Monitoring Treatment Efficacy And Resistance In Breast Cancer Patients Via Circulating Tumor Dna Genomic Profiling.

e12502 Background: Circulating tumor DNA (ctDNA) from liquid biopsy provides a valuable assessment of invasive breast cancer (BC). We evaluated the utility of ctDNA to reflect the efficacy of HER2-targeted trastuzumab in treating HER2+ BC patients, as well as chemotherapy in treating HER2- BC patients, to monitor trastuzumab and chemotherapy resistance mechanisms. Methods: Targeted next-generation sequencing (NGS) of 416 cancer-relevant genes was performed in 41 plasma biopsy samples from 19 HER2+ and 12 HER2- BC patients in a retrospective study. We compared ctDNA somatic mutations and germline mutations for analyzing acquired and innate resistance, respectively. Results: ERBB2 somatic copy number of HER2+ BC patients who developed progressed diseases to HER2-target therapy was significantly higher than those benefited from HER2-target therapy. HER2+ BC patients who developed acquired resistance to trastuzumab showed frequent genomic alterations on ERBB2, TP53, EGFR, NF1 and SETD2 genes. Specifically, in longitudinal analyses, somatic mutations found in the original breast tumor can be detected in the liver metastasis and plasma ctDNA with increased allele frequencies. Newly emerged deleterious mutations occurred when the patient was benefiting from trastuzumab, predicting the poor prognosis. From these newly emerged somatic mutations, EBBB2L869R was further investigated in vitro and contributed to trastuzumab resistance. In the HER2- BC patients with chemotherapy resistance, frequently genetic alterations on TP53, PIK3CA and DNA damage repair genes were observed in ctDNA mutation profiling. Conclusions: liquid biopsy ctDNA, particularly longitudinal analyses, provides insights into targeted therapy efficacy and gene alterations underlying trastuzumab resistance and chemotherapy resistance in HER2+ and HER2- BC patients, respectively.