An Open-label, Multicenter, Phase 2 Dose Optimization and Expansion Study to Evaluate the Safety and Efficacy of BB-1701, an anti-human epidermal growth factor receptor 2 (anti-HER2) antibody-drug conjugate (ADC), in Previously Treated Subjects with 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 at the time of 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 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 benefit from further endocrine therapy.

- ECOG PS 0 or 1.

- Life expectancy of at least 3 months.

- Adequate organ function and laboratory parameters.

Exclusion Criteria

- Inflammatory BC.

- Presence of brain or subdural metastases, unless participants has completed local

therapy and has discontinued the use of corticosteroids for this indication for at

least 4 weeks prior to starting treatment in this study.

- Diagnosed with meningeal carcinomatosis.

- Presence of malignant effusion/ascites requiring drainage for symptom relief.

- 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 hypersensitivity 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) (including asymptomatic Grade 1) or

any clinically significant lung disease (example, chronic obstructive pulmonary

disease), history of pneumonitis/ILD that required systemic steroids, 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) <50% measured by multigated acquisition scan

(MUGA) or echocardiogram.

- Has a corrected QT interval prolongation per Fridericia formula (QTcF) greater than

(>) 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 less

than 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.