Study design and participants

We designed an observational, multicentre clinical study to evaluate the circulating tumor DNA (ctDNA) analysis for patients with late-stage breast cancer. Specifically, the study was conducted in Hunan Cancer Hospital, the Forth Hospital of Changsha and Zhuzhou Central Hospital in China. This study enrolled metastatic TNBC patients who progressed after at least one line of chemotherapy and also enrolled HR-positive or HER2-positive MBC patients who progressed after at least two lines of chemotherapy, antihormone therapy, or anti-HER2 therapy. Therefore, late-line therapy defines the second- or above-line (≥2 lines) in mTNBC patients, and the third- or above-line (≥3 lines) in HR-positive or HER2-positive MBC patients. Patients who had druggable ctDNA abnormalities were assigned to the case group. Patients who did not have ctDNA abnormalities or druggable ctDNA abnormalities were assigned to the control group. The control group received physician-chosen treatment. The case group received druggable ctDNA alteration-guided therapy. This study is registered with ClinicalTrials.gov ({"type":"clinical-trial","attrs":{"text":"NCT05079074","term_id":"NCT05079074"}}NCT05079074). The endpoints (PFS and DCR) were confirmed by a retrospective independent radiologic committee. Investigations were performed in accordance with Chinese laws and regulations and the Helsinki declaration after approval by the local ethics committee at each participating hospital. The current study was approved by the Ethics Committee of Hunan Cancer Hospital, Central South University (approval number 2017YS031) and registered with ClinicalTrials.gov ({"type":"clinical-trial","attrs":{"text":"NCT05079074","term_id":"NCT05079074"}}NCT05079074). The authors declare that they obtained ethics approval and patient consent to participate.

This study recruited 223 consecutive patients with MBC treated at Hunan Cancer Hospital, the Forth Hospital of Changsha and Zhuzhou Hospital Affiliated to Xiangya School of Medicine, Central South University, from December 2016 through June 2019. The patient consent form was approved by the independent ethics committee. Each participant provided written informed consent to participate in the study. The eligibility criteria and exclusion criteria were listed in supplemental protocol file.