A Study of BB-1701 in Previously Treated Participants With Human Epidermal Growth Factor Receptor 2 (HER2)-Positive or HER2-low Unresectable or Metastatic Breast Cancer

Inclusion Criteria * Male or female, aged \>=18 years at the time of informed consent. * Metastatic or unresectable BC that is histologically confirmed to be either HER2-positive (defined as an immunohistochemistry \[IHC\] status of 3+, or a positive in situ hybridization \[ISH\] test \[fluorescence, chromogenic, or silver-enhanced ISH\] if IHC status is 2+) or HER2-low (defined as an IHC status of 1+, or 2+ and negative ISH) per the American Society of Clinical Oncology/College of American Pathology guidelines as documented prior to trastuzumab deruxtecan (T-DXd) treatment. * Must have previously received T-DXd. * Sufficient tumor tissue is required for HER2 status testing at a central laboratory. * Measurable disease per RECIST 1.1 as assessed by the investigator. Participants with bone only disease may be eligible if there is a measurable soft tissue component associated with the bone lesion. * Must have previously received at least 1 but no more than 3 prior chemotherapy-based regimes in the unresectable or metastatic setting. If recurrence occurred during or within 6 months of (neo) adjuvant chemotherapy, this would count as 1 line of chemotherapy. * If HR-positive HER2-low BC, must have previously received endocrine therapy and is not expected to further benefit from it. * ECOG PS 0 or 1. * Life expectancy of at least 3 months. * Adequate organ function and laboratory parameters. Exclusion Criteria * Presence of brain or subdural metastases, unless participant has co…