1%, EGFR L747_P753 DelInsS 2.9%, EGFR C797S 1.4%. NGS indicated that there may be 2 clones present for EGFR C797S but confirmation is required as disease progresses.
Discussion: Complete exon sequencing of cfDNA upon clinical progression uncovered the simultaneous re-emergence of the original driver EGFR exon 19 del and resistance driver mutation EGFR T790M as well as the emergence of the next resistance driver mutation EGFR C797S. The latter mutation interferes with covalent binding of a 3rd generation TKI and under treatment pressure, the tumor appears to have simultaneously evolved two different EGFR C797S clones at low MAFs. cfDNA can be a dynamic measure of tumor response, easily obtained from a patient on any visit with potential for detection before macroscopic/radiographic evidence of progression, as demonstrated in this case with tumor evolution of the EGFR T790M and C797S alterations.
Citation Format: Kathryn F. Mileham, Qing Zhang, Carol J. Farhangfar, Daniel E. Haggstrom, Stephen Fairclough, Oliver A. Zill, Daniel R. Carrizosa, Richard B. Lanman, Edward S. Kim. Development of EGFR C797S mutation in serial liquid biopsy assessments in the clinical practice setting. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C80.