105P - Prospective testing of circulating tumour DNA in metastatic breast cancer facilitates clinical trial enrollment and precision oncology

The increasing availability of targeted agents for treatment of metastatic breast cancer (mBC) necessitates accurate and timely molecular characterisation of disease. As a minimally invasive test, circulating tumour DNA (ctDNA) is well positioned to overcome many of the limitations associated with traditional tumor biopsies. Here, we established a program to assess the feasibility of routine prospective ctDNA testing for the clinical management of mBC patients. Detection of somatic mutations from patient plasma was performed using a multiplexed droplet digital PCR (ddPCR) approach to identify hotspot mutations in PIK3CA, ESR1, ERBB2 and AKT1. In parallel, a subset of samples were also analysed via next generation sequencing (targeted amplicon (TA) sequencing and low-coverage whole-genome sequencing). Results were discussed at a multidisciplinary breast cancer meeting prior to therapy selection. 234 mBC patients were enrolled on this study, with a median age at diagnosis of 54 years (28-80) and a median of 2 lines of prior therapy. The average turnaround time for ctDNA testing using ddPCR was 9 days (1-49). Using ddPCR, 80/234 (34.2%) patients had ≥1 mutation identified, with 52/234 (22.2%) patients having an alteration in PIK3CA, 35/234 (15.0%) in ESR1, 9/234 (3.8%) in AKT1 and 2/234 (0.9%) in ERBB2. TA sequencing performed in the first 159 patients, identified actionable mutations (classified using the OncoKB database) in 63 patients (39.6%) and showed that a mean variant allele fraction of>5% was significantly associated with inferior overall survival (Hazard ratio: 1.8; 95% Confidence interval: 1.1-3.1; p