ARX788 in HER2-positive, Metastatic Breast Cancer Subjects (ACE-Breast-03)

Inclusion Criteria

• Key Inclusion Criteria: - Age ≥ 18 years and older - Life expectancy ≥ 6 months - Unresectable or metastatic breast cancer subjects - Presence of at least one measurable lesion per RECIST v 1.1 - Subjects must have HER2 positive breast cancer per ASCO-CAP guidelines, documented in a CLIA lab pathology report - Subjects must have had prior treatment with no more than 5 prior regimens of systemic treatment HER-2 targeting therapy or chemotherapy in the metastatic setting. One of these prior treatments must have been treatment with T-DXd. - Subjects with stable brain metastases - Acute toxicities from any prior therapy, surgery, or radiotherapy must have resolved to Grade ≤1 as per the NCI-CTCAE v 5.0, except alopecia, vitiligo, Grade 2 peripheral neuropathy, or endocrine toxicities that are stable on hormone replacement. - Adequate organ functions - Willing and able to understand and sign an informed consent inform and to comply with all aspects of the protocol Key Exclusion Criteria: Any subject who meets any of the following criteria is excluded from the study: - History of allergic reactions to any component of ARX788. - Prior history of interstitial lung disease, pneumonitis, or other clinically significant lung disease. Any requirement for supplemental oxygen. - Any active ocular infections or chronic corneal disorders - History of congestive heart failure, unstable angina pectoris, unstable atrial fibrillation, cardiac arrhythmia, or myocardial infarction within 6 months prior to enrollment - Grade 3 to 4 peripheral neuropathy (NCI CTCAE v 5.0). - History of unstable central nervous system (CNS) metastases - Radiotherapy outside of the brain administered < 7 days prior to first dose of ARX788 - Current severe, uncontrolled systemic disease (eg, clinically significant cardiovascular, pulmonary, or metabolic diseases) - Any uncontrollable intercurrent illness, infection (including subjects with active, symptomatic Covid-19 infections), or other conditions that could limit study compliance or interfere with assessments - Exposure to any other investigational or commercial anticancer agents or therapies administered with the intention to treat malignancy within 14 days before the first dose of ARX788