Study of ISB 1302 (CD3 Bispecific Ab) in HER2-positive Metastatic Breast Cancer

Inclusion Criteria:

* Females with HER2-positive \[IHC 2 +, with FISH confirmation\] or 3+ \[IHC or FISH\] metastatic breast cancer that has progressed on last therapy. No more than 4 lines of therapy in metastatic setting (of which no more than 2 lines should be anti-HER2 antibody-based therapy).
* Measurable disease, defined as per RECIST v1.1.
* Eastern Cooperative Oncology Group (ECOG ) performance-status score of 2 or less
* Adequate bone marrow, renal, and liver function.
* Recovered from any previous surgery and no history of major surgery within the last 28 days prior to start of study drug
* Must be willing to undergo pre-treatment and on-treatment biopsies in Part 1 and Part 2.

Exclusion Criteria:

* Any suspected or proven immunocompromised state, or infections, such as history of positive human immunodeficiency virus (HIV), known active or chronic hepatitis B virus (HBV) or hepatitis C virus (HCV).
* Any history or evidence of clinically significant cardiovascular disease.
* Evidence of clinically significant cardiovascular and respiratory conditions
* Previous antineoplastic treatment with immune checkpoint regulator or comparable immunotherapy within 8 weeks of starting study drug.
* Chemotherapy, radiotherapy, molecular-targeted therapy, or biological therapies (including HER2-directed therapies) within 4 weeks of starting study drug
* Hormone therapy within 2 weeks of starting study medications.
* Diagnosed with another malignancy that requires active therapy
* Brain metastases that require directed therapy.
* Has not recovered from any therapy related toxicities from previous treatments.
* Use of any investigational drug within 4 weeks from the start of study drug.
* Any condition that, in the opinion of the Investigator, would interfere with evaluation of the study drug or interpretation of subject safety or study results.