A single arm, phase 2 Study of ARX788 in HER2-positive metastatic breast cancer patients who were previously treated with T-DXd. The ARX788 will be administered every 3 weeks (Q3W) intravenous (IV) infusion.

Inclusion Criteria:

1. 18 to 75 years old (including upper and lower limits), male or female;
2. Unresectable locally advanced, recurrent or metastatic BC;
3. Tissue samples determined to be HER2 positive (defined as IHC3+ or FISH+);
4. Received T-DXd treatment in the advanced stage;
5. Adequate bone marrow, liver, kidney and coagulation function;
6. Voluntarily sign the informed consent, have good compliance and are willing to comply with the follow-up visit.

Exclusion Criteria:

1. With meningeal metastases or disseminated brain metastases or active brain metastases;
2. Has interstitial lung disease requiring steroid therapy, a history of drug-induced interstitial lung disease, a history of radiation pneumonitis, or any evidence indicating clinically active interstitial lung disease;
3. Has any eye disease that require medical intervention such as keratitis, corneal diseases or active eye infection;
4. Has cardiac insufficiency;
5. Has received any systemic anti-tumor therapy (with the exception of endocrine therapy, with an interval of at least 7 days) within 28 days (or at least 5 half-lives) before the first use of the investigational product;