This is an exploratory, ctDNA-guided, multi-stage clinical study designed to evaluate the clinical value of treatment decision-making based on circulating tumor DNA (ctDNA) variant allele frequency (VAF) dynamics in patients with HER2-negative metastatic breast cancer.

All enrolled patients will receive antibody-drug conjugate (ADC) therapy for two cycles in Stage 1, with serial ctDNA assessments performed prior to Cycle 1 and Cycle 2. In Stage 2, patients without disease progression will be assigned to different treatment strategies based on the percentage reduction in mean ctDNA VAF.

The study aims to assess whether ctDNA VAF dynamics can serve as an early biomarker to guide treatment intensification and improve clinical outcomes in metastatic breast cancer.

Inclusion Criteria:

• Age 18-75 years；ECOG performance status 0-1； Histologically or cytologically confirmed HER2-negative metastatic breast cancer； At least one measurable lesion per RECIST v1.1； Eligible for ADC therapy； Adequate organ and bone marrow function； Life expectancy ≥3 months； Willingness to provide tumor tissue and blood samples； Signed informed consent

Exclusion Criteria:

• Prior treatment with ADC targeting the same antigen and payload；History of grade ≥3 immune-related adverse events； Active or untreated CNS metastases； Active autoimmune disease requiring systemic therapy； Clinically significant cardiovascular disease； Active interstitial lung disease； Active infection including tuberculosis, HIV, hepatitis B or C； Pregnancy or breastfeeding；Investigator-determined conditions that may interfere with study participation