ctDNA-guided Treatment Decision-making

This trial is testing whether using a blood test called ctDNA (circulating tumor DNA) to detect traces of cancer in the bloodstream can guide treatment decisions for people with HER2-negative metastatic breast cancer who are candidates for antibody-drug conjugate (ADC) therapy. The idea is that real-time tumor monitoring may allow more personalized treatment adjustments. **You may be eligible if...** - You are 18 to 75 years old with a good performance status - You have HER2-negative metastatic breast cancer confirmed by biopsy - You are eligible for ADC therapy - You have at least one measurable tumor - You are willing to provide tumor tissue and blood samples **You may NOT be eligible if...** - You have previously received an ADC targeting the same cancer protein - You have had severe immune-related side effects (grade 3 or higher) from prior immunotherapy - You have active or untreated brain metastases - You have active autoimmune disease requiring systemic treatment - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.