The goal of this clinical trial is to learn if SHR-A1811 combine with Pyrotinib is safe and tolerable for patients with HER2 positive breast cancer. It will also learn about the anti-tumor efficacy of this combination therapy. Participants will take SHR-A1811 and pyrotinib every three weeks, until disease progression or intolerable toxicity.

Inclusion Criteria:

• Age: ≥18 years old and ≤70 years old;
• Pathologically confirmed HER2-positive locally advanced or metastatic breast cancer;
• Progression during or after prior HER2-targeted therapy and chemotherapy in the advanced/metastatic setting OR progression during/within 12 months after completion of trastuzumab and taxane-based chemotherapy in the neoadjuvant/adjuvant setting OR after 2 cycles of trastuzumab and taxane-based chemotherapy in the neoadjuvant setting with suboptimal response (assessed as non-response) for locally advanced disease;
• At least one measurable lesion per RECIST v1.1 criteria.
• ECOG performance status of 0 or 1.
• Adequate Organ Function
• Female subjects of childbearing potential must have a negative serum pregnancy test within 7 days before the first dose. Male subjects with partners of childbearing potential must be surgically sterile or agree to use effective contraception during the trial. Sperm donation is not permitted during the study period.
• Subjects must voluntarily enroll in this study, sign the informed consent form, demonstrate good compliance, and be willing to cooperate with follow-up.

Exclusion Criteria:

• Prior Pyrotinib in Advanced/Metastatic Setting;
• Prior Tropoisomerase I Inhibitor ADC Therapy;
• Active CNS Metastases;
• Other Malignancy;
• Recent Major Surgery/Trauma;
• Interstitial Lung Disease (ILD)/Severe Pulmonary Conditions;
• Significant Cardiac Disease;
• Inability to swallow oral medication, chronic diarrhea, intestinal obstruction, or other factors significantly affecting drug ingestion or absorption;
• Known hypersensitivity to any component of the investigational drugs in this study protocol;
• Immunodeficiency/Organ Transplant;
• Uncontrolled Third-Space Fluid Accumulation;
• Pregnancy/Breastfeeding/Contraception;
• Severe concomitant illness or comorbid conditions that may interfere with planned treatment or preclude study participation, as assessed by the investigator. This includes, but is not limited to, active hepatitis B or pulmonary infection requiring treatment.