Abstract 509: HER2 genomic amplification in circulating tumor DNA from patients with cetuximab-resistant colorectal cancer

Abstract

Abstract Background: Patients with metastatic colorectal cancer (mCRC) harboring wild-type KRAS benefit from epidermal growth factor receptor (EGFR)-targeted therapy. However, patients who are treated with anti-EGFR antibodies will eventually develop the resistance to those agents. We previously found that HER2 amplification was one of the mechanisms conferring resistance to anti-EGFR antibody therapy and could therefore be a potential therapeutic target (Yonesaka et al, STM 2011). The aim of this study was to detect HER2 amplification in circulating tumor DNA (ctDNA) from patients with CRC and acquired resistance to anti-EGFR antibody therapy. Methods: We analyzed plasma ctDNA using digital polymerase chain reaction (PCR) from 18 patients with CRC, who had been treated with anti-EGFR antibody-based therapy (cetuximab) and subsequently acquired resistant cetuximab. We aimed to obtain at least 100 droplets for HER2 and EFTUD2 to accurately assess the ratio. Digital PCR data were analyzed using the QuantaSoft analytical software package (Bio-Rad). The copy number concentration of each gene (HER2 and EFTUD2) was estimated from the Poisson distribution. HER2 amplification with digital PCR was defined as a HER2 ratio (HER2/EFTUD2 copy number ratio) of 1.25 accordingly (Gevensleven H et al, CCR 2013). HER2 gene copy number was analyzed using fluorescence in situ hybridization also in tumor samples before and after acquisition of resistance to cetuximab-based therapy. Results: Our data showed various HER2 copy number in plasma ctDNA from CRC patients (median: 1.09; range: 0.94-5.18). 22% (4/18) of patients in the cohort exhibited HER2 amplification. One of these patients was found to be positive for HER2 amplification in matched tumor specimens collected after cetuximab therapy, at which point the patient had acquired cetuximab resistance, despite being negative for HER2 amplification prior to therapy (pre: 1.14, post: 2.78). Conclusion: Analysis of plasma ctDNA by digital PCR could be useful for detecting HER2 amplification in patients with CRC who were resistant to anti-EGFR antibody therapy. We are now conducting another, large-scale study for evaluating HER2 amplification by ctDNA in CRC patients using digital PCR. Citation Format: NAOKI TAKEGAWA. HER2 genomic amplification in circulating tumor DNA from patients with cetuximab-resistant colorectal cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 509.