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

Open to people ages 18 years and up

• 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.

• Presence of brain or subdural metastases, unless participant has completed local therapy and has discontinued the use of corticosteroids for this indication for at least 2 weeks prior to starting treatment in this study.
• Diagnosed with meningeal carcinomatosis.
• Received anticancer therapy (chemotherapy or other systemic anticancer therapies, immunotherapy, radiation therapy, etc) or an investigational drug or device within the past 28 days or 5 half-lives, whichever is shorter.
• Prior treatment with eribulin.
• Any prior allergic reactions of Grade >=3 to monoclonal antibodies or contraindication to the receipt of corticosteroids or any of the excipients (investigators should refer to the prescribing information for the selected corticosteroid).
• Residual toxic effects of prior therapies or surgical procedures that is Grade >=2 (except alopecia or anemia).
• Grade >=2 peripheral neuropathy or history of Grade >=3 peripheral neuropathy or discontinued any prior treatment due to peripheral neuropathy.
• Active pneumonitis/interstitial lung disease (ILD) or any clinically significant lung disease (example, chronic obstructive pulmonary disease), history of Grade >=2 pneumonitis/ILD, or received radiotherapy to lung fields within 12 months of Cycle 1 Day 1 of study treatment.
• Congestive heart failure greater than (>) New York Heart Association Class II or left ventricular ejection fraction (LVEF) less than (<) 50 percent (%) measured by multigated acquisition scan (MUGA) or echocardiogram.
• Has a corrected QT interval prolongation per Fridericia formula (QTcF) >470 millisecond (ms) (for both males and females) based on screening triplicate 12-lead ECG.
• Concomitant active infection requiring systemic treatment, except:
  • If known to be human immunodeficiency virus (HIV)-positive, must be on anti-HIV therapy for at least 4 weeks and have a clusters of differentiation 4+ T-cell (CD4+) count >=350 cells per microliter (cells/mcL) and an HIV viral load <400 copies per milliliter (copies/mL).
  • If meets the criteria for anti-hepatitis B virus (HBV) therapy, must agree to take anti-HBV therapy, if known to be HBV-positive as defined by positive hepatitis B surface antigen or hepatitis B core antibody. HBV viral load must be undetectable.
  • If known to be hepatitis C virus (HCV)-positive must have completed curative therapy for HCV. HCV viral load must be undetectable.
• Known history of active bacillus tuberculosis (TB).
• Any medical or other condition which, in the opinion of the investigator would preclude the participant's participation in the clinical study.