Utility of circulating tumor DNA (ctDNA) to inform treatment of patients with metastatic breast cancer.

1042 Background: Circulating tumor DNA (ctDNA) can overcome limitations of tissue biopsy (tbx) in patients with metastatic breast cancer (mBC). While tbx provides a snapshot of a tumor’s molecular profile from one site and time, ctDNA provides a more comprehensive landscape of the tumor genome. Serial liquid biopsies (lbx) can be easily obtained for monitoring clinical response and resistance over time. The Individualized Molecular Analyses Guide Efforts in Breast Cancer (IMAGE) II study evaluates tbx and serial lbx methods for tumor genomic profiling in mBC. Methods: IMAGE II (NCT02965755) is a prospective, multi-center trial to evaluate the clinical utility of lbx versus tbx. Eligible individuals had mBC of any subtype, with progression requiring therapy change. Tbx sequencing per standard of care was required unless biopsy was not technically feasible. Upon enrollment, we collected baseline plasma for tissue-agnostic comprehensive genomic profiling (CGP) by Foundation Medicine. Both lbx and tbx sequencing results were reviewed in real time by the Johns Hopkins Molecular Tumor Board to provide treatment recommendations. Clinical data was recorded. We collected serial lbx samples 1-2 weeks post initiation of next line of therapy, at first restaging, and at progression. Here we present the analysis of data collected at enrollment, and serial lbx results will be reported at a later date. Results: Total enrollment was 199 pts, of whom 194 were women (median age = 57 years, range 27-86 years). There were 140 HR+HER2-, 4 HR-HER2+, 20 HR+HER2+ and 35 triple negative (TNBC) cases. Metastases were found in bone only in 19 pts, or included lung in 99 pts, liver in 97 pts, and brain in 14 pts. Median prior lines of metastatic therapy was 2, with 44 pts (22.1%) having been on 4 or more, including antibody-drug conjugates (14 pts), immunotherapy (2), cytotoxic chemotherapy (74), endocrine (88), and targeted therapy (12). 9 pts were newly diagnosed with mBC at the time of enrollment. 110 pts (55%) had tbx sequencing. In lbx-based CGP results from 190 patients, the genes most frequently altered were TP53(90 patients), PIK3CA (63), ESR1 (54). Copy number amplifications were found in FGFR1 (21 patients) and FGFR3 (12), and copy number losses were found in PTEN in 3 patients. Of the 87 patients whose baseline lbx was profiled using FoundationOne Liquid CDx which allows calculation of ctDNA tumor fraction (TF), 69 (79.3%) pts had detectable ctDNA TF. 60 patients (69.0%) had ctDNA TF at least 1%. We will present the frequency of clinically actionable mutations detected via lbx vs tbx genomic profiling, accounting for sites of metastases, receptor subtype and prior therapy exposure, as well as the association between ctDNA TF and sites of disease. Conclusions: A high rate of ctDNA TF detection in pts with mBC highlights the value of lbx for CGP in this setting for detecting multiple alteration classes across mBC subtypes and metastatic sites. Clinical trial information: NCT02965755 .