ARX788 for Treating Patients With HER2-low Locally Advanced Unresectable or Metastatic Breast Cancer

Inclusion Criteria: * Male or female participants age 18 years or greater with ability to provide written informed consent for the study. * Eastern Cooperative Oncology Group (ECOG) score of 0-2. * Estimated life expectancy of at least at 6 months per investigator assessment. * Ability to understand and the willingness to sign a written informed consent document. * Pathologically documented HER2-low locally advanced unresectable or metastatic breast cancer (MBC). NOTE: human epidermal growth factor receptor 2 (HER2)-low status determined by HER2 immunohistochemistry (IHC) 1+ or 2+ and no evidence of HER2 gene amplification by in situ hybridization (ISH)/fluorescence in situ hybridization (FISH), which can be documented from any tumor sample during the patient's cancer treatment history (early-stage or metastatic). * Cohort 1: Participants with hormone receptor positive (HR+)/HER2-low locally advanced unresectable or MBC. HR+ status defined as estrogen receptor \>= 10% and/or progesterone receptor ≥ 10% and HER2 low. * Cohort 2: Participants with hormone receptor negative (HR-)/HER2-low locally advanced unresectable or MBC. Considered HR- if estrogen receptor (ER) and progesterone receptor (PR) \< 10% and HER2-low. * Presence of at least one measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) version (v) 1.1. NOTE: Participant's with at least one measurable lytic bone lesion are eligible. * Availability of tumor block or formalin-fixed paraffin-em…