Background Circulating tumor DNA (ctDNA) analysis allows non-invasive detection of tumor mutations and amplifications in advanced breast cancer. Multiple technologies have been developed to analyse ctDNA and here we compared two leading ctDNA detection technologies, enhanced Tagged-Amplicon Sequencing™ (eTAm-Seq™) and digital PCR (dPCR) assays, in advanced breast cancer.

Methods We recruited a cohort of 35 women with advanced breast cancer, of whom 23 had two separate blood samples taken in a standard EDTA tube processed immediately or in preservative Streck tubes processed up to 120 hours after venipuncture. Digital PCR was conducted with assays for hotspot actionable mutations in 3 known drivers in breast cancer: PIK3CA exon 9 and 20, ESR1 ligand binding domain and AKT1 (c.49G>A; p.E17K), and ctDNA sequencing was conducted with eTAm-Seq method using a 35-gene panel including cancer hotspots, entire coding regions and copy number variants (CNVs).

Results Across both assays, 37 mutations were detected in 35 patients, with PIK3CA mutation in 13 patients (37%), ESR1 mutations in 10 patients (29%), and no AKT1 mutations. ESR1 mutations were polyclonal in 8 patients, with ctDNA eTAm-Seq method revealing substantially more diversity in mutations, with up to 8 individual mutations detected in a patient. There was 96.15% agreement for PIK3CA mutation detection between assays (Kappa 0.89, 95% CI 0.743 to 1.000), 100% agreement for ESR1 mutations (Kappa 1.00, 95% CI 1.000 to 1.000). There was very high correlation in mutation allele frequency between eTAm-Seq and dPCR (r=0.93, 95%CI 0.86 to 0.96, p<0.0001). The sensitivity and specificity for HER2 amplification detection by eTAm-Seq was 100% compared to tumor HER2 status. Comparison of immediate processing and Streck tubes revealed 97.92% agreement (Kappa 0.95, 95% CI 0.868 to 1.000) for mutation calling.

Conclusions This study demonstrates that ctDNA analysis using eTAmSeq and dPCR have very high agreement in mutation detection in patients with advanced breast cancer patients. Streck tubes present a robust alternative to immediate processing of samples. eTAm-Seq and digital PCR have high clinical validity in mutation detection.

Citation Format: Isaac Garcia-Murillas, Matthew Beanney, Michael Epstein, Karen Howarth, Andrew Lawson, Sarah Hrebien, Emma Green, Nitzan Rosenfeld, Nick Turner. Comparison of enhanced Tagged-Amplicon Sequencing and digital PCR for circulating tumor DNA analysis in advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2743. doi:10.1158/1538-7445.AM2017-2743