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

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).


Eligibility Criteria:
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.