The primary purpose of the Dose Optimization (Part 1) of this study is to assess the safety and tolerability of BB-1701 and to determine the recommended dose (RD) of BB-1701 for Dose Expansion (Part 2). The primary purpose of Dose Expansion (Part 2) is to assess the antitumor activity of BB-1701 at RD in the selected population(s) of breast cancer (BC).

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 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.

Eisai's data sharing commitment and further information on how to request data can be found on our website http://eisaiclinicaltrials.com/.