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

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