Advanced Breast Cancer With Ultra-Low HER2 Expression

This is a single-center, single-arm, open-label phase II exploratory study. A total of 30 patients aged 18-75 years with ECOG PS 0-1, pathologically confirmed advanced breast cancer with ultra-low HER2 expression will be enrolled. Eligible patients must have at least one measurable lesion and have received 1-2 lines of prior systemic therapy for advanced disease. All participants receive intravenous SHR-A1811 at 4.8 mg/kg every 3 weeks, until disease progression, intolerable adverse events or other discontinuation criteria. The primary endpoint is objective response rate (ORR). Secondary endpoints include progression-free survival (PFS), overall survival (OS), duration of response (DOR), disease control rate (DCR), clinical benefit rate (CBR), and safety profile.